Multi-objective optimization of a metal hydride reactor coupled with phase change materials for fast hydrogen sorption time

Recently, the utilization of phase change materials (PCM) for the heat storage/recovery of the metal hydride's reaction heat has received increasing attention. However, the poor heat management process makes hydrogen sorption very slow during heat recycling. In this work, the H2 charging/discharging performance of a metal hydride tank (MHT) filled with LaNi5 and equipped with a paraffin-based (RT35) PCM finned jacket as a passive heat management medium is numerically investigated. Using a two-dimensional mathematical model validated with our in-house experiments, the effects of design parameters such as PCM thermophysical properties and the fin size on hydrogen charging/discharging times of the MHT are investigated systematically. The results showed that the PCM's melting point and apparent heat capacity have a conflicting impact on the hydrogen sorption times, i.e., the low melting point and high specific heat capacity reduce the H2 charging time. In contrast, the hydrogen discharging time follows the opposite trend. As a result, a multi-objective optimization was conducted to simultaneously minimize the H2 charging/discharging times using the thermal properties and size of the PCM. The optimum solutions selected from the Pareto front show that the PCM melting point should be around 42-43 degrees C for fixed hydrogen ab/desorption pressures of 10/1.5 bar. Moreover, the comparison between the MHT-PCM using the optimized PCM and reference PCM showed that the former reduced the hydrogen charging/discharging times by 48.6 % and 4 %, respectively. At the same time, the hydrogen storage efficiency of the optimal design is 100 % as compared to 96 % for the reference design. Besides, among the practical PCMs, inorganic PCMs (salt hydrates and eutectics) display favorable hydrogen charging time below 3000 s at the expense of hydrogen discharging time (above 7000 s) in our case study.