hr. J. Hwhgm En‘nrr~y, Vol. 21, No. 3, pp. 213 ‘21, 1996 Copyright @ International Association for Hydrogen Energy Elsevier Science Ltd 0360-3199(95)00064-X Prmtcd in Great Britain. All rights reserved 0360.3 199!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, P. D. LUND and M. T. HAGSTROM Helsinki University of Technology, Department of Technical Physics, FIN-02150 Espoo, Finland (Rrceiredjbr 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. C eff hair K df k, k, qc %o, R 1’1 1’2 T T, T T2 T amb AH,, t NOMENCLATURE Effective heal. capacity (J-C- ‘) Convective heat transfer coefficient Pm ) -2-c-1 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 reac- tion (J mol- ‘) Thermal time constant (h) INTRODUCTION 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 con- structed [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 con- version 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