Mechanism Analysis on Stress Accumulation in Cylindrical Vertical-Placed Metal Hydride Reactor

doi:10.4236/epe.2011.34059

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Energy and Power En gi neering, 2011, 3, 490-498

doi:10.4236/epe.2011.34059 Published Online September 2011 (http://www.SciRP.org/journal/epe)

Mechanism Analysis on Str e ss Accumulation in Cylindrical

Vertical-Placed Metal Hydride Reactor

Xiaochen Hu, Zhaogang Qi, Feng Qin, Jiangping Chen

Institute of Refrigeration and Cryogenics Engineering,

Shanghai Jiao Tong University, Shanghai, China

E-mail: [email protected]

Received June 29, 2011; revised July 30, 2011; accepted August 15, 2011

Abstract

It’s known that the pulverization-densification mechanism of metal hydride may cause the stress accumula-

tion in metal hydrides reactors. In this paper, this idea is proved based on granulometry and a new idea of

cycling compression effect is presented, which is caused by the friction between wall and metal hydrides.

Through theoretical analysis, the cycling compression effects is shown to increase the localized packing rate

from top to down in vertical-placed reactors and thus lead to the maximum deformation in the bottom of re-

actors, proving that it is the interaction of pulverization-densification effect and cycling compression effect

resulting in the stress problems of vertical-placed reactors. Further study points that the effective methods

relieving the cycling compress effect are to decrease hydrogen absorption/desorption cycle number, slender-

ness ratio of reactor, wall friction factor and initial packing rate, or to lower the thermal conductivity and the

volume expansion coefficient of metal hydrides.

Keywords: Metal Hydride, Stress Accumulation, Pulverization, Densification

1. Introduction

The pulverization and expansion of metal hydrides during

its absorption/desorption cycles could decrease the reliabil-

ity of the reactor directly and greatly, becoming one of the

key obstacles to the development and application of hy-

drogen energy. Many studies have been done to explain

and solve this problem. In Nakamura Y. et al.’s study, the

expansion ratio for AB5 type metal hydrides can be larger

than 1.2 during the hydrogen absorption, being the domi-

nant factors in the intrinsic powder degradation [1]. B. Y.

Ao et al. also found in their test that the LaNi5 alloy’s vol-

ume expansion ratio was up to 24% after absorbing hydro-

gen, causing the density of alloy from the top to bottom of

the bed higher and higher and thus the stress greater and

greater after several cycles [2]. They indicated that the in-

crease of initial packing rate and decrease of the wall

thickness could help relieving the stress accumulation. Na-

sako K. et al. presented that the localized stress would be

generated at the bottom of the reactor with an alloy packing

rate of 40% (v/v), and increased with each absorption/de-

sorption cycle [3]. They then proved that the stress accu-

mulation depend on the amount of hydrogen absorption/

desorption cycles and on the initial packing rate. Other

experimental studies also proved that the actual stress could

rise up acutely with the increase of localized relative den-

sity, depth of reactor and cycle number et al. [4-6]. As a

conclusion, the pulverization-densification mechanism is

proposed and widely accepted: during the process of hy-

drogen absorption/desorption cycles, since the metal hy-

drides powder is crushing up gradually, the finer and finer

particles becomes gathering into to the bottom of the reac-

tor under the pull of gravity, which finally cause the local-

ize relative density of metal hydrides powder to increase

and self-densification to take place after several repeated

manner [4]. Hence, in order to smooth the expansion of

metal hydrides and the congestion of stress, researchers

have carried out a lot of attempt, for example, designing

special construction of reactor, like “rabloc” and “free

space” in the reactor advised by Block F. R. et al. [7] and

Bernauer O. et al. [8], respectively.

Although a lot of investigations have indicated that the

stress accumulation is clarified by the pulverization-

densification of metal hydrides and the absorption/de-

sorption cycles, few surveys have been presented about

the reason how the absorption/desorption cycles could

help the concentration of stress. In this study, a new idea

of cycling compression effect is proposed to explain the

principle of absorption/desorption cycles on the stress

accumulation. Cycling compression effect is caused by

X. C. HU ET AL.491

the friction between bed wall and metal hydrides powder.

Through the model simulation, its mechanism has been

described and the factors which perform influence to the

effect have been also studied.

2. Experimental

Figure 1 shows the structure of the metal hydrides reac-

tor. The reactor made of stainless steel was filled with

La0.6Y0.4Ni4.8Mn0.2. The alloy was prepared in a vacuum

arc remelting furnace under argon (99% pure) atmos-

phere. The purity of the metal used is as follows: La is

99% pure, Y and Ni is 99.5% pure, Mn is 99.7% pure.

The alloys were annealed at 1323 K for 12 h in the vac-

uumed quartz tubes. They were then activated in the

method of surface modification.

In the first step, the distribution of metal hydrides

powder in different location was measured using micro-

scope and 25 hydrogen absorption/desorption cycles

were processing in the vertical-placed reactor with 51.5

vol% initial packing rate. After each cycle, the particle

distribution was measured with the metal hydrides sam-

ple extracted in the location of 10 mm, 63 mm and 78

mm away from the bottom of the bed, using B-spline

curve to smooth the histogram data. The particle distri-

bution after 25th cycles using the metal hydrides powder

with initial size ranging from 212 μm to 380 μm was

considered as the basis set for comparison

The hydrogen absorption/desorption cycles was tested

in the measurement as Figure 2. And the reactor was

placed vertically. The absorption stage and desorption

Figure 1. Structure of thin-wall cylindrical reactor. (A:

Hydrogen entrance; B: Stainless steel sintered filter; C:

Alloy; D: Stainless steel cylinder; E: End cap; F: Joint

socket).

Figure 2. The reactor cycling strain test bench. (A: Hydro-

gen cylinder; B: Pressure regulator; C: Ball valve; D: Hy-

drogen reference cylinder; E: Pressure sensor; F: Vacuum

pump; G: Thermostatic water bath; H: Pt100 RTD; I:

Thin-wall sample vessel; J: Compensating vessel; K: Biaxial,

90° stacked rosettes; L: Strain amplifier; M: Data acquisi-

tion system; N: Computer).

stage were lasting for one hour each. The hydrogen-inlet

pressure and water bath temperature were 3 MPa and 313

K, respectively, ensuring the maximum hydrogen content

per mass alloy around 1 molmol−1.

3. The Pulverization-Densification Effect

Figure 3 shows the metal hydride particle size distribu-

tions at different height in the reactor. With the sampling

point’s dropping, a left shift is found in the distribution

curve obviously, which means the proportion increase of

fine particles. The cumulative distribution at 10 mm, 63

mm and 78 mm height is 24.6%, 16.2% and 9.4% in the

range of 1 to 5 μm particle diameter and 11.1%, 21.9%

and 21.1% in the range of 20 to 80 μm, when the distri-

bution at the basis set is 11.6% and 28.8%, respectively.

The above kind of distribution is mainly caused by the

pulverization and particle sedimentation of metal hydride.

During the absorption/desorption cycles, gravity pro-

vides a driving force for the sedimentation of particles.

At the same time, the extrusion, stretching, isolation and

fragmentation between particles as well as the frequent

110100

Differential volume fraction (%)

Particle diameter (



10 mm

63 mm

78 mm

Baseline

42 5035

Figure 3. Particle size distributions at different height in the

reactor.

X. C. HU ET AL.

492

scaling of particle distance led by absorption/desorption

cycles accelerate the particle deposition. However, the

particles of large and medium size are easily blocked

during the sinking process, while the fine particles can

sink sufficiently because of its small size. In the test, the

fully sedimentation of fine particles whose diameter is

less than 5 μm from the middle and top of the bed leads

to the proportion increase of fine particles in the bottom

of the bed. On the other hand, the proportion of the me-

dium size particles from 10 to 20 μm increases in the

middle of the bed because of their inadequate sedimenta-

tion, making the curves of 63 mm and 78 mm distribute

between those of 10 mm and basis set. From the results,

the effect of particles sedimentation performs more ob-

viously with the increasing height in the bed. And in the

bottom of the bed, since the packing rate is increased as

the sink of more and more fine particles, the appearance

of stress accumulation presents more prominent than

those in the top and middle of the bed. So to decrease the

height of metal hydrides filled can be an effective

method to reduce the stresses in the bed.

4. The Effect of Cycling Compression

4.1. The Ideal Model of Cycling Compression

Effect

Before analysis, various assumptions made for the sim-

plified modeling are given below.

 The relative density of metal hydrides is assumed

constant in the radial direction and axial-variable

relative density is considered only.

 Considering the mass of hydrogen absorbed/desorbed

in the process, the “alloy composition packing den-

sity” ρa is defined as the mass of both metal hydrides

and hydrogen per unit volume in the bed. That is, af-

ter the desorption process has finished completely, the

ρa is then consistent with the packing density of metal

hydrides in the bed.

 During the cycle, the process of absorption/desorp-

tion is assumed synchronous in each part, which

means the localized alloy composition packing den-

sity and localized packing density are homogeneous

in the same area.

Figure 4 shows the ideal process of cycling compres-

sion effect in one absorption/desorption cycle. The metal

hydrides in the bed are divided into two parts: Part A

near the hydrogen-inlet and Part B far away the hydro-

gen-inlet, with black spots showing the density of alloys

approximately. Since the absorbing/desorbing velocity of

metal hydrides is quite fast, it is suitable to argue that the

absorption/desorption in Part A is processing earlier than

that in Part B. On step a, metal hydrides in two parts are

all loose and uniform. As the absorption process is start-

ing up (step b), metal hydrides in Part A begin to absorb

hydrogen, then expand and squeeze those in Part B,

which leading to the increase of ρa in Part B and the de-

crease of ρa in Part A. The compression deformation of

metal hydrides happened in Part B includes irreversible

plastic deformation and reversible elastic deformation [2].

And then metal hydrides in Part B start the process of

absorption, expansion and reversion (step c) which make

the ρa in Part B decrease and the ρa in Part A recovery

appreciably. However, this recovery procedure is hin-

dered by the friction force produced between fully-ex-

pansive metal hydrides in Part A and the bed wall. The ρa

in both parts can’t resume to the initial state before ab-

sorption and the ρa in Part B still keeps larger than that in

Part A (step d). During the desorption process on step e,

metal hydrides in Part A is starting to shrink firstly when

those in Part B is keeping on recovering its elastic de-

formation part, which is also held back by the friction

force occurring between fully-expansive metal hydrides

in Part B and the bed wall. Along with the progress of

cycle, metal hydrides in Part B are then desorbing hy-

drogen, shrinking and finally retain the plastic deforma-

tion with no hydrogen. Compared with the status of

pre-cycle, the interface of two parts is downward shifting

and the localized packing rate of metal hydrides in Part B

is higher than that in Part A. That is, for the metal hy-

dride reactor bed, the cycling compression effect would

cause the localized packing rate of alloys growing from

the hydrogen-inlet to the bottom of the bed.

4.2. The Modeling for the Practical Process of

Cycling Compression Effect

4.2.1. The Calculation Principle

In order to get quantitative verification and analysis on

the cycling compression effect, the absorption and de-

sorption processes of metal hydrides in the reactor bed

Figure 4. The ideal process of cycling compression effect.

(Step (a) initial state before absorption; (b) absorption of

Part A; (c) absorption of Part B; (d) after absorption; (e)

desorption process; (f) after desorption).

X. C. HU ET AL.

493

ption cycles, respectively. It can be observed that after

multiple cycles the localized relative density of metal

hydrides is growing from the hydrogen-inlet (right side)

to another closed bottom (left side) and on the radial di-

rection, the localized relative density of left-part metal

hydrides is increasing while that of right-part is decreas-

ing. The maximum localized relative density appears in

the corner where the metal hydrides powder is transiting

to the bottom of the bed (top left corner in Figure 7) and

the minimum value is on the top right corner near the

hydrogen inlet. In additional, compared with that after 5

cycles, the localized relative density after 50 cycles is

obviously higher in each part of the bed. These results

are consistent with the above analysis on the ideal model.

So it can be proved that there exists the cycling compres-

sion effect caused by the friction force between ex-

panded metal hydrides powder and the bed wall in the

reactor.

were modeled on the finite element analysis software

MSC. MARC. It uses the Shima yield function to de-

scribe the plastic yield phenomenon of metal hydrides

powders approximately [9]:

1/2



















(1)

where σy is the unidirectional yield stress,



 is the de-

viatoric stress tensor and p is the iso-static pressure. β

and γ are the related parameters of metal hydrides. They

can be determined through uniaxial compression test and

iso-static pressure test.

To the metal hydride powder, its plastic yield is occur-

ring in the hydrogen absorption process. However, its

expansion during hydrogen absorbing stage is a chemical

reaction process, making it hard to establish the model

directly. Considering the similarity between mass trans-

fer and heat transfer process in the reactor bed, it is pro-

posed to replace the hydrogen source with heat source

and use the thermal expansion coefficient instead of the

volume expansion coefficient in the model. The influ-

ence law of absorption and desorption cycle on localized

packing rate of metal hydrides in the bed is studied in

this simulation model, without regard to the effect of

pulverization-densification.

Study on the localized relative density in selected

spots has been carried out, too. Shown on Figure 8, six

paths are chosen in the model: Path a is from the bottom

to the hydrogen-inlet along the symmetry axis; Path r1 to

Path r5 are pointing to the bed wall along the radial di-

rection, whose spacing with the bottom of bed is 1, 15,

Table 1. Parameters of initial condition and metal hydrides.

4.2.2. Model and Boundary Co ndi tions Fraction coefficient 0.2

Thermal conductivity of metal hydride 20 W/(m·K)

Initial packing rate of metal hydrides 40 vol%

Thermal expansion rate 20 vol%

Linear expansion coefficient 0.000333/˚C

Since it is assumed that the effect of cycling compression

is axisymmetric in the cylindrical reactor, only half of

the reactor has been modeled as shown in Figure 5.

Defining the inner wall as rough wall surface, the fric-

tion force exists between metal hydrides powder and

medial surface of the reactor bed. The parameters of ini-

tial condition and system are given in Table 1. In the

model, a temperature constraint shown in Figure 5 is

also added on the right side of metal hydrides to simulate

the process that hydrogen starts to go into the powder

from the inlet on the right side of the reactor bed. Figure

6 gives the constraint of time on temperature. The high-

est temperature of heat source is 200˚C.

4.2.3. Simulati on C oncl usi o n and Di scussi o n

Figure 7 shows the contour map on localized relative

density of metal hydrides after 5 and 50 absorption/desor- Figure 6. Temperature constraint of time in the model.

Figure 5. Calculation model of cycling compression effect.

X. C. HU ET AL.

494

(a)

(b)

Figure 7. The contour map of localized relative density. (a) After 5 cycles. (b) After 50 cycles.

Figure 8. Analysis spots and analysis paths for the packed alloy.

30, 45 and 59 mm, respectively. Meanwhile, seven spots

are selected in the paths.

0 102030405060

0.40

0.41

0.42

0.43

Axial location (mm)

Relative density

1st cycle

5th cycle

10th cycle

25th cycle

50th cycle

Figure 8 shows the localized relative density in each

path. In Figure 9(a), it is observed that the localized

relative density in Path a is decreasing from the bottom

to the inlet where exists fluctuation within a narrow

range and as the increase of the cycle numbers it is

growing up with different amplitude in all locations of

Path a. It is mainly due to the radial compression of

metal hydrides powder and since the cycling compres-

sion effect is reacting on the symmetry axis, the increase

amplitude of the localized relative density at the bottom

is higher than that near the inlet. Figure 9(b) reveals the

distribution of localized relative density on Path r1 to

Path r5 after 5 cycles and the localized relative density

increases radially along Path r1 to Path r3 contrary to

that along Path 4 and Path 5. This is the result of the

friction force between metal hydrides powder and the

bed wall, which make the cycling compression effect

reacting on the regions near the bed wall more signifi-

cantly.

(a)

01234567

0.399

0.401

0.403

0.405

0.407

0.409

Relative density

Radial location (mm)

Path r1

Path r2

Path r3

Path r4

Path r5

Figure 10 shows the localized relative density time

history curves on #a1 and #a5 spot, presenting an obvi-

ous rise of localized relative density in the bottom. Fig-

ure 11 indicates another time history curves of axial and

radial displacement on spot #e5 near the inlet and with

the progress of cycling metal hydrides at the terminal

point of each cycle are keep shrinking in the radial direc-

tion under compressing and keep stretching in the axial

direction caused by radial compressing as well as axial

(b)

Figure 9. Distribution of localized relative density on alloy

analysis paths. (a) Path a; (b) Path r1 to Path r5 after 5 cy-

cles.

X. C. HU ET AL.495

06001200 1800 2400 3000

0.396

0.400

0.404

0.408

0.34

0.36

0.39

0.41

#a5 Localized relative density

#a1 Localized relative density

Time (s)

# a1

# a5

2400 2550 2700 2850 3000

-1.0x105

-8.0x104

-6.0x104

-4.0x104

-2.0x104

0.0

2.0x104

4.0x104

Contact friction force (Pa)

Time (s)

# e1

# e2

# e3

# e4

# e5

Figure 10. Time history curves of localized relative density

on spot #a1 and #a5.

06001200 1800 2400 3000

0.0

0.5

1.0

1.5

2.0

-0.055

-0.040

-0.025

-0.010

0.005

Time (s)

Radial displacement (mm)

Axial displacement (mm)

Axial displacement

Radial displacement

Figure 11. Time history curves of displacement on spot #e5.

rubbing until they separate with the bed wall. Further, the

time history of friction force in spots #e1 to #e5 in the

fifth cycle is studied as shown on Figure 12. The friction

force performs more notable in the spots near the inlet,

where the friction force is negative during the absorption

process and reaching the maximum in the end of the ab-

sorbing stage, being an obstacle to the stretch of metal

hydrides powder. In the desorption process, the friction

force is decreasing gradually and transiting to be positive

in the middle stage, indicating that the friction force is

turning to an obstacle to the shrinkage of metal hydrides

powder. These results are also consentaneous compared

with the above theoretical analysis on cycling compres-

sion effect. Moreover, a negative peak of friction force

appears on spots #e2 and #e3 in the later desorption stage

and the time node of negative peak is delaying with the

left shift of test spots. This is because the stress release

process of metal hydrides during desorption is asyn-

chrony on the radial direction. After 2900 seconds the

friction force on each spot becomes zero, meaning that

the metal hydrides powder is separated from the bed wall

in the later stage of cycle, confirming the displacement

change on radial direction of #e5 on Figure 11.

4.2.4. Influencing Factor of the Cycling Compression

Effect

4.2.4.1. Slenderness Ratio of Reactor

It has been proved that an effective method of decreasing

Figure 12. Time history curves of friction force on spots #e1

to #e5 in the fifth cycle.

the stress is to reduce the vertical height of reactor

[10,11]. Considering the deformation of metal hydrides

powder on the radial direction as above analysis, the

slenderness ratio of reactor is used as a structural factor

of the cycling compression effect. Figure 13 shows the

distribution of localized relative density on Path a with

different reactor slenderness ratios after 5 cycles, whose

abscissa is relative location (relative location is the ratio

of test spot coordinate and total metal hydrides powder

length). It is observed that with the increasing of reactor

slenderness ratio the localized relative density is growing

up remarkably but its amplitude is slowdown. It is be-

cause the contact area of metal hydrides powder and re-

actor is increasing with the slenderness ratio, making the

effect of friction more obviously and the localized rela-

tive density larger. However, the increase of friction and

reactor length also has the hysteresis effect on the de-

veloping process of the cycling compression effect.

Hence, to decrease the slenderness ratio of reactor ap-

propriately can help relieve the cycling compression ef-

fect.

4.2.4.2. Cycle Number of Times

Figure 14 shows the effect of cycle number on the cy-

cling compression effect. In Figure 14(a), it is shown

that the relative density of metal hydrides is increasing

with the cycle number of times on spots #a1 and #a5.

Similarly, the axial and radial displacement of spot #e5

shown in Figure 14(b) is also increasing. These two re-

sults present that the cycling compression effect increase

with the cycle number and it may cause that the localized

relative density in the bottom of reactor exceed the al-

lowed value after a certain cycle number.

4.2.4.3. Thermal Conductivity of Metal Hydrides

Figure 15 shows the distribution of relative density in

Path a after 5 cycles with changing thermal conductivity

X. C. HU ET AL.

496

0.0 0.1 0.2 0.30.4 0.5 0.6 0.7 0.8 0.9 1.0

0.402

0.403

0.404

0.405

0.406

0.407

0.408

0.409

Relative density

Relative locatio

l /d=2.33

l /d=3.26

l /d=4.29

l /d=5.40

l /d=6.60

Figure 13. Localized relative density on Path a with differ-

ent reactor slenderness ratios after 5 cycles.

0 102030405

0.40

0.41

0.42

0.43

Relative density

Cycle number of times

# a1

# a5

(a)

0 1020304050

0.0

0.7

1.4

2.1

2.8

3.5

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

Cycle number of times

Radial displacement (mm)

Axial displacement (mm)

Axial

Radial

(b)

Figure 14. Influence of cycle number of times in cycling

compression effect. (a) Spots #a1 and #a5; (b) Spot #e5.

of metal hydrides. As is shown, the relative density in the

same location increase with the thermal conductivity but

the rising amplitude is decreasing gradually. This is be-

cause the lower mass-transfer velocity made the axial

expansion of metal hydrides powder develop completely

before the friction force between expanded metal hy-

drides powder and the bed wall becomes strong, de-

creasing the axial plastic deformation of metal hydrides

powder. Therefore, to choose a reasonable mass-transfer

velocity of metal hydrides can help inhibit the cycling

compression effect.

4.2.4.4. Volume Expansion Coefficient of Metal

Hydrides

Figure 16 presents the distribution of relative density in

Path a after 5 cycles with volume expansion coefficient

of metal hydrides. Obviously, the cycling compression

effect was strengthened with the increase of volume ex-

pansion coefficient.

4.2.4.5. Wall Friction Factor

Figure 17 shows the distribution of relative density in

Path a after 5 cycles with different wall friction factor

between metal hydrides powder and the bed wall. These

distribution curves present a decreasing tendency basi-

cally and the localized relative density of metal hydrides

powder in the same location increased with the wall fric-

tion factor. It indicates that the wall friction factor has

0 10203040506

0.400

0.402

0.404

0.406

0.408

0.410

k = 10 w/(m

k = 20 w/(m (Baseline)

k = 50 w/(m

Axial location away from Symmetry Axis (mm)

Relative density

Figure 15. Influence of alloy thermal conductivity (mass

transfer rate) in cycling compression effect.

0 10203040506

0.400

0.403

0.406

0.409

0.412

0.415

a= 10 vol%

a= 20 vol% (Baseline)

a= 30 vol%

Axial location away from Symmetry Axis (mm)

Relative density

Figure 16. Influence of alloy expansion in cycling compres-

sion effect.

X. C. HU ET AL.497

0 10203040506

0.400

0.403

0.406

0.409

Relative density

Axial location away from Symmetry Axis (mm)



= 0.15



= 0.2 (Baseline)



= 0.3



= 0.5

Figure 17. Influence of wall friction factors in cycling com-

pression effect.

promotion on the cycling compression effect. On the

other hand, the shape of curves makes a great difference

as shown in the figure: the larger the wall friction factor,

the straighter the curve in the location near the bottom of

the bed. This is mainly caused by the obstruction of fric-

tion force on the stretching motion of metal hydrides

powder during the absorption process. Larger friction

factor would limit the axial stretching of metal hydrides

powder. Furthermore, according to the generalized Hook’s

law, the radial expansion of metal hydrides powder in-

creases inevitably with larger friction factor, rising the

amount of compression up and finally resulting in the

growth of the localized relative density.

4.2.4.6. Initial Packing Rate

Figure 18 shows the effect of initial packing rate on

relative density in Path a after 5 cycles. Here, the rela-

tive density increment of metal hydrides is used in the

ordinate. As shown, if the initial packing rate is lower,

the relative density increment will become larger, which

means that the cycling compression effect presents much

more obvious.

0 10203040506

-2

Relative density increment (%)



= 20 vol%



= 40 vol% (Baseline)



= 60 vol%

Axial location (mm)

Figure 18. Influence of initial packing fractions in cycling

compression effect.

5. Conclusions

1) The stress accumulation in the metal hydrides reactors

is due to the interaction of pulverization-densification

effect and cycling compression effect. In the vertically-

set reactor, both effects cause the metal hydrides powder

gather to the bottom of the bed body.

2) In the vertically-set metal hydrides reactor, the cy-

cling compression effect becomes more obvious from the

top to the bottom. More and more fine particles are de-

positing to the bottom, making the relative density larger

and stress accumulation prominent. Hence, decreasing

the height of the reactor can reduce the stress.

3) The cycling compression effect is caused by both

the expansion of metal hydrides and friction force be-

tween metal hydrides powder and the bed wall. To re-

lieve the influence of cycling compression effect, it is

helpful to decrease cycle number, the slenderness ratio of

reactor, wall friction factor and initial packing rate. On

the metal hydrides side, the cycling compression effect

also can be decreased with the metal hydrides of lower

thermal conductivity and volume expansion coefficient.

6. Acknowledgements

The work has been supported by the National Natural

Science Foundation of China (NO. 50776094) and High-

Tech Research and Development Project of China (No.

2006AA05Z135). The authors thank the Institute of

Metal Research, Chinese Academy of Sciences, for ap-

plying the metal hydrides alloys.

7. References

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and I. Uehara, “Lattice Expanding Behavior and Degra-

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doi:10.1016/S0925-8388(97)00503-3

[2] B. Y. Ao, S. X. Chen and G. Q. Jiang, “A Study on Wall

Stresses Induced by Lani5 Alloy Hydrogen Absorp-

tion-Desorption Cycles,” Journal of Alloys and Com-

pounds, Vol. 390, No. 1-2, 2005, pp. 122-126.

doi:10.1016/j.jallcom.2004.05.092

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