Cryo Ads Sys

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System simulation models for on-board hydrogenstorage systems

Sudarshan Kumar a,*, Mandhapati Raju a,b, V. Senthil Kumar c

aChemical Sciences and Materials Systems Lab, General Motors Global R&D, Warren, MI 48090, USAbOptimal CAE Inc., Plymouth, MI 48170, USAc India Science Laboratory, General Motors Global R&D, Creator Building, International Technology Park, Bangalore 560066, India

a r t i c l e i n f o

Article history:

Received 22 November 2010

Received in revised form

8 April 2011

Accepted 21 April 2011

Available online 12 June 2011

Keywords:

Hydrogen storage

Cryo-adsorption

Sodium alanate

System simulation models

a b s t r a c t

System simulation models for automotive on-board hydrogen storage systems provide

a measure of the ability of an engineered system and storage media to meet system

performance targets. Thoughtful engineering design for a particular storage media can

help the system achieve desired performance goals. This paper presents system simulation

models for two different advanced hydrogen storage technologies e a cryo-adsorption

system and a metal hydride system. AX-21 superactivated carbon and sodium alanate are

employed as representative storage media for the cryo-adsorbent system and the metal

hydride system respectively. Lumped parameter models incorporating guidance from

detailed transport models are employed in building the system simulation models.

Simulation results to test the storage systems’ ability to meet fuel cell demand for

different drive cycles and varying operating conditions are presented. Systems are engi-

neered to provide the ability to refuel a vehicle in a short time guided by DOE targets.

Gravimetric and volumetric hydrogen densities are computed for the engineered systems

and compared to the DOE system goals.

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

reserved.

1. Introduction

System level models for two hydrogen storage systems were

developed as part of a DOE funded project for evaluating the

performance of cutting-edge hydrogen storage technologies.

This paper presents the system level performance of the two

systems e metal hydride and cryo-adsorbent systems. The

system design and dynamic performance of the two systems

is presented along with a brief literature review for each of

these systems. The performance of the two systems is

compared with respect to the DOE targets. The drive cycle

simulations are tested on an integrated vehicle level model

framework. This vehicle level model framework [1] consists of

three primary modules e a vehicle level module, a fuel cell

module and a storage system module. Different storage

system models can be used in this integrated framework for

evaluation on a consistent basis. The fuel cell model used in

this framework is adapted from the fuel cell modeling work

of Pukrushpan et al [2]. and the vehicle level model is an

Excel based model integrated into the vehicle level module

of the framework.

In this paper we report on the system level models and

system simulations using this integrated framework for two

separate systems e a cryo-adsorbent system using the acti-

vated carbon AX-21 and ametal hydride system using sodium

alanate. Both systems are designed to carry w 5 kg of usable

* Corresponding author. Tel.: þ1 586 986 1614; fax: þ1 586 986 1910.E-mail address: [email protected] (S. Kumar).

Avai lab le at www.sc iencedi rect .com

journa l homepage : www.e lsev ie r . com/ loca te /he

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 8 6 2e2 8 7 3

0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2011.04.182


hydrogen. System level dynamics during drive cycle simula-

tions are presented for the two systems and the ability of the

storage system to deliver the required amount of hydrogen

demanded by the fuel cell during drive cycle simulations is

examined under different conditions. Finally, performance of

these two systems in meeting the DOE gravimetric and volu-

metric energy density targets is evaluated.

2. AX-21 based cryo-adsorbent storagesystem

On-board storage of hydrogen by adsorption at low tempera-

tures and moderately high pressures (77K, up to 60 bar) is

considered viable and competitive with other storage tech-

nologies including liquid hydrogen, compressed gas, and

metallic or complex hydrides [3]. At these conditions,

superactivated carbons like AX-21 offer good gravimetric

capacity and fast and reversible kinetics. For example, AX-21

has a reversible hydrogen storage capacity of about 5.8 wt%

at 77 K and 35 bar [4]. AX-21 as an adsorbent material has

been studied extensively and has been considered to assess

the tank performance in previous studies [5e7]. An allied

technology is the cryo-compressed storage, wherein

hydrogen is stored inside a pressure vessel at 250e360 atm

pressures and 50e300 K temperatures, without any

adsorbent material [8,9].

Consider a fuel-tank with an initial operating condition of

35 bar and 80 K, and a fuel cell operating at 3 bar. The four

processes occurring in a cryo-adsorber fuel-tank are refueling,

discharge, dormancy, and venting. These fuel-tank processes

occur over different time scales: refueling over a fewminutes,

discharge over a few hours, dormancy over a few days, and

venting over a fewweeks. In our previous studies [10,11] it was

shown that refueling, the fastest process is quasi-static i.e.

local equilibrium conditions prevail. Hence, the slower

processes are also quasi-static.When themolecular processes

are fast, slow processes are expected to have negligible

internal gradients and are generally amenable for a lumped

parameter analysis. Hence, a quasi-static lumped parameter

model for the cryo-adsorber fuel-tank was developed in [10].

That model is used in the current work to study the drive

cycle discharge simulations for a cryo-adsorption hydrogen

storage tank. However, during a drive cycle discharge, the

hydrogen demand fluctuates rapidly. Therefore, in this

work, the quasi-static approximation is relaxed and an

adsorption kinetic model is developed and employed.

During discharge hydrogen is desorbed from the adsorbent

bed. Since desorption is an endothermic process, we need to

addheat during discharge to avoid very low tank temperatures

and maintain fast desorption kinetics. Heat can be added into

the tank by heating a part of the recirculating gas, as shown in

Fig. 1. Since the gas is in intimate contact with the bed, this

mode of heating is expected to be efficient. An alternate way

of adding the heat is through the use of a jacketed or

embedded electrical heater. Although such external heating

will be an electrical load penalty on the fuel cell, it might be

beneficial in terms of gravimetric/volumetric capacities of

the system, since hot gas recirculation loop, along with the

recirculation pump etc., are expected to be bulkier and

heavier than this alternative option. At the level of lumped

parameter description, hot gas recirculation and electrical

heater are mathematically equivalent. Hence, the lumped

parameter adsorption kinetic model developed in the current

work can be used to include hot gas circulation or external

heating during discharge for on-board implementation.

2.1. Adsorption system model development

This model uses the mass balance, energy balance and

adsorption kinetics to develop the time evolution of pressure,

temperature and adsorbate concentration. The hydrogen

content in the bed at any time is the sum of the gaseous and

adsorbed hydrogen i.e. mH2 ðtÞ ¼ msqðtÞ þ Vb3trgðT;PÞ:

2.1.1. Transient mass balanceThe rate of change of hydrogen content of the tank balances

the net flow into the tank. Hence the transient mass balance

for hydrogen is given by

dmH2

dt¼ _mf � _m (1)

i:e:msdqdt

þ Vb3tdrgdt

¼ _mf � _m: (2)

The time derivative of density is expressed in terms of the

temperature and pressure time derivatives as drg=dt ¼�rgaPgdT=dtþ rgkTgdP=dt. Using this result the transient mass

balance simplifies to

A11dTdt

þA12dPdt

þ A13dqdt

¼ B1; (3)

where A11 ¼ �Vb3trgaPg, A12 ¼ Vb3trgkTg, A13 ¼ ms, and

B1 ¼ _mf � _m.

2.1.2. Transient energy balanceThe thermal masses associated with the fuel-tank are the gas

phase, adsorbed phase, adsorbent, pressure vessel including

the bed restrainers and other bed internals, insulation layer,

outer shell and ambient, as shown in Fig. 2. The insulation

layer isolates the inner thermal masses (gas, adsorbed

phase, adsorbent, and pressure vessel) from the outer ones

(shell and ambient). The transient energy balance for the

Cryo bed

Radiator

AirFuel Cell

H2

H2Anode

Cathode

Coolant

,T

, T

Qh

,Tf

: Net H2 output to fuel-cell

om

fm

fm

om

om

Fig. 1 e Schematic of a cryo-adsorber bed with hot gas

recirculation.

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 8 6 2e2 8 7 3 2863


inner thermal masses (‘system’), with the assumption of

a constant average heat of adsorption is

mwdHw

dtþms

dHs

dtþ�msqþ Vb3trg

� dHg

dtþmsDHa

dqdt

� Vb3tdPdt

¼ _Qh þ _Ql: (4)

The time derivative of gas enthalpy is written in terms of

temperature and pressure time derivatives as dHg=dt ¼CPgdT=dtþ vgð1� aPgTÞdP=dt. Similarly, for the solid phases

(pressure vessel and adsorbent), neglecting the thermal

expansion of the material, gives dHw=dt ¼ CPwdT=dtþ vwdP=dt

and dHs=dt ¼ CPsdT=dtþ vsdP=dt. Using these equations and

rearranging the transient energy balance simplifies to

A21dTdt

þA22dPdt

þ A23dqdt

¼ B2; (5)

where

A21 ¼ mwCpw þmsCps þ ðmsqþ Vb3trgÞCPg,

A22 ¼ mwvw þmsvs þ ðmsqvg þ Vb3tÞð1� aPgTÞ � Vb3t

and A23 ¼ msDHa ,and B2 ¼ _Qh þ _Ql.

The assumed heat leak has the form _Ql ¼ ðTN � TÞ=Reff . A

typical value of Reff ¼ 74:0K=W is used so that the heat leak

into the tank during typical dormancy conditions is about 3

Watts, as in [10].

2.1.3. Adsorption kineticsIn our earlier study [10], the quasi-static kinetics i.e.

qðtÞzq�½TðtÞ;PðtÞ� was used. A constant discharge is typically

quasi-static [10]. Hence, actual desorption kinetics is not very

important for constant discharge case. However, in a real

drive cycle hydrogen demand variations occur at time scales

of a second or less and quasi-static conditions may not

prevail. Therefore, in this paper we use the Linear Driving

Force (LDF) model with Glueckauf’s approximation [12,13]:

dqdt

¼ 15Da

R2ðq� � qÞ (6)

Sircar and Hufton [13] show that the LDF model can be

used to capture adsorption transients. We have used a

representative value of Da=R2z1:52� 10�2 s�1 for activated

carbon at near liquid nitrogen temperature [14] and also

note that the adsorbate diffusivities could vary with

temperature as in [15].

A Simulink�modelwasdeveloped to compute the transient

temperature, pressure and weight fraction by solving the

mass balance, energy balance and the adsorption kinetics. The

property correlations used in this model are functions of

temperature and pressure, described in [16]. The bulk density

and the skeletal density of AX-21 are taken as 0.27 g/cc

and 2.2 g/cc respectively [17]. The heat of adsorption is taken

as 6.0 kJ/mol [17]. The vessel material is assumed to

be aluminum; properties of aluminum at cryogenic

temperatures are taken from Marquardt and Radebaugh [18].

2.2. Drive cycle simulations

In general, there is no unique relationship between the

amount of hydrogen discharged and heat input because it is

possible to discharge a particular amount of hydrogen with or

without heating the bed. However, in order to remove most of

the hydrogen within the storage system with the final pres-

sure above the fuel cell pressure, it is necessary to heat up the

adsorbent material. It is possible to assume a constant heat

input or time varying heat input proportional to the hydrogen

demand depending on the scenario being studied. In this

paper, we assume a constant heat input and performed

simulations for both the FTP75 and US06 drive cycles [19]. The

drive cycle simulations are performed for a compact vehicle in

the vehicle level framework developed by the HSECoE team.

Fig. 3 shows the vehicle speed and fuel consumption rate

(g/s) for the two drive cycles.

The FTP75 cycle is amild & short duration cycle consuming

only 159.36 g of H2 in 1874 s (or 31.25 min), with an average

hydrogen demand of 0.085 g/s. Assuming that a significant

fraction of the discharged hydrogen is desorbed from the

adsorbed phase, the added heat must supply the heat of

adsorption of the desorbed hydrogen. For AX-21 with an

average heat of adsorption of 3.0 � 106 J/kg, and

0.085 � 10�3 kg/s average discharge rate, the necessary heat-

ing rate is 0.204 kW assuming 80% of the discharged hydrogen

is desorbed. The US06 cycle is a shorter but more aggressive

cycle than FTP75. It consumes 155.15 g of H2 in 601 s with an

average demand of 0.258 g/s. For AX-21, with an average heat

of adsorption of 3.0 � 106 J/kg and 0.258 � 10�3 kg/s average

discharge rate, the heating rate is calculated to be 0.62 kW,

again assuming that 80% of the discharged hydrogen is des-

orbed hydrogen.

Fig. 4 shows drive cycle simulation for a single cycle of

FTP75. Fig. 4(a) shows the net fuel consumption rate in g/s.

The oscillations in the fuel cell demand causes oscillations

in the bed pressure. Comparing the gaseous and adsorbed

phase loads for the single drive cycle (Fig. 4(d)), it is seen

that the gas phase responds to the demand fluctuations and

the adsorbed phase responds to the steady demand. As

noted earlier, the heating rate mainly needs to target the

heat of desorption. Hence, the heating rate need not be

altered in shorter time scales to meet the fluctuating

demand. It needs to be changed only if the average demand

changes over longer periods of time, as long as there is

sufficient hold-up in the gas phase. Model formulation and

Ambient

Header

Outer shell

Insulation

Pressurevessel

Collector

Adsorbent bed

Flow out

Flow in

Fig. 2 e Sectional view of a cryo-adsorber bed.



results apply equally to both the recirculation gas heating and

electrical heating of the bed.

To study the discharge of a tankwith about 5 kg useable H2,

the cycle is repeated continuously. The drive cycle simula-

tions are presented for both FTP75 and US06 drive cycles. The

hydrogen demand, temperature, pressure and tank load

evolutions for such a sequence of FTP75 and US06 cycles are

shown in Figs. 5 and 6 respectively.

2.3. Hot gas recirculation versus electrical heating

Hot gas recirculation takes advantage of the intimate contact

between gas and solid in a porous bed. Since porous beds tend

to have low thermal conductivity, such an intimate gas-solid

contact is an efficient way of heating up the bed. However, gas

recirculation requires additional elements including piping,

insulation, a blower or compressor on the recirculation loop,

along with a heat exchanger, and valves. These components

add to the capital cost and lower the gravimetric capacity at

system level. In addition, minimizing the heat leak into the

tank through the recirculation loop, while recirculation is not

on, could be an engineering challenge for cryogenic systems.

Electrical heating, on the other hand, requires few additional

components. Hence, it may be possible to achieve better

gravimetric capacity, easier control, and probably lower

capital cost. However, there is an electrical penalty on the fuel

cell system which could be at least partially obviated by

thermal integration of the cold hydrogen from the storage unit

with the fuel cell cooling system or by energy recovery

through expanding the high pressure gas from the cryo-

adsorber to fuel cell feed pressure. Heat leaks into the tank, or

leaks from the piping should be significantly lower in this

design.

2.4. System weight and volume

A viable on-board hydrogen storage system must have high

gravimetric and volumetric storage densities. A heavy storage

system results in the so-called mass compounding effect as

heavier supporting components are needed to fit the system

in the vehicle. In addition, a bulkier system results in lower

passenger or trunk space. The DOE has specified system

gravimetric and volume density targets for hydrogen storage

systems - for 2010 these targets are 0.045 kg H2/kg of system

mass and 0.028 kg/L of system volume. In the following, we

calculate approximate system gravimetric and volumetric

densities for a base case design. The cryo-adsorbent system

considered is a relatively low-pressure system and the storage

vessel can bemade of a hydrogen compatible aluminumalloy.

Because of low temperatures, the storage vessel will need to

be insulated with multi-layer vacuum insulation enclosed in

an outer vessel. We consider a system that can deliver 5 kg of

usable hydrogen, with the ‘empty’ conditions specified to be

135 K and 3 bar Table 1 gives information on weights and

volume of various parts and components of the system and

shows that the gravimetric density is 3.3 wt% and the

Fig. 3 e Speed and fuel cell consumption for FTP75 and US06 drive cycles.



volumetric density is 13.1 kg/L.We have used this base case as

an illustration. When we consider different designs, we need

to revise the different masses (adsorbent, inner and outer

vessel masses, etc.). Note that a lumped parameter cryo-

adsorber model distinguishes different tank designs just

through the masses involved.

3. Sodium alanate based storage system

The absorption and desorption of Ti-doped sodium alanate

[20] can be described as a two-step reaction [21] given below

NaAlH441=3Na3AlH6 þ 2=3AlþH2 (7)

1=3Na3AlH64NaHþ 1=3Alþ 1=2H2 (8)

First stage is the decomposition of NaAlH4 (sodium

aluminum tetrahydride, or the tet phase) and the second stage

is the decomposition of Na3AlH6 (sodium aluminum hexahy-

dride, or the hex phase). The theoretical capacity of sodium

alanate is 5.6 wt% but its practical storage capacity is much

smaller than this. Luo and Gross [22] report that themaximum

hydrogen weight percent in their sample is 3.9%. The present

paper incorporates thekineticspresentedbyLuoandGross [22].

The system level implementation of sodium alanate based

hydrogen storage system is different from that of high pres-

sure metal hydride storage systems. The primary reason is

that the heat of absorption/desorption for sodium alanate is

much higher compared to high pressure metal hydrides like

Ti1.1CrMn. The performance of the high pressure metal

hydride system has been demonstrated [23] using a system

level model on a Matlab/Simulink platform. The advantage

of the high pressure metal hydride systems lies in their

operation near the fuel cell stack temperature. Hence the

heating of the bed can be achieved by using the same

radiator fluid used for cooling the fuel cell. However for

sodium alanate system, high temperatures are required for

decomposition. Temperatures around 180e200 �C [24] are

required to decompose the hex phase to meet a practical

drive cycle. The bed is heated to this high temperature by

passing a portion of the hydrogen to the combustor to heat

up the heat exchanger fluid, which in turn heats up the bed.

A buffer tank is also needed to supply H2 during periods

when the bed is not able to supply sufficient H2 to the fuel

cell. We have assumed a buffer tank capacity of 100 g. This

number was arrived at by considering the hydrogen needed

for vehicle operation under conditions of cold start and low

tank pressure. Gas phase hydrogen is needed to warm up

the hydrogen storage system, and to supply H2 to the fuel

cell until the storage system is warm enough to desorb

hydrogen from the sodium alanate in the tank. Earlier

efforts for system level modeling for sodium alanate

considered only the low temperature decomposition [25] of

the tetrahydride phase. However this limits the storage

capacity of the system to a maximum theoretical capacity of

time (hr)

Net

H2

tofu

elC

ell(

g/s)

0 0.2 0.4 0.60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

time (hr)

Pres

sure

(bar

)

0 0.1 0.2 0.3 0.4 0.5 0.634

34.5

35

35.5

36

time (hr)

gase

ous

H2

(kg)

adso

rbed

H2

(kg)

0 0.1 0.2 0.3 0.4 0.5 0.62.3

2.35

2.4

3.3

3.35

3.4

adsorbed H2gaseous H2

time (hr)

Tem

pera

ture

(K)

0 0.1 0.2 0.3 0.4 0.5 0.680

81

82

83

84

a b

c d

Fig. 4 e Variation of (a) Net hydrogen demand from the fuel cell (b) pressure (c) temperature and (d) hydrogen content in the

bed for a single FTP75 drive cycle.



3.7% by weight. Recent modeling efforts [24,26,27] of alanate

storage bed include a catalytic burner and incorporate both

the tet and the hex phase decomposition. Dedrick et al. [26]

considered a shell and tube heat exchanger with the alanate

in the tubes and the cooling fluid in the shell while Raju and

Kumar [24] considered a shell and tube heat exchanger with

the alanate in the shell and the cooling fluid in the tubes.

Results for drive cycle simulations for a Chevrolet Equinox

vehicle were presented. This present work is an extension of

the same with a full scale system of w5 kg usable H2 using

dual bed strategy. The drive cycle simulations are run within

the framework of the vehicle level model developed by the

DOE Hydrogen Storage Engineering Center of Excellence

(HSECoE) team. In addition, the system level targets are

evaluated for this system.

3.1. Description of the storage system

Fig. 7 shows a schematic flow sheet of alanate storage system

in a fuel cell vehicle. The storage system consists of two beds,

each of approximately 5 kg usable hydrogen. Fig. 8 shows the

cross-sectional view of the storage bed, which consists of

alanate in the shell and coolant through the tubes. The

tubes are interconnected by fins to provide efficient heat

transfer.

The details of the system level modeling for sodium ala-

nate storage system including bed design, bed properties and

alanate properties are presented in [24]. Both refueling and

drive cycle simulations have been studied. Here only a brief

synopsis of the system level modeling strategy is provided.

The emphasis in this paper is to evaluate the performance

of the system dynamics of a dual bed system that provides

w5 kg of total usable hydrogen. The dual bed system is of

interest because a single bed system can be quite large and

difficult to accommodate in a vehicle. In addition, the

control system necessary for a dual bed system is more

complex than that for a single bed. The control system used

in the simulations includes a recharging of the buffer tank

to 150 bar when the bed is hot and is able to deliver

hydrogen at rates higher than those demanded by the fuel cell.

During refueling, both hydrogen and coolant are supplied

at the refueling station. The coolant is passed through the

tubes to provide for efficient cooling during refueling. Since

the kinetics of absorption is slow for sodiumalanate system, it

is not possible to achieve the refueling of the bed in the DOE

target refueling time of 4.2 min. Instead, a refueling time of

10.5 min (based on 40% refueling rate of the target value) is

chosen. A two-dimensional model is developed in COMSOL to

simulate the refueling of the bed [24]. In addition, overall heat

transfer coefficients are extracted from the two-dimensional

COMSOL model, which can be incorporated into the lumped

parameter model for desorption. During refueling, it is

ensured [24] that the local temperature within the bed does

not shoot above 500 K to avoid sintering of the bed due to

melting of alanate.

The drive cycle simulation is performed using a lumped

parameter model in Matlab/Simulink. All the components

including the storage beds, buffer tank, catalytic combustor,


bed for FTP75 drive cycle.



and oil loop are included in the model. Various component

level equations are described in detail in [24] and the initial

state of the bed is taken as the state at the end of refueling.

During driving, the bed is heated by passing hot fluid

through the tubes. The fluid is in turn heated by a catalytic

combustor. A small amount of hydrogen is burnt in

a catalytic combustor. Minimum amount of oil that needs to

be carried by the vehicle is calculated based on the total

volume occupied by the cooling fluid flow path in the bed.

To account for the volume of any connecting tubes outside

the bed, we include an additional 50% oil volume. Based on

this volume estimation, the vehicle needs to carry 13 kg of

heating oil, which is part of the storage system. During

driving, the oil is pumped through the storage bed tubes at

a flow rate of 2 LPM per tube. The oil passes through

a catalytic burner, where it is heated. The oil temperature is

set to a maximum of 450 K during the tet phase decomposition

and 470 K during the hex phase decomposition. A 12 kW

catalytic burner is provided for heating the oil. The efficiency

of the burner is assumed to be 90%. A buffer tank carrying

100 g of hydrogen at 150 bar and 300 K is provided.

A flow control strategy for the dual bed storage system is

adopted to control the flow of hydrogen between different

components. Hydrogen is supplied to the fuel cell and burner

either by the storage bed or by the buffer tank depending on

the state of these storage components. The strategy employed

in the current storage system is shown in Fig. 9. Initially the

first storage bed is given control to supply the hydrogen to

the fuel cell and burner. It will supply hydrogen as long as

the bed pressure does not fall below 1.1 times the fuel cell

cut-off pressure. When the bed pressure reaches below this

limit, the control is transferred to the second bed to supply

the hydrogen from its gas phase. Note that the heating fluid

at this time is flowing through the first bed. The heating


bed for US06 drive cycle.

Table 1 e System weight and volume for the cryo-adsorbent system.

System Temp & Pressure 77 K, 35 bar

Final pressure 3 bar

Adsorbent volume (L) 250

Total usable H2 5 kg

Adsorbent mass (kg) 67.5

Total inner volume (L) 275

Cylindrical part L (cm) 59.5

2 Hemispheres D (cm) 59.5

INNER VESSEL & OUTER VESSEL Material Aluminum 6061

Inner vessel mass (kg) 46.1

Outer vessel mass (kg) 11.1

Insulation mass (kg) e MLVSI (1” thick) 12

BOP components (kg) 15

Total mass (kg) 151.7

Outer volume(L) 380.3

Gravimetric capacity (kg/kg) 0.0330

Volumetric density (kg/L) 0.0131



fluid is routed to the second bed only when the first bed is

almost empty. When the control is transferred to the second

bed, the second bed tries to supply the hydrogen from its

gas phase as it cannot supply the absorbed hydrogen since

there is no heating. If the gas phase hydrogen in the second

bed is unable to supply the fuel cell demand, then the

control is transferred to the buffer tank to supply

the hydrogen demand. Meanwhile pressure builds within

the first bed due to hydrogen desorption reactions. When

the bed pressure exceeds twice the cut-off pressure, the

control is again transferred back to the first bed. When

the first bed is almost empty, the heating fluid is rerouted to

the second bed and the control is transferred to the second

bed to supply the hydrogen. At this stage, whenever the

pressure in the second bed falls below 1.1 times the fuel cell

cut-off pressure, the control is transferred to the buffer

storage tank to supply hydrogen. Because of heating and

hydrogen desorption, pressure in the second bed starts

increasing and as soon as the bed pressure exceeds twice

the fuel cell cut-off pressure, control is transferred back to

the second bed. This strategy is chosen to ensure continuous

supply of hydrogen to the fuel cell and at the same time

extract most of the absorbed hydrogen from the storage bed.

3.2. Drive cycle simulations

The vehicle level model has different drive cycle options to

evaluate the performance of the storage system. FTP75 and

US06 drive cycles are chosen to evaluate the dynamic

performance of the storage system during real driving condi-

tions. The cycles are periodically repeated to run a full tank to

empty tank simulation. Simulations start with a nearly full

tank based on a refueling time of 10.5 min and bed tempera-

ture set at 390 K. The tank is considered empty and the

simulation stops when the pressure in each of the beds and

the buffer falls below the fuel cell cut-off pressure.

Fig. 10 shows the system level dynamic performance during

FTP75 drive cycle simulation. Fig. 10(a) shows the variation of

the bed pressures and the buffer pressure. In the beginning,

hydrogen is extracted from the first bed. Heating fluid is

supplied to the bed. While the bed is getting heated up (see

Fig. 10(b)), the gas phase hydrogen in the first bed supplies

the hydrogen to the fuel cell. This leads to a drop in the

pressure of the bed. As the bed gets heated up, the rate of

desorption (tetrahydride phase) increases and hence the

pressure in the bed starts rebuilding after a short while. The

fluctuations in the bed pressure are due to the fluctuations in

the fuel cell demand. Once the tetrahydride phase is almost

converted to the hexahydride phase, the decomposition of

hexahydride phase begins. The kink in the red line of

Fig. 10(c) at 5 h is due to this transition. The temperature

of the heating oil is increased to 470 K during this transition.

This causes a rise in bed temperature (Fig. 10(b)) at 5 h. The

bed pressure drops to the equilibrium pressure (Fig. 10(a)) of

the hexahydride phase at the current bed temperature. Even

at this high temperature the rate of decomposition is small.

Eventually when the hexahydride phase decomposition is

almost complete, the pressure in the first bed drops to the

Alanate bed

Buffer Volume

Fuel Cell

Catalytic heater

H2

H2

Heating fluid

Anode

Cathode

CoolantRadiator

Air

Oil tank

Fig. 7 e Schematic of sodium alanate based dual bed hydrogen storage system.

Fig. 8 e Cross-section of the alanate storage bed.



fuel cell cut-off pressure. Once the bed pressure drops, then

the second bed is called in. The heating oil is rerouted to the

second bed. Hence the temperature of the second bed starts

rising. Similar dynamic behavior is observed for the second

bed as that of the first bed. Eventually when the hexahydride

phase decomposition is near completion in the second bed,

buffer supplies the hydrogen to the fuel cell. The total driving

time for the FTP75 is approximately 16 h.

Fig. 11 shows the system level dynamic performance

during US06 drive cycle simulation. US06 is an aggressive

drive cycle compared to FTP75. Fig. 11(a) shows the variation

of the bed pressures and the buffer pressure. The

fluctuations in the bed pressure for US06 drive cycle are

larger compared to those for the FTP75 cycle. At first,

hydrogen is extracted from the first bed. Heating fluid at

450 K is supplied to the first bed but the bed does not heat

up to that temperature. This is due to the cooling produced

by excess hydrogen demand which prevents the bed from

heating quickly. Once the tetrahydride phase is almost

converted to the hexahydride phase, the decomposition of

hexahydride phase begins. Since the rate of hexahydride

phase decomposition is low, the bed cannot supply the

hydrogen demand. Consequently, the control is switched

from the first bed to the second bed when the pressure in

the first bed falls below 1.1 times the fuel cell cut-off

pressure. Note that the heating oil is still being supplied to

the first bed and the second bed is not being heated up. The

gas phase hydrogen in the second bed now supplies the

hydrogen to the fuel cell. This results in a drop in second

1 _

2

bed 1 is not empty

heating fluid flows through bed 1

> 1.1

First bed supplies the H to the fuel cell and burner

Control is transferred to eith

bed cut off

if

if P P

else

2 _

2

er bed 2 or buffer

In the meanwhile, pressure is building up in bed 1 due to heating

> 1.1

Second bed supplies the H from its free volume to the fuelbed cut offif P P

2

cell and burner

No heating is supplied to this bed

Buffer supplies the H to the fuel cell and burner

Once the first bed pressure rea

else

end

_

2 _

2

ches 2 , the control is transferred back to first bed

bed 2 is not empty

heating fluid flows through bed 1

> 1.1

Second bed supplies the H to the fuel

cut off

bed cut off

P

elseif

if P P

cell and burner

Control is transferred to buffer

In the meanwhile, pressure is building up in bed 2 due to heating

Once the second bed pressure reache

else

end

_

2

s 2 , the control is transferred back to first bed

bed 1 and bed 2 are empty

Buffer supplies the H to the fuel cell and burner till buffer is emptied

cut offP

elseif

end

Fig. 9 e Control system for dual bed system.

Fig. 10 e System performance for FTP75 drive cycle.



bed pressure as well as some desorption in the second bed.

Correspondingly, the temperature and weight fraction of

absorbed hydrogen drop slightly. Meanwhile, the first bed

gets heated up and the pressure in the bed builds up. If the

pressure in the first bed exceeds twice the fuel cell cut-off

pressure, control is shifted to the first bed. This shifting of

control back and forth continues till the first bed becomes

almost empty. This results in fluctuations in the first and

second bed pressures during the range of 2e3.5 h of driving

time. Once the first bed is almost empty, the second bed is

called in. The heating oil is rerouted to the second bed.

Hence the temperature of the second bed starts rising.

Eventually when the hexahydride phase decomposition

starts, the second bed takes the help of buffer to supply the

hydrogen demand as explained by the control strategy. The

excess hydrogen demand eventually leads to an empty

buffer even before the second bed is completely empty. The

second bed can no longer supply the hydrogen demand even

though there is absorbed hydrogen still present in the

hexahydride phase. Hence for aggressive drive cycles, it is

difficult to extract all the absorbed hydrogen.

3.3. System weight and volume

Table 2 below shows the preliminary estimation of gravimetric

and volumetric density of the current sodium alanate storage

system. Based on the FTP75 drive cycle simulation, it is

assumed that each bed will deliver roughly 2.75 kg of usable

H2. Two such beds are used to deliver a total of 5.5 kg usable

H2. The vessel is made of an inner liner and an outer layer of

carbon composite. The thickness of the materials are chosen

to withstand pressures of 150 bar and temperatures of 180 �C.The cooling tubes and the fins are made of aluminum. The

gravimetric capacity for this system is roughly 0.012 kg H2/kg.

It is quite far below the DOE 2010 target of 0.045 kg H2/kg.

There has to be a significant improvement in the hydrogen

absorption capacity of metal hydrides in order to meet the

DOE target. The volumetric capacity for this system is

0.0148 kg H2/L, which is also below the DOE 2010 target value

of 0.028 kg H2/L.

4. Relative merits of the two storage systems

The two storage systems operate at entirely different oper-

ating conditions. Each system has its relative merits and

demerits. Overall the performance of the cryo-adsorbent

system is much better in terms of gravimetric capacity as

compared to the metal hydride system. The volumetric

capacities for the two systems are nearly identical. In addi-

tion, there are some important distinguishing features of the

two systems that should be noted.

4.1. Cold start capability

Cryo-adsorbent system can handle cold start at very low

temperatures. However, in the case of metal hydride systems

like sodium alanate, cold start is a challenge. If the car hasFig. 11 e System performance for US06 drive cycle.

Table 2 e System weight and volume for the sodiumalanate system.

Bed specifications units Value

Number of beds 2

deliverable hydrogen kg 5.5

Length (alanate packing) mm 1000

Actual length of the bed mm 1292.0

Diameter of the bed (inner) mm 416.0

Diameter of the bed (outer) mm 436.9

Shell material Composite

carbon

No of cooling tubes 24.0

Diameter of cooling tubes (inner) mm 20.0

Weight of alanate kg 200.00

weight of shell include liner kg 44.00

weight of tubes and fins kg 137.00

accessories (manifolds, end

plates etc)

kg 33.70

pump/HEX/burner kg 8.00

pump/HEX/burner volume liters 8.00

BOP mass kg 16.85

Oil mass kg 13.00

Buffer kg 5.05

Buffer volume liters 11.30

Total weight of the bed kg 381.00

Total volume of the beds liters 351.21

Total system volume liters 370.51

Total system mass (tubes,

plates, shell/insulation, alanate)

kg 457.60

Gravimetric density kg/kg 0.012

Volumetric density kg/liter 0.0148



been parked for a long time during peak winter days, the bed

will cool down. During start up, the bed may not be able to

supply the hydrogen and the buffer tank will need to supply

the required hydrogen demand. Size of the buffer tank will

decide whether the system will be able to handle cold start

conditions. Fig. 12 shows cold start simulation results for the

FTP75 drive cycle. The bed and the heating fluid is assumed

to be at ambient temperature assumed to be �20 �C. Sincethe bed is cold, there is no desorption in the beginning. Gas

phase hydrogen in the first bed supplies the fuel cell

demand. Hence the pressure in the first bed falls rapidly as

shown in Fig. 12(a). In the meantime, the bed is heated. The

bed takes a long time (Fig. 12 (b)) to heat up for two reasons -

the low initial bed temperature and the heat needed for the

endothermic desorption reaction. As shown in Fig. 12(a), the

bed pressure almost falls to cut-off pressure before the bed

pressure starts increasing. If the bed pressure falls below

cut-off, then the gas phase hydrogen in the second bed will

supply the fuel cell demand before hydrogen can be

desorbed from the first bed to supply the hydrogen demand.

As the temperature increases, the bed pressure slowly rises.

Once the bed temperature reaches 450 K, the system will

perform normally as shown in Fig. 12.

4.2. Hydrogen overhead

In the case of cryo-adsorbent system, hot gas recirculation can

be used to heat up the bed during the discharge cycle. A small

heat exchanger to warm up the cold hydrogen using heat

exchange with the ambient would be sufficient. However, as

the storage system warms up, venting may be necessary

resulting in a loss of hydrogen to the atmosphere. The amount

of hydrogen vented depends on the total heat leak into the

system. In the case of sodium alanate storage system,

hydrogen has to be burnt for heating up the bed during driving

for supplying the heat of desorption and in transient heating

of the bed from the initial temperature of the bed to the

desorption temperature. The amount of hydrogen burnt can

be substantial because of the significant enthalpy (w40 kJ/mol

H2) of hydrogen desorption and the need to keep the system at

a high temperature (140 �C) for speeding up the kinetics to

supply the hydrogen demand.

5. Summary

Lumped parameter system simulation models are developed

for the cryo-adsorption and metal hydride hydrogen storage

systems. For the cryo-adsorbent system, the model solves the

mass, energy balances and adsorption kinetics to compute

temperature, pressure and adsorbate concentration. The

adsorption kinetics included is the linear driving force model

with Glueckauf’s approximation. Simulations for the FTP75

and US06 drive cycle demand are performed and the

temperature, pressure, adsorbate concentration, adsorbed

and gaseous hydrogen content in the tank are presented.

Simulation results show that the gas phase responds to the

demand fluctuations and the adsorbed phase responds to the

average demand. Hence, the heating rate need not be altered

in shorter time scales to meet the fluctuating demand. In

a cryogenic adsorption storage unit, an electrical heater could

be more optimal (in the sense of heat leak, gravimetric and

volumetric capacities and cost) than a hot gas recirculation

system, since the heating rate needs to change on longer time

scales than the fluctuating demand.

For the metal hydride based system, a dual bed storage

system is considered to supplyw 5 kg of usable hydrogen. The

system performance of the dual bed storage system is shown

for the FTP75 and US06 drive cycle demands. It is shown that

the usable hydrogen for a given system depends on the drive

cycle, with aggressive cycles like US06 resulting in lower

usable hydrogen. The gravimetric and volumetric capacities of

the two storage systems are evaluated and the relative merits

and demerits of the two systems are presented.

Acknowledgments

This work was performed under DOE contract DE-FC36-

09GO19003 as GM’s contribution to the DOE Hydrogen Storage

Engineering Center of Excellence (HSECoE). The authorswould

like to acknowledge the support of Ned Stetson, Monterey

Gardiner and Jesse Adams of DOE and DonAnton of SRNL. The

authors would like to thank Lincoln Composites for supplying

data on shell design and thickness for the given operating

conditions of the storage systems. The authors also

acknowledge Mei Cai and Scott Jorgensen of General MotorsFig. 12 e Cold start simulation for FTP75 drive cycle.



for their valuable suggestions and HSECoE team members for

contributing to the development of vehicle level model.

Nomenclature

T; P Temperature and Pressure K, bar

L;R Length and radius of the adsorbent bed, m

ms;Vb Mass and volume of the adsorbent bed kg, m3

mH2 Mass of hydrogen in the bed, kg

mw; vw Mass and specific volume of outer shell kg, m3/kg_mf ; _mo Mass flow rate of H2 in the feed and outlet

streams, kg/s

3t Porosity of the bed, m3/m3

rb bed densities kg/m3

rg;vg Gas density and specific volume, kg/m3, m3/kg

mg Gas viscosity, Pa s

aPg; kTg Isobaric thermal expansion coefficient and

isothermal compressibility, 1/K, 1/bar

Hg;Hq;Hs;Hw Specific enthalpy of gas, adsorbate, adsorbent

and outer shell, J/kg

CPg;CPs;Cpw Specific heat capacity of gas, adsorbent and outer

shell, J/kg/K_Qh;

_Ql Heat flux supplied, and heat flux leak into the

system, W

q; q� Excess adsorbate concentration and its equilibrium

value, kg, H2/kg adsorbent

DHa Heat of adsorption, J/kg H2 adsorbed

Da Effective diffusivity of the adsorbate in the adsorbent

particle, m2/s

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