Evaluation of metal hydride machines for heat pumping and cooling applications: Evaluation des machines à hydrure métallique dans les applications de pompes à chaleur et de refroidissement

Evaluation of metal hydride machines for heat pumping andcooling applications

E. Willers*, M. Groll

Institut fuÈr Kernenergetik und Energiesysteme der UniversitaÈt Stuttgart (IKE), Pfaffenwaldring 31, 70550 Stuttgart, Germany

Received 5 March 1997; received in revised form 1 August 1997

Abstract

Three different schemes of metal hydride solid sorption devices for heat pumping and cooling applications are presented and

compared based on theoretical evaluations. Key parameters obtained from experimental and simulation results from coupled

metal hydride reaction beds have been used for the theoretical evaluation. The single (HS) and double stage (HD) devices show

reasonable performances, but they require many moving parts. Using high performance reaction beds, e.g. a capillary tube

bundle reaction bed, cycle times of about 5±10 min can be obtained with these devices. This corresponds to a speci®c power

output of 100±200 W kg21 (HS) or 150±300 W kg21 (HD), referred to the total hydride inventory of the machine. The multi-

hydride-thermal-wave (HW) system has a lower speci®c power output, but it offers signi®cant advantages like modest hardware

effort, low pumping power and a very wide operating temperature range. q 1998 Elsevier Science Ltd and IIR. All rights

reserved.

Keywords: Metal hydrides; Absorption; Refrigeration; Multi-stage systems

Evaluation des machines aÁ hydrure meÂtallique dans les applicationsde pompes aÁ chaleur et de refroidissement

ResumeÂ

On aÁ preÂsenteÂ et compareÂ, aÁ partir d'eÂvaluations theÂoriques, trois diagrammes diffeÂrents d'appareils aÁ sorption solide aÁ

l'hydrure meÂtallique utiliseÂ pour le pompage de chaleur et le refroidissement. Pour l'eÂvaluation theÂorique, on a utiliseÂ les

parameÁtres cleÂs obtenus de facËon expeÂrimentale et par simulation avec des lits de reÂaction coupleÂs metal-hydrure. Les

appareils aÁ un eÂtage (HS) et aÁ deux eÂtages (HD) preÂsentent des performances satisfaisantes mais neÂcessitent beaucoup de

pieÂces mobiles. En utilisant des lits de reÂaction aÁ haute performance, par exemple un lit de reÂaction aÁ faisceau de tubes

capillaires, des temps de cycle d'environ 5 aÁ 10 minutes peuvent eÃtre obtenus. Ceci correspond aÁ une puissance utile de 100 aÁ

200 W kg21 (HS) ou 150 aÁ 300 W kg21 (HD) par rapport aÁ la masse total d'hydrure dans la machine. Le systeÁme multi-hydrure

aÁ ondes thermiques (HW) a une puissance speÂci®que infeÂrieure, mais comporte des avantages signi®catifs tels des besoins en

mateÂriel modestes, une puissance de pompage faible et une gamme de tempeÂrature de fonctionnement treÁs large. q 1998

Elsevier Science Ltd and IIR. All rights reserved.

Keywords: Hydrure meÂtallique; Absorption; ReÂfrigeÂration; SysteÁme bi-eÂtageÂ

International Journal of Refrigeration 22 (1999) 47±58

0140-7007/99/$ - see front matter q 1998 Elsevier Science Ltd and IIR. All rights reserved.

PII: S0140-7007(97)00056-X

* Corresponding author.

Nomenclature

A Surface area (m2)

c Speci®c heat capacity (J kg21 K21)

COPC Coef®cient of performance, cooling

COPH Coef®cient of performance, heating

m Mass (kg)

mÇ Mass ¯ow (kg s21)

n Number of kilomoles

NTU Number of transfer units

Q Heat (J)

T Temperature (K)

Greek letters

a Heat transfer coef®cient (W m22 K21)

DH Reaction enthalpy (J kmol21 H2)

DS Reaction entropy (J kmol21 H2 K21)

DT Temperature difference (K)

z Ef®ciency

l Thermal conductivity (W m21 K21)

t Cycle time (s)

r Density (kg m23)

f Internal heat recovery rate

Indices

I Circuit I

II Circuit II

A Reactor A

Abs Absorption

av Average

B Reactor B

C Reactor C

Cool Cooling temperature level

Des Desorption

Drive Driving heat temperature level

Heat Heating temperature level

inlet Reaction bed inlet

m Medium temperature level

max Maximum exchange of hydrogen

max Maximum temperature of the heat transfer ¯uid in

the heat exchanger

min Minimum temperature of the heat transfer ¯uid in

the heat exchanger

outlet Reaction bed outlet

RB Reaction bed

1. Introduction

Thermally driven sorption machines are attractive

systems for the rational use of energy [1]. They become

even more attractive if the driving heat is waste heat, or

other low grade heat, which can be upgraded to more

valuable temperature levels or if various user demands can

be satis®ed simultaneously.

Appropriate working pairs are zeolite±water [2±4],

salts±ammonia (ammoniated salts) [5±7], activated

carbon±methanol [8], metal±hydrogen (metal hydrides)

[9,10] and some other materials in solid sorption systems,

as well as ammonia±water [11] or water±lithium bromide

[12] in liquid sorption systems.

The temperature range for different applications depends

both on the working pair and on the basic machine scheme.

Compared with single-stage machines, multi-stage machines

lead to a higher temperature lift and/or to higher COPs.

Metal hydrides offer a wide range of potential applica-

tions because there exist hydrides with equilibrium tempera-

tures (for 1 bar equilibrium pressure) between 160 K and

800 K and above [13].

A theoretical `static' evaluation method is used for the

comparison. This method is based on a representation of

the thermodynamic cycle in the van't Hoff diagram and

employs energy balances for the cycle. This simpli®ed analy-

sis is justi®ed, because the technical reaction kinetics are not

mass transfer limited. With this method, very good results are

obtained if the key parameters (e.g. driving temperature

differences, heat transfer coef®cients, etc.) are carefully

selected. The key parameters used in this article have been

derived from both experimental results and the respective

numerical simulations of coupled reaction beds [14±17].

2. Metal hydride heat pumps and cooling devices

There are different machine schemes for thermally driven

metal hydride machines. Some of them have recently been

applied in an air-conditioning system [14] and in heat trans-

formers [15,16]. Three different systems will be discussed:

² the single stage system (HS)

² the double stage system (HD)

² the multi-hydride-thermal-wave system (HW).

All systems have the following speci®c characteristics.

Their useful cold and heat, respectively the heat released

to the ambient, are completely related to sorption process of

low temperature hydrides. Compared with liquid sorption

systems or solid sorption systems with evaporation and

condensation of the working ¯uid, the evaporation is

replaced by the desorption and the condensation is replaced

by the absorption of the low temperature hydride. This leads

to a disadvantage concerning the weight of the system,

which is nearly twice as high as for a system working

with evaporator and condenser.

Additionally, most metal hydrides show a hysteresis

between absorption and desorption as well as a pressure

plateau slope during the phase transition from metal to

metal hydride (see Fig. 1). The amount of hydrogen,

which is usually exchanged between coupled reaction

beds, is characterized by the length of the plateau, indicated

by Dx/xmax. These material-dependent characteristics cause

higher driving and cooling temperatures (Fig. 2) than a

machine with hysteresis-free sorbents can provide, thus

E. Willers, M. Groll / International Journal of Refrigeration 22 (1999) 47±5848

decreasing the thermodynamic ef®ciency of the metal

hydride device. They usually can be improved by a heat

treatment. The temperature limits for a hysteresis-free

device are shown with dotted lines (driving temperature:

TDrive*, cooling temperature: TCool*).

The performance of these machines is a function of the

properties of the metal hydrides, like enthalpy, hysteresis

and plateau slope, as well as of the ef®ciency of the heat

transfer to, from and within the reaction beds. These factors

have a strong in¯uence on the energetic coef®cients of

performance both for cooling

COPC � QCool=QDrive �1�

and for heating

COPH � QHeat=QDrive �2�

2.1. The single-stage system (HS)

The single-stage system comprises two pairs of reactors

containing hydrides A and B (Fig. 2). A and B are coupled

on the hydrogen side. In the ®rst half-cycle hydride B is

desorbed by applying QDrive at temperature level TDrive.

The desorbed hydrogen ¯ows to hydride A owing to a driv-

ing pressure difference (not shown in Fig. 2). The absorption

heat is released to the ambient or to the user depending on

the temperature level Tm. The second half-cycle is

performed at lower pressure. QCool at temperature TCool is

taken up by the desorbing low temperature hydride A,

thus providing useful cold. The released hydrogen is

absorbed in hydride B, and the absorption heat generates

useful heat or waste heat at temperature Tm. Useful heat

(heat pumping mode) is produced in each half-cycle, i.e.

quasi-continuously. Useful cold (cooling mode) is produced

only in one half-cycle. To obtain a quasi-continuous cold

supply, two pairs of reactors have to be used and operated in

parallel with a phase shift.

Owing to the periodic absorption/desorption process, the

performance of the device can be increased by internal heat

recovery if two pairs of reactors are employed. The higher

the temperature lift, the more important becomes the inter-

nal heat recovery. For the internal heat recovery, liquid heat

transfer ¯uids have advantages over gases. If a liquid is

employed as the heat transfer ¯uid, the generated cold can

be used directly or it can be pumped to the user, e.g. for

cooling of ceilings or chemical processes.

Owing to thermodynamics, the heat transfer ¯uid

temperature should approach the reaction temperature as

close as possible (Fig. 2), i.e. the driving temperature differ-

ence between the liquid (e.g. ¯owing in an annular gap) and

the geometric centre of the reaction beds should be kept

small (about 3±5 K) while maintaining a homogeneous

reaction along the reaction beds. Therefore, the required

small temperature differences of the heat transfer ¯uid

between inlet and outlet of the reaction beds (about

E. Willers, M. Groll / International Journal of Refrigeration 22 (1999) 47±58 49

Fig. 2. Basic scheme and van't Hoff diagram of a single-stage system (HS).

Fig. 2. ScheÂma de principe et diagramme de van't Hoff d'un systeÁme aÁ un eÂtage (HS).

Fig. 1. Plateau slope and hysteresis during the phase transition

metal to metal-hydride.

Fig. 1. Pente aplatie et hysteÂreÂsis pendant la phase de transition

metal/hydrure meÂtallique.

2±3 K) can be obtained by large mass ¯ows. Under these

conditions, acceptable cycle times of about 1200±1600 s

can be obtained if conventional reaction beds are used

[15±17]. The large liquid ¯ow can be achieved by intensive

pumping. Owing to the large mass ¯ows in the annular gap

around the reaction bed and the respective pressure drops, a

relatively high pumping power of up to 50 W kW21Cold is

necessary to run the system.


Fig. 3. Double-stage system (HD): star-scheme (the numbers 1 to 6 refer to the corner points of the thermodynamic processes in Fig. 4.

Fig. 3. SysteÁme aÁ deux eÂtages (HD); scheÂma eÂtoile (les chiffres se reÂferent aux points limitrophes des processus thermodynamiques dans Fig. 4.

Fig. 4. Double-stage system (HD): van't Hoff diagrams for schemes 1 and 2.

Fig. 4. SysteÁme aÁ deux eÂtages (HD); diagrammes de van't Hoff pour les scheÂmas 1 et 2.

If gas is used as heat transfer ¯uid [14] then very large

volume ¯ows with large temperature differences between

the gas and the metal hydride reaction beds are necessary.

An ef®cient internal heat recovery between both half-cycles

is not possible. This causes relatively low COPs.

The hardware for the single-stage device (with internal

heat recovery) consists of: 16 ¯uid valves, two hydrogen

valves, three pumps, and three reservoirs/heat exchangers.

If two separated loops are necessary, four reservoirs/heat

exchangers and four pumps are needed. In this case, the

system can be designed as non-pressurized, e.g. at about

1 bar, which can be advantageous. If oil is used, e.g. in

the high temperature loop, the respective heat transfer char-

acteristics are much worse than for water. Thus, higher driv-

ing temperatures are necessary to ensure a relatively short

cycle time.

Theoretically, cooling temperatures between 10 and

2508C can be obtained with a single-stage system. The

driving temperature has not to be very high for air-condi-

tioning purposes (e.g. 90±1158C). In the case of very low

cooling temperatures however, it increases up to . 2008Cfor deep freezing applications (see Table 2) [18]. Then, the

in¯uence of the thermal masses becomes more important

and the COPs decrease. The internal heat recovery with a

single-stage system cannot be higher than 50%. In practice,

a value of 45% can be obtained.

The COP calculation is based on an internal heat recovery

rate f and an exchange of nH2moles of hydrogen which is

due to Dx/xmax (Fig. 1). Thus, the amount of driving heat is

QDrive � nH2DHB;Des 1 �1 2 f�cp;RBmRB�TDrive 2 THeat� �3�

the amount of useful cold

QCool � nH2DHA;Des 2 �1 2 f�cp;RBmRB�THeat 2 TCool� �4�

and the amount of useful heat

QHeat � nH2DHA;Abs 2 �1 2 f�cp;RBmRB�THeat 2 TCool�

1 nH2DHB;Abs 1 �1 2 f�cp;RBmRB�TDrive 2 THeat�

�5�

2.2. The double-stage system (HD)

The double-stage system employs three different metal

hydrides A, B and C. In a special `star'-scheme design

(Fig. 3) there are six interconnected reactors (A1, A2, B1,

B2, C1, C2). Each hydride is connected on the hydrogen

side with the two other hydrides. All reactors are simulta-

neously in operation. Between each two coupled reactors

there is one hydrogen valve. The star-scheme operates in

two half-cycles and allows continuous cold generation.

Two basic schemes can be realized with the six reactor

star-scheme. Scheme 1 (see van't Hoff diagrams in Fig. 4) is

appropriate for high COPs. Driving heat is put in once,

useful heat is generated three times and useful cold is

obtained twice. There is a large temperature difference

required between the driving temperature level and the

useful heat temperature level. Scheme 1 is the reference

scheme for the double stage data (HD).

Scheme 2 is appropriate for relatively low driving

temperatures and/or high ambient or heating temperatures.

It provides a high temperature lift between cooling tempera-

ture and ambient temperature. Driving heat is put in twice,

useful heat is generated three times and useful cold is

obtained only once. Depending on the purpose of the

system, the hydrides have to be selected carefully.

More hardware is required for the double-stage device

than for the single-stage device: 24 ¯uid valves are required,

six hydrogen valves, three or four pumps and three or four

reservoirs/heat exchangers.

Because there is always a large temperature difference

between the highest and the lowest temperature level, it is

very dif®cult to ®nd a suitable heat transfer ¯uid covering

the whole temperature range which offers good heat transfer

and thermodynamic characteristics.

For scheme 1, COPC is expressed by

COPC � QCool;A 1 QCool;B

QDrive;C

�6�

and COPH by

COPH � QHeat;A 1 QHeat;B 1 QHeat;C

QDrive;C

�7�

The amounts of heat are calculated in the same way as for

the single-stage system.

For scheme 2, the COPs are calculated as:

COPC � QCool;A

QDrive;B 1 QDrive;C

�8�

COPH � QHeat;A 1 QHeat;B 1 QHeat;C

QDrive;B 1 QDrive;C

�9�

2.3. The multi-hydride-thermal-wave system (HW)

The basic system consists of two circuits, a high tempera-

ture circuit I and a low temperature circuit II (Fig. 5) [19].

Each circuit comprises two reactors with identical reaction

beds and two heat exchangers, one for heat input and one for

heat output. Each high temperature reactor HT1, HT2 is

coupled on the hydrogen side with the respective low

temperature reactor LT1, LT2. The reaction beds are ®lled

with different metal hydrides. As an example, seven differ-

ent metal hydrides (H1±H7) are contained in each of the

reactors HT1/2 and two different metal hydrides (H8, H9) in

each of the reactors LT1/2 (Figs. 5±7). The equilibrium

temperature differences from one hydride to the next at

uniform pressure in each reaction bed should be similar

(about 15±25 K). The heat transfer ¯uid is pumped by a

reversible two-channel proportionating pump.

The basic idea of this design is that a sharp temperature

and reaction wave moves along the length of each hydride


section of a reaction bed. The heat transfer ¯uid ¯ows

slowly in a very small annular gap around the reaction

bed. The temperature change of the heat transfer ¯uid

while passing the reaction bed is great owing to the different

hydride equilibrium temperatures at uniform pressure.

The operation comprises two half-cycles; in between the

reversible pump is reversed and internal heat recovery takes

place. The low temperature circuit II is discussed ®rst

(Fig. 6). We start the sequence, arbitrarily, with the central

heat exchanger HE3. In HE3 the ¯uid is cooled down from

THeat,II,max to either THeat,II,min or Tamb. The ¯uid ¯ow passes

through reaction bed LT2, where it is cooled down to the

cooling temperature TCool,min due to the desorption of hydro-

gen from the hydrides H8 and H9. In HE4, useful cold QCool

is taken up by the heat transfer ¯uid. While passing through

LT1, the ¯uid is heated up owing to the absorption of hydro-

gen in the hydrides H9 and H8. After entering HE3 the

circuit II is closed.

For the high temperature circuit I, the same principle

holds (Fig. 7). We start the sequence with the desorbing

high temperature reaction bed HT1. In this reaction bed

there is a cascaded decrease of the heat transfer ¯uid

temperature from TDrive,max to THeat,I,min. In HE2 useful

heat is released by cooling down the heat transfer ¯uid to


Fig. 6. HW-system: temperature pro®le in circuit II, one half-cycle.

Fig. 6. SysteÁme HW: pro®l de la tempeÂrature dans le circuit II, un demi-cycle.

Fig. 5. Multi-hydride-thermal-wave (HW) scheme in the van't Hoff diagram.

Fig. 5. ScheÂma multi-hydrure-ondes thermiques dans le diagramme de van't Hoff.

THeat,I,min. During the absorption of hydrogen in reaction bed

HT2, the heat transfer ¯uid is heated up to TDrive,min. In HE1,

the driving heat QDrive is supplied, and the heat transfer ¯uid

is heated up to TDrive,max.

When the exchange of hydrogen is ®nished, the direction

of liquid ¯ow is reversed and the internal heat recovery

takes place. After the internal heat recovery, the next half-

cycle starts.

It is obvious that, in both circuits, very different heat

transfer ¯uid mass ¯ows are required at such large tempera-

ture differences. The smaller the temperature drop in a reac-

tion bed, the higher is the mass ¯ow in the annular gap

around this bed. This is shown in the mass ¯ow calculations

(Eq. 12). With the multi-hydride-thermal-wave scheme very

low cooling temperatures can be obtained by combining

several low temperature hydrides, while the driving

temperatures are not necessarily high. This is possible

because the absorbing reaction bed LT1 absorbs not neces-

sarily at ambient temperatures. The inlet temperature of the

heat transfer ¯uid can be much lower than ambient tempera-

ture, thus avoiding very high hydrogen pressures in the

system and, as a consequence, very high driving tempera-

tures. Therefore, the in¯uence of the thermal masses in both

loops can be reduced. Additionally, an internal heat recov-

ery of more than 50% is possible. Moreover, owing to the

slowly ¯owing heat transfer ¯uid, the parasitic electric

power for pumping is very low. On the other hand, the

speci®c power output is also low (see Ref. [20] and the

section `Comparison of systems').

The amounts of heat needed to determine the thermal

performance are calculated in a completely different way.

This is due to the fact that the heat which is transferred from

and to the reaction beds is not directly related to the heat

transferred in the heat exchangers. It is, rather, related to

external constraints like minimum ambient temperature or

maximum cooling temperature.

The heat necessary to desorb the high temperature reac-

tion bed at high pressure is calculated as

QRB;I;input � nH2DHDes 1 �1 2 f�mcp;RB;IDTRB;I �10�

The heat which is transferred to the desorbing reaction bed

at low pressure is

QRB;II;input � nH2DHDes 2 �1 2 f�mcp;RB;IIDTRB;II �11�

The respective mass ¯ow of the heat transfer ¯uids in the

respective circuits during one half-cycle t /2 can be deter-

mined by

_mHTF;I=II �2QRB;input;I=II

tcp;HTF;I=IIuTinlet 2 Toutletu�12�

It is evident that with every improvement of the internal

heat recovery (f in Eqs. (10) and (11)) both the heat transfer

¯uid mass ¯ow can be reduced in the high temperature

circuit (Eq. 10) and increased in the low temperature circuit

(Eq. 11). The COPs are calculated based on the temperature

differences in the heat exchangers HE1 to HE4:

COPC � _mHTF;IIuTinlet;HE4 2 Toutlet;HE4u_mHTF;IuTinlet;HE1 2 Toutlet;HE1u

�13�

COPH �_mHTF;IIuTinlet;HE2 2 Toutlet;HE2u 1 _mHTF;IIuTinlet;HE3 2 Toutlet;HE3u

_mHTF;IuTinlet;HE1 2 Toutlet;HE1u�14�

A multi-hydride-thermal-wave device for combined heat-

ing/cooling has been taken into operation in February 1997.


Fig. 7. HW-system: temperature pro®le in circuit I, one half-cycle.

Fig. 7. SysteÁme HW: pro®l de la tempeÂrature dans le circuit I, un demi-cycle.

Measured COPs are not yet available. Respective results

will be published in the near future.

The longer the cycle, the smaller is the mass ¯ow and the

(speci®c) power output of the system. In this case, the

number of transfer units (NTU) between the reactor and

the heat transfer ¯uid [21]

NTU � aA

_mcp;HTF

�15�

is increased if the heat transfer coef®cient is not strongly

dependent on the heat transfer ¯uid velocity, e.g. in a very

small annular gap. The higher NTU causes a better de®ned

thermal and reaction wave in the reactors.

The better de®ned thermal wave also causes a closer

approach of the heat transfer ¯uid temperature to the reactor

temperature. This leads to lower heat transfer ¯uid outlet

temperatures in desorbing reactors and higher outlet

temperatures in absorbing reactors. Thus, T in HE1 becomes

smaller and T in HE4 becomes larger (Eq. 13). Both effects

contribute to a higher COPC. The same principle holds for

the heat transfer in HE2 and HE3, thus increasing the COPH.

If a higher power output is required, the mass ¯ow of the

heat transfer ¯uid has to be increased and the COPs become

smaller.

2.4. General considerations concerning an exergetic

comparison of the systems

An exergetic comparison of systems like the ones

discussed above with such different system speci®c proper-

ties is not simple. An evaluation of minimum cooling, heat-

ing (ambient) or driving heat temperatures following

generally the simple formula

zCarnot � 1=TM 2 1=TDrive

1=TCool 2 1=TM

�16�

leads in a wrong direction. It depends strongly on the system

which is used. A most important aspect is whether the cool-

ing (the same holds for heating) occurs at isothermal condi-

tions, i.e. the heat transfer is achieved by a phase change or

by transfer of sensible heat. The latter is employed in the

systems discussed (Fig. 7). The single- and double-stage

devices provide nearly isothermal cold and heat (Figs. 2

and 4) owing to large liquid mass ¯ows. The heat transfer

¯uid is pumped in a loop through an absorbing/desorbing

reactor, is heated up/cooled down slightly and is ®nally

recooled/reheated slightly in a heat exchanger. Under exer-

getic aspects, this is comparable with the temperature beha-

viour of a phase change heat exchanger and Eq. (16) can be

applied for determination of the exergetic ef®ciency.

The multi-hydride-thermal-wave system is characterized

by large temperature differences (Fig. 5) and small mass

¯ows. The heat transfer ¯uid is pumped through a loop

with two reactors and two heat exchangers (Figs. 6 and 7).

If counter-current or cross-current heat exchangers are used

in the device, it becomes evident that in some applications

the heat transfer ¯uid inlet temperatures to the heat exchan-

gers (HE2,3,4) can be applied to the user, thus leading to a

higher ef®ciency (Fig. 8). For example, in the air-condition-

ing mode the heat transfer ¯uid is heated up in the counter-

current heat exchanger HE4 to a temperature several

degrees below the incoming air, which is cooled down to

a temperature close to the inlet temperature of the heat

transfer ¯uid. In this case, the exergetic losses can be smal-

ler than the exergetic losses of (quasi-)isothermal heat

exchangers like evaporators or condensers [22]. Therefore,

Eq. (16) is not applicable for precise calculation of the

Carnot ef®ciency, because there are no quasi-isothermal

heat sources and sinks.

The systems are compared in Fig. 8 in the h,T-diagram for

the applications air-conditioning (cooling air from 258C to

58C) and water-chilling (cooling water from 128C to 68C).

The diagrams show that for the large temperature difference

in air-conditioning the sensible±sensible heat exchange for

HW is far better than for HS and HD with respect to exergy

losses, while in the water chilling machine with its small


Fig. 8. (T,h)-diagrams for sensible±sensible heat exchange; exergy losses are indicated by shaded areas.

Fig. 8. (T,h) diagrammes pour l'eÂchange de chaleur sensible±sensible et sensible-latente; les pertes d'exergie sont indiqueÂes par des zones griseÂes.

temperature difference the exergy losses are comparable.

Applied to different applications, the above-discussed

systems can be classi®ed concerning their applicability for

the respective tasks. This is shown in Table 1.

Owing to the particular character of the multi-hydride-

thermal-wave device, the selection of suitable control

volumes is of great importance. The machine has to be

equipped with counter-current heat exchangers in order to

achieve its speci®c advantages. The selection of the heat

exchanger type for single-stage (HS) and double-stage

(HD) systems has no signi®cant in¯uence on the perfor-

mance of the device.

3. Development of metal hydride reaction beds

At IKE, intensive research concerning metal hydride

reaction beds has been carried out. The heat and mass trans-

fer in coupled reaction beds has been investigated. Usually,

the limiting factor for the reaction is the heat transfer from

the heat transfer ¯uid to the bulk of the hydride bed. By

implementing different heat transfer matrices, like alumi-

nium foam, copper cassettes or other metallic materials, as

ef®cient heat conduction structures [15±17], the effective

thermal conductivity of the reaction bed can be improved

from about 1 W m21 K21 to about 8 W m21 K21. The

performance data listed in the comparison tables [20] have

been calculated based on an effective thermal conductivity

of 8 W m21 K21.

Another attempt to improve the effective thermal conduc-

tivity of a hydride bed are porous metallic-matrix hydride

(PMH) compacts [23]. They consist of a mixture of metal

hydride powder and metal (Al or Cu) powder which has

been pressed to pellets and sintered. Their effective thermal

conductivity can be very high, typically up to

20 W m21 K21. One problem, however, is that the porosity

is rather low, and the mass transfer (hydrogen transport)

limitation of the technical reaction kinetics has to be

considered.

Another very promising solution is the so-called

expanded-graphite hydride pellets developed by Le Carbone

Lorraine. These pellets have a high porosity and a non-

isotropic thermal conductivity, with about 25 W m21 K21

in the axial direction and , 1 W m21 K21 in the radial

direction [24].

A completely different solution for fast reaction beds is

the reduction of the heat transfer distances between the heat

transfer ¯uid and the hydride [25,26]. Capillary tube bundle

reaction beds (Fig. 9) have been developed and tested at

IKE. The average heat transfer distance was about 1 mm.

The cycle times with these reaction beds have been


Table 1

Qualitative exergetic performance of the systems ( 1 : good, o: medium, 2 : bad) using co-current (CO) or counter-current (CC) heat

exchangers for different applications

Tableau 1.

Performances exergeÂtiques qualitatives des systeÁmes ( 1 : bonne; o: moyenne; 2 : mauvais) utilisant des eÂchangeurs de chaleur co-courant

(CO) ou contre-courant (CC) pour des diffeÂrentes applications

Application Temp. from¼to (8C) HS HD HW

CO CC CO CC CO CC

Chilling of water 12 to 6 o 1 o 1 2 o

Chilling of air 25 to 5 2 o o 2 o o 2 1

Heating of air 20 to 40 2 o o 2 o o o 1

Heating of water 20 to 80 2 a oa 2 a oa 2 a 1

Heating of water 20 to 40 2 o o 2 o o o 1

a Only possible with signi®cantly higher driving temperatures

Fig. 9. Photograph of a capillary tube bundle reactor.

Fig. 9. Photographie d'un reÂacteur aÁ faisceau de tubes capillaires.

measured to be 5±10 min (half-cycle time: 120±270 s),

while the COP remains unchanged.

Note: both latter solutions have not been taken into

account in the comparison tables [20] because they are not

yet commercially available; however, they are included in

the comparisons in Table 2. When they can be produced at

competitive conditions, i.e. in a cost-effective way, a signif-

icant improvement of metal hydride machines will be

obtained. Metal hydrides are expensive and de®ne a

substantial part of the total investment costs of a device.

Increased power density means a reduced hydride inventory

and thus reduced investment costs. With the capillary tube

bundle reactors, the weight- and volume-speci®c power

outputs can be tripled for single- and double-stage systems

[26] and doubled for multi-hydride-thermal-wave systems

[27], and the investment costs for hydride machines can be

substantially reduced.

4. Comparison of the systems

In Table 2, the temperature limits, COPC, apparative

effort and performance data for the respective scheme

employing two different types of reaction bed are shown.

Metal hydrides used for the different applications are

usually of the type AB5 or AB2. We can cover all calculated

AC applications with LaNi52xAlyMnx2y and similar alloys,

e.g.:

HS: B: LaNi4.95Al0.05 A: Ti0.98Zr0.02V0.43Fe0.09Cr0.05Mn1.5

HD: C: LaNi4.7Sn0.3 B: LaNi4.95Al0.05 A: Ti0.98Zr0.02V0.43Fe0.09Cr0.05Mn1.5

HW: H1: LaNi4.3Al0.4Mn0.3 H2: LaNi4.4Al0.34Mn0.26 H3: LaNi4.5Al0.29Mn0.21

H4: LaNi4.7Sn0.3 H5:LaNi4.75Al0.25 H6:LaNi4.85Al0.15

H7: LaNi5 H8: La0.555Co0.03Pr0.12Nd0.295Ni5 H9: Ti0.98Zr0.02V0.43Fe0.09Cr0.05Mn1.5

Very low temperatures can be obtained, e.g. by using

Ti1.2Cr1.9Mn0.1 and related materials on the low temperature

side and e.g B: LaNi4.95Al0.05 on the high temperature side of

a single-stage system.

Among others, Ref. [18] gives a good overview of many

metal hydrides. It can used for selection of pairs of metal

hydride alloys for single-stage systems, and for selection of

single alloys for multi-stage systems. A valuable tool for

metal hydride selection is a data bank, of which one is

shown in Ref. [28]. Another data bank is presently built

up at IKE and is currently accomplished. It provides

complete information about the properties relevant for

applications, e.g. entropy, enthalpy of hydride formation,

maximum hydrogen capacity, mathematical expressions

for plateau slope, information about cyclic stability, etc.

for a large number of hydrides.

However, metal hydrides must be improved in various

respects (besides lowering their production costs), e.g.

higher reversible H2-storage capacity without affecting reac-

tion kinetics, reduced hysteresis, ¯atter plateau slopes and

an improved cyclic stability.

5. Conclusions

Three different metal hydride devices have been evalu-

ated, the single-stage (HS) and the double-stage (HD)

system and the novel multi-hydride-thermal-wave (HW)

concept. A wide range of working temperatures can be

covered with metal hydride devices, both with respect to

the driving heat, and to the useful heat and cold. Single-

and double-stage systems work, in general, with three

®xed temperature levels, so the useful heat and cold is avail-

able at one level, independent of the heat exchanger. The

multi-hydride-thermal-wave system uses the sensible heat

of the heat transfer ¯uid for generation of heat and cold. By

employing counter-current or cross-current heat exchangers

the best results can be obtained. The exergetic comparison

of the systems requires the analysis of the complete applica-

tion. If the exergetic ef®ciency is not carefully determined,

the existing advantageous thermodynamic performance of

the systems may not become evident, especially for the

multi-hydride-thermal-wave system.

Fast reaction beds, e.g. the capillary tube bundle reac-

tion bed, have been developed at IKE. The cycle time

has been reduced to 5 min with no loss of ef®ciency.

With such fast reactors, metal hydride systems become

signi®cantly more competitive: their power density will

increase and, for a given power output, less of the

expensive metal hydride inventory is needed; therefore,

the investment costs are reduced. The values in the

comparison tables [20] do not consider these fast

reaction beds. The respective power output values will

be increased of a factor of 2 to 4 by using the fast

capillary tube bundle reaction beds due to the shortened

cycle times, as is shown in Table 2. Future work

should comprise the use of such reaction beds in

metal hydride applications in order to reach a higher

competitiveness.



Tab

le2

Ov

ervie

wo

fth

ete

mp

erat

ure

ran

ge

of

som

ep

ote

nti

alap

pli

cati

on

sfo

rth

edif

fere

nt

syst

ems,

thei

rre

quir

edhar

dw

are

and

the

per

form

ance

dat

a(c

ooli

ng)

for

dif

fere

nt

reac

tion

bed

s

Ta

ble

au

2.

Rev

ue

de

lag

am

me

de

tem

peÂ r

atu

red

an

sce

rta

ines

ap

pli

cati

on

sp

ote

nti

elle

sdes

dif

feÂre

nts

syst

eÁ mes

,le

ur

com

ple

xiteÂ

,et

les

donneÂ e

sco

nce

rnant

laper

form

ance

(en

refr

oid

isse

men

t)des

dif

feÂre

nts

lits

de

reÂa

ctio

n

Tem

per

atu

rera

ng

e(8

C)

for

cooli

ng/h

eati

ng

appli

cati

ons

CO

PC

Har

dw

are

Per

form

ance

dat

a

Nu

mb

ero

fre

quir

edkey

com

ponen

tsC

onven

tional

fast

reac

tion

bed

sN

ovel

hig

hper

form

ance

bed

s

Cool

Hea

tA

mb.

Dri

ve

Rea

ctors

Pum

ps

Gas

val

ves

Liq

uid

val

ves

Cycl

e

tim

e

(min

)

Wei

ght

spec

.

cooli

ng

pow

er

�Wkg

21

hydri

de�

Volu

met

ric

cooli

ng

pow

er

�Wl2

1re

acto

r�

Cycl

e

tim

e

(min

)

Wei

ght

spec

.

cooli

ng

pow

er

�Wkg

21

hydri

de�

Volu

met

ric

cooli

ng

pow

er

�Wl2

1re

acto

r�

HS

34

03

51

15

0.5

34

3/4

216

20

±30

55

to40

110

to80

5to

10

200

to100

400

to200

25

02

15

0.2

35

to20

70

to40

120

to60

240

to120

HD

I3

21

00

.83

63/4

624

20

±30

85

to60

170

to120

5to

10

300

to150

600

to300

21

02

20

0.5

670

to45

140

to90

240

to120

480

to240

HD

II3

85

0.3

220

±30

45

to30

90

to60

5to

10

150

to75

300

to150

25

01

45

0.1

530

to20

60

to40

100

to50

200

to100

HW

32

15

0.8

41/2

0±

31

±3

30

±45

30

to20

60

to40

15

to30

100

to50

200

to100

25

02

15

0.2

512

to8

25

to16

25

to12

50

to25

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Documents

Evaluation of metal hydride machines for heat pumping and cooling applications: Evaluation des machines à hydrure métallique dans les applications de pompes à chaleur et de refroidissement