Y. Komazaki

Effective thermal conductivity of metal hydride beds

Abstract An experimental investigation aimed at improving the effective thermal conductivity of activated metal hydride is reported. Of the many material configurations which have been used to enhance heat transmission through metal hydride beds, a three-dimensional structure of a porous aluminium composite has shown the best results giving transmissions a factor of 9 or 10 greater than those of the unmodified hydride bed. The experimental results were used to derive equations for estimating the effective thermal conductivity as a function of the system pressure, the temperature, the hydride composition and the void fraction of the materials used.

Operating characteristics of a metal hydride heat pump for generating cooled air

Abstract A metal hydride heat pump was constructed and the dependence of its operating characteristics on such variables as the pressure-temperature-concentration and kinetic properties of paired hydrides, the heat transfer performance of the heat exchangers, the amount of hydrogen gas with which the system was charged, the cycle time, the mass flow rate of the air supply and the operating temperature was investigated. The optimum operation conditions and parameters were evaluated and will be used as guidelines for future heat pump designs.

Development of a double-stage heat pump: experimental and analytical surveys

Abstract A double-stage heat pump using three different kinds of metal hydrides was developedto demonstrate the practical applicability of the hydriding-dehydriding reactions to the generation of high temperature heat through the dehydriding reaction, under lower temperature conditions, with high operational performance. Extensive research was carried out on the design, construction and operation,and the operating performance of two experimental demonstration units with different capacities (7.72 kW and 77 kW) was analysed. The emphasis of this work was the generation of high temperature steam in thetemperature range 120–150°C using a low grade heat source at a temperature of 80°C and ambient air as the cooling medium. Experimental and analytical results of the operational performance of thetesting unit was 7.72kW capacity are described in detail.

The dynamics of hydrogen transfer in a metal hydride heat pump

Abstract The hydrogen transfer between metal hydride reactors in a heat pump has been studied experimentally using two different pairs of hydrides. It is shown that the sorption kinetics and the heat transfer processes in both reactors are coupled and that they together determine the rate of hydrogen transfer, although heat transfer resistances dominate in the present set-up. Convection on the fluid side of the reactors is not a negligible resistance to heat transfer.

Dynamic behaviour of paired metal hydrides: I. Experimental method and results

Operational characteristics of metal hydride heat pumps are strongly dependent on the P-T-C behaviour of the paired metal hydrides under operational conditions. Changing operational conditions are directly related to the available amount of hydrogen which can be transferred between the paired metal hydrides. Maximum hydrogen transfer rates must be obtained for all operational modes and conditions to attain a high system performance. Therefore, the amount of hydrogen transferred is correlated with the actual behaviour of each metal hydride together with the static P-T-C behaviour to analyse heat pump operations under dynamic conditions. The purpose of this paper is to extend the scope of our detailed study to dynamic behaviour of systems in order to facilitate the design of metal hydride heat pumps.

Equilibrium properties of TiZrFeMn hydrides

Abstract Quaternary alloys with the formula Ti1−xZrx(Fe1−yMny)z were investigated with the aim of obtaining hydriding alloys suitable for thermal energy conversion systems operating under relatively high temperature conditions (120–200°C). The effects on the pressure-temperature-composition relations of changing x, y and z were studied. Ti0.5Zr0.5(Fe0.2Mn0.8)1.5 hydride had a very low hysteresis (hysteresis factor, 0.955) and a high hydrogen capacity ( [H] [M] = 1.14 (2.0 wt.%) ) at temperatures above 100°C.

Dynamic behaviour of paired metal hydrides. II: Analytical survey of the experimental results

Abstract The operational characteristics of a paired metal hydride system are strongly dependent on the amount of hydrogen gas which is transferred from one metal hydride as hydrogen donor to the other metal hydride as hydrogen acceptor. In such a dehydriding-hydriding process the amount of hydrogen gas which is transferable between the paired metal hydrides is considered as the result of the dynamic pressure-temperature-composition (P-T-C) behaviours in such a combined system. These dynamic P-T-C behaviours are studied from the experimental P-C and P-T diagrams in order to analyse the effect of operational parameters which influence the system characteristics. Correlations between dynamic P-C behaviours and static P-C relationships revealed that the actual pressure difference (defined as the dynamic pressure driving force) determines the pressure levels of the given system and the amount of hydrogen transferred through dehydriding and hydriding reactions. Three different types of pressure driving forces under equilibrium (static) conditions are also discussed in connection with the dynamic pressure driving force which is obtained from the experimental dynamic study of a paired metal hydride system.

Kinetic properties of aluminium- and iron-substituted misch metal-nickel hydrides☆

Abstract The kinetics of the hydriding and dehydriding reactions in aluminium- and iron-substituted misch metal-nickel hydrides were studied. The experimental data were correlated as a function of pressure, temperature and hydride composition. The extent of the reaction depended more strongly on the system pressure and temperature than on the hydride composition. The effect of these factors on the rate constant was evaluated using a pressure-corrected constant called the N value. A temperature at which the maximum hydriding rate is obtained exists in each isobaric relation between the N value and the temperature in the hydriding reaction.

The effective thermal conductivity of a metalhydride bed packed in a multiple-waved sheet metal structure

Abstract The effective thermal conductivity (ETC) of a hydrogen-MmNi4.51Al0.47 hydride system * was measured under equilibrium pressure conditions between approximately 0.001 and 5 MPa. In the system, a multilayer waved sheet structure made of copper was used to ensure improved heat transfer in the metal hydride bed where the contact surfaces of copper sheets were soldered by applying amorphous thin films placed between the copper sheet and the inner surfaces of the reactor tubes. The maximum value obtained in this experiment was found to give an ETC of14.0 W m−1 K−1 by packing the metal hydride powder in a tubular space of which only 6% was occupied by the metallic structure used for improving the ETC. There was no distinguishable difference in the effective thermal conductivity between the hydriding and dehydriding processes. The effect of temperature was observed to be small in the range 303–363 K. The ETC increased by approximately 2.5 W m−1 K−1 owing to the volume expansion of the metal hydride particles and the increase in the contact surface of the particles. An empirical equation is proposed to estimate the ETC of the proposed metalhydride composite by considering the effects of the thermal conductivities of the metal hydride bed itself and the metallic structure used to improve the ETC as a function of the hydrogen concentration of the metal hydride.

Design of a new reactor system for the kinetic study of metal hydrides under isothermal conditions

Abstract A highly heat sensitive reactor was designed to study the reaction kinetics of metalhydrides under isothermal conditions. In the reactor, heats equivalent to those which accompanied the hydriding and dehydriding reactions were transferred to or from the reactor in order to maintain the temperature of the metal hydride bed as constant as possible. This temperature control was achieved by a heat exchanger system in which carbon dioxide and nitrogen gases were used as the cooling and heating media respectively and the gas flow rates were controlled by activating a pneumatically operated valve using a PID control. Using this technique, the temperature changes were kept within ±0.1°C. Onlyat the very beginning of the reaction was the temperature found to deviate 2°C for a few seconds owing to the slow response time of the reactor. Using this new reactor, hydriding and dehydriding reactions of the LaNi4.7Al0.3−H system were measured by a stepwise method. Experimental data taken under quasi-isothermal conditions were compared with data obtained under nonisothermal conditions. Thermal effects on the reaction kinetics were examined in detail.