Contribution degree analysis of phase change capsule size in packed bed

The phase change capsule packed bed has been widely studied by many scholars because of its simple structure and high heat transfer rate. This study aims to obtain the effect of phase change capsule size on the thermal properties of the packed bed and the variation of the contribution degree in the heat storage process. In this paper, three three-dimensional models of heat storage packed beds with different phase change capsules sizes are established and the thermal storage process is numerically simulated by Fluent. The simulation results are compared with the experimental results, and the results are in good agreement. By monitoring the temperature of the heat transfer fluid (HTF), the temperature of the phase change material (PCM), and the liquid phase fraction in the packed beds with different phase change capsule sizes, the indicators for characterizing thermal characteristics of heat storage devices, such as heat storage rate, heat storage density, instantaneous energy efficiency, and exergy efficiency, can be calculated. The comparative analysis shows that the thermal properties of the packed bed improve as the capsule size decreases. Then the contribution degree of phase change capsule size to the heat storage process of the packed bed is obtained by the chain rule. Results indicated that as the phase change capsule size decreases, its contribution degree increases gradually. Still, the degree of increase decreases, and the gain from the change in capsule size decreases. The contribution degree of the phase change capsule size is also numerically less than that of the inlet mass flow and temperature. In general, with the decrease of the phase change capsule size, the thermodynamic performance of the packed bed and the contribution degree of the phase change capsule size increase significantly, but the power consumption and cost of packed beds increase.