Investigating the dynamic thermal performance of a novel PCM to earth-air heat exchanger: Developing numerical model and comparing thermal performance

Integrating phase change materials (PCMs) into earth-air heat exchangers (EAHE) to form PCMEAHE systems effectively enhances the efficiency of natural energy utilization. This study proposes a novel PCM-EAHE configuration for the first time, incorporating multiple annular PCM layers and a single cylindrical PCM layer within the duct. Mathematical models for four distinct scenarios were developed and validated against experimental data. The findings indicate that the proposed system outperforms existing PCM-EAHE systems in terms of temperature drop, cooling capacity, average coefficient of performance, and temperature drop factor. Furthermore, positioning multiple annular PCM units along the centerline of the pipe (detached from the pipe wall) enhances the cooling and heating performance of the system, while placing a layer of PCM units on the inner wall of the pipe mitigates heat buildup in the soil surrounding the buried pipe. At the same outlet air temperature, this innovative structural design increases the fresh air handling volume and reduces the length of buried ducts by 44 %-54.5 % compared to existing systems. Over five months of continuous operation in Chongqing, the system demonstrated a maximum temperature drop of 6.91 degrees C, with the maximum liquid fraction reaching 0.34 and a maximum cooling capacity of 5795.14 W. This research contributes to the advancement of natural energy resource exploitation.