4. ^^I_

SAN-857-1

EVALUATION OF PRACTICABILITY OF A RADIOISOTOPE THERMAL CONVERTER FOR AN ARTIFICIAL HEART DEVICE

Phase 1 Final Report

April 1972

TRWSystems Group Redondo Beach, California

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED*

UNITED STATES ATOMIC ENERGY COMMISSION • TECHNICAL INFORMATION CENTER

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Pt nud n >h» Un lad Stoi«t o' Am r USAEC lachn eol Inioi'Mit on Cini*

S A N - 8 5 7 - 1 D i s t r i b u t i o n C a t e g o r y U C - 8 4

EVALUATION OF PRACTICABILITY OF A RADIOISOTOPE THERMAL CONVERTER FOR

AN ARTIFICIAL HEART DEVICE

PHASE 1 FINAL REPORT

IAN R. JONES, MALCOLM G. RIDGWAY,

AND D.R. SNOKE

TRW SYSTEMS GHOUP

O N E S P A C E P A R K • B E O O fM O O B E A C H • C A L I F O R N I A

DATE PUBLISHED - APRIL 1972

PREPARED FOR THE U.S. ATOMIC ENERGY COMMISSION

DIVISION OF APPLIED TECHNOLOGY

UNDER CONTRACT AT{04-3)-857

NOTICE

This report contains information of a preliminary nature and was prepared primarily for internal use at the originating installation. It is subject to re­vision or correction and therefore does not repre­sent a final report. It is passed to the recipient in confidence and should not be abstracted or further disclosed without the approval of the originating installation or USAEC Technical Information Center, Oak Ridge, TN 37830

DlSTRiBL'TION 0^ THIS DOCUMENT IS UNLUv/llTED

I hi

CONTENTS

Page

INTRODUCTION AND SUMMARY 1-1

1. 1 Descr ip t ion of P r e f e r r e d System Concept 1-4

1.1.1 Hybrid Heat Engine 1-4 1.1.2 Moto r /Rec ip roca to r Unit 1-6

1. 1.3 Automatic Actuator 1-7

1. 2 Growth Capabili ty 1-10

1.3 Sensit ivi ty Analysis 1-12

1.4 Conclusions 1-13

SYSTEM DESIGN CONSIDERATIONS 2-1

2. 1 Groundru les 2-1

2. 2 Sys tem Terminology 2-3

2. 3 Al te rna te Conceptual Approaches 2-3

2. 3. 1 System Load C h a r a c t e r i s t i c s 2-5

2. 4 Waste Heat Management 2-12

2 .4 . 1 Heat Generat ion Rates 2-12 2. 4. 2 Related Exper imen ta l Studies 2-14

2.4.3 Peak Heat Reject ion Rates 2-17

2. 5 Packaging Considera t ions 2-17

2, 5. 1 Packaging Configuration 2-18 2. 5. 2 Packaging M a t e r i a l s 2-18

2.6 Gas Management 2-19

2. 7 T h e r m a l Insulation 2-20

2. 7. 1 F ibrous Insulation 2-20 2. 7. 2 Foi l Insulation 2-22 2. 7. 3 T h e r m a l Insulation Selection 2-24 2.7.4 O v e r t e m p e r a t u r e P r o t e c t i o n 2-24

2. 8 Ene rgy Storage 2-25

2. 8. 1 T h e r m a l Energy Storage Mate r i a l (TESM) 2-25

2 . 8 . 2 E l ec t rochemica l Ene rgy Storage 2-28

2.9 Radioisotope Capsule Design 2-32

COMPONENT SUBSYSTEMS 3-1

3. I Power Conditioning and Control 3-1

3. 1. 1 Blood Pump Interface 3-1 3. 1. 2 Blood P u m p Fil l ing Requ i remen t s 3-3 3 . 1 . 3 Actuation of the Blood Pump 3-4 3. 1. 4 Power Conditioning 3-13 3 . 1 . 5 P e r f o r m a n c e Summary 3-23

ii

7-

CONTENTS (Continued)

Page

3. 2 Engine Subsys tems 3-27 3. 2. 1 Gas Reciprocat ing Engines 3-29 3. 2. 2 Linear Vapor 3-70 3. 2. 3 Rotary Vapor 3-91 3 . 2 . 4 T h e r m o e l e c t r i c s 3-148 3. 2. 5 Hybrid 3-166

RELIABILITY OF THE CANDIDATE COMPONENTS 4-1

4. 1 P r e m a t u r e Fa i lu re Rel iabi l i ty Modeling 4-2

4. 1. 1 Vapor /Gas Bear ings 4-5 4. 1. 2 Expansion Turbine 4-6 4. 1. 3 Bellow Seals and Pumps 4-7 4. 1.4 P r e c i s i o n Bal l /Hydrodynamic Sleeve

Bear ings 4-8 4. 1. 5 E lec t ron ic Components and Solid State

Devices 4-10 4. 1. 6 Gear Boxes /Speed Reducers 4-10 4. 1. 7 High Energy Density Bat te r ies 4-11 4 . 1 . 8 PCCS Ci rcu l a r Drive Cam 4-12

4. 2 Summary of the P r e m a t u r e Fa i lu re Reliabili ty E s t i m a t e s for the Candidate Systems 4-13

4 , 2 . 1 System Rel iabi l i ty Math Model Results 4-13 4. 2. 2 Hybrid The rma l Conver te r 4-13 4. 2. 3 Hybrid The rma l Conver te r with

Bat te ry 4-18 4 . 2 . 4 T h e r m o e l e c t r i c / B a t t e r y Thermal

Conver te r 4-18 4. 2. 5 Rotary Vapor Engine 4-20 4. 2. 6 E l e c t r i c a l PCU 4-20 4. 2. 7 Mechanical Actuator for E lec t r i ca l

Sys tems 4-20 4. 2. 8 Gas Reciprocat ing Engine 4-22 4. 2. 9 TESM 4-22 4 . 2 . 10 Gas Reciprocat ing Engine PCU 4-22 4 . 2 . 11 Gas Reciprocat ing Engine Actuators 4-22 4. 2. 12 Linear Vapor Engine and PCU 4-26 4. 2. 13 Linear Vapor Engine Actuator 4-26

4. 3 Reliabil i ty Against Wearout 4-26

4. 4 Confidence in the Rel iabi l i ty E s t i m a t e s 4-29

CANDIDATE SYSTEM DESCRIPTION 5-1

i i i

CONTENTS (Continued)

P a g

EVALUATION CRITERIA AND SCORING

METHODOLOGY 6-1

CANDIDATE SCORING 7-1

SENSITIVITY ANALYSIS 8-1

8, 1 Introduction 8-1

8. 2 Distr ibution of C r i t e r i a by Category 8-2

8, 3 Effects of C r i t e r i a Scoring Revis ions 8-4

8. 4 Al te rna te Scoring Techniques 8-7 8. 5 Conclusions 8-9

REFERENCES 9-1

APPENDIX A-I

^

Acknowledgments

Of the many TRW personnel who contributed to the Phase I Pro jec t ,

the following dese rve sepa ra t e ment ion : J . P . Aha, R. D. Baggenstoss ,

J. E. Boretz , D. J. Dunivin, K. E. Green, A. R. Halpern, L. M. Osborne,

S. R. Rocklin, G. D. Shaw, and B. A. Snoke.

In addition, two of our consul tants m e r i t special mention: Dr. Y. Nose

of the Cleveland Clinic Foundation, and Dr. T. Finkels te in of Trans

Computer Assoc i a t e s .

r

Abs t r ac t

The objective of the p rog ram was to de te rmine the pract icabi l i ty

of developing a radioisotope t h e r m a l conver t e r for an ar t i f ic ia l h e a r t

device. The the rma l conver t e r , including the radioisotope heat sou rce ,

and al l a s soc ia ted power conditioning and control s y s t e m s a r e to be

fully implantable and capable of functioning for ten y e a r s with high

re l iabi l i ty . The device m u s t supply the n e c e s s a r y mechanica l ,

hydraul ic , or pneumatic power for a blood pump which provides 100%

of the left ven t r i cu la r pumping function.

The pr incipal design groundru les included a daily average power

level of 2 .8 watts del ivered to the blood pump and the following design

max ima: 60-wat t heat sou rce , 1. 5- l i te r volume, 3 .0 -k i l og ram weight,

and peak power level of 4. 44 wat t s .

The concepts evaluated included var ious mechaniza t ions of gas

rec ip roca t ing , vapor r ec ip roca t ing , vapor ro t a ry , and t h e r m o e l e c t r i c

cyc les , as well as var ious combinat ions of these dev ices . A total of

eight different t he rma l conver t e r s y s t e m s were found capable of

meet ing al l the design g roundru le s .

These eight sy s t ems were compared using previously-defined

evaluat ion c r i t e r i a and scor ing p r o c e d u r e s . The p re fe r red concept

is an a l l - e l ec t r i c -ou tpu t hybrid engine which cons i s t s of a t h e r m o ­

e l ec t r i c s tage operated the rma l ly in s e r i e s and e lec t r i ca l ly in pa ra l l e l

with an organic vapor tu rbogene ra to r . The hybrid engine powers a

m o t o r / r e c i p r o c a t o r unit which in turn mechanica l ly ac tua tes a Kwan-

Gett- type blood pump. The weight and volume of the ent i re t h e r m a l

conver ter (exclusive of the blood pump) a r e 1. 83 kg and 1. 33 l i t e r s .

The t h e r m a l conver te r r e q u i r e s a 49-wat t heat sou rce and has an overa l l

h e a t - t o - m e c h a n i c a l convers ion efficiency of a l i t t le over nine percen t .

v i

1. INTRODUCTION AND SUMMARY

The overa l l objective of the p r o g r a m is to de te rmine the p r a c t i c a -

bilitv of developing a fully implantable radioisotope t h e r m a l conver te r to

power a h e a r t - a s s i s t pump that is capable of assuming 100 percent of the

l e f t -ven t r icu la r pumping function, and has a high probabili tv of operat ing

acceptably for 10 y e a r s . The objective of this f i rs t phase of the study

was to evaluate all the poss ib le concepts and recommend a p re fe r r ed

sys tem for m o r e detai led evaluat ion in Phase II.

The Phase I p rac t icab i l i ty evaluat ion of a radioisotope t he rma l con­

v e r t e r was c a r r i e d out under the design groundru les l is ted in Table 1-1,

In addition to these physical and per fo rmance specif icat ions, we imposed

the r e q u i r e m e n t that all candidate s y s t e m s utilize fibrous t he rma l insulation

Tab le 1-1 . l^r inu ' I')csign G r o u n d r u l e s

P o w e r supp ly to s u p p o r t blood p u m p in 1 00"'(i-assist left v e n t r i c u l a r funct ional r e p l a c e m e n t m o d e ( i . e . , to take ove r 100";) of lef t v e n t r i c u l a r work not s i m p l y c a p t u r e 100'^', of c a r d i a c output)

N o m i n a l a v e r a g e output power 2 . 8 1 wa t t s (to 60% ef f ic ien t K w a n - G e t t -type of blood pump)

r>

M a x i m u m hea t s o u r c e

Outpu t power r a n g e

E n e r g y s t o r a g e

M a x i m u m blood flow r a t e c a p a b i l i t y

Blood p r e s s u r e a t p u m p output

M a x i m u m p u m p cyc le r a t e

P a c k a g e

M a x i m u m to ta l volume (excep t blood pump)

M a x i m u m Weight

D u r a b i l i t y

F i l l i n g p e r f o r m a n c t

60 wa t t s ( t h e r m a l )

2 . 22 to 4 . 44 w a t t s a c c o r d i n g to d a i l y power prof i le

S. 58 w a t t - h o u r s a t input to blood p u m p

12 l i t e r s / m i n u t e

100 m m m e r c u r y (mean)

120 c y c l e s / m i n u t e

fully i m p l a n t a b l e , t o t a l l y self con t a ined

1 5 l i t e r s

3, 0 k i l o g r a m s

s y s t e m l i f e t ime 10 y e a r s m i n i m u m with r e a s o n a b l e l e v e l of r e l i a b i l i t y

lef t a r t e r i a l p r e s s u r e m a i n t a i n e d b e t w e e n 0 and 18 m m m e r c u r y g a u g e

1-1

y

with an iner t gas fill s ince we do not feel confident that vacuum

super insula t ion can be made re l iab le enough for 10-year operat ion. We

also imposed the r e q u i r e m e n t that the t he rma l converter surface a r ea

be adequate to p e r m i t re ject ing al l waste heat d i rec t ly to the body fluids. 2

We as sumed a value of 0. 07 w a t t / c m for the allowable heat flux a c r o s s

the conta iner . However, we did a s s e s s the effects of using supe r insu la ­

tion or a l t e rna te waste heat reject ion techniques.

We found a total of eight candidate t h e r m a l conver t e r sy s t ems

capable of meet ing al l the design g roundru les . The ma in c h a r a c t e r i s t i c s o

these eight sy s t ems a re summar i zed in Table 1-2. More complete

desc r ip t i ons a re contained in Section 5 of this r epo r t . When they a r e

compared on the bas i s of 24 evaluat ion c r i t e r i a , the a l l - e l e c t r i c hybrid

t h e r m o e l e c t r i c and ro t a ry vapor sy s t em (without a bat tery) emerged as

the m o s t sa t i s fac tory sys t em concept.

Since the candidate designs using a t h e r m a l to e l ec t r i ca l convers ion

stage genera te a constant level of power, any energy s to rage m u s t be

downs t ream of the main t he rma l engine. S t a t e -o f - the -a r t n ickel -cadmiurn

s torage b a t t e r i e s a r e too heavy, while the promis ing h igh-energy-dens i ty

solid e lectrolyte ba t t e r i e s a r e not yet sufficiently well developed. In a

ce r t a in sense , one can cons ider the t h e r m o e l e c t r i c s tage in the selected

design as subst i tut ing for the energy s torage function on an i n t e r i m bas i s

There is one pa r t i cu l a r major s y s t e m - l e v e l advantage to all of the

candidate e l ec t r i ca l s y s t e m s . This pe r t a ins to the i r abili ty to efficiently

match the var ia t ion in the load on the sys t em during the period of a

single ejection s t roke . During this f i r s t phase of the study, we

a s sumed , as d i rec ted , that the blood pump opera tes agains t a constant

b a c k p r e s s u r e of 100 mm Hg. But in rea l i ty , the p r e s s u r e va r i e s from

about 80 m m at the beginning of systole to something between 120 and

180 m m Hg at the end of systole depending on the output power leve l .

There fo re , the candidate approaches with cons tan t - force r a t h e r than

constant d i sp lacement c h a r a c t e r i s t i c s m u s t be p re se t to del iver the

output a t peak r a t h e r than average p r e s s u r e . All except the e l e c t r i c a l

candidates have this mi sma tched cons tant - force type of output

c h a r a c t e r i s t i c . Since the excess work is not r e c o v e r a b l e , the t rue

power r e q u i r e m e n t s for these engines \w 11 be somewhere between 20%

' " /

Table 1-2. Candidate Systems

Gas Reciprocat ing

Vapor Reciprocat ing

Rotary Vapor

The rmoe lec t r i c s

T h e r m o e l e c t r i c s and Rotary Vapor

Synchronous

TESM

Long life engines with output r e q m r e s design not yet proved in this s

48 watts 1.42 l i t e r s 1.78 kg Variable Frequency 60 - 120 bpm

None

mechanical concepts ize range

57 watts 1.40 l i t e r s 1.34 kg 120 bpm

Nonsynchronous

TESM

48. 9 watts 1. 23 l i t e r s 1.75 kg 120 bpm

Large heat source requi red

Bat te ry

41 watts 1. 06 l i t e r s 1. 46 kg 120 bpm (most compact)

54 watts 1. 38 l i t e r s 1.51 kg 120 bpm

37 watts 1. 4 l i t e r s 1. 84 kg 120 bpm (most efficient)

None

54. 4 watts 1. 36 l i t e r s 1. 57 kg 120 bpm

Large heat source requi red

Most s imple design but too inefficient

Theoret ical ly m o s t - r e l i a b l e but l a rge heat source required

49 watts 1. 33 l i t e r s 1. 83 kg 120 bpm (most p r a c t i c ­able)

and 80% g r e a t e r than calculated according to the Phase I g roundru les .

For the a l l - e l e c t r i c a l s y s t e m s , however , the e l ec t romechan ica l

energy s to rage and t r ans fe r c h a r a c t e r i s t i c s a re such that the sys t em will

follow the var ia t ions in the b a c k p r e s s u r e and, the re fo re , the blood pump

energy reqxairements can leg i t imate ly be calculated for the specified 100

m m average b a c k p r e s s u r e .

1. 1 DESCRIPTION OF P R E F E R R E D SYSTEM CONCEPT

The hybrid t he rma l conver t e r sys t em is compr i sed of three major

components, namely :

• Hybrid Heat Engine

• Mo to r /Rec ip roca to r Unit

• Automatic Actuator

This sys t em ha s projected weight of 1.83 kg and occupies a total

volume of 1.33 l i t e r s .

1.1.1 Hybrid Heat Engine

The hybrid heat engine subsys tem (Figure 1-1) includes a

49-wat t heat source which inco rpora t e s a vent and cap i l l a ry tube

a s s e m b l y for r e l e a s e of the helium vented from the heat source into

the abdominal cavity. Mounted on each end of this cy l indr ica l capsule

a r e two cascaded t h e r m o e l e c t r i c conver ters- These conve r t e r s a r e

composed of 24 s i l i con -ge rman ium h i g h - t e m p e r a t u r e couples operat ing

with a hot jxonction t e m p e r a t u r e of 17 42 F (950 C) and a cold junction

t e m p e r a t u r e of 932 F (500 C). The cascaded l o w - t e m p e r a t u r e conver te r

cons is t s of 60 2N/TAGS couples opera t ing at a hot junction t e m p e r a t u r e

of 887°F (475°C) and a cold junction t e m p e r a t u r e of 464°F (240°C).

Under these conditions, these conve r t e r s provide an output power of 3.21/

watts at 3. 64 volts and an overa l l efficiency of 6. 55%. Combined with

this t h e r m o e l e c t r i c conver te r is a r o t a r y vapor cycle tu rbogenera to r

operat ing a t a peak cycle t e m p e r a t u r e of 430 F . This t e m p e r a t u r e

is compatible with the cold junction t e m p e r a t u r e of the 2N/TAGS

t h e r m o e l e c t r i c couples . The r o t a r y vapor cycle sys tem uses CP-34

(thiophene) as a working fluid. Operat ing between a peak cycle t e m p e r a ­

tu re of 430 F (at 250 psia) and a condensing t e m p e r a t u r e of 116°F (4 psia

1-4

t h i s uni t i s c a p a b l e of p r o v i d i n g 7. 24 w a t t s a t 15 vo l t s and a c h i e v e s an

o v e r a l l e f f ic iency of 15. 75%. T h e m a i n c o m p o n e n t s of t h i s r o t a r y vapo r

c y c l e s y s t e m a r e shown in F i g u r e 1 -1 . When i m p l a n t e d in the body , the

eng ine wi l l be m o u n t e d v/ith the t u r b o g e n e r a t o r shaft in the v e r t i c a l a x i s .

The t u r b o g e n e r a t o r o p e r a t e s at 9 6 , 0 0 0 r p m and i s s i m i l a r in s i z e and d e s i g n

to u n i t s a l r e a d y bu i l t by A i r e s e a r c h M a n u f a c t u r i n g Co . wh ich o p e r a t e a t

s p e e d s up to 220 , 000 r p m .

The e n g i n e un i t i s i n s u l a t e d wi th f i b r o u s M i n - K i n s u l a t i o n f i l led wi th

xenon g a s . The w a s t e h e a t i s r e j e c t e d t h r o u g h c o n d e n s e r t u b e s on t h e

i n s i d e of t h e t i t a n i u m c o n t a i n e r . T h e s u r f a c e a r e a s of both of the c o n t a i n e r

p a c k a g e s i s suf f ic ient to p e r m i t h e a t r e j e c t i o n to t h e l oca l body t i s s u e s at 2

flux l e v e l s l e s s t h a n 0. 07 w a t t / c m .

In o r d e r to c o m b i n e the o u t p u t s f r o m the two e l e c t r i c a l g e n e r a t o r s ,

the ou tpu t f r o m the t h e r m o e l e c t r i c un i t m u s t be c o n v e r t e d to 1 5 v o l t s ,

r e d u c i n g the e l e c t r i c a l p o w e r ou tpu t f r o m 3 . 2 1 to 2. 6l w a t t s • The

o v e r a l l c o n v e r s i o n e f f i c i ency ( t h e r m a l to e l e c t r i c a l ) a c h i e v e d by the

h y b r i d h e a t e n g i n e i s 20 . 5%.

MAGNETIC COUPtING THROUGH HERMETIC

SEAt

9 000 RPM PERMANENT MAGNET BRUSHtESS DC MOTOR RECIPROCATING DRUM-CAM

SHUTTtE (120 RPM)

GAS BEARINGS

COMPOUND PtANETARY GEAR SPEED REDUCER

-3.5-

MECHANICAt OUTPUT CONNECTOR

HARDENED GUIDE PIN

STATIONARY tINEAR GUIDE BEAMS

Figure 1-2. Moto r /Rec ip roca to r Unit

1.1.2 Moto r /Rec ip roca to r Unit

The m o t o r / r e c i p r o c a t o r unit is shown in F igure 1-2. This unit con­

s i s t s of a b r u s h l e s s dc motor operat ing at 15 vdc and 9,000 rpm. The

motor is s i m i l a r in design to units used for the Minuteman guidance and

control gy rocompass . These uni ts have been operat ing continuously at

16,000 rpm for over 4 y e a r s now. The rotor is of the permanent magnet ic

field type and the s ta tor r e s e m b l e s a conventional a r m a t u r e winding without

a mechanical commuta tor . The motor opera tes on gas bear ings in an

he rmet ica l ly sealed housing. The motor is coupled to a se t of reduction

gea r s through a magnet ic coupling. The reduction ge a r s d e c r e a s e the

motor output speed to 120 rpm cind dr ive a ro t a ry cam. This ro t a ry motion

converted to rec ip roca t ing motion by a cajn-follower opera t ing in a

continuous e l l ip t ical groove provided on the inner surface of the r o t a r y

Cam. The cam follower i m p a r t s rec iproca t ing motion to a rod which has

an overa l l s t roke of 1. 3 i nches . A bellows sea l i s provided to r e t a in

the cam and rec iproca t ing cam follower lubr icant . An e lec t ron ic control

sys t em is a lso provided to implement var ious control functions. This

is shown in F igure 1-3. A tu rbogenera to r speed control is provided.

1-6

/ ^

WMMTOR IS . ? 7««

SPCID CmTROllER

BRUSHLtSS 0 C

MOTO"

MOTM EllCTIWtrC

CUCIIITS

1 ^n

Figure 1-3, E lec t ron ic Controls Block Diagram

When the output voltage, which is d i rec t ly re la ted to genera to r speed,

r i s e s above a ce r t a in se t point, a c i rcu i t consist ing of a f i l ter , a

compara to r with h y s t e r e s i s , and a power switch, turn on a pa ras i t i c

load. When the load is added, the tu rbogenera to r speed is reduced which

reduces the output vol tage. The c i rcu i t is configured to provide speed

control within 4^2%. As was previously mentioned, a dc - to -dc conver te r

i s included to i n c r e a s e the t h e r m o e l e c t r i c conver te r output voltage from

3. 64 to 15 vol t s . To avoid a m i s m a t c h of impedances with the two

para l le l power suppl ies , a l oad - sha r ing e lec t ronics c i rcui t is provided.

This pe rmi t s switching from one power source to the other at a very

high r a t e . Final ly , a motor overspeed control is used to ensure con­

stant speed output from the b r u s h l e s s dc motor . With the exception of the

s p e e d - c o n t r o l l e r , all the e lec t ron ic controls will be mounted on the m o t o r -

cam r e c i p r o c a t o r sy s t em. The overa l l efficiency of the m o t o r / r e c i p r o c a t o r

i s 59%- Several a l te rna te m o t o r / r e c i p r o c a t o r configurations a re d iscussed

in Se ction 3 . 1 . 4 . 2 .

1 .1 .3 Automatic Actuator

The combined blood pump/actuator unit is shown in Figure 1-4. The

dimensions of the pump were derived by scaling the Kwan-Gett design for

a stroke volume displacement of 125 m i s . (Volumetric efficiency assumed

to be 80% and a blood volume per stroke of 100 ml — 12 1/min at 120 cpm. )

We have considered it permiss ible to include a part of the blood pump

actuator within the physical envelope of the pump unit, provided that we do

not change the functional or material interface between the blood pump and

the blood.

The electrical motor/reciprocator unit generates a mechanical back-

and-forth motion which is transmitted to the pump actuator through a flexi­

ble plastic-coated braided metal cable. A spring within the actuator unit

1-7 /3

Figure 1-4. Combined Blood P u m p / Automatic Actuator Unit

provides a force b ias which effectively main ta ins the cable in tension which

is the p r e f e r r e d operat ing mode during all phases of the pumping cycle .

With this des ign, no s e n s o r s a r e requ i red to mainta in control of the

punnping action, the output power is automat ica l ly modulated by a var ia t ion

in the pumping duty cycle (between 25 and 50% at a fixed r a t e of 120 b e a t s /

minute) . The duty cycle is automatical ly regulated by the novel ac tuator

design and the control led cons tan t - speed c h a r a c t e r i s t i c of the e l ec t r i c

mo to r .

The m o s t impor tan t fea ture of the automatic ac tuator i s the additional

fluid r e s e r v o i r which is contained behind the flexible membrsme located

inside the per fora ted casing which f o r m s the lower surface of the com­

bined blood pump/ac tua to r unit . This flexible r e s e r v o i r , or make -up

r e s e r v o i r as shown on the f igure , al lows the power piston in the unit to be

physical ly decoupled f rom the blood pumping m e m b r a n e . There fo re , the

piston goes through a d isp lacement cycle which is not locked to the blood

)-8

inflow r a t f and tb-^ r ' '8u'/ i j :g d i sp lacement ol tn*. pui. ^Ui^ membrcine. Thus

the power pis ton is continuously cycled through a fijc^d volumetr ic d i sp lace ­

ment at a constant r a t e of 120 cyc les p e r minu te .

At the onset of the filling p h a s e , the pumping m e m b r a n e and the

r e s e r v o i r m e m b r a n e a r e max ima l ly displaced. As the power piston begins

i ts downward d i sp lacemen t , the blood pump begins to fill at a r a t e d e t e r ­

mined by the left a t r i a l p r e s s u r e , and the re fo re , co r re sponds to the phys io­

logical demand. If this inflow r a t e is l e s s than the max imum, the constant

suction genera ted by the power pis ton will begin to reduce the p r e s s u r e in

the blood pump. However , as soon as the p r e s s u r e begins to t r y to fall

below body ambient , the m e m b r a n e enclosing the m a k e - u p r e s e r v o i r begins

to co l lapse , maintaining the pump p r e s s u r e very c lose to the body r e f e r ­

ence p r e s s u r e .

By the end of the filling cyc le , the m a k e - u p r e s e r v o i r has been

depleted by an amount equal to the difference between the max imum s t roke

volume and the value cor responding to the actual demand at this t ime .

When the power pis ton r e v e r s e s d i rec t ion , the ejection phase begins with

ini t ial flow back into the low b a c k - p r e s s u r e of the m a k e - u p r e s e r v o i r .

When the r e s e r v o i r chamber is ref i l led , the p r e s s u r e r i s e s until the

forward-f low valve in the blood pump opens and the blood is d i scharged

into the a o r t a . At the complet ion of the pis ton ups t roke , the pumping m e m ­

brane i s r e tu rned exact ly to the m a x i m a l d i sp lacement posi t ion, and all of

the blood which was admit ted during filling is d i scharged .

In shor t , the ac tua tor au tomat ica l ly de l ive r s a pa r t i a l s t roke , a c c o r d ­

ing to the physiological demand; i t p rov ides the posi t ive filling action n e c ­

e s s a r y to main ta in s y s t e m cont ro l ("pass ive autoregulat ion") with the

manda to ry ant isuct ion cont ro l ; and it p r e s e n t s a very s imple mechan ica l

in te r face to the ac tua tor d r ive unit .

Careful maj iagement of the fluid volume behind the power piston is

n e c e s s a r y in o r d e r to avoid significant power l o s s e s . A gas fill (nitrogen

or carbon dioxide) w^hich is contained in a high quality rubber m e m b r a n e

sac (butyl o r poss ib ly viton) is nominal ly used to accommodate the volu­

m e t r i c change, pa r t l y by expanding into a m e m b r a n e - e n c l o s e d space s u r ­

rounding the power connect ion cable and pa r t l y by gas compres s ion .

1-9

The power piston itself is specia l ly designed so that it fits very

c losely behind the blood pumping m e m b r a n e , leaving only a ve ry thin

layer of liquid to even out the pumping s t r e s s e s and to prec lude d i rec t

physical contact during pumping. The overa l l d i ame te r of the piston is

about 4 inches and it r e q u i r e s a s t roke of 1 . 33 inches to d isplace the

full 125 m l . The piston has a double m e m b r a n e const ruct ion with a

sma l l t rapped volume of liquid or l ight g r e a s e . This feature prevents

the m e m b r a n e s from tending to fold or wrinkle as the effective length

of the m e m b r a n e s change during the pumping cycle . With the t rapped

volume, t' e m b r a n e s a r e smoothly bowed out into a m o r e convex

coniip ,xon in the in t e rmed ia t e pos i t ions . At the top and bot tom

posi t ions , the inner m e m b r a n e is pulled a lmos t flat. Without the

double l aye r , the swept volxome would be reduced by the m e m b r a n e s

being deflected into a r e v e r s i n g concave shape by the flow r e s i s t a n c e s

of the d isplaced f lmds.

The overa l l efficiency of the automat ic ac tua tor is 76%. This

value, coupled with the 59% efficiency of the m o t o r / a c t u a t o r , t r a n s f o r m s

the 9. 85 watts of e l ec t r i c a l power from the hybrid engine into the

requ i red 4 .44 watts of hydraul ic power into the blood pump. Models of

the hybrid engine, m o t o r / r e c i p r o c a t o r , and automat ic ac tua tor a r e

shown together in F igure 1-5.

1 . 2 GROWTH CAPABILITY

One of the m o s t a t t r ac t ive a spec t s of our p r e f e r r ed concept is i ts

growth capabil i ty. As can be seen in F igure 1-6, t h e r e a r e a number of

different ways that the sys t em can "g row" in t e r m s of pe r fo rmance and

re l iab i l i ty . (The hybrid sys t em a l ready has the highest re l iab i l i ty

ra t ing among the candidates) .

One option would be to go to a p u r e r o t a r y sys tem when minor

improvemen t s a r e made in e i ther engine, pump, or ac tua tor efficiency.

The heat source s ize requ i red to do this w^ith p re sen t ly predic ted or

a s sumed efficiencies is only a few watts over the 60-wat t l imi t .

Other opportuni t ies a r i s e if the h igh-energy-dens i ty solid e lec t rolyte

b a t t e r i e s now under development at TRW (sodium-sulfur) and Argonne

National Labora to ry ( l i th ium-se lenium) become state of the a r t . (The

1-10

JO.

--<i

Figure 1-5. Thermal Converter

SIMPLEST DESIGN

/ ROTARY \ SMALLEST PHYSICAL

\ BATTERY / S'^E

HIGHEST THEORETICAL

RELIABILITY

Figu re 1-6. Growth Capabili ty

energy density of NiCd b a t t e r i e s i s too low to p e r m i t the i r ut i l izat ion. ) One

option would be to re ta in the hybrid des ign, but reduce the heat source to

37 wat t s . Two other options a r e to go pu re r o t a r y or pu re t h e r m o e l e c t r i c .

The la t te r option would probably yield the highest poss ib le sys tem

rel iabi l i ty .

K 10-year - l i fe t ime super insula t ion packages become avai lable , the

potential exis ts for a further reduction in size and weight. However , we

have found that for the m o r e compact s y s t e m s , a dec reased insulation

thickness will not r e su l t in a corresponding reduction in conver te r volume

since the conver te r size i s c lose to opt imum when specific gravity and

allowable heat flux fac tors a r e taken into account.

1.3 SENSITIVITY ANALYSIS

An analys is was c a r r i e d out to de te rmine the sensi t ivi ty of our

selection p r o c e s s to subjective assumpt ions . While the init ial pe rcen tage -

point spread ainong the top candidates was sma l l , all the logically induced

per turba t ions in scoring left the hybrid sys tem in the top- ranked position

by an increased nnargin.

1-.U' / «

1.4 CONCLUSIONS

In summary the p r e f e r r e d candidate approach can be seen to

demons t r a t e s e v e r a l c l ea r advan tages .

• All of the components a r e capable of providing 10-year durabi l i ty using exis t ing , demons t ra t ed s t a t e -o f - t he -a r t technology.

• The output of the s y s t e m provides an efficient match with the load c h a r a c t e r i s t i c of the positive d i sp lacement blood pump. This r educes the heat source r e q u i r e m e n t by a factor of a lmos t two compared with non-matched sys t ems .

• The novel ac tua tor design efficiently provides overa l l sy s t em contro l using the exper imen ta l ly -p roved 'pass ive autoregulat ion ' approach .

• The design approach is technical ly flexible and capable of incorpora t ing and taking advantage of technology such as solid e lec t ro ly te b a t t e r i e s which will probably be developed in the re la t ive ly n e a r future.

1-13

/I

millerc

Text Box

BLANK

2. SYSTEM DESIGN CONSIDERATIONS

2. 1 GROUNDRULES

The p r i m e design groundru les for the Phase 1 study a re l i s ted in

Table 2.1-1. In con t ras t with previous design studies, these groundrules

emphas ize the capabili ty of the design for long life and high re l iabi l i ty in

addition to acceptable functional pe r fo rmance , min imum size and weight,

and high overa l l t h e r m a l convers ion efficiency. Since the objective of

the pro jec t is to demons t r a t e p rac t icab i l i ty , in addition to feasibili ty, we

have de l ibera te ly emphas ized s y s t e m s utilizing technology with good

development predic tabi l i ty .

Consider ing the key re l iab i l i ty quest ions of wear -ou t and p rema tu re

fa i lure , which a r e d i scussed in some detai l in Section 4, i t is clear that

these fundamental sy s t em qual i t ies tend to be in conflict with seve ra l

specified per formance goals . Rel iabi l i ty can be enhanced by conservat ive

design and carefully control led fabr icat ion at the expense of p a r a m e t e r s

such as s ize , weight and unit product ion cost .

As a f i rs t s tep towards the re l i ab i l i ty goals , all of the s t r e s s e d

p a r t s in the s y s t e m s , and the s ea l s in pa r t i cu l a r , mus t be designed with

conserva t ive s t r e s s m a r g i n s . This r equ i r emen t will be pa r t i cu la r ly

ha rd to mee t in the case of the synchronous rec iproca t ing gas cycle

engines because of the re la t ively high force levels which must be deve l ­

oped by the prinne mover during the ejection p h a s e s . The mechanica l

s t r e s s e s a r e lower in the nonsynchronous r e c i p r o c a t o r s because the

s t roke work l eve l s a r e lower at the h igher engine speeds . The technical

feasibi l i ty of the nonsynchronous gas cycle engines has been demons t ra ted

in t h e r m a l engine p r o g r a m s that have been underw^ay for seve ra l y e a r s .

Severed key a r e a s re la t ing to the quest ion of a p rac t i ca l design for long

life and high re l iab i l i ty a r e s t i l l under study.

In looking for a l te rna te candida tes , a sys t em with an a l l - s t a t i c

t h e r m a l conver te r i s an obvious approach with a c lea r potential

re l i ab i l i ty edge, although the na tu re of the sys t em output d ic ta tes that

a l l of the s y s t e m s will have at l eas t some moving p a r t s . Another a r e a

w h e r e r ecen t advances re la t ing to long-l ife dynamic conve r t e r s justify

2-1

Table 2.1-1. P r i m e Design Groundrules

• Power supply to support blood ven t r i cu la r functional rep lace i 100% of left ven t r i cu la r work i ca rd iac output)

• Nominal ave rage output power (to 60% efficient Kwan-Gett type of blood pump)

• Maximum heat source s ize

• Output power range

• Energy s to r age capacity

• Maximum blood flow r a t e capabil i ty

Blood p r e s s u r e at pump output

Maximum pump cycle r a t e

• Package

Maximum total volume (except blood pump)

Maximum total weight

• Durabi l i ty

• Fi l l ing P e r f o r m a n c e

pump in 100%-assis t left l en t mode ( i . e . , to take over ot s imply capture 100% of

2 .81 watts

60 watts ( thermal)

2. 22 to 4. 44 watts according to specified daily power profile

5. 58 wa t t -hour s , at input to blood pump

12 l i t e r s / m i n u t e

100 m m m e r c u r y (mean)

120 cyc l e s /minu te

fully implantable , totally self-contained

1. 5 l i t e r s

3. 0 k i lograms

s y s t e m l i fet ime 10 y e a r s min imum with r easonab le level of re l iab i l i ty

left a r t e r i a l p r e s s u r e m a i n ­tained between 0 and 18 m m m e r c u r y gauge.

Z- L

cons idera t ion for this application is that of hydrodynamic bear ings for

rotat ing mach ine ry . These techniques which a r e now considered to be

s t a t e - o f - t h e - a r t for commerc ia l products , have excellent p romise for

very low wear and long life.

2 .2 SYSTEM TECHNOLOGY

The terminology which has been used throughout this study is

shown in F igure 2.2-1. The total t he rmal conver te r sys tem which is the

m e c h a n i s m by which t h e r m a l energy from the radioisotope heat source

is conver ted to another energy form suitable for pumping blood is

subdivided into two p r i m a r y s u b s y s t e m s :

• The engine subsys tem which is defined as containing the p r ime power converter (or engine) and engine modulator (if any), and the energy s to rage unit ( thermal energy s to rage m a t e r i a l or e l ec t rochemica l ba t te ry) .

• The power conditioning and control subsys tem which is defined as containing the power conditioning units (if any), all n e c e s s a r y control s enso r s and signal condi t ioners , and the blood pump ac tua tor .

2. 3 ALTERNATE CONCEPTUAL APPROACHES

There a r e seve ra l bas ica l ly different approaches to designing a

t h e r m a l conver te r sy s t em to power a posi t ive d isplacement type of

a r t i f ic ia l blood pump. The p r i m a r y power conver te r can be designed

to genera te pu lsa t i l e power that can be applied d i rec t ly to the pump in

a so-ca l led synchronous operat ing mode; or the main power unit can

be designed to take a role m o r e like that of a "boi le rhouse" supplying

a m o r e or l e s s constant level of power that is converted by a second

s tage into the requ i red pulsa t i le output format . Sys tems of this second

type a r e said to ope ra te in the nonsynchronous mode.

Other bas ic differences include, of course , the type of t he rma l

engine used for the p r i m a r y convers ion and the different ways and extent

to which the va r ious Bystems can s to re energy in o rde r to most

efficiently meet the need to vary both the mean and the instantaneous

output power demands . This l a t t e r fea ture which r e l a t e s to the t i m e -

varying na tu re of the load p re sen ted to the sys tem by this type of blood

punnp, has been genera l ly neglected in vir tual ly all of the documented

2-3

1,3

THERMAL CONVERTER SYSTEM

r n THERMAL

POWER

HEAT A HEAT iSOURCa

MAIN POWER UNIT 8F ENGINE SUBSYSTEM

• PRIME POWER CON VERTER (ENGINE)

ENERGY STORAGE (TESM OR BATTERY)

ENGINE MODULATION UNIT

CONTROL SIGNALS

\

POWER CONDITIONING AND CONTROL

SUBSYSTEM

POWER CONDITIONING UNIT

AUTOMATIC ACTUATOR

CONTROL SIGNALS

MECHANICAL HYDRAULIC, ELECTRICAL

POWER

BLOOD PUMP

Pav = 2.81 WATTS

HYDRAULIC OUTPUT POWER

Figure 2.2-1. Sys tem Terminology

sys t em studies to date . Yet in t e r m s of the impact on overal l efficiency,

th is pa r t i cu la r c h a r a c t e r i s t i c could prove to be more significant than the

differences in convers ion efficiency between most of the candidate heat

engines .

Another genera l ly neglected sys t em- leve l considerat ion also

r e l a t e s to the t ime-va ry ing na ture of the load presen ted by the blood

pump and that is the discontinuous volumetr ic d isplacement that -will

r e su l t from a compres s ib l e pneumat ic link at any point in the power

t r a n s m i s s i o n chain. A d i r ec t consequence of th is factor i s the se r ious

disadvantage inherent in the use of a synchronous reciprocat ing gas cycle

engine, such as the free piston modified Stirling engine, with an output in

the form of a gas p r e s s u r e dr ive .

Thus seve ra l impor tant genera l conclusions regarding selection of

the best sys t em design approach can be drawn from considerat ion of the

output power r equ i r emen t s and the t ime-vary ing na ture of the load

p resen ted by the blood pump.

2. 3. 1 System Load C h a r a c t e r i s t i c s

The re a re t h r ee aspec t s to the load cha rac t e r i s t i c which have

pa r t i cu l a r bearing on maximizing the per formance of the total sys tem.

F i r s t , the b a c k - p r e s s u r e during the blood ejection s t roke va r i e s

typical ly between 80 and 120 m m Hg at the lower levels of physical

activity and may reach peak levels as high as 160 to 180 m m Hg during

the m o r e active pe r iods . As a resu l t , sys t ems that produce constant

force levels (which include m o s t of the candidate approaches) , mus t be

se t so that they a r e providing excess ive force (and therefore excess ive

power) for some par t of the t ime .

A second impor tant aspect of the load cha rac t e r i s t i c is the s tep-

function changes in the blood pump back p r e s s u r e between the end of the

ejection phase and the beginning of the filling phase, and again between

the end of the filling phase and the beginning of ejection. If the re is a

compress ib l e pneumatic link anywhere in the power t r a n s m i s s i o n t r a in

between the p r i m e mover and the blood pump, the finite t ime requi red

to change the p r e s s u r e in th is link e i ther up or down, leads to a differen­

t ia l p r e s s u r e si tuation which causes the blood pump to "pause . " In the

2-5

2 6"

case of a synchronous rec iproca t ing gas cycle engine, we found that the

combined sum of these "dwell" pe r iods could not be reduced to l e s s than

50% of the t ime avai lable in the total cycle. Half of the per iod al located

to ejection pas sed before the power piston in the actuator driving the

blood pump could begin actual eject ion. A s imi l a r s i tuat ion was found to

occur on the filling s t roke with the power pis ton s ta t ionary for the f i rs t

half of th is per iod a lso . This phenomenon i s caused by the approximately

sinusoidal na ture of the output p r e s s u r e d r ive . There appeared to be

no way to design this type of engine to provide an output which is m o r e of

a square wave, without generat ing excess ive power and thus sacrif icing

a grea t deal of convers ion efficiency.

The th i rd aspect is one which is fair ly commonly acknowledged,

and that is the var ia t ion in the mean power demand assoc ia ted with

different levels of physica l act ivi ty. Sys tems that have the abili ty to s to re

considerable amounts of energy at some par t of the convers ion chain can,

to a g r e a t e r or l e s s e r extent depending on the s torage capacity, del iver

levels of power w^hich match the daily var ia t ion in demand. Sys tems

with no l o n g - t e r m s to rage capaci ty mus t continuously gene ra t e power at

the level corresponding to the m a x i m u m specified demand.

T h e r e will always be a p rac t i ca l p rob l em assoc ia ted with specifying

the amount of s to rage n e c e s s a r y to provide for not only different life

s tyles of the r ec ip i en t s , but day- to -day and l o n g e r - t e r m var ia t ions in the

activity profi les of different individuals . There fore , energy s torage

sys tems capable of efficiently providing additional s to rage over and above

the reasonable m i n i m u m a re p r e f e r r e d . The re a r e two p rac t i c a l methods

of s tor ing energy within the sys tem and these a r e d i scussed m o r e

specifically in Section 2.8.

2. 3. 1, 1 Output F o r c e Matching

F igure 2.3-1 shows a simplified model of the sys t em load which

i l l u s t r a t e s the e s sen t i a l c h a r a c t e r i s t i c s being d i scussed . During the

ejection phase , the back p r e s s u r e ref lected back through the blood pump

to the output power piston r i s e s f rom a low (diastolic) level to a peak

(systolic) level, and the work pe r fo rmed during this per iod is sub­

stantial ly that r equ i red to pump the blood up to th is potent ial energy level .

2-6

The mean functional head which is used to compute the equivalent s t roke

work r e p r e s e n t s an average value of the back p r e s s u r e during the ejection

per iod.

Thus the force requ i red to just ove rcome the back p r e s s u r e and

eject the blood follows the changing back p r e s s u r e and likewise inc rease

during the per iod of ejection. If a constant force i s applied to the power

pis ton during ejection, then it mus t be set high enough to overcome the

peak back p r e s s u r e o therwise i t will s ta l l and fail to d i scharge some f r a c ­

tion of the s t roke volume. Ensur ing that al l of the blood is always d i scharg

during per iods of i nc reased act ivi ty when the peak back p r e s s u r e may r i s e

DIASTOLIC

PEAK SYSTOLIC

EJECTION

LEFT ATRIUM - INFLOW PRESSURE HEAD OF

BLOOD FLOW 8-10 MM Hg

FROM PULMONARY

CIRCULATION

SYSTOLIC LEVEL _

(1^0 l50"„",n."Hg)

FUNCTIONAL MEAN OUTFLOW PRESSURVHEAD'

DIASTOLIC LEVEL / HO n.n"i H g " " /

4r PUMP MEMBRANE '

POWER PISTO ~4« ^

BLOOD OUTFLOW

TO ~~~ SYSTEMIC

CIRCULATION

100 MM Hg

POSITIVE DISPLACEMENT BLOOD, PUMP

[lOOMLS STROKE VOLUME

BLOOD PUMP ACTUATOR 125 MLS STROKE VOLUME

Figure 2.3-1. Simplified Model of Load C h a r a c t e r i s t i c s

to say 180 mm Hg, r equ i r e s that the sys tem be set so that it is providing a

cons iderab le exces s of force, and there fore power, for mos t of the t ime

The output c h a r a c t e r i s t i c which comes c loses t to the ideal would

eject the blood at a nea r constant flow r a t e . This r equ i r e s that the power

p is ton be dr iven as f rom a c rank with nea r - cons t an t angular velocity A

r o t a r y - d r i v e sys tem, with mechan ica l i ne r t i a sufficient to absorb the

2 - 7

^7

b a c k p r e s s u r e v a r i a t i o n wi thou t a p p r e c i a b l y c h a n g i n g speed , h a s s u c h a

c h a r a c t e r i s t i c and t h u s m a t c h e s the d e l i v e r e d p o w e r to the i n s t a n t a n e o u s

d e m a n d . By way of c o n t r a s t , the s y n c h r o n o u s r a m s t e a m engine , the

s y n c h r o n o u s e x p a n s i o n s t e a m eng ine , h y d r a u l i c and p n e u m a t i c p r e s s u r e

s t o r a g e s y s t e m s , s o l e n o i d and p i e z o e l e c t r i c s y s t e m s a i l have m i s m a t c h e d

output f o r c e c h a r a c t e r i s t i c s . T h e o v e r a l l e f f i c iency p e n a l t y a s s o c i a t e d

wi th t h i s f o r c e m i s m a t c h c a n be e s t i m a t e d by c o m p a r i n g the b a c k p r e s s u r e

c o r r e s p o n d i n g to the a c t u a l p r e s e t f o r c e l e v e l (which should be c l o s e to

180 m m Hg), w i th the da i l y a v e r a g e of the m e a n func t iona l b a c k p r e s s u r e ,

w h i c h i s s p e c i f i e d a s 100 m m Hg . A n o t h e r d r a w b a c k of s y s t e m s wi th a

m i s m a t c h e d f o r c e ou tpu t i s t h a t t h e e x c e s s p o w e r i s d i s s i p a t e d k i n e t i c a l l y

in the b lood d u r i n g t h e e j e c t i o n p e r i o d , c o n t r i b u t i n g to the r i s k of

m e c h a n i c a l l y d a m a g i n g t h e b lood c e l l s .

2. 3. 1. 2 P n e u m a t i c P o w e r D r i v e

If t he p r e s s u r e w a v e ou tpu t f r o m a s y n c h r o n o u s r e c i p r o c a t i n g g a s

cyc l e e n g i n e i s u s e d to d r i v e the p o w e r p i s t o n , t h e f i l l ing and e j e c t i o n

p h a s e s of the p u m p i n g c y c l e a r e s i g n i f i c a n t l y s h o r t e n e d by a d v e r s e

p r e s s u r e r e l a t i o n s h i p s w h i c h c a u s e t h e p o w e r p i s t o n , and t h u s t h e b lood

punnp, t o " p a u s e . " T h e r e s u l t i n g d i s t o r t i o n of t h e b lood p u m p d i s p l a c e m e n t

c h a r a c t e r i s t i c is s h o w n in F i g u r e 2 . 3 - 2 .

T h e s e c o n d u n d e s i r a b l e f e a t u r e of p n e u m a t i c d r i v i n g i s c o m m o n to

bo th s y n c h r o n o u s s y s t e m s , w h e r e g a s o r v a p o r i s u s e d in t h e coupl ing

b e t w e e n t h e e n g i n e p i s t o n and t h e ou tpu t p o w e r p i s t o n , and the n o n -

s y n c h r o n o u s s y s t e m s w h i c h s t o r e fluid e n e r g y at a c o n s t a n t p r e s s u r e . The

c o m p r e s s i o n w o r k c o n t a i n e d in t h e f luid u s e d a s t h e p o s i t i v e d i s p l a c e m e n t

m e d i u m canno t be e f f i c i en t l y r e c o v e r e d , s i n c e t h e c o m p r e s s e d g a s m u s t

be r e m o v e d r a p i d l y at t he end of t h e e j e c t i o n s t r o k e in o r d e r to a c h i e v e

t h e l o w b a c k p r e s s u r e in the b lood p u m p r e q u i r e d for a d e q u a t e

f i l l ing p e r f o r m a n c e .

In c u r r e n t p n e u m a t i c - p o w e r e d blood p u m p i n g s y s t e m s us ing

e x t e r n a l p o w e r s u p p l i e s , t h i s e x t r a e n e r g y i s d i s c a r d e d by d u m p i n g t h e

g a s to a l a r g e s ink s u c h a s the a t m o s p h e r e in o r d e r to m e e t the fi l l ing

p h a s e r e q u i r e m e n t s .

PQ Po Pb Pb

PRESSURE OR

FORCE

POWER PISTON

DISPLACEMENT

IDEAL SYSTEM

DWELL

GAS DRIVE SYSTEM OPTIMIZED PUSH-PULL GAS DRIVE SYSTEM

Figure 2. 3-2. "Dwell" Pe r iods With Gas Drive

In s u m m a r y , c o n s i d e r a t i o n of t h e n a t u r e of t h e s y s t e m load

c h a r a c t e r i s t i c s l e a d us to t h e fo l lowing g e n e r a l c o n c l u s i o n s :

(1) S y s t e m s w h i c h d r i v e the ou tpu t p o w e r p i s t o n at , o r c l o s e t o , c o n s t a n t a n g u l a r v e l o c i t y wi th s h o r t - t e r m e n e r g y s t o r a g e in t h e f o r m of a m e c h a n i c a l " f l y w h e e l , " o r the e q u i v a l e n t , a r e c a p a b l e of p r o v i d i n g t h e i d e a l load m a t c h i n g c h a r a c t e r i s t i c . All t he e l e c t r i c a l c a n d i d a t e s h a v e been d e s i g n e d wi th t h i s c h a r a c t e r i s t i c .

(2) S y s t e m s wi th f o r c e - l i m i t e d ou tpu t d r i v e c h a r a c t e r i s t i c s , s u c h a s the n o n s y n c h r o n o u s d e s i g n s t h a t u s e s t o r e d Quid e n e r g y m u s t be d e s i g n e d to m e e t t h e m a x i m u m ( s y s t o l i c ) b a c k p r e s s u r e r a t h e r t h a n t h e m e a n p r e s s u r e l e v e l s . T h i s r e s u l t s in an a d d i t i o n a l e f f i c i ency f a c t o r wh ich canno t be g r e a t e r t h a n 0. 83 (i, e, , 100 /120) and m a y be a s low a s 0. 56 ( i , e, , 100 /180) in o r d e r to p e r f o r m a d e q u a t e l y u n d e r t h e s p e c i f i e d r a n g e of p h y s i c a l a c t i v i t y l e v e l s .

(3) S y n c h r o n o u s e n g i n e s m u s t be d e s i g n e d •with a m e c h a n i c a l o r h y d r a u l i c ( i n c o m p r e s s i b l e ) coup l ing m e d i u m b e t w e e n t h e e n g i n e p i s t o n and the ou tpu t p o w e r p i s t o n in o r d e r to avo id d w e l l p e r i o d s and a c o n s e q u e n t r e d u c t i o n in both f i l l i n g and e j e c t i o n p e r i o d s .

(4) P n e u m a t i c l i n k s in t h e ou tpu t p o w e r t r a i n l e a d to a d d i t i o n a l p o w e r l o s s e s r e s u l t i n g f r o m n o n r e c o v e r y of the c o m p r e s s i o n w o r k .

2 . 3 . 1 . 3 S u m m a r y of G e n e r a l S y s t e m C o n s i d e r a t i o n s

T h e a p p r o x i m a t e o v e r a l l s y s t e m e f f i c iency m u l t i p l i e r s to t ake

a c c o u n t of the l o s s e s a s s o c i a t e d wi th bo th l a c k of e n e r g y s t o r a g e

c a p a c i t y and ou tpu t f o r c e m i s m a t c h e s a r e g iven in T a b l e 2 . 3 - 1 . T h e

0 . 7 0 v a l u e r e p r e s e n t s a va lue b e t w e e n the n u n n b e r s d e r i v e d e a r l i e r

( 0 . 5 6 and 0 . 8 3 ) and c o r r e s p o n d s to a p e a k (or s y s t o l i c ) p r e s s u r e of

143 m m Hg .

T h e m o s t o b v i o u s p o t e n t i a l a d v a n t a g e of s y s t e m s us ing a

s y n c h r o n o u s eng ine i s avoid ing the c o m p o u n d i n g of tAvo o r m o r e s t a g e s of

p o w e r c o n v e r s i o n . H o w e v e r , u n d e r the r e a l i s t i c g r o u n d r u l e s of t h i s

s tudy , the po t en t i a l a d v a n t a g e in o v e r a l l c o n v e r s i o n e f f i c i ency i s

a p p a r e n t l y ou twe ighed by a n u m b e r of o t h e r f a c t o r s r e l a t i n g to the

d i f f icul ty of d e s i g n i n g a s i n g l e - s t a g e c o n v e r t e r t ha t a d e q u a t e l y m e e t s a l l

of t h e s y s t e m r e q u i r e m e n t s . T h e n o n s y n c h r o n o u s d e s i g n a p p r o a c h a l l o w s

2 10

Table 2.3-1. Impact of Load Matching and Energy Storage Cha rac t e r i s t i c s on Overall System Efficiency

System Efficiency Equals

All of candidates using e lec t r i ca l genera tors j Q X power conversion efficiency and ba t te ry s torage a r e perfectly matched.

Synchronous and non-synchronous rec iprocat ing engine candidates using TESM can accomnnodate „ _„ daily changes m mean output power but cannot match changes in back p r e s s u r e during pumping.

E lec t r i ca l genera tor candidates without bat tery s torage cannot accommodate changes m mean 0.60 output power level .

Synchronous and non-synchronous rec iprocat ing engine candidates without TESM cannot accommodate 0.42 ei ther changes m mean output power or changes m back p r e s s u r e during pumping.

J

considera t ion of non- rec ip roca t ing t he rma l c o n v e r t e r s and selection of

an opt imum speed range for the engine.

P a r t i c u l a r design problems with synchronous engines include

achieving high efficiency when the output power is modulated over a 2: 1

range, efficiently controll ing the power modulation and achieving

adequate control of the back p r e s s u r e and volumetr ic var ia t ions during

the filling phase ,

2 ,4 WASTE HEAT MANAGEMENT

Physiologica l s tudies using seve ra l different species of

exper imenta l an ima l s have es tabl i shed guidelines for the levels of heat

flux that can be to le ra ted at the sur faces of ar t i f ic ia l devices implanted

within the body. These findings a r e d i scussed in detai l in Section 2. 4, 2,

2.4.1 Heat Generat ion Rates

General ly it has been a s sumed that at l eas t some of the waste heat

from an ar t i f ic ia l h e a r t device power supply must be t r a n s f e r r e d

d i r e c t l y into the main blood s t r e a m through e i ther a tube-type of heat

exchanger placed in s e r i e s with the blood pump, or by a special

modification of the pump itself. The ra t ionale for this approach

appea r s to be, f i r s t , that there is a na tura l , physiological p recedent

since the hea r t itself gives up 10 to 15 wat ts of heat to the throughflow;

and second, the logical considerat ion that with only a smal l tennperature

r i s e acceptable , the g rea tes t level of heat re jec t ion can be at tained

where the mass flow ra te of coolant i s g r e a t e s t . The exper imenta l

evidence summar i zed in 2. 4. 2 is that heat fluxes in the range of 2

0.8 to 0.9 w a t t / c m can be re jec ted to the main blood flow while the

l imit ing ra te to the surrounding muscu la tu re and o ther body t i s sues is 2

in the range of 0.05 to 0.10 w a t t / c m .

Considera t ion of severa l f ac to r s re la t ing to the groundrules and

r equ i r emen t s of this specific study, however , lead us to the following

general conclusions on which we propose an init ial design approach to

waste heat management which is s imply to re jec t all of the waste hea t

through the meta l container to the surrounding body t i s s u e s .

2-12

• F o r a 7 0 - k g m s ub j ec t , the m a x i m u m a l l o w a b l e t h e r m a l i n v e n t o r y of the h e a t s o u r c e i s spec i f i ed to be 60 w a t t s . The s u r f a c e a r e a of a s p h e r e equ i \ a , l en t to the spec i f i ed v o l u m e t r i c l i m i t of 1. 5 l i t e r s i s 634 c m ^ . The c o r r e s p o n d i n g m e a n flux l e v e l of 0 . 0 9 5 w a t t / c m , i n d i c a t e s the p o s s i b i l i t y of a c h i e v i n g m e a n t h e r m a l f lux l e v e l s in the e x p e r i m e n t a l l y v e r i f i e d r a n g e of 0 . 0 5 to 0. 10 w a t t s / c m ^ .

A c t u a l c a n d i d a t e s y s t e m s i z e s a r e be low the 1 . 5 - l i t e r linnit bu t t h e y a l s o u s e h e a t s o u r c e s l e s s than 60 w a t t s and p a c k a g e g e o m e t r i e s t ha t ha \ e s u r f a c e a r e a s g r e a t e r than the v o l u m e - e q u i v a l e n t s p h e r e .

• The d e s i g n goal of an o v e r a l l spec i f i c gravi ty c l o s e to uni ty \\i 11 tend to m a i n t a i n this h e a t s o u r c e / s u r f a c e a r e a r a t i o f a i r l y c o n s t a n t , e v e n a s o v e r a l l s y s t e m ef f ic iency i s i m p r o v e d d u r i n g d e v e l o p m e n t .

• E v e n wi th a s p e c i a l - p u r p o s e h e a t e x c h a n g e r in the s y s t e m d e s i g n , t h e r e wil l be an i n e v i t a b l e a m o u n t of h e a t l e a k a g e ( e s t i m a t e d by an e a r l i e r s tudy c o n t r a c t o r to be abou t 40 to 50%) t h r o u g h the i n s u l a t i o n and in to the s u r r o u n d i n g t i s s u e s . T h i s i s p a r t i c u l a r l y t r u e wi th in ou r s y s t e m d e s i g n g r o u n d r u l e s s i nce the low spec i f i c g r a v i t y r e q u i r e m e n t m a k e s the u s e of the l e s s c o m p a c t f i b r o u s i n s u l a t i o n a v e r y a t t r a c t i v e d e s i g n a p p r o a c h . B e c a u s e of t h e s e h e a t - l e a k a g e p a t h s , i t would be di f f icul t to t r a n s p o r t m o r e than about 50% of the w a s t e h e a t to a s e p a r a t e h e a t - e x c h a n g e r .

T h e r e i s a d i s a d v a n t a g e in u s ing a h e a t e x c h a n g e r in the b lood pu inp b e c a u s e un l ike the n a t u r a l h e a r t wh ich r e j e c t s the g r e a t e r p a r t of i t s t h e r m a l load to the c o r o n a r y c i r c u l a t i o n and thus d i r e c t l y in to the blood c i r c u l a t i n g t h r o u g h the l u n g s , the h e a t r e j e c t e d to a left h e a r t a s s i s t p u m p would be c a r r i e d in to the s y s t e m i c c i r c u l a t i o n and t h u s l a r g e l y b a c k in to the a b d o m i n a l c a v i t y . An a b d o m i n a l l y l o c a t e d t u b e - t y p e h e a t e x c h a n g e r would be a l o g i c a l e n g i n e e r i n g so lu t ion to t h i s q u e s t i o n . The a r g u m e n t s a g a i n s t t h i s a p p r o a c h c e n t e r l a r g e l y a r o u n d the r i s k a s s o c i a t e d with a n o t h e r s i t e of s u r g i c a l a n a s t a m o s i s , the i n c l u s i o n of a n o t h e r p o t e n t i a l l y d a m a g i n g m a t e r i a l i n t e r f a c e with the b lood , and the a d d i t i o n a l c o m p l i c a t i o n a t i n s t a l l a t i o n . O u r m e d i c a l c o n s u l t i n g g r o u p w a s u n a n i m o u s in t h e i r p r e f e r e n c e fo r u s i n g the o u t e r wa l l of the c o n t a i n e r a s the p r i m e body h e a t e x c h a n g e r .

In a d d i t i o n to t h e s e s p e c i a l s y s t e m c o n s i d e r a t i o n s , t h e r e i s a good

p h y s i o l o g i c a l c a s e for r e j e c t i n g h e a t v ia the c o n d u c t i o n and c o n v e c t i o n

p a t h s p r o v i d e d by the s u r r o u n d i n g t i s s u e s and body f l u i d s . The h e a t

p r o d u c t i o n in the t r u n k c o r e u s u a l l y a c c o u n t s fo r o v e r half of the b o d y ' s

b a s a l h e a t p r o d u c t i o n so t h e r e a r e e s t a b l i s h e d n a t u r a l p h y s i o l o g i c a l

2-13

62>

p a t h w a y s for s i g n i f i c a n t l e v e l s of h e a t flow f r o m th i s r e g i o n . The a l w a y s -

p r e s e n t t i s s u e f lu ids e n s u r e good t h e r m a l coup l ing b e t w e e n the i m p l a n t e d

p a c k a g e and the s u r r o u n d i n g m u s c l e s and v i s c e r a . P e r h a p s the m o s t

p e r t i n e n t e x p e r i m e n t a l e v i d e n c e to d a t e on a l l o w a b l e h e a t f lux l e v e l s

c o m e s f r o m l o n g - t e r m s t u d i e s c a r r i e d out on d o g s by the B o s t o n G r o u p

( R e f e r e n c e 7). Two i s o t o p e s o u r c e s (16 and 24 w a t t s ) w e r e a t t a c h e d to t u b e -

type h e a t e x c h a n g e r s t ha t w e r e c o n n e c t e d in to the a b d o m i n a l a o r t a . The 2

b l o o d - e x p o s e d s u r f a c e had an a r e a of 11,4 c m a n d the e x t e r n a l s u r ' ' a c e to 2

which the t i s s u e w a s e x p o s e d had an a r e a of abou t 120 c m . A c c o r d i n g to

one of the a u t h o r s (page 909, r e f e r e n c e 7), a s m u c h a s 50% of the h e a t w a s

r e j e c t e d t h r o u g h the s u r f a c e a r e a s s u r r o u n d e d by t i s s u e . F o r the two h e a t

s o u r c e s i z e s , th i s r e s u l t s in a c h r o n i c t h e r m a l f l u x l e v e l of 0,07 and 0,10

w a t t / c m , r e s p e c t i v e l y in the t i s s u e s . T h e e x p e r i m e n t s w e r e con t inued for 22

and 26 m o n t h s , r e s p e c t i v e l y , b e f o r e t hey w e r e t e r m i n a t e d for c a u s e s not r e l a t e c

to the e x p e r i m e n t a l e x p o s u r e . The t i s s u e s showed no e f fec t s tha t cou ld be

a t t r i b u t e d to the e x p o s u r e . T h e b a l a n c e of the h e a t w a s r e j e c t e d

d i r e c t l y to the b lood s t r e a m v i a the i n s i d e s u r f a c e of the i m p l a n t at a 2

f lux d e n s i t y of 0 . 7 and 1,0 w a t t / c m , r e s p e c t i v e l y .

The P i e r c e F o u n d a t i o n g r o u p s p e c i f i c a l l y s t u d i e d t h i s m o d e of

d i s s i p a t i o n u s ing e l e c t r i c a l l y - h e a t e d flat p a c k a g e s i m p l a n t e d wi th in the

a b d o m i n a l c a v i t y of 4 0 - k g s h e e p . T h e y found t h a t at l e v e l s of abou t

0, 6 w a t t / k g m ( e q u i v a l e n t to 42 w a t t s for a n o m i n a l 70 kg h u m a n ) t h e

a n i m a l s e q u i l i b r a t e d w i t h only m a r g i n a l i n c r e a s e s in body t e m p e r a t u r e .

Aga in , the e x c e s s h e a t w a s r e j e c t e d v i a the r e s p i r a t o r y s y s t e m , s i n c e

s h e e p a r e not p r o v i d e d wi th any body s u r f a c e h e a t r e j e c t i o n m e c h a n i s m s .

2 .4,2 R e l a t e d E x p e r i m e n t a l S tud ie s

T h e e x p e r i m e n t a l s t u d i e s r e l a t i n g to m a x i m u m t o t a l l e v e l s of

add i t i ona l e n d o g e n o u s h e a t and m a x i m u m t h e r m a l flux d e n s i t i e s at the

t i s s u e s t ha t c a n be p h y s i o l o g i c a l l y a c c o m m o d a t e d h a v e b e e n c a r r i e d out

p r i m a r i l y a t t h r e e c e n t e r s : B a t t e l l e ' s P a c i f i c N o r t h w e s t L a b o r a t o r i e s in

R i c h l a n d , Wash ing ton ( R e f e r e n c e s 1 - 3); by a g r o u p in B o s t o n

c o m p r i s e d of staff meiTibers f r o m T h e C h i l d r e n ' s H o s p i t a l M e d i c a l

C e n t e r , S e a r s S u r g i c a l R e s e a r c h L a b o r a t o r i e s and T h e r m o E l e c t r o n

C o r p o r a t i o n ( R e f e r e n c e s 4 - 8); and by the John B. P i e r c e F o u n d a t i o n

L a b o r a t o r i e s in New Haven, C o n n e c t i c u t ( R e f e i e n c e s 9 ' 12).

2-14

All t h r e e groups have invest igated fairly extensively the ability of

dogs (and in the Cose of PNL, Hanford min ia tu re swine) to accommodate

tube- type blood-cooled heat exchanger s connected into the descending

aor ta . Thei r findings (which included a number extending over pe r iods

of 12 to 24 months) genera l ly supported the conclusion that the animal

subjects can adapt chroniccully to an additional t h e r m a l load up to about

0. 8 to 1.1 w a t t s / k g m of body weight, although for the l a r g e r sources

there was an ini t ial to le rance p rob lem during the f i r s t s eve ra l days . In

dogs, the exces s heat is re jec ted via the r e s p i r a t o r y route which is a

p r e f e r r e d mode for the dog, r a t h e r than through the skin. If the findings

coTild be ex t rapola ted d i rec t ly , th is would cor respond to a to le rab le heat

source in the range of 55 to 70 wat ts (nominal 70 kgm body weight). The

PNL g roup ' s findings w e r e somewhat l e s s confident about t he rma l burdens

in e x c e s s of 20 wat t s even though they repor ted 2 to 3-month survival for

an imals with 80- and 100-watt s o u r c e s . They repor ted that 50 to 80 kgm

swine with 60-watt s o u r c e s had p r o b l e m s accommodating unusually w a r m

envi ronmenta l t e m p e r a t u r e s and mild infections.

None of the g roups found evidence of g r o s s physiological , b iochemical

o r h is to logica l anomal ies that could be re la ted to the t h e r m a l burden,

though t h e r e were numerous p r o b l e m s assoc ia ted with the thrombogenic

na tu r e of the foreign m a t e r i a l su r faces exposed to the blood and some

difficulties with physica l acceptance of the implanted packages .

Dr . Rawson of the John B . P i e r c e Foundation L a bo ra to r i e s i s

quoted on Page Via of Reference 5 as s tat ing that vascu la r ized t i ssue

can be conditioned to d i ss ipa te a heat flux of 0. 1 w a t t / c m ' ' with a

source t e m p e r a t u r e of 43 C, Thei r study, however, exposed a very

significant new facet to the quest ion of was te heat management , which is

the re la t ive ly rapid encapsulat ion of the w a r m implant with highly

vascu la r i zed t i s s u e s r a t h e r than the m o r e typical avascu la r , t he rmal ly

insulat ing sca r t i s sue capsu le . Of major significance is the fact that

this is c u r r e n t l y being invest igated (Reference 8) as a technique for

s t imulat ing co l l a t e ra l c i rcula t ion in the damaged hea r t muscle , using local

heating via a venous ca the ter . P rev ious to these repor t s , t e m p e r a t u r e s of

about 45 C were cons idered noxious and potentially damaging to body t i s sues ,

2-15

It has been specula ted that the cap i l la ry formation is t r i gge red by a local

shor tage of oxygen m the t i s sues brought about by inc reas ing the local ra te

of t i s sue me tabo l i sm and, therefore , an excess ive consumption of oxygen.

While it IS not poss ible to state ca tegor ica l ly at this t ime that

l a rge , w a r m implan ts in humans will behave in an analogous manner ,

these findings suggest that heat can be re jec ted safely to body t i s s u e s 2

at l eve ls of about 0.07 w a t t s / c m . However, if an a l t e rna te mode of

heat re jec t ion is p r e f e r r e d , the mos t a t t r ac t ive approach is to add a

heat exchange surface to the blood pump. Adopting this design option

would i nc r ea se the volume of the nominal se lec ted design by about 4%

and the total weight by about 6,6%. As d i scussed l a t e r , the re would be

a negligible i n c r e a s e m the power r e qu i r e d .

2-16

. 4. 3 Peak Heat Rejection Rates

The heat sources for the eight candidate systems that were finally

evaluated l ie in a range between 37 and 57 watts. For the physical body

size of the recipient model specified for the study, there i s evidence

from experiments with animals to the effect that these levels are

acceptable as a chronic thermal body burden. However, how the body

will adapt to temporary thermal loads higher than this for relatively

short periods throughout the day is not so easy to predict. Systems

using thermal energy storage reject heat at levels from 28 to 56% higher

than the mean during per iods of max imum physical act ivi ty . The mean

is approximate ly the same as the t h e r m a l power of the heat sou rce .

At first consideration, it might be argued that in relative terms

this i s not l ikely to be a problem, since the body itself i s generating

more heat at these t imes and the natural heat dissipation channels are

fully opened. However, there could be a problem in coupling this extra

heat from the site in the abdominal cavity into the natural heat distribu­

tion channels because of a redistribution of the blood flow between the

different body segments. During exerc i se , the flow to the abdominal

organs tends to i n c r e a s e only marginally with significant increases in

the flow to the skin and the outer body layers . Unless there i s a natural

compensation, the additional rejected heat could lead to locally elevated

t e m p e r a t u r e s . This quest ion does not a r i s e with the non-nnodulated

sys t ems or those using ba t t e ry s torage since the re jec ted heat level

does not va ry apprec iably at different output power l eve l s ,

2. 5 PACKAGING CONSIDERATIONS

Some packaging considerations cannot be adequately evaluated at

this time. For example, conformity of the package with body spaces in

the chest and the abdomen, the exact nature of the interconnections

between the packages, and the na ture of any external surface mountings

have been a s s u m e d to be design cons idera t ions common to all of the

candidates. Therefore no attempt has been made to differentiate the

candidates on these points. Other considerations, including overall

volume, specific gravity and maximum anterior-posterior dimension,

have been specifically evaluated.

2-17

J7

F o r consis tency in a r r iv ing at a set of comparab le overa l l sys t em

des igns we have followed a set of common guidel ines .

2. 5. 1 Packaging Configuration

Within the volume, specific gravi ty , A-P dimension and to some

extent, the surface a r e a cons t ra in t a ssoc ia ted with heat re ject ion to the

surrovinding body t i s s u e s , we have a s sumed that the complete sys tem will

consis t of at l eas t two in te rconnected p a r t s . The pumping unit must be

located as close as poss ib le to the si te of the natura l hea r t , while the

power unit i tself can probably be m o r e readi ly accommodated in the lower

abdomen. Since those p r e f e r r e d s i t e s a r e at leas t 5 to 7 inches apar t in

the m a t u r e human, we have allowed nominal configurat ions with a third,

sma l l e r package located in the in te rmedia te a rea . All of the package

configurations a r e cons idered reasonable nominal des igns in the absence

of m o r e specific r e q u i r e m e n t s . They a r e part ly respons ive to cer ta in

of the p r i m a r y design r e q u i r e m e n t s but they a r e not proposed at this

t ime a s opt imum conf igurat ions .

Because the power unit will be prefueled and operat ing at the t ime

of instal lat ion, we have, where feasible , included the capaci ty to

mechanical ly uncouple the pump dr ive unit (the ac tua tor ) from the power

unit or power condit ioner , since we considered that this would provide a

convenient way of allowing the surgeon to connect and p r ime the blood

pump. This mechanica l coupling is a design feature to be explored .

2 . 5 . 2 Packaging Ma te r i a l s

The main package of all of the candidate s y s t e m s is enclosed in a

he rme t i ca l ly sealed can of c o m m e r c i a l l y pure t i tanium. The re a r e t h r ee

genera l a r e a s of cons idera t ion that lead to the choice of t i tanium. F i r s t ,

it is potent ial ly capable of withstanding the co r ro s ive environment within

the body for 10 y e a r s , second it is not harmful to body t i s s u e s , and

th i rd , it can be fabr icated using conventional techniques .

The body offers a su rp r i s ing ly host i le environment to i.^etals, and

only those with cons iderab le r e s i s t a n c e to co r ros ion in sal ine solut ions,

such as t i tanium, coba l t - ch romium al loys, and to a l e s s e r extent,

s t a in le s s s teel can be cons idered . Ti tan ium is far m o r e r e s i s t a n t than

2-18

J ,.1

i t s g e n e r a l r e a c t i v i t y ( a s i n d i c a t e d by i t s p o s i t i o n in the e l e c t r o c h e m i c a l

s e r i e s ) would s u g g e s t . T h i s i s b e c a u s e , l i ke s t a i n l e s s s t e e l , it f o r m s a

p r o t e c t i v e ox ide l a y e r when i t i s e x p o s e d to an e l e c t r o l y t i c so lu t ion . Of

the m a t e r i a l s w h i c h a r e m o s t c o m m o n l y u s e d for i n t e r n a l p r o s t h e s e s ,

p u r e t i t a n i u m i s the m o s t c o r r o s i o n - r e s i s t a n t , be ing v i r t u a l l y i m m u n e to

a t t a c k . Both s t a i n l e s s s t e e l and t h e c o b a l t - c h r o m e a l l o y s d e g e n e r a t e

u n d e r c e r t a i n s t r e s s c o n d i t i o n s . S t a i n l e s s s t e e l i s u s u a l l y the m o s t

s e r i o u s l y a f fec ted .

In the a b s e n c e of c o r r o s i o n , t h e r e is l i t t l e to c h o o s e b e t w e e n t i t a ­

n i u m and the o t h e r m a t e r i a l s wi th r e s p e c t to t i s s u e a c c e p t a b i l i t y .

As wi th a l l m e t a l i m p l a n t s t h e r e wi l l be a slow m i g r a t i o n of m e t a l l i c

ions in to the t i s s u e . H o w e v e r , t h i s s low d i s s o l u t i o n d o e s not a p p e a r to

be h a r m f u l .

It i s a n t i c i p a t e d tha t the o u t e r m o s t s u r f a c e of the innplanted p a c k a g e

wi l l be a " j a c k e t " of an o p e n - w e a v e i n e r t t ex t i l e m a t e r i a l such a s D a c r o n

m e s h i m p r e g n a t e d wi th m e d i c a l g r a d e s i l i cone r u b b e r . T h i s wi l l a l low

the s u r r o u n d i n g t i s s u e s to g row in to the p o r o u s s u r f a c e p r o v i d i n g a

s t r o n g m e c h a n i c a l b o n d .

2 . 6 GAS M A N A G E M E N T

G a s e o u s i s o t o p i c d e c a y p r o d u c t s a r e v e n t e d f r o m the h e a t s o u r c e

and c o n d u c t e d to the o u t s i d e of the p o w e r uni t t h r o u g h a m e t a l l i c c a p i l l a r y

t u b e . If t h e ven t d e s i g n i n c o r p o r a t e d in to the h e a t s o u r c e i s of the

n o n s e l e c t i v e t ype a s m a l l p o r t i o n of the ven t ed g a s , the r a d o n f r a c t i o n ,

w i l l be r a d i o a c t i v e . H o w e v e r , p r o v i d e d tha t t he p a t h l eng th of the c a p i l l a r

i s g r e a t e r t h a n 5 o r 6 i n c h e s the c a l c u l a t e d flow r a t e i s such tha t t h e

a c t i v i t y l e v e l w i l l have d e c a y e d to an a c c e p t a b l e l e v e l (16 h a l f - l i v e s ) ,

b e f o r e i t r e a c h e s t h e o u t s i d e of the c o n t a i n e r .

The m a j o r g a s e o u s d e c a y p r o d u c t i s h e l i u m wh ich i s not r a d i o a c t i v e

and p r e s e n t s no b i o l o g i c a l h a z a r d . It w i l l be r e l e a s e d at a m o r e o r l e s s

s t e a d y r a t e of about 2 x 1 0 " " s t a n d a r d cc p e r s e c o n d . If a l l of the h e l i u m

g e n e r a t e d o v e r 10 y e a r s w i th in a 5 0 - w a t t c a p s u l e i s r e l e a s e d f r o m the

fuel i t wi l l a m o u n t to a t o t a l of about 625 s e e s .

H e l i u m i s c o m p a t i b l e wi th body t i s s u e s and the p r o p o s e d gas

m a n a g e m e n t a p p r o a c h i s to l e a k the g a s to the s u r r o u n d i n g body

2-19

31

environment through a he l ium-pe rmeab le membrane . The capi l lary will t e r ­

minated on the outside of the main container , where it will pass through a

length of solft si l icone rubber into the lumen of a smal l d iamete r plast ic

tube which will act as a r e s e r v o i r f rom which the helium can diffuse into the

surrounding body t i s s u e s . The tubing will be mounted around the outside

of the conta iner and appropr ia te ly in tegra ted with the final D a c r o n - m e s h

and s i l i cone- rubber "jacket . " The ex te rna l length of the gas -ven t

capi l la ry pene t r a t e s the soft rubber s topper which will provide an

adequate fluid r e s i s t an t seeil excluding body fluids f rom the gas space

within the plas t ic tube. The re a r e s eve ra l candidate m a t e r i a l s for the

plast ic diffuser such as Teflon, polyethylene, and polyethylene terephthala te

which a r e compatible with body t i s s u e s . Assuming that only one side of

the plast ic ac ts as a diffusion "window" a 12-inch length of 1/4-inch OD

polyethylene tube with a 0 .050- inch- thick wall will adequately match the

max imum anticipated hel ium venting r a t e ,

2 .7 THERMAL INSULATION

The advantages and disadvantages of fibrous and foil type insulat ions

were investigated to de te rmine the opt imum insulation for use in the

engine subbys tems . The impor tan t p a r a m e t e r s studied were weight,

s ize , t h e r m a l conductivity, fabr icabi l i ty , ease of assembly , and

rel iabi l i ty .

2. 7. 1 F ib rous Insulation

Min-K 2020 (Reference 13) is a nonmetal l ic insulation which cons is t s

p r i m a r i l y of submicron s i l ica , t i tanium dioxide opacifier for radiat ion

blockage, and two types of si l ica f ibers (microquar tz and as t roquar tz) for

re inforcement . The insulat ion contains no binder , t he re fo re , no p r e c o n ­

ditioning is r equ i red to remove the binder which in the pas t caused con­

taminat ion with usage in var ious s y s t e m s . In fact, the Min-K formulat ions

1999, 2002, and 2020 were specifically designed for lead te l lur ide and

s i l i con -ge rman ium the rmoe lec t r i c energy convers ion devices to reduce

excess ive outgassing and potential poisoning of the thprmoelFct»-ic e lements

by impur i t i es within the insulat ion.

T h e r m a l conductivity data a re available for Min-K 2020 in a i r ,

2 - 2 0

*f O

argon, vacuum (Reference 14) and xenon. The data a r e plotted in F igure

2,7-1,

400 600 too low M A N TEMHUTUn - 'F

Figu re 2 . 7 - 1 . T h e r m a l Concductivity Versus Mean T e m p e r a t u r e

As can be seen from this f igure, the use of xenon as the Min-K fill

gas r e su l t s in lower insulation t he rma l conductivit ies than a r e at tainable

with a i r or argon fill g a s . * Min-K in a vacuum, of course , has lowest

conductivity; however, maintaining a stat ic vacuum for 10 years i s

difficult, although the conductivity i n c r e a s e of fibrous insulation with

inc reas ing gas p r e s s u r e is not ca tas t rophic a s it is with super insulat ion.

Ease of fabr icat ion and a s s e m b l y of Min-K is a lso an advantage since

the insulat ion can be molded and machined into var ious g e o m e t r i e s .

The nominal densi ty of Min-K 2020 is 20 Ib/ft^. Lower Min-K

dens i t ies a r e available at the expense of mechanical s trength. Load-

*However, the difference is not subs tant ia l . Fo r the candidate concept, if a rgon were substi tuted for xenon, the heat l o s se s would inc rease by 1.4 wa t t s .

2-21

y/

b e a r i n g r e q u i r e m e n t s for the c a n d i d a t e t h e r m a l c o n v e r t e r s y s t e m s a r e

s m a l l and the M i n - K 2020 should be s u i t a b l e for t h i s a p p l i c a t i o n .

S ince t h e M i n - K i n s u l a t i o n a b s o r b s m o i s t u r e f r o m the a i r , a s s e m b l y

p r o c e d u r e s wou ld i n c l u d e a b a k e o u t a t t e m p e r a t u r e in a v a c u u m wi th

a s s e m b l y of the i n s u l a t i o n and we ld ing of the t h e r m a l c o n v e r t e r s y s t e m

p a c k a g e in a d r y box f looded w i t h xenon g a s . If e l e c t r o n b e a m w e l d i n g

is r e q u i r e d for p a c k a g e s e a l i n g , the we ld ing \vould be done in a v a c u u m

and then the p a c k a g e b a c k - f i l l e d wi th x e n o n .

S h r i n k a g e d o e s no t a p p e a r to b e a p r o b l e m even a t the h i g h e s t

a n t i c i p a t e d s y s t e m o p e r a t i n g t e m p e r a t u r e s . F o r e x a m p l e , a SiGe t h e r m o ­

e l e c t r i c c o n v e r t e r u s i n g M i n - K 2020 o p e r a t i n g a t 9 5 0 - 1 0 0 0 C, w h i c h h a s

b e e n on t e s t fo r 24, 000 h o u r s , s h o w s no i n c r e a s e in h e a t l o s s . ( R e f e r e n c e

15). S h o r t e r - t e r m t e s t s have d e m o n s t r a t e d no m e a s u r a b l e s h r i n k a g e

be low abou t 850 C .

2 . 7 . 2 F o i l I n s u l a t i o n

One type of foi l i n s u l a t i o n c o n s i s t s of m e t a l l i c fo i l s wh ich h a v e been

s p r a y e d wi th a th in coa t i ng of c e r a m i c ox ide . The fo i l s a r e r o l l e d o r

f o r m e d in to c y l i n d e r s w i t h the r e q u i r e d n u m b e r of l a y e r s and then

p a c k a g e d in to a l e a k - t i g h t c o n t a i n e r and e v a c u a t e d . The oxide c o a t i n g

h a s a low t h e r m a l c o n d u c t i v i t y and a c t s a s a s p a c e r b e t w e e n the m e t a l

f o i l s . Hea t f r o m i n s i d e the i n s u l a t i o n can only be c o n d u c t e d t h r o u g h the

a r e a s w h e r e the ox ide c o n t a c t s the n e x t l a y e r of foi l and by r a d i a t i o n ,

A h igh v a c u u m ( 1 x 1 0 m m H g ) i s r e q u i r e d for e f f ic ien t p e r f o r m a n c e . T h i s

type of foi l i n s u l a t i o n is be ing d e v e l o p e d by T h e r m o E l e c t r o n C o r p o r a t i o n .

A s i m i l a r type of fo i l i n s u l a t i o n i s be ing d e v e l o p e d by Linde D i v i ­

s ion of Union C a r b i d e C o r p o r a t i o n . F i b r o u s g l a s s o r q u a r t z p a p e r i s

u s e d b e t w e e n the m e t a l fo i l s i n s t e a d of the s p r a y e d c e r a m i c ox ide . T h i s

type a l s o r e q u i r e s a h igh v a c u u m .

The m e a n t h e r n n a i c o n d u c t i v i t i e s for bo th of t h e s e t y p e s ( R e f e r ­

e n c e s 16 and 17) a r e shown in F i g u r e 2.7.2 v e r s u s s o u r c e t e m p e r a t u r e .

A c o m p a r i s o n of F i g u r e s 2 . 7 - 1 and 2 . 7 - 2 s h o w s t h a t t he t h e r m a l

c o n d u c t i v i t y for the foi l type i n s u l a t i o n s i s l o w e r t h a n for the f i b r o u s

Z - 2 2

;*-\ Of

X 3 <e

> > X i z o u

1 I

z

I

.001

oooe

.0006

,000*

.0002

,0001

00006

,nflon«

-

- •

1

r , Ci, - REFKASIL QUARTZ

(140O-FMAX)

^ y^

^ /

Tl - ZrOj _

(40 LAYERS)

/ / / /

^^^ ^^^/ •

/

' - " [ y X X

/ y

J /

/ /

/ / / / / /

/ / ^

^ / 1/

? /

/ 1 / A l - 106 GLASS

' (900'FMAX)

^

/

/ X -> / / V

' ^ b ^ /^^

tf^i.i. -Zr02 '80 LAYERS)

1 1

600 800 1000

SOURCE TEMPERATURE - 'F

Figu re 2 . 7 - 2 . Vacuum Foil Insulation Thernnal Conductivity V e r s u s Source T e m p e r a t u r e

insulat ions by a factor of approximate ly 100. There fore , if vacuum foil

insulat ion w e r e used to insulate the candidate t h e r m a l conver te r s y s t e m s ,

the insulat ion th ickness could be appreciably dec reased .

The approximate dens i t i e s of the var ious foil insulat ions a r e as

follows:

Foi l s

Al

Stainless Steel

Ti

Cu

Insulation Densi t ies (Ib/ft^)

7 . 9

22.7

13.4

26. 3

2-23

fi

Compar ing these dens i t i e s with the Min-K density of 20 lb/ft shows that

a vacuum foil insulat ion sy s t em for the candidate t h e r m a l conver te r

s y s t e m s would be a l ighter weight s y s t e m because of the thinner insula­

tion th ickness requi red ; in some c a s e s , depending on the metal l ic foil

used, the density would also be lower.

Application of vacuum foil insulation to anything but flat sur faces

is difficult. If tubes a r e to be brought through the insulation or if the

surface to be insulated has p r o t u b e r a n c e s , fabricat ion and assembly

become complex and a good s ta t ic vacuum is difficult to maintain. Also,

l o s se s at c o r n e r s and edges where the foils a r e discontinuous subs tan t i ­

ally reduce the overa l l insulation efficiency. Separa te s t r u c t u r a l support

of the insulated components must also be provided since the load bear ing

c h a r a c t e r i s t i c s of the vacuum foil insulation a r e poor and the heat los ses

i nc r ea se as loading on the foils i n c r e a s e s . S t ruc tu ra l supports provide

additional heat leaks and fur ther reduce overal l sy s t em efficiency.

2 . 7 . 3 The rma l Insulat ion Selection

Based on the foregoing advantages and d isadvantages of f ibrous

v e r s u s super insula t ion, a decis ion was made to use only the fibrous

type of insulat ion. The re l iabi l i ty a s soc ia t ed with the 10-year m a i n ­

tenance under stat ic conditions of the high vacuum n e c e s s a r y for the

effective pe r fo rmance of super insula t ion appeared to be low. In

addition, the insulat ion efficiency improvement potentially available with

super insula t ion is only par t ly re l izable because of edge l o s s e s and the

inability of the super insula t ion to provide load-bear ing capabi l i ty .

Final ly , s y s t e m - l e v e l ana lyses showed that the possible size reduct ion

is l a rge ly offset by a cons iderab le i n c r e a s e in the specific gravity of

the package. However, for the sake of comple teness we did conn pare

the c h a r a c t e r i s t i c s of the se lec ted sys t em concept with both f ibrous

insulat ion and super insu la t ion .

2 . 7 . 4 O v e r t e m p e r a t u r e Pro tec t ion

In the event of a sys tem malfunction or fa i lure , some method of

prevent ing heat source overheat ing mus t be provided. Fo r the selected

sys tem concept, a s imple pass ive t h e r m a l fuse a p p e a r s p rac t i ca l and is

desc r ibed in Section 3 . 2 . 5 . 4 .

Z - ? 4 (i ii-

2 . 8 E N E R G Y S T O R A G E

By p r o v i d i n g c e r t a i n t y p e s of l o n g - t e r m energy s t o r a g e , the eng ine

s u b s y s t e m s c a n be d e s i g n e d to g e n e r a t e p o w e r a t a l eve l c o r r e s p o n d i n g to

the a v e r a g e d a i l y b lood p u m p r e q u i r e m e n t s r a t h e r than t h e d e m a n d s

a s s o c i a t e d wi th m a x i m u m p h y s i c a l a c t i v i t i e s . In o r d e r to a c c o m m o d a t e

v a r i a t i o n s m the s p e c i f i e d d a i l y p o w e r d e m a n d p r o f i l e , the e n e r g y s t o r a g e

d e v i c e m u s t be su f f i c i en t to d e l i v e r and s t o r e a t l e a s t 5 . 6 w a t t - h o u r s of

e n e r g y , a s m e a s u r e d a t the inpu t to the b lood p u m p .

M e c h a n i c a l , p n e u m a t i c , and h y d r a u l i c m e t h o d s do not p r o v i d e

su f f i c i en t e n e r g y d e n s i t y to m e e t the a b o v e r e q u i r e m e n t . E n e r g y s t o r a g e

m the f o r m of t h e r m a l e n e r g y s t o r a g e m a t e r i a l (TESM) o r e l e c t r o ­

c h e m i c a l s t o r a g e in t h e f o r m of s e c o n d a r y b a t t e r i e s a r e the two f o r m s

t h a t h a v e b e e n c o n s i d e r e d . B e c a u s e t h e T E S M m u s t b e l o c a t e d a t the

s a m e p o s i t i o n in the c o n v e r s i o n c h a i n a s the hea t s o u r c e , it su f f e r s

the c o n v e r s i o n i n e f f i c i e n c y of the e n t i r e t h e r m a l c o n v e r t e r (engine

s u b s y s t e m p l u s p o w e r c o n d i t i o n i n g and c o n t r o l s u b s y s t e m ) . The

s e c o n d a r y b a t t e r y , h o w e v e r , i s d o w n s t r e a m of the e l e c t r i c a l g e n e r a t o r and

t h e r e f o r e , a s m a l l e r a m o u n t of s t o r e d e n e r g y i s n e e d e d to p r o v i d e the

e q u i v a l e n t of 5 .6 w a t t - h o u r s a t the b lood p u m p . Us ing t y p i c a l f i g u r e s

for the s y s t e m c o n v e r s i o n e f f i c i e n c i e s to i l l u s t r a t e t h i s f a c t o r , we find

tha t

• T E S M r e q u i r e s a p p r o x i m a t e l y 73 w a t t - h o u r s b a s e d on a 7 . 5% o v e r a l l t h e r m a l c o n v e r t e r e f f i c i ency .

• B a t t e r i e s r e q u i r e a p p r o x i m a t e l y 12. 5 w a t t - h o u r s b a s e d on a 45% P C C S e f f i c i e n c y .

2 8 1 T h e r m a l E n e r g y S t o r a g e M a t e r i a l (TESM)

The T E S M u s e s the l a t e n t h e a t of fus ion of the m a t e r i a l to a b s o r b

e x c e s s t h e r m a l e n e r g y i s o t h e r m a l l y . T h i s p r o c e s s i s c o m p l e t e l y r e v e r s i ­

b l e and t h e o r e t i c a l l y t h e r e i s no l i m i t to the n u m b e r of t i m e s it c an be

r e p e a t e d . T h e t e m p e r a t u r e of the T E S M u s e d m u s t be c o m p a t i b l e w i th

the eng ine and w o r k i n g f lu id . I d e a l l y , the h e a t s o u r c e of the t h e r m a l

c o n v e r t e r i s s i z e d for the a v e r a g e p o w e r r e q u i r e m e n t of the b lood p u m p .

T h e n w h e n the b lood p u m p p o w e r i s b e l o w the da i l y a v e r a g e , the T E S M

wi l l a b s o r b the e x c e s s t h e r m a l e n e r g y wh ich wi l l t end to m e l t the T E S M .

2-25

D u r i n g p e r i o d s w h e n the b lood p u m p p o w e r r e q u i r e m e n t s a r e above the

da i l y a v e r a g e , the a d d i t i o n a l h e a t e n e r g y wi l l be s u p p l i e d by the

s o l i d i f i c a t i o n of the T E S M . U t i l i z a t i o n of TESM p r e s u m e s tha t the

eng ine can be o p e r a t e d no t only in a v a r i a b l e p o w e r m o d e but a t r e a s o n a b l y

c o n s t a n t e f f i c i e n c y . V a r i a b l e p o w e r c a n be o b t a i n e d e i t h e r by v a r y i n g

the p o w e r ou tpu t p e r s t r o k e a t f ixed f r e q u e n c y o r v a r y i n g the f r e q u e n c y

if t he p o w e r p e r s t r o k e i s f ixed.

If the T E S M is s i z e d for the da i ly load p r o f i l e and the r e c i p i e n t

p o w e r p r o f i l e m e e t s the da i ly a v e r a g e , no a d d i t i o n a l c o n t r o l s wi l l be

r e q u i r e d o t h e r t h a n eng ine c o n t r o l s . H o w e v e r , if t he da i ly p o w e r p r o f i l e

i s not m a i n t a i n e d , s o m e d e v i c e s u c h a s a hea t p ipe m a y be r e q u i r e d to

p r e v e n t h e a t s o u r c e t e n x p e r a t u r e e x c u r s i o n s . A h e a t p ipe could be p l a c e d

b e t w e e n the T E S M and the o u t s i d e c o n t a i n e r . The hea t p ipe would be

p r e s s u r i z e d so t h a t it would not t r a n s m i t hea t u n t i l t he t e m p e r a t u r e of

the T E S M r e a c h e s a g iven v a l u e . Once the h igh t e m p e r a t u r e is r e a c h e d ,

the T E S M h e a t c a n be t r a n s f e r r e d v ia t h e hea t p ipe to t h i s c o n t a i n e r w h i c h

wi l l r e j e c t t he h e a t d i r e c t l y to the body.

The T E S M n a a t e r i a l s c o n s i d e r e d for u s e vi th the c a n d i d a t e s y s t e m s

a l e shown in Tab le 2 . 8 - 1 . F i g u r e s 2 . 8 - 1 and 2 . 8 - 2 i n d i c a t e v o l u m e and

we igh t of the T E S M m a t e r i a l v e r s u s w a t t - h o u r s .

T a b l e 2 . 8 - 1 . TESM M a t e r i a l s C o n s i d e r e d

Mater ia l

L i F / L i C l

Al

L i F / N a F

LiH

Cu

Melting Temp (°F)

930

1216

1250

1252

1981

C a l / c c

362

207, 5

442

480

375

I

C a l / g r a m

165

76, 8

170

610

42

2-26

M

12

8

6

4

7

— -

1

>

7 /

/ /

/ /

/ /

/ /

/

yu-^

i^y^ n

/ /

/ A I / /

yy

xy yy y

1

/ L I F / L . C J / / '

\y y

J / x L,F/NaF / *

^ ^ ^ ^

- - ^ ^ y

^ ^VM

1

1 1 ! T

1 20 30 40 50 «0 70 80 90 100

WAnS - HOURS

Figu re 2. 8 - 1 . TESM Volume Versus Wat t -Hours

1 2

o

7-

/ / C u

/ 1

t

/ /

/ /

^

——_——

^

/

/ /

/ • •

/ /

/

i^^

1

1

1

1

k i

l^ iy^

/ ^

^ ; : ^

L,F/L,C«

k 4 ] 1 1

L j

LiH

i

JoF h

—

- - =

^ ,^^

—

20 30 40 JO 60 70 80 90 100

WATTS - HOU«S

Figure 2 . 8 - 2 . TESM Weight Ver sus Wat t -Hours

2-27

Lf n I

2 , 8 . 2 E l e c t r o c h e m i c a l E n e r g y S t o r a g e

The s e c o n d a r y e l e c t r o c h e m i c a l e n e r g y s t o r a g e d e v i c e s m u s t be

c a p a b l e of supp ly ing a d d i t i o n a l p o w e r w h e n blood p u m p r e q u i r e m e n t s a r e

in e x c e s s of the da i ly a v e r a g e and s t o r i n g p o w e r w h e n the blood p u m p

p o w e r r e q u i r e m e n t s fa l l be low the da i l y a v e r a g e . T h i s m e t h o d of e n e r g y

s t o r a g e i s p a r t i c u l a r l y a t t r a c t i v e s i n c e the eng ine can be s i z e d *'or the

a v e r a g e p o w e r l e v e l and o p e r a t e d at f ixed load. The c y c l e l i fe , spec i f i c 3

e n e r g y ( w - h r / l b ) , and e n e r g y d e n s i t y ( w - h r / c m ) shou ld be a s h igh a s

p o s s i b l e . If t he s e c o n d a r y b a t t e r y i s to be in c l o s e c o n t a c t w i t h the

i s o t o p e h e a t s o u r c e , as in the c a s e of s e v e r a l of the d e s i g n s , t hen

r a d i a t i o n e f fec t s m u s t a l s o be c o n s i d e r e d a s m u s t the p r a c t i c a l p r o b l e m s

r e l a t e d to c o n t r o l l i n g the c h a r g i n g r a t e of the b a t t e r y .

On the b a s i s of t h e s e r e q u i r e m e n t s , t h r e e t y p e s of s e c o n d a r y c e l l s a r

c o n s i d e r e d ; n i c k e l - c a d m i u m , l i t h i u m - s e l e n i u m and s o d i u m - s u l f u r s y s t e m s .

O the r b a t t e r i e s s u c h a s s i l v e r - c a d m i u m and s i l v e r - z i n c a r e no t c o n s i d e r e a

b e c a u s e of t h e i r p o o r c y c l e l i fe c h a r a c t e r i s t i c s .

E x t e n s i v e d e v e l o p m e n t w o r k h a s b e e n p e r f o r m e d on the n i c k e l -

c a d m i u m b a t t e r y by the a e r o s p a c e i n d u s t r y and a g r e a t d e a l of p e r f o r m a n c e

i n f o r m a t i o n i s a v a i l a b l e . F i g u r e 2 8.3 shows the d e p t h - o f - d i s c h a r g e

v e r s u s the c y c l e l ife of a n i c k e l - c a d m i u m c e l l . T h e s e d a t a i n d i c a t e tha t

for a 20% d e p t h - o f - d i s c h a r g e a t 100 F the n i c k e l - c a d m i u m c e l l w i l l l a s t

for only 3000 c y c l e s , and the low s p e c i f i c e n e r g y (15 w - h r / l b ) i n d i c a t e s

that a h e a v y un i t be r e q u i r e d . At 20% d e p t h - o f - d i s c h a r g e , the w e i g h t

of the b a t t e r y to p r o v i d e 1 2 . 5 w - h o u r s of s t o r e d e n e r g y would be about

4.0 l b s . At 40% d e p t h - o f - d i s c h a r g e , th i s we igh t cou ld be r e d u c e d to

2 lbs but the c y c l e l i fe would d e c r e a s e to abou t 1500 c y c l e s . A f u r t h e r

d r a w b a c k to n i c k e l c a d m i u m s y s t e m s i s t he r e l a t i v e l y c o m p l i c a t e d

e l e c t r o n i c s r e q u i r e d to c o n t r o l the c h a r g i n g c o n d i t i o n s .

The o t h e r two t y p e s of b a t t e r i e s u t i l i z e fu sed s a l t o r so l id e l e c t r o l y t e

c e l l s w h i c h a r e m o r e e f f ic ien t a t h i g h e r t e m p e r a t u r e s . T h e s e c e l l s can

be o p e r a t e d to 100% d e p t h - o f - d i s c h a r g e and t h e y do no t r e q u i r e e l a b o r a t e

c h a r g i n g c o n t r o l c i r c u i t s . Unl ike the n i c k e l - c a d m i u ; n c e l l wh ich c o n t a i n s

a p o l y m e r i c m a t e r i a l a s a s e p a r a t o r , t h e s e c e l l s do no t n e e d a c o n v e n t i o n a l

s e p a r a t o r . The l i t h i u m - s e l e n i u m ce l l u s e s a L i F - L i C l - L i I e n t e c t i c in an

i m m o b i l i z e d m o l t e n s t a t e a s an e l e c t r o l y t e , A spec i f i c e n e r g y c l o s e to

c "28

3 yK 5 YB

F i g u r e 2 . 8 - 3 . S e c o n d a r y B a t t e r y - N i C d - C y c l e Life

50 w - h r s / l b h a s been d e m o n s t r a t e d at A r g o n n e Nat iona l L a b o r a t o r y with

the c e l l o p e r a t i n g at 7 10 F and t e s t e d t h r o u g h 300 c h a r g e / d i s c h a r g e

c y c l e s .

The s o d i u m - s u l f u r c e l l u s e s a th in f i lm of s i n t e r e d p o l y c r y s t a l l m .

a l u m i n a a s the e l e c t r o l y t e . The c e l l i s o p e r a t e d a t 570 F wi th p r o j e c t t r t

spec i f i c e n e r g y of abou t 150 w - h r s / l b . T h e s e s y s t e m s a r e u n d e r

d e v e l o p m e n t at TRW, F o r d , GE, and the Navy D e p a r t m e n t .

A c o m p a r i s o n b e t w e e n l i t h i u m - s e l e n i u m and s o d i u m - s u l f u r c e l l s

shows tliat the s o d i u m - s u l f u r c e l l h a s a n u m b e r of a d v a n t a g e s .

• It i s l e s s e x p e n s i v e ( i . e . , c o s t of s o d i u m i s $0 , Z5 / lb v s . $ 8 . 0 0 / l b for l i t h i u m ; s o d i u m is abundan t and su lphur c o s t s abou t $ 0 . 0 7 / l b ) .

• It c an be o r i e n t e d in any d i r e c t i o n b e c a u s e of c o m p l e t e s e p a r a t i o n of the a n o d e and c a t h o d e c o m p a r t m e n t s .

• I t can be h e r m e t i c a l l y s e a l e d b e c a u s e it d o e s not o f fgass d u r i n g c h a r g i n g and d i s c h a r g i n g .

• It i s no t s e n s i t i v e to s t o r a g e t e m p e r a t u r e .

2-29

/f

• It can be d i scharged at both high and low r a t e s .

• It has a long shelf l i fe.

The neutron and gamma radia t ions at the levels anticipated will not

damage the cel l m a t e r i a l s . Operat ing cel ls at higher t e m p e r a t u r e does

not affect the i r pe r fo rmance but it may d e c r e a s e rel iabi l i ty because of

the m a t e r i a l compatibi l i ty p r o b l e m s .

A sodium-sulphur cel l weighing about 0.15 lb is capable of

mieeting the energy s torage r e q u i r e m e n t s for the candidate e l ec t r i ca l

s y s t e m s . F i g u r e s 2 .8 -4 and 2 .85 -5 compare the weight and volume of

the a l te rna te s y s t e m s .

X O UJ

>

0.5

0.4

0.3

0.2

0.1

J

/

/

/ Nl<

f

:KEL -

1

_^ ^ " ^

CADM

^^^-^

.,..

UM (1

-

00° F)

^

LITHIL

^^

^

)M - SE

^

SOD

LENIU

^

lUM -

M (710

SULPH

° F ) _ ^

JR (57C

. *

»F)

^

- - ^

2 4 6 8 10 12

BATTERY ENERGY STORAGE - WATT-HRS

Figu re 2. 8-4. Candidate Bat te ry Weight Ver sus Energy Storage

14

: - 30

S' ^

3

- J O

>

NICKEL -

— -

/

- CADt\

/

AIUM (

'^

100-F)

/

/

/

/

1 !

^ y

X

r

^

1 LITHIUM •

1

xi y^

^

1 1 - SELENIUM (7I0°F)

1 \ y

( .

I y

• ^

SODIUM - SULPHUR (570''F)

- -

i

1 1 1 4 6 8 10

BATTERY ENERGY STORAGE - WATT-HRS

12

F i g u r e 2. 8-5 . Candidate Ba t te ry Volume Ver sus Energy Storage

The re a r e st i l l s eve ra l p r ac t i ca l p rob lems assoc ia ted with the

fabr icat ion of sodium-sulphur c e l l s . These p rob lems a r e assoc ia ted

with:

• Selecting the solid e lec t ro ly te (i. e. , sodium aluminate at s e v e r a l different composi t ions can be used) ,

• Reproducibi l i ty in fabr icat ing the e lec t ro ly te .

• Developing c e r a m i c - t o - m e t a l seals and c e r a m i c - t o - c e r a m i c s e a l s .

• Selecting a p roper c u r r e n t co l lec tor .

• Optimizing the design for min imum weight and volume.

2-31

iV

While the r equ i r ed ba t t e ry technology cur ren t ly ex i s t s at the labora tory

demons t ra t ion level , there is no off- the-shelf ba t te ry sys tem which is

capable of meet ing the weight and life r e q u i r e m e n t s for this application .

Based on a review of the c u r r e n t technical s ta tus , it i s est imated that a

prototype ba t t e ry could be developed within 2 y e a r s , A production model

could be made avai lable in about 3 y e a r s .

2 .9 RADIOISOTOPE CAPSULE DESIGN

The design c h a r a c t e r i s t i c s of the vented radioisotope capsule used

throughout the study were supplied by the AEC and a r e shown below:

Fuel Pel le t : ^^^PuO^ ^^ (90% ^^^Pu)

Density 9. 5 g/cc Specific Power 0 .45 wa t t / g

Ta-lOW Container

In ternal Volume taken as Pe l l e t Volunne P lus 10% Wall Th ickness 0. 1 5 cm (0. 060 in) Densi ty Ta-lOW 16.83 g/cc

P t -20Rh Container

C lea rance between Conta iners taken as 0 .025 cm (0.010 in) Wall Th ickness 0. 05 cm (0. 020 in) Densi ty P t -20Rh 18.74 g/cc

The AEC-suppl ied va lues for capsule weight and volume a s a

function of t h e r m a l inventory a r e as follows:

The rma l , Power , Watts

Capsule Diamete r , cm L / D = 1 2.59

3 Capsule Volume, cm"

Capsule Weight, G r a m s

30

2.59

13.6

156

40

2.79

17.1

196

50

2.99

21.0

234

60

3.15

24.6

274

2 - 3 2

b^^'

3. COMPONENT SUBSYSTEMS

3. 1 POWER CONDITIONING AND CONTROL

In addition to the gene ra l r equ i r emen t s that cover all sy s t em

components , such as long life and high rel iabi l i ty , there is a further

set of r e q u i r e m e n t s for each of the power conditioning and control sub­

sys t ems de te rmined by cons t ra in t s of the blood pump interface and the

conver t e r output r e q u i r e m e n t s , A th i rd set of r equ i r emen t s r e su l t s

f rom the specific cont ro l and power conditioning needs of the c o r r e ­

sponding engine subsys tem.

3. 1. 1 Blood Pump Interface

The Kwan-Gett type of blood pump was specified as the unit on

which to model the in terface for the purpose of this study. This is a

posi t ive d i sp lacement unit, i n t e rmed ia te between a d iaphragm d isp lacer

and a piston d i sp l ace r design. Since a p r i m e goal of the overa l l sy s t em

design is to min imize the total implanted volunne we have cons idered it

legi t imate to i n se r t a p a r t of the t h e r m a l conver t e r sys t em within the

envelope of the pump unit, provided that the pump function and physica l

appearance of the unit r ema in ident ical as seen by the blood. Because

of the fragil i ty of the pump m e m b r a n e s , we disallowed the possibi l i ty

of d i rec t ly actuating the m e m b r a n e Avithout an in te rmedia te fluid layer

to uniformly d i s t r ibu te the mechan ica l s t r e s s e s . A simplified drawing

of this type of pump unit is i l lus t ra ted in F igure 3.1-1.

A vo lumet r ic efficienty of 80% is assumed as a reasonable p r o p o r ­

tion of the specified 60% ove ra l l pow^er t r ans fe r efficiency of the pump.

The re fo re , to accommodate the requ i red maximum blood flow ra te of

12 l i t e r s / m i n u t e at the specified max imum pump cycle ra te of 120 cpna,

the design has been scaled to provide a s t roke volume d isp lacement of

125 m i l l i l i t e r s . The remain ing mechan ica l /hydrau l i c efficiency factor

of 75% r e s u l t s in a back p r e s s u r e during pumping of 133. 3 m m of

m e r c u r y (mm Hg) at the input to the blood pump, corresponding to the

specified mean functional back p r e s s u r e of 100 m m Hg at the output of

the blood pump (the aor ta ) .

3-1 6'>

INFLOW CONNECTOR

6.6 cm

PNEUMATIC DRIVE VERSION SHOWN, SCALED FOR 125 cm STROKE VOLUMF

Figure 3.1-1. Kwan-Gett Pos i t ive Displacement Blood Pump

Since the pump will probably be insta l led in the chest and the

po\ver unit in the abdomen, it is des i r ab le to provide a coupling in the

connecting line which can be made quickly and eas i ly during the ins t a l l a ­

tion p r o c e d u r e . It is envisioned that the power unit will be a l ready in

operat ion at this t ime , so a s imple mechanica l connection would be

convenient in allowing the surgeon to p r i m e the blood pump by hand before

it is energ ized by connecting it to the power unit. We have r e f e r r ed to

this r equ i r emen t as a "d ry" coupling indicating a p re fe rence for a mechan­

ical l inkage. Connecting fluid l ines with the i r inherent ins ta l la t ion p r o b ­

lems that may involve balancing fluid inventor ies a r e not an a t t rac t ive

a l t e rna te .

T^vo impor tan t cons t r a in t s on the PCCS des igns have a l ready been

d i scussed as overa l l sy s t em design cons idera t ions in Section 2. The f i r s t ,

which is the need to match the power format with the load prof i le , p r e ­

cludes (as we d i scussed) the use of in t e rmed ia te power s to rage in devices

such as pneumat ic or mechan ica l spr ings \vhich have fo rce - l imi t ed

c h a r a c t e r i s t i c s , un less they a r e combined or in tegrated with other devices

to produce a sa t i s fac tory net force c h a r a c t e r i s t i c . The second cons t ra in t

also r e l a t e s to the blood punnp load c h a r a c t e r i s t i c and r e q u i r e s that the

force r i s e - t i m e during ejection, and the force fa l l - t ime at the beginning of

the fill pe r iod , be sufficiently rapid so that the re is a min imum reduction

3-2

^ /

in the ne t l e n g t h of e a c h of t h e s e p h a s e s . As we showed in Sec t ion 2 - 3 , a

n o n c o m p r e s s i b l e p o s i t i v e - d i s p l a c e m e n t l i nkage in the pow^er t r a i n r e d u c e s

t h i s p rob l enn . C o m p r e s s i b l e p n e u m a t i c s e g m e n t s ca l l for c a r e f u l d e s i g n

to e n s u r e t h a t the e f fec t ive p o w e r duty c y c l e and fi l l ing p e r i o d s a r e not

a p p r e c i a b l y d e g r a d e d .

3, 1. 2 Blood P u m p F i l l i n g R e q u i r e m e n t s

S e v e r a l a p p r o a c h e s to o v e r a l l c o n t r o l of the blood p u m p (i. e, ,

e n s u r i n g tha t the output b lood flow r a t e c o r r e s p o n d s to the p h y s i o l o g i c a l

d e m a n d at e a c h m o m e n t in t i m e ) have been p r o p o s e d . The m e t h o d adop ted

for the p u r p o s e s of t h i s s t udy , " p a s s i v e a u t o r e g u l a t i o n , " p r o b a b l y r e p r e ­

s e n t s the s i m p l e s t and m o s t p r a c t i c a l a p p r o a c h . The t e c h n i q u e has p r o v e d

to be s a t i s f a c t o r y in r e c e n t e x p e r i m e n t s w h e r e a n i m a l s have b e e n s u p ­

p o r t e d for p e r i o d s of s e v e r a l h u n d r e d h o u r s w i th t o t a l h e a r t r e p l a c e m e n t

p u m p s .

The e s s e n t i a l r e q u i r e m e n t s of the " p a s s i v e a u t o r e g u l a t i o n " a p p r o a c h

a r e (a) to m a i n t a i n the b lood in - f low r a t e p r o p o r t i o n a l to the s o u r c e (or

in - f low) p r e s s u r e head in the left a t r i u m , and (b) to e n s u r e tha t a l l of the

a d m i t t e d b lood i s d i s c h a r g e d d u r i n g the fol lowing e j e c t i o n p h a s e . T o

a c c o m p l i s h t h i s s a t i s f a c t o r i l y and e n s u r e an o v e r a l l fi l l ing r a t e c o m m e n ­

s u r a t e wi th the r e q u i r e d m a x i m u m flow r a t e , the s y s t e m m u s t p r o v i d e a

p o s i t i v e f i l l ing f o r c e to offset the h y d r a u l i c r e s i s t a n c e s and i n e r t i a

i n h e r e n t in a l l a r t i f i c i a l b lood p u m p s . Since the s o u r c e p r e s s u r e i s

af fec ted by c h a n g e s in the body a m b i e n t p r e s s u r e in the i m m e d i a t e l y s u r ­

rounding body f lu ids ( p o s t u r a l e f f e c t s , n a t u r a l r e s p i r a t i o n , c h a n g e s in

a t m o s p h e r i c p r e s s u r e , e t c . ), the b i a s p r e s s u r e i n t r o d u c e d by th i s p o s i ­

t ive f i l l ing a c t i o n m u s t s i m i l a r l y be r e f e r e n c e d to ( i n t r a t h o r a c i c ) body

a m b i e n t , p r e f e r a b l y f r o m a s i t e c l o s e to the f i l l ing s o u r c e , i. e. , the

left a t r i u m / p u l m o n a r y ve in c o m p l e x . One p a r t i c u l a r l y i m p o r t a n t c o n ­

s t r a i n t , h o w e v e r , c o m p l i c a t e s the d e s i g n of the p o s i t i v e f i l l ing a c t i o n ,

and tha t i s t he need to avoid g e n e r a t i n g suc t ion (nega t ive p r e s s u r e ) at the

input to the b lood p u m p . N e g a t i v e p r e s s u r e s l ead to u n d e s i r a b l e p h y s i o ­

l o g i c a l c o m p l i c a t i o n s wi th in the p u l m o n a r y c i r c u l a t i o n and a l s o to the

p o s s i b i l i t y of the v e i n s c o l l a p s i n g and d a m a g i n g the p u m p in- f low v a l v e .

3-3

6'^

There a re two bas ica l ly different ways of operating the blood pump:

ei ther by maintaining a fixed pump cycle ra te and allowing the s t roke

volume to vary as a resu l t of different in-flow ra t e s (net flow rate will

then reflect the metabol ic dennand); or allow the pump to fill completely

each cycle , then ini t iate the d i scharge phase by using an end-of-fi l l

detector . In this case the pump frequency reflects the metabolic demand

and de t e rmines the output flow ra te . For the nominal daily power profile

specified for the study, both approaches a r e sa t i s fac tory . The var iab le

frequency mode would be more complicated to innplement if a g r e a t e r than

3:1 flow ra t io were requ i red . This is the resu l t of the pump operat ion

being l imited Avithin a frequency range which is cons t ra ined at the high

end by the filling pe r fo rmance of the blood punnp, and at the low end by

the medica l requi renient to maintain a minimuim pulse ra te in the

range of 45 to 50 beats per minute.

Some designs a r e m o r e easi ly implemented at va r iab le frequency

and some at fixed frequency. For example , synchronous rec iprocat ing

engines have a fixed s t roke length and so a re e a s i e r to implement on a

var iable frequency b a s i s . Devices such as solenoids and p iezoe lec t r i c

d r i v e r s can be readi ly designed to produce fixed pulses of power that also

fit more readily into the var iable frequency r eg ime .

3, 1, 3 Actuation of the Blood Pump

The evolution of the actuator design concepts developed for the two

pump operating modes and each of the specific pow^er convers ion

approaches is i l lus t ra ted in F igure 3.1-2. The goal was to develop prac t ica l

designs for units which efficiently and automatical ly performi as many as

possible of the functional r equ i r emen t s jus t descr ibed . A second goal was

to minimize the complexity of the interface with the power convers ion

stage in e i ther the engine (synchronous sys tems) or the power conditioning

unit (nonsynchronous sys t ems) .

Two famil ies of automatic ac tua tors w^ere developed. The Model 1

vers ions Ccin be used in e i ther the fixed or the var iab le frequency sys t ems

and the Model 2 ve r s ions only where the operating frequency is fixed.

The s imples t approach is to pump hydraul ic fluid between the

abdominal power unit and the blood pump. However this r e q u i r e s ac tua to r s

3-4

r

NOTE- THE PHYSICAL SIZE OF THE MODIFIED BLOOD

PUMP IS PROJECTED TO BE ABOUT 170 c m ' {10 in )

THIS FIGURE HAS BEEV SUBTRACTED FROM THE

ESTIMATED VOLUMES OF THE CONFIGL RATIONS

SHOWN HERE TO ARRIVE AT THE VALUES INDICATED.

MODEL I lANTI-SUCTION VALVE DESIGNS)

^ 580 cm- 0.798 Kgmi

, ^ _ ^ .

KWAN-GETT BLOOD PUMP

80% VOLUMETRIC EfFICIENCY '60%OVERALL)

MAXIMUM FREQUENCY 120 BPM

STROKE VOLUME 125 cm^

MODEL 2 (MAKE-UP RESERVOIR DeSIGNS)i

Figure 3.1-2. Blood Pump/Actua to r Units

which a re la rge and heavy with r e spec t to the total systenn budgets .

Bellows must be used as d i sp lacement ampli f iers when the engine cannot

provide the s t roke length requ i red by the output poAver piston. Cer ta in

specia l types of bellows a r e feasible for 10-year l i fe t imes , but they must

be operated with ve ry low d isp lacement pe r convolution and there fore a r e

relat ively bulky. General ly a more compact actuator can be designed

using mechanica l devices (cables and push rods) to t r ans fe r the power.

Careful management of the fluid behind the output power piston is

required to avoid cons iderable power l o s s e s , and all of the des igns make

use of flexible (compliance) r e s e r v o i r s located at the outer surface of the

unit which expand displacing body fluids. All of the des igns use gas ,

r a ther than liquid, in this space in o rde r to minimize frict ion and iner t ia l

l o s ses . The re a re s e v e r a l acceptable gas /mennbrane combinat ions that

appear capable of providing sa t i s fac tory pe r fo rmance over a 10-year

per iod. The degradat ion of per formance , with the max imum specified

changes in ambient p r e s s u r e , falls within the acceptable l imi t s .

All of the Model 1 automatic actuator designs use a spec ia l valve

operated by body fluid p r e s s u r e to achieve both ajitisuction protec t ion

and automatic output flow ra te control . The Model 2 designs use a

r e s e r v o i r of fluid maintained at body ambient p r e s s u r e to achieve the

same goals . The second design approach is potential ly more compact

than the f i r s t s ince the po\ver pis ton can sha re some of the volume Avithin

the blood pump envelope.

3, 1. 3. 1 Model 1 Automatic Actuators

The working pr inc ip le of the Model 1 automatic ac tua tors is i l lus ­

t ra ted in F igure 3.1-3. A collapsible " segment" is in terposed between the

fluid volume in the blood pump and the output power piston which provides

the back and forth volumetr ic d i sp lacement . The outer wal l of the

p r e s s u r e - s e n s i t i v e segment is exposed to the local body fluid p r e s s u r e so

that it tends to co l lapse , thus stopping flow and isolat ing the blood pump

from the power pis ton, whenever the fluid p r e s s u r e within the " segmen t "

falls below body ambient . Since blood wil l flow at a more or l e s s steady

rate during filling, the valving action wil l tend to be a s e r i e s of shor t

on-off cyc le s , resul t ing in a "f lut ter" mode of operat ion with the power

piston being displaced under a s teady applied force at a mean r a t e matching

the blood in-flow r a t e . 3-6

Si

BLOOD INFLOW

ANTIVACUUM VALVE PREVENTS POWER PISTON PULLING PRESSURE IN BLOOD PUMP BELOW BODY AMBIENT PRESSURE

- ANTIVAC VALVE PROVIDES PROTECTION AGAINST SUCTION DURING POSITIVE ACTION FILLING

- DISPLACEMENT OF POWER PISTON IS ADEQUATE MEASURE OF FILLING RATE

- AUTOMATIC CONTROL ACTION IF ENGINE OPERATED AT END OF STROKE (VARIABLE FREQUENCY) OR IF ENGINE RUN FIXED FREQUENCY (VARIABLE STROKE)

Figure 3 .1-3. P r inc ip l e of Model 1 Automatic Actuators

The ant isuct ion valve concept, t he re fo re , r equ i r e s a posi t ive filling

action such as a spr ing- loaded r e t u r n s t roke on the power piston, but it

p reven ts the genera t ion of a sus ta ined negative p r e s s u r e behind the blood

pump m e m b r a n e . At the same t i m e , this valve action couples the d i s ­

p lacement r a t e of the pow^er pis ton to the blood f i l l ing- ra te -con t ro l led

d isp lacement of the blood pump m e m b r a n e .

Automatic s y s t e m operat ion can be achieved e i ther by using an end-

of-s t roke s e n s o r opera ted by the povi^er piston to init iate a s e r i e s of

identical power pu lses or by s imply allowing the high p r e s s u r e fluid dr ive

to d i scha rge the pa r t - fu l l blood pump at fixed in te rva l s . The var iab le

length s t roke automat ica l ly m e t e r s the c o r r e c t amount of fluid f rom the

high p r e s s u r e accumula tor and thus regu la tes the amount of power

del ivered pe r s t roke .

Th ree v e r s i o n s of this ac tuator design were developed; one for short

mechan ica l s t r o k e s , one for long s t r o k e s , and one for the high p r e s s u r e

fluid d r ive . The synchronous vapor cycle engine opera tes at va r iab le

frequency (60-120 cpm) and furnishes a full 1. 33-inch mechanica l s t roke

on initiation by a noncontacting pump-ful l sensor . The Model IM auto­

mat ic ac tua tor (F igu re 3.1-4) is des igned to t r ans fe r this power pulse to

3 -7

BODY AMBIENT PRESSURE

r ~ ~ 1 POWER PISTON CANNOT DISPLACE j ~ ^ FLUID AT GREATER RATE THAN INFLOW ' , ' OF BLOOD TO PUMP ALLOWS

BLOOD PUMP HOUSING

BLOOD PUMP

ANTI-SUCTION VALVE

COMPLIANCE SPACE

COMPLIANCE SAC AND PERFORATED HOUSING

POWER PISTON

MECHANICAL COUPLING TO ENGINE OUTPUT (l.?3 IN STROKE)

Figure 3.1-4. Model IM Automatic Actuator

the blood pump. Pos i t ive filling action resu l t s f rom a pulling force on

the mechanica l coupling provided by a spr ing action within the engine

subsys tem.

The nonsynchronous gas cycle engine ope ra te s at a fixed frequency

of 120 cpm which is p r e s e t by adjustment of a hydraul ical ly powered auto­

matic t i m e r / v a l v e w^hich wil l be desc r ibed la te r . The antisuction valve

action e n s u r e s that the pump fills during the (approxinnately) 0. 25-second

filling phase , to a level de te rmined by the in-flow blood p r e s s u r e . A

posi t ive filling force is provided by pe rmanen t ly connecting the upper face

of the r ec ip roca to r p is ton (F igu re 3.1-5) which has a s m a l l e r effective a r e a

than the lower face, to the high p r e s s u r e side of the accumula tor . This

force acts continuously, requir ing an additional force to neu t ra l i ze this

bias during the ejection phase . This mechan i sm is long because of the

need to support the roll ing sea l , but it is nnore efficient than using an

equivalent r e t u r n spr ing . In this design the option of a "d ry" connection

was sacr i f iced to reduce s ize and weight.

3-8

GO

BLOOD PUMP HOUSING

BLOOD PUMP

ANTI-SUCTION VALVE

COMPLIANCE SPACE POWER PISTON

COMPLIANCE SAC AND PERFORATED HOUSING

ROLLING DIAPHRAGM SEAL

PISTON RECIPROCATOR

HIGH-PRESSURE FLUID LINES COUPLED TO ENGINE SUBSYSTEM

Figure 3.1-5. Model I F Automatic Actuator

A sys t em using a synchronous gas cycle engine must be operated

in a var iab le frequency mode because it is imprac t icab le to opera te with

a va r iab le s t roke length or s t roke pow^er. The var iable speed engine

design r e q u i r e s a cont ro l s ignal for a heat flow regula tor consist ing of a

pa i r of ga s -d i sp l acemen t - con t ro l l ed heat pipes in tegra l with the engine /

heat source in te r face . Obtaining an appropr ia te control signal r e q u i r e s a

complex ac tuator design using a combination of a push rod with a sp r ing -

control led sepa ra t ion section to allow m e a s u r e m e n t of the mi sma tch

between the engine frequency and the pow^er demand, and a bellows d i s ­

p lacement amplif ier to match the shor t s t roke available f rom the engine.

The design of this ac tuator is not shown because this sys t em design was

elinninated before the final evaluation.

3. 1. 3. 2 Model 2 Automatic Actua tors

A novel fea ture of the second group of automatic actuator designs is

the addition of a fluid r e s e r v o i r which allows the power piston to go

through a d i sp lacement cycle which is not locked to the blood in-flow ra te

and the resu l t ing d i sp lacement of the pumping m e m b r a n e . This decouplin

3-9

Ipl

between the power piston and pumping m e m b r a n e r e su l t s in a unit which

va r i e s slightly in physica l s ize according to the flow ra te as well as the

usual cyclic change. A n iembrane within a cage- l ike container su r round­

ing the main body of the blood pump/ac tua to r is used to form a make-up

r e s e r v o i r and it is this m e m b r a n e which undergoes additional sma l l

phys ica l pulsa t ions according to the flow ra te . In this physica l configura­

tion the fluid within the r e s e r v o i r is maintained at a p r e s s u r e c lose to

body ambient .

The power piston in this design of ac tuator (F igu res 3.1-6 and 3.1-7) is

cycled through a fixed vo lumet r ic d i sp lacement at a constant frequency

(120 cpm). At the beginning of the filling phase , the pumping m e m b r a n e

and the r e s e r v o i r m e m b r a n e a re at the i r maximum disp lacement pos i t ions .

The downward d i sp lacement ra te of the power piston is fixed and equal to

the in-flow^ ra te cor responding to the max imum blood pumping ra te

(12 1/min). If the physiological demand is such that blood flows into the

pump, under the control led p r e s s u r e gradient ( le f t -a t r ium p r e s s u r e to

body ambient) at a s lower r a t e , the p r e s s u r e in the blood pump will begin

to fall. However, as soon as the p r e s s u r e within the blood pump begins

to fall below body ainbient, the m e m b r a n e enclosing the make-up r e s e r v o i r

begins to col lapse inaintaining the p r e s s u r e very close to ambient . At the

end of the filling cycle , the make-up r e s e r v o i r has been depleted by an

amount equal to the difference between the max imum s t roke volume 3 (125 cm ) and the actual s t roke volume, cor responding to the physiological

demand at that p a r t i c u l a r t ime .

When the power pis ton r e v e r s e s d i rec t ion and begins the ejection

phase , the ini t ia l flow is back into the make -up r e s e r v o i r , until the

r e s e r v o i r is refil led at which t ime the p r e s s u r e within the pump rapidly

r i s e s to the net pump b a c k - p r e s s u r e level and blood ejection begins . At

the end of the piston s t roke the pumping m e m b r a n e is re tu rned exactly to

its init ial max imum d i sp lacement posi t ion and all of the admit ted blood is

d i scharged .

The make-ufj r e s e r v o i r design thus automatical ly de l ive r s a pa r t i a l

s t roke according to the physiological demand, p rovides posi t ive filling

action with adequate antisuction cont ro l , and p r e s e n t s a ve ry s imple

mechanica l requi re inen t to the ac tuator dr ive unit (i. e. , fixed frequency,

fixed s t roke rec ip roca t ing motion).

SUITABLE FOR ELECTRICAL NON-SYNCHRONOUS SYSTEMS AND NON-MODULATED VERSIONS OF ALL CANDIDATE SYSTEMS

BLOOD INFLOW

POWER PISTON

AMBIENT

MAKE-UP RESERVOIR PROVIDES FLUID AT BODY AMBIENT PRESSURE TO COMPENSATE FOR DIFFERENCE BETWEEN THE DISPLACEMENT RATE OF THE POWER PISTON DURING FILLING AND THE BLOOD INFLOW RATE

MAKE-UP RESERVOIR PROVIDES PROTECTION AGAINST SUCTION DURING POSITIVE ACTION FILLING

AT FIXED FREQUENCY THIS DESIGN ALLOWS A FIXED DISPLACEMENT AND PRESENTS A LOAD WITH A VARIABLE DUTY CYCLE TO THE UNIT DRIVING THE POWER PISTON

Figure 3.1-6. P r inc ip le of Model 2 Automatic Actuators

POWER PISTON COMPLETES UPSTROKE BY EJECTING BLOOD INTO AORTA

3. POWER PISTON BEGINS UPSTROKE If AGAINST LOW BACKPRESSURE

I . POWER PISTON BEGINS DOWNSTROKE

0 PUMP FILLING BEGINS

END OF EJECTION

MAKE RESERVOIR

DEPLETED IN VOLUME BY

DIFFERENCE BEIWEE ACTUAL STROKE A N D

MAXIMUM STROKE

MAKE UP RESERVOIR COLLAPSES

TO MAINTAIN PRESSURE IN PUMF

CLOSE TO AMBIENT

2. POWER PISTON COMPlETtS DOWNSIROKl

COMPLIANCE VOLUME MAXIMALLY DISTENDED

END ( » I U L I N u

Figure 3.1-7. Complete Pump/Actua tor Cycle

3-11 (,h

The power requ i red to d i scharge each s t roke co r responds to the

output blood flow ra te and the mean back p r e s s u r e — provided that the

power piston d isp lacement ra te is maintained constant when the make-up

r e s e r v o i r , which p r e s e n t s a negligible load, is being refilled. If the

piston speeds up appreciably when it is not loaded, additional power will

be diss ipated kinet ical ly within the actuator .

The ini t ial ve r s ion of this actuator design, the Model 2M,

F igure 3.1-8, was designed for the nonsynchronous (e lec t r ica l ) candidate

sys t ems . The power piston is special ly designed so that it fits very close

behind the pumping m e m b r a n e , w^ith a min imum protec t ive layer of fluid

to even out the pumping s t r e s s e s and prevent d i rec t physica l contact during

pumping. The effective d i ame te r of the piston is about 4 inches and it

r equ i re s a 1. 33-inch s t roke to d isplace 125 cm of fluid. A double m e m ­

brane design is used for the piston. At the top and bottom posi t ions the

inner mernbrane is pulled a lmost flat while the outer membrane is

deflected into a convex or bulged shape by a volume of t rapped liquid.

Because of the p a r t i c u l a r geomet ry of the design, the effective width of

BLOOD PUMP HOUSING

POWER PISTON

MAKE-UP RESERVOIR (LIQUID FILLED)

RESERVOIR MEMBRANE AND PERFORATED HOUSING

COMPLIANCE SPACE (GAS FILLED)

COATED STAINLESS STEEL CABLE

— MECHANICAL COUPLING TO ENGINE OUTPUT

(1.33 I N . STROKE)

Figure 3.1-8. Model 2M Automatic Actuator

3-12

tf

the p i s t o n c h a n g e s d u r i n g the d i s p l a c e m e n t c y c l e . The l iquid t r a p p e d

b e t w e e n the two m e m b r a n e l a y e r s , h o w e v e r , s m o o t h l y bows the m e m ­

b r a n e s into a " f a t t e r " s e c t i o n at the m i d - p o s i t i o n w h e r e the e f fec t ive

d i a m e t e r i s at a m i n i m u m , thus p r e c l u d i n g any t endency to fold o r

w r i n k l e wh ich would r e d u c e the d u r a b i l i t y of the uni t .

F o r c o m p a r a b l e e f f i c i ency , m e c h a n i c a l p o w e r t r a n s f e r r e s u l t s in

the m o s t c o m p a c t a c t u a t o r d e s i g n s . A p l a s t i c - c o a t e d s t a i n l e s s s t e e l

c a b l e is u s e d to t r a n s f e r b a c k - a n d - f o r t h mo t ion w^ith a 1. 3 3 - i n c h s t r o k e

f r o m t h e e l e c t r i c a l l y d r i v e n r e c i p r o c a t o r . It i s not n e c e s s a r y to t r a n s m i t

a p u s h - t h e n - p u l l f o r c e t h r o u g h t h i s c a b l e b e c a u s e a f o r c e b i a s is p r o v i d e d

by a s p r i n g to m a i n t a i n the c a b l e in t e n s i o n du r ing the e n t i r e c y c l e .

T h e s w e p t v o l u m e beh ind the p o w e r p i s t o n i s f i l led w i th a g a s s u c h

a s c a r b o n d iox ide w h i c h is p a r t l y c o m p r e s s e d and p a r t l y p u m p e d into the

v o l u m e b e t w e e n the p o w e r t r a n s m i s s i o n c a b l e and the o u t e r m e m b r a n e .

F i n a l o p t i m i z a t i o n of t h i s p a r t i c u l a r d e s i g n f e a t u r e r e q u i r e s m o r e

d e t a i l e d s tudy .

A p u s h - p u l l v e r s i o n of the Mode l 2M a c t u a t o r ( F i g u r e 3.1-9) is u s e d

wi th the n o n m o d u l a t e d l i n e a r v a p o r s y s t e m . Two m o r e v e i s i o n s , Mode l 2B

and 2F w^ere d e s i g n e d for a s y n c h r o n o u s g a s r e c i p r o c a t i n g s y s t e m and a

n o n s y n c h r o n o u s g a s r e c i p r o c a t i n g s y s t e m , r e s p e c t i v e l y . The Mode l 2 F

is shown in F i g u r e 3.1-10. T h e s y n c h r o n o u s gas r e c i p r o c a t i n g c a n d i d a t e

w a s not c o n s i d e r e d in the f ina l e v a l u a t i o n .

The e s s e n t i a l c h a r a c t e r i s t i c s of the s i x a c t u a t o r d e s i g n s a r e

s u m m a r i z e d in T a b l e 3.1-1.

3. 1, 4 P o w e r Cond i t i on ing

The s y n c h r o n o u s eng ine c a n d i d a t e s i n t e r f a c e d i r e c t l y t h r o u g h a

m e c h a n i c a l c o n n e c t i o n w i t h the a p p r o p r i a t e b lood p u m p a c t u a t o r . No

i n t e r m e d i a t e p o w e r cond i t i on ing o r c o n v e r s i o n is n e c e s s a r y . Both the

e l e c t r i c a l and h y d r a u l i c n o n s y n c h r o n o u s c a n d i d a t e s r e q u i r e f u r t h e r

p r o c e s s i n g . The n o n s y n c h r o n o u s g a s c y c l e eng ine (gas r e c i p r o c a t o r /

T E S M s y s t e m ) i n t e r f a c e s w i th a h y d r a u l i c p o w e r s t o r a g e uni t w h i c h a l s o

func t ions a s a s e n s o r for the eng ine p o w e r output i n o d u l a t o r . In the c a s e

of the c a n d i d a t e s w i th e l e c t r i c a l o u t p u t s , the e l e c t r i c a l p o w e r f r o m the

eng ine s u b s y s t e m m u s t be c o n v e r t e d in to the r e c i p r o c a t i n g m e c h a n i c a l

3 -13

MAKE-UP RESERVOIR (LIQUID FILLED)

RESERVOIR MEMBRANE AND PERFORATED HOUSING

POWER PISTON

COMPLIANCE SPACE (GAS FILLED)

PUSH ROD

MECHANICAL COUPLING TO ENGINE (1.33 I N . STROKE)

ure 3.1-9. P u s h - P u l l Vers ion of Model 2M Automatic Actuator

POWER PISTON

MAKE UP RESERVOIR (LIQUID FILLED)

RESERVOIR MtMBRANE AND PERFORATED HOUSING

COMPLIANCE SPACE (GAS FILLED)

ROLLING DIAPHRAGM SEAL

PISTON RECIPROCATOR

HIGH PRESSURE FLUID LINES COUPLED TO ENGINE SUBSYSTEM

Figure 3.1-10. Model 2F Automatic Actuator

3 - 1 4

^ ^

Table 3.1-1. Actuator Design C h a r a c t e r i s t i c s

Model

I M

I F

I B *

2M

2 F

2 B *

Candidate System

Linear Vapor /TESM

Gas Rec ip roca t ing /TESM

Synchronous Gas Recip rocating / TESM

All Modulated and Non-modulated E l ec t r i c a l Candidates and Linear Vapor

Gas Reciprocat ing

Nonmodulated, Synchronous Gas Reciprocat ing Cycle

Size (cm3)

267

277

325

103

236

226

Wt (kg)

0.367

0.381

0.494

0. 159

0.336

0.322

Effi­ciency

(%)

76

76

59

76

76

69

Comments

Complex, control loop losses

Spring ra te l o s ses

*These candidate s y s t e m s Avere e l iminated before the final evaluation.

raotion requ i red to opera te the blood pump actuator . The energy s torage

unit (battery) is defined as p a r t of the engine subsys tem,

3. 1, 4. 1 Hydraulic Power Conditioning

The design of the hydraul ic power conditioning unit is shown in

F igure 3.1-11. It cons i s t s of a combined high p r e s s u r e / l o w p r e s s u r e

accumula tor which accepts the 750 cpm punnping output from the engine

subsys tem and a p r e s e t hydraul ic t i m e r / r o t a r y valve. The e s sen t i a l

fea tures of the design a r e (a) the la rge s ize of the pneumat ic spring

requi red to provide the n e c e s s a r y force b ias , and (b) the compact design

of the hydraul ic t i m e r / s w i t c h unit .

The accumula tor p r e s e n t s a design challenge even assuming r e l a -3 t ively opt imis t ic reS|-onse tirnf^s from the engine. Storage for 7.5 cm oi

fluid at the in t e rmed ia t e fluid p r e s s u r e (180 psig) , which r e p r e s e n t s power

for only 2 to 4 s t rokes of the blood pump, r equ i r e s a unit with an outer

d i ame te r of alm.ost 3 inches . The force b ias requ i red for this unit is

beyond the capabil i ty of even the bes t nnechanical spr ings and a bellows

3-15

7

HYDRAULIC LINE FOR RETURN FORCE

PULSATILE HYDRAULIC POWER OUTPUT TO ACTUATOR

ROTARY VALVE

5.9 cms

TIMER/HYDRAULIC SWITCH UNIT

FLUIDIC GEAR MOTOR

HIGH PRESSURE (180 PSD

ACCUMULATOR

GAS PRECHARGE FOR

PNEUMATIC SPRING

HIGH

ENGINE INTERFACE LOW

Figure 3.1-11. Hydraulic Power Condit ioner

pneumat ic spr ing is proposed. However, a ve ry conserva t ive ly ra ted

design is r equ i r ed since with an engine speed of 750 cpm the output

ripple over 10 y e a r s wil l cycle the device through s m a l l excurs ions 9

about 4 x 10 t i m e s .

F o r the gas rec ip roca t ing sys t em, the accumula tor must be p r o ­

vided with a check-va lve control led d i ss ipa t ive bypass . F o r the modu­

lated, gas r ec ip roca t i ng /TESM sys t em a mechan ica l output f rom the

bellows is r equ i red to opera te the engine modulation unit (speed control ) .

These des ign de ta i l s a r e not shown in the f igure .

A g e a r - v a n e motor running f rom the high p r e s s u r e supply cyc les

the ro t a ry valve, through a gear reduct ion, at 120 cpm. If s i l icone fluid

is used r a t h e r than wa te r as the in t e rmed ia t e high p r e s s u r e hydraul ic

fluid, then re la t ive ly high efficiency and 10-year life can be pro jec ted for

th is design.

3-16

1,9

The basic d imens ions of the combined unit a r e 5. 9 c m s by 7. 9 c m s 3 3

in d i a m e t e r . This bas ic volume (234 c m ) i s i nc reased to 261 cm when the d iss ipa t ive bypass for the gas rec ip roca t ing sys tem is included and

3 to 285 cm when the volume requ i red for the additional modulator for the gas r ec ip roca t i ng /TESM sys t em is added. The weights cor responding to

these modificat ions a r e 0 .408, 0 .463, and 0.508 kg respect ively . The

net power t r ans fe r efficiency for all ve r s ions is projected to be 83%.

3. 1. 4. 2 E lec t r i c M o t o r / R e c i p r o c a t o r Unit

The re a r e s e v e r a l a l t e rna te ways in which the e l ec t r i c a l power

output f rom the engine s u b s y s t e m s can be conver ted to the rec iproca t ing

mechanica l motion requ i red by the automatic actuator . Among the design

approaches that have been explored specifically for e lec t r i ca l ly dr iven

blood pumps a r e p i ezoe lec t r i c convers ion , high-efficiency solenoids ,

va r iab le re luc tance o sc i l l a to r s and var ious mo to r -d r iven devices . For

s e v e r a l r ea sons which have been d i scussed in Section 2 . 3 , we have selecte

a design approach using a cons tan t - speed motor driving a d r u m - c a m

r e c i p r o c a t o r , which when combined w^ith the novel (Model 2M) design of

automatic ac tuator offers a number of significant advantages. In s u m m a r y

these advantages a r e :

• S h o r t - T e r m Load Matching. The constant speed operat ion allows the mechan ica l i ne r t i a of rotating p a r t s to be used to provide the total sy s t em with an output c h a r a c t e r i s t i c which is not fo rce - l imi t ed . Even if the back p r e s s u r e during ejection v a r i e s , the actuator wil l not s ta l l and it will draw power f rom the sy s t em at a level corresponding to the mean ra the r than the peak back p r e s s u r e . Also it is poss ib le to achieve higher overa l l convers ion efficiencies with a constant speed of operat ion r a t h e r than with a control mode requir ing s t a r t - s t o p or v a r i a b l e - s p e e d operat ion.

• Sinnple, Automatic Operat ion. No specific control s e n s o r s or power-modula t ing devices a re requ i red with this p a r t i c u l a r combination of components . The m o t o r / r e c i p r o c a t o r runs at a p r e s e t constant speed (and mechanica l s t roke) even when the de l ivered output power is var ied over a 2:1 range . The load var ia t ion is p a s s e d to the engine subsys t em as simply a v a r i a ­tion in the e l e c t r i c a l load, i. e. , a varying demand for e lec t r i c c u r r e n t .

3-17

• U s e s M o r e C o m p a c t A u t o m a t i c A c t u a t o r , The c o n s t a n t f r e ­quency o p e r a t i n g m o d e w h i c h i s p o s s i b l e w i th the M o d e l 2 a u t o m a t i c a c t u a t o r s a l l o w s e l i m i n a t i o n of the a n t i v a c u u m va lve c o n f i g u r a t i o n and t h e r e f o r e m o r e c o m p a c t a c t u a t o r d e s i g n s w i t h the p o w e r p i s t o n w i th in the n o m i n a l e n v e l o p e of the blood p u m p . E l e c t r i c a l c o n v e r t e r s w i t h f ixed p u l s e p o w e r f o r m a t s s u c h as the s o l e n o i d o r the p i e z o e l e c t r i c d e v i c e s r e q u i r e a v a r i a b l e - f r e q u e n c y m o d e of o p e r a t i o n and u s e of the l e s s c o m p a c t Mode l 1 a c t u a t o r s . At the s y s t e m l eve l , the d i f f e r e n c e b e t w e e n the two a c t u a t o r s is 164 c m and 0.209 kg.

• F l e x i b i l i t y . S ince the p r e s e t m o t o r s p e e d d e t e r m i n e s t h e output p u l s e r a t e , it a l s o p r e d e t e r m i n e s the i n a x i m u m (and m i n i m u m ) blood flow r a t e s . T h e o p e r a t i n g r a n g e of ou tput flow r a t e s c a n then be v a r i e d w i th in a s m a l l r a n g e (up to the f r e q u e n c y linnit s e t by the f i l l ing c h a r a c t e r i s t i c s of the blood p u m p ) v e r y s i m p l y p r i o r to i n s t a l l a t i o n by ad jus t ing the p r e s e t m o t o r s p e e d . When blood p u m p s w i th b e t t e r f i l l ing c h a r a c t e r ­i s t i c s a r e d e v e l o p e d , t h e y c a n be i n c o r p o r a t e d wi thou t s i g n i f i ­c a n t s y s t e m r e d e s i g n . A h i g h e r f r e q u e n c y of o p e r a t i o n would a c h i e v e the s a m e flow r a t e p e r f o r m a n c e w i th a p h y s i c a l l y s m a l l e r p u m p un i t , o r i n c r e a s e d flow p e r f o r m a n c e f r o m the s a m e s i z e of p u m p .

A n o t h e r r e f i n e m e n t t ha t c a n be i n c o r p o r a t e d i s to d e s i g n the g r o o v e in the c a m - d r u m ( d i s c u s s e d l a t e r ) to p r o v i d e a b lood p r e s s u r e w a v e f o r m w h i c h i s a c l o s e m a t c h to the n a t u r a l p h y s i o l o g i c a l w a v e f o r m .

• S t a t e - o i - l n e - A r t T e c h n o l o g y . The c h o s e n d e s i g n a p p r o a c h i s b a s e d on e x i s t i n g and p r o v e n t e c h n i q u e s . T h e r e i s no q u e s t i o n t h a t t he d e v i c e can be d e v e l o p e d wi th in the p r o j e c t e d s i z e and w e i g h t .

T h e p r e f e i r c J n i e c h a n i c a l d e s i g n of the m o t o r / r e c i p r o c a t o r un i t ,

wi th p h y s i c a l d i m e n s i o n s , i s shown m F i g u r e 3.1-12. T h e b r u s h l e s s dc

e l e c t r i c m o t o r i s d e s i g n e d to run at a c o n s t a n t s p e e d of a b o u t 9, 000 r p m .

It r u n s in an h e r m e t i c a l l y s e a l e d h o u s i n g on s e l f - e n e r g i z i n g c o m p l i a n t m u l t i

foil type gas b e a r i n g s . R o t a r y m o t i o n i s coup led t h r o u g h the a l u m i n u m end

face by m e a n s of a doub le p l a t e m a g n e t i c coup l ing a b o u t 1 inch in d i a m e t e r .

The nnotor b e a r i n g s u r f a c e s a r e of fse t to c o m p e n s a t e for t h e a x i a l t h r u s t

r e s u l t i n g f r o m th i s c o u p l i n g . As no ted in the s e c t i o n on t u r b o g e n e r a t o r

d e s i g n (Sec t i on 3. J 3), the b e a r i n g d e s i g n is c o n s i d e r e d s t n t . : , - o f - t h e - a r t .

The b e a r i n g i s d e s i g n e d to b e c o m e a e r o d y n a m i c a l l y s u p p o r t e d at about

1, 000 r p m and at full o p e r a t i n g s p e e d t h e r e i s no p h y s i c a l c o n t a c t w h a t s o ­

e v e r b e t w e e n the s u r f a c e s . S ince the m o t o r r u n s c o n t i n u o u s l y at a

^-18 70

MAGNETIC COUPLING THROUGH HERMETIC

SEAL

9 000 RPM PERMANENT MAGNET BRUSHLESS DC MOTOR RECIPROCATING DRUM-CAM

SHUTTLE (,20 RPM) MECHANICAL

OUTPUT CONNECTOR

HARDENED GUIDE PIN

GAS BEARINGS

75-1 COMPOUND PLANETARY

GEAR SPEED REDUCER

'STATIONARY LINEAR GUIDE BEAMS

• 3.5 • Inch

Figure 3.1-12. Moto r /Rec ip roca to r Unit

constant speed (within 2 or 3%), a min imum wear situation will exist .

Exposing the motor to mechanica l shocks g r e a t e r than about 3g will cause

a momenta ry "brush ing" between the rotating Teflon-coated foils and the

s ta t ionary surface before the ro tor r e s t o r e s itself to the stable no-contact

posit ion. However this bear ing design is commonly used for applications

involving many s t a r t - s t o p cycles and it is felt that the motor will easi ly

withstand a 10-year mechanica l shock environment such as will be

encountered in the body.

To achieve high convers ion efficiency, the motor uses s a m a r i u m -

cobalt pe rmanen t magnets on the ro tor and an i ron less s ta tor . It is

e lec t ronica l ly commutated by switching signals orginated by a position

encoder built into the motor . Calculat ions indicate that a motor

efficiency well in excess of 70% can be achieved. This figure includes

the commutat ion l o s se s and the magnet ic coupling

Under the projec ted opera t iona l load conditions, the s tored

momentum of the ro tor will ensu re vi r tual ly constant speed operation.

3-19

7/

In the w o r s t - c a s e si tuation, when the actuator p r e s e n t s a n o - b a c k p r e s s u r e

situation for the f i r s t par t of the ejection phase , the change in speed is

calculated to be 2. 3%.

The 75:1 ro t a ry speed reduction f rom 9000 to 120 rpm is achieved

using a very compact compound p lane ta ry gear t ra in (Figure

3.1-14). The app iopr i a t e mechanica l s t r e s s levels for each of the

components a r e calculated to be wel l within the allowable design loads

for regula r high-quali ty gea r and bear ing m a t e r i a l s . The moving p a r t s

a re sp lash- lubr ica ted with lightweight oil. The power t r ans fe r efficiency

including projec ted bear ing cuid windage l o s se s is 85%.

(a ) (b)

F igure 3.1-13. Moto r /Rec ip roca to r at (a) S ta r t -of -S t roke (b) Mid-Stroke and (c) End-of-Stroke

The c a m - d r u m runs in a se t of combined r a d i a l / t h r u s t filna-

lubricated porous bronze bea r ings . The 120 rpm ro ta ry motion is

converted to a rec iproca t ing mechan ica l d i sp lacer with a fixed a m p l i ­

tude of 1. 33 inches by means of a continuous guide or slot machined in

3-20

7 ^

MAGNETIC COUPLING - - ^

INPUT

CIKCUIAII C A M QliTPUT

i I-

.1719 INCH P D.

INPUT - »O0O RPM

P O W t K - .013 H.P.

INPUT TOKQUI

GEAR RATIO

WHERE

= .091 IN-LBS

1 - — "d bd

1

^ - I - - 1 = bd 75

1

75

74

75

LET B • C GEARS » 20 TEETH O N 64 D.P.

A - 74 TEETH O N 64 D.P.

O 75 TEETH O N 64 D.P.

OUTPUT TORQUE • 75 ( . 0 9 1 ) - 6 8 I N - LBS

M A X . ALLOWABLE TOOTH L O A D I N G - 1000 LBS/ 1" TOOTH WIDTH

INPUT TOOTH L O A D I N G

OUTPUT TOOTH L O A D I N G --

l O R Q U E ' L

TOOTH WIDTH

.091

1.17

2 _

03

6J 1.17

2

—,— » .08

.155

.03

11.6

.08

5.2 «

1" TOOTH WIDTH

145 f

• TOOTH WIDTH

Figure 3.1-14. E l e c t r i c a l PCCS Gear Box Detai ls

the inner surface . A shutt le (F igure 3.1-13) which is free to move back an

for th along a set of s ta t ionary guides (w^hich a re designed to absorb the

rad ia l forces) is provided with a hardened guide pin which engages in the

machined slot on the rotat ing d rum. The continuous el l ipt ical configura­

tion of the machined slot is such that the shuttle is moved smoothly

through one back-and- fo rward rec iproca t ing cycle for each revolution of

the d rum. A special ly designed long-life bellows is used to seal the

lubr icant within the r e c i p r o c a t o r unit. This bellows is at tached to the

shutt le unit and is located concent r ic with the long axis of the unit within

the s ta t ionary guides. A mechan ica l output is obtained by connecting the

cable of the Model 2M ac tua tor to a mechan ica l connecting post attached

to the inside surface of the bel lows.

The overa l l length of the unit i s 3. 5 inches and it is 2 inches in 3 3

dianneter and 10.9 in (179 cm ) in volume. The calculated weight is

3-21

7i

0.88 lb (0.4 kg). The overa l l power t r ans fe r efficiency from e lec t r i ca l

input to mechanica l output i s 59%. The total surface a r ea of the unit is 2

about 180 cm and, there fore , the mean flux through the surface is l e s s

than 0. 04 w a t t / c m .

3 . 1 . 4 . 3 E lec t r i ca l Control for the Reciprocat ing Vapor Candidate Systems

The l iner v a p o r / T E S M sys t em and the l iner vapor sys tem a r e both

control led e l ec t r i ca l ly . The solenoid pump and the smal l thernnoelectr ic

module a r e cons idered to be pa r t of the engine subsys t ems . The engine ra te

control for the l iner vapor /TESM sys tem is a non-contact ing type of

e lec t r i ca l switch which physical ly senses when the mechanica l ac tua tor

dr ive r e a c h e s the pump-full and the end-of - s t roke pos i t ions . The weight

of this switch together with the n e c e s s a r y e lec t ronic c i r cu i t ry is 0.041 kg.

This f igure must be added to the bas ic weight of the IM model ac tua tor

(0.367 kg) to obtain the total PCCS weight (0.408 kg). These additional

smal l p a r t s can be eas i ly accommodated within the exist ing package

vo lumes .

F o r the l iner vapor sys tem, which runs at a fixed s t roke ra te of

120 cpm, the e l e c t r i c a l s t roke ini t iat ion and t e rmina t ion s ignals a r e

generated by a p r e s e t f ixed- ra te e lec t ronic osc i l l a to r . The additional

weight is es t imated to be 0.035 kg, and it is again eas i ly acconnmodated

within the engine subsys tem package envelope. The total PCCS weight in

this case is the weight of the Model 2M a c t u a t o r (0. 160 kg) plus the

additional e lec t ron ic osc i l l a tor module (0. 035 kg) which is equal to 0. 195 kg

3 . 1 . 4 . 4 Al terna te Mechanical Configurations

The physical c h a r a c t e r of the a r r a n g e m e n t s used to i l lus t r a t e the

mechanical interface between the different ac tua tors and the modified

Kwan-Gett blood pump mus t be cons idered only a s nominal des igns at th is

t i m e . The nnechanical configurations will be opt imized when a m o r e

detailed specification is ava i lab le .

Two a l t e rna te genera l configurations have been cons idered for the

m o t o r / r e c i p r o c a t o r unit (the p r e f e r r e d design approach) . One is the

t r a n s m i s s i o n of mechanica l power via a rotat ing r a t h e r than a push-pul l

l inkage. The second is the impact of changing the d i rec t ion of the

mechanica l d r ive with r e spec t to the d i rec t ion of d i sp lacement of the blood

3 - 2 2

7/

p u m p m e m b r a n e .

In o r d e r to u s e a r o t a r y c a b l e c o n n e c t i o n , the r o t a r y - t o - l m e a r m o t i o n

c o n v e r t e r (the r e c i p r o c a t o r p a r t of the unit) m u s t be p h y s i c a l l y l o c a t e d

on the a c t u a t o r . A spec i f i c d e t a i l e d t n e c h a n i c a l d e s i g n with the d r u m - c a m

p h y s i c a l l y l o c a t e d w i t h m the a c t u a t o r w a s not a t t e m p t e d s ince it i s c l e a r

t ha t t h i s a l t e r n a t e c o n f i g u r a t i o n would have a m i n o r i m p a c t on the

o v e r a l l s i z e , we igh t , and p e r f o r m a n c e of the u n i t . The l u b r i c a t i o n f lu ids

would be r e t a i n e d m t h e g e a r b o x m t h i s c a s e by a s c r e w type s e a l r a t h e r

t han the b e l l o w s u s e d m the n o m i n a l d e s i g n . T h i s would p e r m i t a b l ight ly

m o r e c o m p a c t d e s i g n bu t t h i s opt ion w a s not p r e f e r r e d b e c a u s e i t wi l l

i n c r e a s e the s i z e and w e i g h t of the i n t r a t h o r a c i c p o r t i o n of the c o n v e r t e r

s y s t e m . H o w e v e r , w i th t h i s a r r a n g e m e n t , the coup l ing cab l e i s no

l o n g e r n e c e s s a r i l y p a r a l l e l to the p u m p m e m b r a n e d i s p l a c e m e n t d i r e c t i o n .

A c c o r d i n g to the d a t a s u p p l i e d by one c a b l e m a n u f a c t u r e r , the powe r

t r a n s m i s s i o n e f f i c i ency i s c o n s t a n t a t abou t 95% for to t a l bend a n g l e s up

to 90° .

An a l t e r n a t e type of r e c i p r o c a t o r unit u s e s an e p i c y c l i c g e a r p r i n c i p l e

r a t h e r t h a n t h e d r u m - c a m a p p r o a c h In th i s c a s e , the s h a p e of the m o d i f i e d

i n t r a t h o r a c i c p a c k a g e is l e s s s y m m e t r i c a l , but the cab le d r i v e r u n s at

r i g h t a n g l e s to the d i r e c t i o n of the p u m p m e m b r a n e d i s p l a c e m e n t S ince ,

a g a i n the c a b l e can b e f l exed up to 90 wi thou t s i g n i f i c a n t w e a r o r e f f ic iency

pena l ty , t h e s e two b a s i c c o n f i g u r a t i o n s p e r m i t c o m p l e t e f r e e d o m in s e l e c t i n g

the c a b l e c o n n e c t i o n a n g l e One d r a w b a c k wi th the e p i c y c l i c r e c i p r o c a t o r

IS t h e n e e d to l u b r i c a t e the g e a r s w h e n i t i s m o u n t e d on t h e a c t u a t o r , w h e r e a s

the d r u m - c a m unit can b e r u n wi th d r y l u b r i c a n t . T h e need for a s e a l s u c h

a s a b e l l o w s a t the r e c i p r o c a t i n g m e c h a n i c a l ou tpu t m a k e s th i s opt ion l e s s

a t t r a c t i v e t h a n the d r u m - c a m .

3. 1. 5 P e r f o r m a u n c e S u m m a r y

The m a j o r d e s i g n f e a t u r e s of the p o w e r cond i t ion ing and c o n t r o l

s u b s y s t e m s fo r e a c h of the c a n d i d a t e s t ha t w e r e f ina l ly e v a l u a t e d a r e

b r i e f l y s u m m a r i z e d b e l o w .

3 -23

3. 1. 5. 1 Linear Vapo r /TESM System

Engine Output:

Modulation:

Engine Control :

Actuator;

Packaging:

Power T r a n s f e r Efficiency:

3. I. 5. 2 Linear Vapor System

Engine Oj.tput:

Reciprocat ing mechanica l d r ive , 1. 33-inch s t roke at va r iab le f r e ­quency (60-120 cpm).

Power del ivered on push s t roke . P re load spring in engine requi red to provide force for filling action.

By var ia t ion in s t roke frequency. Constant s t roke work.

E l ec t r i c a l control sys t em requ i r e s s ignal at end of filling s t roke to init iate the power s t roke . Non-contact s^vitch is ins tailed within engine subsys tem package.

Model IM. Antisuction valve vers ion with flexible mechanica l push rod in p las t i c -coa ted guide.

3 Single package, 267 cm , 0.408 kgm

76%.

Reciprocat ing mechanica l d r ive , 1. 33-inch s t roke at constant f r e ­quency (120 cpm). Power de l ivered on push s t roke . P re loaded spring in engine requ i red to provide filling force .

Modulation and Engine Contro l :

Actuator :

Packaging:

Power T r a n s f e r Efficiency:

None, p r e s e t e l e c t r i c a l t i m e r . Exces s s t roke power d iss ipa ted in ac tua tor /b lood pump.

Model 2M. M a k e - u p - r e s e r v o i r ve r ­sion with flexible mechan ica l push rod in p l a s t i c -coa ted guide. No spr ing .

3 Single package, 103 cm , 0.195 kg.

76%.

3. 1. 5. 3 Gas Rec ip roca t ing /TESM System

Engine Output:

Modulation:

Pumped high p r e s s u r e (180 psi) hydraul ic fluid del ivered at a var iab le flow ra te de te rmined by the engine speed (nominally 750 cpm).

By var ia t ion of engine speed.

3-24

7^

Engine Control : Mechanical signal f rom accumula tor varying engine dead space .

Actuator :

Power Conditioning:

Packaging:

Model I F . Antisuction valve vers ion with piston r ec ip roca to r located behind the output power piston. Power t r a n s m i s s i o n through a pa i r of h i g h - p r e s s u r e l ines . Size 277 cm -', weight 0. 381 kg efficiency 76%.

Combined high pressure/ low^ p r e s ­s u r e accumulator with fluid l ines and r o t a r y valve located within the engine subsys t em package. Size 285 cm^, weight 0.508 kg, efficiency 83%.

Actuator permanent ly connected to power-condi t ioning unit which is located within the engine subsys tem package. Size 569 cm3, weight 0.889 kg.

Power T r a n s f e r Efficiency:

• ^ Gas Reciproca t ing Sys tem

Engine Output:

Modulation and Engine Cont ro l :

Actuator :

Power Conditioning:

Packaging:

Power T r a n s f e r Efficiency:

63%.

Pumped high p r e s s u r e (180 psi) hydraul ic fluid de l ivered at fixed (maximum) flow ra t e .

None. Engine speed p r e s e t at 750 cpm.

Model 2F . M a k e - u p - r e s e r v o i r v e r ­sion with piston r e c i p r o c a t o r located behind the output power pis ton. Power t r a n s m i s s i o n through a pa i r of high p r e s s u r e l ines . Size 236 cm^, weight 0.335 kg, efficiency 76%.

Same as modulated vers ion except that the modulator output is rep laced with a check-va lve control led d i s -s ipat ive bypass . Size 261 cm-^, weight 0.463 kg, efficiency 83%.

Same as modulated vers ion . Size 497 cm^, weight 0.798 kg.

63%.

3-25

nt]

3. 1. 5. 5 Modulated and Nonmodulated E lec t r i ca l Sys tems

Engine Subsys tem Output: DC e l ec t r i c a l power at 15 vol ts .

Modulation: P a s s i v e ; by load var ia t ion .

Engine Contro l :

Actuator :

Power Conditioning:

Packaging:

Power T r a n s f e r Efficiency:

None. P r e s e t to genera te 6. 2 w^atts (9. 9 wat ts for sys tem without ba t te ry) .

Model 2M. M a k e - u p - r e s e r v o i r v e r ­sion with spr ing biased cable t r a n s ­miss ion , mechanical ly connected to the mechanica l output of the motor / r e c ip roca to r unit. Size: 103 cm-^, weight 0, 160 kg, efficiency 76%.

E l e c t r i c a l to r ec ip roca t ing -mechanica l output with 1. 33-inch s t roke at 120 cpm. Constant speed dc pe rmanen t magnet gas bear ing motor driving a d r u m - c a m r e c i p r o ­ca tor through compound p lane tary gear speed r educe r . Nominally packaged sepa ra t e f rom the main power unit (engine subsys tem) . The b a t t e r y l e s s s y s t e m has additional e lec t ron ic c i r cu i t r y to cont ro l the load sharing between the two gene­r a t o r s . Size: 180 cm^, weight 0.4 kg, efficiency 59%.

Nominally s epa ra t e packages for the ac tuator and m o t o r / r e c i p r o c a t o r unit ^vith mechan ica l connection. Armored cable to c a r r y e l ec t r i ca l pow^er between the main power unit and the m o t o r / r e c i p r o c a t o r . Except in the cas( of the t h e r m o e l e c t r i c / b a t t e r y sys t em design, where the m o t o r / r e c i p r o c a t o r i s physical ly in tegra ted with the stat ic genera to r in the main power unit . Size 283 cm^, weight 0,560 kg.

45%.

3-26

71

3 .2 E N G I N E SUBSYSTEMS

T h e t h e r m a l c o n v e r t e r eng ine s u b s y s t e m s c o n v e r t hea t f r om the

r a d i o i s o t o p e in to h y d r a u l i c , p n e u m a t i c , e l e c t r i c a l o r m e c h a n i c a l e n e r g y

for t h e p o w e r cond i t i on ing and c o n t r o l s u b s y s t e m o r for d i r e c t a c t i v a t i o n

of the blood p u m p . The m o s t p r o m i s i n g c l a s s e s of eng ine s u b s y s t e m s

s e l e c t e d for u s e in the t h e r m a l c o n v e r t e r a r e :

• G a s r e c i p r o c a t i n g

• L i n e a r v a p o r

• R o t a r y v a p o r

• T h e r m o e l e c t r i c s

• H y b r i d ( c o n s i s t i n g of t h e r m o e l e c t r i c s and r o t a r y v a p o r d e v i c e s )

T h e r m o e l e c t r i c e l e m e n t s a r e a l s o u s e d wi th the r e c i p r o c a t i n g

d e v i c e s on s e v e r a l of the c a n d i d a t e s y s t e m s for e l e c t r i c a l c o n t r o l p o w e r .

As u s e d in t h i s s t u d y , the t e r m h y b r i d r e f e r s to s u b s y s t e m s in which the

funct ion of p r i m a r y p o w e r g e n e r a t i o n is s h a r e d b e t w e e n two d i f fe ren t type

of e n g i n e s .

T h e eng ine s u b s y s t e m s e x a m i n e d w e r e d r a w n f r o m t h r e e b a s i c

s o u r c e s :

• E n g i n e s be ing d e v e l o p e d by o t h e r s s p e c i f i c a l l y for a r t i f i c i a l h e a r t d e v i c e s

• E n g i n e s t ha t h a v e b e e n d e v e l o p e d for o t h e r a p p l i c a t i o n s and w h o s e c h a r a c t e r i s t i c s m a k e the d e s i g n s a t t r a c t i v e for t h i s a p p l i c a t i o n

• E n g i n e s u t i l i z i n g new a p p r o a c h e s , but b a s e d on p r o v e n t e c h n o l o g y

E a c h type of eng ine e x a m i n e d in the gas r e c i p r o c a t i n g , l i n e a r v a p o r

r o t a r y v a p o r , t h e r m o e l e c t r i c and h y b r i d c l a s s a r e d i s c u s s e d in Sec t i ons

3 . 2. 1 to 3 . 2. 5, r e s p e c t i v e l y .

3-27

7f

Although many engine designs were evaluated and a r e descr ibed in

detai l , mos t were not cons idered legi t imate candidates for scoring because

they were deficient in one or m o r e of the following r e s p e c t s :

10-year- l i fe design

Proven technology design

Design mus t mee t AEC groundrules

Design mus t mee t TRW groundrules

The re must be a reasonable level of confidence that the engine design being considered will be capable of operat ing for a 10-year period.

Information mus t be available to show that the design is feasible and that the requis i te technology is cu r ren t ly avai lable .

• 60-watt heat source m a x i m u m

• Volume less than 1. 5 l i t e r s

• Weight l ess than 3 kg

• Specific gravi ty l ess than 2. 0

• F ib rous insulat ion (Min-K with xenon fill gas) through­out r a the r than vacuum foil insulation (see Section 2, 7).

• Heat re ject ion to body t i s sues with container designed to l imit heat flux to 0.07 watt/ cm^ or l e s s (see Section 2 .5 ) .

• Energy s torage mus t be f rom e i ther TESM or h igh-ene rgy-densi ty ba t t e r i e s (see Section 2, 8),

• The engine output must be compatible with a power con­ditioning and control s u b s y s ­t em as p resen ted in Section 3. 1,

3-28

F

3 . 2 . 1 Gas Reciprocat ing Engines

3 .2 . 1. 1 His tor ical Background

The basic pr inc ip les of regenera t ive reciprocat ing engines date back

many y e a r s . Until the last few y e a r s , however, p r o g r e s s has been slow

and has been confined to s izes and power levels outside the prac t ica l range

for ar t i f ic ia l hear t appl icat ions . Milestones marking engine innovations

a r e l is ted in Table 3 .2 . 1-1.

The f i r s t d i sp l ace r - type engine, built by Stirl ing in 1817, has been

revived in modern form in s izes of about 50 hp pe r cylinder. Stirl ing also

built a double-act ing machine with a d i sp lace r , A regenera t ive gas

machine using pistons with no d i s p l a c e r s was invented in 1870. Because

the pistons were s ing le -ac t ing , the specific power was low.

The f i r s t l iquid /vapor engine was built in 1930. This engine i n t r o ­

duced the liquid r e g e n e r a t o r , recent ly t e rmed a " t idal" r egenera to r and

desc r ip t ive of its changing liquid level . A p r e s s u r e genera to r , with out­

put in the fo rm of p r e s s u r i z e d gas to pump up a tank, was built in 1937.

The f i rs t mul t i -cyc le , mu l t i - cy l inde r machine came in 1945. Instead of

conventional mechanica l l inkages , it had a swash plate and connecting

l inks for rec iproca t ing motion.

Table 3 . 2 . 1-1. Regenerat ive Reciprocat ing Engine Development Miles tones

Single-Cyl inder Disp lacer

Double-Acting Pis tons with Disp lacer

Single-Acting Pis tons without Disp lacer

Liquid Regenera to r

Gas P r e s s u r e Genera to r

Mul t i -Cyl inder , Double-Acting Machine

Resonant F r e e - P i s t o n Mult i -Cycle Variable

Resonant F r e e - P i s t o n Constant Volume

Resonant F r e e - P i s t o n Single-Cycle Variable

E lec t r i ca l ly -Cont ro l l ed F r e e - P i s t o n

Volume

Volume

1817

1840

1870

1930

1937

1945

1964

1966

1967

1969

3-29

Up to that t ime , engine mechanica l output had always been del ivered

by a crankshaft . The f i r s t f ree-p is ton engine, proposed and constructed

in 1964, was a mul t i - cyc le va r i ab le -vo lume machine . In 1966, a f ree -

piston constant -volume p r e s s u r e genera tor was built. This was proposed

for ar t i f ic ia l hea r t applicat ions and has rece ived further engineering

development since then.

The resonant f reo-pis ton s ingle-cyc le machine with va r able volume

was first mechanized in 1967. Final ly , an e lec t r i ca l ly -con t ro l l ed f ree -

piston engine was built in 1969. This was a s ingle-cycle engine with a

liquid r egene ra to r .

3 . 2 . 1 . 2 Cycle and Working Fluid Selection

Regenerat ion

Regenerat ion is an essen t i a l feature of gas rec iprocat ing mach ines .

It is accomplished by a duct connecting high- and low- t empera tu re work­

ing spaces . Typical ly, the duct is filled with a nnatrix of fine wi re to

provide la rge heat t r an s f e r surface a r e a . As gas flows through the r egen­

e r a t o r f rom hot to cold volumes , energy is deposited in the m a t r i x . This

energy is re turned to the gas when the flow r e v e r s e s . In smal l engines ,

the m a t r i x may uc omitted if the narrov^ passage avails a r e designed to

fulfill the t h e r m a l regenera t ion function.

Carnot efficiency can be achieved only if a l l heat supply takes place

at the peak cycle t e m p e r a t u r e , and all heat re ject ion at the lowest t e m p e r ­

a tu re . If ex te rna l heat exchange occu r s at in te rmedia te t e m p e r a t u r e s ,

engine efficiency is degraded. It is the function of the r e g e n e r a t o r to

e l iminate o r min imize heat exchange at in t e rmed ia te t e m p e r a t u r e s .

Engine Volume Variat ion

A s imple way to accompl ish regenera t ion in a rec iproca t ing gas

engine is to cycle a regenera t ive d i sp l ace r back and forth in a volume of

gas heated at one end and cooled at the o ther . The rec iproca t ion alone

will cause a fluctuation in p r e s s u r e , with a p r e s s u r e ra t io of approximate ly

1.2 pe r s tage . The p r e s s u r i z e d gas can be used to charge ais ar cumula tor

or to actuate a pumping mechanisnn d i rec t ly . This operat ing mode will be

r e f e r r ed to as "constant vo lume" ,

-i-30

n

A second opera t ing mode combines rec iprocat ing flow through a

r egene ra to r with vo lumet r i c changes in the main volume of gas . Com­

p r e s s i o n and expansion a r e phased to supplement p r e s s u r e var ia t ions due

to rec ip roca t ion of the r e g e n e r a t o r a lone, achieving a much higher p r e s ­

s u r e ra t io ( typical ly 2.2). This mode of operat ion will be cal led "var iab le

vo lume" .

Both "constant vo lume" and "var iab le volume" refer not to the

individual component working spaces , which mus t always be varying in all

mechaniza t ions of St ir l ing engines , but r a the r to the total volume. Vary ­

ing the total volume of the working fluid automatical ly subjects it to a

per iodic r i s e and fall in p r e s s u r e .

The va r i ab le volume operat ing mode was selected for this p r o g r a m

because , a s shown in Table 3 . 2 , 1-2, i t s work output i s an order of

magnitude h igher than for a constant volume device of the same physical

d imens ions and cycle condi t ions . E x p r e s s i o n s for power output used to

ca lcula te specific power a r e a s follows:

Constant p Volume out T ^ (^3 -^2)^1 T) P V

max

1

/ m i n I

\ max /

Var iab le Volume out

TTK s i n J-

T + K + m (1 T ) P V max

4T^ Vi~^^(i+ Ji- p'^)

where

; T^ + K^ + 2 T K cos a

T + K ^m X-j = pis ton posi t ion when outlet valve c loses

X_ = pis ton posi t ion when outlet valve opens

V = swept volume

Y = rat io of specific hea ts

See Table 3 ,2 . 1-2 for o ther symbols

3-31

$3

Table 3 . 2 . 1-2. Specific Power Compar ison

P a r a m e t e r

Max. Temp. (Tj^^^)

Min. Temp. (T^^^^)

Maximum P r e s s u r e (P ) ^ max

Cylinder Diamete r

Disp lacer Extension Diamete r

Stroke (S)

F rac t iona l Stroke (X3 - X2)/S

Clearance Volume (K V)

Phase Angle {0)

Volume Ratio (K)

T e m p e r a t u r e Ratio (T)

P r e s s u r e Ratio

Minimum P r e s s u r e

Power

Specific Power

Units

°F

° F

p s i

inches

inches

inches

cubic inches

d e g r e e s

—

—

—

p s i

I b - i n / c y c l e / c u . in.

j o u l e / c y c l e / c u . in.

Constant Volume

1200

100

500

1. 0

0 . 5

1. 0

0.25

0. 5

—

—

0. 337

1. 16

430

12

1. 36

Variable Volume

1200

100

500

1. 0

0. 5

1.0

0. 5

90

1.0

0. 337

2 . 2

227

174

19.5

Working Fluid Selection

A rec iproca t ing gas engine can be opera ted not only by using s ing le -

phase g a s e s , but a l so with a two-phase ( l iquid/vapor) working fluid. The

advantage of using a gas is that 100 percen t regenera t ion would be poss ib le

if the gas were ideal . This may be seen from a t e m p e r a t u r e - e n t r o p y

char t for an ideal gas showing all polytropic l ines pa ra l l e l to each o ther .

i M ^1/

This means that i so the rma l p r o c e s s e s at different t e m p e r a t u r e s can be

connected by any pair of a r b i t r a r y polytropic p r o c e s s e s with perfect regen­

era t ion. All of the heat given up during cooling can then be recovered

during heat ing.

In con t ras t , the t e m p e r a t u r e - e n t r o p y d i ag ram for a superheated

vapor shows differences in specific hea ts for expansion and compress ion .

A so-ca l led "pinch t e m p e r a t u r e " exis ts which de t e rmines to what extent

regenera t ion is poss ib le . In the l iquid /vapor region the differences a r e

even g r e a t e r . How^ever, the vapor sy s t ems have the advantage that very

large p r e s s u r e different ia ls can be achieved. While gas regenera t ive

cycles a r e l imi ted to p r e s s u r e differences of a fe^v hundred pounds pe r

square inch, vapor cycles under s i m i l a r conditions may go as high as

seve ra l thousand pounds per square inch.

Gas Working F lu ids . Ideal gases a r e most des i r ab le for r e g e n e r a ­

tive machines because of s y m m e t r y in the i r t empera tu re -en tha lpy d ia ­

g r a m s . The s ame amount of enthalpy is exchanged during t rans i t ion from

one t e m p e r a t u r e to another for equal p r e s s u r e changes in an ideal gas .

In non- ideal gases , heat recept ion and rejection in the r egene ra to r a r e not

balanced, so that additional heat mus t be supplied or rejected external ly .

Engine speed is the mos t per t inen t design var iab le affecting the

choice of gaseous working fluid. At high speeds , hydrogen and he l ium

appear to be the bes t choices . At low ( synchronous) speeds , the choice

is not as c l e a r - c u t . Gas select ion fac to r s influenced by speed include

v iscous l o s s e s , gas leakage pas t the piston, and heat t r ans fe r as

l imited by conduction. F a c t o r s insens i t ive to speed include loss of gas

by diffusion, and r e g e n e r a t o r heat s torage capaci ty . The impact of

these fac tors is s u m m a r i z e d below:

• Diffusion L o s s e s : Hydrogen diffuses appreciably through m e t a l s at typical engine t e m p e r a t u r e s (1200 F) and, t he re fo re , mus t be e l imina ted from further considera t ion for this r e a s o n .

• Regenera to r Capaci ty: The amount of heat which must a l t e rna t e ly be s tored and re leased in the r e g e n e r a t o r is p ropor t iona l to the working fluid heat capacity. The Cp/R ra t io is 2. 5 for monatomic gases and 3. 5 for d ia tomic g a s e s . Accordingly, monatomic gases should

3 -33

r e q u i r e l e s s r egene ra t ion . A t r ea tmen t of this factor has not been found in the l i t e r a t u r e .

• Heat T r a n s f e r ; In re la t ively h igh-speed mach ines , p e r ­formance may be linnited by heat t r an s f e r to and from the working fluid in the heat exchangers and r e ge ne ra to r . Accordingly, in these mach ines , high the rma l conduc­tivity in the working fluid is de s i r ed . A plot of t he rma l conductivity v e r s u s molecu la r weight (Figure 3 .2 . 1-1) shows low molecu la r weight to be decidedly advantageous .

• Leakage; As engine speed is reduced, the impor tance of gas t h e r m a l conductance d iminishes because the re is m o r e t ime p e r cycle to accompl i sh the requi red heat t r a n s f e r . T h e r e is a lso m o r e t ime , however , for gas to leak around p i s tons , and at low speeds leakage can become the l imit ing factor. This effect is apparent in F igure 3 , 2 . 1-2 in which engine torque is plotted v e r s u s speed for three working fluids. (Reference 18) At low speed the torque with ni t rogen is higher than with hydrogen o r helium.

• Viscous L o s s e s ; F o r synchronous mach ines , the work­ing fluid gas may be selected without regard to viscous l o s s e s , since these lo s ses a r e negligible at synchronous speed. At h igher speeds the gas se lec ted will not profoundly affect the viscous lo s ses since the v i scos i t i e s of candidate gases do not differ marked ly (Figure 3 .2 . 1-3). Only hydrogen, which is not a candidate , has an excep­tionally low value .

Vapor Working F lu ids . Regenera t ive rec iproca t ing machines using

vapor working fluids a r e designed with smal l s t r o k e s , because vapor

volume grea t ly exceeds liquid volume. This is an advantage since

smal l s t rokes can be achieved by means of a bellows r a the r than a piston.

It is a disadvantage in that dead volumes a r e d i spropor t iona te ly l a rge

compared with swept vo lumes , and engine operat ion is fixed within a

fair ly na r row range with l i t t le flexibility. It has been shown that in r egen­

e ra t ive machines with vapor cycles the liquid interface in the r e g e n e r a t o r

can move through a long d is tance when the pis ton is d isplaced through a

smal l d i s tance . The locat ion of this in ter face is c r i t i c a l , since the engine

might s tal l complete ly if the in terface drif ts ve ry far toward the hot or

cold s ide .

3 -34 '

I

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c 1-1

n I

0 2

ra

Hj

T

cu

01

3 w

^

c n

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^-

Q,

3 G

. T

O

CO

•->

•;•

O

3

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Figure 3. 2. 1-3. Viscosi ty of Gases

T h e r m o d y n a m i c h e a t b a l a n c e s a r e i l l u s t r a t e d in a t e m p e r a t u r e -

e n t h a l p y d i a g r a m ( F i g u r e 3 , 2 . 1-4) for a h y p o t h e t i c a l l i q u i d / v a p o r r e g e n ­

e r a t i v e e n g i n e . A t h e r m o d y n a m i c c y c l e i s p e r f o r m e d in the s e q u e n c e

A B C D E F G H wi th w a t e r / s t e a m o p e r a t i n g b e t w e e n 2 1 Z " F and 1000 F .

Two c o n s t a n t - v o l u m e l i n e s at s p e c i f i c v o l u m e s of 5 cubic feet p e r pound

and 0. 015 cub ic foot p e r pound a r e c o n n e c t e d to the i s o t h e r m s . At point

H, h e a t i s added at c o n s t a n t v o l u m e unti l e v a p o r a t i o n is c o m p l e t e , wi th

p r e s s u r e r i s i n g f r o m 14. 7 to 2 , 300 p s i . Addi t iona l hea t is added up to a

p r e s s u r e of 4 , 500 p s i . T h e p r e s s u r e d r o p s to 175 ps i in the i s o t h e r m a l

p r o c e s s b e t w e e n C and D and to 14. 7 p s i in the e n s u i n g c o n s t a n t v o l u m e

p r o c e s s . F i n a l l y , an i s o t h e r m a l p r o c e s s r e s t o r e s in i t i a l c o n d i t i o n s .

T h e r m o d y n a m i c c a l c u l a t i o n s show t h e e n t h a l p y a v a i l a b l e d u r i n g hea

r e j e c t i o n at c o n s t a n t v o l u m e to be out of b a l a n c e wi th the en tha lpy n e e d e d

to b r i n g the w o r k i n g fluid up to the h i g h e r t e m p e r a t u r e . Most of the hea t

i s r e q u i r e d d u r i n g e v a p o r a t i o n w^hich o c c u r s only b e t w e e n H and B, whi le

t h e r e i s i n a d e q u a t e c a p a b i l i t y to r e c o v e r t h i s a m o u n t of h e a t d u r i n g

c o n d e n s a t i o n wh ich o c c u r s only be t \veen F and G.

It c an be shown tha t the o v e r a l l e f f i c iency of t h i s cyc l e is

36 p e r c e n t , c o m p a r e d wi th a C a r n o t e f f ic iency of 54 p e r c e n t .

T h u s , a r e l a t i v e e f f i c i ency of only 67 p e r c e n t could be a c h i e v e d for t h i s

c y c l e e v e n if it could be m e c h a n i z e d .

If the d e s i r a b l e p r o p e r t y of v a p o r ( l a r g e change in spec i f i c v o l u m e

upon e v a p o r a t i o n ) i s to be e x p l o i t e d , the chief d i s a d v a n t a g e ( i m b a l a n c e

b e t w e e n h e a t r e j e c t i o n d u r i n g e x p a n s i o n and hea t r e c e p t i o n d u r i n g c o m ­

p r e s s i o n ) m u s t be o v e r c o m e . It i s obv ious t h a t , w h e r e e v a p o r a t i o n and

c o n d e n s a t i o n t ake p l ace , an u n s y m m e t r i c a l p r o c e s s d i a g r a m m u s t be

found in o r d e r to r e a l i z e a h i g h e r t h e o r e t i c a l e f f ic iency .

3 . 3 . 1 . 3 C o n f i g u r a t i o n S e l e c t i o n

C o n f i g u r a t i o n s for r e c i p r o c a t i n g r e g e n e r a t i v e m a c h i n e s a r e

c l a s s i f i e d and s c h e m a t i c a l l y r e p r e s e n t e d in F i g u r e 2 .2 .1 -5 . T h e

d e s i g n v a r i a n t s a r e g r o u p e d a c c o r d i n g to the n u m b e r of r e c i p r o c a t i n g

e l e m e n t s .

3-37

1530 BTU/LB D

1000°F

340''F 1185 BTU/LB

90 PSI

G 370 BTU/LB

ENTROPY

Figure 3 .2 . 1-4. T e m p e r a t u r e - E n t r o p y Diagram for Hypothetical Vapor Cycle

3-38

90

ONE RECIPROCATING

ELEMENT

THERMAL COMPRESSION

MECHANICAL COMPRESSION

00 I

OJ vO

RECIPROCATING REGENERATIVE

MACHINES

TWO RECIPROCATING ELEMENTS AND MECHANICAL

COMPRESSION

PLUG/ORIFICE DRIVE

FOUR RECIPROCATING ELEMENTS AND MECHANICAL

COMPRESSION

Figure 3 . 2 . 1-5. Design Options for Regenerat ive Machines

The two s imples t mechanica l ar rangennents (No's . 1 and 2) use a

single free piston. D i a m e t e r s a r e equal on both s ides of the piston, so

that these engines opera te on the constant -volume pr inciple at a p r e s s u r e

rat io of about 1. 25. The only cons tant -volume machines a r e in the f irs t

group (one r ec ip roca to r ) .

If d i a m e t e r s a r e unequal on the two s ides of the piston, the machine

is a va r i ab le -vo lume device , and energy for actuating the piston can be

obtained from the expansion of the working fluid itself. In this c a se , the

space on the l a r g e r d i a m e t e r side functions as an expansion cyl inder and

the o ther space as an equivalent feed pump. P r e s s u r e rat ios above 2 a r e

typical , h igher than for the constant volume device, because compress ion

and expansion effects a r e added to heating and cooling.

In the second group (two r ec ip roca to r s ) the re a r e two basic s ingle-

cycle v a r i a n t s : conventional l ink-opera ted engines (No. 4) and f ree-p is ton

engines (No 's . 5 and 6). Until recent ly , all c losed-cyc le rec iproca t ing gas

engines were mechanica l ly phased, with each piston connected to a c r ank ­

shaft by connecting rods or o ther l inks . This was n e c e s s a r y not only to

maintain the proper phase difference between pistons in gas engines with

more than one r ec ip roca to r , but a lso to smooth out power fluctuations in

s ingle-cycle m a c h i n e s .

In s ingle-cycle mach ines , power output i s negative during a port ion

of the cyc le . The engine mus t be kept running with a flywheel to s tore and

del iver energy . The flywheel may be e l iminated or reduced in size if

more than one cycle o p e r a t e s on a single crankshaf t . Power flow is

always posit ive if m o r e than two cyc les a r e used in the same mach ine .

Two-cycle (No. 7) and four-cyc le (No. 8) v e r s i o n s of the t w o - r e c i p r o c a t o r

group a r e feas ible , a s well a s the fou r - cyc l e , f ou r - r ec ip roca to r engines

(No. 9).

Mechanical ly-Linked V e r s u s F r e e P is ton Configurations

Stir l ing engines perfected for l a r g e r power levels a r e all based on

crank m e c h a n i s m s connected to f lywheels . For a s ingle-cycle machine ,

posit ive power is avai lable for l i t t le m o r e than half cycle t i m e . To keep

the engine running smoothly, it i s n e c e s s a r y to s tore about ten t imes the

average energy developed during one cyc le .

3-40

The power range of s ingle-cycle Stir l ing engines with mechanica l ly -

linked pis tons has been re la t ive ly l imi ted . One reason for this is that

flyweeel energy s torage capabili ty d e c r e a s e s with the square of the

dimension, un less speed is cor respondingly increased. Therefore ,

requ i red flywheel speeds a n d / o r weights beconne imprac t i ca l for smal l

engines . In addition, if a mechanica l linkage such as a rhombic dr ive

is employed, then the c r ankcase mus t be lubr ica ted and sealed from the

working volume of the engine. This becomes a major design problem and

a major source of engine unre l iab i l i ty . F o r these r ea sons we chose to

cons ider only f r ee -p i s ton configurations of Stir l ing-type engines . F o r c e -

d i sp lacement invest igat ions for a number of possible a r r a n g e m e n t s a r e

desc r ibed in the following p a r a g r a p h s . The ana lyses a r e based on

a s sumed sinusoidal d i sp lacemen t s and i so the rmal opera t ion.

Nomenc la tu re . Conventional nomencla ture for examining the var ious

mechaniza t ions of r egene ra t ive mach ines is given in F igure 3 .2 . 1-6. The

sketch shows the t h r ee bas ic engine volumes; the heated expansion

volume, the r e g e n e r a t o r , and the cooled compres s ion volume. In the

f igures , expansion and c o m p r e s s i o n volunnes a r e connected to imaginary

c ranks rotat ing at w r a d i a n s / s e c with a re la t ive phase d isplacement of a

and with<<)=wt the re fe rence l ine . Equat ions for ins tantaneous p r e s s u r e (p,

or , in nondimensional form, \p) and for power (P) a r e as follows:

2WRT P " V ( T + K + V) [1 + P COS (<t) - 6)] ^ ^

"^ 2WRT ( r + K + v) [1 + p cos ((|) - 6)] ^ '

P - ^WRTCsincot + K sin (ut - a ) ] i ^ - i n / s e c (3) ^ ~ (T + K + w) [1 + p cos (cot - e)] i " / s e c [i)

W = weight of gas in engine, lbs

T = t e m p e r a t u r e of heat re ject ion, °R

w = rotat ional speed, r a d / s e c

3-41

P

EXPANSION SPACE V

REGENERATOR V

COMPRESSION SPACE V

Figure 3 . 2 . 1 - 6 . Basic Regenera t ive Cycle with Nomencla tu re

t = t ime , sec

T = t e m p e r a t u r e rat io = T

K = volume ra t io (compress ion /expans ion)

cold hot , R

K = c lea rance rat io = dead vo lume/expans ion volume

p = J r^ + K^ + 2 T K cos a/{r + K + v)

<t> = crank angle = cot

Q" = phase angle (between expansion and compres s ion volume vec to rs )

V = 4 K T / ( 1 + T )

e = tan"^ [K sin a/(T + K cos a)]

P r e s s u r e (ijj) from Equation (2) is plotted in F igure 3 . 2. 1-7 for

values of the p a r a m e t e r s indicated. P r e s s u r e is plotted against volume

for the expansion space , compres s ion space , and total engine in F igure

3.2.1-8. These plots a r e all c losed curves for one complete cycle.

The enclosed a r e a is a m e a s u r e of the net work per cycle . The work is

posit ive if the a r e a is c i r c u m s c r i b e d c lockwise , negative if

counterc lockwise .

It is seen from the p r e s s u r e - v o l u m e plots that , over one cycle , wo

is done by the expansion space , work is done on the compres s ion space ,

and the difference is the net engine work. Because the c o m p r e s s i o n spac

net work per cycle is negative in this ins tance , the c o m p r e s s o r piston

mus t be dr iven using a flywheel with link mechanisnn.

3-42

f/

1.5

> a.

< 2 O 1/1

z UJ

o 2 O Z

— • I — — i II. • • I I I •

V2 » 3 T / 2

K = 0.75 r = 0.337 Kr = 0.5a = 90°

Figure 3 . 2 . 1-7. P r e s s u r e Versus Crank Angle

2 f

Single Cycle Machine. The mos t common s ingle-cycle machine is

the d i sp lacer design shown schemat ica l ly in Figure 3 2.1-9 The design

on the top is the conventional configuration (Stirl ing engine), that on the

bottom, the modification for free piston operat ion The force-d isp lace­

ment d i ag ram for the conventional Stir l ing engine (F igure 3 2.1-8), showing

posi t ive net cycle power, applies for the piston of this machine However,

s ince d i sp lacer power flow is theore t ica l ly zero, it must be driven by the

crankshaft .

3-43

fs'

a! 5

* .5

t " \

\ NEGATIVE \ ^

T

AREA = -20.2

k --Jb

EXPANSION SPACE VOLUME 0.5

COMPRESSION SPACE VOLUME I 1.5

TOTAL ENGINE VOLUME

.75, T - .3373, Kr .5 a ^ 90°F

Figure 3.2.1-8. Basic Indicator Diagrams for Variable Volume Machines

F r e e P i s t o n O p e r a t i o n

In f r e e - p i s t o n m a c h i n e s , a s e l f - a c t u a t e d d i s p l a c e r is d e s i r e d . In

the d e s i g n m o d i f i c a t i o n to a c h i e v e t h i s ( F i g u r e 3 . 2 . 1-9, l o w e r ske t ch ) ,

EXPANSION SPACE V

COMPRESSION REGENERATOR SPACE V

_ \ _REF:iJNE_

EXPANSION SPACE V

REGENERATOR V^

I COMPRESSION SPACE V

IlTiiLiinnTn';

y^'^'^^^y^^^'^^^y^^^

/ (" t - 0]

ijiuit„.if,ifj,ftfm.

' \ REF. LINE

DISPLACER EXTENSION

F i g u r e 3 . 2 . 1-9. S t i r l i n g E n g i n e With and Without P i s t o n E x t e n s i o n

the d i s p l a c e r h a s a s m a l l e r d i a m e t e r e x t e n s i o n rod p r o t r u d i n g t h r o u g h the

p i s t o n . T h i s c a u s e s c y c l i c w o r k to be s h a r e d b e t w e e n the p i s t o n and d i s ­

p l a c e r e x t e n s i o n r o d . In t h i s c o n f i g u r a t i o n , t h e c o r r e c t s t r o k e v e c t o r s for

d i s p l a c e r (S ) and p i s t o n (S ) a r e ^ e ^ p

V

e A c

(A - A ) c r

3-45

^7

where A and A denote cyl inder and rod a r e a s , and v and v denote d i s -c r ^ e p p lacer and piston swept volume v e c t o r s , respec t ive ly . Indicator d i ag rams

for the piston and the d i sp l ace r extension rod a r e given in F igu re 3 . 2 . 1-10.

The net work is posi t ive for each component and sums exact ly to the piston

work in F igure 3 . 2 . 1-8. Introduct ion of the extension rod thus enables

f ree-pis ton opera t ion .

Single-Cyl inder , Two-Cycle Machine. A s ingle-cyl inder , two-cycle

machine is shown schemat ica l ly in F igure 3.2.1-11. Since the two cycles

a r e 180 out of phase , one equation with a sign change in the t r igonomet r i c

t e r m applies for the p r e s s u r e in both cyc l e s . The force acting on the

inner piston is der ived f rom the p r e s s u r e difference between i|; and i|j^.

Cycle Avork in expansion and compres s ion spaces is de te rmined from

p r e s s u r e - d i s p l a c e m e n t plots for two c a s e s . In the f i rs t , the cyl inder is

taken to be s ta t ionary and al l t h r e e p is tons move. In the second, the

outer two pis tons a r e s ta t ionary and the inner piston and cyl inder move.

a) Sta t ionary Cylinder

F o r the s ta t ionary cyl inder , the inner piston d i sp lacement is r ep re sen t ed by Vel- The differential p r e s s u r e (ijji - 4^2) acting on the inner piston is plotted against Vel on the let t in F igu re 3 . 2 . 1-12, showing work in the expansion spaces to be posi t ive and twice the net magnitude of F i g u r e 3 . 2 . 1-8. Although the force is in the right d i rec t ion for only 95 pe rcen t of the s t roke , a spr ing force along the dashed line would make the piston self-acting in all pos i t ions . A s i m i l a r f o r ce -d i sp l acemen t d i a g r a m for the outer pis ton pa i r is given on the r ight in F igu re 3 . 2 . 1-12. Net work in the c o m p r e s s i o n space is negative and i t s magnitude double that of F igu re 3 . 2 . 1-8. Again, such a design would r e q u i r e a link m e c h a n i s m to the outer p is tons since ex te rna l work is needed to dr ive t h e m .

b) Moving Inner P i s ton and Cylinder

F o r the des ign with a movable cyl inder and s ta t ionary outer p i s tons , the power developed by the pis ton and cyl inder is

3-46

VOLUME K = .75 T = ,3373, Kr= .5 « = 100°

Figure 3 . 2 . 1-10. Indiv^ator Diagram for Stir l ing Engine With Digp»1a'.f'r Extension

PtD- ^ = (4JX - 4JL )(V + V ) —^ P i s t o n ' <|) ^^-TT e c A

% l i n d e r = ^ <t> ^ ^<|, - .> ^ c ( - ^ )

where Ae and Ac r e p r e s e n t expansion and compres s ion space pis ton a r e a s . Net work p e r cycle is obtained by in tegrat ing these poAver exp re s s ions over one cycle . If the cyl inder net work pe r cycle is to be posi t ive , Ae mus t be g r e a t e r than AQ. This follows because energy is absorbed in the compres s ion space and there fore (4^4) - ^(\>--n) dVc is negat ive . The p r e s s u r e - v o l u m e

d i a g r a m s in F igure 3 . 2 . 1-13 show that equal amounts of posi t ive net work a r e done by the piston and cyl inder .

2WRT Pi " (»C+T+i;)[l +PC05 {^-6J\

2WRT 2 " (K+T+l/) [l - p cos (<|)-e)]

F i g u r e 3 . 2. 1-11. T w o - C y d e , Single-Cyl inder Machine

Mult icyl inder Machines

Mult icyl inder configurations analyzed include a four -cy l inder , four­

cycle machine (Figure 3 . 2 . 1-14) and a two-cyl inder , four-cycle machine

(Figure 3 . 2 . 1 - 1 5 ) . In each f igure, only one of the cy l inders is shown, the

o the rs being equivalent by s y m m e t r y . P r e s s u r e - d i s p l a c e m e n t ( indicator)

d i ag rams for one cyl inder in each of these configurations a r e given in

F igu re s 3 . 2 . 1 - 1 6 and 3. 2 .1-17 •

3-48 / ^ ^

^

in

O I

S

DISPLACED VOLUME DISPUCED VOLUME

Figure 3 . 2 . 1 - 1 2 . Indicator Diagrams for Single-Cylinder , Two-Cycle Machine (Stationary Cyl inders)

(J1 o

V

S 0 a.

s

-.5

- I

^ \

\ \

Ss^osrriVE > v

V N AREA = 38.4 CM^

\ \

\

: ^

^ 0 a.

s to X

% a I

I - .5

- I

/

/

'y

AREA = 39.0 /

/ /

'osmvE y

I 1.5

PISTON DISPLACEMENT

.5

CYLINDER DISPLACEMENT

K - .75 T - .337 Kr - .5a=90*

A« /Ac -2

Figure 3 . 2 . 1-13. Indicator Diagrams for Single-Cylinder , Two-Cycle Machine (Moving C y l i n d e r - S t a t i o n a r y Outer Pis tons)

, = V { I + c o s < t ) / 2

2WRT Pi = 1 (K + T + I') f 1 + pcos (<;>-e)l

Figure 3 .2 . 1-14. P i s ton /Cyl inder for F o u r - C y c l e , Four -Cy l inde r Machine

2WRT 3 ( i + K + I')!! + pcos (<j)- e ^ i i /4 ) ] t

Vg2 = 5" V (1-cos (t>)

P2= 2WRT/ [ l - p c o s ((|.-e)J (T + K + f)

' g , = ^ V ( l + c o s 4 . )

p, = 2WRT/fl + pcos (4.-6)] ( i + K + f )

2WRT P4 ( T + K + ri)H + pco$ (<|)-e -11/4)]

Figure 3 . 2 . 1 - 1 5 . P i s ton /Cy l inde r for F o u r - C y c l e , Two-Cyl inder Machine

3-51

/O^

-.5

n-V

V AREA-39.8

\

A to to

a.

s

-.5

.5 DIMENSION LESS VOLUME RATIO

K = .75, r ' .3373, Kr » .5, « - 90°

-1

^ AREA-78.1 CM^

\

\

\J DIMENSION LESS VOLUME RATIO

IC-.15 T - .3373 K r - . 5 a - 9 0 °

Figure 3 . 2 . 1 - 1 6

Indicator Diagram for F o u r - C y c l e , Four -Cy l inde r Machine

F igure 3 . 2 . 1-17

Indicator D iag ram for F o u r - C y c l e Two-Cyl inder Machine

3-52

/ ^ ^

The i n d i c a t o r d i a g r a m for t h e f o u r - c y l i n d e r , f o u r - c y c l e m a c h i n e

( F i g u r e 3.2.1-16) s h o w s p o s i t i v e p o w e r c o n v e r s i o n , but wi th f o r c e s in the

w r o n g d i r e c t i o n for a s u b s t a n t i a l p o r t i o n of the s t r o k e . H e n c e , a s p r i n g -

i n e r t i a r e t u r n and h i g h - s p e e d o p e r a t i o n a r e r e q u i r e d . With s i m p l e s p r i n g

c o n t r o l , p o s i t i v e e n e r g y flow can be ef fec ted for the whole pa th excep t

o v e r the s m a l l i n v e r t e d l o o p , r e p r e s e n t i n g about 15 p e r c e n t of the s t r o k e .

S ince the i n d i c a t o r d i a g r a m for the t w o - c y l i n d e r , f o u r - c y c l e m a c h i n e

( F i g u r e 3 , 2 . 1-17) i s s y m m e t r i c a l , a s p r i n g r e t u r n for th i s m a c h i n e can

p r o v i d e p o s i t i v e e n e r g y flow o v e r t h e e n t i r e c y c l e .

P e r f o r m a n c e C o m p a r i s o n s

In a s y n c h r o n o u s p o w e r s y s t e m t h e r e should be a p o s i t i v e p o w e r

ou tpu t f r o m the m e c h a n i s m at e a c h i n s t a n t of t i m e . In high s p e e d n o n -

s y n c h r o n o u s e n g i n e s th i s is not e s s e n t i a l b e c a u s e the p e r i o d s of n e g a t i v e

power can be a c c o m m o d a t e d by s t o r e d k ine t i c e n e r g y . In low speed

s y n c h r o n o u s m a c h i n e s the only p r a c t i c a l e n e r g y s t o r a g e i s in the f o r m of

p o t e n t i a l e n e r g y , such a s s p r i n g f o r c e s o r the p n e u m a t i c c o m p r e s s i o n of

a b o u n c e c h a m b e r .

E f f i c i e n c i e s of four e n g i n e s w e r e c o m p a r e d . Two c o n f i g u r a t i o n s

w e r e the s i n g l e - c y c l e , s i n g l e - c y l i n d e r c o n s t a n t v o l u m e and v a r i a b l e

v o l u m e d e s i g n s o p e r a t i n g in the n o n s y n c h r o n o u s ( h i g h - s p e e d ) m o d e . The

o t h e r two c o n f i g u r a t i o n s w e r e o p e r a t i n g in the s y n c h r o n o u s m o d e wi th

one be ing a t w o - c y c l e , o n e - c y l i n d e r d e s i g n and the o t h e r be ing a f o u r - c y c l e

t w o - c y l i n d e r d e s i g n . T a b l e 3 , 2 . 1-3 i d e n t i f i e s the four e n g i n e s .

C o n f i g u r a t i o n A i s the s i m p l e s t m e c h a n i c a l a r r a n g e m e n t and

u t i l i z e s only one s ing l e f r e e p i s t o n w i th n e a r l y equa l d i a m e t e r s on both

e n d s . T h i s i s a c o n s t a n t v o l u m e d e v i c e and a s m e n t i o n e d p r e v i o u s l y can

p r o d u c e p r e s s u r e r a t i o s of abou t 1.2 to 1.

T h e low p r e s s u r e r a t i o i s a r e s u l t of u s ing only the r e l a t i v e m o v e ­

m e n t of w o r k i n g f luid b e t w e e n the ho t and cold r e g i o n s to p r o d u c e p r e s s u r e

v a r i a t i o n s . T h i s type of e n g i n e i s c u r r e n t l y be ing d e v e l o p e d for the

a r t i f i c i a l h e a r t p o w e r supp ly a p p l i c a t i o n by A e r o j e t and McDonne l l D o u g l a s .

T h e r e a r e two known w a y s of e n e r g i z i n g the p i s ton m o v e m e n t : by a p lug /

o r i f i c e d r i v e o r by a p i s t o n e x t e n s i o n . The l a t t e r o p e r a t e s in con junc t ion

wi th f l u i d - o p e r a t e d v a l v e s and s e c o n d a r y low and h igh p r e s s u r e s t o r a g e .

3-53

Table 3.2,1-3. Comparative Regenerative Machine

1 Configu

1 ^ 1 ^ 1 ^ 1 °

rat ion Cycles

1

1

2

4

Cylinders

1

1

1

2

Operat ing Mode

Non-synchronous

Non-synchronous

Synchronous

Synchronous

Volume

Constant

Var iab le

Var iab le

Var iab le

Fi gure 3.2.1-5 1 Sketch No. 1

2

5

7 1

8 1

When two free pis tons a r e used a s in Configuration B, higher p r e s s u r e

ra t ios of the o r d e r of 2:1 a r e typical because connpression and expansion

of the working fluid a r e added to the heating and cooling effects. Single-

cycle f r ee -p i s ton ve r s ions have been cons t ruc ted which depend mainly on

r ec ip roca to r m a s s and spring stiffness to maintain osci l lat ion and to provide

piston coordinat ion. This design has to be opera ted at a high frequency,

since no obvious way of maintaining piston movement at hea r t beat

frequency i s known at p r e s e n t .

Operat ion at hear t frequency has the potential for higher overa l l

efficiency by e l iminat ing the r equ i r emen t for a hydraul ic or pneumatic

frequency convers ion m e c h a n i s m . While this has not yet been achieved

in p r ac t i c e , feas ible approaches (Configuration C and D) a r e outlined

he re for evaluat ion.

F r o m the viewpoint of mechan ica l efficiency, it is more des i rab le

to use the pr incip le of piston operat ion only (the Rider design) ra ther

than of d i sp l ace r operat ion (the Stir l ing design) to achieve the same

cyclic sequence of events. With double-act ing pistons, a fully self-

actuating slow speed machine can then be designed, (Configuration C

or F igu re 3 . 2 . 1-18),

In this configuration the radioisotope heat source is embedded in

the inner pis ton. This piston osc i l l a t e s in a cylinder which c a r r i e s

two r e g e n e r a t o r s at the ends and is surrounded by insulat ion. This

cyl inder is connected by bellows to the s ta t ionary end p ieces , so it is

a l so able to r e c i p r o c a t e in r e s p o n s e to the changing p r e s s u r e . The space

between the outer casing and the inner a s sembly is filled with hydraulic

fluid and the whole a s s e m b l y r e c i p r o c a t e s so that the liquid can be d i rec ted

to the PCCS.

An a l t e rna t ive configuration is shown as Configuration D in F igure

3 . 2 . 1-19. The two stepped pis tons with in terconnect ions a r e combined in

such a way that four cycles a r e pe r fo rmed s imultaneously in the mechan i sm.

The radioisotope is in two separa te in te rna l capsules which provide heat

for the four expansion s p a c e s . These expansion spaces a r e connected by

four r egene ra t ive ducts to the outer , s m a l l - d i a m e t e r compres s ion spaces

where heat i s r e j ec ted . The two pis tons a r e coordinated so that they

3-55

ISOTONIC LIQUID SPACE

w OUTER CASING BELLOWS

MOVABLE END PLATE

OUTER CASING

INNER CYLINDER BELLOWS

REGENERATOR

INSULATION

RADIOISOTOPE

ENCAPSULATION

INNER MOVABLE CYLINDER

REGENERATIVE MATRIX

INNER CYLINDER BELLOWS

SLIDING SEAL

LIQUID PRESSURE PULSE OUTPUT

Figure 3, 2. 1-18. Two-Cycle , Single-Cylinder Machine

INPUT/OUTPUT

i

CLEARANCE REGENERATOR

-EXPANSION/COMPRESSION TRANSFER DUCTS

INPUT/OUTPUT

Figure 3. 2. 1-19. F o u r - C y c l e , Two-Cylinder Machine

3 - 5 7

/eft

opera te with a re la t ive phase d isp lacement of ninety deg ree s . The

power take-off to the PCCS cons i s t s of a l te rna t ing pneumatic p r e s s u r e

pulses supplied to the opposite faces of an actuat ing pis ton.

Heat Loss Breakdown

A breakdown of the contributing l o s se s is given below and resu l t s

a r e s u m m a r i z e d in Table 3.2.1-4.

• Heat Loss from Isotope Through Insulation. A 60-watt heat source is a s sumed . In Configurations A and B, with the hea t source or iented so that heat flows in only one d i r e c ­tion, the insulat ion lo s s i s a s sumed to be 10 wa t t s . In Configuration C, since heat flow is pe rmi t ted in two d i rec t ions , the a s s u m e d l o s s i s reduced to 6 wa t t s . In Configuration D, l o s se s occur only along the par t ia l c i r cumfe rence nea r the cen te r , fur ther reducing the es t imated insulat ion loss to 4 wa t t s .

• Heat Loss from Isotope Along Engine. The heat loss along the engine is d i rec t ly propor t ional to surface a r e a and inverse ly propor t ional to path length. The re fo re , it v a r i e s d i rec t ly as the l inea r d imension L. Taking 8 watts for Configuration A and B, the corresponding l o s se s for Configuration C a r e

3/

2 X 8 X yjO. 5 = 12, 7

and for C o n f i g u r a t i o n D

4 X 8 X yjO.ZS = 20, 1,

3-58

• '

Table 3 .2 , 1-4. Engine Efficiency Comparison

1. D i r e c t Heat L o s s F r o m I s o t o p e to Body T h r o u g h Insulat ion

2 . Heat L o s s F r o m Isotope Along T h e r m a l C o n v e r t e r to Cooled P o r t i o n s of the Engine

3. Enthalpy L o s s e s by L e a k a g e of Working F lu id Along N o n - P o w e r P r o d u c i n g P a t h s

4 . Carnot Heat R e j e c t i o n (Ideal)

5. Lack of C o r r e s p o n d e n c e B e t w e e n Actual and Carnot Cyc l e

6, A e r o d y a n m i c F l o w L o s s e s in T h e r m a l C o n v e r t e r

7. Mechanica l F r i c t i o n L o s s e s

8. XCiscel laneous L o s s e s - R e g e n e r a t o r Inef f i c i ency

9. A l lowance for H y d r o s t a t i c B e a r i n g s

Engine F.fficiency

A Constant Volume

T h e r m o c o m p r e s s o r

0 . 8 3

0 . 8 7

1.0

0 . 6 6

0. 70

0 . 9 0

0 . 9 0

0 . 8 1

0 . 8 0

0. 18

B Variable Volume

T h e r m o c o m p r e s s o r

0 . 8 3

0 . 8 7

0. 90

0. 66

0 . 8 0

1.0

0 . 8 0

0 . 8 6

0 . 8 0

0. 19

C T u o - C > ( . l r O p p o s e d

P i s t o n Mach ine

0 . 9 0

0. 79

0. 87

0 . 6 6

0 . 8 0

1.0

0 . 9 0

0 . 8 5

0 . 8 0

0 . 2 0

- •- • ••-• 1

D F o u r - C \ c l e T w o -

Cyhndf r M a c h i n e

0. 93

0 67

0 89

0 . 6 6

0 . 8 0

1.0

0 95

0. 86

0. 80

0. 19

Heat Loss by Enthalpy Transpor t , Losses due to enthalpy t r an spo r t depend on gas leakage. F o r Configuration A, leakage is ze ro , since there a r e no in ternal p r e s s u r e dif­ferences in the engine. Configuration B is used as the basel ine and a loss of 3 watts is assumed for it. Losses in the other des igns a r e scaled from the propor t ional i ty of leakage to ( P r e s s u r e Difference) x ( P e r i m e t e r of Gap) ~ (Length of Flow Path) . P e r i m e t e r s in Configura­tions C and D a r e scaled from Configuration B by the rat io of l inear d imens ions . Because effective p r e s s u r e differ­ence is g r e a t e r with cycles out of phase by 180 deg, an addi­tional loss factor of 1,25 is applied for Configurations C and D. Configuration B l o s se s a r e then mult ipl ied by the following fac to r s :

2 For Configuration C: 2 x ^0. 5 x 1.25 = 1.98

3i F o r Configuration D: 4 x >/0,25 x 1,25 = 3,25

Carnot Efficiency Limit , The same cycle t e m p e r a t u r e l imi ts (1200°F and 100°F) a r e a s sumed for al l des igns . The "Carnot l o s s " is then 60 x 560/1660 = 30. 5 wat t s .

Deviations from Ideal Cycle. Correspondence between actual and ideal cycles is a s sumed to be in the range of 0. 7 to 0, 8, With leakage l o s s e s accounted for e l s e ­where , the main cause of inefficiency is heat t r a n s f e r a c r o s s the cyl inder wal ls . Hence the valve-ope rated machine (Configuration A) is placed at the lower end of the range and the o thers at the high end.

Aerodynamic L o s s e s . A 10% aerodynamic loss (6 watts) is a s sumed for the gas floAV through valves (Configura­tion A) and zero loss for the low speed devices .

Mechanical F r i c t i on L o s s e s . Mechanical efficiency may be independent of speed, with bet ter hydrodynamic b e a r ­ing p r o p e r t i e s at high speed compensat ing for i nc reased V x P fac tors . Hence, the main considera t ion is the number moving p a r t s p e r cycle . These a r e 1, 2, I, and 0. 5 for Configurations A, B, C, and D, respect ive ly . With con­stant loss pe r component, the corresponding mechanica l efficiencies a r e taken to be 0, 90, 0. 80, 0, 90, and 0, 95.

Regenera to r Inefficiency. Regenera to r effectiveness will be ve ry high (0. 96) for low speed devices and somewhat lower (0. 90) for high speed dev ices . F o r p rac t i ca l machines these effectiveness values a r e fur ther reduced by 10 percen t .

Hydrosta t ic Bea r ings . Hydrosta t ic bear ings a r e to be used as detai led in a l a t e r sect ion. The assoc ia ted power loss is allowed for by an efficiency factor of 0. 80 for all des igns .

3-60

All four engine configurations have comparable efficiencies . AH

other things being equal, the potentially higher power conditioning and

control subsys tem efficiency of the synchronous engines should favor

thei r select ion. However, as mentioned previously, the synchronous

des igns (Configurations C and D) have not been reduced to prac t ice and

involve many development p rob lems ; one of which is synchronizing the

engine output to match the r equ i r emen t s of the blood pump. One approach

is to modulate the heat conducted to the engine by providing two para l le l

heat paths fronn the heat source; one by conduction d i rec t ly to the TESM,

the other by a heat pipe in terposed between the heat source and the engine.

The ex te rna l sur faces of the engine expansion space form the condensing

c h a m b e r s for sodium in the heat pipe. The heat pipe is backfilled with

argon, the p r e s s u r e of which es tab l i shes the effective length of the con­

dense r . By using a pneunaatic signal from the blood punnp, it would be

possible to regula te the heat flow through the heat pipe as required .

Another approach is to opera te the engine at constant speed and have a

mechanica l connection between the piston and blood pump. Both rate

and s t roke of the blood pump a r e constant . In this case , no energy

s torage is provided and engine power equals the maximum demand of the

blood pump with the excess power being diss ipated at the blood pump.

Since nei ther of the synchronous engines nor the means of control ­

ling them have been reduced to p rac t i ce , they were not considered as

legi t imate candidate sy s t ems for evaluation,

3 . 2 , 1 , 4 Component Design

Pis tons Versus Bellows

Reciprocat ing gas engines depend upon volume changes in a quantity

of gas . These changes can be effected e i ther by p is ton-cyl inder a s s e m ­

bl ies or by bel lows. Both mechaniza t ions a r e ent i re ly p rac t ica l and the

re la t ive pe r fo rmance of these components was therefore invest igated.

Bellows have no leakage and theore t ica l ly no friction; however, they

a r e subject to some mechanica l h y s t e r e s i s . They a r e l imited p r i m a r i l y

by life. At re la t ively high speeds (above 500 s t rokes per minute) thei r

use may be ruled out by fatigue. Another disadvantage of bellows is that

the gas between the convolutions adds to sys tem dead volume. This is

3-61

il2>

par t i cu la r ly de le te r ious in Stir l ing engines , degrading the p r e s s u r e ra t io

and specific power.

Bellows a r e m o r e sui table for shor t s t rokes with l a rge d i a m e t e r s .

They a r e mos t effective ^vith tw^o-phase working fluids, where smal l

volume changes occur with la rge p r e s s u r e var ia t ions .

P i s ton -cy l inde r a s s e m b l i e s may be dis t inguished by the type of

rec iprocat ing m e m b e r , which may be e i ther a d i sp lace r or a piston. D i s -

p l ace r s a r e c h a r a c t e r i z e d by equal p r e s s u r e s on opposite ends and t h e r e ­

fore need li t t le power to r ec ip roca t e . The only force opposing the i r

motion is ae rodynamic friction due to gas d i sp lacement . With d i s p l a c e r s ,

it is poss ib le to mount r e g e n e r a t o r s e i the r in tegra l ly within the d i sp l ace r

or in an external duct through which the working fluid is made to flow.

Bear ings

The use of p i s ton-cy l inder a s s e m b l i e s depends mainly on the i r

capabili ty for t e n - y e a r opera t ion without undue wear . In conventional

applications, a hydrodynamic lubr ica t ing film, such as an oil film, is

normal ly used to reduce wear . Such bear ings function mainly during the

high velocity per iods of the s t roke . With rec iproca t ing e l emen t s , a c o m ­

plete stop is exper ienced at each end. Operation then e i ther r e v e r t s to a

hydros ta t ic mode or the bear ing film b reaks down. If t he re is a me ta l l i c

contact a t these points, wear cannot be avoided. Accordingly, hydros ta t ic

gas bear ings a r e preferab le , s ince the i r functioning does not depend on the

maintenance of high re la t ive motion between the rec ip roca t ing pa r t s . How­

ever, the operat ion of a hydros ta t ic bear ing does r e q u i r e a steady leakage

from the bear ing pad to both ends of the rec iproca t ing naember in o r d e r

to provide a constant flotation action and el iminate m e t a l - t o - m e t a l con­

tact . We have a s sumed that it will be prac t icable to design these bea r ings

so that except dur ing brief i n t e rva l s , the p r e s s u r e in this space is always

above the p r e s s u r e in the engine or in the sump.

Regenera to r

Calculat ions indicate that the w i r e m a t r i x r e g e n e r a t o r cus tomar i ly

used in Stirl ing engines is not n e c e s s a r i l y mos t efficient in min ia tu re

Stirl ing engines. Disp lace r and cyl inder wall o r pis ton and cyl inder wall

3-62 /

s u r f a c e s m a y p r o v i d e suf f i c ien t h e a t t r a n s f e r a r e a to r e g e n e r a t e the g a s

f lowing in t h e a n n u l a r g a p betw^een t h e m .

In the v e r y s m a l l e n g i n e s u n d e r c o n s i d e r a t i o n h e r e , h e a t t r a n s f e r

f r o m the w a l l s of a m o v i n g d i s p l a c e r o r p i s t o n , and t h o s e of the c y l i n d e r

i s d e e m e d to be su f f i c i en t . M o s t of the h e a t e x c h a n g e r s a r e m a d e a s

s i m p l e t u b u l a r p a s s a g e s , o r a s a n n u l a r e l e m e n t s b e t w e e n c y l i n d e r w a l l s

and p i s t o n s u r f a c e s ,

3, 2, 1. 5 Systenn D e s i g n S u m m a r y

Two g a s r e c i p r o c a t i n g eng ine c o n c e p t s w e r e s e l e c t e d a s c a n d i d a t e s

fo r e v a l u a t i o n . The m e c h a n i c a l c o m p r e s s i o n f r e e p i s t o n n o n s y n c h r o n o u s

eng ine w a s t h e b a s i c eng ine d e s i g n . T h e f i r s t d e s i g n o p e r a t e s at c o n s t a n t

s p e e d and c o n s t a n t p o w e r ou tpu t , s u p p l y i n g a h igh p r e s s u r e (180 ps ia )

h y d r a u l i c f luid to the p o w e r c o n d i t i o n i n g uni t . The s e c o n d d e s i g n i s s innila

to the f i r s t b u t u t i l i z e s T E S M in o r d e r to d e c r e a s e the h e a t s o u r c e s i z e .

T h i s d e s i g n i s m o d u l a t e d by c o n t r o l l i n g the w o r k i n g fluid v o l u m e in the

e n g i n e . Both c a n d i d a t e d e s i g n s a r e d i s c u s s e d be low.

G a s R e c i p r o c a t i n g E n g i n e

T h e d e s i g n of the g a s r e c i p r o c a t i n g n o n s y n c h r o n o u s , n o n m o d u l a t e d

g a s c y c l e eng ine i s shown in F i g u r e 3, 2, 1-20 and the s y s t e m d e s i g n c h a r ­

a c t e r i s t i c s in T a b l e 3. 2, 1-5,

T h e g a s eng ine c o n t a i n s a 54, 4 - w a t t s p h e r i c a l h e a t s o u r c e (I. 4 inch

d i a m e t e r ) wi th in the u p p e r end of the s p h e r i c a l eng ine h o u s i n g . A g a p

b e t w e e n the h e a t s o u r c e and eng ine h o u s i n g a l l o w s h e l i u m to c i r c u l a t e and

p r o v i d e s t h e r m a l c o n t a c t b e t w e e n h e a t s o u r c e and h o u s i n g with m i n i m a l

c l e a r a n c e v o l u m e . X e n o n - f i l l e d M i n - K i n s u l a t i o n s u r r o u n d s the h e a t

s o u r c e and the h e a t e d p o r t i o n s of the eng ine c y l i n d e r . F i n s a r e u s e d

to m i n i m i z e the t e m p e r a t u r e d i f f e r e n t i a l b e t w e e n the s p h e r i c a l p a r t of

the eng ine h o u s i n g and the h e a t e d l e n g t h of the eng ine c y l i n d e r .

T o r e d u c e h e a t c o n d u c t i o n b e t w e e n hot and cold e n d s of the c y l i n d e r ,

the c y l i n d e r i n n e r w a l l o v e r the o n e - i n c h w o r k i n g l eng th of the r e g e n e r a t o r

i s r e d u c e d by s p e c i a l c o n s t r u c t i o n to a t h i c k n e s s of 0. 003 inch . In th i s

r e g i o n the w a l l i s r e i n f o r c e d wi th r a d i a l s t i f f e n e r s s p a c e d a long the a x i s .

The 0, 0 1 0 - i n c h o u t e r w a l l and the s t i f f e n e r s a c t a s the m a i n load c a r r y i n g

3 -63

CLEARANCE REGENERATOR SECTION OF CYLINDER

HOLLOW CORE DISPLACER (WITH INTERNAL BAFFLES)

INSULATION (XENON FILLED

HEAT SOURCE

ANNULAR GAP BETWEEN

CYLINDER AND HEAT SOURCE

CONDUaiVE FINS FOR X HEAD END OF CYLINDER

INSULATION OUTER CONTAINER

INNER AND OUTER CONVERTER CONTAINERS

ANNULAR COLD END HEAT EXCHANGER

DISPLACER ROD CUTAWAY SHOWING GAS BEARING ORIFICES

POWER PISTON

OUTLET CHECK VALVE

SPRINGS

BELLOWS PUMP

—- LP FROM PCU

XENON FILLED

- ^ HP TO PCU

TUBULAR TISSUE HEAT EXCHANGER

ANNULUS, FILLED WITH ISOTONIC SOLUTION TO

DISTRIBUTE HEAT

Figure 3 .2 . 1-20. Nonsynchronous, Nonmodulated The rma l Conver ter

Table 3 , 2 . 1 - 5 , Design S u m m a r y Nonsynchronous , Nonmodulated Single-Cyl inder , Single-Cycle Gas Engine

Component

Heat Source

Engine

P a r a m e t e r

T h e r m a l

T e m p e r a t u r e

D i a m e t e r

Power Density

Relat ive Specific Power

Efficiency

P i s t o n Disp lacement

Bore

Stroke

Maximum P r e s s u r e

Minimum P r e s s u r e

Maximum T e m p e r a t u r e

Min imum T e m p e r a t u r e

Power Output (into PCCS)

Cycle Rate

Insulat ion Efficiency

Tota l Weight

Total Volume

Dimens ions

Units

watts

°F

inches

•2

j ou le s / in

%

%

in3

inches

inches

p s i

p s i

° F

°F

watts

cyc l e /min

%

g r a m s

cc

inches

Value

54, 4

1300

1.4

14.45

60

16

0.08

0 . 5

0. 54

500

190

1200

160

7.05

750

90

772

870

3.04 X 3. 04 X 5. 74

3-65

//7

m e m b e r s and c i r cumferen t i a l s lots cut in the outer wall act as heat dams .

Beyond the r e g e n e r a t o r region the inner wall th ickness i n c r e a s e s to

0. 025 inch. Calculat ions for this cons t ruc t ion indicate heat conduction

l o s se s to be 2. 8 wat t s , approximate ly five percent of the heat source

output.

The d i sp lace r piston is of hollow const ruct ion to l imi t heat conduc­

tion between expansion and compres s ion ends of the engine. The hot end

of the d i sp l ace r is contoured to match the shape of the heat source . The

cyl indr ical wal ls of the d i sp lace r a r e thin (0, 003 inch) while the ends and

in te rna l radia t ion shields a r e 0. 010 inch. Holes in the d i sp lace r and

shields reduce the differential p r e s s u r e . Ihe gap between the d i sp l ace r

and engine cyl inder walls ac t s as a c l ea r ance r e g e n e r a t o r .

The ho l low-core d i sp l ace r i s approximate ly I. 5 inches long by

0. 5 inch d i a m e t e r . The d i ame te r of the d i sp l ace r extension rod pass ing

through the power piston is 0. 25 inch. One spring at tached to the d i s ­

p lacer rod is r e c e s s e d internal ly and connects the piston to the d i sp lace r

rod. Another spr ing is in te rna l ly r e c e s s e d within the power piston and

connected to the hydraul ic bel lows. These spr ings a s s i s t in maintaining

the p r o p e r phase re la t ionship and rebound energy for the piston and

d i sp l ace r .

The power conditioning sys tem is a separa te package in which the

pneumatic p r e s s u r e of the engine is conver ted to hydraul ic power at the

bellows. Relatively low p r e s s u r e isotonic fluid is introduced through the

inlet check valve fronn the PCCS and c o m p r e s s e d to 180 psi by the engine.

The fluid flows out of the engine through the outlet check valve into the

annular cold-end heat exchanger where it picks up waste heat. It is then

d i scharged into a tubular heat exchanger within an annular gap formed by

an inner and outer conta iner . This gap is a lso filled with an isotonic

solution which abso rbs the waste heat and d i s t r ibu tes it over the en t i re 2

outer surface of the package at a nnaximum heat flux of 0, 06 w a t t / c m ,

Gas bear ing support i s provided to min imize power piston wear by

bleeding gas from the connpression space through the end of the d i sp l ace r

rod and engine cyl inder wall, A s e r i e s of differential flow or i f ices act as

unidirect ional va lves to control the flow of gas . Developing a sa t i s fac tory

3-66 .

gas bear ing support and minimizing its effect on the engine per formance a r c

cons idered major design unce r t a in t i e s .

Gas Rec iproca t ing /TESM Engine

C h a r a c t e r i s t i c s of the gas r ec ip roca t ing /TESM, nrjodulated, non-

synchronous engine a r e given in Table 3. 2. 1-6. Its design (F igure 3. 2. 1-21)

r e s e m b l e s the nonmodulated sys tem but differs as follows:

• The engine is modulated by controlling dead space volume in the compres s ion end by means of a p r e s s u r e signal from the PCU.

• P a r t of the PCU is an integral par t of the engine package and a l so ac t s as a d i s t r i bu to r for waste heat reject ion.

• Energy s to rage has been added, TESM capacity is 53, 3 w a t t - h o u r s . Internal TESM fins mainta in heat t r a n s f e r r a t e s during solidification.

The use of TESM lowers the required heat source t he rma l power.

However, the reduction in heat source s ize is offset by the need for engine

speed modulation. Consequently, efficiency is va r iab le and slightly lower

than for a nonmodulated engine. Maximum efficiency occurs at the average

power level and is l e s s at the lower and h igher leve ls .

The signal to modulate engine power and frequency is proport ional

to the d i sp lacement of the d iaphragm between high and low p r e s s u r e accu­

m u l a t o r s . An a l t e rna te mode of modulat ion was cons idered using heat

pipes and varying the i r conductance by means of an argon bellows at the

condenser end. This scheme was re jec ted , however , because of i ts

complexity.

The engine has five moving p a r t s : d i sp lacer , power piston, bellows

and two liquid check va lves . Gas bear inge a r e used to reduce wear and a re

again cons idered to be a major design uncer ta inty . The d i sp lacer piston

is centered within the cyl inder by the power piston and aerodynamic forces .

M e t a l - t o - m e t a l contact may occur at the end of the s t roke when velocity is

z e r o . Since the two pistons a r e never s imultaneously at top or bottom dead

cen te r , however , one is a lways moving to provide aerodynannic fo rces .

Wearout should be the ma jo r re l iabi l i ty considerat ion. The spr ings

and bellows have been designed to main ta in the fluctuating s t r e s s e s within

fatigue l imi ts of the m a t e r i a l s .

Table 3. 2. 1-6. Design Summary Gas Rec iproca t ing /TESM Engine

Component

Heat Source

Engine

P a r a m e t e r

The rma l Power

T e m p e r a t u r e

Diamete r

TESM (L iF /NaF)

Power Density

Relat ive Specific Power

Efficiency (Max/Min)

P i s ton Displacement

Bore

Stroke

Maximum P r e s s u r e

Minimum P r e s s u r e

Maximum T e m p e r a t u r e

Minimum T e m p e r a t u r e

Power Output (into PCCS)

Cycle Rate

Insulation Efficiency

Total Weight

Total Volume

Dimensions

Units

wat ts

" F

inches

wa t t -hou r s

jou le s / in^

%

%

in3

inches

inches

ps i

ps i

° F

° F

wat ts

c y c l e / m i n

%

g r a m s

cc

inches

Value

48 .9

1300

1.31

53 .3

14.45

50

12/8

0. 07

0 .5

0 .337

500

189

1200

160

4. I - 6, 7

480 - 1000

90

874

681

3. 34 X 3. 34 X 5. 34

3-68

PCU HIGH&LOW PRESSURE ACCUMULMOR

PCU VARIABLE GAS VOLUME

PCU GAS TRANSFER TUBE TO MODULATE ENGINE POWER

OJ I

o

HEAT SOURCE

INSULATION (XENON FILLED) HOLLOW-CORE

DISPLACER (WITH INTERNAL

BAFFLES)

/ OUTER

CONTAINER

^, TO PCU MOTORA'ALVE TIMER

CHECK VALVE

SPRINGS

BELLOWS PUMP

FROM PCU MOTOI^VALVE TIMER

XENON ATMOSPHERE

DISPLACER ROD SHOWING GAS BEARING ORIFICES

F i g u r e 3 . 2. 1 -21 . N o n s y n c h r o n o u s M o d u l a t e d T h e r m a l C o n v e r t e r

3. 2. 2 Linear Vapor

3, 2, 2, 1 His to r ica l Background

The technology for the l inea r vapor engine dates back over 200 y e a r s .

This technology includes data for var ious engine types ; expansion and non-

expansion, condensing and noncondensing, and single and mult iexpansion.

Considerable data a r e a lso available '" on the use of the vapor engine as an

energy convers ion device for an ar t i f ic ia l hea r t a s s i s t sys tem. These

data include both analyt ical and exper imenta l r e su l t s for t h r ee dis t inct

types of vapor engines al l operat ing with different cycle condit ions. These

th ree engines a r e t e r m e d the rec iproca t ing , r a m , and tidal r egene ra to r

engine (TRE). An efficiency s u m m a r y for these th ree engines is p resen ted

in Table 3. 2. 2 - 1 .

Table 3 . 2 . 2 - 1 . Art i f ic ia l Hear t Vapor Engine His tory

Engine Type

Reciprocat ing

Reciprocat ing

R a m

R a m

T R E

Working Fluid

Wate r

Organic (CP-34)

Wate r

Water

Water

r p m

I'iOO

1500

120

1500

50

Boihng P r e s s u r e

and T e m p e r a t u r e

(psia)

2 5 0

2 5 0

95

95

160

C F )

9 0 0

5 0 0

9 0 0

9 0 0

9 0 0

Ideal Cycle

(%)

2 8

2 4

13

13

14

M

Mechanical (%)

4 4

75

91

86

N A "

achine

T h e r m a l

71

59

89

84

N A «

Total (%)

31

44

81

72

32

Pump Volumet r i c

(%)

79

80

74

74

N A *

Overa l l (The rma l to Hydraul ic)

(%)

- 7

- 8 . 5

- 8

- 7

^ 4 . 5

NA ~ Not ava i lab le

The rec iproca t ing engine is r ep resen ta t ive of the conventional type

piston engine uti l izing a connecting rod, crankshaf t , and flywheel. The

engine design cons i s t s of a combined vapor and hydraul ic piston connected by a

un iversa l crankshaft al l housed within a single cyl inder . A vapor exhaust

valve is located within the vapor pis ton. The engine employs the Rankine cycle

Data f rom The rmo Elec t ron Corpora t ion , Waltham, Mass .

3-70

which p e r m i t s high ideal eff iciencies, A flywheel is used to average out

the p r e s s u r e var ia t ions during expansion and to provide for piston re tu rn .

Using water as the working fluid, the m e a s u r e d overa l l efficiency

( the rmal to hydraulic) was~7%. The low efficiency (25% of ideal) was cause

by mechanica l inefficiencies a s soc ia ted with the friction of wa te r - lub r i ca t ed

bea r ings . Additional p rob lems were a lso encountered because of fluid

leakage and wear r a t e s . The longest repor ted run, using water as the

working fluid, was l e s s than 4 days .

To reduce the frict ion on the bear ing sur faces , an organic working

fluid (Monsanto CP-34) was used. To enhance i ts lubr icat ing p rope r t i e s

and reduce wear r a t e s , s i l icone oil was added, resul t ing in higher mechan­

ical efficiencies as shown in Table 3, 2, 2-1. The the rma l efficiency in

th is case was reduced because of p r e s s u r e drops through inlet and exhaust

va lves caused by the higher m a s s flow requ i red with the organic fluids.

In shor t , while high ideal efficiencies of 24 to 28% were possible

with the r ec ip roca t ing engine, efficiencies of only about one -qua r t e r of

ideal were a t ta inable . Additional p rob lems included flywheel weight,

design complexity, and lifetinne and re l iabi l i ty because of bear ing and

pis ton sea l wea r and number of moving p a r t s .

To reduce the fr ict ion and w e a r of bea r ings , and to maintain long-

t e r m p e r f o r m a n c e , a c o n s t a n t - p r e s s u r e engine, t e r m e d the r a m engine,

was developed. This was accompl i shed by using a f ree-p is ton engine that

e l iminated the need for a crankshaf t , connecting rod, flywheel and b e a r ­

ings . This engine opera ted with the full vapor p r e s s u r e exer ted on the

pis ton at al l t i m e s . Since t he re is no expansion work, the cycle efficiency

is approximate ly one-half that of the expansion engine.

In the design of the r a m engine a th ree - faced piston was used which

combines the vapor engine, hydraul ic pump and feed pump into a common

a s s e m b l y . A spr ing was used to r e t u r n the piston. Measured efficiencies

for the r a m engine a r e shown in Table 3, 2, 2-1, Although the ideal

cycle efficiency of the r a m engine was approximate ly one-half that of the

r ec ip roca t ing engine, s i m i l a r ove ra l l efficiencies were obtained. This

was due to the i n c r e a s e d machine (mechanical and thermal ) efficiency

3-71

that r e su l t ed f rom the s imp le r design used in the r a m engine. Even though

the b e a r i n g s , flywheel, crankshaft , and connecting rod were e l iminated,

piston sea l s caused wear and leakage p r o b l e m s . In addition, t h e r m a l

l o s s e s were st i l l p r e sen t between the vapor and hydraul ic fluid because

of conduction l o s s e s in the piston and cyl inder .

To e l iminate the need for valves o r sliding sea ls an e lec t ron ica l ly

control led vapor cycle engine, t e r m e d the "tidal r e g e n e r a t o r engine" or

TRE, was developed. The TRE is a vapor cycle analog of the regenera t ive

gas cycle engine which combines some of the c h a r a c t e r i s t i c s of the Rankine

engine with those of the St i r l ing engine. The advantage of the TRE is i ts

s imple mechanica l na tu re . It contains ne i ther valves nor sliding sea l s and

the only moving p a r t s a r e the be l lows, b ina ry solenoid, and interface piston

The vapor cycle is bas ica l ly a sealed column filled with working

fluid, having a d i sp lace r bellows at the cold end and a piston output be l ­

lows at the hot end. Located between the d i sp lace r and piston output

bellows in the following o rde r a r e a condenser , liquid (tidal) r e g e n e r a t o r ,

boi ler , vapor r e g e n e r a t o r , and s u p e r h e a t e r .

The engine cycle begins with the liquid level at the in ter face of the

condenser and liquid r e g e n e r a t o r . As the d i sp l ace r bel lows, operated by

a b inary (double-acting) solenoid, m o v e s , liquid is shifted f rom the con­

d e n s e r through the liquid r e g e n e r a t o r to the boi ler . The liquid is v a p o r ­

ized until the en t i re column reaches a new and higher p r e s s u r e level . The

piston output bellows then pushes on an interface piston (the piston is used

to s e p a r a t e the high t e m p e r a t u r e vapor f rom the low t e m p e r a t u r e hydraul ic

fluid) which in tu rn pushes on the hydraul ic fluid be l lows, sending high

p r e s s u r e fluid to the blood pump. At the complet ion of the power s t r o k e ,

the d i s p l a c e r bel lows moves in the opposite d i rec t ion lowering the liquid

level in the sea led colunnn. As the liquid level in the column r e c e d e s , the

vapor is r e tu rned to the condenser at constant volume. As the vapor con­

d e n s e s , the column o r s y s t e m p r e s s u r e d e c r e a s e s . When the s y s t e m

p r e s s u r e is a m i n i m u m , the hydraul ic fluid is re tu rned by the spr ing in

the blood pump. The hydraul ic fluid in tu rn r e tu rns the hydraul ic fluid

bellows and in ter face pis ton to "top dead c e n t e r " which then comple tes the

cycle .

3-72

/ 2 ^

During the operat ion of the TRE, both the vapor and liquid r e g e n e r ­

ator r egene ra t e much of the heat except for a l a rge fract ion of the heat

of vapor iza t ion. Since the TRE ope ra t e s with the absence of any expansion

work, ideal cycle efficiencies comparab le to the r am engine a r e obtained.

By using a higher deg ree of superhea t , improved TRE efficiencies, com­

pared to the r a m , a r e poss ib le . Higher TRE efficiencies a r e a lso poss ible

if a working fluid mix tu re (such as ammonia and water) is used that has a

noncons t an t - t empe ra tu r e vapor iza t ion curve , since m o r e heat of vapor iza ­

tion is avai lable for regenera t ion .

The efficiency of the TRE with wate r as the working fluid is shown in

Table 3 , 2 . 2 - 1 , The m e a s u r e d overa l l efficiency of the TRE was approxi ­

ma te ly one-half that of the rec ip roca t ing and r a m engine. Data were a lso

avai lable for F luor ino l , which had an efficiency comparable to wate r . The

efficiency degradat ion for the TRE is probably caused by r egene ra to r inef­

f ic ienc ies , t h e r m a l l o s s e s , and bellow^s l o s s e s . Besides the low m e a s u r e d

eff ic iencies , the TRE had a l a rge volume {'^Z. 35 l i t e r s excluding the blood

pump) and appea r s sens i t ive to gravi ty effects, shock and vibrat ion. K a

mixed working fluid were used to i n c r e a s e the cycle efficiency, fluid

breakdown or decomposi t ion could p re sen t a p rob lem because of the smal l

volume of working fluid.

3. 2, 2, 2 Working Fluid Select ion

The choice of the working fluid and its re la ted thermodynamic cycle

c h a r a c t e r i s t i c s for a heat engine is of fundamental impor tance . F a c t o r s

which influence i ts choice a r e cycle efficiency, operat ing t empe ra tu r e and

p r e s s u r e , t h e r m a l s tabi l i ty , toxicity, and co r ros ivenes s. The candidates

that w e r e invest igated included: w a t e r , Monsanto CP-34 , Fluorinol 51 and

85, and va r ious o ther organic fluids.

The ideal cycle efficiencies for each of the working fluids were

invest igated as a function of inlet t e m p e r a t u r e , inlet p r e s s u r e and con­

d e n s e r p r e s s u r e for both an expansion and nonexpansion cycle. An example

of a typical data se t i s shown in F i g u r e 3. 2. 2 - 1 . The ideal efficiency of

w a t e r is plotted for an expansion and nonexpansion cycle as a function of

inlet t e m p e r a t u r e and p r e s s u r e . The r e su l t s of these invest igations showed

that for a given inlet and condenser p r e s s u r e , s i m i l a r cycle efficiencies

3-73

would be obtained with both water and the organic fluids; but with the

organic f luids, the sanne cycle efficiency could be obtained at a sub­

stant ia l ly lower t e m p e r a t u r e .

Hence, on the bas i s of the the rmodynamic cycle c h a r a c t e r i s t i c s , the

organic working fluids appea r potent ial ly supe r io r to wa te r . Unfortunately,

the use of an organic fluid in a l inea r vapor rec iprocat ing engine does

p r e sen t o ther potential p r o b l e m s , A p r i m e concern is avoiding decompos i ­

tion resul t ing from overheat ing the working fluid. This type of p rob lem is

common to rec iproca t ing engines that opera te at hear tbea t frequency.

Since the engine ope ra t e s at low speed, the working fluid flow is pulsat i le

and not continuous. This makes it difficult to design a boi ler that will not

overheat the working fluid. F o r this r eason , wa te r was chosen as the

p r e f e r r e d working fluid for the l i nea r

vapor engine. Other des i r ab l e c h a r a c ­

t e r i s t i c s of ^vater a r e : good cycle effi­

ciency, t h e r m a l s tabi l i ty , nontoxicity,

and the avai labi l i ty of a l a rge amount of

p r i o r operat ing exper ience .

3. 2. 2. 3 Cycle Selection

Tw^o cycles were invest igated for

use in the l inear vapor engine. One was

the Rankine or complete expansion cycle.

The other was the nonexpansion cycle ,

some t imes t e r m e d the r a m cycle . Both of

these cycles have typical ly been used

with a fluid that is a l te rna t ive ly vapo r -

"0 loo 200 300 400 500 60O ized and condensed. The i r main advan-INIET PRESSURE, PSIA

tage is the sma l l fraction of the total F igure 3 , 2 . 2 - 1 .

Compar i son of Expansion and engine work requ i red for pump work. Nonexpansion Cycle The p r i m a r y components for both cycles Efficiency

a r e the s a m e : bo i le r , condenser , r egen­

e r a t o r , feed pump and engine.

A compar i son of the ideal efficiencies for these two cycles is shown

in F igure 3 . 2 . 2 - 1 . In the ideal Rankine cycle , the working fluid unde r ­

goes a complete i sen t rop ic expansion between the bo i le r and condenser

3-74 / 1 is> '}^

p r e s s u r e . In the nonexpansion cycle , a constant p r e s s u r e is maintained

during the en t i re s t roke , as shown in F igure 3 . 2 . 2 - 2 . During this p r o c ­

e s s , no work is obtained from the in te rna l energy of the working fluid as

in the expansion cycle . F igure 3. 2, 2-1 shows that the ideal efficiency of

the nonexpansion cycle is about one-half that of the expansion cycle.

To be able to fully uti l ize all the available cycle energy, the shape of

the engine output pulse mus t be proper ly matched with the blood pump.

Since the blood punnp r e q u i r e s a c lose - to -cons tan t p r e s s u r e actuating

pulse-", a s imi l a r engine output pulse is required . In the expansion cycle,

the working fluid undergoes an i sent ropic expansion (PV^= constant) , as

shown in F igure 3. 2. 2-2 , where the working fluid expands isentropical ly

from 95 to 15 psia. This produces a var iab le force as shown in

F igure 3. 2. 2 -3 . The force ranges f rom 129 to 20 pounds with a mean

effective force of 47 pounds. For a 1-inch s t roke , this is equivalent to

VOLUME, IN

Figure 3 . 2 . 2 - 2 . P r e s s u r e - V o l u m e Relat ionship for Expansion and Nonexpansion Cycle

*As d i scussed in Section 2. 3 this r equ i r emen t is mis leading and for some s y s t e m s may resu l t in an underes t ima t ion of the t rue power r equ i r emen t s by 20 to 80%.

3-75

/^7

a work output of 47 in-lb. Since the output of the engine will be used to

either pressurize an accumulator or drive the blood pump directly, the

maximum force the engine can exert over its entire stroke is 20 pounds.

If a larger force were required, the engine would be unable to maintain

this force during its entire stroke. The result is that less than one-half

of the total available work output from the expansion engine can be utilized

as useful work.

Two methods can be used to overcome this problem. One is to store

the unavailable energy by the use of a flywheel. This method was ruled out

based upon the complexity and additional weight such a design would intro -

duce. A second method is to use a compound or multiexpansion engine.

In this type of engine, the working

fluid is allowed to expand in various

stages by passing it from one piston

to another by the use of an interceding

receiver. It is possible, through the

use of multiple piston-receiver com­

binations, to reduce the output from

an expansion cycle into a form that

approximates a constant force. An

analysis of this arrangement shows

that the potential gains are offset by

the pressure losses of transferring

the fluid from one piston to another

via the receiver.

It was concluded that the cycle "O .2 .4 .« .8 1.0

STJOKE, INCHES fTom wWch the most work could be

Figure 3. 2. 2-3. Force Relation- obtained, and still meet the blood ship for Expansion and Nonexpan- p^^p requirements, was the non-sion Cycle

expansion cycle. Figures 3. 2. 2-2

and 3. 2. 2-3 show that in the non-

expansion cycle, a constant pressure or force is exerted during the entire

stroke. Also, for the same operating conditions, the nonexpansion cycle

produces 32 pounds of force for the entire stroke connpared to 20 pounds

for the expansion cycle. The nonexpansion engine has several other

3-76

1 1 RANKINE (EXPANSION) CYCLE

RAM(h

\

\

\

\

lONEXPANS

V

\ ON) CYCLE

« ^ _

d e s i r a b l e c h a r a c t e r i s t i c s b e s i d e s a c o n s t a n t - p r e s s u r e output p u l s e . Two

of the m o s t i m p o r t a n t a r e t ha t a c o n s t a n t eng ine t e m p e r a t u r e i s nna in ta ined

w h i c h g r e a t l y nnininnizes c y l i n d e r c o n d e n s a t i o n , and, for a given c y l i n d e r

s i z e , the m a x i m u m p o s s i b l e w o r k i s ob ta ined .

To s e l e c t the o p e r a t i n g t e m p e r a t u r e and p r e s s u r e s for the n o n e x p a n ­

s ion c y c l e , r e f e r e n c e i s m a d e to F i g u r e 3 . 2 . 2 - 1 . T h i s f igu re was c o n ­

s t r u c t e d for a c o n d e n s e r t e m p e r a t u r e and p r e s s u r e of 142 ° F and 3 p s i a .

S ince a p p r o x i m a t e l y 30 ° F of s u b c o o l i n g is r e q u i r e d to p r e v e n t c a v i t a t i o n

of the w o r k i n g fluid d u r i n g p u m p i n g , it w a s felt t ha t the c o n d e n s e r t e m p e r a ­

t u r e i s r e a s o n a b l e c o m p a r e d to the body t e m p e r a t u r e . F i g u r e 3, 2. 2 - 1

s h o w s tha t the e f f i c i ency of the n o n e x p a n s i o n cyc l e is s e n s i t i v e to in le t

t e m p e r a t u r e but i s r e l a t i v e l y i n s e n s i t i v e to in le t p r e s s u r e . S ince it was

d e s i r a b l e to ob t a in the m a x i m u m e f f i c i ency , an o p e r a t i n g t e m p e r a t u r e and

p r e s s u r e of 9 0 0 ° F and 95 p s i a w e r e s e l e c t e d . T h i s r e s u l t s in an i d e a l

c y c l e e f f i c i ency of 14. 2%.

3, 2. 2. 3 C o n f i g u r a t i o n S e l e c t i o n

Two d i f f e r en t m e t h o d s e x i s t for o p e r a t i n g the n o n e x p a n s i o n e n g i n e .

In one m e t h o d t h e w o r k i n g fluid i s p r e s s u r i z e d , h e a t e d and cooled e x t e r n a l

to the e n g i n e . In the o t h e r t h i s i s a c c o m p l i s h e d wi th in the eng ine i t se l f .

T h e s e two m e t h o d s a r e t e r m e d the r a m and t ida l r e g e n e r a t o r eng ine ( T R E ) ,

r e s p e c t i v e l y . A t e m p e r a t u r e - e n t r o p y d i a g r a m for e a c h eng ine i s shown in

F i g u r e 3. 2 . 2 - 4 . Both e n g i n e s r e l y on r e g e n e r a t i o n to i n c r e a s e the cyc l e

ENTROPY, BTU'LB - "F ENIROPV, STU'U --F

(a) (b)

F i g u r e 3 . 2. 2 - 4 . C o m p a r i s o n of Two Nonexpans ion T h e r m o d y n a m i c C y c l e s : R a m (a) and T R E (b)

3-77

/ 2 ^

efficiency. In the r a m , regenera t ion occu r s at constant p r e s s u r e and in

the TRE at constant specific volume. In the l a t t e r case a l a r g e r amount of

heat is avai lable for regenera t ion . In the r a m a feed pump, r e g e n e r a t o r ,

condenser , boi ler , and engine intake and exhaust valves a r e requ i red . For

the TRE, a r e g e n e r a t o r , condenser , bo i le r and solenoid-actuated d i sp l ace r

bellows a r e requi red . The advantage of the TRE is that the feed pump and

engine intake and exhaust valve a r e e l iminated in favor of the d i sp l ace r

bel lows. The TRE is shown in F igure 3. 2. 2 - 5 . Its opera t ion is s i m i l a r

to that desc r ibed in Section 3. 2. 2. 1, except blood pump actuat ion is

accompl ished mechanica l ly ins tead of hydraul ical ly . To mee t our power

r equ i r emen t s the engine must be capable of del iver ing 5. 84 wat t s . This

power is based on an automat ic ac tua tor efficiency of 76% and a blood

pump power of 4 .44 wat t s . The ideal efficiency of the TRE is 14. 8%. To

mee t the volume and power r equ i r emen t s for this application an overa l l

cycle efficiency of 10. 3% is r equ i red . This efficiency is twice the p r e ­

viously repor ted value. Even though the TRE has the advantage of e l i m i ­

nating the feed pump and engine exhaust and intake valves, it is bel ieved

that based on the cu r r en t l y avai lable exper imenta l data it does not have

the capabil i ty of meet ing our power r e q u i r e m e n t s .

IDEAL CYCLE

MACHINE THERMAL MECHANICAL

TOTAL

OVERALL ENGINE PCCS

81% 85% 69%

10.3°'. 76%

DESIGN CHARACTERISTICS

ENGINE SPEED 120 RPM

ENGINE POWER OUTPUT 5.84 WATTS

VOLUME ENGINE PCCS

TOTAL

85.2 I N , 6.3 IN : ;

91 .5 I N ' ' ( I . 5 LITERS)

4" DIA.

I f V

o n n n H n » n i i t i n n n n n n » n n n H n n n n

VAPOR BELLOWS

VAPOR REGENERATOR

CONDENSER J^ TrTTTTTi ftnnnniinLiiinounuoLJLJtinunnn n t X

MECHANICAL LINE TO

BLOOD PUMP

SOLENOID

Figure 3 . 2 . 2 - 5 . Tidal Regenera to r Engine (TRE)

3-78

For the r a m engine, the engine output can e i ther be in the form of

hydraul ic or mechan ica l power. If the output is hydraulic, it can be used

to e i ther dr ive the blood pump d i rec t ly or p r e s s u r i z e an accumula tor .

However, the l a t t e r type of design is unacceptable since an isotope inven­

tory of 71 watts will be requ i red . F igu re 3. 2. 2-6 shows that a volumetr ic

efficiency of 76 to 78% can be obtained

with a hydraul ic pump. This type of

inefficiency is e l iminated by using a

mechanical link that d i rec t ly connects

the output of the engine to the blood

pump.

Since a mechanical , r a ther than

a hydraul ic link is used to dr ive the

blood pump, e l ec t r i ca l power is needed

for the engine intake and exhaust valves.

In addition, e l ec t r i ca l power is used

to opera te the feed pump as the engine

design prec ludes the use of e i ther

mechanica l or hydraul ic power. The

e l ec t r i ca l power is provided by a

t he rmoe lec t r i c module.

z S 300

i 250

S 200

S 150

— —

-

—1

/

^ ^ .

IDEAL

A-

FLOW RAT

/ A C

'

• ^ EFF

/

/

- /

TUAL FLOV

ICIENCY

.

r

1 V RATE

80 *

Z

78 O

80 100 120

PUMP SPEED RPM

F i g u r e 3. 2. 2 -6 . Hydraulic Pump Flow C h a r a c t e r i s t i c s Based on the actuator and

engine efficiency, two designs of a

l inear vapor engine were selected.

One ope ra t e s with a va r iab le speed at hea r t beat r a t e , the other with a

fixed output at 120 bpm. The configuration for both is the s ame , except

one design u se s TESM. By using TESM the isotope inventory is reduced,

but a m o r e complicated engine and ac tua tor design a r e requi red since the

engine ope ra t e s at va r i ab le speed.

3. 2. 2. 4 Component Select ion and Design

As mentioned p rev ious ly , the components for the two designs of the

l inea r vapor engine a r e the s a m e except one design uses TESM and the

o ther does not. The m a j o r components in each design a r e : engine, e l e c ­

t r i c a l feed pump, r e g e n e r a t o r , condenser , boi le r , t he rmoe lec t r i c module,

and e l ec t r i ca l ly -ac tua ted engine intake and exhaust va lves . In both designs

3-79

/ ^ /

the same m a x i m u m power output is requi red since the automat ic ac tua tor

efficiency is the s a m e . Thus , the same size components a r e used in both

designs s ince the components must be designed for m a x i m u m engine output.

Both des igns have been optimized for the same operat ing conditions of

95 psia at 900°F engine inlet conditions and 3 psia and 142 °F condenser

conditions. The following p r e s e n t s a d i scuss ion of the se lec t ion and

design of each component in the l inear vapor engine.

Engine

A number of different engine configuration a l t e rna t ives were inves t i ­

gated. They included piston v e r s u s bel lows, linked v e r s u s f ree , and

crankshaft v e r s u s spr ing re tu rn . Analysis of a linked piston with a c rank­

shaft and flywheel r e t u r n showed that bes ides excess ive fr ict ion l o s s e s the

overa l l weight of the des ign was prohibi t ive . The friction l o s se s a r e

pa r t i cu l a r ly acute s ince the only avai lable lubr icant is water . In cons ide r ­

ing the use of a p i s ton-cy l inder combination or a belloAvs, the fluid leakage

ra t e s w e r e examined.

Since such a sma l l quantity of working fluid is p r e sen t , it was found

that even a minute leakage ra t e could resu l t in a l a rge degradat ion in cycle

efficiency. Analysis of the leakage r a t e s showed that r ings mus t be used

if a p i s ton-cy l inder combination is cons idered . It was found that without

r ings a c l ea rance of 0. 0001-inch was requ i red to reduce the leakage to

l e s s than 3%. C lea rances of this s ize would probably cause excess ive

frict ion and tend to cause engine se i zu re on heating because of differential

t he rma l expansion. Since the use of pis ton rings is requ i red , the p rob lem

of wear mus t be cons idered . Even though w a t e r - l u b r i c a t e d graphi te r ings

would be acceptab le , the p rob lem of wear and eventual leakage would not

be e l iminated. To c i rcumvent this p rob lem, it was decided to use bellows

instead of a p i s ton-cy l inder combination. The spr ing ra te of the bellows

is used to r e tu rn the bellows to the i r or iginal posi t ion. Data avai lable on

welded meta l l i c bellows show that both the d e s i r e d spr ing r a t e s and l i fe­

t imes a r e feasible . Since the bellows act as a he rme t i ca l l y sealed device ,

fluid leakage l o s se s a r e complete ly e l iminated . Also the welded bellows

act as a nonfriction device which avoids w e a r . Thus the use of bellows

e l imina tes two of the ma jo r p rob lems that were p re sen t in previous vapor

engines of this type.

3 -80

Since t h e e n g i n e i s m e c h a n i c a l l y c o n n e c t e d to the blood p u m p , t h e i r

s t r o k e s m u s t m a t c h . T h e s i z e of t h e b e l l o w s i s b a s e d on the m a x i m u m 3

v a p o r flow r a t e , w h i c h i s 41 in / m i n . F o r a 1. 3 - i n c h s t r o k e , t he m e a n

e f f ec t ive d i a m e t e r of t h e b e l l o w s i s 0. 5 8 - i n c h .

C o n d e n s e r

T h e c o n d e n s e r and s u b c o o l e r h a v e b e e n d e s i g n e d to t r a n s f e r the h e a t

f r o m t h e c o n d e n s i n g v a p o r to the p a c k a g e w a l l s w h e r e it i s t h e n r e j e c t e d to

t h e body f lu ids and t i s s u e . In t h e c o n d e n s e r t h e v a p o r i s coo led f r o m a

s a t u r a t e d v a p o r to a s a t u r a t e d l iqu id a t 3 p s i a , and i s t h e n s u b c o o l e d a p p r o x -

m a t e l y 30 ° F to p r e v e n t c a v i t a t i o n of the fluid d u r i n g p u m p i n g . The c o n ­

d e n s e r >vas d e s i g n e d b a s e d on a m a x i m u m a l l o w a b l e p a c k a g e t e m p e r a t u r e 2

and h e a t r e j e c t i o n s u r f a c e a r e a of 107. 6 ° F (42°C) and 0. 07 w a t t / c m ,

r e s p e c t i v e l y .

T h e m a x i m u m c o n d e n s e r h e a t l oad i s 46 w a t t s . The h e a t t r a n s f e r

coef f ic ien t f r o m the v a p o r to t h e p a c k a g e w a l l was c a l c u l a t e d u s i n g the

e q u a t i o n ( R e f e r e n c e 19)

0. 065

h m

w^here

h = m e a n h e a t t r a n s f e r coe f f i c i en t m

C = s p e c i f i c h e a t P

u = v i s c o s i t y

k = t h e r m a l c o n d u c t i v i t y

Pp = d e n s i t y of l i qu id

p = d e n s i t y of v a p o r

f = f r i c t i o n f a c t o r

G = 0. 58 of m a s s v e l o c i t y (ve loc i t y - a r e a of t ube )

3-81

/^3

and the liquid p r o p e r t i e s of the working fluid. The condenser s ize was de te rmined by use of the convective heat t r a n s f e r equation (Q = h A AT)

' m s

and a tube wall effect iveness factor of 0. 7. The condenser is 30 inches

long with a 0. 0625-inch ID and a 0. 020-inch wall . In o r d e r to re jec t the

heat uniformly over the requ i red package surface a r e a , the condenser tube

mus t be finned. Boi ler

P r ev ious bo i l e r des igns for s t e a m cycle engines in th is s ize range

have proven to be inadequate to achieve 100% s t eam quality or the des i r ed

degree of superhea t . This deficiency may have been caused by heat t r a n s ­

fer f rom the bo i l e r tube to the vapor only, neglect ing the liquid fraction.

However, in a compara t ive ly low speed rec iproca t ing engine, the flow

through the bo i le r may be in the slug flow reg ime and the hydrodynamic

behavior of the liquid d i spe r s ion cannot be ignored.

The formulat ion for the heat t r a n s f e r coefficient (Reference 20) for

slug flow is as follows:

1 - x ( l - p^/p^)

where

h

^f

D

X

Pf

Pv

—

=

=

=

=

=

heat t r a n s f e r coefficient

t h e r m a l conductivity of fluid

tube d i a m e t e r

s t e a m quality

densi ty of the fluid

densi ty of the vapor

The boi ler tube s ize including the s upe rhe a t e r , using the heat t r an s f e r

coefficient and the convection heat t r a n s f e r equation Q = h A AT, was

de te rmined to be 18. 7 feet long for a 0. 0625-inch ID.

3-82

By using the Di t tus -Boe l te r equation

h = 0. 023(Re)°*^ (Pr ) ° ' ^

where

h = heat t r an s f e r coefficient

Re = Reynolds number

P r = Prandt l number

and assuming heat t r an s f e r to the vapor only, the length of the boi ler and

supe rhea t e r for a 0. 0625-inch ID tube would be 14. 4 feet. As seen, the

design for slug flow resu l t s in a longer boi ler length. However, slug flow

may only exist for s t eam quali t ies up to 30%; the re fo re , the required

length will probably be between the two calculated r e s u l t s .

A different approach to the boi ler configuration would be a packed

bed boi ler in which an annular housing packed with spheres made from a

high conductivity m a t e r i a l is located around the heat source . Heat flows

from the heat source walls to the sphe res and then to the fluid. The con­

cept is a t t r ac t ive since the sphe res in the packed bed provide a la rge heat

t r an s f e r surface for the fluid. Data on packed bed boiling tv/o-phase flow

a r e m e a g e r . However, if the approach of considering heat t r ans fe r to the

vapor only is taken, the graph in Reference 21, which per ta ins to gas flow

through an infinite, randomly s tacked, sphere ma t r i x , may be used to

de te rmine the requ i red boi le r design. Again, this may be an inadequate

approach for the s ame reasons as in the tube-type boiler design. A packed

bed boi ler design would appear to i nc r ea se the weight and size of the engine

sy s t em over the tube bo i le r design because of the weight of the ma te r i a l

uti l ized in the packed bed (spheres) and the requi rement of a sufficient

annular volume to achieve the boiling and superheat ing of the fluid. How­

eve r , if hot spots a r e a p rob l em in the tube type boi ler because of slug

flow, a packed bed boi ler approach may prove to be a t t r ac t ive .

3-83

Regenera to r

Regenerat ion is used in mos t vapor cycles to i n c r e a s e the overa l l

sys t em efficiency. When a nonexpansion cycle is used, regenera t ion

becomes quite impor tan t . Without it, the cycle efficiency could be reduced

by a lmos t one-s ix th . The r e g e n e r a t o r design se lected for this engine is a

conventional vapor - to - l iqu id concentr ic tube counterflow design. The

liquid f rom the pump en t e r s into the annulus of the outer tube where it pick

up heat through the tube wall f rom the vapor which is flowing in the inner

tube. In sizing the r e g e n e r a t o r , considera t ion was given to the r e q u i r e ­

ments of a low p r e s s u r e drop on the vapor side to negate p rob lems with

the liquid pump. Because of the low flow r a t e , the p r e s s u r e drop on the

liquid side will be negligible.

The design cons is t s of a 0. 08-inch ID inner tube (vapor side) with a

0. 020-inch wall and an outer tube with an ID of 0. 16-inch and an OD of

0 .20- inch . The length of the tube is 3 inches .

T h e r m o e l e c t r i c Module

The e l ec t r i ca l power for the engine feed pump, intake and exhaust

va lves , and control unit is provided by a t h e r m o e l e c t r i c module. The

module is located in s e r i e s the rmal ly between the heat source and boi le r .

The per t inent c h a r a c t e r i s t i c s of the t he rmoe lec t r i c module a r e :

Hot cap t e m p e r a t u r e

Cold cap t e m p e r a t u r e

T h e r m o e l e c t r i c m a t e r i a l

Element s ize N

P

Number of couples

Number of s t r ings

E lec t r i ca l power output

Load voltage

Conversion efficiency

1500' 'F

950' 'F

SiGe

0. 11-inch d i ame te r x 0. 2- inch long

0. 09- inch d i ame te r x 0. 2- inch long

14

2

1. 14 wat ts

0. 7 volt

2 .0%

3-84

/«^<^

Feed Punnp

An e l ec t r i ca l feed pump is used to pump the liquid from the condenser

to the boi ler . This p r e s s u r e range is from 3 to 95 psia. The feed pump

was de te rmined to have a 9% convers ion efficiency from e lec t r i ca l to

mechanica l to hydraul ic power . This requ i res a 0.21 watt e l ec t r i ca l input.

The feed pump cons i s t s of an e l ec t r i ca l l y -d r iven solenoid with a s t roke of

0. 10-inch and d i a m e t e r of 0. 18-inch.

Engine Intake and Exhaust Valves

The engine intake and exhaust valves a r e located external to the

engine and a r e actuated by an e lec t r i ca l ly powered solenoid. A detailed

design for these valves was not under taken, but a s imple spool valve or s im­

i l a r device can be used to regulate the flow of vapor into and out of the

engine. The valve would be operated by a solenoid which would have only

two interlocking posi t ions . The e l ec t r i ca l power required to operate the

solenoid will be l e s s than 0. 5 watt.

TESM

In the design of the synchronous vapor engine, a the rmal energy

s torage m a t e r i a l is used to reduce the isotope inventory. The ma te r i a l

u se s the latent heat of fusion to abso rb excess t he rma l energy i so thermal ly .

The m a t e r i a l chosen was L i F / L i C l since it has a melt ing point (930°F)

sl ightly higher than the bo i le r t e m p e r a t u r e (900°F). This m a t e r i a l has

an energy dens i ty of 87. 5 Avatt-hr/lb and a volumetr ic densi ty of 3

6 .9 w a t t - h r / i n . Since 69 w a t t - h o u r s of t he rma l s torage a r e requi red , 3

the weight and volume of TESM a re 0. 79 lb and 10 in , respect ively .

3. 2. 2. 5 Sys tem Design S u m m a r y

Two des igns of a l inea r vapor engine were selected. One opera tes

with a va r iab le speed at hea r tbea t r a t e . This design uses TESM and

de l ive r s a power output equivalent to the average blood pump r e q u i r e m e n t s .

The other o p e r a t e s with a fixed speed and output at 120 bpm. This design

de l ive r s a constant power output equivalent to the maximum blood pump

requ i r emen t . The components for both designs a r e the same except one

u se s TESM to reduce the isotope inventory.

3-85

137

The insulat ion used in both s y s t e m s was Min-K 2020 filled with an

iner t gas (xenon). The package s izes were de te rmined using the ave rage

isotope inventory and were based on a nnaximum allowable package and 2

heat reject ion sur face a r e a of 107. 6 ° F (42°C) and 0 .07 w a t t / c m , r e s p e c ­tively. A s u m m a r y of the per t inent design c h a r a c t e r i s t i c s for each s y s ­tem is given below.

Linear Vapor Engine /TESM

The design for this engine is shown in F igu re 3. 2. 2 -7 . An e l ec t r i c a l

control sy s t em is used and i ts power is supplied by a t h e r m o e l e c t r i c

module. The engine cons i s t s of a bellows with a mechanica l output dr ive

that is d i rec t ly coupled to the blood pump. Bellows a r e used to e l iminate

fluid leakage and mechan ica l fr ict ion and wear .

The engine ope ra t e s with the vapor from the bo i le r driving the engine

bel lows. Exhaust vapor f rom the engine p a s s e s through a r e g e n e r a t o r into

a condenser where it is condensed and subcooled. The condenser is

designed to re ject the heat to the package wal ls where it is then rejected to

the body fluids and t i s s u e . The fluid from the condenser p a s s e s through

the feed pump where it is pumped through the r e g e n e r a t o r into the boi le r .

The engine is control led by regulat ing the vapor flow ra te into and out of

CONDENSER MIN-K XENON BORER

FILLED INLET-EXHAUST

VALVE

MECHANICAL DRIVE TO

BLOOD PUMP

•FEEDPUMP

REGENERATOR

ELECTRO JIC CONTROL PACKAGE

F i g u r e 3 . 2 , 2 - 7 . L inea r Vapor Engine /TESM

3-86

/:3^

the engine. Since the engine operates with a constant pressure, the inlet

valve is open during the entire engine stroke. At the end of the stroke the

inlet valve is closed and the exhaust is opened. The exhaust valve remains

open during the entire exhaust stroke. By regulating the inlet and exhaust

rate, the engine speed can be varied over the desired range.

The engine power output and isotope heat input as a function of engine

speed are shown in Figure 3. 2. 2. 8. The heat source fuel inventory was

based on the average blood pump power requirements. The heat source

inventory is 48 watts. To accommodate periods of peak activity, 69 watt-

hours of TESM are required. The overall engine efficiency versus engine

power output is shown in Figure 3.2. 2-9. The engine efficiency is seen to

increase with power output. The average daily efficiency of the engine is

8. 0%. Table 3. 2. 2-2 summarizes the pertinent engine design character­

istics and a system weight breakdown is presented in Table 3. 2. 2-3. The

volume of the engine subsystem is 1. 152 liters and the weight is 1. 36 kg.

Linear Vapor Engine

The vapor engine operates at constant

frequency and delivers constant power at

all times. The poAver output is equivalent

to the maximum blood pump requirements.

A schematic of the engine is shown in

Figure 3.2.2-10 and its operation is sim­

ilar to that of the previous engine. Since

the engine delivers maximum power, a

larger heat source inventory is required.

The advantages to be gained by delivering

a constant power output are that the need

for thermal energy storage is eliminated

and the engine controls are simplified

since the engine operates at constant fre­

quency. In addition, the problem of varia­

tion in the daily power profile on the heat F i g u r e 3 . 2 . 2 - 8 . • i.- 4. 1 • 4. j

T • ,, „ - r- • /rr^T-c-Kt rejection rate are eliminated. Linear Vapor Engine/TESM •'

^ 60

i 2 50

t

2 40

30

8

<

^ ' 0£

0 2

/

i .

E

y

/ /

I-4GINE PC

/ /

T^

WER OUT

^ " ^

A /POWER INPUT

/ •

U T , / ^

• ' ^

^ ^ ' ' ^ P C C S ACTUATOR POWER OUTPUT

60 80 100 120 ENGINE SPEED, RPM

3-87

1^9

8.8

2 3 4 5 ENGINE POWER OUTPUT, WATTS

F i g u r e 3. 2. 2 -9 . L inea r Vapor Engine /TESM

Table 3 . 2 . 2 - 2 . L inea r Vapor Engine /TESM

Operat ing Conditions

Boi ler Condenser Subcooler

Efficiency

Ideal Cycle Machine Overa l l

Engine C h a r a c t e r i s t i c s

Speed Stroke D i a m e t e r Power Output

Feed Pump

Stroke D iame te r Power Requi rement

95 ps ia , 900°F 3 ps ia , 142 "F 3 ps ia , 115°F

14. 2% 49 to 60% 7 to 8. 6%

60 to 120 bpm 1. 3 inches 0. 58 inch 2. 7 to 5.9 watts

0. 10 inch 0. 18 inch 0 .21 watt

3-88

fH

Table 3 . 2 . 2 - 2 . L inear Vapor Engine/TESM (Continued)

T h e r m o e l e c t r i c Modul

Mate r i a l

e

Number of Couples E l ec t r i ca l Power Voltage

Engine Subsys tem

Volume Weight Power Input TESM

Output

SiGe 14 0. 96 watt 0. 6 volt

1. 152 l i t e r s 1. 36 kg 48 watts 69 wat t -hours

Table 3. 2. 2 - 3 . Weight S u m m a r y

Engine

Heat Source

T h e r m o e l e c t r i c Module

TESM

Boiler

Engine Inlet and Exhaust Valve

Regenera to r

Feed Pump

Condenser

Engine (Bellows and Lines)

E lec t ron ic Package

Outer Housing

Insulation

Total

L inear Vapor / TESM (lb)

0.47

0.40

0.79

0. 13

0. 11

0.03

0 .04

0.03

0. 10

0. 17

0. 16

0.57

3.01

(1.36 kg)

Linear Vapor (lb)

0. 55

0.40

-

0. 13

0. 11

0 .03

0. 04

0.03

0. 10

0. 17

0. 16

0 .78

2. 51

(1. 14 kg)

3-89 / < / /

r CONDENSER REGENIRATOR

-K X E N O N FILLED

"D U~\ U V 0 0 0 -

i-> n n n r> n q

THERMOELECTRIC MODULE INLET-EXHAUST

VALVE MECHANICAL DRIVE

TO BLOOD PUMP

Figure 3 . 2 . 2 - 1 0 . L inear Vapor Engine

A s u m m a r y of the engine design c h a r a c t e r i s t i c s is p resen ted in

Table 3. 2 . 2 - 4 . The isotope inventory is 57 wat ts and the volume of the

engine subsys t em is 1. 365 l i t e r s . Since the need for TESM was e l iminated ,

the weight of the sys t em was reduced as shown in Table 3. 2. 2 - 3 .

Table 3 . 2 . 2 - 4 . L inear Vapor Engine

Operating Conditions

Boiler Condenser Subcooler

Efficiency

Ideal Cycle Machine Overall

Engine Character is t ics

Speed Stroke Diameter Power Output

Feed Pump

Stroke Diameter Power Requirement

Thermoe lec tr i c Module

Material Number of Couples Electr ical Power Output Voltage

Engine Subsystem

Volume Weight Power Input

95 psia , 900''F 3 psia , U Z - F 3 psia , 115'F

14. 2% 72% 10.2%

120 bpm 1. 3 inches 0. 58 inch 5. 9 watts

0. 10 inch 0. 18 inch 0. 2 1 watt

SiGe 14 1. 14 watts 0. 7 volt

I .36S l i t ers 1. 14 Kg 57 watts

3-90 / ^ Z -

3. 2. 3 Rotary Vapor

3 . 2 . 3 . 1 His tor ica l Background

One of the poss ible approaches to the conversion of the rmal energy

into e l ec t r i ca l o r mechanica l energy is the use of the ro ta ry Rankine cycle.

This cycle is a t t r ac t ive for this application since there a r e various t h e r m o ­

dynamic and heat t r a n s f e r advantages in using a working fluid m both its

liquid and vapor phases . An impor tan t advantage of the Rankine cycle is

the g rea t ly reduced ra t io of compres s ion work to expansion work which

resu l t s from pumping a liquid r a t h e r than a gas . Fo r example , in an ideal

Rankine cycle , this ra t io can be l e s s than 1. 0% when water is the working

fluid. This compares with the 30% rat io in a typical Stirl ing cycle. Thus,

a Rankine cycle can be essent ia l ly independent of the efficiency of the com­

pres s ion p r o c e s s (i. e. , pump work). In addition, in the cycle the con­

dense r and boi ler both involve e i ther a condensing or boiling fluid, thus

increas ing the overal l heat t r a n s f e r coefficient. Both Stirl ing and Brayton

cycles requi re gas heat exchange r s , thus requir ing g rea t e r heat t r ans fe r

a r e a .

In the past cons idera t ion was given to the use of a ro ta ry expander

dr iven hydraul ic pump. It was felt that in o rde r to achieve as high an

efficiency as poss ib le , a posit ive d isplacement expander would be

requi red . Steam was selected as the working fluid because of its high

energy content and its compatibi l i ty with heat source and body t empe ra tu r e

cons t ra in t s . Because of i ts continuous unidirect ional motion, the e las t ic

response of a cur ta in (slide type) valve and a continuously open exhaust ,

the potential p rob lem of des t ruc t ive action of t rapped liquid could be e l i m i ­

nated, permi t t ing the use of "wet vapor" in the sys tem. A sys tem of this

type, as proposed by West inghouse, had an es t imated overal l efficiency of

8%. This design was excess ive ly heavy (17. 5 pounds including heat

source) , mechanica l ly complex ( ro ta ry and cur ta in va lves , chain and

sprocket wheels , e tc . ) and v e r y sensi t ive to t he rma l conduction l o s se s

(low conductivity t i tanium alloy housing requi red) . The la t te r condition

l imited the max imum cycle operat ing t e m p e r a t u r e to 389°F (218 psia) .

This coupled to a condensing t e m p e r a t u r e of 150°F (3.47 psia) to m i n i ­

mize the m o i s t u r e content at the end of the expansion p roces s resul ted in

a Carnot efficiency of only 36%. Thus , this sys tem, at an es t imated

3-91

overa l l efficiency of 8%, would achieve only 22% of the Carnot efficiency.

The use of working fluids other than water was not considered. However,

a so-ca l led advanced engine concept operat ing at an inc reased peak cycle

t e m p e r a t u r e of 500°F and with the t i tanium alloy engine casing replaced

with a f iber-f i l led Teflon m a t e r i a l was considered. This configuration

was es t imated to achieve an overa l l efficiency of 12. 7%. Ho^vever, the

previous ly mentioned shor tcomings (excess ive weight and mechanica l

complexity) st i l l exis ted. Work on this heat engine concept was t e rmina ted

by the NHLI and no additional data on any future development efforts were

avai lable .

The second approach that can be cons idered for the ro ta ry Rankine

cycle sys t em is tu rbomachinery . This concept had been looked at only in

a c u r s o r y manne r by previous inves t iga tors and s u m m a r i l y d i smi s sed as

not being applicable to this low power output application. It was felt that

efficiencies would be too low. In addition, the high rotat ive speed requi red

would necess i t a t e the use of reduction ge a r s in o rde r to dr ive the blood

pump at synchronous speed. A second factor which may have contr ibuted

to lack of i n t e r e s t in tu rbomach ine ry was that if a gene ra to r were ut i l ized,

the the rnna l - to -e l ec t r i ca l conver t e r had to be coupled to an e l e c t r i c a l - t o -

mechan ica l /hydrau l i c conver te r to opera te the blood pump. It was pos tu­

lated that this sy s t em could not be competi t ive with a d i rec t t h e r m a l - t o -

mechanica l conver t e r device . The re a r e s eve ra l f ac to r s , which when

examined in a m o r e comprehens ive m a n n e r than has been done previous ly ,

resul t in a r r iv ing at a different conclusion. F i r s t , component efficiencies

need not be low. F o r example , as will be d i scussed l a t e r in this sect ion,

turbine efficiencies of 55 to 60% and gene ra to r efficiencies of 90 to 95%

can be achieved even at the low power levels involved. In addition, as was

previously d i scussed , the pump efficiency is not c r i t i ca l in the Rankine

cycle . However, even for this component, acceptable efficiencies in the

range of 30 to 40% can be obtained using p rope r design techniques . Also

overlooked in previous a s s e s s m e n t s was the cons iderable potential for

achieving higher turbine and overa l l cycle efficiencies through the use of

high molecu la r weight working fluids. Because the thermodynamic c h a r ­

a c t e r i s t i c s of many of these fluids p e r m i t the use of regenera t ion , a s i g ­

nificant i n c r e a s e in overa l l cycle efficiency can be achieved. In addit ion,

for those sys t ems providing an e l ec t r i ca l output, with a t u rbogene ra to r ,

3-92

the use of a secondary ba t t e ry for energy s torage is a lso feasible. This

pe rmi t s sizing of the sys t em for blood pump average power requ i rements

and resu l t s in a further reduction in heat source the rma l r equ i remen t s .

Final ly , high speed tu rbomach ine ry components a r e capable of providing

high power dens i t ies (wat ts / lb) and specific volumes (watts/cu in. ) and by

utilizing hydrodynamic type b e a r i n g s , long life and low noise levels .

In select ing a candidate r o t a r y Rankine cycle sys tem both turbine

and ro ta ry expander dr iven sys t ems were invest igated. A d iscuss ion of

the types of engine configurations considered follows.

3 . 2 . 3 . 2 Engine Configurations

T h e r e a r e s eve ra l engine configurations that can be initially con­

s idered for this applicat ion. They a r e :

• Tu rbopump-a l t e rna to r (ac e lec t r i ca l power output)

• Tu rbopump-gene ra to r (dc e lec t r ica l power output)

• Turbopump (hydraulic power output)

• Turbopump (mechanical power output)

• Rotary expander -pump (hydraulic power output)

• Rotary expander (mechanical power output)

The turbopump configurations a r e cha rac t e r i zed by operat ion at high speed

(i. e. , 96, 000 rpm) . In ca ses where an e lec t r i ca l output is provided they

also pe rmi t a wide choice of e l ec t r i c a l - t o -mechan ica l o r hydraulic energy

convers ion dev ices . In addition, they a r e compatible for use with second­

a r y ba t t e r i e s for e l ec t rochemica l energy s torage . This la t te r capability

pe rmi t s sizing the sys tem for the average ra the r than peak pump power

r e q u i r e m e n t s . The turbopump can also be employed to supply a hydraulic

output to a hydraul ic conver t e r fronn the main cycle pump. This approach

would be compat ible with a high p r e s s u r e hydraulic conver te r sys tem capa­

ble of operat ing on the Rankine cycle working fluid. However, this system

does not have the potential for energy s torage and must be designed to p ro ­

vide peak power continuously. Finally, the turbopump can provide mechan­

ical output power and through a suitable speed r educe r be coupled to a

mechanica l or hydraul ic conve r t e r . This configuration is a l so a peak

3-93

power output, d i ss ipa t ive type. Without energy s torage capabil i ty, these

la t te r two configurations r e su l t in requi r ing a significantly l a r g e r heat

source than those providing an e l ec t r i ca l output with a ba t te ry .

An a l t e rna te approach is to use a posi t ive d isp lacement ro t a ry

expander . This is cha rac t e r i zed by low speed (i. e. , 390 to 6000 rpm)

operat ion. This p e r m i t s d i rec t coupling to a hydraul ic pump or a low gea r

ratio speed r educe r capable of providing a mechanica l output. These s y s ­

t ems a lso a r e not capable of energy s torage and have all the sy s t em sho r t ­

comings assoc ia ted with peak power output, d iss ipat ive types .

A p r e l i m i n a r y review of these s ix configurations was m a d e . It

became apparent that the t u rbopump-gene ra to r (or a l t e rna to r ) was supe r io r

to the turbopump alone in t e r m s of m i n i m u m heat source s ize and sys t em

flexibility. Hence, emphas i s was placed upon investigating va r ious con­

figurations of th is type. The low speed, ro t a ry expander configurations

a r e inherent ly heavy, mechanica l ly complex, and l a rge in volume. How­

ever , because of the i r capabil i ty for d i r ec t coupling to a hydraul ic o r

mechanica l conve r t e r , these configurations were a lso retained for fur ther

ana lys i s . A m o r e detai led d i scuss ion of these sy s t ems is provided in

Section 3. 2. 3. 5.

3. 2. 3. 3 Selection of Working Fluid

The re is a wide var ie ty of poss ib le Rankine cycle working fluids that

could be cons idered for this applicat ion. However, when one cons ide rs the

low power level involved and the cons t ra in t s imposed by body implantat ion,

the number of choices na r rows cons iderably . In the l a t t e r ca tegory , in

addition to the obvious r equ i r emen t s of weight, volume, specific gravi ty ,

and heat re ject ion t e m p e r a t u r e l im i t s , an additional over r id ing c o n s i d e r a ­

tion is overa l l efficiency. Another impor tan t factor is that the working

fluid should be imperv ious for the 10-year life with r e spec t to pyrolyt ic

and radiolytic decomposi t ion.

The mos t commonly used fluid in the Rankine cycle is s t eam. How­

ever , except for use with the posi t ive d isp lacement r o t a r y expander , s t e a m

3-94

is not a ve ry a t t r ac t ive fluid for use with smal l s ing le -d isc tu rb ines . Its

ve ry high nozzle efflux velocity for a modera te t e m p e r a t u r e drop resu l t s

in imprac t i ca l l y high ro to r speeds in o rde r to achieve a reasonable level

of efficiency. An addit ional shor tcoming of s t eam is that unless a high

degree of superhea t is employed, the expansion p r o c e s s can resul t in

enter ing the wet region of the working fluid and cause turbine e ros ion . A

much higher degree of superhea t (and hence high peak cycle t e m p e r a t u r e s )

is a lso requ i red if any significant cycle pe r fo rmance improvement using

r egene ra t ion is to be achieved. Despite these shor tcomings , s t eam was

re ta ined for use with the ro t a ry expander and for compar i son purposes

with the t u r b o p u m p - g e n e r a t o r configurat ions.

The fluids mos t capable of meet ing the essen t i a l p r e r e qu i s i t e s out­

lined above a r e broadly ca tegor ized as high molecu la r weight (HMW) work­

ing fluids. F lu ids having a high molecu la r weight provide a IOAV nozzle

efflux velocity and a compara t ive ly l a rge m a s s flow rate through the t u r ­

bine. This enables a sa t i s fac tory matching between the nozzle gas exit

veloci ty and the ro to r t ip speed (velocity ratio) at modera te turbine ro ta ­

tive speeds and r e su l t s in a h igher s ing le -d i sc vapor turbine efficiency.

Another a t t r ac t ive feature of these fluids is that the slope of the sa tura ted

vapor line in the t e m p e r a t u r e - e n t r o p y d i ag ram is e i ther ve r t i ca l or has a

posi t ive d s / d T s lope. This not only a s s u r e s that i sen t ropic expansion

does not resu l t in "wet" vapor , but in an actual cycle pe rmi t s the use of

eff ic iency- improving r egene ra t ion without the need for excess ive super ­

heating in the bo i le r . Another impor tan t r equ i remen t is t he rma l s tabi l i ty

at the peak cycle t e m p e r a t u r e for prolonged pe r iods of t ime (10 yea r s ) and

chemica l i n e r t n e s s with r e spec t to the m a t e r i a l s used in the tu rbopump-

gene ra to r . In addition, for this applicat ion where the heat source will be

medica l g rade plutonium-238 dioxide, it is impor tan t that the threshold

values for these working fluids be sufficiently high so that no adve r se

effects on fluid composi t ion, v i scos i ty , densi ty , o r heat t r a n s f e r c h a r a c ­

t e r i s t i c s a r e exper ienced .

With the above c r i t e r i a in mind, a review and analys is of the t h e r m o ­

dynamic c h a r a c t e r i s t i c s of approx imate ly 25 of these fluids was conducted.

3-95

//7

Based upon this invest igat ion, four fluids were selected for cons idera t ion

for use with var ious potential candidate ro t a ry Rankine cycle s y s t e m s .

These , as well a s s t e a m , a r e l i s ted in Table 3. 2. 3 - 1 . The factors con­

s idered in select ing these organic fluids included:

• Reasonable i sen t ropic work avai lable at peak cycle t e m p e r a t u r e s in the 400 to 750°F range

• Condensing t e m p e r a t u r e s in the range of 115 to 350' 'F to be consis tent with body heat re ject ion capabi l i t ies and p rac t i ca l heat exchanger designs

• Avoidance of fluids requi r ing ex t r eme ly high condenser vacuum to a s s u r e adequate pump net posi t ive suction head

• T h e r m a l s tabi l i ty and sufficient imperv iousness to 10 yea r in tegra ted radiat ion dose levels from the heat sou rce .

At this point, the final se lec t ion of a working fluid for this appl ica­

tion involves an a s s e s s m e n t of the cycle conditions to be uti l ized and the i r

resu l tan t impact on component and overa l l sy s t em efficiencies. This is

d i scussed in Sections 3. 2, 3. 4 and 3. 2. 3. 5.

3 . 2 . 3 . 4 Cycle Conditions

The select ion of cycle operat ing conditions is contingent upon many

fac to r s . These include the rmodynamic c h a r a c t e r i s t i c s of the working

fluid, achievement of acceptable Carnot and Rankine cycle eff ic iencies ,

and compatibi l i ty with sy s t em component c h a r a c t e r i s t i c s and body phys io ­

logical cons t ra in t s to insu re m a x i m u m per fo rmance at m i n i m u m weight

Table 3 .2 . 3 - 1 . Candidate Rota ry Vapor Cycle Working Fluids

Fluid

S team

Monsanto CP-34 (Thiophene)

Biphenyl

F r e o n 11

Fluor inol 85

Molecular Weight

18

84

154.2

137.4

87.74

3-96

1^?

and volume. Since these factors a r e in t e r r e l a t ed , it is usually n e c e s s a r y

to i t e ra t e s e v e r a l t imes before a r r iv ing at acceptable cycle conditions.

In addition to analyt ical a s s e s s m e n t s , exper ience and judgment a lso play

an impor tant role in a r r iv ing at a se lect ion.

Based upon the foregoing, the cycle conditions selected for inves t iga­

tion for use with the t u rbopump-gene ra to r configurations a r e shown in

Table 3 , 2 . 3 - 2 . The peak cycle t e m p e r a t u r e s se lec ted for s t eam a r e

c h a r a c t e r i z e d by the fact that all include considerable superheat (from

110° to 300°F) . This was done to i n su re that the s t e a m remained in the

superhea ted or sa tu ra ted vapor condition after i sen t ropic expansion so

t he re would be no danger of turbine blade eros ion caused by impingement

of condensed liquid d rop l e t s . Condensing p r e s s u r e s were selected to p r o ­

vide acceptable heat re ject ion t e m p e r a t u r e s , avoid ex t remely high con­

d e n s e r vacuum, and resu l t in reasonable Carnot cycle efficiencies. The

i sen t rop ic work avai lable for all the s t e a m cycle conditions cons idered is

fair ly high (261 to 500 Btu/ lb) . This resu l t s in high nozzle spouting ve loc­

ity r a t i o s , even at high turbine speeds . In addition, at low power output

l eve l s , the requ i red flow ra te s a r e ex t r eme ly smal l . This has the effect

of producing v e r y low turbine and pump specific speeds , which in turn

leads to low component eff ic iencies . When cons idered as a working fluid

for ro t a ry expande r s , the m a x i m u m cycle operat ing t e m p e r a t u r e s and

p r e s s u r e s mus t be cons iderably reduced to min imize the rma l conduction

l o s se s and t h e r m a l differential expansion design complexity.

The cycle conditions for the HMW fluids impose min imal superhea t

r e q u i r e m e n t s ( i . e . , 5° to 30°F) , The peak cycle operat ing t e m p e r a t u r e s

a r e cons t ra ined p r i m a r i l y by the need to a s s u r e t h e r m a l stabil i ty over a

per iod of 10 y e a r s . Except in the case of biphenyl, where the t he rmody-

namica l ly imposed high condensing t e m p e r a t u r e r equ i r e s a high peak

t e m p e r a t u r e to achieve a reasonable Carnot efficiency, this value was in

all o ther c a se s kept at 500°F or l e s s . By l imiting the HMW fluid cycle

t e m p e r a t u r e to th is va lue , the total decomposi t ion to be expected in 10 y e a r s

will be l e s s than 0. 1% (Figure 3 .2 , 3-1). The HMW working fluids a r e

c h a r a c t e r i z e d by cons iderab ly lower avai lable i sen t ropic work (17. 5 to

175 Btu/ lb) and somewhat lower Carnot cycle efficiencies than that obtain­

able with s t eam. However, the lower i sen t ropic work r equ i re s a higher

3-97

Table 3 ,2 , 3-2, Comparison of Cycle Conditions for Selected Working Fluids

Working Fluid

Peak Cycle Conditions

T e m p e r a t u r e ( ' F l P r e s s u r e (psia)

Condensing Conditions

T e m p e r a t u r e (°F) P r e s s u r e (psia) Carnot Cycle Efficiency (%)

Isent ropic Work Available

(Btu/lb)

Steam

CP-34

Biphenyl

F r e o n 1 1

FUionnol 83

800

800

750

750

500

500

500

430

400

750

390

390

470

470

700

700

500

500

220

450

450

250

200

116. 5

550

550

700

700

115

281

115

2 12

153

150

116

116

100

330

165

120

165

115

1. 5

50

1. 5

14. 7

4

q

4

4

3

1. 5

67

33

15

4. 5

54. 3

41. 2

52. 5

44.2

36. 2

36. 5

40.2

35. 0

34. 9

34. 7

26. 5

31. 8

32. 8

38.2

500

261

462

321

295

70

80

69

70

175

19. 0

22. 5

57

71

100 10"

# •d 10 O X

g o 8 - 1

z z O

2 0.1 s O o o —J

< • 0.01 O

10-" 0£

^ ae UJ

H 10-5 z O h -

U ^

O 5 10-* o (J Q

,„-7 10

0.001 in-° l ,X I I I I 350 400 450 500 550 600 650

PEAK CYCLE TEMPERATURE, °F

Figure 3 .2 . 3 - 1 . T h e r m a l Decomposit ion of Typical HMW Rankine Cycle Working Fluids

cycle m a s s flow ra te . F o r this low pow^er level application, this resu l t s

in improved component efficiencies s ince higher specific speeds a r e

obtainable. In addition, lower nozzle exit veloci t ies a r e obtained which

p e r m i t s achieving h igher turbine velocity ra t ios at reduced rotat ive

speeds . This a lso i n c r e a s e s the turbine efficiency. Final ly, since it is

poss ib le to use regenera t ion with these HMW fluids, the overal l effect is

to be able to achieve h igher Rankine cycle efficiencies than with s team,

despi te the init ial ly lower Carnot cycle efficiencies. For the turbopump-

gene ra to r , the Rankine cycle efficiency is defined as the net e lec t r i ca l

power out divided by the t he rma l heat input to the working fluid. In the

vapor Rankine cycle , the degree of regenera t ion avai lable , as well as the

higher turbine efficiencies achievable , a r e two of the m o r e significant

fac tors in obtaining higher Rankine cycle efficiencies at lower peak cycle

t e m p e r a t u r e s , than with nonregenera t ive s team Rankine cycles .

The de te rmina t ion of the actual efficiencies obtainable for the cycle

conditions outlined in Table 3. 2. 3-2 requ i re a detai led analys is of each

3-99

/6'7

sys tem and its components . This a lso enables one to nnake a s s e s s m e n t s

regarding sys t em weight, volume, and heat source the rma l r equ i r emen t s .

The resu l t s of these ana lyses a r e p resen ted in Section 3, 2. 3. 5,

3, 2. 3, 5 Selection of Candidate Rotary Vapor Cycle Sys tem Configuration ^ ^

In analyzing the var ious ro ta ry Rankine cycle s y s t e m s , the major

emphas is was placed upon invest igat ing the tu rbopump-genera to r configu­

ration. These des igns held the highest p romise of achieving the requi red

per formance within the weight, volume, and heat source the rma l inventory

cons t r a in t s . The ro t a ry expander configuration was a l so analyzed, but

considerat ion in these cases was l imited to s t eam and F r e o n 11 as working

fluids. The approach taken in each case is desc r ibed below.

Turbopump-Genera to r Configurations

The c lass ic method employed for es t imat ing the pe r fo rmance of

turbomachines involves four s imi l a r i ty p a r a m e t e r s . These a r e specific

speed, N , specific d i a m e t e r , D , Reynolds number , R , and Mach

number , M, These a r e defined respec t ive ly a s :

(1) N s

D s

N(V)°'

^^^e./'

iv)'-'

R = °"" e l

M - l

5

75

25

(2)

(3)

(4)

where

N = turbine rotat ive speed, rpm

D = ro to r d i a m e t e r , ft 3

V = vo lumet r ic flow ra te at the turbine outlet , ft / s e c

3-100

H J - i sen t ropic head avai lable for work in turb ine , f t - lb / lb ad

u = rotor tip speed, f t / sec

V = velocity at nozzle exit, f t / sec 3

a - densi ty of fluid at turbine inlet, lb/ft

^ = v iscos i ty of fluid at turbine inlet, Ib / f t - sec

C = sonic velocity at turbine inlet conditions, f t / sec

F o r Reynolds numbers g r e a t e r than 10 and Mach numbers l e s s than

1 (subsonic flow) which exist for the designs considered, the significant

pe r fo rmance p a r a m e t e r s became p r i m a r i l y the specific speed and specific

d i a m e t e r . An in te res t ing aspec t of the specific d i ame te r , D , p a r a m e t e r

is that this value r e p r e s e n t s a c r i t i ca l dimension of the turbomachine , the

turbine ro tor d i ame te r , and in t roduces a geometr ic value into the s i m i ­

l a r i ty concept. This is a dis t inct advantage over use of the turbine ve loc ­

ity ra t io (u/v) only, since it offers an opportunity to recognize relat ions

for the opt imum turbine geomet r ica l values in t e r m s of D , This concept

is used to p resen t avai lable turbine per formance tes t data in convenient

design d i ag rams which depict max imum obtainable efficiency together with

opt imum design geomet ry .

In utilizing these p a r a m e t e r s , engineering judgment must also be

exe rc i s ed since var ious turbine configurations can be considered. These

include:

• Axial Flow — Impulse

Single s tage — par t i a l admiss ion

Single stage — full admiss ion

Curt is type — velocity compounded

• Radial Flow — Centr ipeta l

Impulse blading —approximately 15% react ion

Radial blading — approximate ly 50% react ion

F o r the low power outputs and correspondingly low flow ra tes involved in

this applicat ion, the cent r ipe ta l (inward flow) radial turbine is preferable

to the axial flow turb ine . This configuration, because of i ts s imple ,

rugged const ruct ion and potential ly lower manufacturing cost , becomes

3-101

ex t remely a t t r ac t ive . In addition, as a resul t of cons iderable development

effort, they a r e capable of matching or exceeding the pe r fo rmance of axial

flow tu rb ines , pa r t i cu la r ly in the pa r t i a l admiss ion operat ing r eg imes

common to low flow ra te appl icat ions of this type.

With the cycle conditions es tab l i shed in Table 3 .2 . 3-2 , it is possible

to de te rmine the avai lable i sen t rop ic head, H ,. However , in o r d e r to

size the turbine ro tor and confi rm its overa l l pe r fo rmance , it i s sti l l

n e c e s s a r y to de t e rmine the requ i red cycle flow rate and turbine operat ing

speed. The turbine flow ra te i s re la ted to the work output requi red from

the turbine and is es tab l i shed as follows:

W, = W + W (5) t P g

where

W = turbine work output, B tu /h r

W = cycle pump work input, B tu /h r

W = total gene ra to r work input, B tu /h r

The cycle pump work input is de te rmined by

0. 000583 X A P X Q . , , W = (b )

P ^p

0.000583 X A P X "^ ^,'^- ^^ x K, ^ 2 I

Tip

where

Q = cycle flo^v r a t e , g a l s / m i n

rh = cycle flow ra t e , Ib /min 2

A P = pump p r e s s u r e r i s e , l b / in 3

Pn = densi ty of working fluid at pump inlet , lb/f t

' p = pump overa l l efficiency

K, = constant to conver t to B tu /h r

3-102 /4'/

and the generator work input (assuming electrochemical energy storage)

is expressed by,

- blood pump average power ,_>

^ pc gen

where

n = power conditioning and control system efficiency

Ti = generator efficiency gen " '

In addition, since the turbine work output can also be expressed as,

^ t = ^^t^ad

where

m = cycle flow rate, Ib/min

r) = turbine overall efficiency

Now by substituting these values in Equation (5), and solving for the cycle

flow rate, one obtains the following expression.

W m = L ^ ^ (8)

where

K2 = K x 0. 000583 X 7. 48 = constant

In order to establish the flow rate, at this point it is necessary to initially

assume values for the various system component efficiencies, i. e. , r\ ,

TI , T] and T] . These must later be confirmed by detailed component

design analyses, design charts, and when available, test data. An iter­

ative process must be performed until the assumed efficiency values and

the detailed design analyses values coincide.

The selection of the turbine operating speed is contingent upon

achieving maximum performance from the three combined rotating unit

3-103

(CRU) components at m in imum weight and volume. However, as was

previous ly d i scussed , the pump efficiency is l e ss c r i t i ca l than the turbine

efficiency in the Rankine cycle when endeavoring to achieve max imum

overa l l sy s t em efficiency. In addit ion, pernnanent magnet type genera to r

efficiency is m o r e a function of the ro tor and s ta tor e lec t romagnet ic p r o p ­

e r t i e s than of speed. Hence, as long as pump cavitation l imi ts a r e not

exceeded and gene ra to r ro tor windage and bear ing l o s se s do not become

excess ive , the choice of CRU rotat ive speed is usually based upon m a x i ­

mizing turbine efficiency. By utilizing Equation (I) , the speed can be

a s sumed and with the cycle conditions known, the specific speed, N , of

the turbine can be de te rmined . This value can be used to obtain a f irs t

approximat ion to the turbine efficiency by employing a design char t of the

type shown on F i g u r e 3. 2. 3-2 . The specific d i a m e t e r s , D , can a lso be 5

obtained from this char t ; using the char t and Equation (2), the turbine

ro tor d i a m e t e r can be calculated. If the values de te rmined for efficiency

and d i a m e t e r a r e accep tab le , a fur ther check on turbine efficiency can be

obtained by calculat ing the turbine veloci ty ra t io , U , / C , whe re , 1 o

U, = turbine pe r iphe ra l velocity, f t / s ec

C = spouting velocity f rom nozzle , f t / sec

The spouting velocity f rom the nozzle is given by,

o = % V ^ ^ <9)

where

4^^ = nozzle flow coefficient

With the veloci ty ra t io known, the turbine d i a g r a m efficiency can be

obtained f rom a design char t as shown on F igu re 3. 4. 3 -3 . Final conf i rma­

tion of turbine efficiency mus t u l t imate ly be accompl ished by a deta i led ,

r igorous ana lys i s of t he rmodynamic , ae rodynamic , and heat t r a n s f e r

c h a r a c t e r i s t i c s of the turbine des ign including al l per t inent l o s s e s . How­

ever , for p r e l i m i n a r y des ign p u r p o s e s , it is usual ly sufficient to re ly upon

3-104

t

o

= 0.010^

STEAM DESIGN POINT

CP34 DESIGN POINT

, . .h/D- .015

Dr 03 n , .p»«>^ :• 04 ^ " ^

ADMISSION = 30"-^-"-

^= .30.40.50.60.70.80.90

.6 I 6 10 20 60 100

N.

Figure 3. 2. 3-2. Design Chart for Pa r t i a l Admiss ion Radial - Impulse Turbines

100 -r: " "=^

TURBINE EFFICIENCY,

Vj, %

0.1 0.2 0.3 0.4 0.6 0.8

VELOCITY RATIO, V , / C Q

Figure 3. 2. 3 -3 . Turbine Diagram Efficiency vs Velocity Ratio

the design cha r t s of F i g u r e s 3. 2. 3-2 and 3. 2. 3-3 which have been based

upon a compilat ion of t es t data for the p a r t i c u l a r turbine type se lec ted ,

i. e. , pa r t i a l admiss ion radial impulse turb ine .

Using the above approach and invest igat ing all the cycle conditions

outlined in Table 3 .2 . 3-2, it was poss ible to de te rmine and se lec t the

p r e f e r r e d cycle operat ing mode for each of the working fluids cons idered .

The r e su l t s of these ana lyses a r e shown on Table 3. 2. 3 -3 . In addition to

the varying cycle condi t ions, it should be noted that the requi red turbine

operat ing speed for m a x i m u m turbine efficiency var ied from 72,000 to

144,000 rpm. By re fe r r ing to Table 3 .2 . 3 - 1 , one can see that the turbine

speed is re la ted to the working fluid mo lecu la r weight, ^vith the lower

values requir ing the h igher operat ing speeds . Also, for the cycle condi­

tions se lec ted , only the the rmodynamic c h a r a c t e r i s t i c s of CP-34 and

biphenyl pe rmi t t ed the use of regenera t ion . The overa l l t h e r m a l conver t e r

pe r fo rmance improvement obtainable with regenera t ion is best seen by a

compar i son bet'ween s t eam and CP-34 working fluids. The nonregenera t ive

s t eam Rankine cycle is shown on F igure 3. 2. 3-4 and the regenera t ive

CP-34 cycle is shown on F igure 3. 2. 3 -5 . F r o m Table 3. 2. 3 -3 , it can be

seen that the Carnot efficiencies for these two cycles a r e nea r ly equal

(i. e. , s t e am = 0. 362 v e r s u s CP-34 = 0. 35). However, because of the use

of r egenera t ion and the h igher turbine efficiency achievable with CP-34 ,

the Rankine cycle efficiency (work out - heat input) for CP-34 i s cons ide r ­

ably h igher than for s t eam (i. e. , CP-34 - 0. 201 v e r s u s s t e a m = 0. 135).

A review of the r e su l t s of the ana lyses s u m m a r i z e d in Table 3. 2. 3-3

led to the se lec t ion of CP-34 (thiophene) a s the p r e f e r r e d working fluid for

the t u rbopump-gene ra to r ro t a ry Rankine cycle sys t em. It not only demon­

s t ra ted the highest overa l l t h e r m a l conve r t e r efficiency (i. e. , 16. 0%) but

it a lso achieved this at mode ra t e peak cycle t e m p e r a t u r e s and rotat ive

speeds . The l a t t e r two c h a r a c t e r i s t i c s a r e impor tan t fac tors in achieving

long life.

Ro ta ry -Expande r Configurations

The s imi l a r i t y laws for posi t ive d i sp lacement r o t a r y machines differ

somewhat f rom tu rbomach ines . However , it has been demons t ra t ed that

the i r m a x i m u m obtainable efficiency is a lso a unique function of the specific

speed and specific d i a m e t e r as prev ious ly defined for t u rbomach ines .

3-106

Table 3 .2 . 3-3 . Compar ison of Turbopump-Genera tor Rankine Cycle Systems

Working Fluid

Turbine Speed, rpm

Turbine Inlet T e m p e r a t u r e , °F

Turbine Inlet P r e s s u r e , psia

Superheat at Turbine Inlet, °F

Condensing P r e s s u r e , psia

Condensing T e m p e r a t u r e , °F

Adiabatic Head, Btu/ lb

Adiabatic Head, ft

Flow Rate, Ib /min

Specific Speed

Turbine Wheel Diamete r , inch

Velocity Ratio, U j / C ^

Turbine Efficiency, %, Diagram

Turbine Efficiency, Overal l

Carnot Efficiency, %

Rankine Cycle Efficiency, %

The rma l Conver te r Heat Input, watts

Genera tor E lec t r i ca l Output, wat ts

Overal l The rma l Conver ter Efficiency, %

Steam

144,000

500

220

110

4

153

295

228,000

0. 0033

0.95

1.25

0 .21

62

45

36.2

13. 5

60

4.26

7 .2

CP-34

96,000

430

250

5

4

116

69

53,600

0. 0134

1. 7

1. 125

0.254

70

58

35.0

20 .7*

60

9 . 6

16.0

Biphenyl

72,000

750

166.5

0

1. 5

340

63

49,000

0.0149

2.75

1. 60

0.290

75

63

34. 5

19.4*

60

9. 17

15. 3

F reon 11

72,000

390

550

20

33

120

22. 5

17.500

0.0347

1.65

1.00

0 . 3

76

64

31. 8

14.8

60

8.25

13,75

Fluorinol 85

96,000

470

700

30

4. 5

115

71

55,200

0.014

1. 89

1.00

0.25

70

58

38.2

18.8

60

8. 85

14. 75

With regenera t ion

COMPARISON OF STEAM AND CP34 THERMODYNAMIC CYCLE DIAGRAMS

220 PSIA (500°F) (110° SUPERHEAT)

U)

H-•

o 00

LL. 0

LU

D t—

LU

LU 1—

500

400

300

200

100 ' " " /(153° COND. TEMP)

0.22 1.61 ENTROPY (BTU/LB/°R)

TURBINE EXPANSION

CONDENSING PUMP WORK PREHEATER BOILER SUPERHEATER

.^420

3

^ 115

UJ

1-2 2-3

3-4 4-5

250 PSIA (425°) 5° SUPERHEAT

PSIA (115^ COND. TEMP.)

I " Q.

z< U J >

-.08-.04 0 .04 .08 ENTROPY, BTU/LB/°R

TURBINE EXPANSION 5-6 REGENERATION

(VAPOR SIDE) 6-7 CONDENSING 7-1 PUMP WORK

REGENERATION (LIQUID SIDE)

BOILER (LIQUID PHASE) BOILER (VAPORIZATION

PHASE)

Figure 3. 2. 3-4. Steam (Nonregenerative) Rankine Cycle

F igure 3. 2. 3-5. CP-34 Regenerat ive Rankine Cycle

H o w e v e r , t he d e s i g n c h a r t s to be u s e d for r e l a t i n g N , D and e x p a n d e r

e f f i c i ency m u s t be b a s e d upon p e r f o r m a n c e da ta ob ta ined on t h e s e t ypes of

m a c h i n e s . In a d d i t i o n , t h e r e i s a wide v a r i e t y of p o s i t i v e d i s p l a c e m e n t

type m a c h i n e s . T h e s e i n c l u d e :

• Roo t s type

• E c c e n t r i c r o t o r ( W e s t i n g h o u s e type)

• H e l i - r o t o r ( L y s h o l m - F a i r c h i l d H i l l e r type)

• Modif ied Wanke l t ype

D e t a i l e d i n f o r m a t i o n on the d i f f e r en t l o s s coef f i c ien t s in r o t a r y d i s p l a c e ­

m e n t m a c h i n e s ia r a t h e r l i m i t e d . H e n c e , a c o m p r e h e n s i v e a n a l y s i s of

t h e s e m a c h i n e s for a n u m b e r of w o r k i n g f luids o v e r a wide r a n g e of cyc le

c o n d i t i o n s , a s w a s done wi th t u r b o m a c h i n e s , was not c o n s i d e r e d p r a c t i c a l .

In a d d i t i o n , s i n c e t h e s e a r e b a s i c a l l y low s p e e d m a c h i n e s o p e r a t i n g in the

390 to 24, 000 r p m r e g i m e , t h e y a r e not c o m p a t i b l e with the f a i r l y h igh

s p e e d s r e q u i r e d ( 5 0 , 0 0 0 to 1 5 0 , 0 0 0 r p m ) to ob ta in l igh twe igh t , s m a l l

v o l u m e , p e r m a n e n t m a g n e t g e n e r a t o r s . T h u s , t h e i r output p o w e r m u s t be

coup led to a h y d r a u l i c p u m p and t h e y m u s t be o p e r a t e d in a n o n m o d u l a t e d

f a sh ion wi thout e n e r g y s t o r a g e .

B e c a u s e of the f o r e g o i n g , only two c o n f i g u r a t i o n s w e r e i n v e s t i g a t e d

in d e t a i l . T h e s e w e r e an e c c e n t r i c r o t o r s t e a m e x p a n d e r and a h e l i - r o f o r

e x p a n d e r o p e r a t i n g on F r e o n 11 . T h e c y c l e cond i t i ons w e r e s e l e c t e d to be

c o m p a t i b l e wi th the t e m p e r a t u r e and p r e s s u r e r a t i o l i m i t a t i o n s of the

e x p a n d e r . Only the m e c h a n i c a l output c o n f i g u r a t i o n was c o n s i d e r e d s i n c e

it w a s e s t a b l i s h e d d u r i n g t h i s s tudy tha t a low p r e s s u r e h y d r a u l i c output

would be i n c o n s i s t e n t with a c h i e v i n g an a c c e p t a b l e m i n i m u m weight and

v o l u m e h y d r a u l i c c o n v e r t e r . A c o m p a r i s o n of the p e r f o r m a n c e of t h e s e

two s y s t e m s is shown in T a b l e 3 . 2. 3 - 4 . As can be s e e n f rom t h i s t a b l e ,

t he n e c e s s i t y for t h e s e s y s t e m s to p r o v i d e peak p u l s a t i l e p o w e r c o n t i n u ­

o u s l y i m p o s e s a s e v e r e p e n a l t y on t h e hea t s o u r c e t h e r m a l input . T h i s

o c c u r s d e s p i t e the a s s u m p t i o n of e x p a n d e r e f f i c i enc i e s a s h igh a s 65% and

u t i l i z i n g a m e c h a n i c a l a c t u a t o r c o n v e r s i o n e f f ic iency of 55%. T h e r e f o r e ,

it i s c o n c l u d e d tha t t h e s e n o n m o d u l a t e d , r o t a r y e x p a n d e r s y s t e m s without

e n e r g y s t o r a g e a r e not c a p a b l e of s t a y i n g wi th in m a x i m u m a l l o w a b l e hea t

s o u r c e c o n s t r a i n t s .

3-109

Table 3 .2 . 3-4. Compar i son of Rotary Expander-Mechanica l Output Rankine Cycle Sys tems

Expander Type

Working Fluid

Expander Speed, rpm

Expander Inlet T e m p e r ­a tu r e , °F

Expander Inlet P r e s ­s u r e , ps ia

Superheat at Expander Inlet, "F

Condensing P r e s s u r e , psia

Condensing T e m p e r ­a tu re , °F

Adiabatic Head, Btu/ lb

Adiabatic Head, ft

Flow Rate , Ib /min

Specific Speed

Expander Outside Diamete r , inch

Expander Efficiency, %, (est imated)

Input Power to Mechanical Actua tor , watts

Carnot Efficiency, %

Rankine Cycle Efficiency, %

T h e r m a l Conver te r Heat Input, watts

1 Overal l T h e r m a l Conver te r Efficiency, %

Eccen t r i c Rotor

Steam

390

500

218

111

3 . 5

147

299

233,000

0.00475

0.0035

1.3

0. 65

16.2

36 .8

16.8

105

15.4

Hel i -Rotor

F r e o n 11

24,000

390

550

20

67

165

19

14,800

0.0955

0. 575

0. 300

0 .65

16.2

26. 5

14. 15

162

10.0

Hel i -Rotor

F reon 11

24,000

280

180

30

30

115

15.5

12,100

0. 100

1.05

0. 500

0.65

16.2

22 .3

10.9

164

9 . 9

3-110 , ^

Configuration Selection

Based upon the foregoing evaluation, the tu rbopump-genera to r was

se lec ted in p re fe rence to the ro ta ry expander . The cycle , using CP-34

as a working fluid, resu l ted in the highest overal l sys tem efficiency.

Hence, this configuration was se lec ted as the candidate ro ta ry vapor cycle

sys t em.

3 . 2 . 3 . 6 Component Design

The components requi red for implementa t ion of the CP-34 ro ta ry

vapor cycle s y s t e m include the following:

• Inward-flow radial impulse turbine

• Centrifugal pump

• P e r m a n e n t magnet , b r u s h l e s s genera to r

• Boi ler

• Condenser

• Regenera to r

• Secondary ba t t e ry

• Speed controls

These components were chosen on the bas i s of achieving max imum

pe r fo rmance for the requi red cycle condit ions. In addition, emphas is was

placed on achieving long operat ing life. By selecting component e lements

such as noncontacting vapor b e a r i n g s , operating at very low s t r e s s l eve l s ,

and m o d e r a t e peak cycle t e m p e r a t u r e s , adequate design marg ins can be

achieved to mee t the 10-year goal.

The main e lement compris ing the ro ta ry vapor cycle sys tem is the

t u rbopump-gene ra to r o r s imply tu rbogene ra to r unit (TGU). The TGU is

shown in F igu re 3. 2. 3-6. It cons is t s of a rad ia l , inward-flow, im pu l se -

type tu rb ine , a centrifugal pump, and a permanent magnet dc genera to r .

They a r e a l l mounted on a single shaft rotating at 96,000 rpm. The m a x i -

jnum p e r i p h e r a l veloci ty, which o c c u r s at the turbine outside dianneter, is

only 470 f t / s e c . This r e p r e s e n t s an ex t r eme ly conservat ive design since

the typical ro to r pe r i phe ra l veloci t ies for many tu rbomach inery appl ica­

tions a r e two to four t i m e s g r e a t e r and occur at peak cycle t e m p e r a t u r e s

3-111 / ^ 3

Figure 3. 2. 3-6. T u r b o g e n e r a t o r - P u m p Unit, Rotary Rankine Cycle

that a r e cons iderably higher . This is shown in F igure 3. 2. 3-7 for var ious

t e r r e s t r i a l and a i r c ra f t appl ica t ions . Since all the rotat ing components

a r e mounted on a common shaft, this heat engine cons i s t s of only one

moving pa r t . This fea ture , when coupled with the use of se l f -energ iz ing ,

compliant-foi l , hydrodynamic gas bea r ings r e su l t s in a design with high

inherent re l iabi l i ty and long life. The c h a r a c t e r i s t i c s of the components

compr is ing the TGU, as well as the overa l l s y s t e m components (excluding

the heat sou rce ) , a r e desc r ibed below.

3-112

/kef

RO

TOR

PER

IPH

ERA

L V

EL

OC

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, F

T/?

EC

f-

CA

ND

IDA

TE

TU

RB

OP

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P-G

EN

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R

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FOR

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GA

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RO

TAR

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AN

KIN

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CY

CLE

SY

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M

CLO

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D C

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U

EU

C.M

OT

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DR

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N H

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CO

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RE

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OR

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SIN

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HR

OU

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D,

BR

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D

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AL

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AB

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RE

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PR

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TYP

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A

IRC

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ET

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NG

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TU

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S

8 g

TE

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8

8 G

AS

TU

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15

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1

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STE

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The achievement of this level of pe r fo rmance in the turbine is

contingent upon an accu ra t e a s s e s s m e n t of the var ious sy s t em p e r f o r m ­

ance degradat ion fac tors . Analyses were per formed to confirm the val id­

ity of the es t i raated loss factors and to de te rmine if sufficient design

m a r g i n s exis t for this low power level t u rbopump-gene ra to r . The th ree

a r e a s of concern were bear ing l o s s e s , t he rma l l o s s e s , and leakage l o s s e s .

These a r e t r ea t ed separa te ly in the following d iscuss ion .

Bearing Design and L o s s e s . The vapor bear ings under cons ide ra ­

tion for this design a r e a se l f -energ iz ing , compliant , multifoil , hydro -

dynamic type. These bear ings ut i l ize full fluid film lubr icat ion. In a

closed, he rme t i ca l l y sealed sys t em of the type envisioned for this appl ica­

tion, the re should be no difficulty in achieving the 10-year (87, 500 hours)

operat ing life. They a r e state of the a r t , having been used on the DC-10

Air Cycle System (53,000 rpm) , the NASA/Lewis Brayton cycle t u r b o -

a l t e r n a t o r (48,000 to 64,000 rpm) and a high speed (180,000 to 220,000

rpm) , 7 .8 watt , r e v e r s e Brayton re f r igera t ion cycle . Because of t h i s ,

the use of these type bear ings const i tu tes a low development r i sk . During

operat ion the re is no physical contact between the shaft and the bea r ings

and the lubricat ing film is mainta ined hydrodynamical ly ra ther than hyd ro -

s ta t ica l ly . The genera l a r r a n g e m e n t of these bear ings is as shown in

F igure 3 .2 . 3-8 . These a r e overlapping foil conical bear ings which a r e

capable of sustaining both radial and th rus t loads . These conical bear ings

a r e composed of s ix thin Teflon-coated sp r ing -me ta l blades or foils. The

foils a r e equally spaced within a conical bushing, and each foil extends or

wraps 135 deg rees within the bushing. Pins a r e spot-welded to the end of

the s p r i n g - s t e e l blades and inse r t ed into the slots by a p r e s s fit to re ta in

these sp r ing - s t ee l foils. A 60-degree cone angle is normal ly used. The

steel foils a r e fabr icated from 0. 0007-inch thick h e a t - t r e a t e d "Havar" , a

spr ing m a t e r i a l avai lable from the Elgin Watch Company and coated with

0 .002- inch thick Teflon using a DuPont p r o c e s s . The s teel foils a r e

formed with a punch and die.

In opera t ion, the spring m a t e r i a l s provide stiffness in the bea r ings

by res i s t ing the hydrodynamic p r e s s u r e developed by the rotat ion of the

shaft. The spr ings exer t p r e s s u r e on the shaft while it is s ta t ionary . At

s t a r t - u p , the Teflon coating functions to reduce fr ict ion. The shaft

3-114

BEARING SUPPORT HOUSING

Figure 3. 2. 3-8. Sel f -Energiz ing, Compliant, Multifoil, Hydrodynamic Type Vapor Bearing (Combined Radial and Thrust)

becomes hydrodynamical ly supported at approximately 40,000 rpm. Only

a minute vapor film thickness (perhaps a few mill ionths of an inch) is

requi red to support the shaft. Because the hydrodynamic film p r e s s u r e

c r e a t e s the c lea rance between the bear ing foils and the shaft, the n e c e s ­

sity of machining to very close to le rances is el iminated. In addition, if

an ex terna l ly applied "g" load is exper ienced, this d e c r e a s e s the operating

c lea rance on one side of the bear ing and i nc rea se s it on the opposite side.

This i n c r e a s e s the hydrodynannic p r e s s u r e on the dec reased c learance

side thus providing a se l f -genera ted res tor ing force and stable bear ing

operat ion.

An additional considera t ion in the use of these bear ings is to con­

f irm the i r sa t i s fac tory opera t ion, including the magnitude of the i r l o s s e s ,

utilizing CP-34 (thiophene) vapor . Initial e s t ima tes for these bear ings

were taken as 5%, which for a nominal tu rbogenera tor output of 9. 0 watts

would equal 0.45 watt. This is the total loss for both the radial and th rus t

bea r ings . To verify this value, an analys is was conducted. The d imen­

sions of the bear ings a r e as shown in Figure 3. 2. 3-9. The analysis was

based upon the data l is ted in Table 3. 2. 3-5 .

3-115

Figure 3 .2 . 3-9 . Dimensions for Conical Combined Radial and Thrus t Bear ings

Table 3 .2 . 3-5 . Conical Gas Bear ing Design Data

Total Weight of Rotating Elements

Bear ing Loads :

3g in th rus t d i rec t ion

2g in radial d i rec t ion

Working Fluid

• T e m p e r a t u r e

• P r e s s u r e

• Viscosi ty (at 300°F)

Shaft Rotative Speed

0.0432 lb

0. 1296 lb

0.0864 lb

CP-34 (Thiophene)

300°F

15 psia

0.4068 x 10"8 I b - s e c / i n

96,000 rpm

F o r the above condit ions, the load factor (Figure 3. 2, 3-10) was

de te rmined to be 0 ,296, and the cor responding compress ib i l i ty factor is

460, The curve shown in F igure 3. 2. 3-10 is based upon tes t data obtained

on four different s ize conical bear ings over a speed range from 12,000 to

102,000 rpm. Under these condit ions, the bear ing fi lm th i ckness , h, was

3-116 A?

A O

< o - 1

< UJ

3 0.8

g 0.7 X

II

o^^o 0.5

II

g 0.4 »-u < 0.3

O 0.2

0.1

0 /

y r

y /

> /

/

A y

y

y' y

y^ ^

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

2

COMPRESSIBILITY FACTOR, n . l U

R — X —

Pa h

Figure 3. 2. 3-10. Normal ized Load as a Function of Bearing Compress ib i l i ty Number

_5 calculated to be 2. 38 x 10 inch. The torque , T, was now de termined

using the following ana lys i s :

T = I \2TTr dr

" " l

J ijLujr _ 2 J ' , 2TTr d r

in tegrat ing and evaluating within the l imi ts ,

~ 2 h 4 4 o 1

3-117 /(P^

Now, by substi tuting the above data , the torque was de te rmined to

be 4 .05 X 10 ' ^ in - lb .

The power into the bear ings under these conditions becomes 0 .46 -4

watt (6. 16 X 10 hp) which compare s very well with the value of 0. 45

watt or iginal ly a s sumed . This value is on the conservat ive side since it

is based upon a sustained 3g acce le ra t ion level . If the normal Ig load is

taken as the s t eady-s t a t e bear ing load, the design marg in for this ca lcu­

lated bear ing loss is 1,4.

Final ly , it should be noted that this bear ing is operat ing on s u p e r ­

heated (approximately 125°F of superheat ) vapor . The power loss in the

bear ing will mainta in or slightly i n c r e a s e this superhea t condition. Hence,

no p rob lem from two-phase fluid operat ion is ant icipated.

In s u m m a r y , the pe r fo rmance e s t ima te s made for the gas bear ings

in this applicat ion a r e conserva t ive . In addition, because the technology

upon which they a r e based has been proven on e a r l i e r appl ica t ions , no

difficulties should be exper ienced in the i r implementa t ion .

T h e r m a l L o s s e s . An additional pe r fo rmance degradat ion factor

which m u s t be taken into account is the t he rma l l o s s e s in the turbine

housing. Under s t eady-s t a t e condit ions, the heat loss from the turbine

inlet s c ro l l , which is at 430°F , to the turbine outlet , which is at 240°F ,

is the mos t significant factor . At the design flow rate (for a 6. 25 w out­

put) of 0. 01 Ib /min , the i sen t ropic work avai lable is 41 .5 B tu /h r . The

t h e r m a l loss caused by conduction through the turbine housing was ca lcu­

lated to be 0. 171 B tu /h r (i. e. , 0. 05 watt) . This const i tutes a loss of only

0. 41%. The value used for this loss in confirming the overa l l turbine

efficiency was a s sumed to be 2%. Hence, a conserva t ive design m a r g i n of

five ex i s t s for the proposed tu rbopump-gene ra to r configuration.

Leakage L o s s e s . The turbine leakage l o s s e s for this design have

been es t imated at 2%. This is readi ly achieved by v i r tue of the select ion

of a shrouded, rad ia l , innpulse-type turb ine . The s ta t ic p r e s s u r e differ­

ential between the turbine blading inlet and outlet p a s s e s is negligible

since the overa l l p r e s s u r e drop in an impulse type turbine occurs in the 2

turbine inlet nozzle . A p r e s s u r e differential of 1. 125 lb / i n would be

requi red to produce a flow ra te equal to this leakage l o s s . An ana lys i s

3-118 IP

w a s c o n d u c t e d and the p r e s s u r e d i f f e r e n t i a l a c r o s s the t u r b i n e c l e a r a n c e 2

s p a c e w a s c a l c u l a t e d to be only 0 . 0 2 5 l b / i n . S ince the l e a k a g e flow is

p r o p o r t i o n a l to t h e s q u a r e roo t of the p r e s s u r e d i f f e r e n t i a l , t h i s i s e q u i v a ­

len t to a l e a k a g e l o s s of only 0. 33%. The a d d i t i o n a l l o s s i s the a m o u n t of

w o r k i n g fluid r e q u i r e d for b e a r i n g h e a t r e m o v a l . T h i s va lue w a s c a l c u ­

l a t e d to be a p p r o x i n n a t e l y 1. 7% of the t u r b i n e flow. T h u s , the 2% t u r b i n e

l e a k a g e f a c t o r e s t i m a t e d fo r t h i s d e s i g n a d e q u a t e l y a c c o u n t s for the

a n t i c i p a t e d l o s s e s in t h i s a r e a .

T u r b i n e D i s c S t r e s s e s . A n o t h e r i m p o r t a n t f a c t o r in the t u r b i n e

d e s i g n i s t he f a c t o r of s a f e t y a s s o c i a t e d with the d i s c s t r e s s e s .

T h e c u r r e n t l y e n v i s i o n e d t u r b o g e n e r a t o r for the r o t a r y v a p o r cyc le

s y s t e m o p e r a t e s at 9 6 , 0 0 0 rpnn. It u s e s a t u r b i n e w h e e l wi th an o u t s i d e

d i a m e t e r of 1. 125 i n c h e s . T h i s r e s u l t s in a t ip s p e e d of 470 f t / s e c . If a

u n i f o r m t h i c k n e s s d i s c a t an a v e r a g e t e m p e r a t u r e of 3 0 0 ° F i s c o n s e r v a ­

t i v e l y a s s u m e d , the m a x i m u m a l l o w a b l e t i p s p e e d b a s e d upon the y i e ld

s t r e n g t h of the m a t e r i a l i s 1750 f t / s e c ( F i g u r e 3. 2. 3 -11) . T h i s r e p r e s e n t s

a d e s i g n m a r g i n of 3 . 7 and i s a c o n s e r v a t i v e f i r s t a p p r o x i m a t i o n .

E x p e r i m e n t s h a v e shown, h o w e v e r , tha t a d i s c wi l l b u r s t at s p e e d s a t

wh ich the c e n t r i f u g a l f o r c e of hal f t h e d i s c i n d u c e s an e q u i v a l e n t c o n s t a n t

s t r e s s to o c c u r a c r o s s the s e c t i o n e q u a l to the u l t i m a t e t e n s i l e s t r e n g t h

of the m a t e r i a l . T h u s , in F i g u r e 3 . 2. 3 - 1 2 , if t he p o s i t i o n of the c e n t e r

of g r a v i t y of the hal f d i s c i s o b t a i n e d a t s o m e r a d i u s , r i n c h e s , and w i s

the w e i g h t in p o u n d s of the half d i s c , A i s the s q u a r e i n c h e s of ne t c r o s s -

s e c t i o n a l a r e a , and if cr , t he y i e l d s t r e s s , i s a g a i n t a k e n to be on t h e

c o n s e r v a t i v e s i d e , t h e n .

A w 2 A(r = -7:5 — w r

y 12g

o r t h e s p e e d of r o t a t i o n fo r b u r s t i n g would be

3-119

3500

400 600 800

TURBINE WHEEL TEMPEHATURE, "F

Figure 3 , 2 . 3 - 1 1 . Turbine Disc S t r e s s e s as a Function of Tip Speed and T e m p e r a t u r e

f rom which an e s t ima te of the m a x i m u m speed for any disc can be obtained.

Using this equation and assuming the use of 7075-T6 a luminum at 325°F

average t e m p e r a t u r e (see Table 3 .2 . 3-6 for yield and ul t imate tens i le

s t rength) , the e s t ima ted speed of rotat ion at burs t ing would be approxi ­

mate ly 139,000 rpm. This r e p r e s e n t s a factor of safety of 1.45 for the

design speed of 96,000 rpm. If AISI 301 s t a in less s tee l is cons idered for

the turbine d i s c , the es t imated burs t speed would be 165,000 rpm and the

design m a r g i n i n c r e a s e s to I. 72.

The above values a r e conserva t ive since a m o r e detai led s t r e s s

analys is would show even h igher fac tors of safety. The actual s t r e s s

levels and d is t r ibut ion for the turbine wheel under considera t ion in this

p r o g r a m a r e m o r e c losely r ep re sen t ed by the values shown on F i g ­

ure 3. 2. 3 -13 . The m a x i m u m average tangential s t r e s s for a wheel o p e r a ­

ting at 20% overspeed (120,000 rpm) is 11,250 ps i . F o r 6061-T6,

3-120

/7t-

Figure 3. 2. 3-12 S t r e s s Dis t r ibut ion in Rotating Disc

at an average turbine wheel t e m p e r ­

a tu re of 325°F, this r e p r e s e n t s a

factor of safety of 2. 2 based upon

yield s t rength, and 2. 75 based upon

the ul t imate tensi le s t rength. In

sunnmary, the turbine wheel s t r e s s e s

at speeds up to 120, 000 rpnn a r e fair ly

modest . Hence, operat ion of the tu r ­

bine wheel at 96, 000 rpm over a 10-

yea r period should pose no problem

with r e spec t to life or re l iabi l i ty .

Rotor Cr i t i ca l Speed and

Dynamics . The c r i t i ca l speed of

the rotor in bending may be con­

serva t ive ly es t imated by t rea t ing

the ro tor as a uniform beam with

rigid suppor ts at the bear ings

(hinged-hinged beam). The equa­

tion for c r i t i ca l speed reduces to

f = n TTd

82^

Eg-

where

f = n

d =

g

P

i

c r i t i ca l frequency, r e v / s e c

shaft d i a m e t e r , in.

2 6 shaft modulus of e las t i c i ty , l b / i n (30 x 10 for s t a in l e s s s teel)

2 gravi ta t ional constant , 386 i n / s e c

3 ro to r specific weight, l b / i n (0. 283 for steel)

length of beam or bear ing span, in.

3-121 /y^

T a b l e 3, 2. 3 - 6 . M a t e r i a l P r o p e r t y Da ta

1 Type

1 A l u m i n u m

6 0 6 1 - T 6

7 0 7 5 - T 6

7 0 7 5 - T 7 3

S t a i n l e s s S t ee l

AISI 304

AISI 301 (Sheet)

AISI 301 (Rod)

( 0 . 2 5 H a r d )

(0. 50 Hard )

AISI 303

AISI 302

AISI 431

Yield S t r e n g t h , p s i

(RT)

3 5 , 0 0 0

6 5 , 0 0 0

6 0 , 0 0 0

3 0 , 0 0 0

4 0 , 0 0 0 to 160 ,000

7 5 , 0 0 0

1 1 0 , 0 0 0

3 5 , 0 0 0

3 5 , 0 0 0

1 3 5 , 0 0 0

(3250F)

2 4 , 5 0 0

3 3 , 8 0 0

3 1 , 5 0 0

—

—

—

—

—

—

—

U l t i m a t e T e n s i l e S t r e n g t h , p s i

(RT)

4 2 , 0 0 0

7 5 , 0 0 0

7 0 , 0 0 0

8 5 , 0 0 0 to 150 ,000

105 ,000 to 185 ,000

125 ,000

150 ,000

9 0 , 0 0 0

125 ,000

1 7 5 , 0 0 0

(325°F)

3 1 , 0 0 0

3 9 , 0 0 0

3 6 , 3 0 0

—

—

—

—

—

—

Aging T e m p e r a t u r e

( ° F )

325

250

250

—

—

—

—

—

—

.AVERAGE •RADIAL STRESS -|

.5625 0.500 0.4375 0.375 0.3125 0.250 0.1875 0.1250 0.0625 RADIAL DISTANCE - INCHES

TURBINE DISC PROFILE

SHROUD

ROTATION

Figure 3. 2. 3-13. Es t ima te of Tangential and Radial S t r e s s e s in Rotary Rankine System Radial Turbine

If a min imum shaft d i a m e t e r of 0. 156 inch is conservat ively a s sume

and a bear ing span of 0. 75 inch is taken, the f irs t c r i t ica l speed of the

ro tor becomes 1,322,000 rpm. This r ep re sen t s a design marg in of 1 3. 8

on the nominal operat ing speed of 96,000 rpm. Consequently, no problem

is anticipated with respec t to operat ion nea r a shaft bending cr i t ica l speed

An ana lys i s was a lso made of the tu rbopump-genera to r gyroscopic

moment . If the shaft axis is mounted horizontal ly within the body which is

a s sumed to rotate at the ra te of 1 r a d i a n / s e c (9.55 rpm), the gyroscopic

moment is only 0.57 in- lb . If the shaft axis is mounted vert ical ly, as is

contemplated, the gyroscopic moment would become negligible. In e i ther

case, the gyroscopic moment will have negligible effect on the implanted

heat engine.

3-123 /76'

Pump

The pump to be uti l ized for this application will be of the centrifugal

type. In o rde r to develop the requ i red head of 236 psi (523 ft) a pump out­

side d i ame te r of 0. 625 inch is requi red . This is based upon the d e t e r m i ­

nation that the requ i red pe r iphe ra l velocity at the impe l le r outside d i a m ­

e t e r is given by

U, = 2gAH 2 ijj

where

U_ = pe r iphe ra l velocity at impe l l e r outside d i ame te r , f t / sec

AH = pump p r e s s u r e r i s e , ft (523 ft)

ijj = head coefficient, (0. 70 es t imated) 2

g = gravi ta t ional constant , 32. 2 f t / sec

The impe l l e r outside d i a m e t e r can then be de te rmined from the following

re la t ionships .

720 X U , d . = ^ 2 TTN

where

d» = impe l l e r outside d i a m e t e r , in.

N = pump rotat ive speed, rpm (96, 000 rpm)

The specific speed of this pump can be de te rmined by an equation s i m i l a r

to the tu rb ine , namely ,

0 5 j ^ , N(V)"-

s ^^jO. 75

where H, r e p r e s e n t s the head actual ly produced by the pump, and the o ther

p a r a m e t e r s a r e as defined in Section 3. 2. 3. 5. This gives r i s e to a specific

speed of 1.4. The es t imated pe r fo rmance for th is pump i s shown in

3-124

Figure 3. 2. 3-14. The overa l l pump efficiency is es t imated to be 40%,

The overa l l pump efficiency is the product of the hydraul ic , volumetr ic

(leakage loss) and mechanica l (disc and seal friction) efficiencies. This

is r ep resen ted by:

^ pump (overall)

hydraul ic '^volumetric 'mechanical

0. 5 X 0. 9 X 0. 9

= 4 0 . 5%

These a r e readi ly achievable values for this size pump at its design spec i ­

fic speed. The use of labyrinth sea ls at the pump inlet and both a dynamic

(noncontacting) sea l and vented labyrinth seal at the back of the pump

impe l l e r a r e envisioned for this application.

The net posi t ive suction head requi rement for this pump is a function

of the achievable suction specific speed. Using CP-34 and a pump inlet

inducer , a suction specific speed of 20,000 (on a gpm basis) should be

readi ly achievable . Under these c i r cums tances l e s s than 1 degree of sub-

cooling is requi red to mee t the net posit ive suction head requ i rements of

this pump. The condenser design for this application provides in excess of

60

Z 13 40

y 20

^

DESIGN POINT

^

H = 523 FT N =96000 N - 1 . 4

1 D =0.625

RPM

INCHES

Figure 3. 2. 3-14

CP-34 Vapor Rankine Cycle Pump Perfornnance

0.005 0.010 0.015

PUMP FLOW RATE, LB/MIN

0.020

3-125

/77

this amount of subcooling. Hence, no prob lem from pump cavitat ion

is anticipated in this design.

Genera to r

The gene ra to r proposed for this application is an i ron less p e r m a ­

nent magnet type. This design avoids the potential problems of e a r l i e r

magnet ic s ta tor pe rmanen t -magne t des igns , while providing an e l ec t r i ca l

efficiency of 98 percen t . The s ta tor windings of the i ron le s s s ta tor gen­

e r a t o r a r e s i m i l a r to those of m o r e conventional mach ines . After wind­

ing, the coils a r e hand-formed so that the windings a r e compactly located

nea r the inner d i a m e t e r of the s t a to r . Mechanical support of the s t a to r

is provided by encapsulat ion of the finished windings in a suitable non­

magne t ic , nonconductive m a t e r i a l . Epoxy compounds a r e excel lent for

these pu rposes .

The magnet ic ro tor m a t e r i a l used in th is machine is p la t inum-cobal t

alloy. The magnet is used in a hollow cyl indr ical configuration. End

pieces a r e insta l led at e i ther end of the magnet cyl inder to complete the

ro tor shaft. The end pieces may be of any adequate s t ruc tu r a l m a t e r i a l

with reasonable magnet ic pe rmeab i l i ty . When compared with o ther m a t e ­

r ia ls capable of being s t rongly magnet ized, this magnet m a t e r i a l displays

excellent mechanica l c h a r a c t e r i s t i c s . Yield s t rengths f rom 90, 000 to

170,000 psi a r e obtainable with the p la t inum-cobal t alloy.

The majo r e l ec t r i ca l l o s s e s in this gene ra to r a r e eddy cu r r en t 2

losses and I R l o s s e s . The i ron l e s s s ta tor design v i r tua l ly e l imina tes

core l o s s . In fact, the e l ec t r i ca l l o s se s a r e so sma l l , high efficiency

can be maintained over a wide load range. F o r example , at a 7. 8 watt

output the total e l ec t r i ca l loss is only 0. 233 watt , resul t ing in an e l ec t r i ca l

efficiency of 0. 971.

The gene ra to r incorpora te s a diode chip rec t i f ie r a s sembly , to

convert f rom ac to dc power , whose design is p r i m a r i l y de te rmined by

its t he rma l environment . The t e m p e r a t u r e of the diode chip i tself mus t

be kept within acceptable l im i t s . The dc voltage r equ i r emen t s affect the

rect i f ier design. Should ripple r equ i r emen t s dictate a lower level than

that obtainable from a s imple , s ing le -phase , full-wave br idge , the use

of a f i l ter o r a t h r e e - p h a s e sys t em can be considered.

3-126

/7f

The overa l l e l ec t r i ca l gene ra to r per formance and design c r i t e r i a

a r e shown in F igure 3. 2. 3-15. At an output of 6. 25 watts e l e c t r i c , the

overa l l efficiency including e l ec t r i c a l , windage, and bear ing losses is

0. 90. The gene ra to r has been designed for a nominal voltage of 24 volts

but at no significant penalty in e i ther efficiency or weight, can be

modified to opera te at any in te rmed ia te voltage level from 15 to 28 volts .

This p e r m i t s a high degree of flexibility in integrat ing the e lec t r ica l power

output with the r equ i r emen t s of the e lec t romechan ica l actuator and its

e lec t ron ic con t ro l s .

D.C. GENERATOR SPECIFICATION

(IRONLESS PERMANENT MAGNET TYPE)

2 4 6 8 ELECTRICAL OUTPUT - WATTS

10

• OUTPUT POWER

• SPEED

• OUTPUT VOLTAGE

• NO. OF POLES

• EFFICIENCY

• ESTIMATED DIMENSIONS

STATOR O.D.

STATOR LENGTH

ROTOR O.D.

ROTOR LENGTH

• ESTIMATED WEIGHT

• OUTPUT RECTIFIER

• CORE MATERIAL

• ESTIMATED COST

(IN PRODUCTION)

6.25 WATTS (NOM.)

96,000 RPM

15-28 VOLTS, D.C.

8

0.90

0.95 INCH

0.85 INCH

0.50 INCH

0.35 INCH

0.14 LB

DIODE CHIP

PLATINUM-COBALT

$25 - $50

PERFORMANCE OF PERMANENT MAGNET GENERATOR FOR

ROTARY RANKINE CYCLE

Figu re 3 .2 . 3-15. Rotary Rankine Cycle System E lec t r i ca l Genera tor Pe r fo rmance and Design Cr i t e r i a

3-127

/ 7 f

Heat Exchangers

As indicated in F igure 3. 2. 3-16, the ro ta ry vapor cycle sys tem

will use t h r e e heat exchanger s , i. e. , bo i le r , condenser , and r e g e n e r a t o r .

Both the boi ler and condenser involve two-phase flow. This i n c r e a s e s the

complexity of the ana lyses with r e g a r d to heat t r ans fe r and p r e s s u r e drop.

Fo r example , the boi ler includes a p r e h e a t e r sect ion to i n c r e a s e the

working fluid t e m p e r a t u r e to the sa tu ra t ed liquid level; a two-phase

boiling sect ion where such phenomena as slug flow and incipient,

nuc lea te , annular , fi lm, and spheroidal boiling mus t be taken into con­

s idera t ion; and a supe rhea te r sect ion to r a i s e the vapor t e m p e r a t u r e to

10° F above sa tu ra ted condit ions. S imi la r ly , the condenser mus t be

configured to handle the incoming vapor , the two-phase fluid during the

convers ion of the working fluid f rom a sa tu ra ted vapor to a sa tura ted

liquid, and, finally, to fur ther cool the liquid to 1° to 2° F of subcooling

to a s s u r e providing adequate net posi t ive suction head at the pump inlet.

By compar i son , the r e g e n e r a t o r is fa i r ly s imple to analyze since the

fluids on the liquid and vapor s ides r e m a i n in one phase during the i r flow

through this heat exchanger . The overa l l design c r i t e r i a for these heat

exchangers i s given in Table 3. 2. 3-7. A brief desc r ip t ion of these

components for the r o t a r y vapor cycle s y s t e m follows.

4.44 W, MAX. 2.81 W, AVE. 2.22 W, M I N .

4 PSIA 116°F

BLOOD PUMP

'PC 0.45/

CONDENSER

SPEED REDUCER AND CAM ACTUATOR

4 PSIA 115«'F

41 W^

ISOTOPE HEAT SOURCE

*—AAAAAAAA REGENERATOR

r - » A A A A A A A A

"I—I—r Jk i .

BOILER AND PRE-HEATER

250 PSIA 117''F

250 PSIA 430" F

PUMP •P-DC GENERATOR

DC MOTOR LOAD BUS i BATTERY

TURBINE

6.25 W (ELEC.)

4 PSIA 240''F

CHARGE CONTROL

Figure 3 .2 . 3-16. Rotary Vapor Cycle System with Regeneration and E l e c t r i c a l Power Output (RBE)

3-128 /^^

Table 3 .2 , 3-7. Heat Exchanger Design C r i t e r i a (CP34 Working Fluid)

Component

P r e h e a t e r

Boiler (10 deg superheat )

Regenera to r

• Vapor Side

• Liquid Side

Condenser

Total Weight

Length ( in.)

9 . 0

35.0

4 . 0

30.0

Diameter ( in.)

0.0625 ID

0.0625 ID

-

0.105 ID 0. 145 OD

0. 045 ID 0.085 OD

0.0625 ID

P r e s s u r e Drop (psi)

Negligible

^ 1. 0

0. 114

Weight (lb)

0.01338

0.0431

0.0149

0,06

0, 13141b. (0.0595 kg)

Boi le r . A through-f low s ingle- tube cyl indr ica l boi ler was selected

as an a t t r ac t ive design combination of low weight and smal l volume. The

tube boi ler is wrapped around a cy l indr ica l housing which is grooved in

a sp i ra l fashion. The tube is b r azed into this groove to minimize t he r ­

m a l r e s i s t a n c e . The isotope heat source and high t e m p e r a t u r e ba t tery

a r e located within this cy l indr ica l housing as shown in Figure 3. 2. 3-17.

This a r r a n g e m e n t a lso p e r m i t s the substi tut ion of an e l e c t r i c a l

source during the e a r l i e r phases of bench model conver te r test ing.

This once- through boiler design r ece ives subcooled liquid and

de l i ve r s superhea ted vapor (10 degrees ) to the turbine with one p a s s

through a continuous tube. In the se lec ted boiler design the heat t r ans fe r

m e c h a n i s m from the isotope heat sou rce to the working fluid (CP-34) is

composed of conduction f rom the heat source to the tube wall and con­

vection f rom the tube wall to the liquid. The assumpt ion was made that

the heat t r ans fe r efficiency f rom the heat source to the tube wall i s 90%

and since the pump speed i s 96, 000 r p m , the flow i s continuous.

3-129

CONDENSER RtGENEKATOK

3.25 DIA

7 ElECimCAL

CONNECTOR

7 SPEED

CONTROLLER & BATTERY CHARGE

CONTROL

HEAT SOURCE ENCAPSULANT

CENTRIFUGAL PUMP

Figure 3. 2. 3-17. Rotary Vapor Cycle Heat Engine

Because the fluid en t e r s the boiler in a subcooled s ta te , the fluid mus t

be p rehea ted to the sa tura ted liquid t e m p e r a t u r e before boiling will

commence .

Since the flow ra te is so smal l and tube s i zes mus t be p rac t i ca l ,

the fluid flow in the p rehea t e r will be in a l amina r r eg ime . A 0. 0625-

inch ID tube was selected as the nninimum prac t i ca l s ize and the p r e ­

h e a t e r length was calcula ted by use of the l a m i n a r flow equation

hD W C

k = ' - ^ M T ^ /

1/3

where

h = heat t r ans fe r coefficient

D = tube d i ame te r

k = thernaal conductivity

W = fluid flow r a t e

3-130 /S^

C - specific heat P ^

L = tube length

and the convection heat t r a n s f e r equation

Q = h A AT s

where

Q = heat t r ans fe r ra te

A = surface a r e a s

AT = t e m p e r a t u r e difference between fluid and tube wall

All units in these and subsequent formulae unless otherwise noted a rc in

pounds, feet, hours , Btu, and ' ' F .

The length of the tube was de te rmined to be 9. 0 inches which should

prove to be conserva t ive and allow sufficient length for the p r ehea t e r

since the use of other l amina r flow equations v/ould resul t in shor te r

lengths .

The requ i red length for the boiling section, again assuming a

0. 0625 inch ID was calculated by tw^o different methods to gain insight

into the degree of poss ib le differences from two approaches .

The f i rs t approach a s s u m e s nucleate boiling followed by a dry wall

boiling section that occu r s at approximate ly 70 to 80% quality.

In the nucleate boiling reg ime for the working fluid, the requi red

length can be de te rmined from

Kkfl C „ P^ _Q ^ E l i ,.rj. .1.3 ^ ' < ^ T S ^ T ( P £ - P V ) S A T >

3-131

/3^

where

K = 0. 218

^SAT " Saturat ion t e m p e r a t u r e of fluid

0" = surface tension

^ T c A T = T,„ ,, - Te . ^. = Wall superhea t SAT Wall Saturation '

.- = densi ty of working fluid

subsc r ip t s

i = liquid

V = vapo r

All other nomenc la tu re a r e as p rev ious ly desc r ibed .

In the dry wall region the length is de te rmined by use of the

Di t tus -Boel te r equation using the vapor p r o p e r t i e s :

hD <^.8 i^ = 0 .023 (Re)

where

Re = Reynolds number

\x = v iscos i ty

and the n o r m a l convection heat t r a n s f e r equation. All o ther nomenc la tu re

a r e as prev ious ly desc r ibed .

Evaluation of these formulae resu l ted in a boi ler length of

29, 0 inches .

The second method was based on J. C, Chen ' s formulat ion of an

additive m e c h a n i s m of m i c r o - and macroconvec t ive heat t r an s f e r to

r e p r e s e n t boiling heat t r ans fe r in convective flow. Considering the

macroconvect ive mechan i sm, a modification of the Di t tus -Boe l te r

equation was used to de te rmine the heat t r a n s f e r coefficient:

" m a c = 0 . 0 2 3 ( R e ^ ) ' ' - 8 ( P , ^ ) ° - * ( k , / d ) F

3-132

where

h = macroconvec t ive heat t r ans fe r coefficient mac

P r = P rand t l number

with F as the rat io of the two-phase Reynolds number to the liquid

Reynolds number and a s sumed to be a function of the Mart inel l i p a r a m ­

eter X-p-r which has been verif ied by exper imenta l data. The m i c r o -

convective coefficient was based on the F o r s t e r and Zuber analysis which

r e s u l t s in the following equation for convective boiling:

^ / • ' ^ S «-^%^ 0,49^0.25

hi^ic = Q'Q^^^^ - 0 . 5 , ""o. 29 0 .24 0 .24 ( A T ) ° ' ^ ^ A P ) ^ ' ^ ^ S " ^£ \ P^

where

h . = microconvec t ive heat t r ans fe r coefficient mic

g = acce le ra t ion caused by gravi ty

V = latent heat of vaporizat ion

where S is a suppress ion factor based on the liquid Reynolds number and

F f rom the preceding equation. All other nomencla ture a re the same as

previous ly descr ibed .

These tw^o heat t r a n s f e r coefficients a r e then additive to de te rmine

the total heat t r ans fe r coefficient,

h^. = h . + h T mic mac

Because th is heat t r a n s f e r equation was based on annular or annu la r -mis t

flow in the quality range of 1 to 70%, the length in th is range was d e t e r ­

mined by the above heat t r a n s f e r equation; in the 70 to 100% quality

range , the Di t tus -Boel te r equation was used as in the previous approach.

3 - 3 3 , ^

The total length was then de te rmined to be 23 inches. The 29- inch- length

calculated from the f i rs t approach was therefore a s sumed for the boiler

design and an additional 6 inches was added to insure achieving 10 degree

superheat , s ince co r r e l a t i on of this type can va ry as much as 20%.

The p r e s s u r e drop was de te rmined to be ^^1. 0 ps i by use of the

Mar t ine l l i and Nelson p a r a m e t e r .

Exper imenta t ion is normal ly requ i red to gather data for final design

verif icat ion. The low working fluid flow ra te for the r o t a r y vapor sys tem

may requ i re additional design ana lyses since the n o r m a l boiling formulae

may have to be modified in th is range . The final detailed design must

also consider the poss ibi l i ty of a slug flow phenomenon in the boiler

because of this low flow. However, a design to account for this type

of flow would only i n c r e a s e the weight by an insignificant amount and

should not affect the overa l l design of the sys t em.

Condenser

The condenser has been designed to re jec t heat to the sys tem

package walls where it is then re jec ted to the body fluids and t i s s u e .

The max imum allowable package wall t e m p e r a t u r e was set at 107. 6° F 2

(42° C ) with a m a x i m u m heat re ject ion surface a r e a of 0. 07 w a t t / c m .

The requ i red heat re jec t ion ra t e of the condenser is 29, 5 watts and the

requ i red heat t r an s f e r coefficient f rom the vapor to the package wall

was calculated by:

/ P i f 0 .065 > / - i i C G

Y P^ 2 p m h =

where

mean coefficient of heat t r an s f e r

liquid densi ty

vapor densi ty

3-134

h m

P£ '

P v "

f = frict ion factor

C = specific heat P

G = 0.58 of working fluid flow rate / f low a r ea m °

[1 = v iscos i ty

k = t h e r m a l conductivity

using the liquid p r o p e r t i e s of the working fluid. The condenser size was

de te rmined by use of the convective heat t r ans fe r equation,

Q = h A AT

m s

where

Q = heat t r a n s f e r r a t e A = surface a r e a s AT = t e m p e r a t u r e difference between fluid and tube walls

and a tube wall effect iveness factor of 0. 7, to be 30 inches long vnth. a

0. 065-inch ID and a 0. 020-inch wall . In o rde r to re jec t the heat

uniformly over the r equ i red package surface a r ea , the condenser tube

mus t be finned.

Regene ra to r

The r e g e n e r a t o r for th is s y s t e m is a conventional vapor- to- l iqu id

concent r ic tube counterflow design. The liquid r e c o v e r s heat f rom the

turb ine d i scha rge vapor , that would o therwise be re jec ted to the package

wall , t he reby reducing the re jec ted heat load and inc reas ing the overa l l

s y s t e m efficiency.

In th is design, the liquid f rom the pump on the TGU is brought into

a tube where it p icks up heat through the tube wall f rom the vapor which

i s flowing in the annulus of the outer tube. The sizing of the r egene ra to r

cons idered the r e q u i r e m e n t s of a low p r e s s u r e drop on the vapor side to

negate p rob l ems with the centr ifugal pump. Because of the low flow r a t e ,

the p r e s s u r e drop on the liquid side will be negligible.

'-''' /il

The design cons i s t s of a 0. 045-inch ID inner tube (fluid side) with a

0, 020 inch wall and an outer tube with an ID of 0. 105 and an OD of 0, 145

inch. The length is 4. 0 inches .

Ba t t e ry

The amount of e l ec t rochemica l energy s to rage that would be requ i red

to mee t the cu r r en t load profi le is 12.5 wa t t -hours . This r equ i r emen t is

based upon meeting the m a x i m u m average blood pump r equ i r emen t s and

i s a function of the ove ra l l e l ec t romechan ica l ac tuator efficiency. Two

additional r e q u i r e m e n t s a r e that the ba t t e ry mus t be r echa rgeab le

(secondary type) eind have a life of at leas t 3650 cycles (assuming daily

recharg ing over a 10-year per iod) . Final ly , the energy density ( w - h r / l b )

and the specific volume ( w - h r / c c ) m u s t be high enough to r ema in within

allowable weight and volume cons t r a in t s .

F a c t o r s affecting secondary ba t t e ry pe r fo rmance include d i scharge

r a t e , charging r a t e , depth of d i s c h a r g e , operat ing t e m p e r a t u r e and

charging efficiency. Other impor tan t fac tors a r e shelf life, m a t e r i a l

compatibi l i ty , e l ec t rode seal re l iab i l i ty and venting, if r equ i red .

Various ce l ls have the potent ia l for achieving the d e s i r e d p e r f o r m ­

ance . However, only t h r e e candidate b a t t e r i e s appear p romis ing for

th i s application. They a r e l i s ted in Table 3. 2. 3-8, The es t ima ted

weight and volume of t he se ce l l s as a function of energy s torage a r e

shown on F igures 2. 8-4 and 2. 8-5 . A review of these f igures

r evea l s that the nickel-cadmixim ba t t e ry would be too heavy and

voluminous to be cons idered . The two h i g h - t e m p e r a t u r e ba t t e r i e s , i.e.,

sod ium-su lphur and l i t h ium-se l en ium a r e the mos t sui table . Both these

b a t t e r i e s a r e in the development s tage; however , at the c u r r e n t ra te of

development, these ba t t e r i e s could be avai lable in 2 to 3 y e a r s . Hence,

for th is r o t a r y Rankine cycle sys t em, the sod ium-su lphur , solid

e lec t ro ly te ba t t e ry which ope ra t e s at 570° F was se lected. Because of its

operat ing t e m p e r a t u r e , it can be in te rposed between the heat source and

the 430° F peak cycle t e m p e r a t u r e bo i le r . At 12. 5 wa t t -hou r s of energy

s to rage , th is ba t t e ry will weigh approximate ly 0. 125 pound and r e q u i r e

a volume of 3, 2 in. . Both t he se va lues a r e consis tent with maintaining

the weight and volume cons t ra in t s for the overa l l t h e r m a l conve r t e r .

3-136

Table 3. 2, 3-8. Candidate Secondary Batteries

Nickel-C admium

Sodixim Sulphur (570° F)

Li th ium- Selenium (710° F)

• Safe Sealed Operat ion

• Cycle Life at Leas t 3650 Cycl

• High Energy Density

• Good Depth of Di

• High Recharging

e s

scharge Cha rac t e r i s t i c

Rate and Effi Lciency

• High Power Density (20 to 50 wat t s / lb )

Energy Depth of Density Discharge

(w-hr / lb ) (%)

8 (100° F) 25

100 100

50 100

Relat ive Weight

33

0,68

1.36

Recharging

83% eff.

5-8 hours

90% eff.

100% eff. 1-3 hours

Power Density

(watts / lb)

>50

150

- 9 0

Development

Available

2-3 y e a r s

lO.OOOi TOTAL DECOMPOSITION 20 LESS THAN 0.5%

(EXTRAPOLATED fOR RANKINE CYCLE) FROM MONSANTO C O .

TEST DATA-

TIME I N YEARS

BULK THERMAL STABILITY TESIS

(FLUID REMAINS CONSTANTLY AT PEAK CYCLE

TEMPERATURE

400 SOG 600

PEAK CYCLE TEMPERATURE, °F

Figure 3, 2, 3-18, Tes t Data on T h e r m a l Decomposi t ion of CP34 as a Function of P e a k Cycle T e m p e r a t u r e

Working Fluid Stability

The two fac tors that a r e

pa ramount in a s se s s ing working

fluid stabil i ty a r e pyrolyt ic ( thermal)

and radiolyt ic (radiation) induced

decomposi t ion. In the case of

pyroly t ic decomposi t ion, the s e l e c ­

tion of a compara t ive ly low peak

cycle t e m p e r a t u r e of 430° F a s s u r e s

that no prob lem ex is t s in this a r e a .

As was indicated in F igure 3, 2. 3 - 1 ,

the total decomposi t ion to be

expected in 100, 000 hours (g r ea t e r

than 10 yea r s ) i s 0, 05%. This i s

fur ther confirmed by the t e s t data

shown on F igure 3, 2. 3-18, which

indica tes that tota l t h e r m a l decom­

posi t ion would be l e s s than 0. 5% at

the end of 20 y e a r s . Hence, it i s evident that t h e r m a l decomposi t ion will

not be a p rob l em in th is applicat ion.

The second factor to be cons idered i s that of radiolyt ic decompos i ­

tion. The working fluid being uti l ized for the candidate ro t a ry vapor

cycle i s C P - 3 4 (Thiophene), a product of the Monsanto Co, Its major

p r o p e r t i e s a r e :

Chemica l fo rmula

Molecular v/eight (M)

Gas constant (R)

Ratio of specific hea ts (k)

C . H . S 4 4

84, 13

18,4 f t / °R

1. 11

Specific heat at constant p r e s s u r e 0, 345 Btu / lb - " F (at 400° F) 0. 236 Btu / lb -° F (at 150° F)

To e s t ima te the design m a r g i n for radiolyt ic decomposi t ion it i s

f i r s t n e c e s s a r y to de te rmine the in t eg ra ted neut ron flux (total fluence)

at the 1-Mev level and the total cumulat ive dose of gamma i r rad ia t ion .

3-138 /90

F o r this application, the heat source is medical grade plutonium-2 38

oxide. In addition, the t h e r m a l energy of the heat source may vary

f rom 37 to 60 watts For th is evaluation, the higher value was

a s sumed to be conse rva t ive . F r o m layouts of the vapor Rankine cycle

sys t em, the d is tance f rom the center of the heat source to the fluid in

the boi ler tubes was de te rmined to be 2, 3 cm. Under these conditions

and assuming no attenuation f rom the heat source encapsulant , the

insulat ion, euid the high t e m p e r a t u r e (570° F) ba t tery , the following

neut ron eind g a m m a , 10-year doses w e r e de termined:

Gamma i r r ad ia t ion dose = 2, 75 x 10 •^^•S-^(C) = 2, 75 x 10 rads gm

12 Neutron i r r ad ia t ion dose = 2, 02 x 10 nvt

The th reshold values for organic fluids have been defined as those

va lues where the densi ty, v iscos i ty , and carbon-hydrogen ratio i n c r e a s e s

a r e of such magnitude that the fluid becomes unsuitable as a heat

t r a n s f e r medium. While specific data on thiophene a re cur ren t ly being

developed at Bat te l le Memor i a l Inst i tute, a good compar ison can be made

at th is t ime with four s imi l a r organic fluids for which data a r e available.

These a r e shown in Table 3. 2. 3-9,

Table 3, 2. 3-9. Damage Thresholds for Organic Fluids

Organic Fluid

Monoisopropylbiphenyl

Terphenyls

Biphenyl

Dowtherm "A"

Neutron I r rad ia t ion (nvt)

1 x 1 0 ^ 8

I x l O ^ S

I x l O ^ S

I x l O ^ S

G amma I r radia t ion

r^g« c)

2 X 10^^

4 x 1 0 ^ ^

1 . 4 x 1 0 ^ ^

> 1 X 10^^

3-139

/f/

F r o m these data, a design m a r g i n equivalent of s ix o r d e r s of

magnitude ex is t s for both neut ron and g a m m a i r rad ia t ion , so that

radiolyt ic decomposit ion, as it would affect the thiophene viscosi ty,

densi ty, and carbon-hydrogen ra t io , would be negligible.

The formation of noncondensable gas i s another potential a r e a of 18 concern . However, at exposures as high as 1 x 1 0 nvt, the G (gas)

values (molecules of gas pe r 100 ev of absorbed energy; a r e quite low

for monoisopropylbiphenyl and biphenyl. Fo r example , the avai lable - 3 - 4

G-values for biphenyl a r e 6. 5 x 10 for hydrogen eind 4 x 10 for

acetylene. These a r e 1 to 2 o r d e r s of magnitude l e s s than the values

obtained for alkyldiphenyl (6.2 - 9.5 x 10 ) where gas evolution amounted

to l e s s than 1 c c / g r a m for an i r r ad ia t ion dose of 3. 7 - 4.1 x 10

e r g / g m (C), Since the es t ima ted dose for th is application is four o r d e r s

of magnitude l e s s as well , the gas evolution should also be negligible.

In sunnmary, the neut ron and g a m m a i r r ad i a t i on total doses a re

quite low. In addition, based upon the data available for th reshold

values and G-values for s i m i l a r organic fluids, it appea r s that the

design m a r g i n s for the radiolyt ic decomposi t ion of thiophene a r e m o r e

than adequate.

Turbogenera to r Speed Control

The tu rbogenera tor speed control c i rcu i t is s imple . It is ac t ivated

by the output voltage r i s ing above a ce r t a in set point. Refer r ing to

Figure 3. 2. 3-19, the c i rcu i t is seen to cons is t of th ree major e l ement s ;

a f i l ter , a compara to r with h y s t e r e s i s , and a power switch to tu rn on an

additional pa ras i t i c load.

The fi l ter r emoves any l ine spikes which m a y inadver tent ly be

mis taken by the compara to r for an input voltage change. The input

voltage i s then compared with a r e fe rence voltage, by the compara to r

r e fe rence , and the s ta te of the compara to r changes when the input

exceeds the re fe rence voltage.

In o rde r to reduce unwanted cycling, voltage h y s t e r e s i s i s provided

in the compara to r . The output of the compara to r ac tua tes a t r a n s i s t o r

switch, turning on an additional ex te rna l load. When the load i s added,

3-140

TURBO-GENERATOR

i?4 VDC

OUTPUT

< PARASITIC ? LOAD

REFERENC, VOLTAGE I

POWER SWITCH

Figure 3. 2, 3-19, Turbogenera tor Speed Control

the tu rbogenera to r speed reduces which reduces the output voltage. When

the output falls below the set point, minus h y s t e r e s i s , the switch tu rns

off. This p roduces a s teady dc output voltage with a slight t r iangular

wave on top of the dc. The annplitude of the t r i angu la r wave will be ±2%

and the per iod will depend upon the load applied. This method of control

i s ve ry re l iab le and s imple ; yet it provides the requi red per formance .

The load will be configured and located so as to c a r r y away the

heat energy genera ted without excess ive t e m p e r a t u r e r i s e . The control

will be physical ly placed in the dc m o t o r - a c t u a t o r housing of the

e l ec t romechan ica l ac tuator .

Rotary Vapor Engine T h e r m a l Map

An impor tan t factor in the design of a smal l tu rbopump-genera to r

is the resul t ing t h e r m a l g rad ien t s . With the turbine inlet at 430° F and

the pump inlet at 115°F, adequate t h e r m a l b a r r i e r and cooling techniques

mus t be provided to a s s u r e that the s t eady-s t a t e t e m p e r a t u r e s a re

compatible with the operat ing c h a r a c t e r i s t i c s of each region of the TGU,

Accordingly, a p r e l i m i n a r y t h e r m a l map of the TGU was made . The

r e s u l t s of th is ana lys i s a r e shown on F igure 3. 2, 3-20. In establishing

th i s t h e r m a l map , it was a s sumed that the housing in the region of the

gene ra to r would be mainta ined at 300° F by the insulat ion design. It was

also es tabl ished that the turbine end bear ing would be lubr icated and

3-141

/f^

Figure 3, 2, 3-20. Rota ry Vapor Cycle T h e r m a l Map

cooled by 300° F vapor , bled through a flow control nozzle f rom the

turbine inlet . This bear ing coolant then flows to the pump-end bear ing

and is finally d i rec ted through port ing in the housing to the gas side

of the r e g e n e r a t o r . In th is way, the heat absorbed in the bear ings can

be regenera t ive ly r e tu rned to the pump d ischarge liquid, thus improving

overa l l sy s t em efficiency. The genera to r s ta tor windings and housing

a r e maintained at 300° F by utilizing the pump d i scha rge flow as a

coolant. After removing hea t f rom the gene ra to r , t h i s flow i s then

d i rec ted to the liquid side of the r e g e n e r a t o r .

F r o m the t e m p e r a t u r e s indicated in F igure 3, 2, 3-20, t h e r e a r e

no r e g i m e s where the resul t ing t e m p e r a t u r e g rad ien t s will c r ea t e any

heat t r an s f e r or differential expansion p r o b l e m s , in the final des ign

configuration, additional t h e r m a l b a r r i e r s can be incorpora ted (i, e, , a

cen t ra l hole in the shaft and grooves in the s ta tor housing) if they a r e found

n e c e s s a r y .

'"'' m

3, 2, 3, 7 Candidate Rotary Rankine Sys tem

As a r e su l t of the ana lyses conducted, the r o t a r y vapor sys tem

using C P - 3 4 as a working fluid has been selected as the candidate sys tem

for th is c lass of heat engines . The cycle conditions a r e as shown in

Table 3. 2. 3 -3 . The main e l emen t s compris ing this sys t em a re shown

on F igure 3. 2, 3 -21 . The s y s t e m includes the combined rotating unit

( turbine, punnp, dc gene ra to r ) , t h r e e heat exchangers (boiler, condenser ,

r egene ra to r ) , a solid e lec t ro ly te , e levated t e m p e r a t u r e (570° F) ba t t e ry

and charge control unit, an isotope heat source (Pu 0^ ), the n e c e s s a r

insulat ion (Min-K/xenon) , and solid s ta te e lec t ron ics for TGU speed

control and ba t t e ry charge control . The overa l l sys t em is capable of

producing a 9. 6 wat ts e l ec t r i c a l output when combined with a 60-watt

t h e r m a l heat source for an overa l l s y s t e m efficiency of 16. 0%. However,

min imiza t ion of the heat source t h e r m a l r equ i r emen t s was an impor tant

objective of the p r o g r a m . Hence, it was de te rmined that by providing

a 12. 5 wa t t -h r ba t t e ry and d i rec t ing this sys t em design toward providing

2, 81 watts (average) to the blood pump through a 45% efficient dc mo to r -

d r iven mechan ica l ac tua tor , that the output from the tu rbogenera to r

need be only 6, 25 wat ts , This pe rmi t t ed a reduction in the heat source

size to 41 wat t s t h e r m a l (which s t i l l includes a 10% allowance for heat

losses ) and i s a lso compat ible with the use of Min-K insulat ion.

The overa l l a r r a n g e m e n t of the heat engine for th is sys tem is

shown in F igure 3. 2, 3-22. The components have been a r ranged in a

manne r to r e su l t in m i n i m u m volume and insulat ion r equ i r emen t s . In

addition, t h e r m a l in te r faces have been a r r anged to reduce overa l l heat

l o s s e s . F o r example , the high t e m p e r a t u r e ba t te ry has been designed

as a cy l indr ica l r ecep t ac l e to enclose a high percen tage of the heat

source surface a r e a .

F u r t h e r m o r e , s ince the peak cycle t e m p e r a t u r e for the ro t a ry

vapor cycle working fluid is 430° F , th is pe rmi t t ed a boi ler design

which coxild su r round and be in tegra ted with the 570° F ba t te ry . Another

feature that has been employed is to locate the high t e m p e r a t u r e turbine

and the r e g e n e r a t o r adjacent to the heat source , fur ther minimizing

s y s t e m heat l o s s e s . F inal ly , the condenser , which is operat ing in the

3-143

4.44 W, MAX 2.81 W, AVE. 2.22 W, MIN.,

'PC 0.45 <

BLOOD PUMP

SPEED REDUCER AND CAM ACTUATOR

DC MOTOR

4 PSIA 116»F

4 PSIA 115°F

LOAD BUS

CONDENSER « ^ A A A A A A A • REGENERATOR

r-^AAAAAAAA

41 W.

ISOTOPE HEAT SOURCE

T 1—r • • 4-

BOILER AND PRE-HEATER

250 PSIA 117°F

250 PSIA 430°F

PUMP • ^

DC GENERATOR

rt BATTERY

— TURBINE

6.25 W (ELEC.)

4 PSIA 240»F

CHARGE CONTROL

Figure 3. 2. 3-21. Rotary Vapor Cycle System with Regeneration and Electrical Power Output

CONDtNSER

KOIIER I

VEHTICAl — AXIS

3.25 DIA

ELECTRICAL CONNECTOR

REGENERATOR

INSULATION (MIN-KyXENON)

4 i^//////A

SECONDARY •ATTERY

(SODIUM-SULFUIt) HEAT SOURCE

ENCAPSULANT

SPEED CONTROLLER

ft SATTEDY CHARGE CONTROL

CENTRIFUGAL PUMP

Figure 3. 2. 3-22. Rotary Vapor Cycle Heat Engine

3-144 /f^

v/^

115° to 117°F t e m p e r a t u r e range , has been mounted on the inner surface

of the engine conta iner . This p e r m i t s heat re jec t ion d i rec t ly to the body

fluids and t i s sue , e l iminat ing the need for a sepa ra t e heat reject ion fluid.

The heat source is a vented capsule as descr ibed in Section 2.6.

The envelope d imensions of this sys t em a r e 3.25 inches in

d iamete r by 6 inches long. This r e su l t s in a total volume of 49 cubic

inches (0, 804 l i t e r s ) . The to ta l weight of th is r o t a r y Rankine heat engine

is 1, 91 pounds or 0, 868 kg. (See Table 3, 2. 3-10 for Weight S u m m a r y . )

The output f rom the gene ra to r will be 24 vdc power. This e l ec t r i ca l

ene rgy will not only be used to provide power to a dc m o t o r - d r i v e n

mechan ica l ac tua tor , but will a lso provide energy for recharg ing the

ba t te ry and for the sy s t em e lec t ron ic control c i r cu i t ry . The overa l l

pe r fo rmance for th is sy s t em and a 60-w sys tem a r e shown on

Table 3, 2, 3 -11 , A detai led d i scuss ion ot the c h a r a c t e r i s t i c s of the

components that c o m p r i s e th i s ccindidate heat engine configuration was

given in Section 3, 2, 3. 6,

3-145

Table 3, 2, 3-10, Rota ry Vapor Cycle Sys tem - Weight Summary

Combined Rotating Unit

Pump and Scrol l

Turbine and Housing

Al te rna to r and Housing

Shaft

Heat Exchangers

P r e h e a t e r

Boi ler (10 deg superheat )

Regenera to r

Condenser

Subtotal

Heat Source (41 watts .)

Ba t te ry

Min-K Insulation

Elec t ron ic Controls

Misce l laneous Tubing

Container

Subtotal

Total

0.0362

0,1775

0,2385

0 ,0183

0,0134

0,0431

0,0149

0,0600

0,2270

0,1250

0.4260

0,3600

0,0400

0.1270

0, 6019 lb; 0 ,273 kg

1,3050 lb; 0 ,595 kg

1,9069 lb; 0 ,868 kg

3-146 / / ^

Table 3. 2. 3 -11 . Rotary Vapor Cycle System Pe r fo rmance Summary

Isotope The rma l Power (watts )

E l ec t r i c a l Power Output (watts )

Overa l l Heat Engine Efficiency (%)

Peak Cycle T e m p e r a t u r e (° F)

Peak Cycle P r e s s u r e (psia)

Condensing T e m p e r a t u r e (° F)

Condensing P r e s s u r e (psia)

Maximum Heat Rejected to Body (watts )

Total Weight (lb)

(kg)

Total Volume ( l i ters)

Specific Gravi ty

41

6. 25

15.7

430

250

116

4

34.75

1.91

0.868

0.778

1. 11

60

9 . 6

16.0

430

250

116

4

49 .4

2, 25

1. 02

0. 900

1. 13

3-147

3 . 2 . 4 The rmoelec t r i e s

3 . 2 . 4 . 1 His tor ica l Background

The t h e r m o e l e c t r i c engine nnakes use of a t he rmoe lec t r i c conver te r

to t r ans fo rm heat genera ted by the radioisotope heat source d i rec t ly into

e lec t r ic i ty . The bas ic concept of the t h e r m o e l e c t r i c conver te r has been in u

since the ear ly 1800's . The fabr icat ion of devices producing usable leve ls

of power at r easonab le efficiency became prac t icab le with the availabil i ty

of semiconductor nmaterials and the development of solid state physics

theory in the ea r ly 1950's . Since that per iod, extensive development by

the AEC has resu l ted in the u s e of t h e r m o e l e c t r i c c onve r t e r s for a var ie ty

of applicat ions in space, unde rwa te r , and ca rd iac p a c e m a k e r s where long

life and high re l iabi l i ty under unattended operat ing conditions is manda to ry .

A par t i a l l i s t of uni ts developed under the A E C ' s SNAP p r o g r a m is

given in Table 3 . 2 . 4 - 1. The power level of these uni ts ranges from

mil l iwat ts to seve ra l hundred wat t s . SNAP-3 , a 3-watt unit , has been

operat ing in space for over a decade .

Efficiency of the SNAP units has inc reased significantly. The

e a r l i e s t uni ts using PbTe had efficiencies in the range of 5%. La te r un i t s ,

such as SNAP-21 and SNAP-23, use segmented couples and achieve an

efficiency of 8 to 9%. The recen t development of r e f r ac to ry me ta l hea t

sources with a t e m p e r a t u r e capabili ty of ^ 1000°C allows the cons idera t ion

of cascading which leads to efficiencies in exces s of 12%.

3 . 2 . 4 . 2 Ma te r i a l s Selection

Thermodynamic ally, the t h e r m o e l e c t r i c conver te r may be cons idered

as a heat engine with i ts max imum efficiency l imited to that obtained with

an ideal Carnot engine. Three bas ic p r o p e r t i e s of a t h e r m o e l e c t r i c

m a t e r i a l a r e used in determining the pe r fo rmance of a t h e r m o e l e c t r i c

conver te r ; these a r e the Seebeck coefficient {a), the e l ec t r i ca l r e s i s t iv i ty

(p) and t h e r m a l conductivity (k). The figure of m e r i t for a t h e r m o e l e c t r i c 2

m a t e r i a l (Z) is defined as a /p K , and the theore t i ca l efficiency of a t h e r m o e l e c t r i c m a t e r i a l can be approximated by

T T 7 T X ^ ~ ^ ^ ^ H ' ' T „

ri

3-148

T a b l e 3 . 2 . 4 - 1 . R T G D e v e l o p m e n t H i s t o r y

I

S y s t e m

S N A P - 3

S N A P - 7 ( A , C , E ) ( B , D , F )

S N A P - 9

S N A P - lOA

S N A P - 1 5 A B

S N A P - 1 9

S N A P - 2 1 A B

S N A P - 2 3

S N A P - 2 7

TRANSIT

S y s t e m C o n t r a c t o r

M a r t i n

M a r t i n M a r t i n

M a r t i n

AI

GGA NUMEC

M a r t i n

M a r t i n 3M

W e s t i n g h o u s e

GE

TRW

T / E M a n u f a c t u r e r

3M

M a r t i n M a r t i n

3M

RCA

GGA NUMEC

3 M / M a r t i n

M a r t i n 3M

3M

3M

GGA

T / E M a t e r i a l

P b T e

P b T e P b T e

P b T e

SiGe

B i T e M e t a l l i c s

P b T e

P b T e P b T e

P b T e

P b T e

P b T e

P o w e r L e v e l

3w

lOw 60w

25w

5 00w

1-lOmw 1-lOmw

40w

lOw lOw

5 0w

60w

30w

App l i ca t i on

Space

M a r i n e M a r i n e

Space

Space R e a c t o r

C l a s s i f i e d C l a s s i f i e d

N i m b u s / P i o n e e r

M a r i n e M a r i n e

M a r i n e

A L S E P

Space

where TTT is the hot side t e m p e r a t u r e , T^ is the cold side t e m p e r a t u r e

and

T T ^H - C

H

is the Carnot efficiency. The overa l l efficiency of a t he rmoe lec t r i c

conver te r can be defined as Eff^.^ = rjc^^HM^^T w^here r) , is the Carnot

efficiency, r | j^ i s the theore t ica l naater ial efficiency (a function of Z only)

and HT r e p r e s e n t s the t he rma l and e l ec t r i ca l l o s s e s associa ted with the

fabricat ion of a p rac t ica l device . F r o m the above equation it i s c lea r that

the highest efficiency is obtained by operat ing the device over a l a rge

t e m p e r a t u r e range to obtain the max imum available Carnot efficiency.

For the p re sen t application the hot side t e m p e r a t u r e is l imited to approxi­

mately 1000°C by heat source cons idera t ions and the cold side t e m p e r a t u r e

is fixed at approximately 40°C by physiological cons ide ra t ions . As shown

in the cu rves of F igures 3 . 2 . 4 - 1 and 3 . 2 . 4 - 2 , the re is no single m a t e r i a l

which has a high figure of m e r i t over the en t i re t e m p e r a t u r e range;

the re fo re , to achieve a highly-efficient device , it is n e c e s s a r y to comibine

the m a t e r i a l s in a manner which allows the m a t e r i a l to be opera ted over

the t e m p e r a t u r e range in which

it exhibits i ts max imum efficiency.

Ma te r i a l s may be combined in

two genera l ways , by cascading or

by segment ing.

In the cascading method, the

t h e r m o e l e c t r i c m a t e r i a l s a r e

a r r anged so the elennent size can

be independent for each m a t e r i a l

u sed . Each e lement of one m a t e r i a l

is not neces sa r i l y the rma l ly or

e lec t r i ca l ly in s e r i e s with a single

e lement of the o ther m a t e r i a l .

The e lements of each stage a r e

e lec t r i ca l ly independent of those

in the next stage and the number

of e lements of one stage may

N - MATERIALS

3.0

2.0

5 o

200 400 600 800 TEMPERATURE - °C

Figure 3. 2. 4- 1. F igure of Mer i t Ve r sus T e m p e r a t u r e , N-Mate r i a l s

3-150 ^ ^ 2 -

P - MATERIALS

3.0

5 U. o

^ ISbTe

\i V

( \ AgSt

TAGS

Te

\ T E G S

SiGe

\

TPM-2I7

^ \ >.

200 400 600 800 TEMPERATURE - °C

1000

Figu re 3 . 2 . 4 - 2 . F igu re of Mer i t V e r s u s T e m p e r a t u r e , P - M a t e r i a l s

differ from that of the other s tage .

In the segmenting method

the m a t e r i a l s a r e joined in such

a manner that a single elen:ient

cons is t s of both nnater ia l s . Thus

the number of e lements is identical

in both s tages and each stage of

one m a t e r i a l is thermal ly and

e lec t r ica l ly in s e r i e s with the

corresponding stage of the other

m a t e r i a l .

The choice of segmenting ve r sus

cascading to obtain max imum effi­

ciency depends on the p rope r t i e s

of the m a t e r i a l s . When the

m a t e r i a l s have sinnilar p r o p e r t i e s ,

the segmenting nnethod is

advantageous because it e l imina tes the need for in te r s tage e lec t r i ca l

connections and m i n i m i z e s t h e r m a l and e lec t r i ca l l o s s e s . If the m a t e r i a l s

have widely differing p r o p e r t i e s , the r equ i rement s for maximum efficiency,

opt imum in t e rmed ia t e t e m p e r a t u r e , and opt imum cu r r en t cannot be met

by segment ing because the only p a r a m e t e r which can be var ied is the geometry

of the element . The use of cascading allows an adjustment of both the geometry

and number of couples to yield the opt imum c u r r e n t for each s tage .

Analyses using p resen t ly avai lable m a t e r i a l s have shown that the

maiximvuTi efficiency over the avai lable t e m p e r a t u r e range is obtained by

cascading SiGe with a segmented second stage using the te l lur ide family

of m a t e r i a l s . A schemat ic of th is conve r t e r sys tem is shown in

F i g u r e 3. 2 . 4 - 3 . The ana lys is indica tes that the efficiency of the cascaded

sys t em is 10 to 15% g r e a t e r than that obtained when the two s tages a r e

combined by segment ing. F o r the cold s tage, because the p rope r t i e s of

the m a t e r i a l s a r e quite s i m i l a r , cascading of the t e l lu r ide m a t e r i a l s offers

no significant advantage in efficiency and segmenting m i n i m i z e s fabricat ion

p r o b l e m s .

3-151

HEAT IN

I I I 1 SiGe STAGE

INTERSTAGE MATERIAL

SEGMENTED TELLURIDE STAGE.

N N N

N N N N

1 1 5 1 HEAT REJECTION

F i g u r e 3 . 2 . 4 - 3 . Typical Cascaded/Segmented T h e r m o e l e c t r i c Conver te r

In the p r e sen t design, the SiGe hot junction is opers.ted at 950 C,

which is enough below its m a x i m u m opera t ing tenmperature of 1100 C to

a s s u r e low degradat ion r a t e s . An in te r s t age t e m p e r a t u r e of 500 C was

chosen to be compat ible with the t e m p e r a t u r e capabil i ty of the te l lur ide

m a t e r i a l s .

S\immary of Candidate Ma te r i a l s

When the figure of m e r i t is plotted against t e m p e r a t u r e , the re la t ive

pe r fo rmance of var ious m a t e r i a l s can be compared and in t e r segmen t

t e m p e r a t u r e s can be chosen. Such plots a r e p resen ted in F i g u r e s 3. 2 . 4 - 1

and 3 . 2 . 4 - 2 where Z is plotted against t e m p e r a t u r e for the N-type and

P- type t h e r m o e l e c t r i c m a t e r i a l s p resen t ly under cons idera t ion . The

inherent ly low m a t e r i a l efficiency of SiGe, re la t ive to the t e l lu r ide

m a t e r i a l s , can be seen to be caused by i t s re la t ive ly low figure of m e r i t .

The advantage in cascading SiGe and t e l lu r ide m a t e r i a l s i s a lso apparent

s ince t e l l u r ides cannot ope ra t e above ~ 600°C and SiGe efficiencies a r e

much lower than te l lu r ide efficiencies below about 600°C.

A pronnising high t e m p e r a t u r e t h e r m o e l e c t r i c m a t e r i a l which

p o s s e s s e s a g r e a t e r figure of m e r i t than SiGe, and is capable of operat ing

3-152

at equivalent t e m p e r a t u r e s , has been developed by Minnesota Mining and

Manufacturing Co. for the AEC. This P- type m a t e r i a l , TPM-217 (see

F igure 3 . 2 . 4 - 2 ) , p o s s e s s e s unusual t h e r m o e l e c t r i c p rope r t i e s which have

made the development of an N-type m a t e r i a l unsuccessful to date . Calcu­

lat ions have shown that if an N-type m a t e r i a l can be developed, overa l l

RTG s y s t e m s efficiencies as high as 16% a re possible for a cascaded

T P M - 2 1 7 / t e l l u r i d e g e n e r a t o r .

Segmented Couple Design (Cold Stage)

The des ign and opt imizat ion of segmented couples r equ i r e s analyt ical

techniques which will pe r form a heat balance at each ma te r i a l junction

and opt imize the re la t ive couple s i zes to yield peak eff iciencies. A

computer p r o g r a m was wr i t ten for the TRW Timeshar ing System to perfornn

the i te ra t ive calcula t ions requ i red to optinnize the segmented couple s i ze s .

Six t h e r m o e l e c t r i c m a t e r i a l s w e r e considered for compara t ive

evaluat ions , t h ree N-type and th ree P - t y p e . The m a t e r i a l s considered

were :

P -Type N-Type

• Bismuth Antimony • 2N Lead Tel lur ide (TEGS) Te l lu r ide

• 3N Lead Tel lur ide (TEGS) • TAGS 85

• Lead Te l l u r i de /Ge rman ium • Si lver Antimony Tel lur ide

Te l lu r ide

Junct ion t e m p e r a t u r e s w e r e chosen by using F i g u r e s 3 . 2 . 4 - 1 and 3 . 2 . 4 - 2

and operat ing at the t e m p e r a t u r e at which the f igures of m e r i t of the

segmented m a t e r i a l s c r o s s e d . Fo r ins tance , the t e m p e r a t u r e at the TAGS

and AgSbTe junction would be 265 °C.

Table 3 . 2 . 4 - 2 is a s u m m a r y of the couple efficiencies obtained by

segnnenting va r ious m a t e r i a l combina t ions . The r e s u l t s of the evaluation

indicate that

• P b T e - G e T e / 2 N - TAGS/BiSbTe is the mos t efficient couple combination cons ide red .

• Adding a AgSbTe segment to the TAGS/BiSbTe p-leg r educes the couple efficiency.

• The unsegmented PbTe-GeTe/TAGS couple yields good efficiency.

3-153

Table 3 . 2 . 4 - 2 . Mate r i a l Efficiency for Optimized Segmented Couples T-. = 500°C, T = 50°C H c

p

M A T E R 1

A L S

TAGS/AgSbTe/BiSbTe

TAGS/BiSbTe

TAGS

N - MATERIALS

PbTe-GeTe/2N

11.99

12.30

11.98

3 N / 2 N

11.18

11.41

11.09

PbTe-GeTe

11.73

12.02

11.70

The addition of a AgSbTe segment to the TAGS/BiSbTe P- leg r educes the

couple efficiency because of the var ia t ion in t h e r m o e l e c t r i c p r o p e r t i e s

between AgSbTe and TAGS or BiSbTe. TAGS and BiSbTe have a, p, and k

p rope r t i e s re la t ive ly close to each o ther ; however, AgSbTe p roper t i e s

a r e much different . This difference in t h e r m o e l e c t r i c p rope r t i e s r e s u l t s

in a "loading down" effect when AgSbTe is added to the leg. This "loading

down" effect o v e r r i d e s the figure of m e r i t i n c r e a s e shown in F igure

3 . 2 . 4 - 2 .

Based on the cons idera t ions d i scussed above a hot stage using SiGe N

and P e l emen t s and a cold stage using segmented P b T e - G e T e / 2 N N e lements

andsegnaented TAGS/BiSbTe P e lements w e r e selected for the conceptual

des ign .

The P b T e - G e T e m a t e r i a l has been well cha rac t e r i zed and a l imited

number of devices using the m a t e r i a l have been built; however , l i t t le life

data a r e avai lable . Both the TAGS and BiSbTe m a t e r i a l s have been used

successful ly and long - t e rm data a r e avai lable for these m a t e r i a l s as

individual e l e m e n t s . Although theore t i ca l cons idera t ions indicate that

the re should be no problem in segmenting the m a t e r i a l , no tes t data a r e

ava i lab le . Should difficulties be encountered with the cold stage m a t e r i a l s ,

a m o r e conserva t ive approach using 3N/2N N m a t e r i a l with TAGS P

m a t e r i a l can be taken; the absolute d e c r e a s e in efficiency using this combin

ation is approximate ly 1%.

3-154

Cascading Design Cons idera t ions

When two or m o r e t h e r m o e l e c t r i c m a t e r i a l s or cascaded s tages a re

connected e lec t r i ca l ly in s e r i e s the s tages must be e lec t r ica l ly "matched'

so that one stage does not "load down" the other s tage(s) . Matching can

genera l ly be performed in a t h e r m o e l e c t r i c genera to r by varying the size

and number of t he rmoe lec t r i c couples . Per fec t matching is achieved in

a s e r i e s c i rcu i t by having the rat io (VQJ- /R^) equal for both s tages , how­

ever ana lys is has shown that v i r tua l peak power may be obtained when

(T ) , / ( * I • " For the conver t e r under considerat ion, design of the s tages for

individual loads r e su l t s in

I ^i )seg/\ ^i )siGe ~

and only 70% of peak power when the s tages a r e combined. When the

couple s i zes and number of couples w e r e a l t e red so that

\ 1 'Seg/ \ 1 /SiGe

it was found that peak power was achieved. The resul t ing couple s izes

a r e summar ized in Section 3 . 2 . 4 . 4 .

Superinsulat ion Ver sus F ib rous Min-K Insulation

The choice of a t he rma l insulat ion sys tem for a cascaded RTG for

an ar t i f ic ia l hea r t power source is not s t ra ightforward. Although the

use of super insula t ion reduces the t he rma l shunt l o s s e s , there a r e sever

potential d isadvantages . A s u m m a r y of the m e r i t s of each sys tem is

p resen ted in Table 3.2.4-3.

One in teres t ing tradeoff involves the use of super insulat ion ve r sus

f ibrous insulat ion in the te l lur ide stage of the conve r t e r . Because of the

i nc r ea sed subl imat ion ra t e of t e l lu r ide t h e r m o e l e c t r i c s in vacuum, it is

a s sumed that , if super insula t ion were used in this s tage, the in ters tage

t e m p e r a t u r e of the conver te r mus t not exceed 400°C. However, the use

3-155

Table 3. 2. 4 - 3 . Summary of Insulation Mer i t s

Supe r insulat ion

Advantages Disadvantages

• Low T h e r m a l Conductivity • Must Opera te Tel lur ide Stage Below 400°C

• Potent ia l Outgassing P rob lem

• Potent ia l E lec t r i ca l Shorting P r o b l e m

• Joint L o s s e s

• High Density

Fibrous Insulation

Advantages Disadvantages

• S t a t e -o f - the -Ar t Mater ia l • Higher The rma l Conductivity

• Can Opera te Tel lur ide Stage • Iner t Gas Containment Up to 550°C Required to Opera te Tel lur ide

Stage Above 400°C • No E lec t r i ca l Shorting P rob lem

• Minimal Joint L o s s e s

• Low Density

of ine r t gas filled fibrous insulat ion (Min-K in th is case) allows one to

opera te the in t e r s t age as high as 550°C, (500° C was a s sumed for conse rv ­

a t i sm) . F igure 3. 2 . 4 - 4 is a plot of the individual s tage t h e r m o e l e c t r i c

efficiencies and the total gene ra to r t h e r m o e l e c t r i c efficiency as a function

of the in t e r s t age t e m p e r a t u r e . The figure shows an i n c r e a s e in the total

efficiency as the in te r s t age t e m p e r a t u r e i s i n c r e a s e d , although the hot and

cold junction t e m p e r a t u r e s a r e maintained at 950° and 50°C, r e spec t ive ly .

Using super insula t ion in the te l lu r ide stage (T. = 400°C) r e s u l t s in a total

t he rmoe lec t r i c efficiency of 14.6% w h e r e a s using fibrous insulat ion

(Tj = 500°C) r e su l t s in an efficiency of 15. 8%. The efficiencies repor ted

in F igure 3 . 2 . 4 - 4 include the m a t e r i a l efficiency and e l ec t r i c a l l o s se s but

do not include the shunt heat l o s s e s . When the shunt l o s se s a r e included

3-156

17.0

)6.0

15.0

14.0

13 0

12.0

11.0

10.0

SI.O

8.0

7.0

6.0

5.0

4.0

3.0

^

• ~

-

-

-

-

-

-

"

-

"

-

-

-

1

350 400 450

INTERSTAGE TEMPERATURE - ' C

Figu re 3 . 2 . 4 - 4 . T / E Efficiency Versus In ters tage Tempera tu re

in the analys is the compar i son (assuming a 60-watt heat source) is

as shown below:

F ibrous Insulation Superinsulat ion (Min-K)

The rma l Inventory (w)

End L o s s e s (w)

Heat into SiGe Stage (w)

Shunt Heat in SiGe Stage (w)

Heat into SiGe E lements (w)

Power Output SiGe Stage (w)=i=

Heat into Segmented Stage (w)

Shunt Heat in Segmented Stage (w)

Heat into Segmented E lemen t s (w)

Power Output Segmented Stage (w)**

Total Power Output (w)

Sys tem Efficiency (%)

60

6

54

1

53

2 . 5

50.5

0 . 3

50.2

5. 1

7 . 6

12.6

60

6

54

13.9

40. 1

1.6

52.4

7 .5

44 .9

5 .5

7. 1

11.8

• SiGe Stage Eff iciencies: 4. 8% S/I, 4. 0% F ib rous ! See F igure • •Segmented Stage Eff ic iencies: 10. 25% S/I, 12. 3% Fibrous) 3. 2. 4-4

3-157 2 ^ ^ ^

It should be noted that a super insula t ion sys t em offers only a

marg ina l efficiency advantage over a Min-K sys tem because of the

t empe ra tu r e l imitat ion in the segmented s tage . Other potential p rob lems

such as outgass ing, joint l o s s e s , and e l ec t r i c a l short ing may fur ther

dec rea se the efficiency of a super insula t ion sy s t em and should be con­

s idered when compar ing insulat ion s y s t e m s .

3. 2. 4. 3 Configuration Selection

RTG Geometry

In o rder to achieve a max imum efficiency sys tem, while minimizing

fabricat ion and developmenta l c o s t s , a c lose-packed , f lat-piate conver te r

has been se lected for the conceptual design. This type of design is

considered opt imum for the follo\ving r e a s o n s :

• Fabr ica t ion s impl ic i ty

• Te l lu r ide stage s t r u c t u r a l in tegr i ty

• E a s y - t o - m a i n t a i n compres s ive load on couples

• Minimum insulat ion pene t ra t ions

Severa l conve r t e r g e o m e t r i e s were invest igated p r i o r to se lec t ion

of the f la t -p la te geomet ry . Both sphe r i ca l and cy l indr ica l geome t r i e s

with c lose packed and d is t r ibuted e lements were cons idered . However,

in each ca se , t h e r e was l i t t le difference in overa l l sy s t em efficiency.

Fabr ica t ion s impl ic i ty was the overr id ing cons idera t ion in choosing the

f la t-plate design.

An analys is to de te rmine the heat source geomet ry resu l ted in the

selection of a r ight c i r c u l a r cyl inder wfith an L/D of 1. This shape yields

the smal les t surface a r e a and there fore min imizes the ra t io of insulat ion

a r ea to t h e r m o e l e c t r i c m a t e r i a l a r ea . A schemat ic of the RTG is shown

in F igure 3. 2. 4 - 5 .

The rma l Analyses

T h e r m a l analyses were pe r fo rmed to de te rmine the t h e r m a l

adequacy of the RTG sys t em and de te rmine the effect of varying conver te r

geometry on conver t e r efficiency, t e m p e r a t u r e d is t r ibut ion, and heat

source t e m p e r a t u r e . A simplified two-dimens iona l model of a cascaded

3-158

ARGON FILLED MIN-K INSULATION

Si Ge STAGE SEGMENTED TELLURIDE STAGE

54-WATT HEAT SOURCE

INTERSTAGE PLATE SODIUM-SULFUR BATTERY

Figu re 3. 2. 4 - 5 . RTG Schematic

unit \vas developed for this ana lys i s . Of chief i n t e re s t was the opt imiza­

tion of the thernaal insulat ion requ i red to maintain the conver te r at the

requ i red t ennpera tu res . It is kno\vn that adding insulation will i nc rease

the gene ra to r efficiency; ho\vever, it nnust also be rea l ized that a marg ina l

gain in efficiency may be at the expense of a substant ia l weight penalty.

Table 3. 2. 4-4 p r e s e n t s the r e su l t s of the analyses per formed for

insulat ion th i cknesses of 0. 5, 0. 75 and 1. 5 inches , respect ive ly . The

insulat ion was a s sumed to be Min-K 2020 filled with argon (Reference 14).

Argon was chosen for this ana lys is since it is known to be an excel lent

cover gas capable of inhibiting PbTe sublimation; exper imenta l t h e r m a l

conductivity data w e r e a lso readi ly available. Other gases such as xenon

and krypton a r e poss ib le candidates as fill g a se s . In any ca se , the d i r e c ­

tion chosen is bel ieved to be conserva t ive and the poss ible change to xenon,

krypton or even vacuum operat ion would only enhance the genera to r

t h e r m a l pe r fo rmance . F o r p u r p o s e s of calculating heat source

t e m p e r a t u r e s , the si l icon g e r m a n i u m hot shoes w e r e assumed to be

sepa ra t ed f rom the heat sou rce by a d is tance of 0. 5 mil; conduction

through the argon gas and radia t ion were the sole modes of heat t r ans fe r

cons idered .

The r e s u l t s of the t h e r m a l ana lyses a r e shown in F igure 3. 2. 4-6 .

It should be noted that only a sma l l gain in efficiency is obtained when

the insulat ion th ickness exceeds 1 inch, although the size and weight of

the sys tem i n c r e a s e significantly. In o rde r to compare the th ree des igns

3-159 ^ / /

Table 3. 2. 4 -4 . Summary of T h e r m a l Analysis Resul t s

Insulation Thickness (In)

0.5 0, 75 1. 5

Heat Source T h e r m a l Inventory (w)

Heat Enter ing Conver t e r (w)

Heat Enter ing End Insulation (w)

Maximum Heat Source Surface T e m p e r a t u r e (°C)

SiGe Hot Junction T e m p e r a t u r e (°C)

In te r s tage T e m p e r a t u r e ( C)

Sink T e m p e r a t u r e ( C)

60

45.7

14.3

1004

969

483

50

60

47.7

12. 3

1054

1018

510

50

60

49. 6

10.4

1104

1065

538

50

fa a '

St

5 -

CONVERTER LENGTH =3 .2 IN 60 WATT HEAT SOURCE

> • u

z

n i

10

0 0.3 I.O 1.3 2.0 INSULATION THICKNESS ( I N . )

Figure 3. 2. 4-6. Converter Characterist ics Versus Insulation Thickness

3-160 • z ^ / z -

in Table 3, 2. 4-4 at the s ame opera t ing t e m p e r a t u r e s , the c r o s s - s e c t i o n a l a r e a s of the t h e r m o e l e c t r i c e lements a r e adjusted as requi red . However, the effect of this change on conver t e r weight is negligible,

3. 2. 4. 4 Component Design

System Requ i remen t s

In o rde r to de l iver 2. 81 watts to the blood pump, the following

d i ag ram shows how the heat source t h e r m a l requ i rement is de te rmined .

Blood Pump

2.81 watts t f CCS „ ^ 45(y

M o t o r / R e c i p r o c a t o r

t T / E

Conver t e r

6. 25 watts

n = 11. 6%

54 watts

Heat Source

The t h e r m o e l e c t r i c c o n v e r t e r includes both the t h e r m o e l e c t r i c m a t e r i a l s

and the t h e r m a l insulation; the 11. 6% efficiency r e p r e s e n t s the overa l l

engine efficiency including both t h e r m a l and e l ec t r i c a l l o s se s . The PCCS

efficiency is desc r ibed in Section 3. 1.

P a r a m e t r i c Design Curves aind Selection

F igure 3. 2 . 4 - 7 p r e s e n t s the g raph ica l r e su l t s of an analys is p e r ­

fo rmed to d e t e r m i n e the design c h a r a c t e r i s t i c s of an RTG as a function of

the heat s o u r c e t h e r m a l inventory. Based on previous t h e r m a l ana lyses

repor ted in Section 3 . 4, 4. 3, an insulat ion th ickness of i inch was se lec ted .

Adding the design cons t ra in t that the RTG mus t produce 6. 25 watts (e),

the following RTG design c h a r a c t e r i s t i c s may be read from Figure 3. 2. 4-7

3-161

CASCADED/SEGMENTED CONVERTER MIN-K INSULATION IN ARGON

INSULATION I ! nln'

CONVERTER LENGTH: 3.2 TO 3.4 IN .

OUTER DIAMETER

z , 5

S 4

< ' 5 2 M 1

8 0 13

ill 12

5 " Z 10 l U

5 ' it 8

7

4

3

3 , 2

X

0

u. 8

< 7 * 4

§ ^ 3 O

? 2 2

-

-. ' -^

--

0

— — —" ^

^ ^ - ^

"t--- ''''

iO

SJ.===* ^ ]

— "

j^»^;C'^'

_-t=^=-^==^ _ _ ——

_ _ _^ ——

^^^^ l - * * * ^ ^ ^ "

""

1

(0 50 6-

6. 25 wat ts (e)

54 wat ts (t)

I. 7 lb (includ­ing heat source)

I I . 6%

Power Output:

Power Input:

Weight:

Efficiency:

Outer D iamete r : 3. 3 inches

The rmoe lec t r i c Couple Sizing

The s ize of the t h e r m o e l e c t r i c

couples is dictated by the conver te r

heat flux, desir( ,d junction t e m p e r a ­

t u r e s , and s t ruc tu r a l cons idera t ions .

Theore t ica l ly the power output

of a t h e r m o e l e c t r i c device is inde­

pendent of the geomet ry of the e lement

as long as the rat io of the a r e a of the

e lement to its length is mainta ined.

In a p rac t i ca l device, however, e l ec ­

t r ica l l o s se s in the conductor between

e lements , l o s s e s caused by contact re

s i s tance , and t h e r m a l l o s se s through

the insulat ion, become increas ing ly

impor tan t and cause a d e c r e a s e in

power as e lement length and a r e a a r e

reduced to min imize weight.

The couple s ize chosen for the conceptual design p resen ted h e r e does

not r e p r e s e n t a complete ly optimized sy s t em but was se lec ted to give

couples that a r e producible by avai lable fabricat ion techniques and give

a s s u r a n c e of having s t r u c t u r a l in tegr i ty .

Heat balances were per formed for each s tage to de te rmine the num­

ber of couples requ i red and the a r e a to length of each t h e r m o e l e c t r i c e l e ­

ment . An i t e ra t ive method was employed to achieve an in tegra l number

of couples and e l iminate couple- t runcat ion power l o s s e s . The following

couple c h a r a c t e r i s t i c s r e su l t :

HEAT SOURCE THERMAL INVENTORY - WATTS

Figure 3 . 2 . 4 - 7

RTG Power Supply, RTG Char­a c t e r i s t i c s Versus The rma l

Inventory

3-162 7^/^

0,

0,

0,

0,

0,

0.

0,

6 in.

0212 in^

495 in.

105 in.

0170 in^

432 in.

168 in.

Silicon German ium Stage (8 couples)

Couple Length: 0. 3 in, 2

N-e lement Cross -Sec t iona l Area : 0.0179 in 2

P -e l emen t Cross -Sec t iona l Area : 0.0098 in Segmented Stage (26 couples)

Couple Length:

N-e lement Cross -Sec t iona l Area :

PbTe-GeTe Length:

2N Length:

P - e l emen t Cross -Sec t iona l Area :

TAGS Length:

BiSbTe Length:

Couple Mounting

The SiGe couples a r e me ta l lu rg ica l ly bonded to SiMo hot shoes which

a r e in turn compress ive ly loaded against the heat source . The use of a

bery l l ium oxide coating is n e c e s s a r y to prevent reaction of the SiMo with

the P t -Rh clad. The SiGe couples themse lves a r e evenly spaced over the

flat ends of the heat source and meta l lu rg ica l ly bonded to the in te rs tage

plate . The te l lur ide stage is meta l lu rg ica l ly bonded to the center of the

containment vesse l end cover in a c lose-packed a r r a y intended to enhance

the s t ruc tu ra l in tegr i ty of the unit. The two conver ter modules a r e held in

compres s ion against the heat source by means of tension rods located

between the end cove r s . The ent i re a s sembly is packed with Min-K, back­

filled with argon, and he rme t i ca l ly sealed in a cylinder,

3. 2. 4. 5 System Design Summary

The t h e r m o e l e c t r i c / b a t t e r y engine is designed to minimize the

heat source power r equ i r emen t by sizing the heat source and the rmo­

e lec t r i c conver t e r to provide the average hea r t pump power requi rement ,

2.81 wat ts , with the peak power supplied by the bat tery .

Without the use of a ba t te ry , peak power (4. 44 watts at the blood

pump) must be provided continuously. Since the maximum prac t ica l

efficiency of the t h e r m o e l e c t r i c conver te r is about 12% and the PCCS

3-163

efficiency is about 45%, a conve r t e r capable of producing 9, 85 wat ts (e)

would be requ i red . The cor responding heat source r equ i r emen t , 82 wat ts

(t\ is in excess of the max imum allowable. The use of a 12. 5 watt hour

sodium-sulfur ba t t e ry , Section 2. 8, r e su l t s in a t h e r m o e l e c t r i c conver te r

power r equ i remen t of 6, 25 wat ts (e) and reduces the heat source to

54 watts (t).

Since a waste heat fltix l imita t ion of 0, 07 w a t t s / c m is used, the

container for the engine is l a rge enough to a l so contain the m o t o r /

r e c ip roca to r .

F igu re 3. 2 . 4 - 8 is a layout of the s y s t e m consis t ing of the RTG, the

m o t o r / r e c i p r o c a t o r PCCS and a t h e r m a l ba t t e ry . A s u m m a r y of the

per t inent design fea tures is given below:

Heat Source T h e r m a l Input 54 watts (t)

RTG Power Output

Load Cur ren t

Load Voltage

Blood Punnp Power Input

Volume

Surface Area

Weight

Specific Gravi ty

6.

3 .

1.

2.

25 watts

28 amps

91 volts

81 watts

77 .4 in^

112 in^

2.

1.

, 96 lbs

, 0 6

(e)

(ave)

SiGe Operat ing T e m p e r a t u r e s

Tel lur ide Operat ing T empe ratu res

Maximum Heat Source Surface T e m p e r a t u r e

T j ^ = 950' 'C, T ^ = 525°C

T „ = 500°C, T ^ = 50*'C rl C

1079''C

3-164

^y^

MOTOK/ RECIPROCATOR

F i g u r e 3 . 2 . 4 - 8 . T h e r m o e l e c t r i c / B a t t e r y Engine

The sys tem component weight breakdown is given in Table 3.2.4-5.

The sodium sulfur ba t t e ry provided to meet peak power is contained

within an annular space around the RTG and surrounded by insulation

to mainta in it at a mean t e m p e r a t u r e of 300 C (570 F).

Table 3 . 2 . 4 - 5 . T h e r m o e l e c t r i c / B a t t e r y Weight Summary

Item

Heat Source

Conver te r

SiGe Element

Tel lur ide E lements

In te r s tage P l a t e

E lec t r i ca l Connections

Insulation

Container

Bat te ry

Motor Rec iproca tor

Outer Container

Total

Weight (lb)

0. 551

0. 828

3-165

0. 130

0. 880

0. 562

2. 951

0.081

0. 287

0. 037

0. 017

0. 283

0. 123

^/7

3 , 2 . 5 Hybrid

The t e r m 'hybr id ' engine is used he re to desc r ibe a sys tem in which

the power input to the blood pump is provided by two sepa ra t e t h e r m a l

energy conve r t e r s . Since all the candidate concepts a re , by definition,

capable of operat ing for 10 y e a r s with high rel iabi l i ty , there is no advant­

age to operat ing two conver t e r s in para l le l ( thermally) solely for the sake of

redundancy. At best , the efficiency of two para l le l c o n v e r t e r s can approach

that of a single, l a r g e r device, and then only at the expense of weight and

volume.

In o r d e r for the hybrid concept to be a t t rac t ive , the two the rma l con­

v e r t e r s mus t be physically and thermodynamica l ly suited to operat ing in

s e r i e s ( thermally) at an overa l l efficiency in excess of that at tainable

with e i ther conver t e r alone.

The only two candidate concepts which qualify under these ground-

ru les a r e the ro t a ry vapor and t h e r m o e l e c t r i c s y s t e m s , which we will

hereaf ter re fer to s imply as the 'hybr id ' sys tem. Since both the ro t a ry

vapor and thernnoelectr ic conve r t e r s separa te ly employ ( requi re) a solid

e lec t ro ly te ba t te ry for energy s to rage , an obvious configuration for the

hybrid sys tem would a lso include this component. This hyb r id /ba t t e ry

sys tem is d i scussed in Section 3. 2. 5, 1. However, with the improved over ­

all efficiency of the hybrid combination, it is now poss ib le to cons ider a

sys tem which does not r equ i re a ba t t e ry to r ema in within all of the des ign

groundru les . This option is desc r ibed in Section 3, 2. 5. 2,

The hybrid concept is s imi la r to a thermodynamic binary cycle

in which the re jec t ion t e m p e r a t u r e of one sys tem co r r e sponds to the

peak cycle t e m p e r a t u r e of the second sys tem. In th is application the

t he rmoe lec t r i c module hot junction t e m p e r a t u r e is essent ia l ly equal to

the heat source wall t e m p e r a t u r e , while the second stage t h e r m o e l e c t r i c

cold junction t e m p e r a t u r e is essen t ia l ly the ho t -s ide input to the ro t a ry

vapor cycle . In this manner , all the heat not conver ted to e l ec t r i c i ty in

the t h e r m o e l e c t r i c s tage is sti l l fully avai lable for convers ion in the

ro t a ry vapor s tage.

3-166

^/9

As explained in Section 3. 2. 4, for the optimized the rmoe lec t r i c

configuration the e lements a r e placed on both ends of a right c i r cu la r

cyl inder . Since the ro t a ry vapor cycle boi ler opera tes at a maximum

t e m p e r a t u r e of 430 F , the t h e r m o e l e c t r i c cold junction t empera tu re is

es tab l i shed at 464°F (240°C).

3. 2. 5. 1 Hybr id /Ba t t e ry Engine System

This engine sys tem u t i l i zes the hybrid concept previously descr ibed

and a sodium-sul fur ba t te ry to provide energy s to rage . With this config­

ura t ion , the e l ec t r i c a l output f rom the engine can be designed to provide

the average power requ i red by the blood pump. As d i scussed in the

e l ec t rochemica l energy s torage section, a 12. 5 wat t -hour ba t te ry is

r equ i red for this application. Under these conditions the requi red heat

sou rce for the h y b r i d / b a t t e r y sys tem is only 37 watts the rmal .

In th is configuration, the t h e r m o e l e c t r i c conver te r r e j ec t s heat to

the boi ler of the r o t a r y vapor cycle sys t em. The peak boiler operating

t e m p e r a t u r e is mainta ined at 430 F (221 °C), which l imi t s the min imum

allowable cold junction t e m p e r a t u r e of the 2N/TAGS couples . The oper ­

ating t e m p e r a t u r e s of the individual s tages of the cascaded t h e r m o e l e c ­

t r i c conver t e r and the r o t a r y vapor cycle a r e as follows:

S i l i con-Germanium 2N/TAGS Rotary Vapor Stage Stage Cycle

T „ = 950°C T „ = 475°C T^ ., =:221°C H H Boiler

T „ = 500°C T^ = 240°C T „ , = 4 6 , 7 ° C C C Cond

As CEin be seen, this hybrid concept has been the rmal ly integrated so

that the low t e m p e r a t u r e s ide of each stage can provide the t he rma l

input to the high t e m p e r a t u r e region of the next stage. In addi­

tion, the bat tery , which opera tes at 300 C, can a lso be packaged within

the insulat ion so that the hea t loss from the ba t tery casing can be

ut i l ized as a hea t input to the ro t a ry vajxjr cycle sys tem bo i l e r . F ig ­

u r e 3 . 2 . 5 - 1 shows schemat ica l ly the t h e r m o e l e c t r i c e lements and

ba t te ry a r r a n g e m e n t with r e s p e c t to the isotope heat sou rce . The

des ign approach taken is s i m i l a r to that desc r ibed for the t h e r m o e l e c ­

t r i c conve r t e r sys t em (Section 3 . 2 . 4 ) . This des ign also employs a

3-167

cascaded converter with 63% Si - 37% Ge couples in the high tempera­

ture stage and 2N/TAGS couples in the low temperature stage.

The 2N/TAGS stage is not segmented because the temperature

differential across the elements is not sufficiently large to make seg­

menting effective. For this engine configuration, most of the power is

provided by the rotary vapor cycle turbogenerator. Hence, the most

state-of-the-art thermoelectric mater ials were selected for both stages

because the perfornnances of these materials would be adequate. The

stages are electrically connected in ser ies and the couples within each

stage are connected in a ser ies-paral le l arrangement. The couple sizes

were designed to incorporate electrical matching between stages so as to

provide peak power operation.

MIN-K (INSUIATIGN)

2.45

\\\\\i\\\\\\\\^\\Z3SS.

No S IATTERY

HEAT SOURCE THERMAL INPUT. WATTS

POWER OUTPUT, WATTS

LOAD CURRENT, AMPS

LOAD VOtTAGE, VOITS

TOTAL WEIGHT (INCLUDING RATTERY), LIS

SILICON GERMANIUM STAGE

HOT J U N a i O N TEMPERATURE, *C

COLD JUNCTION TEMPERATURE *C

COUPtE LENGTH. I N

N-ELEMENT AREA, I N '

P-ELEMENT AREA, IN^

NUMKR OF COUPLES

37

2 I I

0.S8

3.64

1.65

500

0.3

0.00279

o.oais« 24

OUTER CAN

2HaAGS-TELLURIDE STAGE

HOT JUNCTION HMPERATURE. *C 47S

COLO JUNCTION TEMPERATUK, * € 240

COUPU LENGTH, I N 0.3

N-ELEMENT AIKA, I N ' 0.0MS3

P-EUMENTAKA. I N * O.Ott?!

NUMKR OF COUPLES 60

Figure 3 . 2 . 5 - 1 . Hybrid-Battery Thermoelectric Converter

3-168

^2-0

The overa l l h y b r i d / b a t t e r y engine sys tem, shown in F igure 3 . 2 . 5 - 2 ,

is capable of meet ing average daily power r e q u i r e m e n t s . The 12.5 watt-

hour sodium-sulfur ba t te ry has been sized to provide the supplementary

peak power r e q u i r e m e n t s . This ba t te ry has an annular configuration,

occupies 3 cubic inches and is ennbedded in the t h e r m a l insulation to

mainta in itself at a mean t e m p e r a t u r e of 300° C. The weight breakdown

for the t h e r m o e l e c t r i c c o n v e r t e r s and bat tery is shown in Table 3 . 2 . 5 - 1 .

Combined with the t h e r m o e l e c t r i c e lements and bat tery is a ro ta ry

vapor cycle tu rbogenera to r operat ing as previously descr ibed in Sec­

tion 3 . 2 . 3 . With a peak cycle t e m p e r a t u r e of 430° F , the boiler for the

ro t a ry cycle can be placed around the t h e r m o e l e c t r i c modu les . The

vapor cycle operat ing conditions and per fo rmance a r e s imi lar to those

d i scussed in Section 3 . 2 . 3 except that the output xx)wer is 4 .55 watts at

15 vol t s . The vapor cycle u s e s C P - 3 4 (thiophene) as a working fluid and

ope ra t e s bet^veen a peak cycle t e m p e r a t u r e of 221° C (at 250 psia) and a

condensing t e m p e r a t u r e of 46 .6°C (at 4 ps ia ) . The t he rmoe lec t r i c con­

v e r t e r ix>wer g r o s s outjjut i s 2. 11 wat ts at 3.64 vol ts , but after

inc reas ing the voltage in a d c / d c conver t e r to 15 volts th is output is

reduced to 1.7 wa t t s .

Thus , the total net e l ec t r i c power being generated by this hybr id /

ba t te ry engine i s 6 .25 w a t t s . The dc m o t o r / r e c i p r o c a t o r descr ibed in

Section 3 . 1.4 combined with the blood punnp actuator has cLn overa l l

efficiency of 45%. The re fo re , the net power avai lable to the blood pump

equals the daily average r e q u i r e m e n t of 2 .81 wa t t s . As mentioned p r e ­

viously, the 12.5 wat t -hour ba t t e ry m a k e s up the difference between this

daily average and the mcLximum blood pump r e q u i r e m e n t s .

In o r d e r to keep the volume of th is sys tem to a min imum, the

f ibrous insulat ion (Min-K with xenon fill gas) th ickness and heat source

w e r e opt imized. With an es t ima ted 10% heat l o s s , the requ i red heat

source would be 34 wat ts t h e r m a l . However, in o rde r to min imize the

engine volume, an additional 3 wat t s was added to the hea t source which

resu l t ed in a 16% heat l o s s , but pe rmi t ted a |Significant reduction in over ­

all volume. The overall c h a r a c t e r i s t i c s of the hybrid /battery system

are summarized in Table 3 . 2 . 5 - 2 .

3-169

SECONDARY BATTERY (SODIUM-SULEUR) CONDENSER \ EXTERNAL CONTAINER

Figure 3 . 2 . 5 - 2 . Hybr id /Ba t t e ry Engine System

Table 3 . 2 . 5 - 1 . Hybr id /Ba t t e ry E n g i n e - T h e r m o e l e c t r i c Conver te r Weight Summary

Heat Source

Silicon G e r m a n i u m

Tel lur ide E lemen t s

SiGe Hot Shoes

SiGe Cold Stack

In te rs tage P la te

Insulat ion

Te l lu r ide Cold Stac

RTG Can

E lec t r i ca l S t raps

Bat te ry

Total Weight

Elennents

k

Weight (lb)

0,405

0 .004

0.037

0.008

0 .026

0.029

0.094

0.076

0.076

0.014

0 .880

1.649 lb (0 .750 kg)

3-170

Table 3 . 2 . 5 - 2 . Hybr id /Ba t t e ry Engine System Design Summary

Heat Source The rma l

Net E lec t r i ca l Power

Input

Output

Overa l l System Efficiency

Total Weight (lb)

(kg)

Total Volume ( l i ters)

Specific Gravi ty

37 watts

6. 25 watts e

16.8%

2.83

1.28

1. 115

1. 15

3. 2, 5. 2 Hybrid Engine System

As mentioned e a r l i e r , the hybrid engine efficiency is adequate to

consider the option of no energy s torage . In this case, the e lec t r ica l

output must be s ized to mee t the peak power requ i rements with the

excess power being d iss ipa ted in the power conditioning and control unit

as requ i red Under these conditions, this sys tem will r equ i re a 49-watt

t he rma l heat source . The advantage to this approach is that this engine

can be developed completely from s t a t e -o f - t he - a r t components. Fur ther ­

more , when the sodium-sulfur ba t te ry becomes available, it would be a

s imple m a t t e r to convert this engine to the sma l l e r heat source, hybr id /

ba t te ry configuration.

This engine opera tes at t e m p e r a t u r e levels and cycle conditions

s imi la r to the hybrid engine with a ba t t e ry . The cascaded the rmoe lec ­

t r i c conver t e r r e j ec t s heat to the ro ta ry vapor cycle boi ler at 221° C

(430°F) and the condensing t e m p e r a t u r e rennains at 46 ,6°C ( 1 1 6 " F ) .

The operat ing t e m p e r a t u r e levels of the individual conver te r s tages

r e m a i n as outlined in Section 3. 2. 5. 1. The major change in this con­

figuration is the different geonnetry associa ted with the l a rge r heat

source and the sizing of the t h e r m o e l e c t r i c couples and the insulation

th icknesses to min imize the overa l l volume. The design c h a r a c t e r i s t i c s

3-171 ^7. 3

of this thermoelectric converter are shown in Figure 3. 2. 5-3. A weight

breakdown of the elennents comprising the thermoelectric converter,

including the heat source, i s shown in Table 3. 2. 5-3.

In this configuration the electrical output power from the rotary

vapor cycle turbogenerator is 7.25 watts at 15 volts . The gross e lec ­

trical output power from the thermoelectric converter i s 3.21 watts at

3 .64 volts . In boosting this output voltage to 15 volts through a dc-to-dc

converter, the net e lectrical prawer output becomes 2.61 watts. Thus the

total e lectrical power output from the hybrid engine system is 9.85 watts .

When supplied to the dc motor-driven reciprocator and blood pump actua­

tor, which have an overall combined efficiency of 45%, the net power

input to the blood pump is 4 .44 watts which corresponds to its p>eak

power requirements.

3-172

OUTER CAN

2 2

HEAT SOURCE THERMAL INPUT, WATTS 49

POWER OUTPUT WATTS 3 21

LOAD CURRENT, AMPS 0.88

LOAD VOLTAGE, VOLTS 3 64

TOTAL WEIGHT, LBS 0.97

SILICON GERMANIUM STAGE

HOT JUNCTION TEMPERATURE, °C 950

COLD JUNCTION TEMPERATURE, -C 500

COUPLE LENGTH, IN 0.3

N-ELEMENT AREA, IN^ 0.00427

P-ELEMENT AREA, IN^ 0.00233

NUMBER OF COUPLES 24

MIN-K (INSULATION)

2N/TAGS STAGE

HOT JUNCTION TEMPERATURE, °C

COLD JUNCTION TEMPERATURE, °C

COUPLE LENGTH, IN

N-ELEMENT AREA, IN^

P-ELEMENT AREA, IN^

NUMBER OF COUPLES

475

240

0 3

0.00977

0 00433

60

Figure 3 . 2 . 5 - 3 . Hybrid Thermoe lec t r i c Converter

Table 3 . 2 . 5 - 3 . Hybrid Engine Thermoe lec t r i c Conver te r Weight Summary

Heat Source

Silicon Gernnanium

Tel lur ide E lements

SiGe Hot Shoes

SiGe Cold Stack

In ters tage Pla te

Insulation

Te l lur ide Cold Stac

RTG Can

E lec t r i ca l S t raps

Total Weight

1

Elements

k

Weight (lb)

0.508

0.006

0.061 ,

0.011

0.043

0.036

0. 101

0. 122

0.067

0.016

0.971 lb (0.440 kg)

3-173

Z^Z^^

The overa l l a r r angemen t of the hybrid engine sys tem is shown in

F igure 3. 2. 5-4. A s u m m a r y of i ts design and per formance c h a r a c t e r i s ­

t i cs is given in Table 3. 2. 5-4. It i s in te res t ing to note that the weight

and volume of this systenn and the hyb r id /ba t t e ry engine sys tem a r e

near ly equivalent. This enhances the i r in terchangeabi l i ty , and the

potential for heat source size reduction when the sodium-sulfur bat tery

becomes avai lable .

INTERNAL CONTAINER

Figure 3 . 2 , 5 - 4 . Hybrid Engine System

Table 3 .2 . 5-4. Design and P e r f o r m a n c e C h a r a c t e r i s t i c s of the Hybrid Engine System

Heat Source The rma l Input

E lec t r i ca l Power Output

Overa l l System Efficiency

Total System Weight (lb)

Total System Weight (kg)

Total System Volume ( l i te rs )

Specific Gravi ty

49 wat ts

9.85 wat t s e 20%

2 . 8

1.27

1.05

1.21

3-174

3 . 2 . 5 . 3 Load Sharing E lec t ron ic s

The hybr id- type heat engines use the e l ec t r i ca l output power from

two sepa ra t e i n -pa ra l l e l s o u r c e s , i . e . , the the rmoe lec t r i c conver ter and

the tu rbogenera to r unit . Fo r max imum per fo rmance , it is des i rab le to

continuously use the en t i re output power from the thernnoelectr ic source

and to supplement this power with the requi red turbogenera tor power to

mee t the load r e q u i r e m e n t s . Since the turbogenera tor i s providing con­

stant output power, at l e s s than peak load condit ions, this energy can be

used to charge the ba t te ry , or d iss ipated through the power conditioning

and control sys t em. To avoid the necess i ty for load matching on an

impedance b a s i s , a fair ly s imple approach is to employ an e lec t ronic

load-shar ing c i rcu i t . This c i rcu i t switches the load (dc motor) fronr\ one

power source to the other at a fairly rapid r a t e . Using this technique

each power genera to r o p e r a t e s at an effectively constant load. A switch­

ing ra t e of four t i m e s the dc motor speed is used so that iner t ia of the

ro tor naakes switching t r a n s i e n t s negl igible . With the dc motor operating

at 9000 r p m , the e lec t ron ic commutat ion is accomplished at 600 Hz.

The tu rbogenera to r unit is provided with an overspeed control

c i rcu i t (see F igure 3 . 2 . 3 - 1 9 of Section 3. 2. 3) which automatical ly p ro ­

t ec t s this unit from load changes and main ta ins the output voltage within

±2%, S imi la r ly , the load-shar ing e lec t ronic c i rcu i t is designed to ma in ­

tain the output voltage from the t h e r m o e l e c t r i c conver ter at the s ame

to l e r ance . F igure 3 . 2 . 5 - 5 is a block d i ag ram showing the main e lements

assoc ia ted with the load sharing and motor commutat ion e l ec t ron ic s .

Also shown in this d i ag ram is a dc - to -dc conver te r used to inc rease the

output voltage from the t h e r m o e l e c t r i c conver te r to 15 vol ts . This

voltage level is des i r ab le in o rde r to achieve the required efficiencies

on the tu rbogenera to r and the dc m o t o r . The dc- to -dc conver te r opera tes at a

switching frequency of 50 kHz and an efficiency of 81.5%. Synchronous

rect i f icat ion is a l so used to reduce l o s s e s .

3-175 ^

BIAS POWER CONVERTER

a 15V TURBO-GENERATOR

3.64V

TURBO­GENERATOR OVERSPEED CONTROL

-•-•

+ I5V

THERMOELECTRIC DC/DC POWER CONVERTER

• 15 V

THERMOELECTRIC THRESHOLD DETECTOR

EXTERNAL LOAD

MOTOR COMMUTATION

ELECTRONICS

POWER DIVIDER PROPORTIONING

CONTROL AND LOGIC

MOTOR) CAM-RECIPROCATOR

POSITION ENCODER

Figure 3 . 2 . 5 - 5 . Load Sharing and Motor Commutat ion Elec t ron ic Block Diagrami

3. 2. 5. 4 Al te rna te T h e r m a l Insulation Systems

As d i scussed in Section 2. 7, f ibrous insulation with a xenon fill

gas was selected as the base l ine t he rma l insulat ion approach for all candi­

date s y s t e m s . However, for the hybrid engine we did c a r r y out addit ional

t h e r m a l ana lyses to de te rmine the effect of a l t e rna te t h e r m a l insiolation

s y s t e m s , including super insula t ion , on sys tem pe r fo rmance . The r e s u l t s

a r e d i scussed below.

The base l ine insulat ion design ( summar i zed in the f i r s t column of

Table 3. 2. 5-5) cons i s t s of a f ibrous insulat ion (Min-K 2020) with a com­

bination iner t gas fill of xenon and argon. Xenon is used as the fill gas

throughout the engine except in the t he rmoe lec t r i c modules , s ince no

long - t e rm data were avai lable . If no incompatibi l i ty ex i s t s , the use of

xenon in the t h e r m o e l e c t r i c package will fur ther reduce the heat source

inventory by a watt or two. The case of using argon throughout the the rmal

conver t e r was a l so examined, and the only change is an i n c r e a s e in the

isotope inventory by 1. 4 wat ts , a s shown in the second column of Table

3. 2. 5-5 .

3-176

-z- ^ ^

» /

Table 3. 2. 5-5. Summary of Hybrid System Per fo rmance with Alternate Insulation Techniques

Heat Loss

Isotope Inventory

Conver ter

Diajneter

Length

System Voltune

System Weight

Specific Gravi ty

F ibrous Insulation

Min-K (Xenon/Argon)

6. 0 watts

49 .0 watts

3. 5 inches

6. 9 inches

1. 33 l i t e r s

4. 03 lb (1.83 kg)

1. 37

Min-K (Argon)

7. 4 watts

50.4 watts

3. 5 inches

6. 9 inches

1. 33 l i t e r s

4. 04 lb (1.83 kg)

1.37

Superinsulation

Ideal

0. 2 watt

43. 2 watts

2. 6 inches

6. 65 inches

0. 86 l i t e r

3.61 lb (1.64 kg)

1. 91

Most Probable

4. 4 watts

47. 4 watts

2. 6 inches

6. 9 inches

0. 88 l i te r

3.68 lb (1.67 kg)

1. 90

T h e r m a l ana lyses were a lso c a r r i e d out to de te rmine the impl ica­

tions of using Linde composi te super insula t ion, which cons i s t s of Mo,

Ni, Cu, and Al foils depending upon the t empe ra tu r e reg ime. Two cases

were examined. The f i r s t was the ideal case which a s s u m e s no edge,

joint, or penet ra t ion l o s s e s , as well as no heat l o s se s through s t ruc tu ra l

suppor t s . This reduces the heat l o s s to 0. 2 watt, compared to the Min-K

(xenon/argon) heat l o s s of 6. 0 wat t s . However, based on past exper ience

with super insula t ion , the second case or nnost probable value for the heat

l o s se s will be about 4. 4 watts (1. 0 watt through the insulat ion and 3. 4 watt?;

through the s t ruc tu r a l supports for the engine components) . There fore ,

the savings in t he rma l inventory for a super insula t ion sys tem will be on

the o rde r of 2 wat ts . The chief advantage of using a super insula t ion s y s ­

tem l ies in the volume and weight savings which annount to 34% and 9%,

respec t ive ly . As might be expected, however, from the d ispar i ty between

the vo lumet r ic and weight reduct ions , the specific gravity i n c r e a s e s fronri

1. 37 to 1. 90.

3. 2. 5. 5 Safety

The bas ic safety philosophy d ic ta tes that the isotope fuel must be

contained under all nornaal operat ing envi ronments as well as during

sys tem malfunctions which reduce or t e rmina te the active removal of

heat from the heat source . Under no rma l operat ion, the heat source

t e m p e r a t u r e is 1970 F. U the t h e r m o e l e c t r i c module fails (open circui t )

the heat source t e m p e r a t u r e r i s e s to 2450 F because of ths loss of Pe l t i e r

cooling. Should both t h e r m o e l e c t r i c module and tu ibogenerator unit fail,

the heat source t e m p e r a t u r e would theore t ica l ly r i s e to 3400 F. * To

a s s u r e containment under this condition, a t he rma l fuse was incorpora ted

to a s s u r e adequate heat r emova l after r o t a r y vapor engine fa i lure . The

the rma l fuse cont r ibutes negligible additional heat loss during no rma l

opera t ions .

•Because of the degradat ion of the t he rma l insulat ion p rope r t i e s at t emper a tures above 2500 F, the peak t e m p e r a t u r e even without some so r t of over t e m p e r a t u r e protect ion would probably be considerably below 3400 F .

3-178

As will be reca l led , the heat source is surrounded by a t he rmo­

e lec t r i c module and a high t e m p e r a t u r e t he rma l insulation sys tem of

Min-K 2020, filled with an iner t gas . The the rmal insulation is designed

to fit snugly around the heat source and the the rmoe lec t r i c module and to

fill the gap between the tu rbogenera to r boi ler unit and the surrounding heat

engine container . The the rma l fuse is located between the boiler and

outer s t r u c t u r e . A number of s m a l l - d i a m e t e r , non- rad ia l holes through

the Min-K insulat ion a r e filled with a lead- t in compound (solder) and

Kaowool, a type of rock-wool insulat ion. The solder is mounted at the

end c loses t to the boi ler so that if the t empe ra tu r e of the boiler exceeds

the melt ing point of the so lder (600°F), the plug will mel t and flow outward

through the Kaowool insulat ion to forna a high-conductivity fin that con­

ducts heat from the boi ler to the outer s t r uc tu r e . This type of rad io­

isotope o v e r t e m p e r a t u r e protect ion has been considered for s imi la r appli­

cations employing Min-K insulation. (Reference 22)

Analyses have indicated that a s t eady-s ta te heat source t empera tu re

of l e s s than 2600 F can be maintained. To meet these the rmal conditions,

four plugs, 0. 125 inch in d i ame te r a r e requi red . However, twice this

number (eight) a r e used to a s s u r e independence of orientat ion. The addi­

tional heat l o s s through these plugs under normal operat ing conditions

is l e s s than 0. 2 watt . While this approach appears feasible, exper imenta l

ver i f icat ion will be n e c e s s a r y because of the lack of the rmal conductivity

data for the Min-K 2020 above 2000°F.

-'" ^V

millerc

Text Box

BLANK

4. RELIABILITY OF THE CANDIDATE COMPONENTS

The re a r e two p r i m a r y fai lure mechan i sms to be considered

in a re l iabi l i ty ana lys i s . The f i rs t is p r e m a t u r e failure which occurs

pr ior to wear -ou t of the component, and the second is wear -ou t

itself. P r e m a t u r e fai lures occur randomly in t ime from predictable

causes . After the device has reached design maturi ty , the causes

of failure a r e e i ther assoc ia ted with ( 1) s ta t i s t ica l occur rence of

s t r e s s e s exceeding the s t rength capabi l i t ies of the device, or

(2) device defects which escape the quality control check points. If the de ­

sign cons t ra in t s on the device weight and volume a r e sufficiently generous ,

the suscept ib i l i ty to the s ta t i s t i ca l occu r r ence of applied s t r e s s exceeding

the m a t e r i a l s t rength capabil i t ies can be made insignificantly smal l . How­

ever, t he re a re few si tuat ions where the re a r e effectively no l imits on design

weight and volume. Similar ly , the probabil i ty of a device escaping quality

control check points is re la ted to the amount of money that can be allocated

forNthis activity. Given sufficient money, a device can be subjected to

repeated nondest ruct ive tes t techniques , meta l lurg ica l examinations, physi­

cal p roper ty measurennents , radiographic inspection, etc. , and p r e ­

m a t u r e fa i lures caused by undetected quality problems can be reduced to

a negligible level . But again, the funds available for inspecting each and

every component a r e finite and the re fo re p r e m a t u r e fai lure r a t e s will a lso

be finite.

The rel iabi l i ty analys is effort on the candidate sys tems therefore

r equ i r e s a s s e s s m e n t of the suscept ibi l i ty of the designs to p r e m a t u r e fail­

u r e . One technique cons is t s of obtaining data on the number of fai lures

that occur as a function of cumulat ive tes t t ime on components s imi la r in

configuration to those being ut i l ized in the design. Data on many gener ic

components were published by Avco Corpora t ion in their Reliabili ty Engi­

neer ing Data Se r i e s , in 1962. These a r e st i l l useful but a m o r e recent

data sou rce , which is updated approximate ly 2 to 4 t imes year ly , is the

"FARADA" or F a i l u r e Rate Data Handbook, which is published by the U. S.

Naval F lee t Miss i le Systems Analysis and Evaluation Group, Corona,

California. This collection includes fai lure r a t e data on m a n - r a t e d equip­

ment or components s imi l a r in design concept, if not s ize , to many of the

components used in the candidate t h e r m a l c o n v e r t e r s . Where s ize ,

4-1

operat ing envi ronments , and other application data for the components a r e

s imi la r , the FARADA data a r e p re fe r red . However, comparable FARADA

components are not always avai lable and few FARADA components a r e

subjected to 10 y e a r s of unin ter rupted use without main tenance . The re ­

fore cons iderable effort was expended to obtain appropr ia te component

fai lure r a t e data .

In o r d e r to ref lec t continued s t a t e - o f - t h e - a r t advances in design,

manufacturing and quality control , the lowest fai lure r a t e value was chosen

in the FARADA tab les when mul t ip le fa i lure r a t e data were avai lable o r ,

in the ca se of the Avco Data Se r i e s , the lower l imit fa i lure r a t e was

chosen. This choice ref lec ts the expected re l iabi l i ty improvements that

will occur over the next 3 to 5 y e a r s .

It should a lso be pointed out that the p r e m a t u r e fa i lure re l iabi l i ty

model dea ls only with the probabi l i ty of observing a ca tas t roph ic , or sud­

den the rma l conver te r fa i lure .

Since mos t of the data a r e based on p r e m a t u r e fa i lure r a t e s for com­

ponents which have been tes ted for less than 10 y e a r s , the accuracy of

the re l iabi l i ty does have some uncer ta in ty on an absolute b a s i s . However

we believe that the compara t ive re l iabi l i ty predic t ions a r e sufficiently

accura te to p e r m i t a valid r e la t ive compar i son of the candidate s y s t e m s .

4. 1 PREMATURE FAILURE RELIABILITY MODELING

The mos t un iversa l ly accepted m e a s u r e of a component p a r t r e l i ab i ­

lity is the fai lure r a t e . A review of the fa i lure r a t e s used by different

companies on different p ro jec t s r evea l s a wide range of var ia t ions for each

component pa r t . This var ia t ion in fai lure ra te is re la ted not only to the

manner in which a component is designed, manufactured, and inspected but

•also to the m a n n e r in which a component is applied in a sys tem and the

environment in which it is opera ted . As might be expected, these fac tors

resu l t in the ass ignment of different fai lure r a t e s by different u s e r s to the

same pa r t .

Several detai led ana lyses have been pe r fo rmed to no rmal i ze the dif­

fe rences in fa i lure r a t e s for a gener ic c l a s s of components by cor re la t ing

instal lat ion environment and observed fa i lure ra te uti l izing a la rge

4-2

observed population. The co r re l a t ions do not pe rmi t an independent

determinat ion of the degree to which different factors make up the

instal lat ion environment, i.e., shock, vibration, t empera tu re , and

humidity. However, they do pe rmi t the determinat ion of cha rac t e r i s t i c

factors (K factors) , which, when multiplied by the generic failure rate,

yield the predic ted failure ra tes for the different installation environ­

ments . Typical K factors a r e p resen ted in Table 4-1. (Reference 23)

When obtaining a fai lure r a t e from the FARADA tables , or other

s imi la r data s e t s , it is n e c e s s a r y to adjust the source data to fit the new

environmental application and the re fo re an environmental K factor for the

the rmal conver te r application is r equ i red . Table 4-2 compares the tem­

p e r a t u r e , v ibrat ion, and shock environments general ly associa ted with the

operat ing modes p resen ted in the table below.

Table 4 - 1 . Resul t s of Corre la t ion Study

Mechan ica l /E lec t romechan ica l Component Operat ing Mode

Satell i te in Orbit

Computer

Ground Equipment

Shipboard Equipment

Rail Mounted Equipment

Aircraf t Equipment (bench tes t )

Miss i le Equipment (bench tes t )

Ai rcraf t Equipment (in-flight)

Miss i le Equipment (in-fhght)

Average K Fac tor

1

1

8

15

22

30

40

50

900

4-3

Table 4 - 2 . Compar i son of T e m p e r a t u r e , Vibration and Shock Environments

Operating Mode

Satell i te in Orbit

Computer

Ground Equipment

Shipboard Equipment

Rail Mounted Equipment

Aircraf t (in-flight)

Environments

K F a c t o r

1

1

8

15

22

50

Vibrat ion

None

None

Mild

Modera te

Modera te

Moderate / Severe

Shock

None

None

Moderate

Moderate

Heavy

Modera te / Severe

T e m p e r a t u r e and

Var ia t ions

Controlled

Controlled

Modera te / Uncontrolled

Modera te / Uncontrolled

Mode ra t e / Uncontrolled

High/ Uncontrolled

The t h e r m a l conve r t e r s will be subjected to mild v ib ra t ions ,

modera t e - to -h igh shock, and m o d e r a t e , but control led t e m p e r a t u r e s

(depending on the component) . This environment has the re fore been

assigned a K factor of 5, i n t e rmed ia t e between the computer and ground

equipment env i ronments .

When using the FARADA fai lure r a t e or other source data in the

rel iabi l i ty models developed for each of the candidate s y s t e m s , the re l i ab i ­

lity predic t ion r e s u l t s a r e desc r ibed as " re l iabi l i ty best e s t i m a t e s " and

a r e assumed to be equivalent to 50% s ta t i s t i ca l confidence level p red ic ­

t ions. This s ta t i s t i ca l confidence level is not to be confused with the engi­

neer ing confidence that i s a sc r ibed to the re l iabi l i ty predic t ions (see Sec­

tion 4. 4) s ince for many of the components ut i l ized in the va r ious candidate

s y s t e m s , no d i rec t ly comparab le fai lure r a t e data exist in the appropr ia t e

s ize and weight range . Therefore , adjustments a r e made to the fai lure

r a t e data to ref lect the ut i l izat ion of s m a l l e r - s i z e d or l ighter -weight com­

ponents. A d i scuss ion of these p a r t s , including fa i lure r a t e data accumu­

lated from other than the FARADA, Avco Data Se r i e s , or MIL-HDBK-217A

data s o u r c e s , i s p resen ted below.

4 - 4

4. 1. 1 Vapor /Gas Bear ings

One of the bes t sou rces for fa i lure r a t e data on v a p o r / g a s bear ings

a r e the Minuteman weapons sy s t em guidance and control packages which

includes s eve ra l bear ings that opera te continuously in the launch s i lo . Of

the s eve ra l gyro appl icat ions in the Minuteman, the 16,000 r p m pendulus

in tegra t ion gyroscope spool bear ing , with a shaft d iamete r of 0. 25 inch

and a length of 0. 43 inch, i s very c lose in s ize and application to the vapor

bea r ings proposed for the r o t a r y engine. Table 4-3 p r e s e n t s the use

h i s to ry on these b e a r i n g s .

Table 4 - 3 . Bear ing Use His tory

Number of Units

160 plus

Average Continuous

Operat ing Time

4 yr

Longest Continuous

Operat ing Time

6 yr +

F a i l u r e Rate (after spin-up)

0.00035/1000 hr

10 yr Reliabil i ty

0 .94

Approximately 90 to 95% of all bear ing fa i lures occur during the

spin-up of the bear ing . The remain ing 5% of fa i lures a r e believed to be

caused by in ternal ly genera ted contaminants such as off-gassing of potting

compounds. Detai led exanaination of fa i lure h is tory indicates the fa i lures

to be random in t ime with no indicat ion of wear -ou t . This is consis tent

with the fact that t he re is no m e t a l - t o - m e t a l contact in the bear ing and

the re fore 10-year life appea r s achievable . Several other data sources

were invest igated and while fa i lure r a t e data were not avai lable , maximum

operat ing t i m e s a r e shown below:

Gas Bear ing Application R P M Demons t ra ted Life

Minuteman Hel ium Blower 24, 000

Brayton Cycle A l t e r n a t o r s 48, 000

Cooling Turbine 6, 000 to 73, 000

Turboa l t e rna to r 180,000 to 220,000

30,900 hr

8,000 + h r

= 7,000 hr 6, 000 hr min imum

6, 000 h r min imum

4-5

Z ^ l

These data for gas bear ings a r e for discontinuous operation and

therefore r e p r e s e n t a m o r e s t r e ss fu l application than a continuous se rv­

ice application of the same total accumula ted se rv ice t ime.

4. 1. 2 Expansion Turbine

Review of the FARADA data was made for the mos t applicable tu r ­

bine fai lure r a t e data. Hot gas turbine data were cons idered inappropr ia te

for ana lys is because the ro t a ry vapor turbine will be subjected to a peak

t e m p e r a t u r e of only 425° F using the C P - 3 4 working fluid. The fai lure

r a t e data chosen for the r o t a r y vapor turbine a r e based on c o m m e r c i a l

a i rc ra f t (DC-9) exper ience but adjusted as shown below:

Operat ing Time ^o. of Unadjusted Adjusted

Component Total F a i l u r e s F a i l u r e Rate F a i l u r e Rate

DC-9 Cooling 561,670 hr 24 44. 5 x 10" F / h r 0. 36 x 10" F / h r Turbine

The adjusted fa i lure r a t e is based on a review of the fai lure modes

exper ienced by turbine a s s e m b l i e s as s u m m a r i z e d below:

Approximate Pe rcen tage F a i l u r e Mode of All F a i l u r e s (%)

Damage (ingestion of 40

foreign m a t t e r )

Bear ing fai lure 5

Lubr ica t ion 2

Overspeed 20

S t ruc tu ra l fa i lure 25

Leakage and impe l l e r wear 8 Total 100

Since a t h e r m a l conver t e r turbine will not be subject to the ingest ion

of foreign m a t t e r , ope ra to r - induced inadver tent overspeed condit ions, or

bea r ing / lubr i ca t ion fa i lu res , roughly 67% of the above fa i lu res exper ienced

on the DC-9 tu rb ines a r e not appl icable . Of the remain ing 33%, 25% of the

4-6

z3f

fa i lures a r e s t r u c t u r a l in na tu re and the re fore re la ted to the turbine blade

s t r e s s e s which a r e propor t iona l to blade speed. Since the ro t a ry vapor

turbine tip velocity is 470 f t / s ec as compared to the 1200 to 1500 f t / sec

velocity of a i r c ra f t t u rb ines , the fa i lure probabil i ty caused by excess ive

s t r e s s conditions becomes negligible as shown below:

APPLIED STRESS AT 1200 FT/SEC

AIRCRAFT TURBINE

MATERIAL STRENGTH

(1500°F)

APPLIED STRESS

AT 470 FT/SEC

FAILURE PROBABILITY =

25% X 44.5 X 10"*^ F/HRS

n . 3 k s i

ROTARY VAPOR TURBINE MATERIAL STRENGTH (325''F)

\ 25.0 ksi

FAILURE PROBABILITY NEGLIGIBLE

Essent ia l ly then the only fa i lure mode that the ro t a ry vapor turbine

might be subjected to i s the remain ing 8% of the a i rc ra f t turbine fa i lures

a t t r ibutable to "leakage and impe l l e r wear . " With the environmental applica­

tion factor of 50 for an a i r c r a f t tu rb ine and an application factor of 5 for

the AHD t h e r m a l conve r t e r , the fa i lure r a t e becomes :

turb ine thernnal conver t e r

= \ , . X 8% x 5/50 tu rb ine a i r c ra f t

= 44. 5 X 10"^ F / h r x 0. 08 x 1/10

= 0 .36 X 10" F / h r

4. 1. 3 Bellow Seals and Pumps

FARADA data , in the c a s e of bellows sea ls and pumps , a r e inade­

quate both in number s of gener ic bel lows t e s t s and types of bellows

(non-meta l l ic v e r s u s hyd ro - fo rmed v e r s u s welded convolutions). Welded

bellows a r e cons idered the only acceptable design for 10-year l ife. There­

fore bellows manufac tu re r s w^ere contacted to de t e rmine the amount of

long cycle life data ava i lab le . Table 4 -4 s u m m a r i z e s the test results on

four different welded bellows designs.

4-7 Z-^j

Table 4 - 4 . Tes t Resu l t s of Four Different Welded Bellows Designs

Bellows Design

MB-21

MB-41

MB-150

MB-151

No. of Units

18

2

10

1 ^

Total Tes t (hrs)

61 ,581

11.419

85,461

24 .504

•

Total No. of Cycles (mill ions)

11,100

2, 100

8,800

2,500

No. of F a i l u r e s

0

0

0

0

Avg. No. of Cycles P e r Unit* (millions)

616

1,050

880

500

Synchronous engine des igns r e q u i r e a min imum of 630 mil l ion cycles for a 10-year life.

To achieve the cycle life shown in Table 4 -4 , two impor tan t r e q u i r e ­

ments , bes ides l imi t ing max imum allowable cyclic s t r e s s e s , must be met .

F i r s t , the al ignment of the longitudinal axis of the top and bottom bellows

convolutions mus t be mainta ined within 0. 005 inch and, second, the bel­

lows mus t be subjected to a 1-million cycle quality control "bu rn - in" to

detect incipient bellows defec ts .

4. 1. 4 P r e c i s i o n Ba l l /Hydrodynamic Sleeve Bear ings

P r e c i s i o n ball bear ing and hydrodynamic s leeve bea r ings a r e both

infer ior , f rom the re l iab i l i ty point of view, to the gas or vapor bear ings

d i scussed e a r l i e r . The pred ic ted design fatigue life on ball bea r ings is

de te rmined by using AFBMA s tandard ana lys is techniques using the equiva­

lent radia l loads, dynamic loads, opera t ing speeds, etc . While the design

fatigue life is cons idered to be the e s t ima ted life which will be exceeded by

90% of a group of ident ical bear ings operat ing under ident ical load condi­

tions (assuming proper mounting, lubr ica t ion and protect ion) , the ave rage

fatigue life is approximate ly five t imes this figure. Thus the var iabi l i ty

or uncertainty in the predic t ion of bear ing life is very high and in

o rde r to p red ic t design fatigue life at a 98 or 99% re l iab i l i ty level r a t h e r

than the 90% level , the p red ic ted life d rops rapidly . F o r the t h e r m a l

conver te r application, if the p red ic ted life is 100 y e a r s based on a 90%-

surv iva l , the pred ic ted life drops to = 20 y e a r s for a 98% probabil i ty of

4 - 8

s u r v i v a l . In o r d e r to k e e p t h e r a d i a l load ing down to m e e t the 1 0 0 - y e a r

l i fe a t 90% r e l i a b i l i t y , b e a r i n g s i z e s for a 20 -pound load a r e qu i t e l a r g e

(0. 75 inch OD).

F o r the p r e m a t u r e f a i l u r e of b e a r i n g s t h e FARADA t a b l e s i n c l u d e

d a t a on s e v e r a l b e a r i n g a p p l i c a t i o n s a s shown be low:

B e a r i n g A p p l i c a t i o n

A t t i t u d e I n s t r u ­m e n t , M i n i a t u r e P r e c i s i o n

50% C u m u l a t i v e Conf idence Leve l T e s t T i m e No. of F a i l u r e R a t e

E n v i r o n m e n t (h r s ) F a i l u r e s ( F / h r )

Spin- M o t o r , 2400 r p m

Ba l l B e a r i n g

A i r c r a f t 5 . 6 5 x 1 0

B o m b i n g C o m p u t e r , A i r c r a f t 2 . 5 6 x 1 0 M i n i a t u r e P r e c i s i o n

A i r c r a f t 1 . 0 1 x 1 0

S u b m a r i n e 1 0 7 3 x 1 0

0

14

0. 123 X 10

3 .52 X 10 -6

14. 8 x 1 0 -6

0 . 0 0 8 3 X 10 -6

The f a i l u r e m o d e s e x p e r i e n c e d by the b e a r i n g s u s e d in the Bombing

C o m p u t e r a r e g iven be low:

F r o z e n ( s e i z e d ) 50%

V i b r a t i o n e x c e s s i v e 2 1 %

O v e r h e a t e d 7%

P r e l o a d l o s t 7%

W o r n 7%

B r i n e l l e d 7%

It should be no ted tha t t h e a b o v e f a i l u r e s did not o c c u r d u r i n g the

o p e r a t i o n a l u s e of the e q u i p m e n t but d u r i n g p r e d e l i v e r y fl ight t e s t e x p e r i ­

e n c e and t h e r e f o r e the a b o v e f a i l u r e s should not in the n o r m a l s e n s e , con­

s t i t u t e b e a r i n g w e a r - o u t but r a t h e r p r e m a t u r e b e a r i n g f a i l u r e s .

The g a s b e a r i n g s d i s c u s s e d e a r l i e r , wh ich have d e m o n s t r a t e d a

r e l i a b i l i t y g r e a t e r t h a n 94% and no w e a r - o u t a f t e r m o r e than 6 y e a r s of

c o n t i n u o u s s e r v i c e , h a v e a s i g n i f i c a n t l y b e t t e r r e l i a b i l i t y r e c o r d than

p r e c i s i o n ba l l b e a r i n g s .

4-9

Z^ <il

In conclusion, prennature ball bear ing fa i lures as d i scussed above

can be a s s e s s e d for re l iab i l i ty , but life predic t ions a r e sensi t ive to the

actual bear ing ins ta l la t ion de ta i l s .

4, 1.5 Elec t ronic Components and Solid State Devices

Numerous sources of fai lure r a t e data exist for the application of

e lectronic pa r t s to different equipments . T e m p e r a t u r e environments a r e

not s eve re for these p a r t s in the typical t he rma l conver te r application but

to ref lect the high shock loading r e q u i r e m e n t s , an applicat ion factor of

5 has been applied to the bas ic l abora to ry fai lure r a t e data compiled by

Mar t in -Mar ie t t a Corp . This probably r e p r e s e n t s some degree of conse r ­

va t i sm in the re l iabi l i ty modeling of the candidate s y s t e m s .

Generic life expectancy of e lec t ronic p a r t s such as diodes and t ran­

s i s to r s in predic ted as being 200, 000 hours . (Reference 24) More recent

exper ience on Minuteman Miss i l e Systems using Hi-Rel e lec t ronic pa r t s subjected

to burn- in and p a r a m e t e r drift sc reen ing indicates that gener ic life expectancy

of capac i to r s , r e s i s t o r s , e tc . , i s in excess of 200, 000 hou r s . Based on

the above data, wea r -ou t of e lec t ronic p a r t s in the p r e sen t applicat ion is

not considered a p rob lem.

4. 1. 6 Gear Boxes/Speed Reducers

A significant quantity of fai lure rate data exis ts on gear a s sembl i e s and

can be used to a s s e s s the motor r e c i p r o c a t o r re l iabi l i ty agains t p r e m a t u r e

fai lure. The data shown in Table 4-5 were ext rac ted from the FARADA

tables for those assenabl ies that had accrued significant amounts of tes t

data on l a rge unit sample s i z e s .

The es t imated life of a gear t r a in is a function of the number of load

cycles , torque l eve l s , tooth sliding speeds , lubr icat ion, m a t e r i a l , e tc . ,

and as with p rec i s ion ball bea r ings , depends on the actual design de ta i l s of

the gear box. F igu re 32.22 of Mechanical Design and Sys tems Handbook, - - 12

page 38, (Reference 24) indicated that gear life of 10 cycles is obtainable

provided gear bending and contact s t r e s s e s a r e control led, A pa r t i cu l a r ly

4-10

Table 4 - 5 . Sample Assembly Tes t Data from FARADA Tables

Application

Aircraft Radar Gear Box

Air Conditioning Gears - Grd.

Blower Fan Gears - Grd

Air Compressor Gears - Grd

No. of Units

4 1 4

7 3 8

1200

6 1 2

Total Test (hr)

2 33 X 10^

3 19 X 10^

5 18 X 10^

2 64 X 10^

No Fai

. of lures

1

1

1

0

Application Factor

1 / 5

5 / 8

5 / 8

5 / 8

Adjusted Failure

Rate (F/hr)

0 .085 X 10-^

0 .019 X 10"^

0 .012 X lO"^

0. 16 X 10'^

10- Year Reliability

0 .992

9. 998

0 .999

0 .9986

impor tan t cons idera t ion for long gear box life is accuracy of manufacturing

and meshing and, the re fo re , r igidi ty of the g e a r s , gear shaft, bear ings

and gear box. F u r t h e r subjective evidence of achieving a 10-year life for

gear boxes is offered in the common wr i s t watch, which by using high-

p rec i s ion l ightly-loaded g e a r s , can achieve long life, even without the

benefit of an he rmet ica l ly sealed environment .

4. 1. 7 High Energy Density Ba t t e r i e s

The two candidate high energy density ba t t e r ies a r e the lithium-

selenium cell which opera te s a t710OFand the sodium-sulfur cell which

ope ra t e s at 570° F . The l i t h ium-se len ium cell has had the benefit of a

longer development t ime , but seve ra l fa i lure modes r emain to be con­

t rol led for a 10-year l ifet ime goal. The f irst problem is associa ted with

the smal l re la t ive s ize of the l i thium ion (as compared to sodium and sulfur

ions) which aggrava tes seal and co r ro s ion -con t ro l p rob l ems . Another

p rob lem is the use of the semi-fused salt s epa ra to r and liquid t r anspor t of

the react ing ions. With the semi- fused salt separa to r it is not known what

degree of capacity loss may occur during numerous cha rge /d i scha rge

cycles and the resul t ing poisoning or chemical short circui t ing of the

reac t ing e lements .

The sodium-sulfur cell, while l e s s advanced than the l i th ium-seleniur

cel l , would appear to have seve ra l advantages . The f irst advantage is

assoc ia ted with the reduced operat ing t e m p e r a t u r e and correspondingly

reduced m a t e r i a l s compatibi l i ty p r o b l e m s . Ceramic sea ls and m a t e r i a l s

4-11

compatibi l i ty technology a r e well developed for today 's sodium-vapor

l amps . Additional applicable technology is being developed for thermionic

applicat ions at t e m p e r a t u r e s as high as 700°C. In addition, since no

semi-fused salt s epa ra to r is requ i red , l i t t le if any ba t te ry capacity loss

is expected in the sodium-sulfur cel l as a function of the number of

c h a r g e / d i s c h a r g e cyc les . Additional development effort is requ i red in the

ce ramic s epa ra to r and the beta a lumina sea l a r e a s at this s tage of sodium-

sulfur ba t te ry technology. In s u m m a r y , while no re l iabi l i ty data on cell

life and p r e m a t u r e fa i lure exist at this t ime on ei ther concept, a rev iew

of the poss ib le cel l fa i lure modes indicates the sodium-sulfur cell to be a

potentially higher re l iabi l i ty ba t te ry design.

4. 1. 8 PCU Ci rcu l a r Drive Cam

Life predic t ions on the d r u m - c a m and follower sy s t em will depend

on specific design detai ls but analogies avai lable in the t r anspor ta t ion

industry indicate that the typical automobile engine cam lobe turns

approximately:

1/2 X 1500 r evs x_60^x 1500 hr ^ 0. 7 x 10^ r e v s / 6 0 , 000 mi min hr 60 ,000 mi

Fo r the t h e r m a l conver te r application the total number of revolut ions (at

significantly lower c a m lobe sur faces s t r e s s e s and rubbing veloci t ies) i s :

50 r e v s x 60 min x 87,400 h r - 2. 6 x 10 r e v s / 1 0 yr min hr 10 yr

Thus 10-year life for a d r u m - c a m appea r s achievable .

The FARADA fa i lure r a t e data for p r e m a t u r e c a m and follower

fa i lures on l a rge p a r t populations is s u m m a r i z e d below:

Application

Truck, AF

Truck, AF

C o m p r e s s o r Air Conditioning

Total Hr

5.05 X 10^

3.18 xlO^

1.48 x 10^

No. of Units

1171

738

343

No. of F a i l u r e s

1

1

0

F a i l u r e Rate ( F / h r )

0. 19 X 10"^

0. 31 x 10-6

0.47 X 10-6 @50% C. L.

4-12

Since the above engine appl icat ions have 12 cam lobes for a 6-cyl inder

engine using one cam shaft, the fa i lure r a t e for the the rma l conver te r

c a m is equal to 1/6 the engine c a m fa i lure r a t e ; or 1/6 x 0. 19 x 10" =

0. 031 X 10"6 F / h r .

4. 2 SUMMARY OF THE PREMATURE FAILURE RELIABILITY ESTI­MATES FOR THE CANDIDATE SYSTEMS

4. 2. 1 System Reliabi l i ty Math Model Resul t s

The final p r e m a t u r e fa i lure re l iabi l i ty modeling r e su l t s for the 8

candidate s y s t e m s together with a brief descr ip t ion of the subsys tems is

shown in Table 4 -6 . Sys tem re l i ab i l i t i e s vary f rom a high of 0. 69 for the

hybrid without ba t t e ry to a low of 0. 49 for the gas rec iproca t ing with

TESM. The following pa r ag raphs d i scuss the sys t ems in m o r e detail .

4. 2. 2 Hybrid T h e r m a l Conver te r

P r i o r to ini t iat ing the hybr id sy s t em re l iabi l i ty math modeling it

was n e c e s s a r y to se lec t the opt imum e lec t r i ca l network for the two power

s o u r c e s . A review of the fa i lure modes for both the tu rbogenera tor and

t h e r m o e l e c t r i c conve r t e r was made and indicated that approximate ly 2 /3

of all t u rbogenera to r fa i lures r e su l t ed in open c i rcu i t or equivalent open

c i rcu i t fa i lures and the remain ing 1/3 of the fa i lures were shor t c i rcui t .

S imi lar ly , t h e r m o e l e c t r i c c o n v e r t e r s of the spr ing- loaded element design

r a r e l y fail s h o r t - c i r c u i t and, given a total sys t em fai lure , fail open -c i r ­

cui t . There fore the two power s o u r c e s were connected e lec t r ica l ly in

pa ra l l e l to p ro t ec t against open c i r cu i t fa i lures and blocking diodes were

used to p ro tec t agains t the a l t e rna to r failing sho r t - c i r cu i t . With this con­

f igurat ion the opt imum re l iab i l i ty configuration was achieved. With 30

SiGe and sixty 2N/TAGS couples the hybrid engine rel iabi l i ty

agains t ca tas t roph ic fa i lure i s p red ic ted as 0. 94 and is shown together

with the e lec t ronic components r equ i r ed for the t h e r m o e l e c t r i c dc - to -dc

voltage stepup conver t e r in Table 4 -7 . Reliabil i ty modeling of the e l ec t r i ­

ca l PCU sys t em for the hybr id engines is shown in Table 4-8 with the

naechanical ac tua tor shown in Table 4 - 9 .

4-13

Table 4 -6 . P r e m a t u r e Fa i lu re Reliabil i ty

1 Candidate 1 System

T / E 1 with Battery

1 1 Rotary Vapor

with Battery

Hybrid with Battery

Hybrid

Gas Reciprocating with TESM

Gas Reciprocating

Linear Vapor with TESM

Linear Vapor

Engine

Two Parallel Redundant Strings of Couples

0.905

(Turbo Alternator)

0.85

Parallel Redundant Engines Fail-Open Protection

0 .94

Parallel Redundant Engines Fail-Open Protection

0.94

0.89

0.89

0.80

0.80

PCU

DC Motor, Planetary, Cam

0.80

DC Motor, Planetary, Cam

0. 80

DC Motor, Planetary, Cam

0.80

DC Motor, Planetary, Cam

0.80

Fluid Timer, High and Low Accumulator

0.85

Fluid Timer, High and Low Accumulator

0.85

Electronic Oscillator

0.81

Electronic Oscillator

0.81

Actuator

Mechanical (2M)

0.90

Mechanical (2M)

0.90

Mechanical (2M)

0.90

Mechanical (2M)

0.90

Hydraulic (IF)

0.66

Hydraulic (2F)

0.66

MechAnical (IM) and Fill Switch

0.88

Mechanical (2M)

0.90

Battery

Na/S

0.90

Na/S

0.90

Na/S

0.90

TESM

LiF/NaF

0.99

LiF/NaF

0.99

Total

0. 58

0. 55

0.61

0.68

0.49

0. 50

0.57

0.58

Table 4 -7 . P r e m a t u r e Fa i lu re Reliabili ty Model for the Hybrid Engines

ADDITIONAL HYBRID COMPONENTS FOR VOLTAGE MATCHING

COMPONENT

TRANSISTORS (DC PULSES)

DIODE (OUTPUT)

TRANSFORMER, 4 WINDING

RESISTORS

CAPACITOR

INDICATORS

NUMBER ITEMS

2

2

1

3

3

1

FAILURE RATE

0.010 X 10-6 FAIR

0.006 X 10-*^

O.IOOX 10-*

0.004 X 10-*

0.004 X 10-*

0.050 X 10-* F/HR

PRIMARY FAILURE MODES

45% SHORT 55% OPEN

50% 50%

~

5% 95%

60% 40%

~

FAILURE RATE SOURCE & ENVIRONMENT

MARTIN-MARIETTA CORP.

MARTIN-MARIETTA CORP.

MARTIN-MARIETTA CORP.

MARTIN-MARIETTA CORP.

MARTIN-MARIETTA CORP.

MARTIN-MARIETTA CORP.

CUMULATIVE OPERATION TIME & FAILURES

~

~

—

~

~

~

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/1

5/1

5/1

5/1

5/1

5/1

ADJUSTED FAILURE RATE

0.10 X 10-* F/HR

0.06

0.50

0.06

0.09

0.25

DC-DC -(0.10 + 0 .06+ 0 .50+ 0 .06+ 0.090+ 0.25) X 10"* X 8.76 X 10^ -1.06 X 10** X 8.76 X 10^ - 9 . 3 X 1 0 ' ^ „ „ , ,

e = e = e = 0.911

R g = 1 - i 1 - e -(1.70 X O'* X 8.76 X 10^)

12 30

1 - 1 - e •(0 48 X lO''" X 8 76 X lo"*)

X e •(0.0O6+0.003 +0.009) X 10"* X 8.76 X 10^

12 30 = (0.981)'^ X (0.9984)''" X 0.998

= 0 . 7 9 X 0 . 9 7 4 X 0 . 9 9 8 = 0.768

^OTAL = ' " < ' - V ( ^ - ' V E ^ ' ' D C - D C )

= 1 - ( 1 - 0.85) (1 - 0.768 X 0.911)

= 1 - ( 0 . 1 5 ) ( 1 - 0 . 7 0 ) = 1 - ( 0 . 1 5 X 0 . 3 0 )

= 0.94

•^DC-X " "RELIABILITY OF VOLTAGE STEP-UP CONVERTER

R., = RELIABILITY OF T/E HYBRID CONVERTER

'*RE = RELIABILITY OF ROTARY VAPOR ENGINE

Table 4 -8 . P r e m a t u r e Fa i lu re Reliabil i ty Model for the T /E and Rotary Vapor Engine PCU

COMPONENT

FRACTIONAL H.P. ELECTRIC MOTOR

HYD«ODYNAMIC BEARINGS

SPUR GEAR SETS

CAM 4 FOLLOWER ASSY

BELLOWS SEAL

NUMBER OF

ITEMS

1

4

4

1

1

FAILURE RATE F/HR

0.31 X 10-* F/HR

0.098 X 10-* F/HR

0.19 X 10-*F/MR

0.19X 10-* F/HR

0.0285 X 10-9 F/CY

PRIMARY FAILURE MODES

BURNED OPENAHORTED

SEIZURE

TOOTH FRACTURE

PREMATURE WEAROUT

LEAKAGE

FAILURE RATE SOURCE 4 ENVIRONMENT

FARADA 352F GRD

FARADA 352A GRD

FARADA 384J GRD

FARADA 352A GRD

METAL BELLOWS LAB DATA

CUMULATIVE OPERATING TIMES 4 FAILURES

3.18 X 10* HRS, 1 F

10.1 X 10* HRS, 1 F

5.18 X 10* HRS, 1 F

5.06 X 10* HRS, 1 F

24.5 X lO' CYCLES, 0 F

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/8

5/8

5/8

5/8 X 1/12

5/1

ADJUSTED FAILURE RATE, F/HR

0.19 X 10-* F/HR

0.24 X 10-* F/HR

0.48 X 10-* F/HR

0.01 X 10-* F/HR

0.142 X 10-' F/ CYCLE® 50% C.L.

U ^ ^-(0.19+ 0.24+ 0.48 +0.01) X 10"*X8.76X 10* ^ ^-{0.142 X 1 0 ' ' x 6 3 0 X 10*)

-0.92 X 8.76 X 10"^ ^ -89.5 X 10"' e X e

= 0.922 X 0.91 = 0.84

ELECTRONIC COMMUTATION COMPONENTS

TRANSISTORS, NPN, (OSCILLATION)

RESISTORS (OSCILLATION)

CAPACITOR -(OSCILLATION)

TRANSISTORS NPN SWITCHING LOW POWER

PC BOARD

LOW TUNING COILS

IC DECODE COUNTER

2

3

1

4

1

2

1

0.010 X 10^ F/HR

0.004 X 10-*

0.004 X 10-*

0.010X 10^

~ 0.010

0.30X 10-*

45% SHORT

5%

60%

60%

~

60%

55% OPEN

95%

40%

40%

40%

MARTIN MARIETTA CORP.

MARTIN MARIETTA CORP.

MARTIN MARIETTA CORP.

MARTIN MARIETTA CORP.

~

MARTIN MARIETTA CORP.

NOT APPLICABLE 5/1

5/1

5/1

5/1

~

5/1

O.IOX I0-*

0.060 X 10-*

0.004 X 10^

0.20 X 10-*

~ O.IOX 10-*

0.15 X I0 - *

„ - c X t -(0.614 X 10"* X 8.76 X 10-* -0 .54X10-2 R = • « • = •

»TOTALPCU-0-'»"' '°- '5"°-~

Table 4 -9 . P r e m a t u r e Fa i lu re Reliabil i ty Model For Actuator (E lec t r i ca l Systems)

COMPONENT

DIAPHRAGMS (REDUNDANT)

COMPLIANCE BAG

SPRING ^

CABLE ROD, SHEATH & ASSY.

TITANIM HOUSING

NUMBER OF

ITEMS

2

1

1

1

\

FAILURE RATE F/HR

0.6 X 10"*

0.1 X 10"6

0.03 X 10-6

0.002 X IQ-*

NEGLIGABLE

PRIMARY FAILURE MODES

TEARS - LEAKAGE

TEARS - LEAKAGE

FATIGUE/BREAKAGE

FATIGUE/BREAKAGE

~

FAILURE RATE SOURCE & ENVIRONMENT

AVCO DATA SERIES

AVCO (ADJUSTED)

FARADA 373 CACR

AVCO DATE SERIES

~

CUMULATIVE OPERATING TIMES & FAILURES

—

~

62.1 X 10* HRS, 2F

~

—

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/1

5/1

5/50

5/1

—

ADJUSTED FAILURE RATE, FAILURES/HR

3.0 X 10"* F/HR/DIA

0.5 X 10"*

0.003 X 10"*

0.01 X 10"*

~

3.0 X lO"* X8.76X 10"''\^

= 1-(0.2305)2 X e - ° - ° ^ *

= 0.9468 X 0.956 = 0.9Q5

X e -(0.5 X 0.003 + 0.01)X 10'* X 8.76 X lO'*

PREMATURE FAILURE RELIABILITY MODEL FOR THE LINEAR VAPOR ENGINE AND TESM ACTUATOR

SAME AS ABOVE EXCEPT FOR ADDITION OF ANTI VACUUM VALVE AND FILL SWITCH

COMPONENT

FILL SWITCH

ANTI VACUUM VALVE

NUMBER OF

ITEMS

1

1

FAILURE RATE F/HR

0 .36X 10"* F/HR

0.85 X 10"* F/HR

PRIMARY FAILURE MODES

OPEN/SHORT

TEARS, RUPTURE

FAILURE RATE SOURCE & ENVIRONMENT

FARADA 354 GRL

FARADA 313 ACFT (FLAPPER VALVE)

CUMULATIVE OPERATING TIMES & FAILURES

5.60 X 10* 2F

1.195 X 10*, IF

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/18

5/50

ADJUSTED FAILURE RATE, FAILURES/HR

0.225 X 10"* F/HR

0.085 X 10"*

R = 0.905 Xe -0.315 X 10"* X 8.76 X 10*

= 0.905 X e - 2 - " ^ ' ° " '

= 0.905 X 0.973 = 0.88

In summary , the t h e r m o e l e c t r i c conver t e r and ro t a ry vapor engines

together p re sen t an advantageous re l iabi l i ty p ic ture as compared to the

other candidate s y s t e m s .

4.2.3 Hybrid T h e r m a l Conver te r with Ba t te ry

F r o m the re l iabi l i ty point of view and having designed the heat

exchangers to prevent body t i s s u e t e m p e r a t u r e s from exceeding 42 C

under w o r s t - c a s e conditions, the p r e s e n c e of a sodium-sul fur ba t t e ry

dec reased the overal l s y s t e m rel iabi l i ty , i r r e s p e c t i v e of how re l iable

the ba t tery is . Although the initial degradat ion mode may be a loss of

capacity or even open-c i rcu i t conditions for individual cells , the most

likely ul t imate fai lure is a s y s t e m - l e v e l shor t c i rcu i t which is not readi ly

isolated without a significant pe r fo rmance penalty. As d i scussed in

pa ragraph 4.1.7 above, the sodium-sul fur ba t t e ry fai lure modes were

analyzed and from this information the re l iabi l i ty of the ba t t e ry for 10

yea r s (when developed) was es t imated as 0.90. The redundancy con t r i ­

bution of the ba t te ry is smal l since in the event of engine fai lure, the

seve ra l hours of survival t ime that would be avai lable from ba t te ry power

would be marg ina l f rom the standpoint of pe rmi t t ing r emed ia l action.

4 , 2 . 4 T h e r m o e l e c t r i c / B a t t e r y The rma l Conver te r

To help e l iminate ca tas t rophic open c i rcu i t conver te r f a i lu res , the

t he rmoe lec t r i c modules a r e e l ec t r i ca l ly connected in two p a r a l l e l s t r ings

of couples for a total of 8 SiGe couples and 28 TAGS, B i S b T e / P b T e - G e T e ,

2N cascaded couples . RCA data on SiGe modules and Snap 19, - 2 1 , and

-23 data on the other couples w e r e used to obtain the couple fa i lure r a t e s .

Since both the RCA and Snap 19 data revea led no open-couple fa i lures to

date, the re l iabi l i ty calculat ions w e r e pe r fo rmed at the 50% confidence

level . (This is a l i t t le m o r e opt imis t ic than the FARADA technique of

assuming one fa i lure for pu rposes of calculat ing a fa i lure r a t e when in

fact no fai lure has been observed . Using 50% confidence l imi t s , this is

equivalent to a 0. 693 fa i lure probabi l i ty when in fact no fa i lure has been

observed. ) As noted in the re l iabi l i ty math model shown in Table 4-10,

l imited tes t data exist for the P b T e - G e T e e lements but based on the s imi ­

lar i ty of P b T e - G e T e fa i lure modes to the 2N e lemen t s , the fa i lure r a t e

4-18

Table 4-10. P r e m a t u r e Fa i lu re Reliabil i ty Model for Thermoe lec t r i c Cascaded Conver ter

COMPONENT

SiGe COUPLES

TAGS < ELEMENTS)

BiSbTe (ELEMENTS)

PbTe-GeTe (ELEMENTS)

2N (ELEMENTS)

MIN-K INSULATION

HOT 4 COLD SHOES

SPRING LOAD­ING PLATE

ELECTRICAL CONNECTIONS

NUMBER ITEMS

8

28

28

28

28

-

72

2

72

FAILURE RATE F/HR

0.693 ^ . „ , ^ , 2 05 X 1 0 6 * 50% C L

0.693

7 2 8 X 1 0 6 * 5 ° ^ ^ ^

0 10X 10-* F/HR

N/A

0.693 7 28 X 1 0 6 * 50% C L

0 0006 F/IN-WELD

-

0 03 X i c r * FAIR

0 00001 X 10"* F/HR

PRIMARY FAILURE MODES

OPEN CIRCUIT-SUBLIMATION

OPEN CIRCUIT-SUBLIMATION RESISTANCE

OPEN CIRCUIT-SUBLIMATION RESISTANCE

OPEN CIRCUIT-SUBLIMATION RESISTANCE

OPEN CIRCUIT-SUBLIMATION RESISTANCE

ARGON LEAKAGE

INTERACTION WITH T/E ELEMENTS

LOSS OF PRELOAD, CREEP

OPEN CIRCUIT

FAILURE RATE SOURCE & ENVIRONMENT

RCA LAB DATA

SNAP 19-LAB

SNAP 21 &23

RCA (LIMITED) DATA

SNAP 19-LAB

TRW SPACE FLIGHTS

-

FARADA 373C ACFT

MARTIN MARIETTA CORP

CUMULATIVE OPERATING TIME 4 FAILURES

2 050 X 10* HRS, OF

7 28 X 10* HRS, OF

20 5 X 10* HRS, 2 F

7 28 X 10* HRS, OF

-

-

62 1 X 10*, 2 FAILURES

-

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/1

5/1

5/1

5 I

5/1

15' OF WELD

-

5/50

5/1

ADJUSTED FAILURE RATE, F4HR

1 70 X 10-* F/HR/ COUPLE

0 48 X 10-* F/HR/ ELEMENT

0 504 X 10-* F/HR/ ELEMENT

*

0 48X 10-* F/HR/ ELEMENT

0 009 X 10-* F/HR

0 009 X 10-* F/HR

0 006 X I 0 - * F/HR

0 0036 X I 0 - * F/HR

R = I - 1 --(1 70 X 10"* X 8 76 X 10^1

1 - 1 - e •(0 504 X 10'* X 8 76 X 10*) |

14

-(0 006 - 0 0036 - 0 009) X lO ' * X 8 76 X 10*

(0 981)^ X (0 9984)'* X 0 998

0 926 X 0 9778 X 0 998 = 0 904

•OPEN CIRCUIT FAILURE RATE ON THE PbTe-GeTe ELEMENTS IS ASSUMED EQUAL TO THE SNAP 19,2n MATERIAL SINCE THE RCA LAB DATA ON PbTe-GeTe ELEMENTS 15 EXTREMELY LIMITED

for the 2N m a t e r i a l was a s sumed . On this bas i s the fa i lure r a t e for the

cascaded por t ion of t h e r m o e l e c t r i c conver t e r i s a s s u m e d equal to the

element port ion exhibiting the highest fa i lure r a t e , namely the BiSbTe

e lements , and the re l iabi l i ty modeling re f lec ts th i s . While the above

assumpt ions r e su l t in lack of complete engineering confidence in the

re l iabi l i ty e s t ima te , s i m i l a r a ssumpt ions w e r e r equ i red in the modeling of

the other candidate s y s t e m s . The deg ree of confidence in the assumpt ions

is ref lected in the Section 4. 4 d i scuss ion .

4 . 2 . 5 Rotary Vapor Engine

The re l iabi l i ty model for this engine is as shown in Table 4 - 1 1 . With

the exception of the turbine fa i lure r a t e data which a r e d i scussed in sec­

tion 4 .1 .2 , l i t t le adjustment of the component failure r a t e s was requi red .

This r e s u l t s from the fact that this engine is gener ica l ly well developed

and the confidence in the re l iab i l i ty e s t i m a t e s (excluding the t h e r m a l bat­

tery) is the re fo re a lmos t as high as for the candidate t h e r m o e l e c t r i c

sys tem

4. 2. 6 E lec t r i ca l PCU

Table 4-8 shows the re l iabi l i ty model for the e l ec t r i ca l PCU, con­

sist ing of a pancake dc e lec t ronica l ly commutated motor , magnet ic

coupling, compound p lane ta ry gear d r ive , and drum-canni r e c i p r o c a t o r .

The gas bear ings a r e s imi l a r to those used in the tu rbogenera to r except

they will opera te at lower t e m p e r a t u r e s and rota t ional speeds . The

FARADA data for hydrodynamic bear ings and the Minuteman Miss i le gyro­

scope bear ing data can be used to genera te the re l iabi l i ty e s t i m a t e s . As

with the tu rbogenera to r , the confidence in the re l iabi l i ty pred ic t ions for

the e l ec t r i ca l units i s higher than for the hydraul ic power conditioning

unit. No l i fe- l imi t ing ball bear ings a r e used. Specific design fea tu res

such as the u s e of lubr ica t ion for the speed r educe r and cam, conse rva ­

tively s t r e s s e d pa r t s , and the h e r m e t i c seal ing of the dc motor , a s s u r e

long life.

4. 2. 7 Mechanical Actuator for E lec t r i ca l Sys tems

The re l iabi l i ty nnodeling for the e l ec t r i ca l sys t em automatic blood

pump actuator (as well as the ac tua tor for the l inear vapor engine with

TESM) is shown in Table 4 - 8 . For tuna te ly the actuator d i aph ragms a r e

4-20

Table 4 - 1 1 . P r e m a t u r e Fa i lu re Reliabil i ty Model for Rotary Vapor Heat Engine

1 COMPONENT

EXPANSION TURBINE RADIAL INFLOW IMPULSE TYPE

HELICAL SCROLL PUMP

DC ALTERNATOR, PRM.MAGNETIC

ZENOR DIODE, RECTIFIER

VAPOR GAS BEARING

BOILER & PRE-HEATER (TUBING)

REGENBIATOR

CONDENSER

INSULATION SYSTEM

NUMBR OF

ITEMS

1

1

1

4

2

1

1

1

1

FAILURE RATE F/HR

29.9 X 10-*

0.31 X 10-*

0.693 , .29Xlo i@50%C.L.

0.054 X 10-*

0.35 X 10-*

0.09 X lOr*

0.693 0.7221 X10*@50%C.L.

0.065 X 10-*

0.0006 F/IN-WELD

PRIMARY FAILURE MODES

STRUCTURAL/CAVIT AT ION & EROSION

STRUCTURALAEAKAGE

66% OPEN, 34% SHORTED

SHORTING 75%, OPEN 25%

SEIZURE VIA CONTAMI­NATION

RUPTUR^EAKAGE

LEAKAGE

LEAKAGE

VACUUM LOSS

FAILURE RATE SOURCE & ENVIRONMENT

FARADA 294 ACFT

FARADA 352 GRD

FARADA 384J GRD

FARADA 355 GRD

MINUTEMAN II & III

FARADA 352K GRD

FARACA 292 ACFT

FARADA 352J GRD

TRW SPACE FLIGHTS

CUMULATIVE OPERATING TIMES & FAILURES

0.0669 X 10* HRS, 2 FAILURES

3.188 X 10* HRS, 1 FAILURE

1.29 X 10* HRS, 0 FAILURES

110. X 10* HRS, 6 FAILURES

5.7 X 10* HRS, 2 FAILURES

10.78 X 10* HRS, 1 FAILURE

0.7221 X 10* HRS, 0 FAILURES

15.2 X 10* HRS, 1 FAILURE

—

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/50 X 1/4*

5/8

5/8

5/8

1 / 1 "

5/8

5/50

5/8

15" OF WELD

ADJUSTED FAILURE RATE, F/HR

0.36 X 10"**

0.19 X 10-*

0.338 X 10-*

0.034 X 10-*

0.70 X 10-***

0.056 XlCr*

0.0965 X 10"*

0.0406 X 10-*

0.009 XlOr-*

R = ^ - t x ^ t ^ -(0.36 + 0.19+0.338 + 0.034 + 0.70+0.056+0.0965+ 0.0406+ 0.009) X 10'* X 87,600 HRS _ -1.82 X 10** X 8.76 X 10*

-15.9 X 10 ,-2 0.85

*SEE DISCUSSION IN SECTION 4.1.2 THE AHD TURBINE TIP SPEED IS APPROX. 450 FTAEC COMPARED TO APPROX. 1200-1500 FTAEC FOR AIRCRAFT ENGINE APPLICATIONS AND APPROX. 3/4 OF ALL TURBINE FAILURES ARE CAUSED BY OVERSPEED/INGESTION OF FOREIGN MATERIAL FOR AIRCRAFT APPLICATIONS.

•'MINUTEMAN MISSILE II & III GUIDANCE AND CONTROL PIGA GYRO DATA - SILO TEMP. CONTROLLED CONDITIONS - MIN. TEMPERATURE VARIATION & VIBRATION EXPOSURE. THEREFORE APPLICATION FACTOR ASSUMED TO BE ~ 1 TO 1, SILO CONDITIONS TO HUMAN CONDITIONS.

redundant, for without this redundancy, the unrel iabi l i ty of the ac tua to r s

would significantly degrade the total sy s t em re l iabi l i ty ,

4. 2. 8 Gas Reciprocat ing Engine

The p r i m a r y unre l iabi l i ty assoc ia ted with the gas rec iproca t ing

engine r e s t s with the re l iabi l i ty of the check valves and p i s ton /d i sp l ace r

bear ing su r faces . Since the he l ium working fluid provides no lubr icat ion,

it is n e c e s s a r y to use two hydros ta t ic bea r ings , one for the cy l inder - to -

piston sur faces and the other for the d i s p l a c e r / p i s t o n sur faces Since

the bellows pump is subjected to much sma l l e r deflections (in the o rde r

of 0. 10-inch) as compared to the bellows sea l in the e l ec t r i ca l PCU

dr ive , the fa i lure r a t e for the pump was reduced by the deflection ra t io of

1/10. The re l iabi l i ty model for the gas rec ip roca t ing engine is shown in

Table 4-12 .

4 . 2 . 9 TESM

Since the L i F / N a F m a t e r i a l is only subjected to a phase change in

the p r o c e s s of r e l eas ing and s tor ing t h e r m a l energy, the p r e m a t u r e fa i lure

re l iabi l i ty contr ibut ion to the re l iab i l i ty math modeling of the s y s t e m is

cons idered negligible. Wearout re l iabi l i ty cons idera t ions dealing with

m a t e r i a l s compatibi l i ty p r o b l e m s a r e d i scussed in Section 4. 3.

4. 2. 10 Gas Reciprocat ing Engine PCU

The re l iab i l i ty ma th model for this PCU is shown in Table 4 -13 . No

p a r t i c u l a r re l iab i l i ty p rob lems a r e envisioned for this PCU with the excep­

tion of the ro t a ry fluid t ime r which mus t p o s s e s s quite tight t o l e r ances

for the r o t a r y plug valve in o rde r to mainta in r easonab le sy s t em efficiency.

These to l e rances in turn adve r se ly effect the PCU re l iabi l i ty .

4. 2. 11 Gas Reciprocat ing Engine Actua tors

The re l iabi l i ty math models for these PCU' s a r e shown in Table 4-14

The only difference between the t^vo ac tua to r s is the p r e s e n c e or absence

of the an t i -vacuum valve. This valve is essen t ia l ly a s imple flapper valve

and has a negligible re l iabi l i ty effect on the re l iabi l i ty math model ing.

The one c r i t i ca l i t em in these ac tua to r s is the rol l ing d iaphragm which

must r e t a in the fluid to ac tua te the shutt le valve pis ton. The rol l ing

d iaphragm, unlike the ac tuator d iaphragm, is not redundant and the r e l i ­

ability modeling r e s u l t s re f lec t s this lack of redundancy.

4-22

Table 4-12. P r e m a t u r e Fa i lu re Reliabil i ty Model for the Gas Reciprocat ing Engine

I

COMPONENT

DISPLACER & CYLINDER

PISTON, 2 BEARING SURFACES

DISPLACER SPRING

PISTON SPRINGS

CLEARANCE REGENERATOR

CHECK VALVES

HEAT EXCHANGER

HYDROSTATIC BEARINGS & ORIFICES

MIN-K INSULA­TION, Xe Gas

BELLOWS

NUMBER OF

ITEMS

1

1

1

2

1

2

1

2

1

1

FAILURE RATE F/HR

0 16 X 10-* F/HR

0 16 X 10-* F/HR

0 0 3 X 1 0 - *

0 03X 10-*

0.693 ^ , - , , r 1 0 722 X 1 0 ^ * 50% C L

0 59X 10-*

0 065 X 10-*

0 010 X 10-*

0 0006X 10-6 F/ IN-WELD

0 0285 X 10-9 p/cY

PRIMARY FAILURE MODES

SEIZURE, EXCESS LEAKAGE

SEIZURE, EXCESS LEAKAGE

FRACTURE

FRACTURE

LEAKAGE

LEAKAGE, STICTION

LEAKAGE

LEAKAGE, BACK FLOW

SEAL LOSS

LEAKAGE

FAILURE RATE SOURCE i ENVIRONMENT

FARADA 352E GRD

FARADA 352E GRD

FARADA 373C ACFT

FARADA 373C ACFT

FARADA 292 ACFT

FARADA 292 ACFT

FARADA 352J GRD

AVCO DATA SERIES

TRW SPACE FLIGHTS

METAL BELLOWS LAB DATA

CUMULATIVE OPERATING TIME & FAILURES

5 93X 10* HRS, 1 F

5 93X 10* HRS, 1 F

62 1 X 10* HRS, 2 FAILURES

62 1 X 10* HRS, 2 FAILURES

10 78 X 106 HRS, 1 FAILURE

1 68 X 1 0 6 H R S , 1 F

15 2 X 10* HRS, 1 F

—

-

24 5 X 109 CYCLES, OF

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/8

5/8

5/50

5/50

5/8

5/50

5/8

5/1

15" OF WELD

5/1 X 1/10*

ADJUSTED FAILURE RATE, F/HR

0 100 X 10-6

2 (0 100 X 10-6)

0 003X 10-*

0 006 X 10-6

0 056 X 10-6

0 118X 10"*

0 041 X 10-*

0 100X 10-*

0 009 X 10"*

0 0142 X 10-9 F/ CYCLE 'a 50% C L

- E X t -(0 100 + 0 200+ 0 003 + 0 006 + 0 056 + 0 118 + 0 041 + 0 100 + 0 009) X 8 76 X 10* ^ -0 0142 X 10 ' ' ( 750X60X 8 76 X 10*) e A e

-(0 633 X 10'*) 8 76 X 10* ^ -0 0142 X 10"' X 3 94 X 1o' X e

0 946 X 0 945

0 89

•THE 5/1 APPLICATION FACTOR REPRESENTS THE DIFFERENCE IN ENVIRONMENTAL STRESSES BETWEEN THE AHD ENVIRONMENT AND THE BELLOWS TEST LABORATORY, WHILE THE 1/10 FACTOR REPRESENTS ADJUSTMENT FOR THE VERY LOW AHD BELLOWS STRESSES AS COMPARED TO HIGHER LABORATORY BELLOWS PUMP STRESSES

Table 4 -13 . P r e m a t u r e Fa i lu re Reliabil i ty Model for the Gas Reciprocat ing Engine PCU

I

COMPONENT

FLUID TIMER

ROTARY PUMP

I GEAR SET

HYDRODYNA­MIC BEARINGS

ROTARY PLUG VALVE

ACCUMULATOR

LOW PRESSURE BELLOWS

HIGH PRESSURE BELLOWS

NUMBER OF

ITEMS

1

1

4

1

1

1

FAILURE RATE

0.501 X 10-6 p/HR

0.19 X 10 6 F/HR

0.098 X 10-*

0.693 _ ^ , „ . . , 0.597 X 1 0 i - ^ ° ^ ° ^• ' -

0.0285 X 10-9 F/cY

0.0285 X 10-9 F/CY

PRIMARY FAILURE MODES

SEIZURE/LEAKAGE

TOOTH FRACTURE

SEIZURE

SEIZURE/LEAKAGE

LEAKAGE/RUPTURE

FRACTUREAATIGUE

FAILURE RATE SOURCE & ENVIRONMENT

FARADA 201 ACFT

FARADA 3845 GRD

FARADA 352A GRD

FARADA 313 ACFT

METAL BELLOWS LAB DATA

METAL BELLOWS LAB DATA

CUMULATIVE OPERATING TIMES & FAILURES

1.99 X 10* HRS, 1 F

5.18 X 10* HRS, 1 F

10.1 X 106 HRS, 1 F

0.597 X 106 HRS, OF

24.5 X lo'CYCLES, OF

24.5 X lO' CYCLES, OF

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/50

5/8

5/8

5/50

5/1 X 1/10

5/1 X 1/10

ADJUSTED FAILURE RATE

0.0501 X 10-6 F -j ij

0.12 X 10-* F/HR

0.24 X 10-6 F/HR

0.117 X 10-6 F/HR

0.0142 X 1 0 - ' F . ' C Y

0.0142 X 10-' F 'CY

- _ -(0.050+ 0.12+ 0.24 +0.117) X 10"* X 8.76 X 10* „ -2(0.0142 X 10'') X 750 X 60 X 8.76 X 10* K — e A e

-0.527 X 10"* X 8.76 X 10* ^ -0.112 = e X •

= 0.955 X 0.894 = 0.85

I

en

Table 4-14. P r e m a t u r e Fa i lu re Reliabil i ty Model for Actuator (Gas Reciprocat ing Systems)

COAitfONENT

SHUHLE VALVE PISTON

ROLL DIAPHRAGM

DIAPHRAGMS (REDUNDANT)

COMPLIANCE BAG

ANTI-VACUUM VALVE

w n CONNEC­TORS (QUICK DISCONNECT)

NUMBER ITEMS

1

1

2

1

1

2

FAILURE RATE

0.693 0 . 2 4 X 1 0 - 6 ® 50% C.L

0.6 X 10-6

0.6 X 10-*

0 1 X 10-*

0 85 X 10-*

0.693 ^ -n-, r , 1 399 X 10* - ^'^" ^ ••

0.693 ^ .(V.. f 1 237 CYCLES * ^ ° ^ " ^ ^

PRIMARY FAILURE MODES

STICTIONAEAKAGE

LEAKAGEAEARS

LEAKAGEAEARS

LEAKAGEAEARS

TEARS, RUPTURE

LEAKAGE

LEAKAGE

FAILURE RATE SOURCE

FARADA 292 ACFT

AVCO DATA SERIES

AVCO DATA SERIES

AVCO (ADJUSTED)

FARADA 313 ACFT

FARADA 250 ACFT

FARADA 309A MSLE

CUMULATIVE OPERATING TIME i FAILURES

0.24 X 10^ HR, OF

~ __

~

1 19 X 10*, IF

1 399 X 10*, OF

237 CYCLES, OF

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/50

5/1

5/1

5/1

5/50

5/50

5/8

ADJUSTED FAILURE RATE, F/HR

0 29 X 10-6 F/HR

3 OX 10-*

3 0 X 1 0 - * F/HR/DIA

0 5 X 1 0 - *

0 085 X 10-*

0 050 X 10-6 F/HR

0 295 X 10^2 F/ CYCLE

R = •{0 29 + 3 0 + 0 5 + 0085 + ( 2 X 0 05)} X lO"* X 8 76 X 10*. ( , . . - 3 OX 10"* X 8 76X r*^ •°T X e •0 29 X 10 X 1

.2 ^-34 8 X 1 0 X 1--(0 2305)^ X 0 997

= 0 706 X 0 947 X 0 997

= 0 66

•THE ANTI-VACUUM VALVE HAS AN INSIGNIFICANT EFFECT ON THE PREMATURE FAILURE RELIABILITY MODELS. THEREFORE THE ABOVE RELIABILITY ESTIMATE APPLIES TO BOTH THE GAS RECIPROCATING ACTUATOR USED WITH TESM AND THE GAS RECIPROCATING AOUATOR USED WITHOUT TESM.

4. 2. 12 Linear Vapor Engine and PCU

The l inear vapor engine with PCU is modeled in Table 4-15 and

ref lects the unre l iabi l i ty of using solenoid check valves and solenoid

shuttle and feed water pump va lves . Redundant solenoids and quad-

redundant check valves would improve re l iabi l i ty significantly. However,

sys tem weight and volume, even v/ith just the redundant solenoids, becomes

prohibi t ive . No re l iabi l i ty es t imat ion adjustments were requ i red for the

basical ly solid s ta te e lec t ron ics and SiGe t h e r m o e l e c t r i c module.

4. 2. 13 Linear Vapor Engine Actuator

The actuator for the l inear vapor engine ^vithout TESM is the s ame

as the actuator for the e l ec t r i ca l sys t ems and is nnodeled in Table 4 -9 .

The actuator for the l inear vapor engine Avith TESM is sho-wn at the bottom

of the s ame table and ref lec ts the addition of an an t i -vacuum valve, and

fill switch senso r .

4. 3 RELIABILITY AGAINST WEAROUT

Each of the candidate sy s t ems was revie^wed to de t e rmine re la t ive

susceptibi l i ty of all the i r components to 14 different poss ib le w^earout

fai lure modes . These 14 different modes were then grouped under 8 gen­

era l ca tegor ies shown in Table 4-16 . Since the 8 ca tegor ies do not have

the same degree of suscept ibi l i ty to wearout nor the same effect on sys­

tem degradat ion and fa i lure , the ca tegor i e s w^ere given different r anges

of poss ib le re l iabi l i ty s c o r e s . Fo r example , both long and shor t s t roke

spr ings have a smal l probabi l i ty of causing sys tem fai lure even if they

do weaken. However, sy s t em sea l s or d iaphragms have not only a higher

probabi l i ty of wearout , but a higher probabi l i ty that wearout will cause

sys tem fa i lure . There fore the al located range of re l iab i l i ty s c o r e s for

spr ings is between 97% and 100%, while sea l s and d iaphragms a r e allo­

cated a range between 90% and 100%. The major subassembl i e s of a

candidate sys t em w e r e revie^ved for the numbers of components suscep­

tible to the 8 wearout ca tegor ies and given a re la t ive s c o r e with r e s p e c t

to the other candidate s y s t e m s . Fo r example , the t h e r m o e l e c t r i c sys tem

with a ba t tery has no dynamic sea l s and the re fore r ece ives a s c o r e of 100.

In con t ras t , both the l inear vapor and gas rec ip roca t ing engines have

mult iple dynamic sea l s which upon wearout can cause complete systenn

4-26

Table 4-15 . P r e m a t u r e Fa i lu re Reliabil i ty Model for the Linear Vapor Engine and PCU

I

COMPONENT

BOILER

SOLENOID SHUT­TLE SPOOL VALVE-2-POSITION

BELLOWS-VAPOR ENGINE

BELLOWS-ACTUATOR ROD

REGENERATOR

ELECTRIC FEED PUMP

SOLENOID VALVE

SPRING

CHECK VALVES

CONDENSER

NUMBER

2

FAILURE RATE F/HR

0 09 X 10-6

1 61 X 10-6

0 0285 X 10-9 F/CY

0 0B85 X 10-9 F/CY

0.693 0 7221X104^^° ' ' ° ' = L

1 61 X 10"* F/HR

0 03 X 10-6

0 59 X 10-6 F/HR

0 065 X 10-6

PRIMARY FAILURE MODES

RUPTUREAEAKAGE

JAMMING, LEAKAGE, BURNOUT

LEAKAGE

LEAKAGE

LEAKAGE

JAMMING, LEAKAGE, BURNOUT

FATIGUEAREAKAGE

STICTION/LEAKAGE

LEAKAGE

FAILURE RATE SOURCE & ENVIRONMENT

FARADA 352K GRD

FARADA 407 GRD

METAL BELLOWS LAB DATA

MHAL BELLOWS LAB DATA

FARADA 292 ACFT

FARADA 407 GRD

FARADA 373C ACFT

FARADA 292 ACFT

FARADA 352J GRD

CUMULATIVE OPERATING TIMES & FAILURES

10 78 X 109 HRS, I F

2 488X 106 HRS, 4 F

24 5 X 109 CYCLES, OF

24 5 X 109 CYCLES, OF

0 7221 X 10* HRS, OF

2 488 X 10* HRS, 1 F

62 1 X 106 HRS, 2 F

1 68X 106 HRS, 1 F

15 2 X 1 0 6 H R S , 1 FAIL

ENVIRONMENTAL APPLICATION FACTOR RATIO

5/8

5/8

5/1

5/1

5/50

5/8

5/50

5/50

5/8

ADJUSTED FAILURE RATE, F/HR

0 056 X 10-6 F/HRS

1 OX 10-* F/HRS

0 142 X 10-9 F/ CYCLE

0 142 X 10-9 F/ CYCLE

0 0965 X 10-6

1 OX 10-6

0 003 X 10-6

0 118 X 10-6

0 0406 X 10-6

R,. -<0.0564 1 0 + 0 142 + 0 096 + 1 04 0 0034 0 Engine " *

118 + 0 0406) X 10"* X 8 76 X 10* _ -2 6 X 10"* X 8 76 X 10* _ -22 8 X lO"^

VAPOR CYCLE ENGINE PCU

= 0 796

SiGe COUPLES (REDUNDANT)

ELECTRICAL CONNECTIONS

OSCILLATOR -120 CYCLE

14

14

0.693 2 05X106 ^ 50% C L

0 00001 X 10-6 F/HR

2 33 X 10-* F/HR

OPEN CIRCUIT/ SUBLIMATION

OPEN CIRCUIT

OPEN/SHORT

RCA LAB DATA

MARTIN MARIETTA CORP

FARADA 364B SHIP

2 05 X 10* HRS, OF

0 858X 10* 2 F

5/1

5/1

5/15

1 70X 10-* F/HR/ COUPLE

0 0007 X 10-* F/HR

0 78X 10-* F/HR

PCU 1 - I I - e -(1 70 X 10"* X 8 76 X 10*) -0 78 X 10"* X 8 76 X 10* ^ ^^^.7 ^ ^-0 068

= 0 874 X 0 93 = 0 813

R.,^..., = 0 813 X 0 796 = 0 65 TOTAL

Table 4-16. Reliabil i ty Against Wearout

I N 00

Components Susceptible To Wear-Out

Valves , Spool, Check Valve and Rotary Valves

Seals

Dynamic Static Hermetic

Meta l - to -Meta l Contact

Sliding Unlubricated Sliding Lubricated Rolling

Diaphragms

S t r e s s e d via A P

Springs

Long Stroke Short Stroke

Bearing

Hydrodynamic Hydrostatic

Rotational S t r e s s e s Cycling S t r e s s e s

Battery Cycle and Seal Life |

Total Rel iabi l i ty Against Wear-Out

T / E with

Battery

100

100 95 98

100 98 99

90

98 99

99 100

98

90

67 1

Rotary Vapor with Battery

100

98 95 98

100 98 99

90

98 99

98 100

98

90

67

Hybrid with

Battery

100

98 95 98

100 98 99

90

98 99

98 100

98

90

66

Hybrid

100

98 100

98

100 98 99

90

98 99

98 100

98

100

78

Gas Reciprocating

with TESM

95

90 100

98

90 98 99

90

97 97

100 98

95

100

57

Gas Reciprocating

95

90 100

98

90 98 99

90

97 97

100 98

95

100

65

Linear Vapor with TESM

90

90 95 98

100 95

100

90

98 97

100 100

95

100

58

Linear Vapor

90

90 100

98

100 95

100

90

98 97

100 100

95

100

61

fai lure and the re fo re these engines a r e given a s co re of 90. The ro t a ry

vapor engine has sea l s between the turbine and the feedwater pump, which

upon wearout , will degrade sy s t em pe r fo rmance but probably not cause

sy s t em fa i lure . On this bas i s the r o t a r y vapor engine was given a s co re

of 98. Similar s c o r e s w e r e given to al l the sys t ems for the 8 ca tegor ies

and the compara t ive probabi l i ty of the sy s t ems not experiencing wearout

in 10 y e a r s ' t ime obtained by s e r i e s modeling the 8 ca tegor ies of re l iabi l i ty

s c o r e s as p r e sen t ed at the bottom of Table 4-16, The re l iabi l i ty against

wearout of the hybrid sy s t em without ba t te ry was bet ter than ei ther the

t h e r m o e l e c t r i c or r o t a r y vapor s y s t e m s alone because of the lack of the

t h e r m a l ba t te ry in the hybrid sy s t em. The stat ic ba t te ry sea l s a r e p r e s ­

ently an unsolved m a t e r i a l s compat ibi l i ty p rob lem and there fore the bat­

te ry const i tu tes an addition s o u r c e for sys t em unrel iabi l i ty against

Avearout.

The redundancy advantages of the hybrid systenn a r e maintained in

the w^earout re l iabi l i ty modeling as well as in the p r e m a t u r e fai lure re l iab i ­

lity model ing. The advantage in wearout re l iabi l i ty occurs p r i m a r i l y

because the redundancy cons i s t s of tvt'o different types of engine w^hich

have a sma l l probabi l i ty of wearout at the same t ime . Fo r example , it i s

unlikely that the t h e r m o e l e c t r i c couples would suffer complete e l ec t r i ca l

power output degradat ion at the s a m e t ime that the tu rbogenera tor of dc

e lec t r i c motor failed. In con t r a s t to the non-redundant candidate sys t em,

the rec ip ien t could obtain r e m e d i a l a s s i s t a n c e p r io r to loss of both of the

engine power suppl ies .

4 . 4 CONFIDENCE IN THE RELIABILITY ESTIMATES

The amount of confidence which can be assoc ia ted with the sys tem

re l iabi l i ty e s t i m a t e s for both wearout and p r e m a t u r e fai lure probabi l i t ies

has been ref lec ted in a r e p r e s e n t a t i v e ' f igure of m e r i t ' . The ' f igure of

m e r i t ' i s p ropor t iona l to the degree to which each of the candidate s y s ­

t ems fulfills the 4 different c r i t e r i a as shown at the headings of Table 4-17.

Each of the candidate s y s t e m s was reviewed on the bas i s of how well the

engine, PCCS, blood pump ac tua tor , TESM or ba t t e ry met the four eva lua­

tion c r i t e r i a . Thus, if a candidate s y s t e m cons is ted of an engine, PCCS,

and ac tuator with no TESM or thernnal bat tery , the total of 20 points a l lo ­

cated to c r i t e r i a #1, 'Expe r imen ta l Data Availabil i ty ' was apport ioned to

4-29

Table 4-17. Confidence in Reliabili ty Es t ima tes

to O

1 Candidate Systems

T/E with Battery

Rotary Vapor with Battery

Hybrid with Battery

Hybrid

Gas Reciprocating

1 with TESM

Gas Reciprocating

Linear Vapor with TESM

Linear Vapor

Criteria No. 1

Experimental Data Available on Actual

Component Hardware in Site and Power Range

(20/45)

Battery

0

Battery

0

Battery

0

Engine

5

TG

3

T G i T / E

4

Engine It T /E

4

TESM

1

PCU

5 .

P C U

5

P C U

s

P C U

5

Engine

I

Engine

1

TESM

1

Actuator

2

Actuator

2

Actuator

2

Actuator

2

P C U

3

PCU

3

Engine

3

Engine

3

Actuator

• Actuator

1

P C U

5

P C U

5

Actuator

0

Actuator

2

Criteria No. 2

Data Available From Two or More

Sources

(5/451

Battery

0

Battery

0

Battery

0

Engine

5

TG

5

TGJ.T/E

5

Engine ii T/E

5

TESM

1

P C U

5

P C U

5

P C U

5

P C U

5

Engine

1

Engine

I

TESM

1

Actuator

0

Actuator

0

Actuator

0

Actuator

0

P C U

3

P C U

3

Engine

3

Engine

3

Actuator

0

Actuator

0

P C U

5

P C U

5

Actuator

0

Actuator

0

Criteria No. 3

Data Scaled From Similar Components

(10/451

Battery

0

Battery

0

Battery

0

Engine

5

TG

5

TCfcT/E

5

Engine S. T/E

5

TESM

0

P C U

5

P C U

5

P C U

5

PCU

5

Engine

1

Engine

1

TESM

0

Actuator

1

Actuator

1

Actuator

I

Actuator

1

P C U

2

PCU

2

Engine

2

Engine

2

Actuator

1

Actuator

1

P C U

5

P C U

5

Actuator

1

Actuator

1

Complexity Level

(10/451

3.85

2.93

2. 73

2. 73

3.03

3.03

3. 90

3.90

Total Raw

Score

23.85

20.43

21. 73

29.36

12 27

13.73

19 15

25.2

Normalized Score

5 . 3

4 . 6

4 .8 1

6.5 1

2 . 8 1

3 1 1

4. 3 1

4 3

the t h r ee subsys t ems on a (5, 5, 5) x 20/15 maximum point count. If the

candidate sy s t ems included a ba t t e ry as well , the 20 points were appor­

tioned on a 5, 5, 5, 5 m a x i m u m point count.

F o r the f i r s t c r i t e r i on , 'Expe r imen ta l Data Availabili ty ' the candi­

date engines w e r e ranked in o rde r of dec reas ing data availabil i ty as

follows r

' F igu re of Meri t ' Engine Data Availabili ty or Point Count

Static The rmoe l ec t r i c

Rotary Vapor

Linear Vapor

Gas Reciproca t ing

High

Modera te ly high

Moderate ly high

Low

5

3

3

1

And s imi l a r l y for the PCU Subsystem:

' F i g u r e of Mer i t ' Engine PCU Data Availabili ty or Point Count

Static T / E & E lec t r i ca l High 5 Rotary Rankine

Vapor Engine T / E & Solid High 5 State Devices

Gas Cycle Fluid T i m e r & Moderate 3 Accumulator

F o r the sys t ems incorpora t ing the sodium-sulfur ba t te ry , a zero point

s co re was given to the ba t te ry s ince t h e r e a r e no existing rel iabi l i ty data

ei ther for the c e r a m i c - t o - m e t a l seal life or re la t ive to p r e m a t u r e fai lure

and cycle life c h a r a c t e r i s t i c s .

Similar rankings were pe r fo rmed on the second and third evaluation

c r i t e r i a 'Data Available from Two or More Sources ' , and 'Data Scaled from

Similar Components ' . Total (raw) s c o r e for the four evaluation c r i t e r i a

and the weighted s co re a r e shown for each candidate sys tem in the last two

columns of Table 4-17 .

4-31

millerc

Text Box

BLANK

5 . CANDIDATE SYSTEM DESCRIPTION

The eight candidate thermal conver te r sys t ems are

the following f igures:

5-1 Gas Reciprocat ing/TESM Conver ter System

5-2 Gas Reciprocat ing Conver ter System

5-3 Linear Vapor /TESM Conver ter System

5-4 Linear Vapor Conver ter System

5-5 Rotary Vapor /Ba t te ry Converter Systenn

5-6 The rmoe lec t r i c /Ba t t e ry Converter System

5-7 Hybr id /Ba t te ry Conver ter System

5-8 Hybrid Conver ter Systemi

2^6^

summarized

GAS RECIPROCATING/TESM ENGINE

BLOOD PUMP ACTUATOR

BLOOD

ANTI-SUCTION VALVE

HIGH PRESSURE OUTLET

•SPRINGS

HIGH AND LOW PRESSURE ACCUMULATOR

DISPLACER PISTON

COMPRESSION SPACE CONTROL SIGNAL INPUT/ OUTPUT

HEAT SOURCE

TESM

INSULATION (MIN K WITH XEN

PCU

HYDRAULIC LINE FOR RETURN FORCE

HYDRAULIC TIMER/'SWITCH UNIT

HIGH PRESSURE i INLET T

. LOW PRESSURE T INLET

CHECK VALVE

GAS BEARING ORIFICE

COLD END

CLEARANCE REGENERATOR

DISPLACER PISTON

XENON FILLED

PULSATILE HYDRAULIC POWER TO ACTUATOR

- ROTARY VALVE

FLUIDIC GEAR MOTOR

I LOW PRESSURE ' OUTLET

SYSl EM DESCRIPT ION

The nonsynchronous, modulated, gas cycle engine is a free piston Stirling engine emplc>ying mechanical com­press ion by means of a separate power piston and a hollow-core displat er piston. A clearance-type regener­a tor , consisting of the mid-cyl inder portions of the annular space between the engine cylinder and displacer piston, regenera tes the heat for the cycle. Power is produced in this engine by heating helium with the isotope and thermal energy storage mater ial (TESM) at the spherical end of the engine cylinder and rejecting heat at the cold end. Var i ­able heat flow input is provided by solidification of the TESM which is a eutectic of I . iF /NaF. The 48 9-watt heat source incorporates a vent and capillary tube assembly for re lease of the decay-produced helium into the abdominal cavity. The displacer piston moves the gas between hot and cold ends of the cylinder causing p re s su re variat ions that produce useful work. The springs at the cold end of the engine cylinder niaintain the proper phasing betvc^en the displacer aiic' power pistr)n. Gas bearings arc used at the cold end to reduce wear between the cylinder,power piston, and displacer piston rod.

Pneumatic- to-hydraul ic power is accomplished by means of a bellows pump and check valves operating at high speed. Lov, p r e s s u r e fluid flc)ws from the PCli into the bellows pump and is ejected at approximately ISOpsia. This rate is contrc)llcd by a pneurnatu signal from the PCI ' into the compression (cold) space of the engine that var ies the amount of working fluid causing a variat ion in the work per stroke and cycle rate within the engine. The engine is mounted within a hermetical ly sealed contain­ment vessel svhich is sized to provide surface a rea so the heat flux to the body t i ssue does not exceed 0. 07 wat t /cm^.

Within the PCU are the fluidic gear and motor / rotary valve. These mechanisms adapt the variable engine output to a constant l.'O bpm high p r e s s u r e , pulsati le, flow for the actuator. The high p res su re and the low p r e s s u r e accumiilatc>r and the dead space controls are in the engine enclo.'.ure. The high pressure accumulator allows maximum storage of 4. 44 watts PCU output power during artificial diastole A modulative signal is provided to the engine to ..illox^ excess jsotope heat to be stored in the TESM

The actuator del ivers power to the blood pump at a variable stroke and constant frequency. An antivacuum valve prevents toe actuator from imposing negative p res sure on the mco-ning blood flow and l imits the stroke of the power piston.

System Data

Engine

Working Fluid

Maximum Tempera ture

Minimum Tempera ture

Maximum P r e s s u r e

Minimum P r e s s u r e

Speed (Variable)

Efficiency

Power Output (Hydraulic)

Output P r e s s u r e

PCCS

Energy Storage (Hydraulic)

Frequency

Efficiency

Volume

Weight

System Summary

Weight

Volume

Specific Gravity

Heat Source Size

TESM

Heliuni

1200°F

1 60' F

500 psia

190 psia

480 to lOOn , p m

8 to 12%

4. 1 to 6. 7 watts

180 psia

1.11 wat t -sec

120 bpm

55 to 63%

0. 552 l i ter

0. 876 Kg

1. 75 Kg (V 86 lb)

1.233 l i ters (75.2 in^)

1.42

48. 9 watts

53. 3 watt-hours

Figure 5 - i . Gas Rec ip roca t ing /TESM Conve r t e r System

5-2 U^

\

PCU

BLOOD PUMP ACTUATOR

HYDRAULIC i LINE FOR I RETURN FORCE t

FLUIDIC GEAR MOTOR-

HIGH PRESSURE • ACCUMULATOR

PULSATILE HYDRAULIC POWER OUTPUT TO ACTUATOR

ROTARY VALVE

HYDRAULIC TIME IV'SWITCH UNIT

LOW PRESSURE ACCUMULATOR

. POWER CONDITIONING UNIT

PRE-CHARGE LINE

GAS RECIPROCATING ENGINE

PUMP CHECK VALVES-

BELLOWS PUMP-

SPRINGS (3)-

GAS BEARING ORIFICES-

DISPLACER PISTON ROD-

COLD END ANNULAR HEAT EXCHANGER-

CLEARANCE REGENERATOR

DISPLACER PISTON

EXPANSION END (HOT) OF CYLINDER •

FINS FOR HEAT TRANSFER TO HOT END OF CYLINDER

ANNULAR HEATING SPACE

HEAT SOURCE

INSULATION. MIN-K AND XENON

•XENON GAS FILL

ISOTONIC FLUID TO DISTRIBUTE HEAT FLOW TO TISSUES

TUBULAR TISSUE HEAT EXCHANGER

SYSTEM DESCRIPTION

The nonsynchronous, nonmodulated, gas cycle engine is a free piston Stirling engine employing mechanical com­press ion by means of a separa te power piston and hollow-core displacer piston. A clearance- type regenerator con­sisting of the mid-cyl inder portions of the annular space between the engine cylinder and displacer piston regener­ates the heat for this cycle. Power is produced in this machine by heating helium with the isotope at the spherical end of the engine cylinder and rejecting heat at the annular cold end heat exchanger. The 54. 4-watt heat source incorporates a vent and capil lary tube assembly for re lease of the deca^-produced helium into the abdominal cavity. The displacer piston moves the gas between the hot and cold ends, and the heating and cooling cause p r e s su re changes to produce work. The springs at the bottom end of the engine maintain the proper phasing between the d i s ­p lacer and power piston. Gas bearings a r e used at the upper end of the engine to reduce wear between the cylinder, power piston, and d isp lacer piston rod. The engine is mounted within a hermet ica l ly sealed containment vessel which is sized to provide surface a rea so the heat flux to the t i ssue does not exceed 0. 07 wat t /cm^.

Pneumat ic- to-hydraul ic power conversion is accom­plished by means of the bellows pump and check valves operating at 750 cpm. Low p r e s s u r e fluid from the power conditioning unit (PCU) flows into the bellows and is ejected at 130 psia into the annular cold end heat exchanger. The high p r e s s u r e fluid then flows into the tubular heat exchanger which rejects heat to the surrounding isotonic fluid and t i s sues . The fluid then flows into the PCU at a power outpi.t of 7 0 watts and at a pulsatile rate of 750 cpm

Within the PCU are the high and low p r e s s u r e accu­mula to r s , fluidic gear motor and hydraulic t imer switch, rotary valve, and energy dissipative loop. These mecha­n i sms adapt the engine output to a 120 cpm high p r e s s u r e , pulsati le flow for the actuator . The high p r e s s u r e accu­mulator allows storage of 4. 44 watts PCU output power during artificial d ias tole . The excess energy difference between the constant engine output of 7. 0 vratts and the var iable actuator output power is dissipated in a check-valve-controlled bypass.

The actuator de l ivers power to the blood pump at a constant stroke and frequency. A make-up rese rvo i r auto­matical ly del ivers a part ial volume, complementary to the varying physiological blood flow, for the constant actuator displacement.

System Data

Engine

Working Fluid

Maximum Tempera tu re

Minimum Tempera tu re

Maximum P r e s s u r e

Minimum P r e s s u r e

Speed (Constant)

Efficiency

Insulation Efficiency

Power Output (Hydraulic)

Outlet P r e s s u r e

PCCS

Energy Storage (Hydraulic)

Frequency

Efficiency (Maximum)

Volume

Weight

System Summary

Weight

Volume

Specific Gravity

Heat Source Size

Helium

1200°F

160°F

SCO psia

190 psia

750 rpm

16%

90%

7. 0 watts

180 psia

1.11 wa t t - sec

120 bpm

63%

0.496 l i ter

0. 799 Kg

1. 57 Kg ( 3 . 4 8 lb)

1. 365 l i ter (83 in^)

1. 15

54. 4 watts

Figure 5-2. Gas Reciprocating Converter System

5-3 ^(^1

BLOOD PUMP ACTUATOR

LINEAR VAPOR/TESM ENGINE

rV

RECIPROCATING ROD

ANTI-SUCTION VALVE

MIN-K INSULATION-

M I N - K / X E N O N INSULATION-

ELECTRICAL FEED PUMP-

REGENERATOR •

ENGINE BELLOWS

ELECTRICAL ACTUATED INLET • AND EXHAUST VALVE

EXTERNAL CONTAINER-

CONDENSER'

BOILER-

IE SM

ISOTOPE HEAT SOURCE

THERMOELECTRIC MODULE-

ELECTRONIC CONTROL PACKAGE

Q o Q Q o Q & a~

SYSTEM DESCRIPTION

The vapor engine uses water as a working fluid in conjunction with a regen. rative nonexpansion cycle. The engine operates with a variable speed at heart rate. The unit del ivers a power output equivalent to the average blood pump requirement and uti l izes a thermal energy storage mater ia l (TESM). The engine consists of a bellows with a mechanical output drive that is direct ly coupled to the blood pump. An electr ical control system is used and its power is supplied by a thermoelec t r ic module. The 48-watt heat source incorporates a vent and capillary tube assembly for re lease of the decay-produced helium into the abdominal cavity. The system components a r e mounted within a hermet ical ly sealed containment vessel which is sized to provide sufficient surface a rea so the heat flux to the t issue does not exceed 0. 07 wat t /cnn ' .

The unit operates with the vapor front the boiler driving the engine bellows. Exhaust vapor from the engine passes thrc^ugh a regenerator then into a condenser where it is condensed and subcooled. The condenser is designed to reject the heat to the package walls where it is then rejected to the body fluids and t i ssue . The fluid from the condenser passes through the feed pump where it is pumped through the regenera tor info the boiler . The unit is con­trolled by regulating the vapor flow rate into and out of the engine. Since the engine opera tes with a constant p r e s ­su re , the inlet valve is open during the ent i re engine s t roke. At the end of the s troke the inlet valve is closed and the exhaust is opened. The exhaust valve remains open during the ent i re exhaust s t roke. By regulating the inlet and exhaust ra te , the engine speed can be varied over the des i red range.

The heat source fuel inventory was based on the ave r ­age blood pump power requi rements . To accommodate periods of peak activity, 69 watt-hours of T&SM are required. The average daily efficiency of the engine is

The actuator unit provides a mechanical push rod linkage with the eng ne. I'owt r is delivered on the push stroke and a preloacied spring in the engine provides the force for returning -he actuator rod during blood pump filling. An ant i suc tun valve in the actuator prevents nega­tive p r e s s u r e from being developed in the blood pump.

The engine has an end of fill stroke switch to initiate the power s troke. As a result , the actuator unit operates at variable frequency (60-120 cpm) depending on the rate of blood flow into the puiTip.

System Data

Operating Conditions

Boiler Condenser

Subcooler

Efficiency

Ideal Cycle Machine

Overall

Engine Charac te r i s t ics

Speed

Stroke

Diameter

Power Output

Feed Pump

Stroke

Diameter

Power Requirement

Thermoelec t r ic Module

Material

Number of Couples

Elect r ica l Power Output

Voltage

Actuator Unit

Efficiency

Weight

Volume

Frequency

System Summary

Weight

Volume

Specific Gravity

Heat Source Size

TESM

95 psia, 900°F

3 psia , 142''F

3 psia , I I S - F

14.2%

49 to 60%

7 to 8. 6%

60 to 120 rpm

1. 3 inch

0 .58 inch

2 . 7 to 5. 9 watt

0. 10 inch

0. 18 inch

0 .21 watt

SiGe

14

0. 96 watt

0. 6 volt

76%

0.4 Kg

0.267 l i ter

60 to 120 bpm

1.77 Kg (3.91 lb)

1.42 l i te rs (86.4 in^)

1.25

48 watts

69 watt-hours

Figure 5-3 . L inea r Vapor /TESM Conve r t e r Sys tem

5-4 U^

LINEAR VAPOR ENGINE

BLOOD PUMP ACTUATOR RECIPROCATING ROD • CONNECTOR

MIN-K INSULATION

MIN-K/XENON INSULATION

ELECTRICAL FEED PUMP

REGENERATOR

ENGINE BELLOWS

EXTERNAL CONTAINER

CONDENSER

ELECTRICAL ACTUATED INLET AND EXHAUST VALVE

BOILER

ISOTOPE HEAT SOURCE

THERMOELECTRIC MODULE

f!h ELECTRONIC CONTROL PACKAGE

SYSTEM DESCRIPTION

The vapor iingine uses water as a working fluid in conjunction with a regenerative nonexpansion cycle. The engine operates at constant frequency (120 bpm) and delivers constant power at all t imes. The power output is equivalent to the maximum blood pump requirements. Since it delivers a constant power output, no thermal energy storage material is required. The unit consists of a bellows with a mechanical output drive that is directly coupled to the blcod pump. An electrical control system is used and its power is supplied by a thermoelectric module. The 57-watt heat source incorporates a vent and capillary tube assembly for release of the decay-produced helium into the abdominal cavity. The system components are mounted with.n a hermetically sealed containment vessel which is sized to provide sufficient surface area so the heat flux to the tissue does not exceed 0,07 watt/cm .

The unit operates with the vapor from the boiler driving the engine bellows. Exhaust vapor from the engine passes through a regenerator into a condenser where it is condensed and subcooled The condenser is designed to reject the heat to the package walls where it is then rejected to the body fluids and tissue. The fluid from the condenser passes through the feed pump where it is pumped through the regenerator into the boiler. The unit is controlled by regulating the vapor flow rate into and out of the engine. Since the engine operates with a constant pressure, the inlet valve is open during the entire engine stroke. At the end of the stroke the inlet valve is closed and the exhaust is opened. The exhaust valve remains open during the entire exhaust stroke.

The linear vapor engine generates a mechanical back-and-forth motion which is transmitted to the pump actuator through a flexible plastic-coated braided metal cable. A spring within the actuator unit provides a force bias which effectively maintains the cable in tension which is the preferred operating mode during all phases of the pumping cycle.

No sensors are required to maintain control of the pumping action, the output power is automatically modu­lated by a variation in the pumping duty cycle (between 25 and 50% at a fixed rate of 120 beats/minute). The duty cycle is automatically regulated by the actuator design and the controlled constant-speed characteristic of the electric motor.

The actuato' automatically delivers a partial stroke, according to the physiological demand; it provides the positive filling action necessary to maintain system con­trol ("passive autoregulation") with the mandatory antisuc-tion control; and it presents a simple mechanical interface to the actuator drive unit.

System Data

Operating Conditions

Boiler Condenser

Subcooler

Efficiency

Ideal Cycle

Machine

Ove rail

Engine Characteristics

Speed

St roke

Diameter

Power Output

Feed Pump

Stroke

Diameter

Power Requirement

Thermoelectric Module

Material

Number of Couples

Electrical Power Output

Voltage

Actuator Unit

Efficiency

Weight

Volume

Frequency

System Summary

Weight

Volume

Specific Gravity

Heat Source Size

95 psia, 900 3 psia, 142*

3 psia, 115''

14. 2%

72%

10. 2%

120 rpm

1.3 inch

0 .58 inch

5. 9 watts

"F F

F

0. 10 inch

0. 18 inch

0.21 watt

SiGe

14

1. 14 watts

0. 7 volt

76%

0. 191 Kg

0. 103 liter

120 bpm

1.333 Kg (2.94 1b)

1. 4 l i ters (85.2 in^)

0.95 S7 watts

Figure 5-4. Linear Vapor Converter System

5-5 ^u?

BLOOD PUMP ACTUATOR

PUSH ROD SHEATH

MOTOR-RECIPROCATOR

BELLOWS SEAL

RECIPROCATING ROD CONNECTOR

ROTARY CAM

RECIPROCATING CAM FOLLOWER

PLANETARY GEARS

MAGNETIC COUPLING

BRUSHLESS DC MOTOR

ROTARY VAPOR/BATTERY ENGINE

EXTERNAL CONTAINER

ISOTOPE HEAT SOURCE

SODIUM-SULFUR BATTERY

BOILER

M I N - K / X E N O N INSULATION

CONDENSER

REGENERATOR

INWARD-FLOW RADIAL TURBINE

DC GENERATOR

SPEED AND BATTERY CHARGE CONTROLS

CENTRIFUGAL PUMP

SYSTEM DKSCRIPIION

rhe ri)tary vapor /bat tery system uses CP i4 as a working fluid in conjunction with a regenerat ive Rankme cycle. The system includes a turbogenerator unit ( tur­bine, pump, dc generator) , three heat exchangers (boiler, condenser , regenerator) , a solid electrolyte battery (sodium-sulfur) , an isotope heat source , insulation (Min-K/xenon) and a i-olid state e lectronics package for speed control and battery charge control. In operation, the working fluid is heated in the boiler to peak cycle con­ditions (430^F and 250 psia) and expanded thrtjugh the tu r ­bine from which work is abst racted. The turbine exhaust vapor, (PSO't and 4 psia ' is passed th rough tht- regener­ator giving up a porti(,n ot its heat, and thus preheating the high p r e s s u r e pump discharge liquid. From the regener a tor , the cycle vapor enters the condenser where it rejects additional heal 1o the body fluids and t i ssue . The working fluid leaves the conde iser as a subcooled liquid and enters the centrifugal pump. The pump increases the liquid p r e s ­sure bac-k to 250 psia and after passing through the liquid side of the regt ne ratc>r is returned to the boiler and the cycle is repeated.

The overall system produces 9. 6 watts e lect r ica l output when combined with a 60-watt thermal heat source at an overal l efficiency of 16%. This ro tary vapor system provides the 2. 81 watts average power to the blood pump with a 41-watt thermal heat source , through a 45%-efficient dc motor-dr iven mechanical actuator . Under these conditions the output from the turbogenera tor need be only 6. 25 watts e lec t r ic . By using a 12. 5 wat t -hr bat tery , the system will meet the maximum blood pump power of 4. 44 watts.

The heat engine components have been arranged to minimize volume and insulation requ i rements . To reduce overall heat losses the bat tery, which opera tes at BTCF, has been designed as a cylindrical receptacle to enclose a high percentage of the heat source surface area . The boi ler , which operates at 430^F, has been placed around the outer surface of the bat tery. The high ternperature turbine and the regenera tor a r e located adjacent to the heat source . Finally, the condenser which operates in the 115* to 117°F t empera tu re range, has been mounted on the inner surface of the engine container permit t ing heat re jec­tion direct ly to the body fluids and t i ssue . The heat flux to the outside of the container does not exceed 0.07 watt/ cm . The heat source incorporates a vent and capil lary tube assembly for re lease of the decay-produced helium into the abdominal cavity.

The turbogenerator unit is provided with a speed con­trol circuit which will autonnatically protect the unit from changes in the load.

The motor / reciprocator unit consists of a 9,000 rpm brushless dc mctor operating on gas bearings m a hermet ­ically sealed housing. The motor is coupled through a nnagnetic clutch and a set of reduction gears to a 120 rpm d r u m - c a m rec iprocatcr . The cam follower impar ts recip­rocating motion to a rod which has an overall s troke of 1, 3 inches.

The e lect r ica l rrvotor/reciprocator unit generates a mechanical back-and-forth motion which is t ransmit ted to the pump actuator through a flexible plas t ic-coated, braided meta l cable. \ spring within the actuator unit p ro ­vides a force bias which effectively maintains the cable in tension which is the preferred operating mode during all phases of the pumping cycle.

No senscjrs a r e required to^maintain control of the pumping action, the output power is automatically modu­lated by a variat ion m the pumping duty cycle (between 25 and 50% at a fixed rate of 120 bea ts /minute) . The duty cycle is automatically regulated by the actuator design and the controlled constant-speed charac te r i s t i c of the e lec t r ic motor.

The actuator automatical ly de l ivers a part ial s t roke, according to the physiological demand; it provides the positive filling action necessary to maintain syslem con trol ( ' 'passive autoregulation } with the mandatory rtnt SIH tion contrcil; and it p resen ts a simple mechanical iriterfncc to the actuator drive unit.

System Data

Rotary Vapor Turbogenera tor

Working Fluid

Peak Cycle Tempera tu r e

Peak Cycle P r e s s u r e

Condensing Tempera tu re

Condensing P r e s s u r e

Power Output

Overall Heat Engine Efficiency

Bat tery

Energy Storage

Output Voltage

Operating Tempera tu r e

Motor /Rec iproca tor

Efficiency

Speed Reduction

Input Voltage

Actuator

Efficiency

System Summary

Weight

Volume

Specific Gravity

Heat Source Size

CP-34

430' 'F

250 psia i i e -F 4 psia

6. 25 watts a( 24 vol ts , dc

15. 7%

12. 5 wa t t -h r s

24 volts

570»F

59%

9000 to 120 rpm

24 vol ts , dc

76%

1.42 Kg (3, 13 lb)

1.07 l iters (64.7 in^)

1.34

41 watts

F i g u r e 5-5 . Ro ta ry V a p o r / B a t t e r y C o n v e r t e r Sys t em

5-6 Z^l?

THERMOELECTRIC/BATTERY ENGINE

RECIPROCATING ROD-CONNECTOR r

MOTOR/RECIPROCATOR •

LEAD TELLURIDE COUPLES-

SiGe COUPLES —

MIN-K (ARGON) INSULATION •

BATTERY

SiGe COUPLES

LEAD TELLURIDE COUPLES

T

j &

= \

±

SYSTEM DESCRIPTION

The thermoelectr ic engine makes use of a thermo­e lec t r ic converte* to transfi . i in heat generated in the radioisotope heat source into e lectr ical power by coTn-pletely static means . The 54 watt heat sciune incorporates a vent and capillary tube assembly for re lease of the decay-produced helium into abdominat cavity. Two stages of thermoelec t r ic semiconductor mate r ia l s have been ar ranged thermally and electr ical ly in s e r i e s to take advan­tage of the optimvm operating It rriperature range of each mate r ia l and thereby produce a high efficiency device. The hot stage of the conver ter consists of silicon-germanium thermoelec t r i cs and the cold stage is made up of segmented telluride ma te r i a l s . The converter is divided into two modules which a re loaded against the flat ends of the right c i rcu lar cylinder heat source ; the heat source and conver te r a r e surrounded by Mm K fibrous insulation which 18 backfilled with an inert gas .

The system components a r e mounted within a h e r m e t ­ically sealed containment vesse l which is sized to provide sufficient surface a r ea such that the heat flux to the t i ssue does not exceed 0 07 w a t t / c m ' . The size of the container with this heat flux is sufficient to also house the motor -rec iprocator . The waste heat from the thermoelec t r i cs and moto r / r ec iprocator i s conducted to the outer walls of the vessel through the mounting plate. A uniform t empera ­tu re is maintained along the surface of the vessel with a minimum weight penalty by proper tapering of the wall thickness. The sodium-sulfur bat tery, provided to meet peak power is contained within an annular space around the RTG and surrounded by insulation to maintain it at a mean t empera tu re of 570' F

The m o t o r / r e c i p r o c a t o r unit consis ts of a 9,000 rpm brush less dc motor operating on gas bearings in a he r ­metical ly sealed housing. The motor is coupled through a magnetic clutch and a set of reduction gea r s to a 120 rpm d rum-cam reciprocator . The cam follower impar ts reciprocat ing motion to a rod which has an overall s t roke of 1. 3 inches.

The electr ical m o t o r / r e c i p r o c a t o r unit generates a rnechanical back-and-forth motion which as t ransmit ted to the pump actuator through a flexible plastic - coated braided metal cable. A spring within the actuator unit provides a force bias which effectively maintains the cable in tension which IS the preferred operating mode during all phases of the piimpirie cycle.

No sensors a r e required to maintain control of the pumping action; the output power is automatically modu­lated by a variation in the pumping duty cycle (between 25 and 50% at a fixed rate of 120 bea ts /minute) . The duty cycle i s automatically regulated by the actuator design and the controlled constant-speed charac te r i s t i c of the e lec t r ic motor .

The actuator automatical ly del ivers a part ial s t roke , according to the physiological demand; it provides the positive filling acnon necessa ry to maintain system con­trol ("Passive autoregulation") with the mandatory ant isuc-tion control; and it p resen t s a s imple mechanical interface to the actuator drive unit.

System Data

Silicon Germanium Stage (8 couples)

0. 3 in. Couple Length

N-element C r o s s -Sectional Area

P-e lement C r o s s -Sectional Area

Segmented Stage (26 couples)

Couple Length

N-element C r o s s -Sectional Area

PbTe-GeTe Length 2N Length

P-e lement C r o s s -Sectional Area

TAGS Length BiSbTe Length

Lead Tel lur ide Operating T empe ratu re s

Maximum Heat Source

Surface Tempera tu re

RTG Power Output

Load Current

Load Voltage

Motor /Cam Power Output

System Summary

Weight

Volume

Specific Gravity

Heat Source Size

0. 0179 in

0. 0098 in

0. 6 i n .

0. 0212 in^

0.495 H. 0. 105 in .

0.0170 in^

0. 432 in. 0, 168 in.

SiGe Operating Tempera tu res Tjj - 950''C, 525'C

T H 5 0 0 ' C , T c 50 'C

1079°C

6.25 wat ts (e)

3. 28 amps

1. 91 vo l v .

2. 81 watts (m)

1.5 Kg (3.31 lb)

1. 37 l i t e r s (83.75 in^)

1. 10

54 watts

Figure 5-6. Thermoelectr ic/Bat tery Converter System

5-7 t-1'

MOTOR-RECIPROCATOR

BLOOD PUMP ACTUATOR

PUSH ROD SHEATH

BELLOWS SEAL

RECIPROCATING ROD CONNECTOR

ROTARY CAM

RECIPROCATING CAM FOLLOWER

PLANETARY GEARS

MAGNETIC COUPLING

BRUSHLESS DC MOTOR

HYBRID BATTERY ENGINE

CONTAINER

MIN-K/XENON INSULATION

THERMOELECTRIC MODULES

ENCAPSULATED HEAT SOURCE

CONDENSER

BOILER

SODIUM-SULFUR BATTERY

REGENERATOR

INWARD-FLOW RADIAL TURBINE

BRUSHLESS DC GENERATOR

CENTRIFUGAL PUMP

SPEED AND BATTERY CHARGE CONTROLS

SYS BM DKSf HiPTiC)\

The hybrid/battery heat engine sviVisybteiri < '>nsisls of a ^7-watt heat source whith mt otporarea a veni and capillary tube assembly for release of 'he dpi^v produced helium into the abdominal ca- lU Mtmni. d .r. each end of this cylindrical capsule art- t * t a^' ad^ d 'h* rnnoelef trie converters. These converte r *- a re > orvi} osed of •'4 silicon germanium, high temperature couples and bO / \ 1 AGS couples ("ombined with this thermoele^ t r ir •. n^Ttet is a rotary vapor cycle turbogenerator operating al a peak cycle temperature of ZZI'C. This temperature is com­patible with the cold junction temperature of the 2N/TAGS thermoelectric couples. A sodium-sulfur battery pro­viding 12.5 watt-hours of electrochemical energy storage, operates at 300°C and is also compatible with the turbo­generator peak cycle temperature. The engine unit is insulated with fibrous Min-K filled with xenon gas. The waste heat is rejected through condenser tubes attached to the inside of the titanium container The surface area of the container package is sufficient to perrriit heat rejection to the local body tissues at ar> acceptable rate of 0. 0? watt/ cm^ In order to share the load, the -jutput frorn the thermoelectric generato*" must be pui fhrt-ugh a d<-to-dc converter to boost it to 15 volts redut mg the power output to 1.7 watts (e). The rotary vapor cycle output is 4 S5 watts. Thus, the total power input to the motor/ reciprocator becomes 6 2^ watts {el The overall effi­ciency fthermal to elettricaU of the Hybrid/battery heat engine is 1"% These outputs att integrated by a i electronic load-shaiing circuit which connetts tKe brushless dc motoi load alternately lo each gcneicttor in a constant voltage, pulse modulated manner I'sing this technique, each generator sees an effectively constant load The switching rate used is approximately four times the dc motor speed so that switching transients are effectively eliminated by the inertia of the motor rotor For a dc motor operating at 9000 RPM, th^ electronic commutation speed selected was 600 Ha

The thermoelectric generator is designed to deliver a constant power output of 2 II watts at 3 64 vdc By employing a dc-to-dc convtrrter, using d 50 kHz con­version frequency, this is increased to 15 vdc to be compatible with the 15 vdc output of the turboge"eicttor Additional circuit logic is provided to maintain the thermoelectric output voltage within desired tolerances The turbogenerator unit is provided with a speed control unit employing a parasitic load bank Hence, at reduced loads, the load-sharing electronic circuitry diverts a portion of the turbogenerator output to the parasitic load bank, thus maintaining this untt at constant speed while matching the overall system output power to the varying load.

The motor/reciprocator unit consists of a 9,000 rpm brushless dc motor operating on gas bearings in a her­metically sealed housing. The motor is coupled through a inagnetic clutch and a set of reduction gears to a 120 rpm drum-cam reciprocator. The cam follower imparts reciprocating motion to a rod which has an overall stroke of 1. 3 inches.

The electrical miotor/reciprocator unit generates a mechanical back-and-forth motion which is transmitted to the pump actuator through a flexible plastic-coated braided metal cable. A spring within the actuator unit provides a force bias which effectively maintains the cable in tension which is the preferred operating mode during all phases of the pumping cycle.

No sensors are required to maintain control of the pumping action, the output power is autonrtatically modu­lated by a variation in the pumping duty cycle (between 2 5 and 50% at a fixed rate of 120 beats/minute). The duty cycle is automatically regulated by the actuator design and the controlled constant-speed characteristics of the electric motor.

The actuator automatically delivers a partial stroke, according to the physiological demand; it provides the positive filling action necessary to maintain system control ("passive autoregulation") with the mandatory antisuction control; and it presents a simple mechanical interface to the actuator drive unit.

System Data

Thermoelectrics

SiGe Hot Junction Cold Junction

2N/TAGS

Hot Junction Cold Junction

Power Output

Power Out of dc to dc Converter

Motor/Reciprocator

950"C 500°C

475 °C 240'C 2. 11 watts 3. 64 volts, 1. 7 watts 15 volts

Efficiency Speed Reduction Dc/dc Conversion

Rotary Vapor Turbogenerator

Working Fluid Peak Cycle Temperature Condensing Temperature Power Output

Efficiency

Battery

Energy Storage Output Voltage Operating Temperature

Actuator

Efficiency

System Summary

Weight

Volume

Specific Gravity Heat Source Size

59%

9,000 to 120 rpm 3. 64 to 15 volts

CP-34 221-0 47°C 4. 55 watts 15. 0 volts, dc

15.25%

12. 5 watt/hr 15 volts 300'C

76%

1.840 Kgm (4. 06 lb) 1.396 liters (85.2 in^) 1.32 37 watts

Figure 5-7. Hybrid/Battery Converter System

5-8

-u' iV

HYBRID CONVERTER SYSTEM SUMMARY

MOTOR-RECIPROCATOR

BELLOWS SEAL*

RECIPROCATING ROD CONNECTOR

ROTARY CAM

RECIPROCATING CAM FOLLOWER

PLANETARY GEARS

MAGNETIC COUPLING

BRUSHLESS DC MOTOR

HYBRID ENGINE

MIN-K INSULATION

THERMOELECTRICS

HEAT SOURCE

CONDENSER'

BOILER

REGENERATOR

TURBINE

ROTARY VAPOR UNIT

PUMP

SPEED CONTROL

SYSTEM DESCRIPTION

The hybrid heat engine subsystem consists of a 49-watt heat source which incorporates a vent and capillary tube assembly for release of the decay-produced helium into the abdominal cavity. Mounted on each end of this cylindrical capsule a re two cascaded thermoelectr ic con­v e r t e r s . These converters a r e composed of 24 silicon-germanium high- temperature couples and 60 2N/TAGS couples. Combined with this thermoelec t r ic converter is a rotary vapor cycle turbogenerator opjerating at a peak cycle temperature of ZZl 'C. This temperature is com­patible with the cold junction tempera ture of the 2N/TAGS thermoelec t r ic couples. The rotary vaoor cycle system uses CP-34 (thiophene) as a working fluid

The engine unit is insulated with fibrous Min-K filled with xenon gas. The waste heat is rejected through con­denser tubes attached to the inside of the titanium con­tainer . The surface a rea of the container package is suffi­cient to permit heat rejection to the local bodv t issues at an acceptable rate of 0. 07 watt/cnn^.

In order to share the load, the output from the thermoelect r ic generator must be put through a dc-to-dc conver ter to boost it to 15 volts, reducing the power output to 2, 61 watts (e). Thus, the total power input to the motor / reciprocator becomes 9. 85 watts ^e). The overall effi­ciency (thermal to electr ical) of the hybrid heat engine is 20% These outputs a re integrated by an electronic load-sharing circuit which connects the brushless dc motor load alternately to each generator in a constani voltage, pulse-modulated manner. Using this technique, each generator sees an effectively constant load The switching rate used is approximately four t imes the dc motor speed so that switching tranbients a r e effectively eliminated by the ineitia of the motor rotor ~or a dc motor operating at 9000 RPM, the electronic com­mutation speed selected was 600 Hz

The thermoelectr ic generator is designed to deliver a constant power output of 3 21 watts at 3 64 vdc By employing a dc-to-dc converter , using a 50 kHz con­version frequency, this is increased lo 15 vdc to be compatible with the J5 vdc output of the turbogenerdtor Additional circuit logic is provided to maintain the thermoelect r ic output voltage within des i red tolerances The turbogenerator unit is provided with a speed control unit employing a parasit ic load bank Hence, at reduced loads, the load-sharing electronic c i rcui t ry diverts a portion of the turbogenerator output to the parasit ic load bank, thus maintaining this unit at constaat speed while matching the overall sys tem output power to the varying load

The moto r / r ec ip roca to r unit consists of a 9,000 rpm brushless dc motor operating on gas bear ings in a h e r ­metically sealed housing. The motor is coupled through a magnetic clutch and a set of reduction gears to a 120 rpm d rum-cam reciprocator . The cam follower imparts reciprocating motion to a rod which has an overall stroke of '1. 3 inches.

The electr ical moto r / r ec ip roca to r unit generates a mechanical back-and-forth motion which is t ransmit ted to the pump a'-'^uator through a flexible plastic-coated braided metal cable. -'A spring within the actuator unit provides a force bias which effectively maintains the cable in tension which IS the preferred operating mode during all phases of the pumping cycle.

No sensors are required to maintain control of the pumping action, the output power is automatically modu­lated by a variation in the pumping duty cycle {between 25 and 50% at a fixed rate of 120 beats /minute) . The duty cycle is automatically regulated by the actuator design and the controlled constant-speed charac te r i s t i cs of the e lect r ic motor .

The actuator automatically del ivers a part ial s t roke, according to the physiological demand; it provides the positive filling action necessa ry to maintain system control ("passive autoregulation") with the mandatory antisuction control; and it presents a simple mechanical interface to the actuator drive unit.

System Data

Thermoelec t r ics

SiGe Hot Junction Cold Junction

2N/TAGS

Hot Junction Cold Junction

Power Output

Efficiency

Power Out of dc to dc

Converter

Motor /Reciprocator

Eihcjency

Speed Reduction

Dc/dc Conversion

950°C 500°C

475 'C 240 'C

3.21 watts 3. 64 volts, dc 6. 55%

2.61 watts

15 volts

59%

9,000 to 120 rpm

3.64 to 15 volts

Rotary Vapor Turbogenerator

Working Fluid

Peak Cycle Tempera ture

Condensing Tempera tu re

Power Output

Efficiency

Actuator

Efficiency

Systenn Summary

Weight

Volume

Specific Gravity

Heat Source Size

CP-34

221'C

47°C 7. 24 watts 15. 0 volts, dc

15.75%

76%

1.83 Kg (4. 03 lb)

1. 33 l i te rs (81.4 in^)

1.37

49 watts

Figure 5-8. Hybrid Conver ter System

5-9

A^

millerc

Text Box

BLANK

6. EVALUATION CRITERIA AND SCORING METHODOLOGY

The candidate conver te r sys tem design options were requ i red to

meet all the design groundrules specified in Table 2-1. In addition, 24

evaluation c r i t e r i a were developed during Task 1 of the Project . These

a r e explained in Appendix A and summar ized in F igure 6-1. Four of

the or iginal 24 evaluation c r i t e r i a have been utilized as additional ground-

rules for defining acceptable design options ra ther than for scoring

candidate sys tems . These include: Pump Fil l ing P r e s s u r e (Al) wherein

all design options must have a mean back p r e s s u r e during pump filling

of 2-10 m m Hg; and Blood Over t empera tu re (B7), Chronic T i s sue Temper '

a ture (C3), and Chronic Blood T e m p e r a t u r e (C4) which a r e obviated by

designing all the thermal conver te r s to reject all heat to the abdominal 2

cavity at no more than 0.07 watt/cnn .

Each of the proposed evaluation c r i t e r i a is not considered to be

equally important in determining the best candidate system design.

Therefore , we have allocated to each pa rame te r a numer ica l weighting

factor which r ep resen t s i ts value (on an a rb i t r a ry scale) re la t ive to the

r e s t of the group of evaluation p a r a m e t e r s . These factors a r e included

along with each c r i t e r ion in F igure 6 - 1 . Table 6-1 p re sen t s the re la t ive

value coefficients in descending o rde r of impor tance .

The process for determining which is the "bes t" candidate is as

follows:

• Each candidate system is examined against the evaluation c r i t e r i a and is allocated a p r imary score against each of the p a r a m e t e r s . If any of the evalua-tion c r i t e r i a fail to es tabl ish a spread between the candidates of more than 10% in the p r i m a r y s c o r e s , the c r i t e r i a in question will be eliminated from the scoring p roces s . This precaution will reduce the tendency of the l e s s discr iminat ing p a r a m e t e r s to reduce the significant difference between the candidates .

• Each of these p r imary s co re s is then nnultiplied by the relat ive value coefficient allocated to that par t icu la r evaluation pa rame te r to a r r i ve at a weighted score in each category.

6-1

I - I RELIABILITY AGAINST RANDOM FAILMf RVC > 10 0 1-2 MLIAMLITY AGAINST WCAI-OUT W C - 9 5

2

. 1 1 1 1

IE^4-YEA« IE GAINST « * N K

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B-3 CONHOCNa LEVELS FOt THE MLIAtlLITY ESTIMATES tVC - * 0

ANE THEK EXKKIMENTAL DATA AVAIUKE F K M TESTS O N ACTUAL COMPONENT HAtDWARE I N THIS POMTER RANGE AND IN THIS PHYSICAL SIZE? O^fiS)

A K THEIC MTA AVAaABLE FROM 2 OR MO«E SOURaS? ( V U )

D D D D

10

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R-5 LEAKAGE OP TOKlC MATERIALS RVC - 3 0

SCOK 10(1 XCOTICAl FAaURE MODE PROIABILITtES) SYSTEM J SYSTEM J

%-i TISSUE OVERTEAAPERATURE RVC - 3 0

>-7 RLOOO OVERTEMPERATUli RVC • I 0

SCORE - 10(1 XCRITKAL FAILURE MODE PROBARILITIES) SYSTEM J SYSTEM J

RELIASILITY AGAINST WEAR-OUT

C-3 CHRONIC TISSUE TEMPERATUKS RVC • I 0 C-4 CHRONIC VlOODf lLM TCMPERATURE RVC 3 0

C-2 » MXIMUM RATE OF HEAT REJECTION RVC

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HAS THE TCCHNOIOOY K I N REDUaD TO PRAOKf " I N THIS POWER RANGE AND PHYSK>L SIZE? aO»

2 HASITBEENREDUaDTO raACTK;£ BY A NUMBER OF DVFERENI OEVEIOPERST (9)

IS THERE COMPARABLE TECHNOLOGY THAT HAS K E N KOUCEDTOPRACTia? S )

ARE THERE EXISTING DEVELOPMENT PKXSRAMS I N CIOSEIV RELATED AREAS' (10)

f N G I N t

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SYSTEM J NORMALIZED ID) SCORE •

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1 A K THERE PRIME COMPONENTS IN THE SYSTEM THAT A K IN THE MAINSTREAM OP ADVANCING TKHNOLCXSY AREAS? (10)

2 WHAT IS THE EXPECTED 3-VEAR IMPROVEMENT IN ONitHAll SYSTEM EFFICIENCY AND SPECIFIC POWER (WATTVU AT UNIT DENSITY) (10)

SYSTEM J NORMALIZED (10) SCORE'

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Figure 6 -1 . Eveduation Criteria

6-2 Z.7^

Table 6 - 1 . Relat ive Value Coefficients Summary

10,0 B-1 Reliabil i ty agains t random failure

9.5 C-6 Specific gravi ty

9 .0 C-5 Total volume

8.5 E-3 Design growth potential

8 .0 B-7 Blood o v e r t e m p e r a t u r e

7 .0 C-2 Maximum ra t e of heat re ject ion

7 .0 D-2 Sensit ivi ty to ambient p r e s s u r e changes

6.5 E -1 Component technology r ead iness

6 .0 E-4 Es t imated unit production cost

6 .0 B-3 Confidence leve ls for the re l iabi l i ty e s t i m a t e s

5 .5 C-8 Isotope inventory

5.5 E-2 Es t imated development cos t s

5 .0 B-6 T i s sue o v e r t e m p e r a t u r e

4 . 5 C-1 Endogenous hea t

4 .5 C-7 Maximum l inear dimension

4 . 0 B-4 Ability to de tec t incipient sys tem failure

3.5 B-2 Rel iabi l i ty agains t wea r -ou t

3 .0 B-5 Leakage of toxic m a t e r i a l s

3 .0 C-4 Chronic blood film t e m p e r a t u r e

2 .5 D-1 E a s e of su rg ica l instal la t ion

2 .0 A-1 Pump filling p r e s s u r e

1,5 D-4 Sensit ivity to e lec t romagnet ic fields

1.0 C-3 Chronic t i s s u e t e m p e r a t u r e s

1.0 D-3 Sensit ivity to mechanical shock

6-3

• The sum of the weighted s c o r e s in each ca tegory , for each of the candidate s y s t e m s , i s cons idered to be a compound figure of m e r i t r ep re sen t ing the re la t ive pe r fo rmance of that sy s t em according to that p a r t i c u l a r re la t ive value scheme .

• The s y s t e m s a r e then ranked with the highest figure of m e r i t r ep re sen t ing the " b e s t " candidate ,

• In Section 8.0 the sens i t iv i ty of the ranking to both judgmenta l and p rocedura l a spec t s of the scor ing methodology i s d i scussed

6-4

11^

7. CANDIDATE SCORING

Tables 7-1 through 7-8 p resen t scoring data for each of the eight

candidate s y s t e m s . Table 7-9 p r e s e n t s a s u m m a r y mat r ix of candidate

s y s t e m s , evaluat ion c r i t e r i a , and compara t ive s c o r e s .

Six of the evaluation c r i t e r i a did not provide g rea t e r than a

10 percent var ia t ion among the candidate sys t ems and a r e considered

"wash-out" c r i t e r i a . These included Instal lat ion t ime (Dl) , Sensit ivity

to Ambient P r e s s u r e Changes (D2), Mechanical Shock (D3), E l e c t r o -

nnagnetic F ie lds (D4) and Product ion Cos t s {E-4). Since four c r i t e r i a

becanne design g roundru les and six were found to be "wash-ou ts" , the

candidates were scored on the rennaining fourteen evaluation c r i t e r i a .

7-1

Table 7-1. Scoring of T h e r m o e l e c t r i c / B a t t e r y System

Evaluation C r i t e r i a

Descr ipt ion

Relative Value

Coefficient

Sys tem Raw

Score

System Weighted

Score Candidate System Da ta /Comments

Al Pump P r e s s u r e

Bl Reliabili ty Random

B2 Reliabili ty Wearout

B3 Reliabili ty Confidence

B4 Fa i lu re Detection

B5 Toxic Mater ia l

B6 Tissue Tempera tu re

B7 Blood Tempera tu re

CI Endogenous Heat

C2 Maximum Heat Rejection

C3 Chronic T i ssue , °F

C4 Chronic Blood, °F

C5 Volume

C6 Specific Gravity

C7 Dimensions

C8 Isotope Inventory

Dl Installation T ime

D2 P r e s s u r e

2 .0

10. 0

3.5

6,0

4 .0

3,0

5,0

8.0

4. 5

7,0

1,0

3.0

9 .0

9 .5

4 .5

5.5

2 .5

7. 0

10. 0

4 .8

4 .0

5 .3

1.0

10. 0

8.9

10.0

4 .5

1.6

0

0

1.8

9 .6

6 .3

0.9

5.4

10. 0

48. 0

14. 0

31.8

4 .0

44. 5

20. 3

11.2

16.2

91.2

28 .4

5.0

Groundrule: Designed for 6 -10mm Hg

Reliabil i ty is 0, 58

Reliabili ty is 0. 67

Subscores a re 12, 2. 5, 5. 5, 3. 85

System runs severa l hours off ba t te ry

Wash c r i te r ion ( less than 10% difference)

Reliabili ty is 0. 89

Groundrule: No heat exchange into blood

54-w heat source

51 . 16-w constant + 1. 73-w bat tery = 52 .89 -w

2 Groundrule: Designed for 0, 07 w / c m

2 Groundrule: Designed for 0. 07 w / c m

1. 372 l i t e r s (83. 75 in^)

Specific Gravity is 1. 10

3. 4 in OD cylinder

54-w heat source

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion ( less than 10%)

Table 7-1 Scoring of T h e r m o e l e c t r i c / B a t t e r y System (Continued)

I

Evaluation C r i t e r i a

Descr ipt ion

D3 Shock

D4 Elec t romagnet ic Fields

E l Technological Readiness

E2 Development Cost

E3 Potential

E4 Product ion Cost

Relative Value

Coefficient

1. 0

1. 5

6.5

5.5

8.5

6. 0

System Raw

Score

10. 0

10.0

6. 1

1. 7

7.5

10. 0

Total

1

System Weighted

Score

39.7

9 .4

51.0

414 7

Candidate System Data /Comments

Wash c r i te r ion (less than 10%)

Wash c r i t e r ion ( less than 10%)

Subscores 11, 3 .2, 3.2, 3.4, 8, 7.8

6. 0 M

Subscores 7, 5

2 6K (wash c r i t e r ion at 10%)

Table 7-2. Scoring of Rotary Vapor/Battery System

Evaluation C r i t e r i a

Descr ipt ion

Al Pump P r e s s u r e

Bl Reliabil i ty Random

B2 Reliabil i ty Wearout

B3 Reliabil i ty Confidence

B4 Fa i lu re Detection

B5 Toxic Mate r i a l

B 6 Tissue Tempera tu re

B7 Blood Tempera tu re

CI Endogenous Heat

C2 Maximum Heat Rejection

C3 Chronic Tissue , °F

C4 Chronic Blood, °F

C5 Volume

C6 Specific Gravity

C7 Dimensions

C8 Isotope inventory

Dl Instsdlation Time

D2 P r e s s u r e

Relative Value

Coefficient

2 .0

10.0

3.5

6.0

4 . 0

3.0

5.0

8 .0

4 . 5

7 .0

1.0

3.0

9 .0

9 .5

4 . 5

5.5

2 .5

7 .0

System Raw Score

10. 0

4. 3

4 .0

4 .7

1.0

9.9

8 .4

10.0

8 .4

2 .8

0

0

7.0

5.0

0

3.0

5.4

10. 0

System Weighted

Score

43. 0

14.0

28 .2

4 .0

42 .0

37.8

19.6

63.0

47 .5

0

16.5

Cajididate System Da ta /Comments

Groundrule: Designed for 6-10mm Hg

Reliabil i ty i s 0. 55

Reliabil i ty is 0. 67

Subscores a r e 10, 2 . 5 , 5 .5 , 2 .93

Sys tem runs seve ra l hours off ba t te ry

Wash cr i te r ion ( less than 10% difference)

Reliabil i ty is 0. 84

Groundrule: No heat exchanger into blood

41-w heat source

38. 2-w constant + 1. 8-w ba t te ry = 40-w

Groundrule: Designed for 0. 07 w / c m

Groundrule: Designed for 0, 07 w / c m

1. 06 l i t e r s (64.7 in^)

Specific Gravity = 1. 34

3. 25 in OD cylinder

41-w heat source

Wash c r i t e r ion ( less than 10%)

Wash cr i te r ion (less than 10%)

^ ^ k .

Table 7-2. Scoring of Rotary Vapor/Battery System (Continued)

I Ul

Evaluation Criteria

Description

D3 Shock

D4 Electromagnetic Fields

E l Technological Readiness

E2 Development Cost

E3 Potential

E4 Production Cost

Relative Value

Coefficient

1.0

1.5

6 .5

5 .5

8 .5

6 .0

System Raw

Score

10. 0

10.0

5.8

1.4

7 .0

10.0

Total

System Weighted

Score

37.7

7 .7

51.0

412.0

Candidate System Data/Comments

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion (less than 10%)

Subscores 10, 2 .8 , 3 .2 , 3 .2 , 7 .8, 7.6

7. 0 M Subscores 6, 6

3. 1 K (wash c r i t e r ion at 10%)

Table 7-3. Scoring of Hybrid/Battery System

Evaluation C r i t e r i a

Descr ip t ion

A l

B l

B2

B3

B4

B5

B6

B7

CI

C2

C3

C4

C5

C 6

C7

C8

D l

D2

Pump P r e s s u r e

Reliabil i ty Random

ReUability Wearout

Reliabil i ty Confidence

Fa i lu re Detection

Toxic Mate r i a l

T issue Tempera tu re

Blood T e m p e r a t u r e

Endogenous Heat

Maximum Heat Rejection

Chronic Tissue , ° F

Chronic Blood, " F

Volume

Specific Gravity

Dimensions

Isotope Inventory

Instal lat ion Time

P r e s s u r e

Relative Value

Coefficient

2 . 0

10.0

3 . 5

6 . 0

4 . 0

3 . 0

5 . 0

8 . 0

4 . 5

7 . 0

1.0

3 . 0

9 . 0

9 . 5

4 . 5

5 . 5

2 . 5

7 . 0

System Raw

Score

10.0

5 . 2

3 . 7

4 . 8

5 . 0

9 . 9

8 . 6

10.0

8 . 6

8 . 4

0

0

1.6

5 . 8

0

3 . 8

5 . 4

10.0

System Weighted

Score

_ -

52.0

13.0

28 .8

20 .0

- -

43 .0

- -

38.7

58 .8

- -

- -

14.4

55. 1

0

20.9

- -

- -

Candidate Sys tem Da ta /Commen t s

Groundrule : Designed for 6-'10mm Hjg

Reliabil i ty is 0. 61

Reliabil i ty i s 0. 66

Subscores a r e 11, 2, 5, 5. 5, 2. 73

Redundant System. Av. 3 day warning

Wash c r i t e r ion ( less than 10% difference)

Reliabil i ty is 0, 86

Groundrule: No heat exchanger into blood

37-w heat source

34. 19-w consteint + 1. 76-w bat tery = 35 .95-w

Groundrule : Designed for 0. 07 w / c m

Groundrule : Designed for 0. 07 w / c m

1. 396 l i t e r s (85. 2 in^)

Specific Gravity - 1. 32

3. 6 in OD cylinder

37-w heat source

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion ( less than 10%) 1

^ ^ p

Table 7-3. Scoring of Hybrid/Battery System (Continued)

Evaluation Criteria

Description

D3 Shock

D4 Electromagnetic Fields

El Technological Readiness

E2 Development Cost

E3 Potential

E4 Production Cost

Relative Value

Coefficient

1.0

1.5

6,5

5 .5

8.5

6 .0

System Raw

Score

10.0

10.0

5 .3

0.9 7.0

10.0

Total

System Weighted

Score

34. 5

5.0

46. 8

431. 0

Candidate System Data/Comments

Wash c r i t e r ion (less than 10%)

Wash c r i t e r ion (less than 10%)

Subscores 9. 4, 2 .2 , 3, 3, 7 .2 , 7.2

8. 5 M

Subscores 6, 5

4.1 K (wash c r i t e r ion at 10%)

Table 7-4, Scor i

Evaluation C r i t e r i a

Descr ipt ion

A l

B l

B2

B3

B4

B5

B6

B7

C I

C2

C3

C4

C5

C6

C7

C8

D l

D2

D3

Pump P r e s s u r e

Reliabili ty Randorri

Reliabil i ty Wearout

Reliabil i ty Confidence

Fa i lu re Detection

Toxic Mate r ia l

T issue Tempera tu re

Blood Tempera tu re

Endogenous Heat

Maximunn Heat Rejection

Chronic Tissue , ° F

Chronic Blood, " F

Volume

Specific Gravity

Dimensions

Isotope i iven to ry

instal lat ion Time

P r e s s u r e

Shock

Relat ive Value

Coefficient

2 . 0

10.0

3 . 5

6 . 0

4 . 0

3 . 0

5 . 0

8 . 0

4 . 5

7 . 0

1.0

3 . 0

9 . 0

9 . 5

4 . 5

5 . 5

2 . 5

7 . 0

1,0

System Raw Score

10. 0

6 . 4

6 . 6

6 . 5

5 . 0

9 . 9

8 . 6

10. 0

8 . 0

2 . 2

0

0

2 . 5

4 . 4

0

1.8

5 . 4

10.0

10.0

of Hybrid Sys tem

Sys tem Weighted

Score

- -

64.0

23. 1

39.0

20 .0

- -

43.0

- -

36.0

15.4

- -

- -

22.5

41 .8

0

9 . 9

- -

- -

- -

Candidate Sys tem Da ta /Comment s

Groundrule : Designed for 6-10mm Hg

Reliabil i ty is 0. 68

Reliabil i ty is 0. 78

Subscores a r e 16, 3. 3, 7. 3, 2. 73

Redundant System. Av. 3 day warning

Wash c r i t e r ion ( less than 10% difference)

Reliabi l i ty is 0. 86

Groundrule : No heat exchanger into blood

49-w heat source

49-w - 2. 22-w = 46. 78-w

Groundrule : Designed for 0. 07 W c m

Groundrule : Designed for 0. 07 w / c m

1. 334 l i t e r s (81.4 in^)

Specific Gravity = 1. 37

3. 5 in OD cylinder

49-w heat source

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion ( less than 10%)

Table 7-4. Scoring of Hybrid Sys tem (Continued)

I

Evaluation C r i t e r i a

Descr ip t ion

D4 Elec t romagne t ic Fie lds

E l Technological Read iness

E2 Development Cost

E3 Potent ia l

E4 Product ion Cost

Relat ive Value

Coefficient

1.5

6 .5

5 .5

8.5

6 .0

Sys tem Raw

Score

10.0

7 .4

1.8

9 .5

10.0

Total

System Weighted

Score

48 .1

9 .9

80 .8

453.5

Ceindidate Sys tem Da ta /Comment s

Wash c r i t e r ion ( less than 10%)

Subscores 14, 3 .6 , 5 .2 , 4 .4 , 8 .8 , 8 .6

5. 5 M

Subscores 9, 10

3. 9 K (wash c r i t e r ion at 10%)

Table 7-5 . Scoring of Gas Rec iproca t ing /TESM System

1 1 Evaluation C r i t e r i a

Descr ip t ion

A l

B l

B2

B3

B4

B5

B6

B7

CI

C2

C3

C4

C5

C6

C7

C8

D l

D2

D3

Pump P r e s s u r e

Reliabil i ty Random

Reliabil i ty Wearout

Reliabil i ty Confidence

Fa i lu re Detection

Toxic Mate r i a l

T issue Tempera tu re

Blood T e m p e r a t u r e

Endogenous Heat

Maximum Heat Rejection

Chronic Tissue , " F

Chronic Blood, " F

Volume

Specific Gravity

Dimensions

Isotope Inventory

Instal lat ion Time

P r e s s u r e

Shock

Relative Value

Coefficient

2 . 0

10.0

3 . 5

6 . 0

4 , 0

3 . 0

5 . 0

8 . 0

4 . 5

7 , 0

1.0

3 . 0

9 , 0

9 . 5

4 . 5

5 . 5

2 . 5

7 , 0

1.0

System Raw Score

10.0

3 . 5

1.6

2 . 8

0

10.0

6 . 4

10.0

8 . 0

0 . 3

0

0

4 . 2

2 . 8

0

1.8

5 . 4

10.0

10.0

System Weighted

Score

_ _

35.0

5 . 6

16.8

0

- -

32.0

- -

36.0

2 . 1

- -

- -

37.8

26 .6

0

9 . 9

—

- -

Candidate Systenn Da ta /Comments

Groundrule : Designed for 6-lOnnm Hg

Reliabil i ty is 0. 50

Reliabil i ty is 0. 57

Subscores a re 7, . 8, 2, 3. 03

System stops with engine fai lure

Wash c r i t e r ion ( less than 10% difference)

Reliabil i ty is 0. 64

Groundrule: No heat exchanger into blood

49-w heat source

76. 28-w max imum re jec ted

Groxondrule: Designed for 0. 07 w / c m

Groundrule : Designed for 0. 07 w / c m

1.233 l i t e r s

Specific Gravity is 1. 42

3. 25 in OD cylinder

49-w heat source

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion ( less than 10%)

Table 7-5. Scoring of Gas Rec iproca t ing /TESM System (Continued)

Evaluation C r i t e r i a

Descr ip t ion

D4 Elec t romagne t ic Fie lds

E l Technological Readiness

E2 Development Cost

E3 Potent ia l

E4 Product ion Cost

Relative Value

Coefficient

1.5

6.5

5.5

8.5

6.0

System Raw

Score

10. 0

6.9

1.4

8. 5

10.0

Total

System Weighted

Score

44 .9

7.7

72 .3

326.7

Candidate System Data /Comments

Wash c r i t e r ion (less than 10%)

Subscores 12,8, 3,4, 3 .8 , 4 .2 , 8 ,8 , 8. 6

7.0 M

Subscores 10, 7

3. 6 K (wash c r i t e r ion at 10%)

Table 7-6. Scoring of Gas Reciprocat ing Sys tem

Evaluation C r i t e r i a

Descr ip t ion

A l

B l

B2

B3

B4

B5

B6

B7

C I

C2

C3

C4

C5

C6

C7

C8

D l

D2

D3

Pump P r e s s u r e

Reliabil i ty Random

Reliabil i ty Wearout

Reliabil i ty Confidence

Fa i lu re Detection

Toxic Ma te r i a l

T issue T e m p e r a t u r e

Blood T e m p e r a t u r e

Endogenous Heat

Maximum Heat Rejection

Chronic Tissue , ° F

Chronic Blood, " F

Volume

Specific Gravity

Dimensions

Isotope inventory

Instal lat ion Time

P r e s s u r e

Shock

Relative Value

Coefficient

2 . 0

10.0

3 . 5

6 . 0

4 . 0

3 . 0

5 . 0

8 . 0

4 . 5

7 . 0

1.0

3 . 0

9 . 0

9 . 5

4 , 5

5 , 5

2 , 5

7 . 0

1.0

System Raw Score

10, 0

3 . 5

3 . 5

3. 1

0

10,0

6 . 4

10.0

4 . 4

1,7

0

0

2 . 0

9 , 5

7. 1

0 . 9

5 . 4

10.0

10. 0

Sys tem Weighted

Score

__

35.0

12 .3

18.6

0

- -

32. 0

- -

19.8

11.9

- -

- -

18.0

90 .3

32. 0

5 . 0

- -

- -

- -

Candidate System Da ta /Comments

Groundrule : Designed for 6-10mm 1%

Reliabil i ty is 0. 50

Reliabil i ty is 0. 65

Subscores a r e 6. 7, 1. 3, 2. 7, 3. 03

Sys tem stops with engine fai lure

Wash c r i t e r ion ( less than 10% difference)

Reliabi l i ty is 0. 64

Groundrule : No heat exchanger into blood

54-w heat source

54. 4-w - 2. 22-w = 52. 18-w

Groundrule : Designed for 0. 07 w / c m

Groundrvile: Designed for 0. 07 w / c m

1. 36 l i t e r s

Specific Gravity = 1 . 1 5

3. 04 in OD cylinder

54-w heat source

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion ( less than 10%)

Table 7-6. Scoring of Gas Reciprocating System (Continued)

I

Evaluation C r i t e r i a

Descr ip t ion

D4 Elec t roraagnet ic F ie lds

E l Technological Readiness

E2 Development Cost

E3 Poten t ia l

E4 Product ion Cost

Relat ive Value

Coefficient

1.5

6 .5

5 .5

8 .5

6 .0

Sys tem Raw

Score

10.0

7.2

1.5

8 .5

10.0

Total

Sys tem Weighted

Score

46 .8

8 .3

72. 3

402 .3

Candidate System Data /Comments

Wash c r i t e r ion ( less than 10%)

Subscores 12.8 , 3 .6 , 5 .2 , 4 ,2 , 8 .8 , 8 .6

6. 5 M

Subscores 10, 7

3. 5 K (wash c r i t e r ion at 10%)

Table 7-7. Sco

1 Evaluation C r i t e r i a

Descr ipt ion

A l

B l

B2

B3

B4

B5

B6

B7

C I

C2

C3

C4

C5

C6

C7

C8

pi p2

Pump P r e s s u r e

Reliabili ty Random

Reliabil i ty Wearout

Reliabil i ty Confidence

Fa i lu re Detection

Toxic Mater ia l

T i s sue Tempera tu re

Blood Tempera tu re

Endogenous Heat

Maximum Heat Rejection

Chronic Tissue , " F

Chronic Blood, " F

Volume

Specific Gravity

Dimensions

Isotope Inventory

Instal lat ion Time

P r e s s u r e

Relative Value

Coefficient

2 . 0

10.0

3 , 5

6 . 0

4 . 0

3 . 0

5 . 0

8 . 0

4 . 5

7 . 0

1.0

3 . 0

9 . 0

9 . 5

4 . 5

5 . 5

2 . 5

7 . 0

ring of Linear Vapor /TESM System

System Raw

Score

10.0

4 . 6

1.8

4 . 3

0

10.0

6 . 5

10.0

8 . 1

0 . 9

0

0

1.2

8 . 0

0

1.9

5 . 4

10.0

Sys tem Weighted

Score

__

46 .0

6 . 3

25.8

0

- -

32.5

- -

36.5

6 . 3

- -

- -

10.8

76.0

0

10.5

- -

Candidate Sys tem Da ta /Comment s

Groundrule: Designed for 6-10mm Hg

Reliabil i ty is 0. 57

Reliabil i ty i s 0, 58

Subscores a r e 9, 2. 3, 4, 3. 9

Sys tem stops with engine fai lure

Wash c r i t e r ion ( less than 10% difference

Reliabil i ty is 0. 65

Grotmdrule: No heat exchanged into blood

48-w heat source

65. 40 - 4. 25 = 61 . 15-w re jec ted 2

Groundrule : Designed for 0. 07 w / c m Groiindrule: Designed for 0. 07 w / c m

1. 42 l i t e r s

Specific Gravity is 1.25

4. 0 in OD cylinder

48-w heat source

Wash c r i t e r ion (less than 10%)

Wash c r i t e r ion ( less than 10%)

Table 7-7. Scoring of Linear Vapor /TESM System (Continued)

I

Evaluation C r i t e r i a

Descr ip t ion

D3 Shock

D4 Elec t romagne t ic F ie lds

E l Technological Readiness

E2 Development Cost

E3 Potent ia l

E4 Product ion Cost

Relat ive Value

Coefficient

1. 0

1.5

6 .5

5.5

8. 5

6 ,0

Sys tem Raw

Score

10. 0

10. 0

7 .2

1.5

8 .5

10.0

Total

Sys tem Weighted

Score

46 .8

8 .3

72 .3

378. 1

Candidate System Data /Comments

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion (less than 10%)

Subscores 13.8 , 3 .6 , 3 .8 , 4 . 2 , 8 .8 , 8.8

6. 5 M

Subscores 10, 7

3.3K (wash c r i t e r ion at 10%)

Table 7-8. Scoring of L inear Vapor Sys tem

Evaluation C r i t e r i a

Descr ipt ion

A l

B l

B2

B3

B4

B5

B6

B7

CI

C2

C3

C 4

C5

C6

C7

C8

D l

D2

Pump P r e s s u r e

Reliabili ty Random

Reliabili ty Wearout

Reliabil i ty Confidence

Fa i lu re Detection

Toxic Mate r ia l

T issue Tempera tu re

Blood T e m p e r a t u r e

Endogenous Heat

Maximum Heat Rejection

Chronic Tissue , ° F

Chronic Blood, ° F

Volume

Specific Gravity

Dimensions

Isotope Inventory

Installation Time

P r e s s u r e

Relative Value

Coefficient

2 . 0

10. 0

3 . 5

6 . 0

4 . 0

3 . 0

5 . 0

8 . 0

4 . 5

7 , 0

1.0

3 . 0

9 . 0

9 . 5

4 . 5

5 . 5

2 . 5

7 . 0

System Raw Score

10. 0

4 . 9

2 . 5

5 . 6

0

10.0

6 . 5

10.0

2 . 1

1,4

0

0

1.4

10.0

6. 1

0 . 5

5 . 4

10. 0

Sys tem Weighted

Score

. . .

49 .0

8 . 8

33.6

0

-

32.5

- -

9 . 5

9 . 8

- -

- -

12.6

95.0

27 .5

2 . 8

- -

- -

Candidate Sys tem Da ta /Comment s

Groundrule : Designed for 6-10 m m Hg

Reliabil i ty is 0. 59

Reliabi l i ty is 0. 61

Subscores a r e 13. 3, 2 .7 , 5 . 3 , 3.9

System stops with engine fedlure

Wash c r i t e r i s ( less than 10% difference)

Reliabil i ty is 0. 65

Groundrule : No heat exchanger into blood

57-w heat source

51 w - 2 .22 w = 54.88 w

Groundrule : Designed for 0. 07 w / c m

Groundrule : Designed for 0. 07 w / c m

1. 50 l i t e r s (85.4 in^)

Specific Gravity is 0. 95

3. 5 inch d iame te r cylinder

57-w heat source

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion (less than 10%)

Table 7-8. Scoring of Linear Vapor System (Continued)

Evaluation Criteria

Description

D3 Shock

D4 Electromagnetic Fields

El Technological Readiness

E2 Development Cost

E3 Potential

E4 Production Cost

Relative Value

Coefficient

1.0

1.5

6.5

5 .5

8 .5

6 .0

System Raw

Score

10.0

10.0

7 .4

1.7

8 .5

10. 0

Total

System Weighted

Score

48.1

9 . 4

72. 3

410. 9

Car.didate System Data/Comments

Wash c r i t e r ion ( less than 10%)

Wash c r i t e r ion ( less than 10%)

Subscores 13.8, 3 .8 , 5 .2 , 4 .2 , 8 .8 . 8.8

6. 0 M Subscores 10, 7

3. 2 K (wash c r i t e r ion at 10%)

Table 7-9. Evaluation Criteria, Weighted Scoring Summary

-J I

00

Candidate Syatema

Criteria

B l

B2

B 3

B 4

B 6

CI

C2

0 5

C6

0 7

C8

E l

E2

E 3

Reliability (Random)

Reliability (Wearout)

Reliability Confidence

Failure Detection

Tisiue Overtemperature

Endogenous Heat

Maximum Heat Rejection

Volume

Specific Gravity

Maxunutn Linear Dimension

Isotope Inventory

Technological Readiness

Development Cost

Growth Potential

TOTAl.

Thermoelectric/ Battery

48.0

U . 0

31.8

4 . 0

44.5

20.3

11.2

16.2

91.2

28.4

5 . 0

39.7

9 . 4

51 0

414 7

Rotary Vapor/ Battery

43 .0

14.0

28.2

4 . 0

42.0

37.8

19.6

63.0

47.5

0

16.5

37.7

7 . 7

51 0

412 0

Hybrid/ Battery

52.0

13.0

28.8

20.0

43.0

38.7

58.8

14.4

55.1

0

20.9

34.5

5 . 0

46 8

431 0

Hybrid

64.0

23. 1

39.0

20.0

43.0

36.0

15.4

22.5

41.8

0

9 . 9

48. 1

9 . 9

80.8

453.5

Gas Reciprocating/ TESM

35.0

5 . 6

16.8

0

32.0

36.0

2 . 1

37.8

26.6

0

9 . 9

44.9

7 . 7

72.3

326.7

Gas Reciprocating

35.0

12.3

18.6

0

32.0

19.8

11.9

18.0

90 3

32.0

5 . 0

46.8

8 . 3

72 3

402 3

Linear Vapor/ TESM

46 0

6 3

25 8

0

32.5

36.5

6 . 3

10 8

76 0

0

10 5

46.8

8 . 3

72 3

378. 1

Linear Vapor

49.0

8 . 8

33 6

0

32 5

9 5

9 . 8

12.6

95.0

27 5

2 8

48 1

9 4

72 3

410.9

NOTE: Groundrule criteria and wash criteria excluded

8. SENSITIVITY ANALYSIS

8. 1 I N T R O D U C T I O N

T h e o b j e c t i v e of a s e n s i t i v i t y a n a l y s i s i s to d e t e r m i n e the s e n s i t i v i t y

of a s e l e c t i o n p r o c e s s to s u b j e c t i v e j u d g m e n t s .

A s d e s c r i b e d e a r l i e r , t h e c a n d i d a t e s c o r i n g m e t h o d o l o g y r e q u i r e s

a p p l i c a t i o n of s o m e 24 e v a l u a t i o n c r i t e r i a , each of wh ich has a p o s s i b l e

s c o r e of z e r o to t e n depend ing on the p a r t i c u l a r va lue of the p a r a m e t e r

be ing e v a l u a t e d . E a c h e v a l u a t i o n c r i t e r i o n in t u r n has a r e l a t i v e v a l u e

coef f i c i en t (weight ing f a c t o r ) a l r e a d y fixed at a v a l u e b e t w e e n one and t e n .

T h e t o t a l n u m e r i c a l s c o r e for e a c h c a n d i d a t e is s i m p l y the s u m of the

p r o d u c t s of e a c h e v a l u a t i o n c r i t e r i a s c o r e (ECS) and i t s r e s p e c t i v e r e l a t i v e

v a l u e coef f i c i en t (RVC) . In t h e o r y , t he E C S ' s a r e the ob jec t ive j u d g m e n t s

( m a x i m u m w e i g h t , v o l u m e , e t c . ) wh i l e the R V C ' s a r e the s u b j e c t i v e j u d g ­

m e n t s ( r e l a t i v e i m p o r t a n c e of w e i g h t v e r s u s v o l u m e , e t c . ).

T h e t r a d i t i o n a l s e n s i t i v i t y a n a l y s i s is c a r r i e d out to d e t e r m i n e t h e

s e n s i t i v i t y of the s e l e c t i o n p r o c e s s to t h e r e l a t i v e i m p o r t a n c e a c c o r d e d the

R V C ' s . T h i s i s u s u a l l y done by chang ing only one RVC at a t i m e whi le keep in

a l l the o t h e r s fixed. Often t h i s is done on a n o r m a l i z e d b a s i s so that t he RVC

not l i m i t e d to i ts m a x i m u m po in t v a l u e , but can a s s u m e any p e r c e n t a g e of the

s u m t o t a l of the R V C ' s . Whi le t h i s i s p o t e n t i a l l y u s e f u l , even with only

14 of the o r i g i n a l 24 e v a l u a t i o n c r i t e r i a r e m a i n i n g , an e n o r m o u s a m o u n t of

d a t a i s g e n e r a t e d in s i m p l y d e t e r m i n i n g t h e s t a b i l i t y of t h e r a n k i n g to thf

p e r t u r b a t i o n of j u s t one R V C . T h e a m o u n t of da ta g e n e r a t e d when two o r

m o r e R V C ' s a r e v a r i e d , g r o w s g e o m e t r i c a l l y . A l so , we find tha t the

a s s u m p t i o n t h a t t h e E C S ' s a r e " c o r r e c t " and f r e e f r o m s u b j e c t i v e u n c e r ­

t a i n t y i s no t r e a l l y v a l i d . One r e a s o n i s t ha t t he e v a l u a t i o n c r i t e r i a s c o r i n g

i s n e c e s s a r i l y an i t e r a t i v e p r o c e s s . A s t h e d e s i g n e r g e t s new i n f o r m a t i o n ,

he should c h a n g e h i s g r o u n d r u l e s . B e c a u s e of the r e q u i r e m e n t for f r e e z i n g

t h e s c o r i n g m e t h o d o l o g y at the end of t h e f i r s t m o n t h of PliasL I, it w a s not

p o s s i b l e to do t h i s , and s o m e s p e c i f i c e x a m p l e s of the p r o b l e m s t h i s can

p r e s e n t w i l l be g iven l a t e r . A s e c o n d r e a s o n i s t h a t the eva lua t i on c r i t e r i a

s c o r i n g c u r v e s a r e s e l d o m l i n e a r o v e r the r a n g e of a c c e p t a b l e v a l u e s , and

t h e r e f o r e , t h e s h a p e of the c u r v e p e r m i t s j u d g m e n t a l l a t i t u d e .

8-1

F o r the above r e a s o n s and the c loseness of the s c o r e s of the top

candidates , we concluded that any a t tempt at a pure ly mechanica l manipu­

lation of the RVC's would not provide the requ i red insight into the sound­

ness of our concept se lect ion p r o c e s s . Ra the r , we chose to under take a

m o r e focused examinat ion of the scor ing methodology and a reevaluat ion

of the ranking in light of se lect ive p e r t u r b a t i o n s .

8.2 DISTRIBUTION OF CRITERIA BY CATEGORY

As stated in Section 6, we or iginal ly had five genera l c l a s s e s of

evaluation c r i t e r i a as shown in the f i rs t two columns of Table 8-1.

In columns 3, 4, and 5, we show the number of or ig inal c r i t e r i a , the

sum of the RVC's for each c l a s s , and the pe rcen tage of the RVC

total r ep re sen t ed by each c l a s s . However, as indicated in Section

6, p r io r to final scor ing some of the evaluation c r i t e r i a w e r e made

into design groundrules and o thers were dropped from the scor ing

because the re was l e s s than a 10 percen t sp r ead in t he i r ECS ' s .

In addition, we purpose ly chose not to no rma l i ze the highest candidate

sco res for each c r i t e r ion to a fixed maximunn (say 10 points) and this

reduces the effective value of the RVC's . Because of this and our r e ­

duction in the number of evaluation c r i t e r i a from 24 to 14, the re was

concern that poss ibly an imbalance would be c r e a t e d among the var ious

c r i t e r i a c l a s s e s . The effect on reducing the number of c r i t e r i a is shown

in columns 6, 7, and 8 of Table 8-1. As can be seen, the only resu l t is

a modes t i n c r e a s e in the impor tance of the physical p a r a m e t e r s as

compared to re l iabi l i ty and p rac t i cab i l i ty cons ide ra t ions . The de facto

reduction in the values of the RVC's, by not normal iz ing to a common

fixed maximum, is shown in the las t th ree columns of Table 8-1 to have

negligible additional impact .

The impl ica t ions of th is can be seen f rom Table 8-2, which is s imply

a review of the final candidate rank ings , broken down by c r i t e r i a c l a s s sub ­

to ta l s . The hybrid sy s t em had the highest subtotal of any of the candidates

in the r e l i ab i l i t y - r e l a t ed (B) and prac t icab i l i ty cons ide ra t ions (E) c l a s s e s

and had next to the lowest subtotal in the phys ica l p a r a m e t e r (C) c l a s s .

However , it was this " C " c r i t e r i a c l a s s that r ece ived heav ie r - than- in tended

emphas is as a r e su l t of our evaluation p r o c e s s . T h e r e f o r e , the fact that

8-2

Table 8 - 1 . Evaluat ion C r i t e r i a Impor tance by Class

CUas

A

B

(',

D

E

Principal Characteris t ic

Pump Filling P re s su re

Reliability Considerations

Physical Paramete rs (weight, volume, isotope inventory, etc.)

Environmental Sensitivity

Practicability Considerations

Total

No. of Cri ter ia

1

7

8

4

4

Original

Actual

SRVC's

2. 0

39.5

44. 0

12. 0

26.5

124. 0

% Total

2

32

35

10

21

100

No. ot Cri teria

0

5

6

0

3

14

Final

Actual

LRVC's

-

28. 5

40.0

-

20. 5

89.0

% Total

-

32

45

-

23

100

Effective

ERVC's

19. 1

28.4

'

13. 9

61. 4

% Total

-

31

46

-

23

100

Table 8-2. Evaluat ion C r i t e r i a Scoring by C la s s

S y s t e m

T h e r m o e l e c t r i c / B a t t e r y

R o t a r y V a p o r / B a t t e r y

H y b r i d / B a t t e r y

H y b r i d

G a s R e c i p r o c a t i n g / T E S M

Gas R e c i p r o c a t i n g

L i n e a r V a p o r / T E S M

L i n e a r Vapor

B

R e l i a b i l i t y

142. 3

131 .2

1 5 6 . 8

189. 1

8 9 . 4

9 7 . 9

110 .6

123 .9

C

P h y s i c a l C h a r a c t e r i s t i c s

1 7 2 . 3

1 8 4 . 4

1 8 7 . 9

1 2 5 . 6

1 1 2 . 4

1 7 7 . 0

140. 1

1 5 7 . 2

E

P r a c t i c a b i l i t y

100.1

96.4

86 .3

138 .8

124 .9

127 .4

127 .4

129 .8

To ta l

414.7

412.0

431.0

4 5 3 . 5

326 .7

402 . 3

378, 1

4 1 0 . 9

Rank

3

4

2

1

8

6

7

5

the hybrid engine s t i l l e m e r g e d with the highest point score indicates that

the select ion p r o c e s s was insens i t ive to this pe r tu rba t ion . It should a lso

be noted that the h y b r i d / b a t t e r y sy s t em, which provides growth potential

for the hybrid s y s t e m , was ranked highest in the " C " c r i t e r i a c l a s s .

8-3

8.3 E F F E C T S OF CRITERIA SCORING REVISIONS

Since our evaluation c r i t e r i a scor ing methodology was frozen ea r ly

enough in P h a s e I to p reven t any subs tant ive i te ra t ion , it was not s u r ­

p r i s ing to find that s e v e r a l c r i t e r i a scor ing cu rves or p r o c e d u r e s w e r e

obsole te .

The mos t obvious example of this was evaluation c r i t e r i a C2, m a x i ­

m u m r a t e of heat re jec t ion . Our or iginal scor ing curve i s shown in

F igure 8 - 1 . As desc r ibed in the Task 1 F ina l Repor t , our reasoning was

that up to about 40 wat ts of heat could be re jec ted through the hea r t pump

d i rec t ly into the blood s t r e a m . Above this va lue , an addit ional blood heat

exchanger would be r e q u i r e d . T h e r e f o r e , the point value dropped p r e ­

cipitously from 8 to 3 points as the 40-wat t value was exceeded. As docu­

mented in detai l in Section 2. 6, -we bel ieve that not only is was te heat re jec

tion to the blood puinp optional, but is may be difficult to achieve in

p r a c t i c e . On the o the r hand, we found tha t the t h e r m a l conve r t e r volume

and specific gravi ty cons idera t ions tended to r e su l t in package surface

a r e a s which w e r e adequate to d i ss ipa te up to 60 w^atts of heat d i rec t ly to

the abdominal fluids and t i s sue without exceeding heat flux values that

appeared safe based on avai lable exper imenta l da ta .

T h e r e f o r e , the dashed curve in F i g u r e 8-1 would be m o r e cons is tent

with our c u r r e n t approach to re ject ing was t e heat . The effect of using this

updated scor ing cu rve is shown in Table 8 -3 . The hybrid sys tem r e m a i n s

the top- ranked candidate by an i n c r e a s e d m a r g i n while the ro t a ry

v a p o r / b a t t e r y s y s t e m r ep l ace s the h y b r i d / b a t t e r y sy s t em as the second-

ranked sys t em. Since the C-2 (maximum ra t e of heat re ject ion) c r i t e r i o n

scor ing was the only obvious candidate for modif icat ion, we next turned to

an examination of the candidate rankings for in te rna l cons is tency .

An obvious quest ion is r a i s ed by the very sma l l pe rcen tage point

spread (in the o r ig ina l scor ing, only 5%) between the hybrid ( ranked

No. 1) and h y b r i d / b a t t e r y (ranked No. 2) s y s t e m s . Th is is suspic ious

since although the h y b r i d / b a t t e r y sys tem has the s ma l l e s t heat sou rce of

all the candidates (37 wa t t s ) , it r e q u i r e s a solid e lec t ro ly te ba t t e ry which

is s t i l l under development . T h e r e f o r e , the hybrid ba t t e ry sys t em should

8-4

RVC = 7.0

20 40 60 80 MAXIMUM HEAT REJECTION RATE

W

Figu re 8 - 1 . Maximum Rate of Heat Rejection

Table 8 -3 . Revised Tota ls and Ranking Using Updated C2 Scoring C r i t e r i o n

Sys tem

T h e r m o e l e c t r i c / B a t t e r y

Rota ry V a p o r / B a t t e r y

H y b r i d / B a t t e r y

Hybrid

Gas Rec ip roca t ing /TESM

Gas Reciproca t ing

L inear Vapor /TESM

Linear Vapor

Or ig ina l Scoring

414.7

412.0

431.0

453 .5

326.7

4 0 2 . 3

378. 1

410 .9

Original Ranking

3

4

2

1

8

6

7

5

Revised C2 Scoring

427.3

448.4

431.0

477 .3

326.7

414 .9

378. 1

419 .3

Revised Ranking

4

2

3

1

8

6

7

5

8-5

^01

rank low in re l iab i l i ty confidence (B3) and technology r e a d i n e s s ( E l ) . How­

ever , a s can be seen in Table 7-9 , the B l and E l s c o r e s for a l l t h r e e

ba t t e ry s y s t e m s a r e quite p r e s e n t a b l e .

Inspect ion of the r e spec t i ve evaluation c r i t e r i a s c o r e sheets showed

that solid e lec t ro ly te b a t t e r i e s w e r e indeed given a ze ro on both counts .

However , for both these c r i t e r i a , scor ing was done separa te ly on a c o m ­

ponent ba s i s (engine, insulat ion, energy s t o r a g e , power conditioning and

control) and the r e s u l t s w e r e added. This i s obviously an i n c o r r e c t p r o ­

cedure s ince both c r i t e r i a r e p r e s e n t confidence fac tors which should be

de te rmined by the product r a the r than the sum of the s e p a r a t e component

confidence f a c t o r s . As with re l i ab i l i ty , the ove ra l l sy s t em value can never

be higher than that of i t s weakes t component . If the B3 and E l s c o r e s a r e

the re fo re de t e rmined by the p roduc t s r a t h e r than the sums of the i r com­

ponent s c o r e s , the r e s u l t s a r e shown in Table 8-4 . As can be seen , the

hybrid sys tem is now a lmos t 13% ahead of the second-p lace candidate and

the to ta l point sp read among the ceindidates i s 33%. The s c o r e s and rank­

ings a r e a lso shown in Table 8-4 for the ca se in which all t h r e e c r i t e r i a

r ev i s ions a r e applied. The hybrid s y s t e m ' s lead is now over 15% and the

point sp read among the candidates exceeds 36%.

Table 8-4. Revised To ta l s and Ranking for Updated B3 and E l Scoring

Sys tem

T h e r m o e l e c t r i c / B a t t e r y

Ro ta ry V a p o r / B a t t e r y

Hybr id /Ba t t e ry

Hybrid

Gas Rec ip roca t ing /TESM

Gas Rec iproca t ing

L inea r Vapor /TESM

Linear Vapor

Revised B3 and E l

Scoring

358.7

369.5

378.3

449 .0

301.0

374.8

346.4

392 .3

Revised Rank

5

6

3

1

8

4

7

2

Revised B 3 , E l and C2 Scoring

371.3

395.9

378.3

472 .8

301.0

387.4

346.4

' 400 .8

Revised Rank

6

3

5

1

8

4

7

2

8-6

8.4 A L T E R N A T E SCORING T E C H N I Q U E S

T h e n e x t q u e s t i o n we a d d r e s s e d w a s w h e t h e r a l t e r n a t e m e t h o d s of

u t i l i z i n g the e v a l u a t i o n c r i t e r i a s c o r e s ( E C S ' s ) and r e l a t i v e v a l u e c o ­

e f f i c i en t s ( R V C ' s ) m i g h t c h a n g e the r e l a t i v e r a n k i n g s of the c a n d i d a t e s .

A s m i g h t be a n t i c i p a t e d by i n s p e c t i o n of the equa t i on E E C S x R V C :

it d o e s not m a t t e r w h e t h e r o r not t h e R V C ' s a r e f i r s t n o r m a l i z e d to uni ty

( o r a n y o t h e r n u m b e r ) . T h e r e l a t i v e n u m e r i c a l s c o r e s of the c a n d i d a t e s ,

w h i c h is wha t un ique ly d e t e r m i n e s the r a n k i n g , wi l l be unchanged . However ,

t h e r e a r e t h r e e d i f f e r en t t e c h n i q u e s for g e n e r a t i n g t h e E C S ' s wh ich can

i n f l uence the r e l a t i v e r a n k i n g . T h e s e i n c l u d e tak ing the r a w s c o r e s ( z e r o

to a m a x i m u m of 10) f r o m the e v a l u a t i o n c r i t e r i a c u r v e s and

(1) U s i n g t h e m n u m e r i c a l l y u n a l t e r e d

(2) A s s i g n i n g a v a l u e of 10 to t h e b e s t c a n d i d a t e and n o r m a l i z i n g t h e s c o r e s of the r e m a i n i n g c a n d i d a t e s

(3) Add ing the s c o r e s and n o r m a l i z i n g to unity

In t h e o r i g i n a l s c o r i n g c a l c u l a t i o n s , m e t h o d #1 w a s c h o s e n s i n c e both

the s e c o n d and t h i r d a p p r o a c h e s t e n d to p r o d u c e E C S ' s t ha t r e f l e c t r e l a t i v e

s t a n d i n g s r a t h e r t h a n how w e l l e a c h c a n d i d a t e has done in m e e t i n g the m a n y

e v a l u a t i o n c r i t e r i a . H o w e v e r , s i n c e m e t h o d #3 is f r e q u e n t l y u s e d in d e c i s i o n

m o d e l i n g , we r e c a l c u l a t e d the s c o r e s for e a c h of the c a n d i d a t e s y s t e m s by

add ing the s c o r e s for e a c h of the c r i t e r i a and n o r m a l i z i n g uni ty ( m e t h o d ^3)

r a t h e r t h a n us ing t h e m n u m e r i c a l l y u n a l t e r e d ( m e t h o d #1).

We a l s o r e p e a t e d the s e l e c t e d c r i t e r i a s c o r i n g r e v i s i o n s (C2, B3, and

El) d e s c r i b e d in S e c t i o n 8 .3 . T h e r e s u l t s a r e shown in T a b l e s 8-5 and 8-6 .

T h e p r i n c i p a l d i f f e r e n c e in t h e o r i g i n a l s c o r i n g u s ing m e t h o d #3 r a t h e r than

#1 i s t h a t t h e h y b r i d b a t t e r y s y s t e m e m e r g e s in f i r s t p l a c e and the h y b r i d

wi thou t a b a t t e r y c o m e s in s e c o n d . H o w e v e r , a p p l i c a t i o n of any o r a l l

of t h e u p d a t e d s c o r i n g c r i t e r i a r e s t o r e s the h y b r i d s y s t e m to f i r s t p l a c e

and p r o d u c e s r o u g h l y t h e s a m e f inal s t a n d i n g a s p r e s e n t e d in T a b l e s 8 - 3 and

8 -4 . T h e p e r c e n t a g e po in t s p r e a d , h o w e v e r , is s o m e w h a t i n c r e a s e d us ing

m e t h o d #3 wi th t h e upda t ed C2, B 3 , a n d E l c r i t e r i a . T h e p e r c e n t a g e s p r e a d

is 20% b e t w e e n f i r s t and s e c o n d p l a c e a n d 44% b e t w e e n f i r s t and l a s t p l a c e ;

w h e r e a s u s i n g m e t h o d #1, t h e c o r r e s p o n d i n g p e r c e n t a g e d i f f e r e n c e s w e r e

15% and 36%.

Table 8-5. Revised Totals and Ranking Using Updated C2 Scoring Cr i t e r ion and Normal ized ECS's (Method #3)

System

T h e r m o e l e c t r i c / B a t t e r y

Rotary V a p o r / B a t t e r y

Hybr id /Ba t t e ry

Hybrid

Gas Rec ip roca t ing /TESM

Gas Reciprocat ing

Linear Vapor /TESM

Linea r Vapor

Original Scoring

11.25

12.28

13.74

12.92

8 .43

10. 54

9.00

10.46

Original Ranking

4

3

1

2

8

5

7

6

Revised C2 Scoring

11.41

12. 95

12.46

13.31

8.38

10. 70

8.87

10. 54

Revised Ranking

4

2

3

1

8

5

7

6

Table 8-6. Revised Totals and Ranking for Updated B3 and E l Scoring and Normal ized ECS's (Method #3)

System

T h e r m o e l e c t r i c / B a t t e r y

Rotary V a p o r / B a t t e r y

Hybr id /Ba t t e ry

Hybrid

Gas Rec ip roca t ing /TESM

Gas Reciprocat ing

Linear Vapor /TESM

Linear Vapor

Revised B3 and E l

Scoring

10.25

11.35

12.75

14.70

8.52

10.64

9.02

11.38

Revised Rank

6

4

2

1

8

5

7

3

Revised B3, E l and C2 Scoring

10.41

12. 02

11.47 .

15. 08

8.47

10.80

8.88

11.46

Revised Rank

6

2

3

1

8

5

7

4

8-8 '30*/

8.5 CONCLUSIONS

Two fac tors governed the type of sensi t ivi ty analys is that we c a r r i e d

out. The f i r s t was that our scor ing c r i t e r i a and methodology had not been

allowed to change as our unders tanding of the problems and solutions

m a t u r e d during Phase I. There fore , severa l of the c r i t e r i a we applied were

no longer appropr ia te in e i ther content or p rocedure . The second

cons idera t ion was the c lo senes s of the candidate s c o r e s ; the top five

candidates w e r e all within 10% of each o ther . Therefore , we could have

Carr ied out a t rad i t ional sensi t ivi ty ana lys is and show^ed myr iad ways of

having any one of these five end up as the top candidate.

Instead we f i r s t noted that even with the original scor ing methodology,

the hybrid sys tem scored highest in both the rel iabi l i ty and pract icabi l i ty

ca t ego r i e s and st i l l re ta ined i t s total point supremacy even though it was

next to l as t in the heavi ly-weighted physical c h a r a c t e r i s t i c s ca tegory. We

a lso noted that the hybr id /ba t t e ry sy s t em, which was one of the growth

potent ia ls for the hybrid sys tem was a lso the highest ranking sys tem on

the b a s i s of physical c h a r a c t e r i s t i c s .

We then p resen ted the logic for rev is ing three of the c r i t e r i a scor ing

techniques because they w e r e e i the r no longer valid, or they produced

in ternal incons is tenc ies . We a lso cons idered an a l t e rna te calculational

method for combining the ECS's and RCV's . Whether these revised scor ing

p r o c e d u r e s w e r e applied s e p a r a t e l y o r in concert , they reaff i rmed the top

ranking of the hybrid s y s t e m by s t a t i s t i ca l ly significant marg ins .

'-' 2,0^'

millerc

Text Box

BLANK

9.0 R E F E R E N C E S

1 "A Study on the Ef fec t s of A d d i t i o n a l E n d o g e n o u s Hea t Re l a t i ng to the A r t i f i c i a l H e a r t , " Annua l R e p o r t , P a c i f i c N o r t h w e s t L a b o r a t o r i e s , Augus t 10, 1967, P B 177 328

2 "A Study on the Effec t of Add i t i ona l E n d o g e n o u s Hea t R e l a t i n g to the A r t i f i c i a l H e a r t , " Annua l R e p o r t , P a c i f i c N o r t h w e s t L a b o r a t o r i e s , D e c e m b e r , 1968, P B 180 941.

3 M. F . G i l l i s and P . C. Walkup, " S t u d i e s on the Ef fec t s of Added E n d o g e n o u s H e a t and on H e a t E x c h a n g e r D e s i g n s , " P r o c e e d i n g s , A r t i f i c i a l H e a r t P r o g r a m C o n f e r e n c e , C h a p t e r 74, June , 1969

4. "Study of the Effec t of Add i t i ona l E n d o g e n o u s Heat , " Annual R e p o r t , T h e r m o E l e c t r o n C o r p o r a t i o n , D e c e m b e r , 1967, P B 177 753

5 "Study of the Ef fec t of Add i t i ona l E n d o g e n o u s Hea t , " Annua l Repo r t , T h e r m o E l e c t r o n C o r p o r a t i o n , Ju ly , 1968, P B 180 157.

6. J . C. N o r m a n , et a l . , " E x p e r i m e n t a l Mode l for Induc ing Acu te and C h r o n i c H y p e r t h e m i a , " V o l . XII T r a n s A m e r . Soc. A r t i f i c i a l Int . O r g a n s , 1966, p. 282.

7. J . C. N o r m a n , et. a l . , "E f f ec t s of I n t r a C o r p o r e a l H e a t and R a d i a t i o n on Dogs, " P r o c e e d i n g , A r t i f i c i a l H e a r t P r o g r a m C o n f e r e n c e , June , 19'^9, C h a p t e r 76.

8. J . C. Nor inan , et. a l . , " H e a t - I n d u c e d M y o c a r d i a l A n g i o g e n e s e s , " Vol. XVII T r a n s . A m e r Soc A r t i f i c i a l Int . O r g a n s , 1971, p. 213.

9. " T h e Study of the Ef fec t s of A d d i t i o n a l E n d o g e n o u s Heat , "Annua l R e p o r t , J o h n B. P i e r c e F o u n d a t i o n , J u l y 1, 1966 to J u n e 30, 1967, P B 176 91 i

10. R R a w s o n , " S t u d i e s of the E f fec t s of A d d i t i o n a l E n d o g e n o u s Heat , " P r o c e e d i n g s , A r t i f i c i a l H e a r t P r o g r a m C o n f e r e n c e , J u n e , 1969. C h a p t e r 75.

11. J. A Stolwijk , "A M a t h e m a t i c a l Mode l of P h y s i o l o g i c a l T e m p e r a t u r e R e g u l a t i o n in Man, " NAS CR-1855 , Augus t , 1971

12. J . C. Schude r , et a l . , " H e a t and E l e c t r o m a g n e t i c E n e r g y T r a n s p o r t t h r o u g h B i o l o g i c a l M a t e r i a l a t L e v e l s R e l e v a n t to the I n t r a - T h o r a c i c A r t i f i c i a l H e a r t , " Vol . XII, T r a n s . A m e r . Soc. A r t i f i c i a l Int . O r g a n s , 1966, p. 275.

13. J . O. C o l l i n s , " P a r t i c u l a t e T h e r m a l I n s u l a t i o n s for T h e r m o e l e c t r i c E n e r g y C o n v e r s i o n D e v i c e s , " J o h n s - M a n v i l l e C o r p o r a t i o n l E C E C R e c o r d 689036, 1968.

14. J . O. Co l l i n s , K. L. J a u n a r j s , and D. R. Reid , "Deve lop 1800F - 400F F i b r o u s - T y p e I n s u l a t i o n for R a d i o i s o t o p e P o w e r S y s t e m s , " F i n a l R e p o r t No. ALO-2661-12 , J o h n s - M a n v i U e R e s e a r c h and E n g i n e e r i n g C e n t e r , M a n v i l l e , New J e r s e y .

9-1

3on

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16. J. Carvalho, J. B. Dunlay, M. L. Paquin, and V. L. Po i r i e r , "Quar t e r ly P r o g r e s s Repor t of R e s e a r c h and Developnnent of Vacuum Foi l Type Insulat ion for Radioisotope Power Sys tems, " T h e r m o Elec t ron C o r p o r a ­tion Report No. 4059-24-70, July, 1969.

17. W. E. Grunert , F . Notard, and R. L. Reid, "Opacified Fibrous Insula­tions, " Union Carb ide Corporat ion, AIAA P a p e r 69-605, June, 1969.

18. G. Flynn, W. H. Pe rc iva l , and F. E. Heflner, "GMR Stir l ing Engine, " T r a n s . SAE, Vol. 68, I960.

19. W. H. McAdams, Heat T r a n s m i s s i o n , Thi rd Edition, McGraw-Hil l , 1954, p. 336.

20. A. Koestel, and R. J. Ziobro, "Two-Phase Spheroidal Heat T rans fe r to M e r c u r y in Vor tex F o r c e d Convection, " TRW Equipment Group, 1968, lECEC Proceed ings , p. 352.

21. W. M. Kays, and A. L. London, Compact Heat Exchangers , Second Edition, McGraw-Hil l , 1964, p. 131.

22. "Isotopes Kilowatt P r o g r a m , T a s k I - Conceptual Design and Evaluation, ORNL-TM-2366, Oak Ridge National Labora tory , January , 1970.

23. D. R. Ea r l e s , and M. F. Eddins, Rel iabi l i ty Engineer ing Data Se r i e s F a i l u r e Rates , Avco Corporat ion, April , 1962.

24. H. A. Rothbart, Mechanical Design and Sys tems Handbook, McGraw-Hill, 1964.

9-2

APPENDIX A

A total of 24 evaluat ion c r i t e r i a w e r e der ived during Task I of the

P r o j e c t . Scoring of the phys io logicaUy-re la ted c r i t e r i a de te rmined

with the a s s i s t a n c e of our medical adv i sory group which consis ted of the

following consul tants :

D r . H. J . C. Swan Di rec to r , Depar tment of Cardiology, Ceda r s -S ina i Medical Center , Ceda r s of Lebanon Hospital Division Los Angeles , California

Dr. Yukihiko Nose Di rec to r , Art i f ic ia l Organs P r o g r a m , and Head, Depar tment of Artificial Organs R e s e a r c h The Cleveland Clinic Foundation Cleveland, Ohio

Dr . J . Van De Water Di rec to r , Card iovascu la r and Thorac ic Surgery City of Hope National Medical Center Duar te , California

H. P . Roth P r i v a t e consultant, Environmental and T h e r m a l Physiology Manhattan Beach, California

The scor ing c u r v e s r e fe renced throughout the text that follows a r e

sunamarized in F igu re A-1 which is the san:ie as F igu re 6 - 1 .

A-1 P U M P FILLING PRESSURE

It i s impor tan t that the p r e s s u r e ref lected back into the left a t r i u m

by the blood pump dur ing the filling phase be maintained between 0 and

18 m m Hg. At p r e s s u r e s g r e a t e r than 18 m m Hg, it is believed that the

rec ip ien t will begin to exper ience pulmonary congestion; at p r e s s u r e s

below ambient, the d is t r ibut ion of blood flow within the lungs is significantly

d i s tu rbed .

Since p r e s s u r e s between 2 and 10 m m Hg were cons idered opt imum,

full s co re was accorded this range with s c o r e s l inear ly dec reas ing to

ze ro at 0 and 18 m m Hg. This became a "wash" c r i t e r i on since the

A-1 ^ < ? ?

DEVICE RELIABILITY

100% 10% SURGICAL RISK

50

CURVE B SHOWS SUPERIOR RELIA­BILITY DURING THE FIRST 9-1/2 YEARS OF OPERATION

CURVE B SHOWS SIGNIFICANT /WEAR-OUT FAILURE RATE

•^ CURVE A SHOWS NEGLIGIBLE WEAR-OUT AT 10 YEARS

YET, CURVE A HAS A SUPERIOR TEN-YEAR RELIABILITY

JL JL 10 YEARS

TIME

Figure A-2. In te r im and L o n g - T e r m Reliabili ty

f ^ I *

pump/ac tua to r units for all candidate concepts were designed to provide

a b a c k p r e s s u r e of 2-10 mnn Hg.

B-1 RELIABILITY AGAINST RANDOM FAILURE AND B-2 RELIABILITY AGAINST WEAROUT

The re l iabi l i ty e s t i m a t e s for each of the candidate sys tem des igns

were based on accepted re l iabi l i ty computat ion p rocedures as desc r ibed

in Section 4 . 0.

In d i scuss ing what might be a reasonable t en -yea r re l iabi l i ty goal

wi th our medica l consult ing group, i t b e c a m e apparent that they would nriuch

prefer an in t e rmed ia t e or s h o r t - t e r m equivalent m e a s u r e of the rel iabi l i ty

pe r fo rmance of the device . This informat ion, r a the r than the equivalent

t e n - y e a r specification, would be genera l ly more useful to them as the

bas i s for thei r r ecommenda t ion of the t r e a t m e n t to the patient . This

pa r t i cu l a r fac tor is i l lus t ra ted in F i g u r e A - 2 . F o r example , they felt

that a device implantat ion p rocedure with a s ta t i s t i ca l life expectancy

(50% rel iabi l i ty) of five y e a r s would be a ve ry acceptable a l te rna t ive to the

patient who has a na tu ra l life expectancy of 6 months or l e s s , provided that

account is taken of the 10% r i sk a s soc i a t ed with the s u r g e r y . Converted

into equivalent t en -yea r re l iabi l i ty t e r m s this r e q u i r e s a device re l iabi l i ty

of only 27.8% [ ^ x (^.p ^ Q J .

If we a s s u m e that the f ive-year re l iabi l i ty is a lmos t totally dominated

by the r andom fai lure modes , the significantly inc reased incidence of w e a r -

out f a i lu res at ten y e a r s will reduce the overa l l sys tem rel iabi l i ty even

fur ther below the 27.8% level .

To de t e rmine what might be r easonab le re l iabi l i ty design goals , we

adopted the following approach . Based on available "mor ta l i ty f rom hea r t

d i s e a s e " s t a t i s t i c s , we took the t e n - y e a r survival probabil i ty for a midd le -

aged ca rd i ac pat ient to be about 50%. This i s conserva t ive since v i r tua l ly

all candidates for this type of t r e a t m e n t w^ill have a much poore r p rognos i s .

Thus, i nc rea s ing his t en -yea r surv iva l probabil i ty f rom 50% to 7 5% would

appear to be a ve ry reasonable goal . Fac to r ing in the surgica l r i sk , this

cal ls for an overa l l re l iabi l i ty p e r f o r m a n c e f rom the device of 83 . 3%

This overa l l pe r fo rmance could be achieved from var ious

combinat ions of the random fa i lure pe r fo rmance and the wea r -ou t

p e r f o r m a n c e .

We can der ive a specific divis ion between w e a r - o u t and random

fai lure pe r fo rmance by setting a goal for overa l l f ive-year re l iab i l i ty .

If the total t en -yea r pe r fo rmance were a t t r ibuted complete ly to random

fai lure modes , the above 83 ,3% would co r respond to a f ive -yea r pe r fo rmance

at the 91.2% leve l . If we again a s s u m e that v i r tua l ly all of the fa i lu res up

to the f ive-year point will be due to random fa i lu res , r a t h e r than wear -ou t ,

then the f ive-year pe r fo rmance mus t be taken to be g r e a t e r than the 91.2%

leve l .

We elected to make the f ive-year goal 95%. Over a t e n - y e a r period,

this level of re l iabi l i ty aga ins t r andom fa i lure i s equivalent to 89. 5%.

The difference between this and the net t en -yea r device re l iabi l i ty of

83. 3% defines the t e n - y e a r re l iab i l i ty aga ins t w e a r - o u t at 93 . 1%

The min imum acceptable t e n - y e a r re l iabi l i ty agains t random fai lure

level was set, as we have d i scussed by the inedical consult ing group at

27 .8%. By definition, the min imum acceptable t e n - y e a r re l iabi l i ty

agains t w e a r - o u t is 50%; a lower level would imply a design l i fet ime of

l ess than ten y e a r s . We there fore adopted the levels s u m m a r i z e d below

as design goals and acceptable min ima for the overal l sy s t em rel iabi l i ty .

Ten yea r device re l iab i l i ty :

• Against random fa i lu res - goal 90%, min imum 27.8%

• Against wea r -ou t - goal 93%, min imum 50%

Device re l iabi l i ty at the ( in te rmedia te ) f ive-year point; • Against random fa i lu res - goal 95%, min imum 55. 5%

• F a i l u r e s due to w e a r - o u t - negligible

Scoring between " m i n i m u m " and "goa l" re l iabi l i ty va lues was

a s s u m e d to be l i n e a r .

B-3 CONFIDENCE LEVELS FOR THE RELIABILITY ESTIMATES

A r ep re sen t a t i ve 'figure of m e r i t ' was genera ted to re f lec t

the level of confidence which could be a s soc ia t ed with each of the sys tem

re l iab i l i ty e s t i m a t e s . A number of semi-quant i t ive c r i t e r i a a r e l i s ted

below. The s c o r e s a l located in r e s p o n s e to the c r i t e r i a w e r e sunnmed

and no rma l i zed to scale of ten .

A-4

31^

rO.833 | 0 . 8 9 5 j -

1. Are the re exper imenta l data available from

t e s t s on actual component ha rdware in

this power range and in this physical s ize?

(20/45) I

2. Are the re data available f rom 2 or more

sou rces? (5/45) I

3. Are the data scaled f rom exper imenta l

invest igat ions of ' s i m i l a r ' components?

(10 /45 ) I

4. What i s the level of complexity of the I

candidate sys tem? (10/45) I

Scoring the response to i tem 4 was determined by adding the s co re s

in each of the following ca t egor i e s and normal iz ing to a scope of 10.

Scoring curves a r e shown in F igure A - 3 . A response of zero in any

ca tegory was scored at 10.

1) Total number of moving pa r t s (curve #2)

2) Total number of c y c l i c a l l y - s t r e s s e d par t s (curve #2)

3) Number of meta l l ic components exposed to co r ros ion s t r e s s (curve #2)

4) Total p a r t s count (curve #1)

5) Number of p a r t s under static s t r e s s (curve #1)

6) Number of joints ( r i sk of fluid leakage) (curve #1)

B-4 ABILITY TO DETECT INCIPIENT SYSTEM FAILURE

Although the ti t le is m i s l e a d i n g , the purpose of this c r i t e r ion was

to take into account the t ime in te rva l available for remedia l action after

a typical sys tem malfunction.

Since the rec ip ient can reasonab ly be expected to undergo physiological

function test ing at 2 - 3 month in t e rva l s , sys t ems with malfunction inc idence-

to -c r i t i ca l i ty in te rva l s in this range were awarded the higher s c o r e s .

Shor ter in te rva l s were given an in te rmedia te score because they allow some

chance of r emed ia l action if the fault can be diagnosed by a special service

or by the rec ip ient himself . It was a s sumed that at l eas t th ree days would

A-5

SCORE

3 4 5 6 7 8 10 ZO 30 50 100

NUMBER O F PARTS

Figure A-3 . Scoring Curve for Complexity Rating Por t ion of Cr i t e r ion B - 3 .

be a highly des i r ab l e in terva l for the l a t t e r case ; and therefore , a point

score of five was awarded this va lue . The score falls l inear ly to ze ro

below the 3-day point and r i s e s l e s s rapidly to 10 above this point .

A fa i lure modes and effects ana lys i s was pe r fo rmed for each candidate

sys tem des ign . F o r rec iproca t ing s y s t e m s without a bat tery, it was a s sumed

that pos t -engine-fa i lure survival t ime would be negl ig ible . F o r those sy s t ems

with energy s torage downs t ream of the engine (ba t te r ies ) , it was es t imated

that s eve ra l hours might be avai lable for r emed ia l ac t ion. The hybrid

sys t ems were a specia l case since the rec ip ient could function essen t ia l ly

indefinitely on the reduced power provided by the ro t a ry vapor stage alone

(assuming fa i lure of the t h e r m o e l e c t r i c s tage) . However, the r i s e in heat

source t e m p e r a t u r e would probably linait the power f rom the t h e r m o e l e c t r i c

stage alone (assuming fai lure of the r o t a r y vapor engine) to seve ra l hours

at mos t . Consider ing the re la t ive fa i lure probabi l i t ies of the two power -

producing components of the hybrid sys tem, the mean survival t ime would

probably be seve ra l days .

A-6

3)i<4

B - 5 L E A K A G E O F TOXIC M A T E R I A L S

The p r o b a b i l i t y of e a c h m a l f u n c t i o n that could l ead to an u n a c c e p t a b l e

c o n c e n t r a t i o n of tox ic m a t e r i a l m the t i s s u e w a s d e t e r m i n e d f r o m a f a i l u r e

i n o d e s and e f f ec t s a n a l y s i s .

T h e s u m of a l l the p r o b a b i l i t i e s fo r e a c h c a n d i d a t e s y s t e m w e r e

a c c u m u l a t e d and the s c o r e c a l c u l a t e d a s fo l lows

S c o r e = 10(1 - Y. c r i t i c a l f a i l u r e m o d e p r o b a b i l i t i e s ) s y s t e m j

s y s t e m j

M a t e r i a l s in the s y s t e m i n v e n t o r y (and p o s s i b l e d e g r a d a t i o n p r o d u c t s )

t ha t could c o n c e i v a b l y be r e l e a s e d to the s u r r o u n d i n g body t i s s u e s w e r e

i d e n t i f i e d . T h e m o s t o b v i o u s tox ic m a t e r i a l s , T E S M , and the s o d i u m and

su l fu r m the ^olid e l e c t r o l y t e b a t t e r y , a r e so l id at body t e m p e r a t u r e and

t h e r e f o r e it i s u n l i k e l y tha t t h e s e m a t e r i a l s would have suff ic ient m o b i l i t y

to b r e a c h the o u t e r t h e r m a l c o n v e r t e r c o n t a i n e r . The only iden t i f i ed toxic

m a t e r i a l tha t would be l iqu id at r o o m t e m p e r a t u r e i s the th iophene ( C P - 3 4 )

w o r k i n g fluid e m p l o y e d m the r o t a r y v a p o r s y s t e m s . The p r o b a b i l i t y of

l e a k a g e t h r o u g h the l eng th and type of w e l d m e n t s i s e s t i m a t e d a t l e s s than

1 %

B - 6 TISSUE O V E R T E M P E R A T U R E

The m o s t p r o b a b l e d e g r a d a t i o n , ma l func t ion and w e a r o u t m o d e s

t h a t cou ld l e a d to t h e r m a l c o n v e r t e r s u r f a c e t e m p e r a t u r e s h i g h e r than the

d e s i g n l i m i t of 43 C, a r e iden t i f i ed in T a b l e 4-18 All of t h e s e m o d e s l e a d

to the r e s u l t t h a t e n e r g y f r o m the h e a t s o u r c e i s not e f f ic ien t ly c o n v e r t e d

lo b lood p u m p w o r k , but r a t h e r b e c o m e s add i t i ona l t h e r m a l e n e r g y to be

d i s s i p a t e d a t the t h e r m a l c o n v e r t e r s u r f a c e . The o v e r t e m p e r a t u r e m o d e s

do no t n e c e s s a r i l y h a v e the s a m e p r o b a b i l i t y of o c c u r r e n c e m a l l c a n d i d a t e

d e s i g n s , and m s o m e d e s i g n o p t i o n s t h e y a r e not even a p p l i c a b l e . T h e r e ­

f o r e , the p r o b a b i l i t i e s of t h e s e m o d e s o c c u r r i n g in e a c h of the c a n d i d a t e

d e s i g n s w a s r a t e d on a b a s i s of 0 to 10% f a i l u r e p r o b a b i l i t y wi th the h i g h e s t

p r o b a b i l i t y of t i s s u e o v e r t e m p e r a t u r e a s s i g n e d 10%. T h e s e p r o b a b i l i t y

r a t e s a r e c l o s e l y r e l a t e d to the w e a r o u t f a i l u r e p r o b a b i l i t i e s s i n c e a l m o s t

a l l w e a r o u t m o d e s a r e c o n c u r r e n t wi th d e c r e a s e d p e r f o r m a n c e and t h e r e ­

f o r e i n c r e a s e d w a s t e h e a t l o a d . In a d d i t i o n , the s y s t e m s with T E S M c a u s e

l a r g e r t e m p e r a t u r e f l u c t u a t i o n s even d u r i n g the c o u r s e of n o r m a l o p e r a t i o n

A - 7

than those sy s t ems incorpora t ing e l ec t rochemica l energy s to rage . T h e r e ­

fore, the TESM sys tems genera l ly have higher t i s sue o v e r t e m p e r a t u r e

probabi l i t i es . The hybrid sys t em has a lower probabil i ty of t i s sue ove r ­

t empe ra tu r e than e i ther the t h e r m o e l e c t r i c or the ro t a ry vapor engines

because as the t h e r m o e l e c t r i c module power drops due to m a t e r i a l d e g r a ­

dation, the los t power is not re jec ted d i rec t ly to the body but becomes an

inc reased t h e r m a l input to the t u r b o g e n e r a t o r . Finally, those sy s t ems that

have re la t ive motion with m e t a l - t o - m e t a l sliding contact on dynamic sea ls

(such as the solenoid valves in the l inear vapor engine) a r e m o r e prone to

wearout and a steadily dec reas ing efficiency, leading to a h igher probabil i ty

of t i s sue o v e r t e m p e r a t u r e .

The s c o r e s were de te rmined f rom the following equation:

Score ^ . = 10 (1 - Z c r i t i ca l fa i lure mode probabi l i t ies) sys t em J ., . ' sy s t em j

The r e s u l t s a r e shown in the bottom row of Table A-1.

B-7 BLOOD OVERTEMPERATURE

The blood heat exchangers were to be designed to accommodate the

max imum ra te of heat reject ion under conditions within the design spec i ­

f ica t ions . This ra te would co r re spond to a specified m a x i m u m blood film

t empe ra tu r e (probably 41 C). However, a l l our candidate concepts were

designed to r e j ec t all the i r waste heat d i rec t ly to the abdominal fluids and

t i s sue at acceptable heat flux l e v e l s . Since the re would be no blood heat

exchanger , this became "wash" c r i t e r i a as d i scussed in de ta i l in Section 2 .4 .

However, a s epa ra t e blood heat exchanger can readi ly be incorpora ted

if requi red .

C-1 ENDOGENOUS HEAT

A special weighted value of the overa l l systena efficiency was used

to es tab l i sh the r equ i red heat sou rce size, which, by definition becomes the

t i m e - a v e r a g e d endogenous heat burden to the body. The 60-wat t upper

l imit is a specified design g roundru le . The full score for va lues up to

about 10 wat ts re f lec t s the " c r e d i t " that i s probably avai lable due to the

reduced work (heat) output of the na tu ra l h e a r t .

A-8 .

V

Table A-1. T issue O v e r t e m p e r a t u r e Fa i lu re Mode Analysis (Fa i lu re Mode Probabi l i ty /Rel iabi l i ty)

P o s s i b l e F a i l u r e Mode Leading to P r o t e i n Damage

Feed Water P u m p Efficiency L o s s

PCU Efficiency Loss Caused by W e a r

Fa t igue of Spr ings

Fa t igue of Bel lows

Fouling of Heat Exchange Sur faces

Engine Efficiency L o s s e s Caused by Wear /Degrada t ion

Efficiency L o s s e s Caused by Valve Leakage

(I - Summat ion of C r i t i c a l F a i l u r e Mode P r o b a b i l i t i e s )

= T i s s u e O v e r t e m p e r a t u r e Rel iabi l i ty

T / E with

Ba t t e ry

F a i l u r e ^^^"^ P r o b a - ^ y ^ b i l i t y ^ / ^

^ ^ Re l iab i l i ty

0 .00 ^ . - ^

^ ^ ^ 1.00

0 .05 ^ ^ . - - ^

^ , , - - - - ' ^ 0.95

0. 02 ^ ^ ^ ^

^^--^'"''^o.oe

0. 02 ^^-'•'^

^ ^ - - " ^ ^ 0. 98

0. 01 ^^--^^

^ ^ - ^ ^ ^ 0. 99

0 .015 ^.-'^^'^

^ , ^ ' • ^ ' ^ 0 . 9 8 5

0 .00 ^ ^ - - ' ^

^ ^ ^ - ^ ^ 1 . 0 0

0.89

R o t a r y Vapor with

Ba t t e ry

F a i l u r e ^ ^ P r o b a - ^ ^ b i h t y ^ ^ ^

^ ^ Rel iabi l i ty

0 .04 ^ - ^

^ - ^ 0 .96

0.05 ^^-^^

^.-^"^ 0 .95

0 .02 ^ — ^

^ ^ - ' ' ' ^ 0. 98

0 .02 ^ ^ . ^ ^

^^-^^ 0. us

0.02 ^ ^ ^ - - - ^

^,,^-^^ 0 ,98

0. 02 ^ . ^ - ^

^ ^ ^ ^ ^ ^ 0 .98

0. 00 ^ ^ - ^ ^

^ , ^ ^ ' ' ^ 1 . 0 0

0. 84

Hybrid with

Ba t t e ry

F a i l u r e ^ ^ P r o b a - ^ ^ b i l i t y ^ ^ ^ ^

^ ^ Rel iabi l i ty

0 .04 ^ - " ^

^ ^ ^ 0. 96

0. 05 ^ ^ - - ^ ^

^^^'^'^ 0. 95

0, 01 ^....-''^

^ 0. 9Q

0. 02 ^^""^"^

^ -""""^ 0. as

0. 02 ^ ^ . - ^ ^

^^^'^'^ 0. 98

0 .01 ^ ^ - - - ^ ^

^ , , - ^ " ' ^ 0 . 9 9

0 .00 ^ ^ - ' ' ^

0.8b

Hybrid

F a i l u r e ^y^ P r o b a - ^ ^ b i l i t y ^ ^ ^

^ / - ^ ^ Rel iab i l i ty

0 .04 ^ ^ ^ ' ' ^

^ ^ ' " ' ^ 0, 96

0. 05 ^ ^ ^ ^ ^

^ ^ ^ ^ ' " ^ 0. 95

0. 01 ^ ^ ^ - - ' ' ^

^^.y^'^ 0. 99

0 .02 ^ ^ - ^ ^

^ 0 .98

0, 02 ^ ^ y ^ '

^^-"^""'^ 0. 98

0. 01 ^ ^ ^ ' ^

^^^^'^^ 0. 99

0. 00 ^ ^ ^ " ^

^,y^^^ 1. 00

0. 86

Gas Rec ip roca t ing

with TESM

F a i l u r e ^ ^ P r o b a - ^ ^ b i l i t y ^ ^ ^

^y^ Rel iabi l i ty

0 .00 ^ ^

^ ^ 1.00

0.05 ^ '

^ , y ' ^ 0. 05

0.00 ^ , . - ^

^^^^^ 0. 01

0. 01 ^ ^ - ^

^^^-^^ O.ol

0. 05 ^^^^^

^^y^'^ 0. - 5

0 .09 ^ ^ ^ ^

^^-^''^ 0 . 9 !

0.06 ^ ^ ^ ^

^ ^ ^ ^ ^ 0. 04

0. 64

Gas Rec iproca t ing

F a i l u r e ^ y ^ P r o b a - ^ ^ bility y ^

^ ^ Rel iabi l i ty

0 .00 ^ ^ ^

^ ^ ^ 1.00

0. 05 ^ ^ . - - ' ^

^ . ^ ^ ^ " ^ 0.95

0. 00 ^ ^ - " ^

^ ^ y ' ^ 0. 91

0. 09 ^^y'''^

^ ^ • ' ^ ^ O.oi

0.05 ^ , y ^

^ ^ - ^ ' ' ^ O . 05

0. 09 ^ „ - ' ^ ^

^ , , ' ^ ^ 0 .91

0 06 ^ ^ ' - " ' ^

^ , , , - - • ' ^ ^ 0 . 9 4

0. b4

L i n e a r Vapor with TESM

F a i l u r e ^ ^ P r o b a - ^ ^ b i l i t y ^ ^ ' ^ ' ^

^ y ^ Rel iabi l i ty

0 .09 ^ ^ " ^ ^

^ • ' ' ^ ' ^ 0. 91

0 .01 ^ ^ y ^

^^^^^^ 0. 09

0. 06 ^

^,^-y^^ 0 04

0. 07 ^ ^ ' • ' ^

0. 05 ^ ^ ^ ^ ^

0.05 ^^^-^^

^^^y^^ 0.O5

0. 08 ^^.^^^

^ ^ - ' - • ' ' ' ^ 0 . 9 2

0 65

L inea r Vapor

F a i l u r e ^^^^ p r o b a - ^y^ b i l i t v x - ^

^ y ^ Rel iabi l i ty

0 .09 ^ ^ ^ ^

^ ^ ' - ' ^ ^ 0 .91

0 .01 ^ ^ . y ^ \

^,y-^^^ 0 .99

0 .07 ^^y^^\

^„y^^ 0 .93

0.07 ^ „ , - - ^ l

^y^^''^ 0 .93

0.05 ^ ^ ^ ^ '

^ y " ^ ^ 0 .95

0 , 0 5 ^ . ^ ' ^ X

^ y ^ ' ^ ^ 0 .95

0 .08 ^^'^^^

— 0 ,92

0.65

C-2 MAXIMUM RATE OF HEAT REJECTION

The max imum ra te of heat re jec t ion o c c u r s at different t imes for

the va r ious s y s t e m s . F o r the ba t t e ry group, it occu r s at peak blood pump

power and is equal to

Q ^ (Battery) = Qv,^-P + ^ " ' ^ ^ max^ '' hs ave ^

fP - P \i\-^ \ \ max ave / \ ' m r a /

^ m r a

^ max ~ Maximum ra te of heat re jec t ion (watts)

Q , = Heat source inventory (watts)

• max ~ Maximum power into blood punnp (4. Z5 watts)

P = Average power into blood pump (2.81 watts)

n = Combined efficiency of m o t o r / r e c i p r o c a t o r m r a , . ' < f

and automat ic ac tua tor

The l a s t two t e r m s on the r ight a r e about -1.05 wat t s ; and therefore ,

the m a x i m u m ra te of heat re jec t ion is a li t t le over one watt l e s s than the

t h e r m a l inventory.

F o r s y s t e m s using t h e r m a l energy s torage u p s t r e a m of the engine,

the Q y g jj a l so occu r s at P _y^3„ and i s given by

Q (TESM) = Q^ -P + T ' " ^ ^ "^^^e ) ( ^ ' \ o t ) max hs ave n

tot

where

n = Overa l l hea t - to -b lood-pump convers ion efficiency at a power level of 4 . 25 wat ts into the blood pump.

Q r ^ , ^ ^^11 genera l ly be higher than Q, for these s y s t e m s . max " i\s '

F o r the s y s t e m s with no energy s torage , Q o c c u r s at the t ime of m i n i m u m power de l ive red to the blood pump (P . ) and i s given by

min

Q max (' o ene rgy s torage) = Q h s ' ^ m i n

A-10

31^

#

F o r t h e s e s y s t e m s Q is a p p r o x i m a t e l y 2 2 w a t t s l e s s than the m a x

h s

The s h a p e of the C - 2 s c o r i n g c u r v e r e f l e c t s the fol lowing c o n s i d ­

e r a t i o n s "

1) The body c a n p r o b a b l y a c c o m m o d a t e a t e m p o r a r i l y r a i s e d h e a t b u r d e n for p e r i o d s c o m p a r a b l e wi th the a n t i c i p a t e d p e r i o d s of s u s t a i n e d g r e a t e r - t h a n - a v e r a g e a c t i v i t y .

2) 5 to 10 w a t t s of w a s t e h e a t i s r e q u i r e d to r e s t o r e the body to i t s o r i g i n a l t h e r m a l s t a t u s p r i o r to the r e d u c t i o n m the h e a r t ' s w o r k load r e s u l t i n g f r o m i n t r o d u c t i o n of the a s s i s t p u m p .

C - 3 CHRONIC TISSUE T E M P E R A T U R E S

The s c o r i n g of t h i s c r i t e r i o n w a s to be b a s e d on the m e a n and p e a k

t e m p e r a t u r e s to w h i c h the t i s s u e s a r e e x p o s e d d u r i n g the n o r m a l , w i t h m -

s p e c i f i c a t i o n o p e r a t i o n of the c a n d i d a t e s y s t e m s

The s c o r e s w e r e to be c o m p u t e d f r o m the C - 3 s c o r i n g c u r v e s a s

fo l lows

S c o r e „ ^ = S c o r e , ^ x F + S c o r e x (1-F ) S y s t e m j p e a k t e m p p e a k m e a n t e m p peak

w h e r e F , i s an e s t i m a t e of t h e f r a c t i o n of the t i m e tha t the e x p o s e d p e a k

s u r f a c e i s a t t he p e a k t e m p e r a t u r e . H o w e v e r , a s i n d i c a t e d m Sec t ion 2 . 4 ,

a l l c a n d i d a t e s y s t e m s w e r e d e s i g n e d to l i m i t t he h e a t f lux to the t i s s u e s

to 0, 07 w / c m and the t e m p e r a t u r e s to 41 C . T h e r e f o r e , t h i s b e c a m e a

" w a s h " c r i t e r i o n m the f ina l s c o r i n g .

C - 4 CHRONIC B L O O D F I L M T E M P E R A T U R E S

T h e m e a n a n d p e a k t e m p e r a t u r e s of any s u r f a c e to w h i c h t h e b lood

IS e x p o s e d d u r i n g n o r m a l , w i t h m - s p e c i f i c a t i o n o p e r a t i o n of the c a n d i d a t e

s y s t e m w e r e to be c a l c u l a t e d m a m a n n e r e x a c t l y a n a l o g o u s to t h a t s p e c i ­

f ied m C - 3 . S ince a l l t h e c a n d i d a t e c o n c e p t s w e r e d e s i g n e d to r e j e c t

a l l the h e a t d i r e c t l y to the a b d o m i n a l t i s s u e s and f lu ids r a t h e r than a b lood

h e a t e x c h a n g e r , the c r i t e r i o n a l s o b e c a m e a " w a s h " . H o w e v e r , a s e p a r a t e

b lood h e a t e x c h a n g e r cou ld r e a d i l y be i n c o r p o r a t e d in to any of the c a n d i d a t e

c o n c e p t s if d e s i r e d .

A-11 , />

C-5 TOTAL VOLUME

The total volume of each of the complete candidate t he rma l conver ­

t e r s y s t e m s (including the blood pump actuator but not the blood pump

itself) was calculated f rom the component engineer ing a n a l y s e s . The C-5

scor ing curve was der ived in consul ta t ion with the medica l advisory group

and re f lec t s the consensus that a volume up to about 1. 0 l i t e r s can be

accommodated without much difficulty within the lower abdomen. The

curve fal ls to the zero at 1. 5 l i t e r s to ref lec t the design groundrule that

th is value be the maximum allowable vo lume.

C-6 SPECIFIC GRAVITY

The overa l l specific gravi ty of the implanted sy s t em was computed

f rom the component engineer ing ana lyses , and the C-6 scor ing curve used

to de r ive a numer i ca l score for each candidate .

A design goal s t rongly r ecommended by our medica l advisory group

is to achieve an overa l l package densi ty c lose to that of body t i s s u e .

Neu t ra l buoyancy will reduce the s t r e s s on the sur rounding body organs

and e l imina te the need for load-bear ing s t ruc tu r a l t i e s . The package would

ideal ly be only loosely t e the red to the skele ta l f r a m e . Surgically, the

concept of f i rm a t tachment to a bone support is poor ly favored. It was

r ecommended that sy s t ems that cannot be reduced to an overa l l specific

gravi ty of l e s s than 2. 0 not be cons ide red .

C-7 MAXIMUM VENTRAL DORSAL DIMENSION

The max imum v e n t r a l - d o r sal d imension of each of the candidate

s y s t e m s was der ived f rom the component engineer ing a n a l y s e s . The shape

of C-7 scor ing curve re f lec ts the surg ica l feasibi l i ty and convenience of

locat ing packages within va r ious p a r t s of the abdominal cavi ty. The o v e r ­

all m a x i m u m of 5 inches r e p r e s e n t s a typical spacing between the abdomina

aor ta and the an te r io r wall of the cavity where a package of c l o s e - t o -

neu t ra l buoyancy might be placed. The 3-inch d imens ion re f lec ts the

n a r r o w e r space avai lable for m o r e dense packages which might have to

be anchored to the pelvic r i m .

C-8 ISOTOPE INVENTORY

Sys tems requ i r ing a sma l l e r amount of radio iso tope a r e obviously

p r e f e r r e d . Many of the consequences of the size of the isotope inventory,

A-12

such a s e n d o g e n o u s h e a t l e v e l s and c o s t p e r uni t a r e spec i f i c a l l y a c c o u n t e d

fo r e l s e w h e r e in the e v a l u a t i o n s c h e m e . Howeve r , the a c t u a l i n v e n t o r y

l e v e l i t s e l f f o r m s a u se fu l o v e r a l l f i g u r e of m e r i t t ha t wi l l g e n e r a l l y t ake

a c c o u n t of s e v e r a l f a c t o r s s u c h a s a s m a l l e r r a d i a t i o n dose to the r e c i p i e n t

and a s m a l l e r e x p o s u r e p e r unit to the g e n e r a l popula t ion , that a r e diff icul t

to q u a n t i t i z e p r e c i s e l y . The s c o r i n g w a s a s s u m e d to v a r y l i n e a r l y frona

t en to z e r o o v e r the 6 0 - w a t t r a n g e .

D-1 E A S E O F SURGICAL I N S T A L L A T I O N

The s h a p e of the c u r v e r e p r e s e n t s the g e n e r a l c o n c e n s u s that t h e r e

IS r e a l l y no t e c h n i c a l b a s i s for l i m i t i n g the t i m e tha t the r e c i p i e n t can

r e m a i n in s u r g e r y s i n c e a c a r d i o p u l m o n a r y b y p a s s is not r e q u i r e d . H o w e v e r ,

t h e r e i s an i n c r e a s i n g r i s k of d e a t h t h a t c a n be c o r r e l a t e d wi th l eng th of

t i m e in s u r g e r y , and t h i s a c c o u n t s l a r g e l y for the dec l in ing s l ope of the

D-1 c u r v e . T h e r e a r e o t h e r f a c t o r s to c o n s i d e r such a s l a b o r and s u r g i c a l

f a c i l i t y c o s t s .

The c a n d i d a t e d e s i g n c o n c e p t s w e r e p r e s e n t e d to m e m b e r s of our

m e d i c a l c o n s u l t i n g g r o u p who e s t i m a t e d the t i m e r e q u i r e d to i n s t a l l e a c h

s y s t e m . T h e r e w e r e i n su f f i c i en t d i f f e r e n c e s a m o n g the c a n d i d a t e c o n c e p t s

to s u g g e s t any s ign i f i can t v a r i a t i o n s in i n s t a l l a t i o n t i m e and t h e r e f o r e t h i s

b e c a m e a " w a s h " c r i t e r i o n .

D - 2 SENSITIVITY T O A M B I E N T P R E S S U R E CHANGES

The o b j e c t i v e of t h i s c r i t e r i o n w a s to d e t e r m i n e the c h a n g e in o u t ­

put p o w e r c o r r e s p o n d i n g to a r e d u c t i o n in the a m b i e n t p r e s s u r e to 523 m m

Hg ( a t m o s p h e r i c p r e s s u r e a t 1 0 , 0 0 0 f ee t o r 31% r e d u c t i o n in the m e a n s e a

l e v e l b a r o m e t r i c p r e s s u r e ) . A 40% l o s s of p o w e r w a s to have b e e n c o n ­

s i d e r e d g r o u n d s for d i s c a r d i n g the c a n d i d a t e . The s e l e c t e d a c t u a t o r c o n ­

c e p t s m a y show a s l i gh t d e c r e a s e in p o w e r wi th d e c r e a s i n g p r e s s u r e ; but

s i n c e a l l c a n d i d a t e c o n c e p t s would h a v e r o u g h l y the s a m e r e s p o n s e , t h i s

b e c a m e a " w a s h " c r i t e r i o n .

D - 3 SENSITIVITY TO M E C H A N I C A L SHOCK

All the c a n d i d a t e c o n c e p t s w e r e d e s i g n e d to be u n s t a l l a b l e due to

m e c h a n i c a l s h o c k . T h e r e f o r e , s i n c e the d u r a t i o n of any p o w e r l o s s would

be s h o r t , e v e n c o m p l e t e (100%) l o s s c a n be t o l e r a t e d . C o n s i d e r a b l e shock

l e v e l s a r e e n c o u n t e r e d w i th in the body in m o s t c o m m o n l i f e s t y l e s . H o w e v e r ,

A-13

the r i s k s of in te r fe rence f rom acce l e ra t ions encountered in e l eva to r s , and

v ibra t ion levels in motor vehic les a r e not cons idered to be significant

h a z a r d s . Since al l the candidate concepts appeared to be comparably

insensi t ive to mechanica l shock, this became a "wash" c r i t e r i on .

D-4 SENSITIVITY TO ELECTROMAGNETIC FIELDS

All the candidate concepts appeared to be insens i t ive to power loss

f rom e lec t romagne t ic radiat ion (or a cor responding inductive field)

encountered c lose to fa i r ly common domenst ic i t e m s such a s power lawn-

mowers and microwave ovens . There fore , this a l so became a "wash"

c r i t e r i o n .

E-1 COMPONENT TECHNOLOGY READINESS

The major components of the candidate s y s t e m s w e r e evaluated

against the s e r i e s of qual i tat ive c r i t e r i a l i s ted below. Relative s c o r e s

were compiled in the m a t r i x fo rmat shown for c r i t e r i o n E - 1 . The sum of

the box s c o r e s were normal ized to a scale of ten .

E-2 ESTIMATED DEVELOPMENT COST

Development cos t s were es t ima ted under the following assumpt ions :

• Phase II of the P rac t i cab i l i t y Evaluation has been successfully completed, including the 5-month t e s t of the Bench Model Con­v e r t e r . (This model , with only minor modification, is a s sumed suitable for ea r ly an imal exper imenta t ion . )

1 The objective of the Development Phase will be to take the technology f rom Bench Model Conver te r s ta tus to a device appropr ia te for l o n g - t e r m animal exper imenta t ion . This phase would extend over a 3-5 year period and include both l o n g - t e r m in v i t ro tes t ing and s h o r t - t e r m animal in vivo tes t ing .

Component development cos ts were e s t ima ted a s follows:

Cost ($M) Component

5. 0 Gas rec ip roca t ing engine 4 . 5 Vapor rec ip roca t ing engine 3 .0 Solid e lec t ro ly te ba t t e ry 2. 5 Rotary vapor engine 1.5 PCCS (e lec t r ica l or gas) 1.5 T h e r m o e l e c t r i c engine (pure or hybrid) 1.0 T h e r m o e l e c t r i c (vapor r ec ip roca t ing control) ' 0 ,5 PCCS (vapor) 0 .5 TESM

- ^ 31^^

S y s t e m - l e v e l d e v e l o p m e n t c o s t s w e r e d e t e r m i n e d f r o m the sum of

the a p p r o p r i a t e c o m p o n e n t - l e v e l d e v e l o p m e n t c o s t s ,

E - 3 DESIGN GROWTH P O T E N T I A L

The p u r p o s e of t h i s c r i t e r i o n w a s to p r o v i d e a m e a n s of tak ing into

a c c o u n t the t e c h n o l o g y s t a t u s and d e s i g n f l ex ib i l i ty of e a c h of the c a n d i d a t e

c o n c e p t s . As for the t e c h n o l o g y s t a t u s , a full 10 po in t s w a s given to the

g a s and v a p o r r e c i p r o c a t i n g c o n c e p t s a l r e a d y u n d e r s tudy by the Na t iona l

H e a r t and Lung I n s t i t u t e . S ince a r o t a r y v a p o r eng ine in t h i s s i ze r a n g e

i s not c u r r e n t l y u n d e r d e v e l o p m e n t , a o n e - p o i n t pena l ty w a s given a l l

s y s t e m s e m p l o y i n g t h i s c o m p o n e n t . T h r e e po in t s w e r e s u b t r a c t e d for

s y s t e m s r e q u i r i n g the u s e of a so l id e l e c t r o l y t e b a t t e r y . F o r the d e s i g n

f l ex ib i l i t y ( i . e . , o p t i o n s a v a i l a b l e for p e r f o r m a n c e i m p r o v e i n e n t ) a full

t en p o i n t s w a s g iven to the h y b r i d wi thou t b a t t e r y b e c a u s e it p r o v i d e d by

f a r the l a r g e s t n u m b e r of d e s i g n op t ions and p o t e n t i a l p e r f o r m a n c e i m p r o v t -

m e n t s . T h e l o w e s t s c o r e of f ive p o i n t s w a s a w a r d e d to the p u r e t h e r m o ­

e l e c t r i c s y s t e m s i n c e p e r f o r m a n c e c h a r a c t e r i s t i c s a r e v e r y wel l c h a r a c t e r ­

i z e d and s ign i f i can t n e a r - t e r m i m p r o v e m e n t s s e e m un l ike ly . I n t e r m e d i a t e

s c o r e s w e r e g iven to s y s t e m s u n d e r d e v e l o p m e n t by NHLI which have

d e m o n s t r a t e d p e r f o r m a n c e c h a r a c t e r i s t i c s tha t have not yet r e a c h e d

p r e d i c t e d l e v e l s .

E - 4 E S T I M A T E D UNIT P R O D U C T I O N COST

As wi th d e v e l o p m e n t c o s t s , p r o d u c t i o n c o s t s in q u a n t i t i e s of

10, OOO/year w e r e e s t i m a t e d a t the c o m p o n e n t l e v e l and then c o m b i n e d a s

a p p r o p r i a t e to d e t e r m i n e s y s t e m - l e v e l c o s t s . A p a c k a g i n g c o s t of $1.0K

w a s added to the c o m p o n e n t - l e v e l c o s t of e a c h s y s t e m . T h e componenv

c o s t s a r e a s shown b e l o w .

A-15

Cost ($K)

2. 1. 1. 1. 1. 0, 0. 0. 0. 0, 0.

0 5 6 0 0

,5 ,5 ,4 ,2 .2 ,1

Component

Gas rec ip roca t ing engine Vapor rec iproca t ing engine Rotary vapor engine T h e r m o e l e c t r i c (pure or hybrid) T h e r m a l conver t e r packaging T h e r m o e l e c t r i c (vapor r ec ip roca t ing control) PCCS (gas) PCCS (elect r ical ) PCCS (vapor) Solid e lec t ro ly te ba t t e ry TESM

Since sy s t em- l eve l cos t s were below $5. OK in case , all sys t ems rece ived

full score , and this became a 'Wash" c r i t e r ion . Development and production

cost e s t i m a t e s for each of the candidates a r e s u m m a r i z e d in Table A-2.

A-16 ^ ^ ^

Table A-2. System Development and Product ion Costs

T h e r m o e l e c t r i c / B a t t e r y

Ro ta ry V a p o r / B a t t e r y

H y b r i d / B a t t e r y

Hybrid

Gas Rec ip roca t ing /TESM

Gas Reciproca t ing

L inea r Vapor /TESM

Linea r Vapor

Development Costs ($) Million

6 . 0

7 . 0

8. 5

5. 5

7 . 0

6 . 5

6 . 5

6 . 0

— - • - - — - -

Product ion Costs ($) Thousand

2 . 6

3. 1

4. 1

3 . 9

3 .6

3. 5

3. 3

3 . 2

A-17