Investigation of heat transfer characteristics in steel fiber-reinforced energy piles utilizing steel tubes for PCM encapsulation

Phase change materials (PCMs) significantly enhance the heat transfer performance of energy piles, with the choice of encapsulation form being crucial to maximizing their potential. This study proposes a novel energy pile encapsulated with PCM in steel tubes (TuPCM pile) and assesses its heat transfer performance (HTP) through laboratory tests. A comprehensive analysis examines how PCM encapsulation, inlet temperatures, flow rates, operation modes, and flow cycles affect the heat transfer characteristics of energy piles. A 3-D numerical model was developed to investigate the heat transfer mechanisms of energy piles. The results show that under cooling conditions, energy piles with steel tube-encapsulated PCM outperform those with PCM steel balls and ordinary ones, with HTP improvements of 16.3 % and 35.7 %, respectively. The HTP of the TuPCM pile increases with the flow rate, showing a 317.1 % improvement at 0.25 m3/h compared to the ordinary pile. The performance of energy piles is proportional to the temperature variation between the circulating fluid and the initial soil temperature. A temperature variation of 20 °C results in a 40.24 % improvement in TuPCM piles over ordinary ones. The 6 h on-18 h off mode enables energy piles to reach optimal performance, showing an 87.13 % improvement over the ordinary one. In multi-cycle modes, the performance of the TuPCM pile decreases by 22.97 % by the tenth cycle, while the performance of the ordinary pile sees a more pronounced decline of 32.82 %. Numerical simulations reveal that PCM energy piles exhibit a greater temperature gradient with the soil, enhancing heat transfer.