Long-term thermal performance investigation of phase change plates (PCPs) employing tunnel lining ground heat exchangers for cool storage

Abstract

Combining tunnel lining ground heat exchangers (GHEs) and phase change plates (PCPs) to extract and store the geothermal energy is a novel and environment-friendly energy utilization method, and then PCPs can be used for space cooling. However, existing investigations have not focused on the long-term thermal performances of PCPs employing tunnel lining GHEs for cool storage. In the present work, a coupled finite element model of tunnel lining GHEs and PCPs was built to study long-term operation performances of PCPs during the cold energy charging operation period. Numerical modeling and programming (i.e., co-simulation) were employed to realize the continuous cold energy charging operation mode of PCPs employing tunnel lining GHEs for cool storage. The results indicate that the solidification efficiency (i.e., cold energy charging efficiency) of PCPs and phase interface evolution within the PCPs under the long-term cold energy charging mode are affected by the number of times for cool storage (N) and thermal conductivity of the surrounding rock (ksr) compared with the short-term mode. The drop rate of the liquid fraction and solidification efficiency of PCPs decrease with an increase in the number of times for cool storage. At N = 1, there is no obvious difference for the solidification efficiency of PCPs with increasing ksr. The solidification efficiency of PCPs under N = 10 improves by approximately 35.3% as ksr rises by 3 W/(m K). Additionally, the solidification time of PCPs increases linearly with N under ksr = 1.22 W/(m K). However, the solidification time is related parabolically to N under ksr = 2.22, 3.22, and 4.22 W/(m K). Finally, the long-term thermal response of the surrounding rock is analyzed, which also verifies the heat transfer characteristics of PCPs under the long-term cold energy charging mode.