Effects of heat transfer on the sorption kinetics of complex hydride reacting systems

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 1 8 9 6 – 1 9 0 3

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Effects of heat transfer on the sorption kinetics of complexhydride reacting systems

Gustavo A. Lozano*, Nico Eigen1, Claude Keller2, Martin Dornheim, Rudiger Bormann

Institute of Materials Research, GKSS Research Centre, Max Planck Strasse 1, D-21502 Geesthacht, Schleswig-Holstein, Germany

a r t i c l e i n f o

Article history:

Received 5 November 2008

Received in revised form

5 December 2008

Accepted 6 December 2008

Available online 21 January 2009

Keywords:

Sodium alanate

Hydrogen storage

Thermal conductivity

Hydride tanks scale-up

* Corresponding author. Tel.: þ49 41 5287 26E-mail address: [email protected] (

1 Present address: MT Aerospace AG, D-862 Present address: Linde Engineering Divis

0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.12.028

a b s t r a c t

In this work, the effect of powder bed size on the absorption and desorption kinetics of

NaAlH4 catalyzed with TiCl4 was studied experimentally. For this purpose, volumetric

titration measurements were performed using cells of different diameters. The tempera-

ture was measured during the process at different positions inside the hydride bed,

providing detailed information about the influence of heat conduction. Experimental

results show that, under the applied conditions up to a critical size, larger diameters can

lead to faster kinetics for the first and second absorption reactions. At larger cell diameters,

however, temperatures up to 200 �C were measured during the first absorption step in the

hydride bed. This leads to a significant delay in the start of the second absorption step,

reducing the overall rate of the process. Reasons for the observed behaviour are discussed

and measures for optimization are proposed.

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction in the order of 2 wt.%, while the 2010 US Department of Energy

Hydrogen is a very promising energy carrier for a compre-

hensive clean-energy concept in mobile applications.

Regarding its use as fuel for the zero-emission vehicle, one of

the main problems is the storage. Hydrogen storage systems

should fulfil the demands for automotive applications i.e. high

gravimetric and volumetric capacities, fast charging and dis-

charging rates at moderate operating conditions and high

safety levels. Metal hydrides offer a safe alternative to

hydrogen storage in compressed or liquid form. In addition,

metal hydrides have higher storage capacity by volume [1].

Room temperature hydrides are advantageous, because they

require low operating pressures and temperatures. The main

drawback is their poor gravimetric hydrogen storage density

43; fax: þ49 41 5287 2625.G.A. Lozano).153 Augsburg, Germany.ion, Linde AG, D-82049 Puational Association for H

(DOE) target is 6 wt.% for hydrogen storage systems. There-

fore, in recent years, research has focused on light weight

complex hydrides, which also have a high capacity by weight,

e.g. LiBH4 about 18 wt.%. However, LiBH4 only decomposes at

temperatures over 400 �C [2]. Sodium alanate, NaAlH4, offers

a suitable compromise with relatively large gravimetric

storage capacity at relatively moderate temperatures. Bogda-

novic and Schwickardi showed in 1996 that hydrogen can be

reversibly stored and released in sodium alanate, if doped

with a suitable catalyst [3]. NaAlH4 is reversibly formed in

a two-stage reaction within the technically favourable

temperature range of up to about 125 �C.

3NaHþAlþ 1.5H2 4 Na3AlH6 (DH¼�47 kJ/mol H2) (1)

llach, Germany.ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

mailto:[email protected]

http://www.elsevier.com/locate/he

Fig. 1 – Cells of different inner diameters utilized for

sorption measurements in Sievert’s apparatus. Inner

diameters from left to right: 2 mm, 4.5 mm, 10 mm and

15.2 mm.

o g e n e n e r g y 3 4 ( 2 0 0 9 ) 1 8 9 6 – 1 9 0 3 1897

Na3AlH6þ 2Alþ 3H2 4 3NaAlH4 (DH¼�37 kJ/mol H2) (2)

Charging of the material is possible within a few minutes if

suitable catalysts and synthesis methods are used. Titanium-

based catalysts in particular have proven to be very effective,

e.g. TiCl3, TiCl4 [4,5]. Furthermore, sodium alanate can be

produced from cost-effective raw materials in kg amounts [6].

Up-scaling of the milling process is considered to be feasible

up to tonnage quantities at low costs [7].

Fundamental quantitative research on kinetics of the

absorption–desorption processes of metal hydrides is nor-

mally carried out with samples of some hundreds of mg in

standard apparatus e.g. Sievert’s apparatus. However, in order

to exploit the properties of sodium alanate in a suitable

hydrogen storage system, it is required to understand the

performance of the material in larger powder beds.

The heat transfer mechanisms and the correlation to the

hydriding behaviour of large material beds are relatively well

investigated on LaNi5 and FeTi-based transition-metal/rare-

earth metal alloys (room temperature hydrides). For some

reactor systems, the hydrogen sorption kinetics is limited by

heat transfer constraints of the reactor. In most cases,

however, the metal hydride is the principal heat transfer

resistance [8,9]. Many studies therefore focus on the deter-

mination of the effective thermal conductivity of the metal

hydride beds [9–12] and demonstrate heat transfer enhance-

ment by using extended areas for heat transfer and by binding

the hydrides into a solid matrix of high conductivity materials

[13–15] as well as the use of external fins [16]. Additionally,

there are numerous studies on dynamic metal hydride storage

systems and simulations [17–25].

On the other hand, the sorption behaviour of large beds of

complex hydrides is relatively unknown. Thermal conduc-

tivity values around 0.5 W m�1 K�1 for sodium alanate have

been reported based on static measurements in powder beds

at different pressures and degree of reaction [26,27], indicating

relatively poor thermal conductivity of light-metal hydrides

even compared to conventional transition-metal/rare-earth

metal hydrides (average values between 1 and 2 W m�1 K�1

under similar conditions are reported [9–11,14]). A brief study

about the bed temperature during the reactions in a scaled-up

test bed of NaAlH4 is reported in [28]. It was shown that poor

heat conductivity of the bed of hydride has a negative effect

on the absorption and desorption kinetics. However, the

determining mechanism for the sorption kinetics on NaAlH4

could not be clarified. Additionally, the effect of additions, e.g.

carbon, on the heat transfer in alanate-based systems is not

clear up to this point.

Therefore, the present work pursues an advanced under-

standing of the factors that influence charging and discharg-

ing kinetics in large beds of NaAlH4. In particular, we study the

effect of carbon additions, temperature and the powder bed

size on the reaction kinetics of NaAlH4 catalyzed with TiCl4.

For this purpose, titration measurements were performed

using cylindrical cells of different diameters. To understand

the underlying mechanisms, additionally, the temperature

was measured during the process at different positions inside

the bed of material, providing detailed information about the

influence of heat conduction and temperature on the reaction

kinetics.

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

2. Experimental procedures

2.1. Materials’ production

The material used for the investigations was produced in

industrial milling equipment optimised towards fast kinetics

[6]. As raw materials, commercial NaH (95%, Sigma Aldrich

Chemie GmbH, Steinheim, Germany), aluminium (99.5%,

Johnson Matthey GmbH & Co. KG, Karlsruhe, Germany) and

2 mol.% aluminium reduced TiCl4 (3TiCl3þAlCl3) (Fluka

Chemie GmbH, Buchs, Switzerland) as catalyst and carbon

powder (Alpha Aesar) as milling agent were used. All handling

including milling was carried out in a glove box with purified

argon atmosphere. Prior to milling, NaH and aluminium were

mixed with a tumbling shaker for 0.5 h in a molar ratio of

1.08:1 according to the reaction

1.08NaHþAlþ 0.02(TiCl3þ 1/3AlCl3) /

NaHþ 1.0067Alþ 0.08NaClþ 0.02Tiþ 0.04H2 (3)

which is expected to occur during milling. Milling was

carried out in a modified vibratory tube mill (ESM 236, Siebt-

echnik, Muhlheim a. d. R., Germany), using 30 mm hardened

balls at a rotational speed of 1000 rpm and a ball-to-powder

ratio (BPR) of 140:1. In order to avoid agglomeration, 100 ml of

cyclohexane was added prior to milling. After milling for 2 h,

the mill was evacuated to dry the samples (batch 1). In order to

investigate the influence of graphite on the hydrogenation

behaviour, a second batch was prepared in the same way as

described before. After ball milling an additional 5 wt.% of

carbon (graphite powder, Alfa Aesar GmbH & Co. KG, Karls-

ruhe, Germany) was added to the second batch and the

composite milled for another 30 min (batch 2).

2.2. Sorption kinetics and cells

The sorption kinetics was characterized by a volumetric

method using a Sievert’s apparatus (HERA, Quebec, Canada),

utilizing cylindrical sample cells of different sizes (Fig. 1).

During all the experiments the external temperature on the

wall of the cells was measured. Cells with inner diameters of

2, 4.5, 10 and 15.2 mm were used. All material used during

these experiments came from the same batch 1 or 2. The

loading of the milled material in each of the cells was done

Fig. 2 – Thermocouples’ positions in the 15.2 mm diameter cell (thermocell) for heat flow analysis of the system during

absorptions and desorptions.


inside an argon glove box. Each cell was loaded with the

milled material homogeneously up to its top and immediately

weighted afterwards (mg precision scale). The weight of the

material in the cells was around 40 mg (2 mm cell), 150 mg

(4 mm cell), 1300 mg (10 mm cell) and 5000 mg (15.2 mm cell).

For the second part of the investigation, the 15.2 mm

diameter cell was additionally equipped with 5 thermocouples,

Fig. 2, what we call a thermocell. The thermocouples used are

type K, with outer diameter 1 mm and fast response grounded

junction. The cell was designed to enable measurements up to

temperatures of 400 �C and pressures up to 200 bar. The

temperature measurements throughout the bed of the mate-

rial with this cell make it possible to analyze the heat flow in the

system during the reactions and to estimate the effective

thermal conductivity of the bed.

Fig. 3 – Hydrogen content of material from batch 1 (a) and

batch 2 (b) during 3rd hydrogen absorption with different

cells at 100 bar in the Sievert’s apparatus. The initial

temperature was 125 8C.

3. Results and discussion

3.1. Absorption in cells of different sizes

The hydrogen absorption process, which corresponds to filling

an automotive tank, is demanded to last only a few minutes

and is therefore much more critical than the discharging

process. Consequently, it will predominantly determine the

design of the heat transfer system. In order to analyze the

effect of powder bed size on kinetics, the third absorption is

compared. In this absorption the material shows almost

steady kinetics with respect to cycling. In this subsection the

analysis is focused on the effect of the cell size on the

absorption profiles. The conclusions and discussion with

respect to the cell size are valid for the results obtained with

material from both batch 1 and batch 2. The absorption

profiles of the batches 1 and 2 are further compared and

analyzed in Section 3.3 (see Figs. 11 and 12).

Fig. 3 shows the absorption profiles of both materials

(batches 1 and 2) obtained with the cells of different dimen-

sions. Although the hydrogen capacities after 20 min are

similar, the slopes of the curves, and therefore the reaction

rates, are notably different. Intermediate diameters of 4.5 mm

and 10 mm lead to the fastest kinetics. This results from

different temperature developments in the beds of material

within the cells during the hydrogenation process. Fig. 4

shows the external wall temperature with respect to absorp-

tion time. The smaller cells exhibit almost isothermal

behaviour, while for the larger cells the outer temperature

raises by about 10–20 �C. This can be explained by ineffective

heat transfer with the surrounding air and a limited heat

capacity of the vessel. The temperature rise caused by limited

heat capacity of the vessel should decrease as the ratio mass

Fig. 4 – Outer wall temperature of material from batch 1

(a) and batch 2 (b) during 3rd hydrogen absorption using

different cells at 100 bar in the Sievert’s apparatus. The

initial temperature was 125 8C.

Fig. 5 – Cross-section of any cell used in the experiments.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 1 8 9 6 – 1 9 0 3 1899

of cell wall to mass of hydride, MCell wall/MHydride, increases,

because the heat capacity of the cell acts as sink for the heat

generated from the reaction. The calculated ratio MCell wall/

MHydride is presented below in Table 1 for the cells. The ratio

was calculated from the cross-section of cell wall and the

hydride bed (Fig. 5) and their respective densities, assuming

infinite cylindrical cells. By comparing results of Table 1 with

those of Fig. 4, it can be concluded that the calculated mass

ratios relate well with the observed rise of the outer wall

temperature. The temperature does not increase in the 2 mm

Table 1 – Ratio of mass of cell wall to mass of hydride forthe experimental cells. The density of the bed material is0.6 g/cm3 and the density of the walls of the cells is 7.85 g/cm3.

Cell Innerdiameter

(mm)

Outerdiameter

(mm)

MCell wall/Mmaterial

(g/g)

1 2 8 196.3

2 4.5 8 28.3

3 10 15.2 17.1

4 15.2 25 22.3

cell, the one with the highest ratio MCell wall/MHydride, whereas

the corresponding lowest ratio in cell 3 leads to the highest

temperature response.

Additionally, due to the low effective thermal conductivity,

temperature profiles inside the bed should develop. Large

beds would therefore have their maximum temperature at

their centre.

The above outlined thermal effects explain the variance of

the performances among the cells (Fig. 3). From the thermal

and kinetics profiles in Figs. 3 and 4 it is noted that a rise in

temperature improves the kinetics at the beginning of the

process, when the first step of hydrogenation proceeds.

Consequently, it improves the hydrogenation kinetics in the

larger cells compared to the smaller cells. Indeed in cell 3, with

the lowest ratio MCell wall/MHydride and the highest rise in the

outer wall temperature, the fastest kinetics for the first step is

observed. Respectively, cell 1 having the highest ratio MCell

wall/MHydride presents no rise in temperature and the slowest

kinetics for step one. Thus, the optimum temperature for

kinetics is substantially higher than the initial temperature of

125 �C but should be lower than the equilibrium temperature

at 100 bar for the first step of hydrogenation (Eq. (1)), which

amounts to about 250–260 �C [29].

At 100 bar, however, the equilibrium temperature for the

second step of hydrogenation (Eq. (2)) is around 170 �C [29]. If

the hydride reaches a temperature higher than 170 �C after the

first step of reaction, the second step will not proceed until the

temperature of the material has decreased significantly below

that temperature. This would cause a delay between the two

steps, and may explain the delays in Fig. 3 for the larger cell

sizes. This conclusion is supported firstly by the hydrogen

content of the material during the delay, which amounts to

around 1.6 wt.% and corresponds to the theoretical hydrogen

capacity of the first step of hydrogenation, and secondly, by

the observation that the time of the delay increases with the

diameter of the cells.

In addition, the time dependence of the outer wall

temperature of cell 3, in Fig. 4, reflects the delay between the

steps. The temperature rises during the first step, almost

20 �C, and decreases when the first reaction is completed. At


the end of the delay, the temperature increases once again.

Both increments can be explained by the exothermic hydro-

genation reactions. However, the absolute value of tempera-

ture rise on the outer wall of the cells does not explain the

delay in the reaction, because the equilibrium temperature is

neither reached nor exceeded. This indicates significantly

higher temperatures in the powder bed. Therefore, experi-

ments with the thermocell (see Section 2.2) were performed in

which the temperature throughout the bed was monitored.

3.2. Absorptions in the thermocell

Sets of experiments were carried out using the thermocell

described in Section 2.2, with material from both batch 1 and

batch 2. Fig. 6 shows the 3rd absorption of carbon containing

sodium alanate, batch 2. The initial temperature was again

125 �C. Quite similar results are obtained with the material of

batch 1, and the description presented below is valid for both

batches.

As soon as the first step of absorption starts, there is

a small peak in temperature, which occurs quite shortly for

some seconds and had not been detected from outer wall

temperature measurements. It can be attributed to hydrogen

heating up upon expansion and subsequent fast flow of heat

into the hydride. At the same time, the exothermic hydroge-

nation of NaH and Al occurs with fast kinetics, leading to

a substantial increase of temperature in the powder bed. As

expected, due to the limited heat conduction in the material,

after about 1 min a temperature profile develops with

a maximum of 210 �C in the centre of the bed (position T1).

This is still lower than the equilibrium temperature of step 1

(Eq. (1)), which is 250–260 �C at the working pressure (100 bar),

but much higher than the equilibrium temperature of step 2

(Eq. (2)), 170 �C. Consequently, after absorbing 1.6 wt.% of

hydrogen, corresponding to the completion of step 1, hydro-

genation does not continue. Therefore, temperatures in the

bed decrease due to the heat flow through the cell wall and the

surrounding air. It is observed that the second step does not

start at the point when the temperature falls below the

Fig. 6 – Hydrogen content and temperature inside the bed

of hydride during 3rd hydrogen absorption at 100 bar,

measured in the Sievert’s apparatus. The initial

temperature was 125 8C. The positions of temperature

measurement are found in Fig. 2.

equilibrium temperature of 170 �C. The temperature

decreases to 140 �C and remains almost constant in the entire

bed. Only after 2 min does further hydrogenation proceed; the

temperature increases correspondingly again and a new

temperature gradient develops throughout the bed. According

to the slower kinetics of step 2 of hydrogenation, the

temperature gradient is not as high as in step 1.

The delay between step 1 and 2, labelled as section b in

Fig. 6, cannot be entirely explained by the constraint related to

the equilibrium temperature for the formation of NaAlH4,

since during the delay the temperature in the bed is at 140 �C.

This is 30 �C lower than the corresponding equilibrium

temperature, and even at this temperature and 100 bar the

material absorbs once again after its end, when section c in

Fig. 6 starts. A possible cause is that heating of the material to

high temperatures in the first hydrogenation step has resulted

in recovery processes of the material, which impede the

nucleation of the NaAlH4 phase. In order to support this

explanation, the totally absorbed material of batch 1 and

batch 2 was desorbed at 125 �C and 5 bar. This results in the

complete decomposition of NaAlH4 into Na3AlH6 and Al with

the respective release of hydrogen, because the equilibrium

pressure for the further dehydrogenation is 3 bar [29]. The

material was then hydrogenated at different initial tempera-

tures (100 �C, 125 �C, 140 �C and 150 �C) by applying 100 bar H2.

In order to start the absorption, the pressure increase in the

cells using the experimental Sievert’s apparatus is performed

within less than a second. Interestingly, the reaction starts

immediately, as shown in Fig. 7, accompanied by temperature

increase. Consequently, the delay between the two steps has

to be caused by the condition of Na3AlH6 or Al after the first

step of hydrogenation and can, for example, be unfavourable

for the NaAlH4 nucleation process.

These findings are important for modelling, design and

layout of a storage tank based on sodium alanate. It has to be

noted that the significant heating upon hydrogenation of the

cells observed in this investigation can be avoided by efficient

cooling using water or oil. Aspiring towards cost effectiveness,

simple constructions, and the consequently large dimensions

Fig. 7 – Absorptions of Na3AlH6/Al at 100 bar H2, previously

formed by desorption of NaAlH4 at 5 bar and 125 8C. Initial

temperatures of the absorptions are indicated for each

curve (100, 125, 140 and 150 8C). T1 is the temperature in

the centre of the hydride bed.

Fig. 8 – Desorption of NaAlH4 at 5 bar and 125 8C in the

thermocell. Initial temperature is 125 8C. For a reaction

time of 10 min, the temperature profile was fitted and the

desorption rate was calculated in order to estimate the

effective thermal conductivity of the bed of material.

Fig. 10 – Estimated effective thermal conductivity of

hydride bed for NaAlH4 with and without carbon addition

during desorption at 5 bar H2 in comparison with literature

data [26]. The temperatures reported are the mean

temperatures of the bed.


of the powder beds, however, will always result in strong

temperature changes in the hydride as in this work. A possible

approach to overcome the detrimental effect observed before

the reaction step 2 would be to increase the cooling intensity

for a short time after the reaction step 1 is completed.

3.3. Estimation of effective thermal conductivity

A further important factor necessary for the layout of

a storage system based on hydrides is the effective thermal

conductivity. The positions of the thermocouples in the ther-

mocell in Fig. 2 were chosen in order to estimate the effective

thermal conductivity of the material bed upon the hydroge-

nation/dehydrogenation reaction in-situ. The measuring

positions were arranged in such a way that a parabolic

temperature profile throughout the bed could be read with

uniform and sufficient temperature variation between the

positions: calling DT the total difference of temperature

Fig. 9 – Example of temperature of the hydride bed with

respect to the distance from the centre corresponding to

the indicated data in Fig. 8. The fitting was used to

estimate the effective thermal conductivity of the bed of

material.

between the material close to the wall of the cell and the

material in the centre (axis) of the cell, T4 reads 0% of DT,

T3 reads 33%, T2 reads 67% and T1 reads 100% of DT. T1 is

located exactly in the centre of the bed.

In order to evaluate the measurements, a homogeneous

cylindrical system is assumed in a stationary process with

only radial conduction, no axial conduction, and a constant

and homogeneous heat generation, as well as a constant

density and heat conduction of the hydride bed. In this case

a parabolic temperature profile is obtained by the standard

heat conduction equation:

T� TC ¼ �_qrr2

4Keff(4)

where T (K) is the temperature at a radial distance r (m) from

the centric axis of the cylinder, TC(K) the temperature in the

axis (centre) of the cylinder, _q the heat generation (W/kg), r the

density of the bed of material (kg/m3), and Keff the effective

thermal conductivity of the bed of material (W m�1 K�1).

Fig. 11 – Absorptions of the material at 100 bar with and

without carbon. Initial temperature is 125 8C. T1 is the

temperature in the centre of the bed.

Fig. 12 – Desorptions of the material with and without

carbon into vacuum. Initial temperature is 125 8C. T1 is the

temperature in the centre of the bed. As noted also in other

investigations, desorption kinetics at 125 8C for the second

step is relatively slow. Therefore on this time scale, only

desorption of the first reaction step is visible.


The effective thermal conductivity of the hydride during

in-situ measurements is estimated using Eq. (4). For this

estimation, a variable m is defined according to Eq. (5) below,

such that Eq. (4) turns into Eq. (6).

m ¼ �_qr

4Keff(5)

T� TC ¼ mr2 (6)

First, m is numerically adjusted to the experimental set of data

(T� TC) vs. r2 taken during an endothermic or exothermic

reaction in a moment with constant temperature profile

(vT/vt¼ 0 inside the bed). After adjusting m, Keff is calculated

by Eq. (5). A density of the bed in the desorbed state of about

0.6 g/cm3 was thereby taken, as experimentally determined._q is calculated from the measured reaction rate and the

enthalpy of reaction, derived from PcT-measurements [29]

_q ¼ �r0Dh (7)

where r0 is the hydrogen sorption rate (gH2 g�1 s�1) and Dh is

the enthalpy of reaction (J/gH2).

The condition vT/vt¼ 0 inside the bed was not valid during

most of the experiments, but it is clearly fulfilled after about

10 min during the desorptions of NaAlH4 at 5 bar and 125 �C,

as shown in Fig. 8. From the experimental temperature

profiles and the desorption rate r0, m and _q are calculated as

above explained. The experimental and fitted temperature

profile is presented in Fig. 9. The estimations of the effective

thermal conductivity were done for the material without and

with carbon, batch 1 and batch 2 respectively. In the moment

of the estimation, the material is a mixture of NaAlH4, Na3AlH6

and Al.

The estimated heat conductivities are summarized in

Fig. 10. The values obtained with our in-situ measurements

for the material without carbon and the values reported in

the literature from static measurements at 5 bar hydrogen

pressure [26] are in very good agreement, and range between

0.5 and 0.6 W m�1 K�1. The mean temperature of the bed

during the measurements does not seem to have a distinct

influence on the values obtained. The differences between

the various measurements are within the accuracy of the

estimation.

The addition of carbon leads to a roughly 50% higher

thermal conductivity, amounting to around 0.8 W m�1 K�1.

Nevertheless, the behaviour during the absorptions and

desorptions is qualitatively the same as without carbon, both

showing a significant delay between the two steps of reactions

as shown in Figs. 11 and 12. However, carbon improves the

kinetics of both absorption and desorption.

The improvement of hydrogenation kinetics and thermal

conductivity by carbon additions is still not sufficient for

technical applications. Therefore, further investigations on

the mechanisms effective in sodium alanate with carbon

additions are of high interest.

4. Conclusions

The size of the hydride bed as well as the container has

a significant effect on the hydrogenation kinetics of NaAlH4

system. This size effect is to be taken into consideration when

designing applications where large quantities of hydrogen are

to be stored. For cells of diameters between 2 mm and

15.2 mm, different absorption curves and temperature profiles

are obtained using a Sievert’s apparatus. For an initial

temperature of 125 �C, larger diameters up to a critical size

between 4.5 mm and 10 mm lead to faster absorption kinetics

due to optimised temperatures developing for the first step of

reaction. However, high peaks in temperature are detected in

the axis of the bed of the material during hydrogenation,

particularly for larger powder bed sizes. A delay between the

two steps of formation of the alanate is present. Although

Na3AlH6 is formed and thermodynamic conditions for the

formation of NaAlH4 are present, hydrogenation does not

proceed. This delay has to be caused by the condition of

Na3AlH6 or Al after the first step and can, for example, be

unfavourable for the NaAlH4 nucleation process.

Heat conductivities of NaAlH4 are measured to be around

0.5 W m�1 K�1, which agree well with values from reported

static measurements. The addition of carbon to the NaAlH4

system leads to higher effective heat conductivity of the

hydride bed. With the addition of 5 wt.% of carbon, the

effective heat conductivity was increased by about 50%.

Furthermore, the addition of carbon slightly improved sorp-

tion kinetics.

Further work will focus on the theoretical analysis of the

requirements of engineering properties of the system and

how they affect the performance of the system, focused on

hydrogen volumetric and gravimetric capacities. The analysis

will be supported by experiments with large beds of material.

5. Outlook

The experimental work presented and the subsequent anal-

ysis have given essential information concerning the


influence of heat transfer during the hydrogenation and

dehydrogenation of sodium alanate. Nevertheless, not only is

further experimental work with possible modifications of the

material required, but also fundamental work focused on the

performance of the system as a function of the physical

properties of the material and the system (heat capacity, heat

conductivity, density) is necessary. In a first approach, and

supported by the performed experiments, calculations and

simulations of a large bed system will lead to the real

requirements of these properties and how they affect the

feasibility of the system, mainly in terms of hydrogen gravi-

metric and volumetric capacities. This theoretical analysis

will guide the modification of the material as well as the

design of the necessary heat transfer system. Experimental

work with a large bed of material will validate these calcula-

tions and will show the feasibility to fulfil the requirements in

a real application.

Acknowledgments

The authors thank Mr. C. Holvey for the experimental support.

Partial funding by the European Project ‘‘Hydrogen Storage

Systems for Automotive Application’’ is gratefully acknowl-

edged by the authors.

r e f e r e n c e s

[1] Schlapbach L, Zuttel A. Hydrogen-storage materials formobile applications. Nature 2001;414:353–8.

[2] Zuttel A, Wenger P, Rentsch S, Sudan P, Mauron Ph,Emmenegger Ch. LiBH4 a new hydrogen storage material. JPower Sources 2003;118:1–7.

[3] Bogdanovic B, Schwickardi M. Ti-doped alkali metalaluminium hydrides as potential novel reversible hydrogenstorage materials. J Alloy Compd 1997;253–254:1–9.

[4] Anton DL. Hydrogen desorption kinetics in transition metalmodified NaAlH4. J Alloy Compd 2003;356–357:400–4.

[5] Sandrock G, Gross K, Thomas G. Effect of Ti-catalyst contenton the reversible hydrogen storage properties of the sodiumalanates. J Alloy Compd 2002;339:299–308.

[6] Eigen N, Gosch F, Dornheim M, Klassen T, Bormann R.Improved hydrogen sorption of sodium alanate by optimizedprocessing. J Alloy Compd 2008;465:310–6.

[7] Eigen N, Keller C, Dornheim M, Klassen T, Bormann R.Industrial production of light metal hydrides for hydrogenstorage. Scr Mater 2007;56:847–51.

[8] Bernauer O, Topler J, Noreus D, Hempelmann R, Richter D.Fundamentals and properties of some Ti/Mn based lavesphase hydrides. Int J Hydrogen Energy 1989;14:187–200.

[9] Goodell PD. Thermal conductivity of hydriding alloy powdersand comparisons of reactor systems. J Less Common Met1980;74:175–84.

[10] Sun D, Deng S. Theoretical descriptions and experimentalmeasurements on the effective thermal conductivity inmetal hydride powder beds. J Less Common Met 1990;160:387–95.

[11] Suissa E, Jacob I, Hadari Z. Experimental measurements andgeneral conclusions on the effective thermal conductivity ofpowdered metal hydrides. J Less Common Met 1984;104:287–95.

[12] Pons M, Dantzer P. Determination of thermal conductivityand wall heat transfer coefficient of hydrogen storagematerials. Int J Hydrogen Energy 1994;19:611–6.

[13] Zhang J, Fischer TS, Ramachandran PV, Gore JP, Mudawar I. Areview of heat transfer issues in hydrogen storagetechnologies. J Heat Transfer 2005;127:1391–9.

[14] Suda S, Komazaki Y, Kobayashi N. Effective thermalconductivity of metal hydride beds. J Less Common Met 1983;89:317–24.

[15] Nagel M, Komazaki Y, Suda S. Effective thermal conductivityof a metal hydride bed augmented with a copper wire matrix.J Less Common Met 1986;120:35–43.

[16] MacDonald BD, Rowe AM. Impacts of external heat transferenhancements on metal hydride storage tanks. Int JHydrogen Energy 2006;31:1721–31.

[17] MacDonald BD, Rowe AM. Experimental and numericalanalysis of dynamic metal hydride hydrogen storagesystems. J Power Sources 2007;174:282–93.

[18] Mohan G, Prakash Maiya M, Srinivasa Murthy S. Performancesimulation of metal hydride hydrogen storage device withembedded filters and heat exchanger tubes. Int J HydrogenEnergy 2007;32:4978–87.

[19] Mellouli S, Askri F, Dhaou H, Jemni A, Ben Nasrallah S. Anovel design of a heat exchanger for a metal-hydrogenreactor. Int J Hydrogen Energy 2007;32:3501–7.

[20] Botzung M, Chaudourne S, Gillia O, Perret C, Latroche M,Percheron-Guegan A, et al. Simulation and experimentalvalidation of a hydrogen storage tank with metal hydrides.Int J Hydrogen Energy 2008;33:98–104.

[21] Kumar Phate A, Prakash Maiya M, Srinivasa Murthy S.Simulation of transient heat and mass transfer duringhydrogen sorption in cylindrical metal hydride beds. Int JHydrogen Energy 2007;32:1969–81.

[22] Nakagawa T, Inomata A, Aoki H, Miura T. Numerical analysisof heat and mass transfer characteristics in the metalhydride bed. Int J Hydrogen Energy 2000;25:339–50.

[23] Muthukumar P, Madhavakrishna U, Dewan Anupam.Parametric studies on a metal hydride based hydrogenstorage device. Int J Hydrogen Energy 2007;32:4988–97.

[24] Gambini M, Manno M, Vellini M. Numerical analysis andperformance assessment of metal hydride-based hydrogenstorage systems. Int J Hydrogen Energy 2008;33:6178–87.

[25] Brown TM, Brouwer J, Samuelsen GS, Holcomb FH, King J.Accurate simplified dynamic model of a metal hydride tank.Int J Hydrogen Energy 2008;33:5596–605.

[26] Dedrick DE, Kanouff MP, Replogle BC, Gross KJ. Thermalproperties characterization of sodium alanates. J AlloyCompd 2005;389:299–305.

[27] Mosher DA, Arsenault S, Tang X, Anton DL. Design,fabrication and testing of NaAlH4 based hydrogen storagesystems. J Alloy Compd 2007;446–447:707–12.

[28] Gross KJ, Majzoub E, Thomas GJ, Sandrock G. Hydridedevelopment for hydrogen storage. In: Proceedings of 2002 U.S. DOE hydrogen program review, Golden, CO. NREL/CP-610-32405.

[29] Bogdanovic B, Brand R, Marjanovic A, Schwikardi M,Tolle J. Metal-doped sodium aluminium hydrides aspotential new hydrogen storage materials. J Alloy Compd2000;302:36–58.

Effects of heat transfer on the sorption kinetics of complex hydride reacting systems

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