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Feasibility study of a metal hydride hydrogen store for a self-sufficient solar hydrogen energy system
hr. J. Hwhgm En‘nrr~y, Vol. 21, No. 3, pp. 213 ‘21, 1996 Copyright @ International Association for Hydrogen Energy Elsevier Science Ltd Prmtcd in Great Britain. All rights reserved 0360-3199(95)00064-X 0360.3199!96 I 15.00 + 0.00
FEASIBILITY STUDY OF A METAL HYDRIDE HYDROGEN STORE FOR A SELF-SUFFICIENT SOLAR HYDROGEN ENERGY SYSTEM J. P. VANHANEN, Helsinki University
of Technology,
P. D. LUND Department
(Rrceiredjbr
and M. T. HAGSTROM
of Technical Physics, FIN-02150 Espoo, Finland
publication 5 Mug 1!)95)
Abstract---The feasibility of using metal hydride hydrogen storage in a self-sufficient solar hydrogen energy system is studied. Several potential commercial and non-commercial metal hydrides are considered to find a material having a low AH value, a low hysteresis effect, gentle P-C-T, plateau slopes and a high hydrogen storage capacity. A 1 N m3 metal hydride container employing a commercial Hydralloy Cl5 metal hydride with the proper P-C -T curves is analysed in more detail. As the thermal behaviour of the container is crucial in our application, steady-state and time-dependent thermal properties of the container are measured and the respective models are derived. The metal hydride container is also tested under realistic conditions to get further operational experience on its technical feasibility. Based on this study, low-temperature metal hydrides seem to he technically and economically feasible for small-scale self-sufficient solar hydrogen systems in which high volumetricenergydensity is needed due to limited space.
NOMENCLATURE C eff hair K df k,
k,
qc %o,
R 1’1 1’2
T T, T T2 Tamb AH,, t
INTRODUCTION
Effective heal. capacity (J-C- ‘) Convective heat transfer coefficient -2-c-1 Pm ) Effective heat loss coefficient (W ‘C- ‘) Effective thermal conductivity of the metal hydride bed (W mm1 ‘C-l) Thermal conductivity of the cladding (W mm’ ‘C-l) Length of the metal hydride column (m) Length of the cylindrical slice cut from the container (m) Flow rate of hydrogen absorbed (mol s- ‘) Heat production per unit volume within the cylindrical slice (W m 3, Heat production within the cylindrical slice (W) Total heat production/consumption (W) Thermal resis,tance (“C W ‘) Inside radius of the cylinder (m) Outside radius of the cylinder (m) Temperature (“C) Temperature in the middle of the cylinder ( C) Temperature at the interface of the metal hydride bed and the cladding (“C) Temperature at the surface of the cylinder (‘C) Ambient temperature (“C) Reaction enthalpy of the hydrogen-metal reaction (J mol- ‘) Thermal time constant (h)
Hydrogen energy technology offers a promising option to store photovoltaic electricity for later use. This option has been studied in recent years in several laboratories Cl-41 including Helsinki Unversity of Technology, where a small-scale solar hydrogen pilot plant has been constructed [S] and its operation has been optimised by comprehensive numerical models [6]. During recent years, special emphasis has been put on the seasonal storage sub-system consisting of a hydrogen production unit, hydrogen store, and a fuel cell for hydrogen conversion back into electricity. In our present pilot plant, hydrogen is stored in a low pressure (30 bar) steel vessel which has proved to be economically the most feasible despite a low energy density. An Interesting option to increase the volumetric energy density of the hydrogen store is to use metal hydrides. Several commercial and non-commercial metal hydride materials have been previously characterised at our laboratory [7]. Based on these characterisations, a tailor-made 1 N m3 metal hydride container filled with a commercial Hydralloy Cl5 metal hydride alloy was ordered from Gesellschaft fiir Elektrometallurgie mbH for further investigations. The objective of this paper is to study the technical feasibility of the metal hydride hydrogen storage as a part of a solar hydrogen system, and to find an application in which the metal hydride storage would be suitable. This paper describes both the characteristics of the 213
J. P. VANHANEN
214
et al.
Table 1. Properties of the metal hydride alloys studied [7] Alloy
Lao.4do.05W
MmNL.5Mno.4s%05 Zr,,,Ti,,,MnFe Zr(b.&ro.A LmNi( 1) Hydralloy Cl 5
28.1 23.9 19.3 19.5 26.9 24.9
30.0 29.9 28.9 29.2. 29.5 30.8
Capacityb (wt %)
Hysteresis”, b
Slope”, b
1.4 [8] 1.2 [9] 1.1 [lo] 1.3 [ll] 1.3 1.5
++ -
++
+ ++
++ +
a PDSC measurements; b according to the references. + = good; + + = excellent; - = poor; - - = useless.
hydride material and the macroscopic properties of the whole metal hydride container. The characteristics of the metal hydride material are based on thermodynamic measurements of a powdered sample with a pressure differential scanning calorimeter (PDSC) [7]. The macroscopic properties, primarily the thermal characteristics of the whole container, were measured on a laboratory-scale test bench and the appropriate thermal models are presented. The performance of the metal hydride container under realistic conditions is also studied, and conclusions on the technical feasibility are made. Finally, the metal hydride storage option is compared with other commercially available hydrogen storage alternatives.
metal
SELF-SUFFICIENT SOLAR HYDROGEN SYSTEM WITH A METAL HYDRIDE HYDROGEN STORE In self-sufficient solar energy systems,long-term energy
storage is usually the bottleneck of the whole system while short-term energy storage can conveniently be carried out by a conventional battery. In self-sufficient solar hydrogen energy systems,excesssummer photovoltaic electricity is converted to hydrogen by an electrolyser, and the hydrogen is stored over the seasone.g. in a steel vessel,a composite cylinder, or a metal hydride container. In the winter, when direct photovoltaic electricity is not available, hydrogen is converted back into electricity by a fuel cell. Thus, hydrogen is used as an energy carrier from the summer to the winter to level the seasonal variations in photovoltaic electricity production. A schematic diagram of a solar hydrogen system with a metal hydride store is shown in Fig. 1.The metal hydride store is charged by a pressurisedelectrolyser by increasing the hydrogen pressure until hydrogen absorption in the metal hydride occurs. Thus, the characteristic absorption pressure of the metal hydride has to be low enough for the pressurised electrolyser to charge it. On the other
PV-array
Load
Metal hydride container I Fig. 1. Schematic diagram of the solar hydrogen energy system with the metal hydride hydrogen store.
HYDROGEN
I --f --t
STORE FOR A SOLAR HYDROGEN
215
SYSTEM
LaNdNiabs. LaNblidas. LmNlabs. LmNlder. c1.5abs.
1 /temperature(K)
Hz concenlration
(wt%)
Fig. 2. Measured can’t Hoff plots of the feasible metal hydrides studied.
Fig. 3. P-C- T-curves of Hydralloy manufilcturer.
hand, the desorption pressure has to be high enough to feed hydrogen into the fuel cell at ambient pressure without external heating. On a macroscopic level, the container must have good heat transfer properties to avoid over-heating and pressure build-up during charging, as well as excessive cooling and pressure drop during discharging.
P-C-T-plateau slopes. Also, the pressure and temperature ranges are confirmed to be applicable to our system, i.e. Hydralloy Cl5 can be charged by a pressurised electrolyser, and discharged into a fuel cell at ambient pressure near room temperature.
COMPARISON
OF POTENTIAL HYDRIDES
METAL
Six metal hydrides near ambient temperature and pressure were characterised by PDSC in order to find the best metal hydride material for our system [7]. The objective was to find a metal hydride material having a low AH value, a low hysteresis effect, gentle P-C Tplateau slopes, and a high hydrogen storage capacity. These properties of the metal hydrides studied are collected in Table 1. On the basis of these results, three albs (MmNi4.sMn,,4,Zro.os, ZrFeo.,,Cro.,,),~ and Zr,,,Ti,,,MnFe) were ruled out because of their large hysteresis effect or sloping P-C T-plateaux, while the three remaining candidates (La,,,,Nd,,,,Ni,, LmNi( l), and Hydralloy C15) were studied further by comparing their respective van? Hoff plots (Fig. 2). In our application, the maximum temperature during charging remains below 50 C, and the minimum temperature during discharging remains above IO ‘C. Within these limits, the absorption pressures of La,,,,Nd,,,,Ni,, LmNi(l), and Hydralloy Cl5 are 8.6 bar, 15.4 bar and 9.8 bar, and the desorption pressures are I.1 bar, 1.5 bar, and 1.3 bar, respectively [7]. Thus, all three candidates seemed to be technically feasible for the application in question. Finally, we selected Hydralloy C15, because the manufacturer was able to make the container according to our specifications at a reasonable price. The feasibility of Hydralloy Cl 5 can be observed from the measured P-C- l”-curves supplied by the manufacturer (Fig. 3). The &CT-curves show clearly the high storage capacity, low hysteresis effect, and the gentle
Cl5
according
to the
THERMAL. CHARACTERISTICS OF THE METAL HYDRIDE CONTAINER UNDER STEADY-STATE CONDITIONS The thermal characteristics of the metal hydride container were studied under steady-state conditions to understand its heat transfer properties. If the heat transfer properties of the container are poor, the container may over-heat during charging which may also lead to too high a hydrogen pressure for the electrolyser to be able to charge it. Similarly, excessive cooling during discharging may lead to too low a hydrogen pressure for the fuel cell. Therefore, we wanted to determine the effective thermal conductivity of the metal hydride bed, and compare it with the convective heat transfer coefficient between the container and the air to judge which one limits the overall heat transfer of the container. The cross-section of the metal hydride container including the central tube for hydrogen delivery, the metal hydride bed. and the steel cladding is depicted in Fig. 4. The container has empty space because the metal hydride material expands during hydrogenation. Furthermore, the effective volume of the hydride material is expected to increase as the particle size of the metal hydride material decreases during the first few hydriding--dehydriding cycles. The temperatures were measured at the surface and inside the container. The inside temperature sensor had been installed in the central tube in the middle ofthe container by the manufacturer while our five surface temperature sensors were able to move easily depending on the requirements of an experiment. The pressure gauge and mass-flow controller were installed in the hydrogen delivery line.
J. P. VANHANEN
216
et ul.
steel cladding, d = 2.3 mm
hydrogen delivery tube
89 mm I 360 mm 644 mm
t
Fig. 4. Cross-section of the metal hydride container.
To estimate the heat transfer properties of the metal hydride bed, we may consider a small cylindrical slice cut from the container, Fig. 5. We determined the heat transfer properties of the cylindrical slice under steadystateconditions by using different constant hydrogen flow rates to charge the metal hydride store. The hydrogen flow rates employed were so low that all the hydrogen fed into the metal hydride absorbed immediately without pressure build-up. Also, the hydride bed was assumed to be porous enough so that significant pressure gradients within the bed are not formed, i.e. the hydrogenation reaction proceeds uniformly within the bed. Thus, the heat production due to the absorption reaction was assumed to be constant and homogeneous. The surface and inside temperatures of the cylindrical slice were determined after the whole container had reached steadystate, i.e. all the temperatures measured remained unchanged. If the cylindrical slice is cut between 18 and 22 cm, we observe that the surface temperature under steady-state conditions along the z-axis is almost constant (Fig. 6).
Thus, the container experiences temperature gradients mainly in the radial direction. Therefore, a one-dimensional steady-state heat equation may be used to estimate the effective heat conductivity of the metal hydride bed, and also to calculate the temperature profile along the r-axis. Hence, the heat conduction equation for the metal hydride bed may be written as 1 d r dr -4
dT rdr
‘I’ +F=O, I >
(1)
and for the steel cladding as
(2)
where T is the temperature, k, the effective conductivity of the metal hydride bed, and 4:’ produced per unit volume within the cylindrical solving the heat conduction equations (1) and the boundary conditions, we obtain
thermal the heat slice. By (2) with
36
201
0
10
20
30
40
50
60
Height (cm)
Fig. 5. Modelled cylindrical slice cut from the metal hydride container (rl = 42.2 mm. rz = 44.5 mm, I, = 40.0 mm).
Fig. 6. The measured surface temperatures of the container under steady-state conditions along the z-axis at different charging flow rates.
HYDROGEN
T(r)=
To-;&
b
STORE FOR A SOLAR HYDROGEN
ZforO
1‘ i) I
(3)
217
SYSTEM
obtained from the literature [ 123. By employing several charging flow rates from 5 to 30 NI h-‘, we obtain experimentally the following values for k, and hai,:
and k, =0.6iO.l T(r) =
(T, - T,) In r + T;ln Mr,lr2)
hLgi,= 8 & 1 W m-2 for r, < r < r2,
41,= q:“.rrrf/,,
(5)
which may also be written as (1, = AH,,r&
.L 1’
(6)
where AH,, is the reaction enthalpy of the hydrogen metal reaction, I is the overall length of the metal hydride column (36 cm), and ri,, is the constant flow rate of hydrogen absorbed within the container during the measurement. The boundary temperatures T,, Ti and T2 may be written as + SW,
+ R, + Rx),
(7)
T, = jy,,,,, + qc(R, + R,)>
(8)
7; = Tam,,+ ycR,,
(9)
and
where Tam,,is the ambient temperature, and R,. R, and R, are the thermal resistances defined as: R, =p
6,
=
‘2
I 4nk,l,’
(10)
W2/rl)
2nk,I,
’
(11)
and R, =p
C.
1 2nr, hai,l, ’
C.
(13) (14)
(4)
where r, and r2 are the inside and the outside radi of the cylinder, respectively,
To = L,
Wm-’
r, - T, In r2
(12)
where k, is the thermal conductivity of the cladding and hai, is the convective heat transfer coefficient between the container and air. The effective thermal conductivity of the metal hydride bed (k,) and the convective heat transfer coefficient (h,,,) were solved using equations (6))(12) while the thermal conductivity of the steel cladding (k2 = 15 W/m’C) was
By substitutmg these values to equations (lO))( 12) we observe that the overall thermal resistance is dominated by the convection term R,. Actually, R, is over 3 times higher than R, However, the effective thermal conductivity of the metal hydride bed is quite low and the value may even decrease as the metal hydride particles crush during the first few cycles. However, as the total heat transfer is restricted by the convection term, any improvemcnt in k, would not improve much the total heat transfer properties of the container. The effect of R, is insignificant due to high thermal conductivity. The accuracy of the above-described model was verified against measurement by using several different constant charging flow rates. The calculated and measured values of T, and T, are presented in Table 2. According to these results, the model gives accurate results, and thus the values fork, and h,,, arc reliable, and also the assumptions made on the heat production within the container seem to be reasonable. The accuracy of this model is good enough for our purpose to study the thermal feasibility of the metal hydride container for the seasonal storage application, ,where hydrogen absorption and desorption flow rates are extremely low. However, a more detailed model taking the spatial variation of the heat production and also the dynamics of the hydrogen-metal reaction into account would be needed especially in cases when higher hydrogen flow rates are employed.
TIME-DEPENDENT THE METAL
THERMAL BEHAVIOUR HYDRIDE CONTAINER
OF
As the thermal time constant of the container is high, a model to describe the transient behaviour of the container is also needed. Therefore, we modelled the time-dependent behaviour of the metal hydride container by using the lumped capacitance method. The lumped capacitance method is generally valid if the thermal resistance due to convection dominates the thermal resistance due to conduction [12]. This condition also means that the temperature of the body can be described or approximated by a single temperature. In this case, this validity condition is met quite well and the container is described by the temperature measured in the middle of the container. The time-dependent heat balance equation of the container may be written as
c
dT(t)
~ err dt
= a,,(t) - K,,,CT(d - Ta,,,,,l.
(15)
J. P. VANHANEN
218
et al.
Table 2. Calculated temperatures at different flow rates verified against the measured temperatures Flow rate (Nl h-‘)
Inside temperature-T,
5.0 10.0 15.0 20.0 30.0
(-C)
Surface temperature-
TamLl ( C)
q (’ C)
Measured
Calculated
Deviation
Measured
Calculated
Deviation
23.9 26.2 29.8 31.5 36.0
23.1 26.1 29.0 31.1 36.2
-0.2 -0.1 -0.8 -0.4 +0.2
23.1 24.7 21.6 29.0 32.1
23.2 25.0 27.4 28.9 33.0
f0.1 +0.3 PO.2 -0.1 +0.3
where Cefl is the effective heat capacity of the container, q,,, is the total heat production or consumption within the container, and K,,, is the effective heat loss coefficient. The general solution of equation (15) is
--t
21.2 21.1 21.5 21.1 21.3
30Nllh
32.0 -
T(r) = e (16) where the constant C, depends on the initial conditions. To solve the parameters Ceff and K,,, experimentally, we used one constant hydrogen flow rate at a time to charge or discharge the metal hydride hydrogen store. As the flow-rates employed were low and the pressure measurement indicated that absorption and desorption occurred immediately, we assumed q,,, to be constant during the constant flow-rate measurement. As also the initial temperature of the container was Tamb,we obtained the following particular solution of equation (15) for the constant hydrogen flow rate measurement.
T(t) = Tarn,,+ p
(1 - e-“‘), err
(17)
where 5 is the thermal time constant of the container defined as
c T=y. cff
nerr
(18)
By employing several constant hydrogen flow rates to charge and discharge the metal hydride store, we obtained the following value for the thermal time constant
T = 2.3 k 0.2 h,
(19)
which corresponds to
K,,, = 0.69 k 0.03 W “C-l,
(20)
C,,, = 5.8 f 0.5 kJ ‘C-l.
(21)
and
Time(h)
Fig. 7. Calculated (solid line) and measured (scatters) temperatures of the container during charging.
The time-dependent thermal behaviour of the container at different constant charging and discharging hydrogen flow rates is illustrated in Figs 7 and 8. Both figures show that the lumped capacitance model describes accurately the time-dependent thermal behaviour of the container. The high effective heat capacity of the container indicates that the container does not usually reach steady-state, especially if the store is charged by the electrolyser, the production of which depends on solar input. Thus, the time-dependent model is crucial in studying the operation of the metal hydride store connected to the solar hydrogen system. PERFORMANCE CONTAINER
OF THE METAL HYDRIDE DURING TEST RUNS
The metal hydride container was tested under realistic conditions to get further operational information. The hydrogen flow rates employed during the test runs were significantly higher than needed in a seasonal storage system in order also to determine the technical limits for shorter periods of time. In the experimental system, the metal hydride container is connected between two mass flow controllers (Fig. 9). These mass flow controllers are controlled by the computer to emulate the hydrogen production and consumption by an electrolyser and a fuel cell, respectively.
HYDROGEN
-
STORE FOR A SOLAR HYDROGEN
lONl/h
3.0
4.0
5.0
6.0
7.0
6.0
Time(h)
Fig. 8. Calculated (solid line) and measured (scatters) temperatures of the container during discharging.
SYSTEM
219
During the first test run, a stochastic hydrogen flowrate was employed for 6.8 h to charge the metal hydride store (Fig. 10). This stochastic hydrogen flow rate describes hydrogen production in a solar-driven electrolyser during variable insolation conditions. The charging flow rate was varied from 1 to 20 Nl h- ’ with an average of 11.5 Nl h- ‘. The pressure in the metal hydride container stayed clearly below 5 bar which indicates that the store can easily be charged by an electrolyser with a maximum pressure of 5 bar when operating near room temperature. The effect of the high thermal time constant can also be observed as the large variations in hydrogen flow rate do not cause any major variations in temperature. The second test run describes hydrogen consumption in a fuel cell connected to a random electric load (Fig. 11). The discharge flow rate was varied from 1 to 20 Nl h-’ for 6.2 h. The average flow rate was 12.0 Nl h-’ corresponding to a discharge time of about 80 h. The temperature of the metal hydride container varied from 14 to 20 C, corresponding to pressure levels of 0.4 and 1.1 bar above atmospheric pressure. Thus, the discharge
PI = pressure indicator PT = pressure transmitter FC = flow controler TT = temperature transmitter Hypro,
~asxtle
Metal h dride container (l&O 1NTP) Fig. 9. Experimental
system during the test run.
J. P. VANHANEN
et al.
the storage capacity needed; storage time; l the location of the application; l weight and volume; l the need for maintenance; and 0 price.
l l
t 5.0 t 2.5 10.0
'= 8
7.5
f
5.0
b: t
2.5
+, LO
I
1 .o
2.0
3.0
4.0
~“10,t-J
5.0
6.0
I
7.0
Time (h)
Fig. 10. Temperature, pressure and hydrogen flow rate during the first test run (charging).
pressure of the metal hydride seems to be applicable to a fuel cell working at ambient pressure.
OTHER
HYDROGEN
STORAGE
ALTERNATIVES
In this paper, the emphasis is put on a metal hydride container, but there are also several alternative methods to store electrolytic hydrogen, for example: l l l l l
gaseous hydrogen; liquid hydrogen; cryoadsorption; zeolites; and chemical compounds.
An important criteria is the energy density. Approximate gravimetric and volumetric energy densities of different hydrogen storage alternatives are presented in Fig. 12. Gaseous hydrogen covers a wide area in the energy density map as the volumetric energy density depends on the storage pressure, and the gravimetric density depends on the material of the vessel. For example, advanced composite vessels may have significantly higher gravimetric energy densities than conventional steel vessels. Metal hydrides usually have better volumetric energy density than gaseous hydrogen storage. The gravimetric energy density of low-temperature metal hydrides is about the same as that of the gaseous hydrogen storage, but with high-temperature metal hydrides, higher gravimetric energy densities may be reached. Liquid hydrogen stored in a cryocontainer has clearly the best gravimetric energy density, but the volumetric energy density is about the same as with metal hydrides. The characteristics of cryoadsorption lie between gaseous hydrogen and metal hydrides, while the energy densities of zeolites are quite modest. Liquid hydrogen and cryoadsorption are suitable only for large applications, as the handling of hydrogen necessitates expensive devices. In the case of gaseous hydrogen, a compressor is needed to reach a high pressure while high-temperature metal hydrides require hightemperature heat. Therefore, low-pressure hydrogen vessels and low-temperature metal hydrides seem to be the most suitable for small-scale applications, as they do not require expensive instruments for gas handling. The
Generally, the best hydrogen storage alternative depends strongly on the application in question. Some important aspects that have an effect on final selection are: 10 30.0 r
1
8 23 6 i?! 1 8
, 15.0 12.5 10.0 7.5 5.0
2 a c a 8 p‘
J 0.0 7.0
Fig. 11. Temperature, pressure and hydrogen flow rate during the second test run (discharging).
0.01 1 0.1
I I 1 10 gravimetric energy density (kWh/kg)
I 100
.
Fig. 12. Energy densities of the hydrogen storage alternatives (including containers).
HYDROGEN
STORE FOR A SOLAR HYDROGEN
cost of a metal hydride container is now about 20 times higher than the price of a 20 bar steel vessel, but the volumetric energy density is also about 20 times higher. Thus, low-temperature metal hydrides seem to be apphcable to systems in which high volumetric energy density is needed due to limited space. CONCLUSION The feasibility of a metal hydride hydrogen store for a self-sufficient solar hydrogen energy system has been studied. The equilibrium pressure of absorption of the metal hydride material must be low enough to be charged by a pressurised electrolyser. On the other hand, the equilibrium pressure of desorption must be high enough to enable the discharging of the store into a fuel cell at ambient pressure. As the equilibrium pressures of the metal hydride depends on temperature, it is crucial that the thermal behaviour of the container is known. Thus, the thermal characteristics of the metal hydride container were measured, and appropriate thermal models were derived. According to the verification measurements with different hydrogen flow rates, the models seem to describe accurately the thermal behaviour of the container in this application. The metal hydride container was also tested under realistic conditions to get further operational experience on its technical feasibility. According to the test runs near room temperature, the metal hydride hydrogen store studied can be charged by a low-pressure (max. 5 bar) electrolyser, and also be discharged into a fuel cell at ambient pressure even though the hydrogen flow rates employed in the test runs were significantly higher than needed in seasonal storage applications. The effect of hydrogen purity was not considered in this study, but it may have an effect on the metal hydride material as impurities gradually decrease the storage capacity. However, the number of cycles needed in this particular case is low, and thus hydro,gen purity should not cause serious problems. Generally, metal hydrides are an interesting alternative for storing hydrogen. They show many advantages such as high volumetric energy density, good safety aspects, the availability of a wide range of temperatures and pressures, easy maintenance, and reliable operation without additional mechanical devices. The disadvantage is the high cost of the metal hydride material. However, the
SYSTEM
221
metal hydride hydrogen store may have a market niche in small-scale solar hydrogen applications in which high volumetric capacity is needed and safety aspects are appreciated. il~kno~(rdgen1rnt.s~~ This work has been financed by the Finnish Ministry of Trade and Industry under the NEMO research programme. REFERENCES I. A. Heinzel and K. Ledjeff. The self-sufficient solar house: hybrid energy storage system. Prw. 1991 S&r World Coqpw, pp. 254332546, Denver, CO (1991).
2. P. A. Lehman and C. E. Chamberlin, Design of a photovoltaichydrogenfuel cell energy system. Int. J. Hydrogen Energy 16, 349-354 (199 1). 3. A. G. Garcia-Conde and F. Rosa, Solar hydrogen production: a Spanish experience. Int. J. H~vlrogen Energ.v 18, 9951000 (1993). 4. S. Galli, G. De Paoli and A. Ciancia, An experimental solar PV-hydrogen -fuel cell energy system: preliminary results. Proc. 10th World Hydrogen Energy Conftirenw, pp. 439 447, Cocoa Beach, FL (1994). 5. P. S. Kauranen, P. D. Lund and J. P. Vanhanen. Development of a self-sufficient solar-hydrogen energy system. Int. J. H.vdrogm Enrrgv 19, 99- 106 (1994). 6. J. P. Vanhanen, P. S. Kauranen, P. D. Lund and L. M. Manninen, Simulation of solar hydrogen energy systems. Solur Energy 53, 267 278 (I 994). 1. M. T. Hagstrom, P. D. Lund and J. P. Vanhanen, Metal hydride hydrogen storage for near ambient temperature and atmospheric pressure applications, a PDSC-study. In/. J. Hydrogen Energy 20, 897 (1995). 8. H. H. van Mal. K. H. J. Buschow and A. R. Miedema, Hydrogen absorption in LaNi, and related compounds: experimental observations and their explanation. J. /essc.&mon Met. 35, 65-76 (1974). 9. Y. Osumi. H. Suzuki. A. Kato. K. Oauro. S. Kawai and M. Kaneko, Hydrogen absorption and desorption characteristics of Mm-NiiAIlM and Mm-Ni Mn-M alloys (Mm = misch metal). J. kss-cownon Met. 89, 2X7-292 (1983). IO. V. K. Sinha, F. Pourarian and W. E. Wallace, Hydrogenation characteristics of Zr, ,Ti,MnFe alloys. J. Irwcommon Mer. 87, 283 -296 (1982). I I. S. Qian and D. 0. Northwood, Thermodynamic characterisation of Zr(Fe,Cr, X)2 H systems. J. less-comnmn Met 147, 149-I 59 (1989). oj’Hrut and 12 F.P. Incropera and D. P. Dewitt, Fundummtuls Muss Trum/>r, 2nd ed. John Wiley, New York (1985).
FEASIBILITY STUDY OF A METAL HYDRIDE HYDROGEN STORE FOR A SELF-SUFFICIENT SOLAR HYDROGEN ENERGY SYSTEM J. P. VANHANEN, Helsinki University
of Technology,
P. D. LUND Department
(Rrceiredjbr
and M. T. HAGSTROM
of Technical Physics, FIN-02150 Espoo, Finland
publication 5 Mug 1!)95)
Abstract---The feasibility of using metal hydride hydrogen storage in a self-sufficient solar hydrogen energy system is studied. Several potential commercial and non-commercial metal hydrides are considered to find a material having a low AH value, a low hysteresis effect, gentle P-C-T, plateau slopes and a high hydrogen storage capacity. A 1 N m3 metal hydride container employing a commercial Hydralloy Cl5 metal hydride with the proper P-C -T curves is analysed in more detail. As the thermal behaviour of the container is crucial in our application, steady-state and time-dependent thermal properties of the container are measured and the respective models are derived. The metal hydride container is also tested under realistic conditions to get further operational experience on its technical feasibility. Based on this study, low-temperature metal hydrides seem to he technically and economically feasible for small-scale self-sufficient solar hydrogen systems in which high volumetricenergydensity is needed due to limited space.
NOMENCLATURE C eff hair K df k,
k,
qc %o,
R 1’1 1’2
T T, T T2 Tamb AH,, t
INTRODUCTION
Effective heal. capacity (J-C- ‘) Convective heat transfer coefficient -2-c-1 Pm ) Effective heat loss coefficient (W ‘C- ‘) Effective thermal conductivity of the metal hydride bed (W mm1 ‘C-l) Thermal conductivity of the cladding (W mm’ ‘C-l) Length of the metal hydride column (m) Length of the cylindrical slice cut from the container (m) Flow rate of hydrogen absorbed (mol s- ‘) Heat production per unit volume within the cylindrical slice (W m 3, Heat production within the cylindrical slice (W) Total heat production/consumption (W) Thermal resis,tance (“C W ‘) Inside radius of the cylinder (m) Outside radius of the cylinder (m) Temperature (“C) Temperature in the middle of the cylinder ( C) Temperature at the interface of the metal hydride bed and the cladding (“C) Temperature at the surface of the cylinder (‘C) Ambient temperature (“C) Reaction enthalpy of the hydrogen-metal reaction (J mol- ‘) Thermal time constant (h)
Hydrogen energy technology offers a promising option to store photovoltaic electricity for later use. This option has been studied in recent years in several laboratories Cl-41 including Helsinki Unversity of Technology, where a small-scale solar hydrogen pilot plant has been constructed [S] and its operation has been optimised by comprehensive numerical models [6]. During recent years, special emphasis has been put on the seasonal storage sub-system consisting of a hydrogen production unit, hydrogen store, and a fuel cell for hydrogen conversion back into electricity. In our present pilot plant, hydrogen is stored in a low pressure (30 bar) steel vessel which has proved to be economically the most feasible despite a low energy density. An Interesting option to increase the volumetric energy density of the hydrogen store is to use metal hydrides. Several commercial and non-commercial metal hydride materials have been previously characterised at our laboratory [7]. Based on these characterisations, a tailor-made 1 N m3 metal hydride container filled with a commercial Hydralloy Cl5 metal hydride alloy was ordered from Gesellschaft fiir Elektrometallurgie mbH for further investigations. The objective of this paper is to study the technical feasibility of the metal hydride hydrogen storage as a part of a solar hydrogen system, and to find an application in which the metal hydride storage would be suitable. This paper describes both the characteristics of the 213
J. P. VANHANEN
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et al.
Table 1. Properties of the metal hydride alloys studied [7] Alloy
Lao.4do.05W
MmNL.5Mno.4s%05 Zr,,,Ti,,,MnFe Zr(b.&ro.A LmNi( 1) Hydralloy Cl 5
28.1 23.9 19.3 19.5 26.9 24.9
30.0 29.9 28.9 29.2. 29.5 30.8
Capacityb (wt %)
Hysteresis”, b
Slope”, b
1.4 [8] 1.2 [9] 1.1 [lo] 1.3 [ll] 1.3 1.5
++ -
++
+ ++
++ +
a PDSC measurements; b according to the references. + = good; + + = excellent; - = poor; - - = useless.
hydride material and the macroscopic properties of the whole metal hydride container. The characteristics of the metal hydride material are based on thermodynamic measurements of a powdered sample with a pressure differential scanning calorimeter (PDSC) [7]. The macroscopic properties, primarily the thermal characteristics of the whole container, were measured on a laboratory-scale test bench and the appropriate thermal models are presented. The performance of the metal hydride container under realistic conditions is also studied, and conclusions on the technical feasibility are made. Finally, the metal hydride storage option is compared with other commercially available hydrogen storage alternatives.
metal
SELF-SUFFICIENT SOLAR HYDROGEN SYSTEM WITH A METAL HYDRIDE HYDROGEN STORE In self-sufficient solar energy systems,long-term energy
storage is usually the bottleneck of the whole system while short-term energy storage can conveniently be carried out by a conventional battery. In self-sufficient solar hydrogen energy systems,excesssummer photovoltaic electricity is converted to hydrogen by an electrolyser, and the hydrogen is stored over the seasone.g. in a steel vessel,a composite cylinder, or a metal hydride container. In the winter, when direct photovoltaic electricity is not available, hydrogen is converted back into electricity by a fuel cell. Thus, hydrogen is used as an energy carrier from the summer to the winter to level the seasonal variations in photovoltaic electricity production. A schematic diagram of a solar hydrogen system with a metal hydride store is shown in Fig. 1.The metal hydride store is charged by a pressurisedelectrolyser by increasing the hydrogen pressure until hydrogen absorption in the metal hydride occurs. Thus, the characteristic absorption pressure of the metal hydride has to be low enough for the pressurised electrolyser to charge it. On the other
PV-array
Load
Metal hydride container I Fig. 1. Schematic diagram of the solar hydrogen energy system with the metal hydride hydrogen store.
HYDROGEN
I --f --t
STORE FOR A SOLAR HYDROGEN
215
SYSTEM
LaNdNiabs. LaNblidas. LmNlabs. LmNlder. c1.5abs.
1 /temperature(K)
Hz concenlration
(wt%)
Fig. 2. Measured can’t Hoff plots of the feasible metal hydrides studied.
Fig. 3. P-C- T-curves of Hydralloy manufilcturer.
hand, the desorption pressure has to be high enough to feed hydrogen into the fuel cell at ambient pressure without external heating. On a macroscopic level, the container must have good heat transfer properties to avoid over-heating and pressure build-up during charging, as well as excessive cooling and pressure drop during discharging.
P-C-T-plateau slopes. Also, the pressure and temperature ranges are confirmed to be applicable to our system, i.e. Hydralloy Cl5 can be charged by a pressurised electrolyser, and discharged into a fuel cell at ambient pressure near room temperature.
COMPARISON
OF POTENTIAL HYDRIDES
METAL
Six metal hydrides near ambient temperature and pressure were characterised by PDSC in order to find the best metal hydride material for our system [7]. The objective was to find a metal hydride material having a low AH value, a low hysteresis effect, gentle P-C Tplateau slopes, and a high hydrogen storage capacity. These properties of the metal hydrides studied are collected in Table 1. On the basis of these results, three albs (MmNi4.sMn,,4,Zro.os, ZrFeo.,,Cro.,,),~ and Zr,,,Ti,,,MnFe) were ruled out because of their large hysteresis effect or sloping P-C T-plateaux, while the three remaining candidates (La,,,,Nd,,,,Ni,, LmNi( l), and Hydralloy C15) were studied further by comparing their respective van? Hoff plots (Fig. 2). In our application, the maximum temperature during charging remains below 50 C, and the minimum temperature during discharging remains above IO ‘C. Within these limits, the absorption pressures of La,,,,Nd,,,,Ni,, LmNi(l), and Hydralloy Cl5 are 8.6 bar, 15.4 bar and 9.8 bar, and the desorption pressures are I.1 bar, 1.5 bar, and 1.3 bar, respectively [7]. Thus, all three candidates seemed to be technically feasible for the application in question. Finally, we selected Hydralloy C15, because the manufacturer was able to make the container according to our specifications at a reasonable price. The feasibility of Hydralloy Cl 5 can be observed from the measured P-C- l”-curves supplied by the manufacturer (Fig. 3). The &CT-curves show clearly the high storage capacity, low hysteresis effect, and the gentle
Cl5
according
to the
THERMAL. CHARACTERISTICS OF THE METAL HYDRIDE CONTAINER UNDER STEADY-STATE CONDITIONS The thermal characteristics of the metal hydride container were studied under steady-state conditions to understand its heat transfer properties. If the heat transfer properties of the container are poor, the container may over-heat during charging which may also lead to too high a hydrogen pressure for the electrolyser to be able to charge it. Similarly, excessive cooling during discharging may lead to too low a hydrogen pressure for the fuel cell. Therefore, we wanted to determine the effective thermal conductivity of the metal hydride bed, and compare it with the convective heat transfer coefficient between the container and the air to judge which one limits the overall heat transfer of the container. The cross-section of the metal hydride container including the central tube for hydrogen delivery, the metal hydride bed. and the steel cladding is depicted in Fig. 4. The container has empty space because the metal hydride material expands during hydrogenation. Furthermore, the effective volume of the hydride material is expected to increase as the particle size of the metal hydride material decreases during the first few hydriding--dehydriding cycles. The temperatures were measured at the surface and inside the container. The inside temperature sensor had been installed in the central tube in the middle ofthe container by the manufacturer while our five surface temperature sensors were able to move easily depending on the requirements of an experiment. The pressure gauge and mass-flow controller were installed in the hydrogen delivery line.
J. P. VANHANEN
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et ul.
steel cladding, d = 2.3 mm
hydrogen delivery tube
89 mm I 360 mm 644 mm
t
Fig. 4. Cross-section of the metal hydride container.
To estimate the heat transfer properties of the metal hydride bed, we may consider a small cylindrical slice cut from the container, Fig. 5. We determined the heat transfer properties of the cylindrical slice under steadystateconditions by using different constant hydrogen flow rates to charge the metal hydride store. The hydrogen flow rates employed were so low that all the hydrogen fed into the metal hydride absorbed immediately without pressure build-up. Also, the hydride bed was assumed to be porous enough so that significant pressure gradients within the bed are not formed, i.e. the hydrogenation reaction proceeds uniformly within the bed. Thus, the heat production due to the absorption reaction was assumed to be constant and homogeneous. The surface and inside temperatures of the cylindrical slice were determined after the whole container had reached steadystate, i.e. all the temperatures measured remained unchanged. If the cylindrical slice is cut between 18 and 22 cm, we observe that the surface temperature under steady-state conditions along the z-axis is almost constant (Fig. 6).
Thus, the container experiences temperature gradients mainly in the radial direction. Therefore, a one-dimensional steady-state heat equation may be used to estimate the effective heat conductivity of the metal hydride bed, and also to calculate the temperature profile along the r-axis. Hence, the heat conduction equation for the metal hydride bed may be written as 1 d r dr -4
dT rdr
‘I’ +F=O, I >
(1)
and for the steel cladding as
(2)
where T is the temperature, k, the effective conductivity of the metal hydride bed, and 4:’ produced per unit volume within the cylindrical solving the heat conduction equations (1) and the boundary conditions, we obtain
thermal the heat slice. By (2) with
36
201
0
10
20
30
40
50
60
Height (cm)
Fig. 5. Modelled cylindrical slice cut from the metal hydride container (rl = 42.2 mm. rz = 44.5 mm, I, = 40.0 mm).
Fig. 6. The measured surface temperatures of the container under steady-state conditions along the z-axis at different charging flow rates.
HYDROGEN
T(r)=
To-;&
b
STORE FOR A SOLAR HYDROGEN
ZforO
1‘ i) I
(3)
217
SYSTEM
obtained from the literature [ 123. By employing several charging flow rates from 5 to 30 NI h-‘, we obtain experimentally the following values for k, and hai,:
and k, =0.6iO.l T(r) =
(T, - T,) In r + T;ln Mr,lr2)
hLgi,= 8 & 1 W m-2 for r, < r < r2,
41,= q:“.rrrf/,,
(5)
which may also be written as (1, = AH,,r&
.L 1’
(6)
where AH,, is the reaction enthalpy of the hydrogen metal reaction, I is the overall length of the metal hydride column (36 cm), and ri,, is the constant flow rate of hydrogen absorbed within the container during the measurement. The boundary temperatures T,, Ti and T2 may be written as + SW,
+ R, + Rx),
(7)
T, = jy,,,,, + qc(R, + R,)>
(8)
7; = Tam,,+ ycR,,
(9)
and
where Tam,,is the ambient temperature, and R,. R, and R, are the thermal resistances defined as: R, =p
6,
=
‘2
I 4nk,l,’
(10)
W2/rl)
2nk,I,
’
(11)
and R, =p
C.
1 2nr, hai,l, ’
C.
(13) (14)
(4)
where r, and r2 are the inside and the outside radi of the cylinder, respectively,
To = L,
Wm-’
r, - T, In r2
(12)
where k, is the thermal conductivity of the cladding and hai, is the convective heat transfer coefficient between the container and air. The effective thermal conductivity of the metal hydride bed (k,) and the convective heat transfer coefficient (h,,,) were solved using equations (6))(12) while the thermal conductivity of the steel cladding (k2 = 15 W/m’C) was
By substitutmg these values to equations (lO))( 12) we observe that the overall thermal resistance is dominated by the convection term R,. Actually, R, is over 3 times higher than R, However, the effective thermal conductivity of the metal hydride bed is quite low and the value may even decrease as the metal hydride particles crush during the first few cycles. However, as the total heat transfer is restricted by the convection term, any improvemcnt in k, would not improve much the total heat transfer properties of the container. The effect of R, is insignificant due to high thermal conductivity. The accuracy of the above-described model was verified against measurement by using several different constant charging flow rates. The calculated and measured values of T, and T, are presented in Table 2. According to these results, the model gives accurate results, and thus the values fork, and h,,, arc reliable, and also the assumptions made on the heat production within the container seem to be reasonable. The accuracy of this model is good enough for our purpose to study the thermal feasibility of the metal hydride container for the seasonal storage application, ,where hydrogen absorption and desorption flow rates are extremely low. However, a more detailed model taking the spatial variation of the heat production and also the dynamics of the hydrogen-metal reaction into account would be needed especially in cases when higher hydrogen flow rates are employed.
TIME-DEPENDENT THE METAL
THERMAL BEHAVIOUR HYDRIDE CONTAINER
OF
As the thermal time constant of the container is high, a model to describe the transient behaviour of the container is also needed. Therefore, we modelled the time-dependent behaviour of the metal hydride container by using the lumped capacitance method. The lumped capacitance method is generally valid if the thermal resistance due to convection dominates the thermal resistance due to conduction [12]. This condition also means that the temperature of the body can be described or approximated by a single temperature. In this case, this validity condition is met quite well and the container is described by the temperature measured in the middle of the container. The time-dependent heat balance equation of the container may be written as
c
dT(t)
~ err dt
= a,,(t) - K,,,CT(d - Ta,,,,,l.
(15)
J. P. VANHANEN
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Table 2. Calculated temperatures at different flow rates verified against the measured temperatures Flow rate (Nl h-‘)
Inside temperature-T,
5.0 10.0 15.0 20.0 30.0
(-C)
Surface temperature-
TamLl ( C)
q (’ C)
Measured
Calculated
Deviation
Measured
Calculated
Deviation
23.9 26.2 29.8 31.5 36.0
23.1 26.1 29.0 31.1 36.2
-0.2 -0.1 -0.8 -0.4 +0.2
23.1 24.7 21.6 29.0 32.1
23.2 25.0 27.4 28.9 33.0
f0.1 +0.3 PO.2 -0.1 +0.3
where Cefl is the effective heat capacity of the container, q,,, is the total heat production or consumption within the container, and K,,, is the effective heat loss coefficient. The general solution of equation (15) is
--t
21.2 21.1 21.5 21.1 21.3
30Nllh
32.0 -
T(r) = e (16) where the constant C, depends on the initial conditions. To solve the parameters Ceff and K,,, experimentally, we used one constant hydrogen flow rate at a time to charge or discharge the metal hydride hydrogen store. As the flow-rates employed were low and the pressure measurement indicated that absorption and desorption occurred immediately, we assumed q,,, to be constant during the constant flow-rate measurement. As also the initial temperature of the container was Tamb,we obtained the following particular solution of equation (15) for the constant hydrogen flow rate measurement.
T(t) = Tarn,,+ p
(1 - e-“‘), err
(17)
where 5 is the thermal time constant of the container defined as
c T=y. cff
nerr
(18)
By employing several constant hydrogen flow rates to charge and discharge the metal hydride store, we obtained the following value for the thermal time constant
T = 2.3 k 0.2 h,
(19)
which corresponds to
K,,, = 0.69 k 0.03 W “C-l,
(20)
C,,, = 5.8 f 0.5 kJ ‘C-l.
(21)
and
Time(h)
Fig. 7. Calculated (solid line) and measured (scatters) temperatures of the container during charging.
The time-dependent thermal behaviour of the container at different constant charging and discharging hydrogen flow rates is illustrated in Figs 7 and 8. Both figures show that the lumped capacitance model describes accurately the time-dependent thermal behaviour of the container. The high effective heat capacity of the container indicates that the container does not usually reach steady-state, especially if the store is charged by the electrolyser, the production of which depends on solar input. Thus, the time-dependent model is crucial in studying the operation of the metal hydride store connected to the solar hydrogen system. PERFORMANCE CONTAINER
OF THE METAL HYDRIDE DURING TEST RUNS
The metal hydride container was tested under realistic conditions to get further operational information. The hydrogen flow rates employed during the test runs were significantly higher than needed in a seasonal storage system in order also to determine the technical limits for shorter periods of time. In the experimental system, the metal hydride container is connected between two mass flow controllers (Fig. 9). These mass flow controllers are controlled by the computer to emulate the hydrogen production and consumption by an electrolyser and a fuel cell, respectively.
HYDROGEN
-
STORE FOR A SOLAR HYDROGEN
lONl/h
3.0
4.0
5.0
6.0
7.0
6.0
Time(h)
Fig. 8. Calculated (solid line) and measured (scatters) temperatures of the container during discharging.
SYSTEM
219
During the first test run, a stochastic hydrogen flowrate was employed for 6.8 h to charge the metal hydride store (Fig. 10). This stochastic hydrogen flow rate describes hydrogen production in a solar-driven electrolyser during variable insolation conditions. The charging flow rate was varied from 1 to 20 Nl h- ’ with an average of 11.5 Nl h- ‘. The pressure in the metal hydride container stayed clearly below 5 bar which indicates that the store can easily be charged by an electrolyser with a maximum pressure of 5 bar when operating near room temperature. The effect of the high thermal time constant can also be observed as the large variations in hydrogen flow rate do not cause any major variations in temperature. The second test run describes hydrogen consumption in a fuel cell connected to a random electric load (Fig. 11). The discharge flow rate was varied from 1 to 20 Nl h-’ for 6.2 h. The average flow rate was 12.0 Nl h-’ corresponding to a discharge time of about 80 h. The temperature of the metal hydride container varied from 14 to 20 C, corresponding to pressure levels of 0.4 and 1.1 bar above atmospheric pressure. Thus, the discharge
PI = pressure indicator PT = pressure transmitter FC = flow controler TT = temperature transmitter Hypro,
~asxtle
Metal h dride container (l&O 1NTP) Fig. 9. Experimental
system during the test run.
J. P. VANHANEN
et al.
the storage capacity needed; storage time; l the location of the application; l weight and volume; l the need for maintenance; and 0 price.
l l
t 5.0 t 2.5 10.0
'= 8
7.5
f
5.0
b: t
2.5
+, LO
I
1 .o
2.0
3.0
4.0
~“10,t-J
5.0
6.0
I
7.0
Time (h)
Fig. 10. Temperature, pressure and hydrogen flow rate during the first test run (charging).
pressure of the metal hydride seems to be applicable to a fuel cell working at ambient pressure.
OTHER
HYDROGEN
STORAGE
ALTERNATIVES
In this paper, the emphasis is put on a metal hydride container, but there are also several alternative methods to store electrolytic hydrogen, for example: l l l l l
gaseous hydrogen; liquid hydrogen; cryoadsorption; zeolites; and chemical compounds.
An important criteria is the energy density. Approximate gravimetric and volumetric energy densities of different hydrogen storage alternatives are presented in Fig. 12. Gaseous hydrogen covers a wide area in the energy density map as the volumetric energy density depends on the storage pressure, and the gravimetric density depends on the material of the vessel. For example, advanced composite vessels may have significantly higher gravimetric energy densities than conventional steel vessels. Metal hydrides usually have better volumetric energy density than gaseous hydrogen storage. The gravimetric energy density of low-temperature metal hydrides is about the same as that of the gaseous hydrogen storage, but with high-temperature metal hydrides, higher gravimetric energy densities may be reached. Liquid hydrogen stored in a cryocontainer has clearly the best gravimetric energy density, but the volumetric energy density is about the same as with metal hydrides. The characteristics of cryoadsorption lie between gaseous hydrogen and metal hydrides, while the energy densities of zeolites are quite modest. Liquid hydrogen and cryoadsorption are suitable only for large applications, as the handling of hydrogen necessitates expensive devices. In the case of gaseous hydrogen, a compressor is needed to reach a high pressure while high-temperature metal hydrides require hightemperature heat. Therefore, low-pressure hydrogen vessels and low-temperature metal hydrides seem to be the most suitable for small-scale applications, as they do not require expensive instruments for gas handling. The
Generally, the best hydrogen storage alternative depends strongly on the application in question. Some important aspects that have an effect on final selection are: 10 30.0 r
1
8 23 6 i?! 1 8
, 15.0 12.5 10.0 7.5 5.0
2 a c a 8 p‘
J 0.0 7.0
Fig. 11. Temperature, pressure and hydrogen flow rate during the second test run (discharging).
0.01 1 0.1
I I 1 10 gravimetric energy density (kWh/kg)
I 100
.
Fig. 12. Energy densities of the hydrogen storage alternatives (including containers).
HYDROGEN
STORE FOR A SOLAR HYDROGEN
cost of a metal hydride container is now about 20 times higher than the price of a 20 bar steel vessel, but the volumetric energy density is also about 20 times higher. Thus, low-temperature metal hydrides seem to be apphcable to systems in which high volumetric energy density is needed due to limited space. CONCLUSION The feasibility of a metal hydride hydrogen store for a self-sufficient solar hydrogen energy system has been studied. The equilibrium pressure of absorption of the metal hydride material must be low enough to be charged by a pressurised electrolyser. On the other hand, the equilibrium pressure of desorption must be high enough to enable the discharging of the store into a fuel cell at ambient pressure. As the equilibrium pressures of the metal hydride depends on temperature, it is crucial that the thermal behaviour of the container is known. Thus, the thermal characteristics of the metal hydride container were measured, and appropriate thermal models were derived. According to the verification measurements with different hydrogen flow rates, the models seem to describe accurately the thermal behaviour of the container in this application. The metal hydride container was also tested under realistic conditions to get further operational experience on its technical feasibility. According to the test runs near room temperature, the metal hydride hydrogen store studied can be charged by a low-pressure (max. 5 bar) electrolyser, and also be discharged into a fuel cell at ambient pressure even though the hydrogen flow rates employed in the test runs were significantly higher than needed in seasonal storage applications. The effect of hydrogen purity was not considered in this study, but it may have an effect on the metal hydride material as impurities gradually decrease the storage capacity. However, the number of cycles needed in this particular case is low, and thus hydro,gen purity should not cause serious problems. Generally, metal hydrides are an interesting alternative for storing hydrogen. They show many advantages such as high volumetric energy density, good safety aspects, the availability of a wide range of temperatures and pressures, easy maintenance, and reliable operation without additional mechanical devices. The disadvantage is the high cost of the metal hydride material. However, the
SYSTEM
221
metal hydride hydrogen store may have a market niche in small-scale solar hydrogen applications in which high volumetric capacity is needed and safety aspects are appreciated. il~kno~(rdgen1rnt.s~~ This work has been financed by the Finnish Ministry of Trade and Industry under the NEMO research programme. REFERENCES I. A. Heinzel and K. Ledjeff. The self-sufficient solar house: hybrid energy storage system. Prw. 1991 S&r World Coqpw, pp. 254332546, Denver, CO (1991).
2. P. A. Lehman and C. E. Chamberlin, Design of a photovoltaichydrogenfuel cell energy system. Int. J. Hydrogen Energy 16, 349-354 (199 1). 3. A. G. Garcia-Conde and F. Rosa, Solar hydrogen production: a Spanish experience. Int. J. H~vlrogen Energ.v 18, 9951000 (1993). 4. S. Galli, G. De Paoli and A. Ciancia, An experimental solar PV-hydrogen -fuel cell energy system: preliminary results. Proc. 10th World Hydrogen Energy Conftirenw, pp. 439 447, Cocoa Beach, FL (1994). 5. P. S. Kauranen, P. D. Lund and J. P. Vanhanen. Development of a self-sufficient solar-hydrogen energy system. Int. J. H.vdrogm Enrrgv 19, 99- 106 (1994). 6. J. P. Vanhanen, P. S. Kauranen, P. D. Lund and L. M. Manninen, Simulation of solar hydrogen energy systems. Solur Energy 53, 267 278 (I 994). 1. M. T. Hagstrom, P. D. Lund and J. P. Vanhanen, Metal hydride hydrogen storage for near ambient temperature and atmospheric pressure applications, a PDSC-study. In/. J. Hydrogen Energy 20, 897 (1995). 8. H. H. van Mal. K. H. J. Buschow and A. R. Miedema, Hydrogen absorption in LaNi, and related compounds: experimental observations and their explanation. J. /essc.&mon Met. 35, 65-76 (1974). 9. Y. Osumi. H. Suzuki. A. Kato. K. Oauro. S. Kawai and M. Kaneko, Hydrogen absorption and desorption characteristics of Mm-NiiAIlM and Mm-Ni Mn-M alloys (Mm = misch metal). J. kss-cownon Met. 89, 2X7-292 (1983). IO. V. K. Sinha, F. Pourarian and W. E. Wallace, Hydrogenation characteristics of Zr, ,Ti,MnFe alloys. J. Irwcommon Mer. 87, 283 -296 (1982). I I. S. Qian and D. 0. Northwood, Thermodynamic characterisation of Zr(Fe,Cr, X)2 H systems. J. less-comnmn Met 147, 149-I 59 (1989). oj’Hrut and 12 F.P. Incropera and D. P. Dewitt, Fundummtuls Muss Trum/>r, 2nd ed. John Wiley, New York (1985).