Part A: A study on the operating characteristics of metal hydride heat pump.
A promising application of hydride technology is the metal hydride chemical heat pump which can be operated with waste heat exhausted from automobiles and industrial plants. However, the transfer of this technology to the industrial application is being delayed due to the low cooling power per unit mass of the system. The Keys to increase the power of the metal hydride heat pump are ⅰ) development of hydride material that has good hydrogenation properties (large heat of formation, large hydrogen storage capacity), ⅱ) fast hydriding-dehydriding reaction in the reactor by improving the design of the reactor, ⅲ) operation in optimum operating conditions.
In this study, a prototype heat pump was constructed using Zr-based Laves phase alloy pairs which have large hydrogen storage capacity, fast reaction kinetics and the operating performance of the system was investigated. The reactor modules were composed of 12 unit reactors made of Cu-tubes with 19.1mm out diameter. To enhance the heat transfer along the radial directions in the tubular reactor, 120 mesh brass screens were inserted in the hydride bed at the intervals of 2mm. $Zr_{0.9}Ti_{0.1}Cr_{0.9}Fe_{1.1}$ and $Zr_{0.9}Ti_{0.1}Fe_{1.4}$ were used as high temperature side alloys, respectively.
The dependences of cooling power outputs on operating parameters such as the hydrogen charged amount, cycle time, air flow rate and heat source temperature were investigated. The optimum operating condition was evaluated as follows; (1) the initial amount of hydrogen charged = 12.8 - 13.2 moles, (2) cycle time: heating time = 4 min and cooling time = 7-8 min, (3) air flow rate ＞ 3.8㎥/min, (4) heat source temperature ＞ 220℃. The maximum power obtained under optimum operating conditions was about 130 kcal/kg-alloy.h.
Part B: A study on the dynamic reaction characteristics of metal hydride in the reactor.
In order to investigate the dynamic reaction properties of hydride in the hydride beds with large mass, the dynamic P-C-T curve of $MmNi_{4.6}Al_{0.2}Fe_{0.2}V_{0.03}$ has been obtained experimentally, using tubular reactors simulating a module of a full-size multitube heat exchanger of a metal hydride heat pump. It is shown that the hysteresis increased with the increase of hydrogen flow rate. By comparing the measured temperature and the estimated temperature obtained from the hydrogen pressure using van't Hoff equation, it is found the dynamic pressure change is attributed to the temperature change of the hydride bed.
Based on the experimental results and the assumption of thermal equilibrium of hydride, a simple qualitative model of the dynamic P-C-T curve is presented. And it can explain the effects of the design parameters such as the thermal conductivity of the hydride bed and the structure of the reactor, and the operation parameters such as hydrogen flow rate and the temperature of cooling fluid on the change of the dynamic P-C-T curve.
In order to simulate the dynamic P-C-T curve of $MmNi_{4.6}Al_{0.2}Fe_{0.2}V_{0.03}$, a mathematical model has been estabilished and solved numerically by the method of the finite domains. The numerical simulation is used to present the time-space evolution of the temperatures, the pressure and the hydrogen concentration of the hydride powders in the reactor and to determine the effects of poeration parameters (hydrogen flow rate, temperature of water bath) and reactor geometry. The local reaction rates are calculated by the product of the average rate and the pressure factor which is assumed to be (In $P_{appl}$ - In $P_{local}$)/(In $P_{appl}$ - In P). $P_{appl}$ is the imaginary hydrogen pressure after time step Δt if the inflowed hydrogen is accumulated as gas phase. P is the local hydrogen pressure where the reaction occurs as average rate. The simulation results agree with the experiments.
Using the dependence of the dynamic P-C-T curves on the temperature of water bath, the pseudo van't Hoff plots are obtained. It is suggested that the MHHP can be designed based on the pseudo van't Hoff plots and the operating characteristics be predicted.
