Investigations on the Heat Transfer Characteristics and Cascaded Optimization Design During Charging Process of Packed Bed Thermal Energy Storage System

The packed bed thermal energy storage (PBTES) system is one of the important method to address the issues of fluctuation and intermittency in the development and utilization of renewable energy, and it can effectively reduce the impact on the power grid during the grid connection of renewable energy. However, the heat transfer characteristic during heat charging process and the optimization design methods need to be further investigated. In this study, a concentric dispersion model considering heat loss is established, and the phase change and heat transfer characteristics of the PBTES system under different mass flow rates are numerically analyzed. Meanwhile, a formula describing the average phase change travel velocity is proposed to quickly evaluate the phase change and heat transfer performances of the PBTES system. Based on the average phase change travel velocity, an optimal design method of the cascaded PBTES system is further proposed. The results show that the temperature difference caused by heat loss of the PBTES system decreases with the increase of mass flow rate, while the heat transfer temperature difference shows inapparent variation. During the charging process, the phase change front position inside the PBTES system changes approximately linearly and the average phase change travel velocity increases from 3.27×10-5 m/s to 1.01×10-4 m/s when the mass flow rate increases from 40 kg/h to 120 kg/h. The optimal filling ratio of phase change material capsule (PCM capsule) in each layer of the cascaded PBTES system is 35:23:42 based on the average phase change travel velocity. Compared with the uniform filling ratio of 1:1:1, the charging time of the cascaded PBTES system with optimal filling ratio is shortened by about 11.4%, and the average heat transfer power shows the increase of about 6.7%. The above results confirm that the phase change travel velocity can effectively characterize the phase change heat transfer characteristics of PBTES system, and the optimization method based on the phase change travel velocity can provide guidance for the design of PBTES system.