Thermal coupling of a high temperature PEM fuel cell with a complex hydride tank

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 4 5 7 ndash 3 4 6 6

Avai lab le a t wwwsc iencedi rec t com

j ourna l homepage wwwe lsev ier com loca te he

Thermal coupling of a high temperature PEM fuel cell witha complex hydride tank

P Pfeifera C Wallab O Jensenc H Hahnb M Fichtnerb

aInstitute for Micro Process Engineering Forschungszentrum Karlsruhe Hermann-von-Helmholtz Platz 1

76344 Eggenstein-Leopoldshafen GermanybInstitute for Nanotechnology Forschungszentrum Karlsruhe Hermann-von-Helmholtz Platz 1 76344 Eggenstein-Leopoldshafen GermanycTechnical University of Denmark Institute for Chemistry Kemitorvet 2800 Kgs Lyngby Denmark

a r t i c l e i n f o

Article history

Received 20 October 2008

Received in revised form

17 February 2009

Accepted 19 February 2009

Available online 17 March 2009

Keywords

Hydride storage

Tank design

System modelling

HT-PEM fuel cell

Sodium alanate

Cerium catalyst

Thermal coupling

Nanocrystalline alanate

Corresponding author Tel thorn49 (0)7247 82E-mail address peterpfeiferimvtfzkde

0360-3199$ ndash see front matter ordf 2009 Interndoi101016jijhydene200902041

a b s t r a c t

Sodium alanate doped with cerium catalyst has been proven to have fast kinetics for

hydrogen ab- and de-sorption as well as a high gravimetric storage density around 5 wt

The kinetics of hydrogen sorption can be improved by preparing the alanate as nano-

crystalline material However the second decomposition step ie the decomposition of the

hexahydride to sodium hydride and aluminium which refers to 18 wt hydrogen is

supposed to happen above 110 C The discharge of the material is thus limited by the level

of heat supplied to the hydride storage tank Therefore we evaluated the possibilities of

a thermal coupling of a high temperature PEM fuel cell operating at 160ndash200 C The starting

temperatures and temperature hold-times before starting fuel cell operation the heat

transfer characteristics of the hydride storage tanks system temperature fuel cell elec-

trical power (including efficiency) as well as alanate kinetics were varied by system

modelling with gPROMS The kinetics of the hydride decomposition was found to have

a major influence on the performance of the system A cumulative output of 08 kWh was

reached in a test run

ordf 2009 International Association for Hydrogen Energy Published by Elsevier Ltd All rights

reserved

1 Introduction current limited capacity in the manufacturing process makes

Automotive applications based on hydrogen-driven fuel cells

need powerful storage systems for hydrogen Therefore

several options for storing the hydrogen have been investi-

gated in the past Most car manufacturers have decided not to

use liquid hydrogen as a storage medium mostly due to the

inevitable boil-off losses of the cryo liquid and the high energy

cost of hydrogen liquification A second storage option is high

pressure composite tanks of up to 700 bar H2 which are

meanwhile considered as state-of-the-art However there are

concerns in case of rupture in accidents Moreover the

4767 fax thorn49 (0)7247 82(P Pfeifer)ational Association for H

it impossible to determine the lifetime or a safe number of

loading cycles with the necessary accuracy and precision For

a probabilistic approach data would be needed from a high

number of life cycle tests which are also not available [1] In

addition the storage density of current systems based on

pressurized or liquefied hydrogen will probably not reach the

international targets for 2010 and beyond due to the fact that

the physical limits have been more or less reached by these

methods [2] Currently the only option to reach higher

reversible storage densities seems to be the storage of

hydrogen in metal hydrides due to the shorter average HndashH

7767

ydrogen Energy Published by Elsevier Ltd All rights reserved

Hydride Tanks

Preheater

Transfer

HT-PEMPump

H2

Oumll

H2

Oumll

H2

Oumll

Fig 1 ndash Simple schematic of the entire system for the

thermal coupling alanate hydride tanks and an HT-PEM


distances which are possible in these materials and due to the

higher safety of such systems [3]

While binary metal hydrides can be handled safely and the

risk potential is limited complex hydrides may react strongly

with water and self-ignition can occur in such a case

However it has also been shown that the flame rates of

ignited hydrogenndashalanate powder clouds are by more than an

order of magnitude below the ones of pure hydrogen [17] This

leads to flames rather than explosions when material is

ejected into a reactive environment Moreover contact with

hot heat transfer oil does not lead to any dangerous situations

rather to safe encapsulation of the material

Hence nanocomposites based on a complex hydride and

transition metal dopant are one of the favourable options as

hydrogen storage material due to the safety properties the

low operation pressure (compared to pressurized tanks) and

the high gravimetric hydrogen content A number of systems

have been identified with hydrogen contents gt3 wt and

thermodynamic properties which would be suitable for on-

board hydrogen storage systems in combination with a fuel

cell [4ndash7] Sodium alanate (NaAlH4) doped with cerium catalyst

has been proven to have fast kinetics for hydrogen ab- and de-

sorption as well as a high gravimetric storage density of

around 5 wt [8] The first two hydrogen exchange reactions

of the compound (1) and (2) are reversible under moderate

temperatures and pressures The third step (3) occurring at

temperatures higher than 400 C is not considered for prac-

tical purposes

3NaAlH4 Na3AlH6thorn 2Althorn 3H2 (1)

Na3AlH6 3NaHthornAlthorn 32H2 (2)

3NaH 3Nathorn 32H2 (3)

However the second decomposition step (2) which refers

to 18 wt hydrogen occurs above 110 C Therefore the

discharge of the material is limited by the level of heat

supplied to the tank

As a consequence the working temperatures and the off-

gas temperature of a low temperature proton exchange

membrane fuel cell (LT-PEMFC) 85ndash90 C are not sufficient to

heat a storage based on complex hydrides to values high

enough for fast hydrogen desorption Moreover it generally

seems as if working temperatures in the range of 100ndash200 C

are needed in order to access the full potential of a complex

hydride storage system

Such temperatures can be provided by the so-called high

temperature PEM fuel cells (HT-PEMFC) [910] Besides the

beneficial operation temperature for a complex hydride tank

there are several compelling technological and commercial

reasons for operating H2air PEM fuel cells at temperatures

above 100 C rates of electrochemical kinetics are enhanced

water management and cooling are simplified useful waste

heat can be recovered and lower quality reformed hydrogen

may be used as the fuel Moreover using the waste heat of

a fuel cell for desorption of hydrogen from a hydride may

increase the overall efficiency of the system considerably in

case the heat of absorption is recovered at the filling station

In this study we evaluate the possibility of a thermal

coupling between a high temperature PEM fuel cell operating

at 160ndash200 C and the alanate hydrogen storage material to

overcome the obstacle that additional heat at elevated

temperature has to be generated to completely discharge the

storage tank The heat supplied by the HT-PEM was assumed

to be transferred to the storage tanks by an oil cycle For

evaluation we used the commercial software package

gPROMS The storage system in the simulation consisted of 4

parallel tanks with approximately 2 kg of sodium alanate

coupled with a 1 kW H2-consumption fuel cell a pre-heater

system and a pump

The heat transfer characteristics were investigated by

changing the geometries of the tanks changing the starting

up procedures and evaluating the electrical power output

from the system Stable working conditions were identified

and the sensitivity of the process to various design parameters

was determined

2 System

An overall scheme of the simulated system (Fig 1) refers to

components which have already been fabricated for an

experimental verification of the obtained data

The main components which have been set up in

gPROMS as different models with interconnection via

a global model to each other are the fuel cell the oil cycle for

heat transfer between the fuel cell and hydride tanks four

hydride tanks in series a pump and a pre-heater system

The pre-heater system includes an electrical heat source

for a second oil cycle where heat transfer is done via a micro

heat exchanger This allows for the fast stop or start of the

00010203040506070809

0 50 100 150 200Current A

Vo

ltag

e p

er cell V

0

100

200

300

400

500

600

700

Po

we

r (S

ta

ck

) [W

]

200degC170degC

150degC

150degC

170degC200degC

Fig 2 ndash Temperature dependent polarisation curves of the

HT-PEM for the simulation (measured at DTU) open

symbols (thick lines) refer to electrical power closed

symbols (thin lines) refer to voltage

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 4 5 7 ndash 3 4 6 6 3459

heat supply by the stop or start of the flow as well as using the

secondary oil cycle as a heat sink during the constant opera-

tion of the whole system at the maximum fuel cell tempera-

ture Direct electric heating would not allow for rapid stopping

of the heating since the heat capacity of the heating elements

would be much higher and electric heating would require

a certain flux of oil in the main cycle to prevent oil degrada-

tion However the main flow should be adjusted to fuel cell

demand and to allow a sufficient heat flow for desorption of

hydrogen in the tank and should be optimised to yield the

highest overall system efficiency Runaway of the whole

system can practically not be prevented without the stop of

hydrogen flow in direct electric heating In the simulation the

behaviour of the pre-heater system was however treated as

ideal The heat was assumed to be transferred without heat

losses ie electrical power was assumed to be identical to the

heat needed in the main oil cycle

3 Components

31 Fuel cell

The HT-PEM fuel cell fabricated by the Technical University of

Denmark (DTU) has a total hydrogen consumption accounting

for 1 kW chemical energy (which is approx 400 W electrical

power) and is equipped with a polybenzimidazole (PBI)

membrane doped with phosphoric acid

The precise composition of the membrane is poly-220-m-

(phenyl) 550-bi-benzimidazole and details about the doping

and the manufacture of the membrane can be found else-

where [1112] The advantages of the membrane material are

that it doesnrsquot have to be humidified and that it provides

increasing electric conductivity up to 200 C (max value

around 007 Scm) which is in the temperature range for the

fast hydrogen release reactions (1) and (2)

The considered stack consists of 10 cells two cooling

plates and two end plates The membrane size is 16 16 cm2

The bipolar plates are made from expanded graphite The

cooling plates are made from 10 mm thick aluminium sheets

and are equipped with 28 cooling channels of 6 3 mm2 cross

section area and 150 mm length This information is neces-

sary for the determination of the heat removal and pressure

drop in the fuel cell For reducing heat losses to the

surroundings the fuel cell is wrapped with a 10 cm thick layer

of mineral wool possessing a heat conductivity of 006 W(mK)

and a heat capacity of 840 J(kgK) at 200 kgm3 density

Fig 2 shows the polarisation curves which have been

applied for calculation of efficiency and electrical power output

in the simulation The desired point of operation is approxi-

mately 200 C and 40 efficiency ie yielding 5 V and 85 A The

major trend from the polarisation curves is that at tempera-

tures lower than 170 C the electrical output is low and

hydrogen conversion will predominantly yield heat However

this circumstance will lead to faster heating-up of the whole

system Due to the fact that the produced water will condense

at around 100 C ndash which might lead to changes in the phos-

phoric acid concentration and thus to irreversible damage of

the membrane ndash the simulation takes into account pre-heating

up to 120 C before hydrogen is released to the fuel cell

32 Alanate (hydride) tanks

Fig 3 shows a sketch and a photo of the hydride tanks with

some of the major dimensions The inner shaded region of the

sketch is filled with alanate and the coloured region is the

surrounding void for oil Each tank has been designed for 500 g

of alanate material

The weight of the tank fabricated at TU HamburgndashHar-

burg was 20 kg with a heat capacity of 500 J(kgK) For oper-

ation the tank is wrapped with a 10 cm thick layer of mineral

wool The properties of the wool are the same as for the

insulation of the fuel cell

33 Pump

The pump a Sterling SIHI ZTK 32-160 which will be applied in

the lab system is by far oversized but is the best compromise

so far as can be seen from the simulations later the

maximum throughput of 3 m3h and the sizeweight is too

high but it provides features such as thermal decoupling of

the motor and paddle wheel up to 3 bar pressure and oper-

ation at high oil temperatures

It will be operated in the lab under bypass operation to

lower the energy demand and introduction of massive fric-

tional heat into the oil From point of operation of the system ndash

ignoring the influence of the pump ndash the temperature after the

pump will be adjusted to the pump inlet temperature by the

pre-heater system (either heating or cooling depending on

the balance of frictional heat and heat losses) Thus the

insulation might be minimal in the lab setup For the simu-

lation a much simpler approach of heat losses by a linear fit

has been performed (see below)

34 Pre-heater system

The pre-heater system which is necessary to heat the fuel

cell and thus the overall system (including the oil) to at least

120 C consists of a standard thermostat from Huber with

a total heat production of up to 3 kW The heat transfer to

the main oil cycle is accomplished in a cross flow micro

114220

450

50

Fig 3 ndash Sketch of the hydride tank construction (top) and photo of the tank (bottom)


heat exchanger from the IMVT of the Forschungszentrum

Karlsruhe a photo and the internal structure are shown in

Fig 4

For the calculation of the pressure drop the following data

are relevant 55 passages (either duplet or single foils) for each

oil cycle 217 mm hydraulic diameter (experimentally deter-

mined) 35 mm channel length and 8 channels per foil doublet

(one passage) for the main oil cycle The data for the

secondary oil cycle are not relevant for the simulation since

ideal heat transfer and no heat losses are assumed (see

section System)

35 Main oil cycle

Three meters of tubing in total have been installed between

the different elements of the system An inner diameter of

Fig 4 ndash Photo and inner structure (stacking scheme) of the micro

and secondary oil cycle

12 mm has been initially calculated to be sufficiently large

enough to yield only a low pressure drop at a 1 m3h oil flux

For insulation purposes a mineral wool is again utilized The

total diameter of the tubing including the wool was assumed

to be 10 cm

The oil in the main cycle is Therminol 59 which has

a relatively low viscosity at room temperature (approx

7 mPas) but must be operated in a closed loop with a thermal

expansion cylinder since the fire point is 154 C For the

simulation a mean oil density of 878 kgm3 has been used in

the desired temperature region Other parameters for the

simulation such as viscosity have been calculated according

to fitted curves So for example the lower limit of viscosity is

048 mPas at 200 C heat capacity ranges from 1680 to 2270 J

(kgK) and heat conductivity ranges from 0121 to 0104 W

(mK) in the desired range of 20ndash200 C respectively

heat exchanger applied for heat transfer between the main


4 Simulation

The aim of the simulation was to identify the main parame-

ters which influence the efficiency and start-up of the system

For the latter four steps of operation had to be distinguished

in the timeline by IF conditions in the simulation during start-

up (see also Fig 5)

I Pre-heating (electrically) to 120 C to obtain the

minimum temperature level of FC operation

II Hold of temperature (electrically) to obtain a minimum

hydrogen pressure of 12 bar(abs) in the hydride tanks

III Fuel cell operation while increasing the system

temperature with waste heat from the fuel cell (pre-

heater is off)

IV Constant fuel cell operation at 200 C with the removal

of excess heat by the pre-heater system

The necessary main equations with respect to the different

sub-models applied in gPROMS for the system components

are given in the following subsections

41 Fuel cell

Main equations for the fuel cell deal with the heat balance in

the fuel cell The change of internal energy U can be written as

a function of reaction enthalpy leading to heat generation

(according to fits and interpolations of polarisation curves)

and removed heat

dUdtfrac14 eth1 hTHORNDHR _Q losses _Qoil (4)

with t time h electrical efficiency of the fuel cell DHR reaction

heat of hydrogen combustion _Q losses heat losses to environ-

ment by natural convection and effluent gases and _Qoil heat

transfer to the oil cycle

This change on the other hand refers to the mean

temperature of the fuel cell according to

dUdtfrac14 mFCcpFC

dTFC

dt(5)

Fig 5 ndash Different modes of operation during the start-up of

the whole system described in terms of the oil temperature

in the main cycle against time

with m mass cp heat capacity and T temperature of the fuel

cell When assuming near ideal heat transfer which should be

possible with the chosen cooling structure the oil should

leave the fuel cell at the mean fuel cell temperature Thus the

heat transfer to the oil is

_Qoil frac14 cpoil _moil

Toilin TFC

and Toilout frac14 TFC (6)

with cp heat capacity _moil mass flux in the oil cycle as well as

Toilin and Toilout temperature of the oil entering or leaving the

fuel cell The heat loss parameter _Q losses summarises the los-

ses by natural convection on the fuel cell surface and those

which occur due to gases entering and leaving the fuel cell at

fuel cell temperatures above the environmental temperature

Tu Equations for losses by natural convection are standard for

cube like devices (using Raleigh Graszlighof and Nusselt

numbers) and are not explicitly presented here The applied

enthalpy streams of the inlet and outlet gases take into

account a conversion of hydrogen with air with three-fold

oxygen excess and full conversion without condensation in

the produced steamndashair mixture (ie only cathode off-gas

anode side operated dead-end)

The pressure drop calculation in the fuel cell takes into

account the temperature dependent oil properties and

includes an IF condition for the distinction between laminar

and turbulent flow The equations used are for flow in tubes

with a corresponding hydrodynamic diameter and a correc-

tion value 4 of 096 at a height to width ratio of 05 in the

cooling channels of the fuel cell

42 H storage tanks

A precise simulation of the alanate tanks must be performed

according to the design three dimensionally We reduced the

problem to a two dimensional one ie in axial (z) and radial (r)

directions by just simulating the cylindrical part Linked

parameters for the heat distribution in this part are the heat

flux to the outer insulation _qins to the tank (considered only in

terms of heat capacity of stainless steel cpstainlessmstainless) and

to the alanate material _qalanate due to simultaneous cooling of

the oil The heat flux can be written as length specific ie in

units of Wm according to

_qinsethzTHORN frac14 kcorraoilethzTHORNpdouteroil

ToilethzTHORN Tins

zRinnerins

and (7)

_qalanateethzTHORN frac14 aoilethzTHORNpdinneroilfrac12ToilethzTHORN TalanateethzRouteralanateTHORN (8)

and the time dependent change of internal energy of Vdiscr

a cylindrical volume element of the stainless steel of the tank

including the oil according to

cpstainlessmstainless

ndiscrzthorn cpoilroilVdiscr

dToilethzTHORN

dtfrac14 _Hin _Hout (9)

with kcorr correction factor for non-ideal cylindrical shape of

the tank aoilethzTHORN length specific heat transfer coefficients

douteroil and dinneroil outer and inner diameter of the annular oil

cross section Routeralanate and Rinnerins the outer radius of the

alanate material and the inner radius of the insulation ndiscrz

number of volume elements roil density of the oil as well as_Hin and _Hout the enthalpy streams in and out of the volume

element


This calculation assumes an equal distribution of the

weight of stainless steel fast temperature equilibration in the

steel part (no temperature gradient in the metal due to high

heat conductivity compared to insulation and alanate) and

equal heat losses over the hydride tank by the number of

discrete elements in z direction ndiscrz To reduce the error

through the latter hypothesis we introduced a correction

factor in equation (7) which has been estimated as the ratio of

total surface area of the tank to outer surface of the cylindrical

part of the tank

The right side of equation (9) has to be calculated by the

change of oil temperature due to flow in the z-direction and

the heat flux from equations (7) and (8)

_Hin frac14 _moilcpoildToilethzTHORN

dz(10)

_Hout frac14

_qins thorn _qalanate

Lcylinder

ndiscrz(11)

with Lcylinder total length of the cylindrical part of the storage

tank

To calculate the heat flux in equations (7) and (8) the

necessary parameters are the convective heat transfer coef-

ficient aoil in the annular gap and the boundary values of

temperature in the insulation and the alanate The aoil value

has been determined according to standard equations of

annular flow with an IF condition for laminar or turbulent flow

distinction The boundary temperatures can only be deter-

mined by consistency of heat flux through the insulation and

to the centre of the alanate

On the side of the insulation the following equations have

been used for establishing energy conservation

Heat transfer to insulation lins

vTins

z r frac14 Rinnerins

vr

frac14 aoil

ToilethzTHORN Tins


(12)

Heat conduction in the insulation rinscpinsvTinsethz rTHORN

vt

frac14

1r

v

vr

rl

vTinsethz rTHORNvr

(13)

Heat transfer to air lins

vTins

z r frac14 Routerins

vr

frac14 aair

Tins


Tu

(14)

Equation (13) only considers radial heat conduction since

the heat conduction coefficient is much lower than the heat

transport by the oil in axial direction Free convection was

calculated by standard equations for cylindrical parts similar

to the case of the fuel cell for the heat removal from the outer

wall of the insulation

Conservation of the heat flux to the inner part of the ala-

nate is more complex since this is overlaid by reaction heat

and heat conduction according to loading The heat flux to

from reaction is again dependent on the decomposition

reaction kinetics and moreover this is related to the pressure

in the reactor which is also linked with the consumption from

the fuel cell The detailed description is as follows

The boundary equation is similar to that of the insulation

except for the fact that the heat conduction coefficient is

position dependent

lalanateethz r frac14 RouteralanateTHORNvTalanateethz r frac14 RouteralanateTHORN

vrfrac14 aoilethToilethzTHORN Talanateethz r frac14 RouteralanateTHORNTHORN (15)

with lalanate heat conductivity of the alanate

In the centre of the tank symmetry must be fulfilled

vTalanateethz r frac14 0THORNvr

frac14 0 (16)

For the temperature distribution in the alanate the change

of internal energy has to be applied with a sinksource term

according to

ralanatecpalanatevTalanateethzrTHORN

vtfrac14

1r

v

vr

rlalanateethzrTHORN

vTalanateethzrTHORNvr

thorn v

vz

lalanateethzrTHORN

vTalanateethzrTHORNvz

thorndcH2ethzrTHORN

dTralanateDHR (17)

Here we neglected the heat transport by convection of the

released hydrogen since it was proven in literature [13] that

the error is less than 1

The heat conduction coefficient which has been applied in

the (zr)-position was calculated upon approximation of data

from Dedrick et al [14]

lNaAlH4 frac14 0037 ln

pH2

thorn 051 (18)

lNa3AlH6frac14 0061 ln

pH2

thorn 050 (19)

lNaH=Al frac14 0068 ln

pH2

thorn 071 (20)

with averaging between the different phases by the molar

fraction x by equation (21)

lalanate frac14 xNaAlH4 lNaAlH4 thorn xNa3AlH6lNa3AlH6

thorn xNaH=AllNaH=Al (21)

The molar fractions can be calculated from the actual

concentration of hydrogen at the (xr)-position

Kinetic equations for the determination of the hydrogen

release (change of hydrogen content in wt of the solid phase

wt H) in dependence of the actual pressure pappl and the

equilibrium pressure of the different phases peq have been

taken from another publication [15]

NaAlH4 formation dethwt HTHORN frac14 625e8 exp

616 kJ

RT

ln

pappl

peq1

eth39wt HTHORN2 (22)

NaAlH4 decomposition dethwt HTHORN

frac14 19e11 exp

83 kJ

RT

ln

peq1

pappl

ethwt H 167THORN (23)


Na3AlH6 formation dethwtHTHORNfrac14102e8exp

562kJ

RT

ln

pappl

p

eq2

eth167wtHTHORN (24)

Na3AlH6 decomposition dethwt HTHORN

frac14 29e10 exp

93 kJ

RT

ln

peq2

pappl

ethwt HTHORN (25)

By an IF condition these different states and the corre-

sponding reaction heat either 367 kJmol per hydrogen

molecule for equation (1) or 466 kJmol per hydrogen mole-

cule for equation (2) have been distinguished at the (zr)-

position Since the temperature gradients in the alanate were

assumed the reaction heat has been set negative or positive

according to adsorption or desorption at the local (zr)-

position

Last but not least the equilibrium and applied pressure

have to be calculated The latter can be determined from the

mass of hydrogen in the gas void mH2 gas

pstorage frac14mH2 gasRsT

Vgas(26)

mH2 gasfrac14ralanateVstorage

100

0BcH2solid

ethtfrac140THORN 1RstorageLstorage

Z t

0

Z Lstorage

0

Z Rstorage

0

cH2solid

dtdzdr

1CAZ t

0

_mH2 gasout (27)

Vgas frac14 Vstorage eth1 3THORNValanate (28)

where the mean temperature T is the integral overall

temperatures in the alanate Vgas and Vstorage are the free

volume of gas between the alanate particles and of gas and

particles respectively and _mH2 gasout is the mass flux of

hydrogen to the fuel cell

The equilibrium pressure has been calculated by Vanrsquot

Hoffrsquos law for the different steps in the decomposition

43 Pump

Since the existing pump in the setup is not the one which

would be used in a real system we decided to simulate this

part as nearly ideal The efficiency was set to a fixed value of

40 leading to the following equations for electrical demand

Pel and heat introduction in the oil _Qel by friction

Pel frac14poil

_Voil

04and _Qel frac14

poil_Voil

1 04(29)

with p and _V being the pressure and the volume flow of oil

respectively

The heat losses of the fuel cell have been calculated

according to size estimation as approximately 40 W at 100 C

This heat loss was assumed to vary linearly with the differ-

ence to environmental temperature (20 C)

The heat capacity of the pump was estimated to be that of

grey iron with a weight of 50 kg (according to the existing

pump) The heat transfer to heat up the pump was treated

ideally ie the oil leaves the pump at the pump temperature

It was possible to individually neglect heat introduction

heat capacity and heat loss by the use of a SWITCH function

(onoff) since the estimations are quite rough

44 Pre-heater

The pre-heater was managed by IF conditions In the initial

phase 3 kW heat were introduced At phase II the heating

power was calculated from the temperature difference of the

oil and the desired oil temperature using the heat capacity

flux During phase III it was zero and in phase IV it was zero

assuming heat removal by an additional heat sink The pres-

sure drop in the heat exchanger was calculated in the same

way as for the fuel cell using different parameters

45 Main oil cycle

Since the heat losses are time dependent and the heat

capacity of the tube and the insulation will contribute to the

start-up properties of the system equations (7)(9)(10)ndash(14)

including standard equations for free convection have been

adapted to the tubing Pressure drop was also calculated with

standard equations

5 Results

Simulation tests were performed for validation with the main

part of the simulation ie the tank itself A small tank with

a 1 cm diameter and corresponding hydrogen release data

according to literature [15] have been compared with the

simulation of the tank and there was quite good accordance

between both eg the time between hydrogen release from

equations (1) and (2) was several 100 s

After this initial validation different parameters have been

evaluated with respect to the operation of the system and its

efficiency The most interesting variations are presented in

the following subsections Conditions for the pump simula-

tion are given for each case in the figure captions

51 Starting temperature

In principle the heating up of the system to 120 C should be

enough for starting fuel cell operation combined with

hydrogen release due to the first decomposition step of

NaAlH4 However the pressure in the hydride tank could still

be insufficient after reaching 120 C which may partly be due

to the slow decomposition kinetics That is why additionally

phase II a hold of system temperature was introduced into

the simulation to obtain 12 bar(abs) hydrogen pressure in the

tank This hold-time however took approximately 1200 s at

120 C according to literature kinetics [15] Alternatively

a higher starting temperature and faster decomposition

kinetics could shorten this time demand considerably

Fig 6 shows the operation time of the fuel cell obtained as

well as heating-up time in terms of the starting temperature

The theoretical limit of 7726 s of fuel cell operation which is

given by the hydrogen amount stored in the set of 4 tanks

0100020003000400050006000700080009000

140 150 160 170 180 190 200 210Starting Temperature [degC]

Ma

x F

C O

pe

ra

tio

n

He

at-u

p T

im

e [s

]

Theoretical FC Limit

Heat-up Time

Operation FC

Fig 6 ndash Simulation results for system operation time and

heating-up time with respect to starting temperature (ie

where fuel cell operation starts) pump conditions heat

dissipation heat loss and heat capacity consideredFig 7 ndash Axial and radial temperature distribution in the

50 mm diameter hydride tank (fully charged) pump

conditions heat dissipation heat loss and heat capacity

considered

Fig 8 ndash Hydrogen content in the alanate in radial direction

versus time (time [ 0 corresponds to start of heating up)

pump conditions heat dissipation heat loss and heat

capacity considered


(assumed reversible storage capacity of 39 wt) can only be

reached with at least 170 C pre-heating At lower tempera-

tures the second decomposition step ie equation (2) is too

slow to supply enough hydrogen for fully discharging the

tank ndash although the system temperature increases when the

fuel cell is in operation

The hold-time of phase II reduces almost to zero at

temperatures around 180 C The heating-up time increases

between 120 C and 200 C starting temperatures by approxi-

mately 1200 s which shows that it is rather impossible to

reduce the starting-time by increasing the starting tempera-

ture However it has a strong effect on the operation time of

the system

The conditions for the pump mainly affect the operation

time (not shown in Fig 6) by frictional heat generation ie

the operation time gets longer due to energy dissipation

and on heating-up time due to the heat capacity of the

pump Neglecting the heat capacity reduces the heating-up

time by a factor of 2 ie at a lower overall system weight

one could envisage a better effect of increasing the starting

temperature The hold-time will not vary with system

weight since it is hydride dependent but heating-up time

will considerably decrease so that the difference in time

gets enlarged

52 Variation of tank geometry

During simulation one of the obvious changes which should

be considered for improvement of the system is the tank

geometry Fig 7 shows the temperature gradient in the stan-

dard (50 mm diameter) tank when reaching 120 C during

heating-up

From Fig 7 it is clear that the hydrogen desorption is

limited by the radial temperature gradient before and after

reaching the starting temperature The axial gradient is

negligible due to a high heat flux in the oil and relatively low

heat transfer in the present system The temperature limited

hydrogen desorption can be validated by plotting the

hydrogen content of the material against time (Fig 8)

When heating starts (timefrac14 0) the concentration is

everywhere the same Then the outer regions of the tank get

hot and hydrogen desorbs so that the colder inner regions of

the tank absorb hydrogen This is however only feasible if the

starting condition is equilibrated hydrogen pressure

(hydrogen in the gas phase)

In principle three different approaches can increase

the mean tank temperature or improve the temperature

gradient

First a reduction of the size of the annular gap would

increase the heat transfer coefficient to the material Under

current conditions (32 mm gap size) the convective heat

coefficient aoil is only 60 Wm2K whereas at a 5 mm gap it

would be 1000 Wm2K This change would result at 120 C

starting temperature (phase II) in a decrease of time for

reaching a 100 C mean tank temperature from 3500 s to 1300 s

or an increase in mean tank temperature from 77 C to 95 C at

-400

-300

-200

-100

0

100

200

300

2000

4000

6000

8000

1200

014

000

1600

018

000

2000

0Zeit [s]

Cu

mu

lative el P

ow

er [W

h]

Time [s]

250 W total

500 W total750 W total

1 kW total125 kW total

1000

0

Fig 10 ndash Cumulative electrical power output for the system

with different fuel cell total power against time pump


not considered system pre-heating to 120 8C hold-time

800 s


1200 s from the start However this improvement has no

effect on the temperature gradient in the alanate but will

increase the pressure drop in the system and decrease the

system efficiency

The second approach would increase mass flux in the main

oil cycle The influence is different to the above approach

since the gap size reduction leads to higher heat transfer

under laminar flow whereas the increase in oil flux leads to

turbulent gap conditions at a flow rate above 1 m3h in the

tank This means that by an increase of flux a comparable

improvement to a size reduction of the annular gap is

possible However the pressure drop in the system ndash espe-

cially in the cold start phase with highly viscous oil ndash would

lower the systemrsquos efficiency

The third approach would be the size reduction of the inner

tube (together with an outer tube size reduction) Here we

obtained the best conditions since this also reduces the

temperature gradient in the alanate itself Reducing the

diameter from 50 mm to 20 mm led to a decrease in maximum

gradient from 19 K to 7 K after 1200 s from start

Another option which we havenrsquot considered is the

mixing or application of high heat conductive material to

the alanate (see also [16]) However the contribution to the

size of the tank and the material interactions cannot be

neglected

53 System temperature

Under fuel cell operating conditions (after reaching phase III)

it is possible to compare the different system temperatures in

terms of efficiency and self-sustaining operation Fig 9 shows

the cumulative heat (produced by the fuel cell and heat losses)

and electrical power (produced by the fuel cell and needed for

pumping)

The optimum conditions in terms of efficiency would be

200 C but when considering an average heat for hydrogen

desorption of 40 kJmol the best operation point is 185 C

since enough heat has to be produced to release the hydrogen

However a self-heating of up to 200 C is still possible in

special cases ie when the heat dissipation of the pump

prevails over heat losses and the first desorption step is in

progress

0

100

200

300

400

500

600

120 140 160 180 200 220Temperature [degC]

El P

ow

er H

eat [W

]

El Power (PFC-PPump)

Heat (ΘFC-Θloss)

Heat for Desorption

Fig 9 ndash Heat and power balance for the system with system

temperature total oil flux in the main cycle 1 m3h pump


not considered

54 FC total power

The fuel cell total power (heat and electricity) can have an

influence on the overall efficiency when looking into the

contributions to the heat and power balance in Fig 9 There-

fore it is obvious to check the fuel cell total power influence

on the overall system efficiency In Fig 10 we varied the total

power between 250 W and 125 kW with pre-heating to 120 C

and 800 s hold-time It can be seen that the operation time

increases since there is lower demand for hydrogen from the

fuel cell and the hydrogen pressure can be maintained long

enough to almost total discharge of the alanate The higher

the fuel cell total power the higher is the remaining hydrogen

content in the alanate at the end due to pressure break-down

On the other hand the lower the total power of the fuel cell

the more energy is consumed by the pumping and pre-heat-

ing At 250 W the overall energy balance is negative

1 kW total power seems to be a good choice because the

bigger the fuel cell the more heat will be needed during

heating up This effect has not been considered in this simu-

lation however a main contribution to the weight of smaller

fuel cell is the end plates They are comparatively heavy since

a pressure resistant housing with leakndashtight cells is necessary

-2-1012345678

020

0040

0060

0080

0010

000

1200

0

Time [s]

Pre

ss

ure

T

an

k [b

ar]

-02-0100102030405060708 C

um

ula

tiv

e e

l P

ow

er [k

Wh

]

Hold 800s

Pre-heatPressure

Power

Fig 11 ndash Cumulative electrical power output and tank

pressure for the system with 1 kW fuel cell total power

against time for adapted kinetics pump conditions heat

dissipation heat loss and heat capacity not considered

system pre-heating to 120 8C hold-time 800 s


55 Alanate kinetics

Although well described the kinetics used in the simulation is

sluggish when compared to more recent systems [8] Slow

kinetics is not beneficial for the systemrsquos efficiency due to long

hold-times (phase II) or high pre-heating temperatures (phase

I) Therefore we tried to adapt the kinetic equations for the

state-of-the-art material which was produced on the basis of

a Ce dopant Changing the pre-exponential factor in equations

(23) and (25) to 095e11 and 152e11 was only partially

successful Thereby we were able to describe the slope but not

the latency region between the first and second decomposi-

tion steps We decided to introduce a small imaginary step of

loading in between the different decomposition steps to avoid

the time delay between the decomposition steps which is

a result from the different term in the literature kinetics The

simulation analogous to Fig 10 is presented in Fig 11 using the

same conditions with a 1 kW total power fuel cell

It can be seen that faster kinetics has a tremendous effect

on the system performance (much higher overall system

output) and that the total required pre-heating and hold-time

are much lower The alanate can be fully discharged at the

lowest pre-heating temperature and at low hold-times

6 Conclusions

An overall system description for a heat coupled high

temperature PEM fuel cell and an alanate hydrogen storage

tank has been performed by the use of the software package

gPROMS The starting temperatures ie the pre-heating and

temperature hold-times before starting fuel cell operation

were found to have a considerable influence on operation time

due to the possible break-down of hydrogen pressure in the

tank The heat transfer characteristics were investigated by

changing geometries of the tanks and further improvement of

the tanks is envisaged for the experimental validation of the

simulation An optimum system temperature of 185 C and

a fuel cell total power of 1 kW were found to fit to a 2 kg ala-

nate tank with respect to efficiency considerations A varia-

tion of alanate decomposition kinetics exhibited superior

performance for state-of-the-art material on the overall

system efficiency Then full alanate discharging was possible

at the minimum FC operation temperature (120 C) and

a cumulative output of 08 kWh was obtained

r e f e r e n c e s

[1] Mair G Final dissemination event of the integrated projectStorHy httpwwwstorhynetfinaleventpdfWS2_PA_BAM-Mairpdf June 3ndash4 2008 ParisFrance

[2] Satyapal S Petrovic J Read C Thomas G Ordaz G The USDepartment of Energyrsquos National Hydrogen Storage Projectprogress towards meeting hydrogen-powered vehiclerequirements Catal Today 2007120246ndash56

[3] Fichtner M Preface to the viewpoint set nanoscale materialsfor hydrogen storage Scripta Mater 200756801ndash2

[4] Bogdanovic B Schwickardi M Ti-doped alkali metalaluminium hydrides as potential novel reversible hydrogenstorage materials J Alloys Compd 1997253-2541ndash13

[5] Chen P Xiong Zh Wu G Liu Y Hu J Luo W MetalndashNndashH systemsfor the hydrogen storage Scripta Mater 200756817ndash22

[6] Fichtner M Nanotechnological aspects in materials forhydrogen storage Adv Eng Mater 20056443ndash55

[7] Vajo JJ Skeith SL Mertens F Reversible storage of hydrogenin destabilized LiBH4 J Phys Chem B 20051093719ndash22

[8] Bogdanovic B Felderhoff M Pommerin A Schuth FSpielkamp N Advanced hydrogen-storage materials basedon Sc- Ce- and Pr-doped NaAlH4 Adv Mater 2006181198ndash201

[9] Zhang J Xie Zh Zhang J Tang Y Songa Ch Navessin T et alHigh temperature PEM fuel cells J Power Sources 2006160872ndash91

[10] Jensen JO Li Q He R Pan C Bjerrum NJ 100ndash200 C polymerfuel cells for use with NaAlH4 J Alloys Compd 2005404ndash406653ndash6

[11] Li Q He R Jensen JO Bjerrum NJ PBI-based polymer membranesfor high temperature fuel cells ndash preparation characterizationand fuel cell demonstration Fuel Cells 20044147

[12] He R Li Q Jensen JO Bjerrum NJ Doping phosphoric acid inpolybenzimidazole membranes for high temperature protonexchange membrane fuel cells J Polym Sci A 2007452989ndash97

[13] Jemni A Nasrallah SB Study of two-dimensional heat andmass transfer during absorption in a metal-hydrogenreactor Int J Hydrogen Energy 19952043ndash52

[14] Dedrick DE Kanouff MP Replogle BC Gross KJ Thermalproperties characterization of sodium alanates J AlloysCompd 2004389299ndash305

[15] Luo W Gross KJ A kinetics model of hydrogen absorptionand desorption in Ti-doped NaAlH4 J Alloys Compd 2004385224ndash31

[16] Kim K Montoya B Razani A Lee KH Metal hydride compactsof improved thermal conductivity Int J Hydrogen Energy200126609ndash13

[17] W Lohstroh M Fichtner W Breitung Complex hydridesas storage materials first safety tests Int J Hydrogen Energyin press doi101016jijhydene200901030

Hydride Tanks

Preheater

Transfer

HT-PEMPump

H2

Oumll

H2

Oumll

H2

Oumll

Fig 1 ndash Simple schematic of the entire system for the

thermal coupling alanate hydride tanks and an HT-PEM


distances which are possible in these materials and due to the

higher safety of such systems [3]

While binary metal hydrides can be handled safely and the

risk potential is limited complex hydrides may react strongly

with water and self-ignition can occur in such a case

However it has also been shown that the flame rates of

ignited hydrogenndashalanate powder clouds are by more than an

order of magnitude below the ones of pure hydrogen [17] This

leads to flames rather than explosions when material is

ejected into a reactive environment Moreover contact with

hot heat transfer oil does not lead to any dangerous situations

rather to safe encapsulation of the material

Hence nanocomposites based on a complex hydride and

transition metal dopant are one of the favourable options as

hydrogen storage material due to the safety properties the

low operation pressure (compared to pressurized tanks) and

the high gravimetric hydrogen content A number of systems

have been identified with hydrogen contents gt3 wt and

thermodynamic properties which would be suitable for on-

board hydrogen storage systems in combination with a fuel

cell [4ndash7] Sodium alanate (NaAlH4) doped with cerium catalyst

has been proven to have fast kinetics for hydrogen ab- and de-

sorption as well as a high gravimetric storage density of

around 5 wt [8] The first two hydrogen exchange reactions

of the compound (1) and (2) are reversible under moderate

temperatures and pressures The third step (3) occurring at

temperatures higher than 400 C is not considered for prac-

tical purposes

3NaAlH4 Na3AlH6thorn 2Althorn 3H2 (1)

Na3AlH6 3NaHthornAlthorn 32H2 (2)

3NaH 3Nathorn 32H2 (3)

However the second decomposition step (2) which refers

to 18 wt hydrogen occurs above 110 C Therefore the

discharge of the material is limited by the level of heat

supplied to the tank

As a consequence the working temperatures and the off-

gas temperature of a low temperature proton exchange

membrane fuel cell (LT-PEMFC) 85ndash90 C are not sufficient to

heat a storage based on complex hydrides to values high

enough for fast hydrogen desorption Moreover it generally

seems as if working temperatures in the range of 100ndash200 C

are needed in order to access the full potential of a complex

hydride storage system

Such temperatures can be provided by the so-called high

temperature PEM fuel cells (HT-PEMFC) [910] Besides the

beneficial operation temperature for a complex hydride tank

there are several compelling technological and commercial

reasons for operating H2air PEM fuel cells at temperatures

above 100 C rates of electrochemical kinetics are enhanced

water management and cooling are simplified useful waste

heat can be recovered and lower quality reformed hydrogen

may be used as the fuel Moreover using the waste heat of

a fuel cell for desorption of hydrogen from a hydride may

increase the overall efficiency of the system considerably in

case the heat of absorption is recovered at the filling station

In this study we evaluate the possibility of a thermal

coupling between a high temperature PEM fuel cell operating

at 160ndash200 C and the alanate hydrogen storage material to

overcome the obstacle that additional heat at elevated

temperature has to be generated to completely discharge the

storage tank The heat supplied by the HT-PEM was assumed

to be transferred to the storage tanks by an oil cycle For

evaluation we used the commercial software package

gPROMS The storage system in the simulation consisted of 4

parallel tanks with approximately 2 kg of sodium alanate

coupled with a 1 kW H2-consumption fuel cell a pre-heater

system and a pump

The heat transfer characteristics were investigated by

changing the geometries of the tanks changing the starting

up procedures and evaluating the electrical power output

from the system Stable working conditions were identified

and the sensitivity of the process to various design parameters

was determined

2 System

An overall scheme of the simulated system (Fig 1) refers to

components which have already been fabricated for an

experimental verification of the obtained data

The main components which have been set up in

gPROMS as different models with interconnection via

a global model to each other are the fuel cell the oil cycle for

heat transfer between the fuel cell and hydride tanks four

hydride tanks in series a pump and a pre-heater system

The pre-heater system includes an electrical heat source

for a second oil cycle where heat transfer is done via a micro

heat exchanger This allows for the fast stop or start of the

00010203040506070809

0 50 100 150 200Current A

Vo

ltag

e p

er cell V

0

100

200

300

400

500

600

700

Po

we

r (S

ta

ck

) [W

]

200degC170degC

150degC

150degC

170degC200degC























3 Components

31 Fuel cell











































of alanate material






33 Pump
























114220

450

50





Fig 4








section System)

35 Main oil cycle









to be 10 cm














4 Simulation





up (see also Fig 5)







heater is off)






41 Fuel cell





and removed heat








dUdtfrac14 mFCcpFC

dTFC

dt(5)










Toilin TFC























42 H storage tanks












ToilethzTHORN Tins

zRinnerins

and (7)







dToilethzTHORN








element











part of the tank





dz(10)

_Hout frac14


Lcylinder

ndiscrz(11)


tank













vTins


vr

frac14 aoil

ToilethzTHORN Tins


(12)


vt

frac14

1r

v

vr

rl

vTinsethz rTHORNvr

(13)


vTins


vr

frac14 aair

Tins


Tu

(14)
















position dependent






frac14 0 (16)



according to


vtfrac14

1r

v

vr

rlalanateethzrTHORN


thorn v

vz

lalanateethzrTHORN


thorndcH2ethzrTHORN

dTralanateDHR (17)








pH2

thorn 051 (18)


pH2

thorn 050 (19)


pH2

thorn 071 (20)













616 kJ

RT

ln

pappl

peq1



frac14 19e11 exp

83 kJ

RT

ln

peq1

pappl




562kJ

RT

ln

pappl

p

eq2

eth167wtHTHORN (24)


frac14 29e10 exp

93 kJ

RT

ln

peq2

pappl

ethwt HTHORN (25)








position





Vgas(26)


100

0BcH2solid


Z t

0

Z Lstorage

0

Z Rstorage

0

cH2solid

dtdzdr

1CAZ t

0

_mH2 gasout (27)









43 Pump






Pel frac14poil

_Voil

04and _Qel frac14

poil_Voil

1 04(29)


respectively












44 Pre-heater









45 Main oil cycle






standard equations

5 Results






























0100020003000400050006000700080009000


Ma

x F

C O

pe

ra

tio

n

He

at-u

p T

im

e [s

]


Heat-up Time

Operation FC







considered




capacity considered














the system











gets enlarged






heating-up
















gradient









-400

-300

-200

-100

0

100

200

300

2000

4000

6000

8000

1200

014

000

1600

018

000

2000

0Zeit [s]

Cu

mu

lative el P

ow

er [W

h]

Time [s]

250 W total



1000

0





800 s





system efficiency





















neglected







pumping)








progress

0

100

200

300

400

500

600


El P

ow

er H

eat [W

]


Heat (ΘFC-Θloss)

Heat for Desorption




not considered

54 FC total power






















-2-1012345678

020

0040

0060

0080

0010

000

1200

0

Time [s]

Pre

ss

ure

T

an

k [b

ar]

-02-0100102030405060708 C

um

ula

tiv

e e

l P

ow

er [k

Wh

]

Hold 800s

Pre-heatPressure

Power







55 Alanate kinetics






















6 Conclusions



















r e f e r e n c e s


















00010203040506070809

0 50 100 150 200Current A

Vo

ltag

e p

er cell V

0

100

200

300

400

500

600

700

Po

we

r (S

ta

ck

) [W

]

200degC170degC

150degC

150degC

170degC200degC























3 Components

31 Fuel cell











































of alanate material






33 Pump
























114220

450

50





Fig 4








section System)

35 Main oil cycle









to be 10 cm














4 Simulation





up (see also Fig 5)







heater is off)






41 Fuel cell





and removed heat








dUdtfrac14 mFCcpFC

dTFC

dt(5)










Toilin TFC























42 H storage tanks












ToilethzTHORN Tins

zRinnerins

and (7)







dToilethzTHORN








element











part of the tank





dz(10)

_Hout frac14


Lcylinder

ndiscrz(11)


tank













vTins


vr

frac14 aoil

ToilethzTHORN Tins


(12)


vt

frac14

1r

v

vr

rl

vTinsethz rTHORNvr

(13)


vTins


vr

frac14 aair

Tins


Tu

(14)
















position dependent






frac14 0 (16)



according to


vtfrac14

1r

v

vr

rlalanateethzrTHORN


thorn v

vz

lalanateethzrTHORN


thorndcH2ethzrTHORN

dTralanateDHR (17)








pH2

thorn 051 (18)


pH2

thorn 050 (19)


pH2

thorn 071 (20)













616 kJ

RT

ln

pappl

peq1



frac14 19e11 exp

83 kJ

RT

ln

peq1

pappl




562kJ

RT

ln

pappl

p

eq2

eth167wtHTHORN (24)


frac14 29e10 exp

93 kJ

RT

ln

peq2

pappl

ethwt HTHORN (25)








position





Vgas(26)


100

0BcH2solid


Z t

0

Z Lstorage

0

Z Rstorage

0

cH2solid

dtdzdr

1CAZ t

0

_mH2 gasout (27)









43 Pump






Pel frac14poil

_Voil

04and _Qel frac14

poil_Voil

1 04(29)


respectively












44 Pre-heater









45 Main oil cycle






standard equations

5 Results






























0100020003000400050006000700080009000


Ma

x F

C O

pe

ra

tio

n

He

at-u

p T

im

e [s

]


Heat-up Time

Operation FC







considered




capacity considered














the system











gets enlarged






heating-up
















gradient









-400

-300

-200

-100

0

100

200

300

2000

4000

6000

8000

1200

014

000

1600

018

000

2000

0Zeit [s]

Cu

mu

lative el P

ow

er [W

h]

Time [s]

250 W total



1000

0





800 s





system efficiency





















neglected







pumping)








progress

0

100

200

300

400

500

600


El P

ow

er H

eat [W

]


Heat (ΘFC-Θloss)

Heat for Desorption




not considered

54 FC total power






















-2-1012345678

020

0040

0060

0080

0010

000

1200

0

Time [s]

Pre

ss

ure

T

an

k [b

ar]

-02-0100102030405060708 C

um

ula

tiv

e e

l P

ow

er [k

Wh

]

Hold 800s

Pre-heatPressure

Power







55 Alanate kinetics






















6 Conclusions



















r e f e r e n c e s


















114220

450

50





Fig 4








section System)

35 Main oil cycle









to be 10 cm














4 Simulation





up (see also Fig 5)







heater is off)






41 Fuel cell





and removed heat








dUdtfrac14 mFCcpFC

dTFC

dt(5)










Toilin TFC























42 H storage tanks












ToilethzTHORN Tins

zRinnerins

and (7)







dToilethzTHORN








element











part of the tank





dz(10)

_Hout frac14


Lcylinder

ndiscrz(11)


tank













vTins


vr

frac14 aoil

ToilethzTHORN Tins


(12)


vt

frac14

1r

v

vr

rl

vTinsethz rTHORNvr

(13)


vTins


vr

frac14 aair

Tins


Tu

(14)
















position dependent






frac14 0 (16)



according to


vtfrac14

1r

v

vr

rlalanateethzrTHORN


thorn v

vz

lalanateethzrTHORN


thorndcH2ethzrTHORN

dTralanateDHR (17)








pH2

thorn 051 (18)


pH2

thorn 050 (19)


pH2

thorn 071 (20)













616 kJ

RT

ln

pappl

peq1



frac14 19e11 exp

83 kJ

RT

ln

peq1

pappl




562kJ

RT

ln

pappl

p

eq2

eth167wtHTHORN (24)


frac14 29e10 exp

93 kJ

RT

ln

peq2

pappl

ethwt HTHORN (25)








position





Vgas(26)


100

0BcH2solid


Z t

0

Z Lstorage

0

Z Rstorage

0

cH2solid

dtdzdr

1CAZ t

0

_mH2 gasout (27)









43 Pump






Pel frac14poil

_Voil

04and _Qel frac14

poil_Voil

1 04(29)


respectively












44 Pre-heater









45 Main oil cycle






standard equations

5 Results






























0100020003000400050006000700080009000


Ma

x F

C O

pe

ra

tio

n

He

at-u

p T

im

e [s

]


Heat-up Time

Operation FC







considered




capacity considered














the system











gets enlarged






heating-up
















gradient









-400

-300

-200

-100

0

100

200

300

2000

4000

6000

8000

1200

014

000

1600

018

000

2000

0Zeit [s]

Cu

mu

lative el P

ow

er [W

h]

Time [s]

250 W total



1000

0





800 s





system efficiency





















neglected







pumping)








progress

0

100

200

300

400

500

600


El P

ow

er H

eat [W

]


Heat (ΘFC-Θloss)

Heat for Desorption




not considered

54 FC total power






















-2-1012345678

020

0040

0060

0080

0010

000

1200

0

Time [s]

Pre

ss

ure

T

an

k [b

ar]

-02-0100102030405060708 C

um

ula

tiv

e e

l P

ow

er [k

Wh

]

Hold 800s

Pre-heatPressure

Power







55 Alanate kinetics






















6 Conclusions



















r e f e r e n c e s



















4 Simulation





up (see also Fig 5)







heater is off)






41 Fuel cell





and removed heat








dUdtfrac14 mFCcpFC

dTFC

dt(5)










Toilin TFC























42 H storage tanks












ToilethzTHORN Tins

zRinnerins

and (7)







dToilethzTHORN








element











part of the tank





dz(10)

_Hout frac14


Lcylinder

ndiscrz(11)


tank













vTins


vr

frac14 aoil

ToilethzTHORN Tins


(12)


vt

frac14

1r

v

vr

rl

vTinsethz rTHORNvr

(13)


vTins


vr

frac14 aair

Tins


Tu

(14)
















position dependent






frac14 0 (16)



according to


vtfrac14

1r

v

vr

rlalanateethzrTHORN


thorn v

vz

lalanateethzrTHORN


thorndcH2ethzrTHORN

dTralanateDHR (17)








pH2

thorn 051 (18)


pH2

thorn 050 (19)


pH2

thorn 071 (20)













616 kJ

RT

ln

pappl

peq1



frac14 19e11 exp

83 kJ

RT

ln

peq1

pappl




562kJ

RT

ln

pappl

p

eq2

eth167wtHTHORN (24)


frac14 29e10 exp

93 kJ

RT

ln

peq2

pappl

ethwt HTHORN (25)








position





Vgas(26)


100

0BcH2solid


Z t

0

Z Lstorage

0

Z Rstorage

0

cH2solid

dtdzdr

1CAZ t

0

_mH2 gasout (27)









43 Pump






Pel frac14poil

_Voil

04and _Qel frac14

poil_Voil

1 04(29)


respectively












44 Pre-heater









45 Main oil cycle






standard equations

5 Results






























0100020003000400050006000700080009000


Ma

x F

C O

pe

ra

tio

n

He

at-u

p T

im

e [s

]


Heat-up Time

Operation FC







considered




capacity considered














the system











gets enlarged






heating-up
















gradient









-400

-300

-200

-100

0

100

200

300

2000

4000

6000

8000

1200

014

000

1600

018

000

2000

0Zeit [s]

Cu

mu

lative el P

ow

er [W

h]

Time [s]

250 W total



1000

0





800 s





system efficiency





















neglected







pumping)








progress

0

100

200

300

400

500

600


El P

ow

er H

eat [W

]


Heat (ΘFC-Θloss)

Heat for Desorption




not considered

54 FC total power






















-2-1012345678

020

0040

0060

0080

0010

000

1200

0

Time [s]

Pre

ss

ure

T

an

k [b

ar]

-02-0100102030405060708 C

um

ula

tiv

e e

l P

ow

er [k

Wh

]

Hold 800s

Pre-heatPressure

Power







55 Alanate kinetics






















6 Conclusions



















r e f e r e n c e s




























part of the tank





dz(10)

_Hout frac14


Lcylinder

ndiscrz(11)


tank













vTins


vr

frac14 aoil

ToilethzTHORN Tins


(12)


vt

frac14

1r

v

vr

rl

vTinsethz rTHORNvr

(13)


vTins


vr

frac14 aair

Tins


Tu

(14)
















position dependent






frac14 0 (16)



according to


vtfrac14

1r

v

vr

rlalanateethzrTHORN


thorn v

vz

lalanateethzrTHORN


thorndcH2ethzrTHORN

dTralanateDHR (17)








pH2

thorn 051 (18)


pH2

thorn 050 (19)


pH2

thorn 071 (20)













616 kJ

RT

ln

pappl

peq1



frac14 19e11 exp

83 kJ

RT

ln

peq1

pappl




562kJ

RT

ln

pappl

p

eq2

eth167wtHTHORN (24)


frac14 29e10 exp

93 kJ

RT

ln

peq2

pappl

ethwt HTHORN (25)








position





Vgas(26)


100

0BcH2solid


Z t

0

Z Lstorage

0

Z Rstorage

0

cH2solid

dtdzdr

1CAZ t

0

_mH2 gasout (27)









43 Pump






Pel frac14poil

_Voil

04and _Qel frac14

poil_Voil

1 04(29)


respectively












44 Pre-heater









45 Main oil cycle






standard equations

5 Results






























0100020003000400050006000700080009000


Ma

x F

C O

pe

ra

tio

n

He

at-u

p T

im

e [s

]


Heat-up Time

Operation FC







considered




capacity considered














the system











gets enlarged






heating-up
















gradient









-400

-300

-200

-100

0

100

200

300

2000

4000

6000

8000

1200

014

000

1600

018

000

2000

0Zeit [s]

Cu

mu

lative el P

ow

er [W

h]

Time [s]

250 W total



1000

0





800 s





system efficiency





















neglected







pumping)








progress

0

100

200

300

400

500

600


El P

ow

er H

eat [W

]


Heat (ΘFC-Θloss)

Heat for Desorption




not considered

54 FC total power






















-2-1012345678

020

0040

0060

0080

0010

000

1200

0

Time [s]

Pre

ss

ure

T

an

k [b

ar]

-02-0100102030405060708 C

um

ula

tiv

e e

l P

ow

er [k

Wh

]

Hold 800s

Pre-heatPressure

Power







55 Alanate kinetics






















6 Conclusions



















r e f e r e n c e s




















562kJ

RT

ln

pappl

p

eq2

eth167wtHTHORN (24)


frac14 29e10 exp

93 kJ

RT

ln

peq2

pappl

ethwt HTHORN (25)








position





Vgas(26)


100

0BcH2solid


Z t

0

Z Lstorage

0

Z Rstorage

0

cH2solid

dtdzdr

1CAZ t

0

_mH2 gasout (27)









43 Pump






Pel frac14poil

_Voil

04and _Qel frac14

poil_Voil

1 04(29)


respectively












44 Pre-heater









45 Main oil cycle






standard equations

5 Results






























0100020003000400050006000700080009000


Ma

x F

C O

pe

ra

tio

n

He

at-u

p T

im

e [s

]


Heat-up Time

Operation FC







considered




capacity considered














the system











gets enlarged






heating-up
















gradient









-400

-300

-200

-100

0

100

200

300

2000

4000

6000

8000

1200

014

000

1600

018

000

2000

0Zeit [s]

Cu

mu

lative el P

ow

er [W

h]

Time [s]

250 W total



1000

0





800 s





system efficiency





















neglected







pumping)








progress

0

100

200

300

400

500

600


El P

ow

er H

eat [W

]


Heat (ΘFC-Θloss)

Heat for Desorption




not considered

54 FC total power






















-2-1012345678

020

0040

0060

0080

0010

000

1200

0

Time [s]

Pre

ss

ure

T

an

k [b

ar]

-02-0100102030405060708 C

um

ula

tiv

e e

l P

ow

er [k

Wh

]

Hold 800s

Pre-heatPressure

Power







55 Alanate kinetics






















6 Conclusions



















r e f e r e n c e s


















0100020003000400050006000700080009000


Ma

x F

C O

pe

ra

tio

n

He

at-u

p T

im

e [s

]


Heat-up Time

Operation FC







considered




capacity considered














the system











gets enlarged






heating-up
















gradient









-400

-300

-200

-100

0

100

200

300

2000

4000

6000

8000

1200

014

000

1600

018

000

2000

0Zeit [s]

Cu

mu

lative el P

ow

er [W

h]

Time [s]

250 W total



1000

0





800 s





system efficiency





















neglected







pumping)








progress

0

100

200

300

400

500

600


El P

ow

er H

eat [W

]


Heat (ΘFC-Θloss)

Heat for Desorption




not considered

54 FC total power






















-2-1012345678

020

0040

0060

0080

0010

000

1200

0

Time [s]

Pre

ss

ure

T

an

k [b

ar]

-02-0100102030405060708 C

um

ula

tiv

e e

l P

ow

er [k

Wh

]

Hold 800s

Pre-heatPressure

Power







55 Alanate kinetics






















6 Conclusions



















r e f e r e n c e s


















-400

-300

-200

-100

0

100

200

300

2000

4000

6000

8000

1200

014

000

1600

018

000

2000

0Zeit [s]

Cu

mu

lative el P

ow

er [W

h]

Time [s]

250 W total



1000

0





800 s





system efficiency





















neglected







pumping)








progress

0

100

200

300

400

500

600


El P

ow

er H

eat [W

]


Heat (ΘFC-Θloss)

Heat for Desorption




not considered

54 FC total power






















-2-1012345678

020

0040

0060

0080

0010

000

1200

0

Time [s]

Pre

ss

ure

T

an

k [b

ar]

-02-0100102030405060708 C

um

ula

tiv

e e

l P

ow

er [k

Wh

]

Hold 800s

Pre-heatPressure

Power







55 Alanate kinetics






















6 Conclusions



















r e f e r e n c e s



















55 Alanate kinetics






















6 Conclusions



















r e f e r e n c e s


















Thermal coupling of a high temperature PEM fuel cell with a complex hydride tank

Documents

Transcript of Thermal coupling of a high temperature PEM fuel cell with a complex hydride tank