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Economics of hydrogen production and liquefactionby geothermal energy

Ceyhun Yilmaz a, Mehmet Kanoglu a,*,1, Ali Bolatturk b, Mohamed Gadalla c

aDepartment of Mechanical Engineering, University of Gaziantep, 27310 Gaziantep, TurkeybDepartment of Mechanical Engineering, Suleyman Demirel University, 32260 Isparta, TurkeycDepartment of Mechanical Engineering, American University of Sharjah, P.O. Box 26666, Sharjah, United Arab Emirates

a r t i c l e i n f o

Article history:

Received 7 January 2011

Accepted 4 June 2011

Available online 13 July 2011

Keywords:

Geothermal energy

Hydrogen production

Hydrogen liquefaction

Hydrogen economy

Water electrolysis

* Corresponding author. Tel.: þ90 342 317250E-mail address: kanoglu@gantep.edu.tr (M

1 Visiting professor at American University0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.06.037

a b s t r a c t

Seven models are considered for the production and liquefaction of hydrogen by

geothermal energy. In these models, we use electrolysis and high-temperature steam

electrolysis processes for hydrogen production, a binary power plant for geothermal power

production, and a pre-cooled LindeeHampson cycle for hydrogen liquefaction. Also, an

absorption cooling system is used for the pre-cooling of hydrogen before the liquefaction

process. A methodology is developed for the economic analysis of the models. It is esti-

mated that the cost of hydrogen production and liquefaction ranges between 0.979 $/kg H2

and 2.615 $/kg H2 depending on the model. The effect of geothermal water temperature on

the cost of hydrogen production and liquefaction is investigated. The results show that the

cost of hydrogen production and liquefaction decreases as the geothermal water temper-

ature increases. Also, capital costs for the models involving hydrogen liquefaction are

greater than those for the models involving hydrogen production only.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction electricity with minimal or no emissions. Different thermo-

Geothermal energy is the thermal energy within the earth’s

interior. Geothermal energy is mostly used to generate elec-

tricity. It is also commonly used for space heating and cooling,

industrial processes, and greenhouse heating [1]. With the

increasing scarcity of fossil fuels and increasing concerns over

the environmental problems they cause, the use of renewable

energy resourceswill likely increase anddiversify. Geothermal

energy appears to be a significant renewable energy source for

solving someof the environmental problemscausedby theuse

of fossil fuels [2]. The technology for producing power from

geothermal resources is well established. Geothermal power

plants tap certain high-temperature resources to generate

8; fax: þ90 342 3601104.. Kanoglu).of Sharjah during 2010e

2011, Hydrogen Energy P

dynamic cycles are used in numerous geothermal power

plants worldwide. The common designs include dry steam,

single-flash, double-flash, binary, and flash/binary cycles.

The total cost of producing hydrogen depends on produc-

tion, liquefaction, storage and distribution costs [3,4]. Sherif

et al. [5] provides some key data on economics of hydrogen

production. Today approximately 9 billion kilograms of

hydrogen are produced annually. More than 95% of the

merchant hydrogen is used for industrial applications in the

chemical, metals, electronics, and space industries. There are

several methods used to produce hydrogen including natural

gas reforming, electrolysis, liquid reforming, high-temperature

electrolysis, photo-biological, and photo-electrochemical.

1011.ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 9 2059

Steamemethane reforming accounts for 80% of the hydrogen

produced. A number of existing and planned demonstration

projects use electrolysis even though it is an energy intensive

process for producing hydrogen. This is because electrolysis

provides a pathway for producing hydrogen from renewable

energy sources. This is accomplishedwhen the electricity from

solar, wind, geothermal, ocean and hydro technologies is used

to produce and store hydrogen [6].

Despite the existence of numerous investigations on the

use of renewable energy sources for hydrogen production,

reports on using geothermal energy sources for hydrogen

liquefaction are limited. Jonsson et al. [7] investigated the

feasibility of using geothermal energy for hydrogen produc-

tion and estimated that using geothermal energy could avoid

16% of the work consumption for electrolysis and 2% for

liquefaction. Sigurvinsson et al. [8] investigated the use of

geothermal heat for the high-temperature electrolysis (HTE)

process. This process includes multiple heat exchangers and

an electrolyzer based on solid oxide fuel cell (SOFC) tech-

nology. A set of relevant parameters were calculated using

a techno-economic optimization methodology.

Mansilla et al. [9] performed a techno-economic optimi-

zation of the upper heat exchanger network in a high-

temperature electrolysis process for hydrogen production.

Heat is extracted by coupling the process either to a high-

temperature reactor or to a geothermal source. Ingason

et al. [10] investigated the most economical ways of

producing hydrogen solely via electrolysis from water, using

electricity from hydro and geothermal. The mixed integer

programming model facilitates the search for optimal choices

from 23 potential power plants, 11 of which are based on

geothermal sources, and 12 are hydropower stations. In

another report, the potential of geothermal resources of the

western United States for producing electricity is investi-

gated. This electricity would be used for the production of

hydrogen [11]. Balta et al. [12], analyzed a high-temperature

electrolysis process where geothermal water is used as the

heat source.

The cost of electricity production from geothermal plants

ranges between $0.045/kWh and $0.074/kWh. Once capital

costs for the plant are recovered, the price of power can

decrease to below $0.05/kWh. The price of geothermal energy

is within the range of water electricity choices available today

when the costs over the lifetime of a plant are considered [13].

In hydrogen production, the most widely used commercial

technology is alkaline water electrolysis. The mean electricity

energy consumption in the electrolysis of water is about

50 kWh/kg hydrogen. The cost of electricity makes large-scale

electrolytic production of hydrogen uneconomical compared

with the steamemethane reforming method. Work is

underway to improve the alkaline water electrolysis tech-

nology with an advanced alkaline electrolyzer that would

increase cell efficiency somewhat and reduce the electricity

requirement to about 43 kWh/kg [14].

Electricity prices have proven to be a major contributor to

the overall hydrogen cost. Cost of hydrogen increases linearly

with increasing cost of electricity. Based on reasonable effi-

ciencies, an electricity cost of slightly less than $0.10/kWh

would be needed to produce and deliver central gaseous

hydrogen that is cost-competitive [15].

Many research and development projects throughout the

world are devoted to sustainable hydrogen production

processes. To be sustainable, a hydrogen production process

must be carried out without consumption of any raw mate-

rials other than water and be driven by forms of energy

produced without greenhouse gas emissions. Low-

temperature electrolysis driven by electricity produced from

renewable energy sources satisfies these criteria. However,

steam reforming of natural gas is currently less expensive

than electrolysis driven by renewable-based electricity [16].

When hydrogen is produced by electrolysis, the necessary

electricity input can be obtained from a geothermal power

plant. The cost of producing electricity from a geothermal

power plant depends on many factors. The more significant

factors are cycle type (single- or double-flash, binary, flash/

binary etc.), resource temperature, mass flow rate of

geothermal water, and well cost. The other parameters

include well spacing, well replacement rate, steam quality,

non-condensable gas content, plant site, plant efficiency, and

load factor. All of these parameters determine the economic

value of a geothermal resource [17].

Kanoglu et al. [18] investigated the use of geothermal

energy for hydrogen liquefaction. Three models were devel-

oped for the analysis including the use of geothermal elec-

tricity for liquefaction, the use of absorption cooling system

for pre-cooling hydrogen gas, and a cogeneration option for

which both geothermal electricity and geothermal heat are

used. In amore recent study, Kanoglu et al. [19] developed four

models for hydrogen production by geothermal energy and

analyzed these models thermodynamically. In this paper, the

seven models developed for using geothermal energy for

hydrogen production and liquefaction are investigated

economically.

2. Thermodynamic and cost analyses

In this section, we consider both ideal (reversible) and non-

ideal (irreversible) operations for thermodynamic and cost

analyses.

2.1. Reversible operations

The maximum specific work that can be obtained by

a geothermal power plant utilizing a resource at temperature

Ts in an environment at T0 is given by

wrev;geo ¼ cðTs � T0Þ � T0cln

�Ts

T0

�(1)

The reaction of water electrolysis is given by

H2Oþ Electrical work/H2 þ 1=2O2. The total energy required

H(T ) is the sum of heat transfer Q(T ) and Gibbs function G(T ).

The change in Gibbs function DG represents the reversible

electrolysis work:

wrev;elec ¼ DGðTÞ ¼ DHðTÞ � QðTÞ (2)

where Q(T ) ¼ TDS(T ) and DH(T ) is the standard enthalpy of

the reaction at T.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 92060

Standard thermodynamic parameters such as DH(T ),

DG(T ) and DS(T ) are functions of temperature. Therefore, the

standard thermodynamic parameters at different tempera-

tures can be calculated using Kirchhoff’s equation, entropy

equation, and the relation between DG and Nernst potential.

These equations are

DHðTÞ ¼ DH0298:15 þ

ZT

298:15

DcPdT (3)

DSðTÞ ¼ S0298:15 þ

ZT

298:15

DcPT

dT (4)

EðTÞ ¼ �DGðTÞnF

(5)

Gibbs free energy equation for electrolysis can be written as

wrev;elec ¼ DGelec; H2O ¼ GH2þ 1=2GO2

� GH20 (6)

The minimum work required for an electrolysis process is

expressed as

wrev;elec ¼ DGelec;H2O

MH2

(7)

where DGelec;H2O is the change in the Gibbs function (in kJ/

kmol) and MH2is the molar mass of hydrogen (in kg/kmol).

For irreversible operations, we can determine the actual

electrolysis work by utilizing the electrolysis efficiency,

wact;elec ¼ wrev;elec

helec

(8)

We used an electrolysis efficiency of 78% for alkaline elec-

trolysis at room temperature (25 �C), and an efficiency of 82%

at 85 �C. The electrolysis efficiency is taken to be 94% for the

high-temperature steam electrolysis [12]. The cost of

producing hydrogen may be expressed as

CH2¼ CelectricityWelec (9)

where Celectricity is the unit cost of electricity (in $/kWh) and

Welec is work requirement for the electrolysis process (in kWh/

kg H2). One can use minimum work requirement for the

electrolysis process for reversible operations and actual work

for irreversible operations. The unit cost of electricity is taken

as 0.06 $/kWh.

The minimum (i.e., reversible) work input for the lique-

faction of a unit mass of a gas in a liquefier (i.e., liquefaction

unit) can be written as

wrev;liq ¼ h2 � h1 � T0ðs2 � s1Þ (10)

where state 1 is the state of the gas and state 2 is the state of

the liquid.

Seven models (i.e., cases) developed for using geothermal

energy for hydrogen production and liquefaction are shown in

Fig. 1. In case 1 (Fig. 1a), geothermal work output is used as the

work input for an electrolysis process. For a geothermal

source temperature of 200 �C and a dead state temperature of

25 �C, the maximum specific work output is calculated from

Eq. (1) to be 157.9 kJ/kg. Taking the inlet state of the liquid to be

1 atm and a saturated liquid, the reversible specificwork input

for the electrolysis of hydrogen can be calculated from Eq. (7)

to be 117,649 kJ/kg. This is equivalent to 32.68 kWh/kg H2.

Then, the cost of hydrogen becomes 1.961 $/kg H2. The elec-

tricity cost is taken 0.06 $/kWh for the analysis of all models.

In case 2 (Fig. 1b), part of geothermal heat is used to produce

workforelectrolysisprocessandtheremaininggeothermalheat

is used to preheat water in an electrolysis process. Geothermal

water enters the plant at 200 �C and leaves at 95 �C. This

geothermal water heats the water for electrolysis from 25 �C to

85 �C. The reversible specific work input for the electrolysis of

hydrogencanbecalculatedtobe111,421kJ/kg (30.95kWh/kgH2).

The corresponding cost of hydrogen is 1.857 $/kg H2.

In case 3 (Fig. 1c), geothermal heat is used to preheat water

in a high-temperature electrolysis process. The water for

electrolysis is heated to 200 �C steam by geothermal water.

This steam is heated to 900 �C by using the heat generated

during electrolysis. The reversible work in this case is calcu-

lated to be 86,672 kJ/kg H2 or 22.96 kWh/kg H2. Then, the cost

of hydrogen becomes 1.377 $/kg H2.

In case 4 (Fig. 1d), part of geothermal heat is used as the

work input for a liquefaction process. The maximum specific

work output from the geothermal plant is calculated from Eq.

(1) to be 157.9 kJ/kg while the minimum work input for

liquefaction is calculated from Eq. (7) to be 11,963 kJ/kg H2

(3.323 kWh/kg H2). Then, the cost of hydrogen liquefaction

becomes 0.1994 $/kg H2.

In case 5 (Fig. 1e), geothermal water is used as the heat

source for an absorption refrigeration system (ARS) which is

used to precool the gas before it is liquefied. For Ts ¼ 200 �C,T0 ¼ 25 �C and TL ¼ �20 �C, the COP of a reversible absorption

refrigerator is determined from COPrev ¼ (1�T0/Ts)[TL/(T0�TL)]

to be 2.08. This pre-cooling process will decrease work con-

sumption during liquefaction. In this case, the reversible work

input for liquefaction is 11,918 kJ/kg H2 (3.311 kWh/kg H2), and

the corresponding cost is 0.1986 $/kg H2.

In case 6 (Fig. 1f), geothermal water at 200 �C is used as the

heat source for a reversible absorption refrigeration system

which is used to precool the gas before it is liquefied. Then, the

geothermal water leaving the generator of the absorption

system at 190 �C is used to produce work in a geothermal

power plant. The pre-cooling temperature of hydrogen gas is

taken to be �20 �C. The reversible analysis shows that

z ¼ 0.1456 kg of gas can be cooled from 25 �C to �20 �C by 1 kg

geothermal water in the absorption system. The reversible

work output is 142.3 kJ/kg water in the power plant and the

reversible work input is 11,918 kJ/kg H2 in the liquefier. This

corresponds to a liquefaction of y ¼ 0.01194 kg ¼ 11.94 g

hydrogen by 1 kg geothermal water at 190 �C. The cost of

hydrogen liquefaction is 0.1986 $/kg H2.

In case 7 (Fig. 1g), part of geothermal work is used for

electrolysis and the remaining part for liquefaction.We define

a parameter b as the ratio of electrolysis work to the lique-

faction work:

b ¼ Helec;prod

Helec;prod þHelec;liq(11)

where

Helec ¼ b� Helec;prodþð1� bÞ �Helec;liq (12)

a

H2

Geothermal power plant

Electrolysis

Geothermal water

Work

Water

O2

b

WorkElectrolysis

Heat exchanger

Water

Geothermal power plant

Heatedwater

H2 O2Geothermalwater

c

Work

Water Heat exchanger

Heat

Steam 900 C High

temperatureelectrolysis

Heater

Heatedsteam

Geothermal water

O2

H2

d

Work

Gas

Liquid

Liquefactioncycle

Geothermal power plant

Geothermal water

e

Cooledgas

Geothermalwater

Gas Liquefaction cycle

Absorptionrefrigeration

cycle

Liquid

Work

f

Geothermal water

Gas Absorption refrigeration

cycle

Cooledgas Liquefaction

cycle Liquid

Work

Geothermal power plant

g

H2

Geothermalwater

Geothermal power plant

Work

Liquefactioncycle

Liquid H2

Water

O2

Electrolysis

Fig. 1 e Seven cases considered for hydrogen production and liquefaction. (a) Case 1: Using geothermal work output as

the work input for an electrolysis process. (b) Case 2: Using part of geothermal heat to produce work for electrolysis

process and remaining heat to preheat the water for electrolysis. (c) Case 3: Using geothermal heat to preheat water in

a high-temperature electrolysis process. (d) Case 4: Using geothermal work output as the work input for a liquefaction

cycle. (e) Case 5: Using geothermal heat in an absorption refrigeration process to precool the gas before its liquefaction.

(f) Case 6: Using part of the geothermal heat for absorption refrigeration to precool the gas and remaining heat to

produce work in a liquefaction cycle. (g) Case 7: Using part of geothermal work for electrolysis and the remaining part

for liquefaction.

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 en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 9 2061

In this case, electrolysis takes place at 25 �C. Therefore, the

minimum work for electrolysis is 117,649 kJ/kg H2 and the

minimum work for liquefaction is 11,963 kJ/kg H2. Then the

parameter b defined in Eq. (11) becomes 9.834. This indicates

that 90.8% of produced work should be used for the electrol-

ysis and the remaining 9.2% for the liquefaction. The

maximum specific work output was calculated as 157.9 kJ/kg.

Then, 143.34 kJ/kg should be used for electrolysis and the

remaining 14.58 kJ/kg for the liquefaction. For 1 kg of

geothermal water at 200 �C, we obtain yprod;H2¼ ð143:34 kJ=

kg waterÞ=ð117;649 kJ=kg H2Þ ¼ 0:001219 kgH2=kg water. Ther-

efore, 1.219 g hydrogen could be produced and liquefied using

Turbine

Condenser

ReinjectionPump

Heatexchanger

Geothermal fluid

Cooledwater

Power

BINARYCYCLE

Fig. 2 e Binary geothermal power plant.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 92062

1 kg of geothermal water. In another words, 820.6 kg of

geothermal water is needed to produce and liquefy 1 kg of

hydrogen. The required electricity to produce and liquefy 1 kg

hydrogen is 29.97 kWh/kg H2 with a corresponding cost of

1.798 $/kg H2.

2.2. Irreversible operations

For the non-ideal (e.g., irreversible) operations of the seven

cases considered, we consider a binary geothermal power

plant with isobutane as the working fluid as shown in Fig. 2.

This is usually the choice of cycle for liquid-dominated

geothermal resources. In this cycle, isobutane is heated and

vaporized in the heat exchanger by geothermal water. Then, it

flows through the turbine, is condensed and pumped back to

the heat exchanger, completing the binary cycle. The heat

exchanger and condenser pressures are taken to be 3000 kPa

and 400 kPa, respectively while the temperature at the turbine

inlet is taken to be 190 �C, which is 10 �C lower than the

geothermal water temperature at the heat exchanger inlet.

Also, isentropic efficiencies of the turbine and pump are taken

to be 80% and 70%, respectively. The present assumptions and

values taken for the analysis of the binary power plant closely

correspond to those encountered in actual power plants using

similar geothermal resources [20,21].

y

Fig. 3 e Pre-cooled LindeeHampson liquefaction cycle. C:

compressor, EV: Expansion valve, I, II, III, IV, and V: Heat

exchangers, LN2: Liquid nitrogen, LN2: Gasified nitrogen, f:

liquid, g: gas, _mf : mass flow rate of liquid withdrawn from

the system, _m: total mass flow rate of hydrogen [13].

For the liquefaction of a gas, we consider the pre-cooled

LindeeHampson cycle because it is a well-known and rela-

tively simple system used for hydrogen liquefaction (Fig. 3).

Makeup gas is mixed with the uncondensed portion of the gas

from the previous cycle, and the mixture at state 1 is

compressed to state 2. Heat is rejected from the compressed

gas to a coolant. The high-pressure gas is cooled to state 3 in

a regenerative counter-flow heat exchanger (I) by the uncon-

densed gas, and is cooled further by flowing through two

nitrogen baths (II and IV) and two regenerative heat

exchangers (III and V) before being throttled to state 8, where

it is a saturated liquidevapor mixture. The liquid is collected

as the desired product, and the vapor is routed through the

bottom half of the cycle. Finally, the gas is mixed with fresh

makeup gas, and the cycle is repeated.

Using an energy balance of heat exchanger V and the

throttling valve taken together, the fraction of the liquefied

gas can be determined to be

fliq ¼ h9 � h6

h9 � hf(13)

Energy balances for the heat exchangers can be written as

h2 � h3 ¼�1� fliq

�ðh11 � h10Þ (14)

h4 � h5 ¼�1� fliq

�ðh10 � h9Þ (15)

h6 � h7 ¼�1� fliq

��h9 � hg

�(16)

Since the gas behaves ideally during compression, the

specific compression work may be determined from

wcomp;in ¼ RT0lnðP2=P1Þhcomp

ðper unit mass of gas in the cycleÞ

(17)

where hcomp is the isothermal efficiency of the compressor, R

is the gas constant and P is the pressure. The numerator

represents the work input for a corresponding isothermal

process. The specific work input to the liquefaction cycle per

unit mass of liquefaction is

wliq;in ¼ wcomp;in

fliqðper unit mass of liquefactionÞ (18)

The parameter ymay be expressed for this irreversible cycle

operation as

¼ wgeo;out

wliq;in þwnitrogen;inðmass liquefied=mass of geothermal

water usedÞ (19)

Here, the parameter y represents themass of gas liquefied per

mass of geothermal water entering the power plant. Also,

wgeo,out is the net work output from the geothermal power

plant per unit mass of geothermal water and is calculated as

wgeo;out ¼ wturbine;out �wpump;in �wparasitic (20)

wherewturbine,out is the work output from the turbine,wpump,in

is the work input to the pump and wparasitic is the parasitic

work used in the plant for various internal and auxiliary uses.

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 en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 9 2063

The parasitic work is mainly comprised of fan work

consumption in the air-cooled condenser. It is assumed that

about 20% of net work output (wturbine,out � wpump,in) is used

satisfy the parasitic work requirements [20]. The term

wnitrogen,in is the work consumed per unit mass of nitrogen

liquefaction for use in the liquefaction cycle.

In the analysis, the compressor exit pressure is taken to be

10MPa and the gas inlet state is taken to be 1 atm (101 kPa) and

25 �C. The nitrogen bath temperatures are taken to be 77.5 K

and 65 K with the corresponding saturation pressures of

101 kPa and 17 kPa, respectively. Also, it is assumed that each

nitrogen stream exits the heat exchanger 15 K lower than it

enters. The isothermal efficiency of the compressor is taken to

be 65% and the effectiveness of the heat exchangers are taken

to be 90%. The specific work consumption of liquid nitrogen is

estimated as 5050 kJ/kg by considering the states of nitrogen at

the inlet and exit and the operation of some current liquefiers.

These values follow theworks of Barron [22], Timmerhaus and

Flynn [23] and Nandi and Sarangi [24].

For the absorption refrigeration system, an ammonia-

water system is considered to allow subzero temperatures in

the system. For the investigation of the second and third cases

in irreversible operation, we need to assume a value for the

COP of the system.Wemay do this by defining the second-law

efficiency as

hII;ARS ¼ COPact

COPrev(21)

where COPact is the actual COP and COPrev is the reversible

COP. We take this second-law efficiency to be 25%, which

appears to be reasonable for processes involving the cooling of

a gas to �20 �C [25,26].

The analysis of the seven cases considered in this study is

performed for irreversible operation of the processes as

described in this section. The hydrogen production and/or

liquefaction costs for a geothermal resource temperature of

200 �C are:

Case 1: 2.615 $/kg H2, Case 2: 2.38 $/kg H2, Case 3: 1.53 $/kg

H2, Case 4: 1.169 $/kg H2

Case 5: 0.979 $/kg H2, Case 6: 0.981 $/kg H2, Case 7: 2.168 $/

kg H2

3. Total cost analysis

In this section, we present a total cost analysis of the seven

models considered. Total cost includes capital cost, operating

and maintenance cost, and electrical cost. The models

considered in this study involve the installations of

a geothermal power plant, electrolysis or high-temperature

electrolysis units, an absorption refrigeration system,

a hydrogen liquefaction plant, and heat exchangers. The costs

for these installations vary according to the system type and

size, and with the location.

3.1. Geothermal power plant

The initial cost of geothermal power production mostly

consists of exploration of the resource and installation of the

plant. The following formula gives an estimate of the capital

cost as a function of plant capacity [27]:

CC ¼ 2500 exp½ � 0:0025ðP� 5Þ� (22)

where CC is the unit capital cost (in $/kW) and P is the plant

capacity (in MW). The capital cost decreases with the plant

size. For a binary geothermal power plant the average capital

cost can be taken as 2700 $/kW [27]. For the analysis of the

models considered in this study, we consider a 20 MW power

plant. Then, the capital cost of geothermal power plant

becomes $54 millions. The following financial structure is

considered to determine the net value of the capital cost:

Life period

20 years

Debt ratio

50%

Internal investment ratio

50%

Debt interest rate

8%

Debt period

15 years

Monthly interest rate

2.75%

Total tax rate

15%

Internal profit rate

15%

Using these values with the Discounted Cash Flow method,

the net value of the capital cost is determined to be $34

millions.

3.2. Electrolyzer unit

We consider a simple alkaline electrolyzer unit for hydrogen

production. Total cost of electrolyzer unit consist of capital

costs (55%), electricity cost (35%) and operating and mainte-

nance cost (10%) [28]. The cost of hydrogen production may

changebetween2.16and6.75$/kgH2dependingonthecapacity

and the unit cost of electricity. Hydrogen production systems

are classified as small capacity (about 20 kg/day), medium

capacity (about 100 kg/day), and large capacity (about 1000 kg/

day) [29]. InModels 1, 2 and7, thehydrogenproductioncapacity

is 11,016 kg H2/day for a 20 MW geothermal power plant.

The minimum electricity required to produce 1 kg

hydrogen may be calculated from Gibbs energy to be

32.68 kWh. For an electrolyzer efficiency of 75%, the actual

electricity requirement becomes 43.6 kWh. Then, the elec-

tricity cost may be expressed as [28]

EC ¼ 43:6 Celectricity (23)

For an average electricity cost of 0.06 $/kWh, the electricity

cost of hydrogen production becomes 2.615 $/kg H2. In Model

1, the rate of hydrogen production was determined to be

0.128 kg H2/s. Then, the annual electricity cost of hydrogen

production becomes.

ð0:128 kg H2=sÞð365� 24� 3600 sÞð2:615 $=kg H2Þ¼ $10; 555;700 or $10:56 million:

The operating and maintenance cost may be expressed as

[30]

MOM ¼ 0:41 H�0:23 (24)

where H is the production capacity (in kg H2/s). In Model 1, the

production rate is 0.128 kg H2/s and thus the operating and

maintenance cost is 0.65 $/kg H2. Taking discount rate 10%,

Table 1 e Cost analysis of models used for hydrogenproduction and liquefaction by geothermal energy.

Model Capitalcost

(million $)

Electricalcost

(million $/year)

Operating andmaintenance cost(million $/year)

1 48.67 9.99 2.5

2 48.90 9.98 2.7

3 23.6 9.21 3.80

4 106 9.99 2.5

5 72.25 9.985 2.74

6 106.25 8.43 2.47

7 120 5.6 3.17

140 160 180 200 220 240 260 280 3000.5

1.0

1.5

2.0

2.5

3.0

3.5

TGeothermal (°C)

Hyd

roge

n C

ost (

$/kg

H2)

a

6

7

8

9

$/kg

H2)

b

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 92064

construction time 1 year, plant life 40 years and production

capacity rate 95%, the cost of hydrogen for the capital cost

may be calculated from [28]

IC ¼ 0:5 H�0:025 (25)

Combining Eqs. (23e25), the cost of hydrogen production is

expressed as

140 160 180 200 220 240 260 280 3000.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

TGeothermal (°C)

Hyd

roge

n C

ost (

$/kg

H2)

a

140 160 180 200 220 240 260 280 3001

2

3

4

5

6

7

8

9

TGeothermal (°C)

Hyd

roge

n C

ost (

$/kg

H2)

b

Fig. 4 e Variation of hydrogen cost with geothermal water

temperature in Case 1. (a) reversible operation. (b)

irreversible operation.

140 160 180 200 220 240 260 280 3000

1

2

3

4

5

TGeothermal (°C)

Hyd

roge

n C

ost (

Fig. 5 e Variation of hydrogen cost with geothermal water

temperature in Case 2. (a) reversible operation. (b)

irreversible operation.

CH2 ;total ¼ EC þMOM þ IC ¼ 43:6 Celectricity þ 0:41 H�0:23 þ 0:5 H�0:025

(26)

In Model 1, the rate of hydrogen production is 0.128 kg H2/s,

and therefore the unit cost of hydrogen is 3.8 $/kg H2. The

corresponding annual cost is 14,209,445 $/year.

3.3. High-temperature electrolyzer unit

For the economics of high-temperature electrolysis, we

consider the Hot Elly process. In this process, the electricity

requirement for high-temperature electrolysis is 46% less

than that in ordinary electrolysis [7]. In Model 3, 0.217 kg H2

(20,000 kJ/s � 91,858 kJ/kg H2 ¼ 0.217 kg H2/s) is produced per

second for a 20-MW power plant. For a 93% operation

capacity, the annual production is 6,385,633 kg H2/year.

From Eq. (26), the unit cost of hydrogen is determined to be

2.6 $/kg H2. The corresponding annual cost is 16,845,970 $/

year.

The electricity consumption for this process was deter-

mined to be 23.57 kWh/H2. For a unit electricity cost of 0.06 $/

140 160 180 200 220 240 260 280 3000.5

1.0

1.5

2.0

2.5

3.0

TGeothermal (°C)

Hyd

roge

n C

ost (

$/kg

H2)

a

140 160 180 200 220 240 260 280 3000.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

TGeothermal (°C)

Hyd

roge

n C

ost (

$/kg

H2)

b

Fig. 6 e Variation of hydrogen cost with geothermal water

temperature in Case 3. (a) reversible operation. (b)

irreversible operation.

140 160 180 200 220 240 260 280 3000.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

TGeothermal (°C)

Liqu

efac

tion

Cos

t ($/

kg H

2)

a

140 160 180 200 220 240 260 280 3000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

TGeothermal (°C)

Liqu

efac

tion

Cos

t ($/

kg H

2)

b

Fig. 7 e Variation of hydrogen liquefaction cost with

geothermal water temperature in Case 4. (a) reversible

operation. (b) irreversible operation.

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 en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 9 2065

kWh, the cost of hydrogen production is 1.412 $/kg H2. For an

annual hydrogen production of 6,522,959 kg H2/year, the

annual electricity cost becomes 9,210,418 $/year. For

a production rate of 0.217 kg H2/s, the operating and mainte-

nance cost is determined from Eq. (24) to be 0.582 $/kg H2. The

annual cost is 3,800,518 $/year and the initial cost of a high-

temperature electrolysis unit with a 20 MW electricity input

is estimated as 23,614,000 $ [7].

3.4. Liquefaction plant

The cost of a liquefaction plant depends on liquefaction

capacity, type of unit, and installation area among others. In

Model 4, the work requirement for hydrogen liquefaction is

70,155 kJ/kg H2 or 19.5 kWh/kg H2, and the electricity cost is

1.167 $/kg H2. For a 20 MW power output from the plant with

a 95% annual capacity, the cost of hydrogen liquefaction is

calculated as 9,964,280 $/year.

The operating and maintenance cost of liquefaction plants

may be determined from [4]

COM ¼ 3:31

�P

110;372

�0:65�301:88_m

�(27)

where P is the power input to the unit (in kW) and _m is the

hydrogen liquefaction rate (in kg/h). The result of Eq. (27) is

0.303 $/kg and the corresponding annual cost is 2,587,127 $/

year.

The capital cost of a hydrogen liquefaction plant with a 16

million kg/year capacity is given as $137 millions [4]. Using

this value, the corresponding cost for the liquefaction unit

considered in our models becomes $71,973,000.

3.5. Absorption cooling system

In Models 5 and 6, hydrogen is cooled by an absorption system

before the liquefaction process. The following relation is

proposed for the total cost of an absorption unit [31,32]

CARS ¼ CCþ ROMð1þ iÞn�1

ið1þ iÞn � JV1

ð1þ iÞn (28)

140 160 180 200 220 240 260 280 3000.192

0.193

0.194

0.195

0.196

0.197

0.198

0.199

0.2

Liqu

efac

tion

cost

($/k

g H

2)

TGeothermal (°C)

a

140 160 180 200 220 240 260 280 3000.77

0.84

0.91

0.98

1.05

1.12

Liqu

efac

tion

cost

($/k

g H

2)

TGeothermal (°C)

b

Fig. 8 e Variation of hydrogen liquefaction cost with

geothermal water temperature in Case 5. (a) reversible

operation. (b) irreversible operation.

140 160 180 200 220 240 260 280 3000.05

0.10

0.15

0.20

0.25

0.30

0.35

Liqu

efac

tion

cost

($/k

g H

2)

TGeothermal (°C)

a

140 160 180 200 220 240 260 280 3000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

TGeothermal (°C)

Liqu

efac

tion

cost

($/k

g H

2)

b

Fig. 9 e Variation of hydrogen liquefaction cost with

geothermal water temperature in Case 6. (a) reversible

operation. (b) irreversible operation.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 92066

where CC is capital cost, RO&M is the operating and mainte-

nance cost, JV is the junk value, i is the interest rate, and n is

the life of the unit. Considering the cooling rate of the

absorption system, the following values are obtained:

CC ¼ $94,391, RO&M ¼ $10,287, JV ¼ $9439, i ¼ 0.02, n ¼ 20 years.

Then, the total cost becomes 256,246 $.

3.6. Cost analysis of models

The results of the cost analysis for all seven models are

obtained based on the analysis above and are listed in Table

1. It appears that the models involving hydrogen liquefaction

(models 4, 5, 6, and 7) cost more than the models involving

hydrogen production (models 1, 2, and 3). This is due to

higher capital cost of liquefaction units. The operating and

maintenance cost is higher in model 3 compared to other

models due to the use of high-temperature electrolysis unit.

Electrical costs are close to each other for the first five models

while it is somewhat less in model 6 and considerably less in

model 7.

4. Results and discussion

We consider a liquid geothermal resource and investigate all

seven cases using both the reversible and irreversible opera-

tions. Fig. 4 gives the effect of geothermal water temperature

on hydrogen cost for reversible and irreversible operations of

case 1. As the temperature of geothermal resource increases,

the cost of hydrogen decreases for both reversible and irre-

versible operations. This is due to higher thermal efficiency of

power generation with increasing geothermal source

temperature. At a geothermalwater temperature of 200 �C, thecost of hydrogen is 1.961 $/kg H2 in the reversible case (Fig. 4a)

and 2.615 $/kg H2 in the irreversible case (Fig. 4b). At

a geothermal water temperature of 240 �C, the costs are

1.336 $/kg H2 and 1.667 $/kg H2 in the reversible and irre-

versible cases, respectively.

Fig. 5 gives the effect of geothermal temperature on the

hydrogen cost for case 2. The trends are similar to those

obtained for case 1. At a temperature of 200 �C, the cost of

hydrogen is 1.857 $/kg H2 and 2.38 $/kg H2 for the ideal and

actual cases, respectively. The corresponding values at 240 �Care about 15% lower than those at 200 �C.

140 160 180 200 220 240 260 280 3000.5

1.0

1.5

2.0

2.5

3.0

3.5y p

rod,

liq,H

2 ($/

kg H

2)

TGeothermal (°C)

a

140 160 180 200 220 240 260 280 3000

1

2

3

4

5

6

7

8

y pro

d,liq

,H2 (

$/kg

H2)

TGeothermal (°C)

b

Fig. 10 e Variation of hydrogen production and liquefaction

cost with geothermal water temperature in Case 7. (a)

reversible operation. (b) irreversible operation.

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 en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 9 2067

Fig. 6 gives the results of the study in case 3 for which

high-temperature electrolysis is used. The trends are similar

to those obtained for case 1 and case 2. At a temperature of

200 �C, the cost of hydrogen is 1.378 $/kg H2 in the reversible

case, and 1.413 $/kg H2 in the irreversible case. The cost of

hydrogen liquefaction as a function of geothermal water

temperature is given in Fig. 7 for both reversible and irre-

versible operations of Case 4. At a geothermal water

temperature of 200 �C, the cost is 0.1994 $/kg H2 in the

reversible case and 1.169 $/kg H2 in the irreversible case. The

analysis done for Case 5 and Case 6 show that hydrogen cost

decreases with increasing geothermal water temperature

(Figs. 8 and 9).

Fig. 10 gives the cost of hydrogen produced and liquefied

as a function of geothermal water temperature for both

reversible and irreversible operations of case 7. At a temper-

ature of 200 �C, the cost is 1.798 $/kg H2 in the reversible case

and 2.173 $/kg H2 in the irreversible case. The costs are

1.253 $/kg H2 in the reversible case and 1.385 $/kg H2 in the

irreversible case at a geothermal temperature of 240 �C. Thesevalues are higher compared to case 3 and lower compared to

cases 1 and 2.

5. Conclusions

The results show that at a geothermal water temperature of

200 �C, the cost of hydrogen production and liquefaction

ranges between 0.979 $/kg H2 and 2.615 $/kg H2 depending on

the model considered. Case 2 performs better than case 1

because we are able to use the waste geothermal water for

preheating of water for electrolysis. This waste water that is at

a relatively high temperature is reinjected back to the ground

in case 1. Case 3 performs better than case 2 because of the

advantages associated with the high-temperature electrol-

ysis. Providing a considerable amount of energy demand in

the form of heat decreases the electricity demand. This, in

turn, decreases the total energy demand in the process.

The analyses of the liquefaction processes in cases 4, 5, and

6 indicate that geothermal absorption cooling for pre-cooling

of hydrogen gas yields significant savings in the work

requirement for hydrogen liquefaction, and is more advanta-

geous than using geothermal work output alone in the lique-

faction cycle. Case 7 allows both the production and

liquefaction of hydrogen at the same site by the same

resource. This also means that the storage of hydrogen is

automatically taken care of on the production site. This may

be advantages for remote geothermal resources where the

transportation of geothermal water to the residential and

commercial sites for heating/cooling purposes is not feasible

for thermodynamic and economic reasons. The total cost

analysis of the seven models reveal that the models involving

hydrogen liquefaction (models 4, 5, 6, and 7) cost more than

those involving hydrogen production (models 1, 2, and 3) due

to the higher capital cost of the liquefaction units. Also, the

greater the number of installations, the greater the capital and

operating and maintenance costs.

Acknowledgment

This study is sponsored by the Scientific and Technological

Research Council of Turkey (TUBITAK) with project number

108M226. This support is greatly appreciated.

Nomenclature

cp specific heat, kJ/kg K

C cost, $/kWh

CO&M operating andmaintenance cost of liquefaction unit,

$/kg H2

CC capital cost, $/kW

COP coefficient of performance

EC electricity cost, $/kWh

f fraction

DG Gibbs function, kJ/kmol

h specific enthalpy, kJ/kg

H enthalpy per unit mole, kJ/kmol

IC capital cost, $/kWh

JV junk value, $/kg H2

i interest rate_m mass flow rate, kg/h

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 0 5 8e2 0 6 92068

M molar mass, kg/kmol

MO&M operating and maintenance cost of electrolyzer unit,

$/kWh

n life time, year

P pressure, kPa

P plant capacity, MW

R gas constant, kJ/kg K

RO&M operating and maintenance cost of absorbtion unit,

$/kg H2

Q heat per unit mole, kJ/kmol

s specific entropy, kJ/kg K

S entropy per unit mole, kJ/kmol K

T temperature, �C or K

w work per unit mass, kJ/kg

w work per unit mole, kJ/kmol_W power, kW

y mass liquefied per unit mass of geothermal water

yprod;liq;H2mass of H2 produced and liquefied/mass of

geothermal water used

Greek

h efficiency

b the ratio of electrolysis work to the liquefactionwork

Subscripts

act actual

ARS absorption refrigeration system

comp compressor

elec electrolysis

f saturated liquid state

g saturated vapor state

geo geothermal

H2 hydrogen

H2O water

in inlet state

liq liquefaction

out outlet state

O2 oxygen

prod production

rev reversible

s source

0 ambient

1,2,3. state numbers

II second-law

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