Efficiency analyses of high temperature thermal energy storage systems of rocks only and rock-PCM capsule combination

A modified transient, one-dimensional, Dispersion-Concentric model is developed to investigate the dynamic performance of high temperature packed-bed thermal energy storage systems using air as the heat transfer fluid. The packed-bed systems of rocks only and rock/PCM capsule combination are investigated, and both the fully charging/discharging processes and the cyclic process are simulated. Four factors including the charging efficiency, the discharging efficiency, the cyclic efficiency and the capacity ratio are introduced in the efficiency analyses. The results show that fully charging-fully discharging operation could not be adopted in most of practical applications due to the considerable energy losses during both the charging and the discharging processes. For the charging/discharging cycle with cut-off temperatures, the practical tank storage capacity is lower than the theoretical storage capacity, and the cut-off temperatures have great influence on the charging efficiency and the capacity ratio. Besides, adding a PCM layer could increase the capacity ratio and the charging/discharging time only if proper cut-off temperatures for the charging/discharging processes are chosen.