International Journal of Hydrogen Energy 30 (2005) 867–877

www.elsevier.com/locate/ijhydene

Net energy analysis of hydrogen storage options

Arindam Sarkar, Rangan Banerjee∗Energy Systems Engineering, Indian Institute of Technology Bombay, Mumbai, Maharashtra 400076, India

Available online 8 December 2004

Abstract

Hydrogen storage is critical for developing viable hydrogen vehicles. This paper compares compressed hydrogen, cryogenichydrogen and metal hydride (Mg and FeTi) options using net energy analysis. A simulation of an Indian vehicle with an urbandrive cycle using a fuel cell stack is carried out to determine the total hydrogen required per km of travel.

Net energy analysis is carried out considering the energy requirements of the storage device and the energy required toproduce and store the hydrogen. From net energy analysis compressed hydrogen is the preferred option. The direct energyrequirement is more than 55% for magnesium hydride as compared to compressed hydrogen due to the combined effect ofincrease in weight and higher heat of desorption.

In addition to volumetric and gravimetric storage density, it is felt that net energy analysis should be also included as anadditional criteria for evaluating any storage option. For metal hydride storage the net energy required to produce the tankshould be minimum. This could be used as a selection criterion to design an optimum metal hydride storage. The performanceof other materials like porous carbon, carbon nanotubes and hybrids can be evaluated using net energy analysis.� 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords:Net energy; Hydrogen storage

1. Introduction

Hydrogen is considered as a potential fuel for the futureby the automobile industry[1]. For successful applicationin automobiles storage of hydrogen is a critical issue. Thethree main options that have emerged are compressed hy-drogen storage, cryogenic storage and solid storage namelyhydrides. All these options have been tested by several auto-mobile companies[1]. In the long term, it is unclear whichof these options would be viable for large scale productionand use.

Ananthachar and Duffy[2] have compared fuel cell vehi-cles with different types of storage and found that onboardcompressed hydrogen storage option is the most energy

∗ Corresponding author. Tel.: +91 22 2576 7883;fax: +91 22 2572 6875.

E-mail address:rangan@me.iitb.ac.in(R. Banerjee).

0360-3199/$30.00� 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2004.10.021

efficient. However, the effect of drive cycle and the weightof different tanks on the hydrogen consumption was not con-sidered. Joshi[3] has done a life cycle analysis of steel andplastic gasoline storage tanks but not for hydrogen storage.Neelis et al.[4] performed detailed energy analysis of dif-ferent hydrogen production routes, transportation and stor-age options and found compressed hydrogen to be most themost energetically efficient.

Besides the energy that is required in the form of hydrogenfor propulsion, additional energy is required to store thehydrogen and manufacture the storage system. Net energy orlife cycle energy analysis can be used to evaluate the energythat has gone into making the tank, producing hydrogen andstoring it.

In most conventional analysis for design of hydrogenstorage systems the gravimetric and volumetric storagedensities are used. In many of these cases the economics isstill emerging. In this scenario net energy analysis will pro-vide an additional tool to evaluate the storage options and

868 A. Sarkar, R. Banerjee / International Journal of Hydrogen Energy 30 (2005) 867–877

Nomenclature

mH2 mass flow rate of hydrogen, kg/s� efficiencyF Faraday’s constant, 96485.34 sA/mol� density of airAf frontal areacd coefficient of aerodynamic dragcrr coefficient of rolling resistanceF force, Nf acceleration, m/s2

g acceleration due to gravity, 9.81 m/s2

hlhv lower heating value of hydrogen, 120 MJ/kgi current, Am mass, kgMH2 molecular mass of hydrogen, 2 gm/molP power, Wt time, su(t) speed as function of time, m/sV voltage, V

recommend potentially viable one. For example it can beparticularly useful in selecting the best metal hydride amongthe different combinations that have been synthesized. Thispaper assesses the net energy usage of the three differenttypes of tanks with two types of metal hydride.

The problem is defined as determining the energy (bothdirect and indirect) required to drive 1 km using the abovetanks in a small passenger vehicle. The direct energy is dueto the hydrogen and the indirect energy is the energy used toproduce and store the hydrogen and manufacture the tank.

To find the mass of hydrogen required, a base case vehiclehas been modelled in MATLAB/ SIMULINK. The powersource has been assumed to be a fuel cell stack that hasbeen modelled separately and integrated within the vehicle.The hydrogen consumption for the three storage options hasbeen found by adding a dead weight equivalent to the tankmass to the vehicle.

2. Modelling of small base case vehicle

Generally two approaches can be used to model the per-formance of a vehicle[5]. They are backward facing ap-proach and forward facing approach. The approaches differin the way the power requested from the power source isestimated.

In the backward facing approach, the performance of thecomponents are evaluated assuming that the vehicle meetsthe desired trace. In this approach force and power is calcu-lated for a given drive cycle. The operation moves againstthe tractive power flow.

The forward facing approach considers the driver com-mand signals. The present speed and the speed required tomeet the trace is calculated and a signal is issued to thepower source. The power source then delivers the power tothe wheels through the transmission. Forward facing algo-rithm tries to simulate the true power and the control signalsand not the requested power.

To model the base case vehicle the backward facing al-gorithm has been chosen due to its simplicity. The powerflow diagram is shown inFig. 1.

Fule Cell Stack Power Conditioning unit

Tractive power flow

Drive Cycle Wheels Motor

Fig. 1. Power flow diagram.

The drive cycle chosen is the Indian urban drive cyclewhich has been extracted from the ADVISOR database[5,6](seeFig. 2). The Indian drive cycle is characterized by lowaverage speed(23.40 km/h) and rapid accelerations (1.73to −2.10 m/s2) as compared to the European urban drivecycle with average speed of 62.44 km/h and accelerationsfrom 0.83 to−1.39 m/s2) [5,6].

After differentiating the drive cycle speed with respect totime, the accelerationf was determined as

f = d(u(t))

dt. (1)

The vehicle was assumed to be plying on a straight roadwith a zero gradient. Three forces were assumed to be actingon the vehicle. They are

(1) Aerodynamic drag (FAerodynamic) given by 0.5�

AFu(t)2cd

(2) Rolling resistance (FFriction) given bymtotalgcrr(3) Inertial force (FInertial) given bymtotalf

mtotal = mglider + mtransmission+ mmotor

+ mf cell stack+ mcargo. (2)

It has been further assumed that no regenerative braking isused. The net force that is acting on the vehicle is the sum ofall the above forces. The wheel power can be evaluated as

Pwheel(t) = Ftotal(t)u(t), (3)

A. Sarkar, R. Banerjee / International Journal of Hydrogen Energy 30 (2005) 867–877 869

0 500 1000 1500 2000 2500 30000

2

4

6

8

10

12

14

16

18

Spe

ed (

m/s

)

Time (s)

Fig. 2. Indian urban drive cycle[6].

0 500 1000 1500 2000 2500 30000

0.5

1

1.5

2

2.5

Pow

er (

kW)

Time (s)

Fig. 3. Power at wheels.

where

Ftotal = FAerodynamic+ FFriction + FInertial. (4)

The vehicle data used for calculating the power requestedfrom the fuel cell stack is shown inTable 1.

Fig. 3 shows the power required at the wheels as a func-tion of time. The maximum power requirement at wheels

is 23 kW. The maximum power requested from the fuelcell stack was determined by considering the transmission,motor and DC/AC convertor losses and was found out tobe 27 kW (seeTable 2). A 40 hp (sufficient surplus ca-pacity motor has been chosen so that the extra power re-quired on addition of tanks can be supplied) induction mo-tor has been chosen from the database of MotorMaster[7,8].

870 A. Sarkar, R. Banerjee / International Journal of Hydrogen Energy 30 (2005) 867–877

0 5 10 15 20 25 300.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Vol

tage

(V

olts

)

Current (Amps)

Fig. 4. Voltage current relationship for a single cell.

Table 1Vehicle data[6]a

Vehicle glider mass(mglider) 600 kgVehicle Cargo weight(mcargo) 150 kgMotor mass(mmotor) 186.88 kgEfficiency of DC/AC converter(�dc/ac) 95%Mass of transmissionb (mtrans) 50 kgTransmission efficiencyc (�trans) 91%Auxiliary consumption 600 WCoefficient of aerodynamic drag (cd) 0.335Coefficient of rolling resistance (crr) 0.009Frontal area (Af ) 2 m2

aThe vehicle is similar to Maruti 800 car, a popular car inIndia, made by Maruti Udyog Limited that uses a 27 kW engine(http://maruti800.marutiudyog.com/specs.asp ).

bThe mass of transmission has been taken to that of Taurustransmission.

cAssumed constant.

3. Modelling of fuel cell

Amphlett et al.[9,10] has developed a model based onsemi-empirical techniques to model a Ballard Mark IV pro-ton exchange membrane fuel cell (PEMFC). The model hasbeen chosen for its good agreement with the experimentalresults. A single Ballard Mark IV cell has been modelledusing the above model. The cell operating conditions areshown inTable 2.

The fuel utilization ratio (moles of hydrogen consumedper mole of hydrogen input to the fuel cell) (1/SR) forhydrogen was taken to be 0.578[9]. With a lower heating

Table 2Fuel cell operating conditions

Cell operating temperaturea 80◦CTotal pressure inside flow channel 1.0 barH2 Pressure 1.0 barO2 Pressure 0.21 bar

(Air is supplied)

aThe cell is assumed to operate at a constant temperature andthe losses due to convection and radiation have been neglected.

value of 120 MJ/kg for hydrogen, the mass flow rate and theefficiency is determined as[11].

�f cell =V × i

hlhvmH2

, (5)

mH2 = i × MH2

2 × F× SR. (6)

The voltage current characteristics is shown inFig. 4. Thevariation of mass flow rate of hydrogen with power is shownin Fig. 5.

4. Integration of fuel cell with the vehicle

A fuel cell stack consisting of 2000 individual cells hav-ing similar characteristics as the individual cell modelledwas chosen. The maximum power output for the stack was33 kW (a higher power rated fuel cell stack has been delib-erately chosen because the effect of addition of weight has

A. Sarkar, R. Banerjee / International Journal of Hydrogen Energy 30 (2005) 867–877 871

0 5 10 150

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Flo

w r

ate

of h

ydro

gen

(gm

/s)

Power (W)

Fig. 5. Hydrogen consumption rate vs. power.

0 500 1000 1500 2000 2500 30000

0.02

0.04

0.06

0.08

0.1

0.12

Cum

ulat

ive

Hyd

roge

n co

nsum

ptio

n (k

g)

Time (s)

Fig. 6. Cumulative hydrogen consumption without tank.

still not been determined). As the weight increases the powerrequirement increases. A weight penalty of 3.8 kg/kW forthe fuel cell was assumed[11]. After integrating the fuel cellmodel within the vehicle model developed earlier the con-sumption of hydrogen has been obtained. The net consump-

tion of hydrogen is determined by integrating with respectto time.Fig. 6 shows the net consumption profile. This fig-ure shows the performance without the storage tanks. Theaddition of the tank increases the dead weight and the hy-drogen consumption rises.

872 A. Sarkar, R. Banerjee / International Journal of Hydrogen Energy 30 (2005) 867–877

505

672 Carbon Fibre & Resin

Aluminium

Fig. 7. Schematic of the compressed hydrogen storage tank (dimensions in mm).

Table 3Composition of the compressed hydrogen tank

Aluminuma 16.05 kgCarbon fiberb 26.68 kgResinc 11.42 kg

Total 56.91 kg

aIt has been assumed that the end caps are oblate with majorto minor axis ratio of 1.5:1. The liner thickness is 0.241 cms atthe cylindrical portion but thickens by a factor of 3.5 times at thedome of the end-caps.

bThe mass of carbon fiber wrapped is assumed to be proportionalto the surface area.

cThe mass of resin has been assumed to be proportional to themass of carbon fibers wrapped.

5. Compressed hydrogen tank

Honda has demonstrated the usage of compressed hydro-gen tank for its fuel cell vehicle FCX. The fuel capacityis 3.75 kg of hydrogen and the pressure inside the tank is34.5 MPa[12]. This capacity has been chosen as a basis forall the storage options.

5.1. Description of the tank

A thin metal lined carbon fiber tank has been chosen asthe base case[13]. The tank has been scaled geometricallymaintaining a constantl/r ratio to hold 3.75 kg of hydrogen.The schematic is shown inFig. 7. The composition of thetank has been evaluated and is shown inTable 3. The massof solenoid valves and pressure control devices have beenneglected.

After the addition of a dead weight equivalent to that ofthe compressed hydrogen tank the simulation results in hy-

Table 4Energy required to produce the compressed hydrogen tank

Material Mass Energy required Total (GJ)(MJ/kg)

Aluminuma 16.05 220.0 3.53Carbon Fiberb 26.68 193.18 5.15Resin (polyamide)c 11.42 137.6 1.57

Total 10.25

aThe energy value is for a main production process only and norecycling has been assumed[24].

bIt has been assumed that carbon fibers have been produced bythermal carbonization of acrylonitrile at 100◦C, and the net energyof acrylonitrile production is 92.43 MJ/kg[25,26].

cThe resin has been assumed to be of the polyamide type withnet production energy of 137.6 MJ/kg[27].

drogen consumption of 0.11 kg per 17.49 km. The cumula-tive consumption plot is shown inFig. 10.

5.2. Net energy analysis for the tank

From Table 3it can be seen that the main componentsof the tank are aluminum, carbon fiber and resin. The netenergy required to produce 1 kg each of aluminum, carbonfiber and resin is shown inTable 4.

The tank is expected to last 10,000 cycles[13]. As perthe simulation a full tank containing 3.75 kg of hydrogenwill provide a range of 600 km. Consequently the tank willlast 6,000,000 km. However, in India the average servicelife of a passenger car is about 20 years with 15,000 km ofannual travel[14]. This implies that for an Indian scenariothe vehicle will dictate the lifespan of the tank which is300,000 km. So, for 1 km of travel the energy used in form

A. Sarkar, R. Banerjee / International Journal of Hydrogen Energy 30 (2005) 867–877 873

753.7

295

Aluminium

Multi Layer Insulation

Aluminium

Fig. 8. Schematic of the cryogenic hydrogen tank (dimensions in mm).

of the energy content of the tank can be estimated at 34.2 kJ(4.6% of the direct energy required).

One of the most common methods of producing hydro-gen is by natural gas reforming. Spath and Margaret[15]reported a detailed life cycle analysis for hydrogen produc-tion via natural gas steam reforming and obtained a value of183.2 MJ/kg of hydrogen. The theoretical energy requiredfor steam reforming and shift reaction is 98.7 MJ/kg of hy-drogen[16]. If it is assumed that the hydrogen coming outof the pressure swing adsorption unit is at 2.5 MPa, thenthe electrical energy required to compress the hydrogen to34.5 MPa is approximately 18.6 MJ/kg.1

Thus the net energy requirement to produce and compress1 kg of hydrogen is 202 MJ/kg. For a compressed hydrogentank travelling 1 km the total energy required is the sum ofthe direct energy and the indirect energy, which is 2043.9 kJ(direct energy is 749 kJ, indirect energy is 1294.9 kJ).

6. Cryogenic hydrogen tank

General Motors among others have demonstrated the useof liquid hydrogen[17]. A representative tank based on LosAlamos/DFVLR design has been chosen for this study[13].

6.1. Description of the tank

The tank has been modified to store 3.75 kg of hydrogenusing the same scheme as in the case of compressed hydro-gen tank.2 The schematic is shown inFig. 8.

1 It has been assumed that hydrogen is coming out of the PSAunit at 27◦C and the process is assumed to be adiabatic (workdone by the compressor= 4.866 MJ/kg) with the efficiency of thecompressor equal to 80% and the thermal to electrical conversionefficiency of 33%.

2 The tank is an aluminum cylinder with elliptical end-caps.The major to minor axis ratio is 3:1. The thickness of inner dewaris 2 mm and the outer dome is 3 mm. The tank holds 3.75 kg (thedensity of liquid hydrogen at 20 K, 1 atm is 71.086 kg/m3) ofhydrogen at 20 K.

Table 5Composition of the cryogenic tank

Aluminuma 15.4 kgPolyesterb 7.5 kg

Total 22.9 kg

aThe surface area of the two shells were evaluated and it wasassumed that the volume of the aluminum is surface area timesthickness, as the thickness is small compared to the tank dimen-sions. The thickness of both the shells and the thickness of MLI(multilayer insulation) is same as the original tank.

bIt has been assumed that the structure of MLI is similar to aspublished by Matsuda[28]. There are 138 layers and the mass ofeach layer is 50 g/m2. The mass of the aluminum shield has beenneglected due to negligible contribution to the overall weight andthe spacer material has been assumed to be polyester.

The composition of the tank has been shown inTable5. The mass of heat exchangers, solenoid valve has beenneglected.

The cryogenic tank will require some additional power tochange the phase of hydrogen and to heat it to ambient con-ditions. The latent heat of vaporization of liquid hydrogen at20 K is 450 kJ/kg and the specific heat of hydrogen has beenassumed to take a constant average value of 13.68 kJ/kg/K[16]. The simulation has been modified to incorporate theadditional heat required (given by the sum of latent heat ofvaporization and the heat required to reach ambient condi-tions).

On addition of a dead weight equivalent to that of thecryogenic hydrogen tank the simulation results in hydrogenconsumption of 0.112 kg per 17.49 km. The cumulative con-sumption plot is shown inFig. 10.

6.2. Net energy analysis for the tank

The main components of the tank are aluminum andpolyester (Table5). The net energy required to produce 1 kgeach of aluminum and polyester is shown inTable 6.

874 A. Sarkar, R. Banerjee / International Journal of Hydrogen Energy 30 (2005) 867–877

Table 6Energy required to produce the tank

Material Mass (kg) Energy required Total (GJ)(MJ/kg)

Aluminum 15.4 220.0 3.4Polyestera 7.5 170.09 1.275

Total 4.675

aBoustead and Hancock[26].

The liquification of hydrogen is an energy intensive pro-cess. Baker and Shaner have reported the energy consump-tion of a large scale hydrogen liquification plant[18]. Theelectrical energy consumption is 39.0 MJ/kg. Thus the to-tal primary energy requirement is 156.1 MJ/kg (a thermalto electrical conversion ratio of 33%). Thus the net energyrequired to produce hydrogen from natural gas and liquifyit is 339.3 MJ/kg.

For cryogenic hydrogen storage tanks at Lawrence Liv-ermore National Laboratory tests confirm a life of about5000 cycles[19]. Here again the vehicle life will decidethe life of the tank. The vehicle life has been taken to be300,000 km. For the tank this implies an embodied energyusage of 15.6 kJ/km (2% of the direct energy).

For 1 km of travel the direct energy required is 768 kJ andthe indirect energy is 2187.6 kJ.

7. Metal hydride tank

Metal hydrides are among the several solid storage op-tions those which store hydrogen by either adsorption or achemical reaction. Currently metal hydrides have been com-mercialized and hydride tanks are commercially available.Daimler Benz, Honda and Mazda are among the severalcar companies to have tested metal hydrides for vehicularpropulsion. Daimler Benz has demonstrated a combinationof low temperature (FeTi) and high temperature hydride(Mg2Ni) tanks to store hydrogen[20].

The uptake capacity of hydrogen in the case of hydridesis dependent on temperature, pressure and alloy composi-tion. It is expected that the hydride tanks will be chargedat ambient conditions. Low temperature FeTi hydride hasbeen chosen as the base case hydride alloy that will be eval-uated for its embodied energy. Another metal that showsmuch higher hydrogen uptake capacity but at higher equilib-rium temperatures is Magnesium(Mg). It will also be eval-uated for its embodied energy.Table 7show some of therelevant properties of both hydrides. The reactions are asfollows:

1.08FeTiH0.1 + H2 ⇀↽ 1.08FeTiH1.95, (7)

Mg + H2 ⇀↽ MgH2. (8)

Table 7Properties of hydrides[29]

Hydride Maximum Dissociation plateau Heat of hydridecomposition H2wt% pressure (MPa) formation (�H )

(MJ/kgH2)

TiFeH1.95a 1.84 0.5–1.0(20◦C) −14.05

MgH2 7.60 0.1(284◦C) −37.25

aAll of the hydrogen is not recoverable. Strickland attained avalue of 1.6[21].

Table 8Composition of FeTi and Mg tank

FeTi tank Mg tank(kg) (kg)

Stainless steel 122.2 92.8Titanium 106.3Iron 124.0Magnesiuma 51.7

Total 352.5 144.5

aThough the theoretical capacity is 7.6 wt% not all the hydrogenis recovered. Pedersen et al. obtained a desorption of 88% of thetheoretical limit[30].

7.1. Description of the tank

A stationary storage tank developed by BNL (BrookhavenNational Laboratory) using FeTi has been used for mobileapplications with little modifications[21]. It has been scaleddown (the scheme used is the same as for other tanks) tohold 3.75 kg of hydrogen.3 The schematic of FeTi tank isshown inFig. 9. The composition of both the tank is shownin Table 8.

Hydrides need additional energy to release hydrogen. Theadditional energy has been assumed to be supplied by elec-tric heaters powered by the fuel cell stack.4 The simulationhas been suitably modified. The hydrogen consumption is0.1407 and 0.1697 kg to travel 17.49 km for FeTi and Mgtanks, respectively. The result of simulation is shown inFig.10.

7.2. Net energy analysis for the tanks

The two hydrides has been separately analyzed for theirnet energy analysis. Their material compositions have been

3 The thickness of steel pipe remains same. The mass of ac-cessories, mainly pipes, has been assumed to be proportional tolength.

4 Since the temperature required for desorption of magnesiumhydride is higher than the fuel cell temperature the exhaust heatof the fuel cell cannot be used. For comparision the exhaust heathas not been used for FeTi hydride also.

A. Sarkar, R. Banerjee / International Journal of Hydrogen Energy 30 (2005) 867–877 875

0.64

153.09

2582

Steel

FeTi

Fig. 9. Schematic of the hydride tank (dimensions in mm).

0 500 1000 1500 2000 2500 30000

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Time (s)

Cum

ulat

ive

hydr

ogen

con

sum

ptio

n (k

g)

Compressed hydrogenCryogenic hydrogenFeTi hydrideMg hydride

Fig. 10. Cumulative hydrogen consumption for various tanks.

determined earlier inTable 8. The embodied energy is shownin Tables 9and10.

The life of the hydride tank is considerably affected by theimpurities present in hydrogen. After 500 cycles a drop ofalmost 50% from the theoretical capacity has been observed[22]. But cycling with pure hydrogen magnesium hydridequickly reactivates it. FeTi hydride is severely affected byimpurities and is difficult to reactivate. Ahn et al.[23] noteda drop in hydrogen adsorption capacity of about 64% afteralmost 8000 cycles. The energy required to recharge hasbeen neglected and it has been assumed that the tanks survivethroughout the vehicle life.

The total energy required for a Mg tank is3001.5 kJ/km (direct energy is 1164 kJ/km, indirect energy is1837 kJ/km) and 2616.4 kJ/km (direct energy is965.4 kJ/km, indirect energy is 1651 kJ/km) for FeTi tank.

Table 9Energy required to produce the FeTi tank

Material Mass (kg) Energy required Total (GJ)(MJ/kg)

Stainless steela 122.2 40 4.8Titaniumb 106.3 409 43.5Ironc 124 40 4.9

Total 53.2

aWeiss et al.[24].bSchuckert et al.[31].cNo distinction has been made between steel and iron and the

amount of energy required to form the alloy and the hydride hasnot been taken into consideration.

876 A. Sarkar, R. Banerjee / International Journal of Hydrogen Energy 30 (2005) 867–877

Table 10Energy required to produce the Mg tank

Material Mass (kg) Energy required Total (GJ)(MJ/kg)

Stainless steel 92.8 40 3.7Magnesiuma 51.7 280 14.5

Total 18.2

aWeiss et al.[24].

Table 11Comparison of different storage options for 1 km ride

Compressed Cryogenic FeTi Mg hydridetank tank hydride

H2 consumption 6.24 6.4 8.04 9.7(gms)

Direct energyrequired to 749 768 965.4 1164travel (kJ)

[0 (Base) 19 216.4 415]a

Energyrequired to 1260.7 2172.7 1473.7 1777produce andstore H2 (kJ)

Energyrequired to 34.2 15.6 177.3 60producetank (kJ) [18.6 0 (Base) 161.7 44.4]b

Total energyrequired (kJ) 2043.9 2956.3 2616.4 3001.5

aThe numbers in bracket indicate the increase in direct energyrequirement from the base case which is compressed hydrogentank.

bThe numbers in bracket indicate the increase in energy require-ment to produce the tank from the base case which is cryogenictank.

8. Results

The results of the net energy analysis for the four tanksmentioned are shown inTable 11. Hydrogen consumption isfound to be the lowest for compressed storage tanks. Takingthis as the base case the effect of increase in weight canbe determined. The results show that hydrides consume themost hydrogen per km of travel. This is due to the combinedeffect of greater mass of the tank and the energy required todesorb hydrogen. The energy required to produce the tankis the lowest for the cryogenic hydrogen tank. This is takenas the base case to evaluate the effect of embodied energyin the tanks.

Table 12Comparison of different storage options against DOE targets

Parameters DOE Compressed Cryogenic FeTi Mgtargets tank tank tank tank

Mass of H2 perunit system mass 0.065 0.062 0.14 0.011 0.025(kg/kg)Mass of H2 perunit system volume 0.036 0.002 0.036 0.046 0.081(kg/l)

The four tanks can also be evaluated against Departmentof Energy (DOE, USA) targets. The results have been shownin Table 12. On a per unit volume basis, hydride tanks clearlyare way ahead of others. But on a per unit weight basis theyperform poorly. Cryogenic hydrogen fulfils both criteria.

9. Conclusions

(1) The energy embodied in hydride tanks is higher thanthe other two options. Magnesium which has shownthe highest hydrogen adsorption capacity among all hy-drides requires almost four times the energy that isrequired for a cryogenic storage tank. In the case ofiron–titanium hydride the situation is worse. The em-bodied energy of the cryogenic storage tank is the small-est among the options explored.

(2) Though the embodied energy for cryogenic hydrogentank is the lowest and the hydrogen consumption is alsolowest among the options explored, the energy requiredto store hydrogen is the highest. This is because of thehigh energy requirement of the hydrogen liquefactionprocess.

(3) Overall, among the existing technologies compressedhydrogen option seems to be the most favorable for longterm viability. The total energy required for compressedgas option is lowest.

(4) The effect of heat of desorption has also been captured.The hydrogen consumption of magnesium hydride tankis more than that of iron–titanium hydride though themass of tank is lower. This is due to the higher heat ofdesorption of magnesium hydride.

(5) It is recommended that net energy analysis be used as anadditional selection criterion for storage option. Differ-ent metal hydrides or metal hydride combinations canalso be evaluated on this basis. This can be further ex-tended to evaluate other solid storage options like car-bon nanotubes, graphitic nanofibers, carbon nanobeads,etc.

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