Numerical analysis of the thermal performance of packed bed thermal energy storage in adiabatic compressed air energy storage systems

Thermal Energy Storage plays a significant role in Adiabatic Compressed Air Energy Storage. There is a limited understanding of how the operational conditions of Adiabatic Compressed Air Energy Storage affect the performance of packed-bed Thermal Energy Storage and vice versa. In this study, packed-bed rock Thermal Energy Storage units operating at different air pressures integrated into a large-scale, four-stage Adiabatic Compressed Air Energy Storage system were examined over a single charging and discharging cycle and over five consecutive cycles. A transient, two-dimensional axisymmetric numerical model of packed-bed rock Thermal Energy Storage was developed, and the charging, standby, and discharging phases were simulated. The results show that all packed-bed Thermal Energy Storage systems of the same size can effectively store and release thermal energy at various pressures. The thermal energy from the compressed air is absorbed and stored with no significant change in the outlet air temperature at around 298 K during the first two hours of charging. A slight rise in the outlet air temperature was observed at the end of the three-hour charging stage. Results also indicate that during five consecutive cycles in the Thermal Energy Storage unit at the lowest pressure, the amount of energy stored at the end of the three-hour charging stage increases slightly from 118.7 kJ/kg in the first cycle to 127.5 kJ/kg in the second cycle, 129.7 kJ/kg in the third cycle, 130.3 kJ/kg in the fourth cycle, and 130.5 kJ/kg in the fifth cycle.