Metal Hydride, Heat Pump, Effective Thermal Conductivity, Optimization
Answering the call for the eco-friendly energy development, the thermally driven metal hydride heat pump (MHHP) is one of the feasible approaches for its great potential in the recovery of low-grade heat without greenhouse-gas emissions, making it a leading contender among different alternate fuels. The main challenge for the practical application of MHHP is the relatively poor system performance, which is mainly considered due to the low effective thermal conductivity (ETC) of metal hydride reaction bed. Therefore, aiming for improving heat transfer performance, previous researches have proved that inserting high-conductivity sheet such as Metal Hydride (MH) sheet and Aluminum Foam (AF) sheet into the reactor is one of the most practical methods. However,there is no significant improvement of system performance when enhancing the effective thermal conductivity (E.T.C) of reaction bed. Thus for the purpose of improving the system performance of MHHP, this paper first identifies the relationship between the ETC of metal hydride reaction bed and the system in terms of two performance indicators: COP and cooling power (kW/kg) with a numerical model.Â By adjusting the cycle time, it is shown that when cycle time increasing, the available hydrogen capacity increases, and thus the heat output increases, resulting in a better coefficient of performance. However, once a certain cycle time has been reached it has less effect on the hydrogen capacity. In addition, the heat loss caused by the systemâ€™s heat capacity is proportional to cycle time, and the average flow rate of hydrogen would be decreased when cycle time increasing, allowing COP to reduce. Therefore, there existed an optimal cycle time. As an important factor affecting cycle time, the ETC is related mathematically to find out the optimal value for a better performance of MHHP. The experimental confirmation is conducted eventually, and the comparison of results shows a reasonable agreement with an acceptable error range. To sum up, the optimization of the ETC based on the relationship with the system performance is proposed in this paper, which can help to find the optimal value overall situation toward to a more efficiency system of MHHP.