Optimizing the restoration performance of pipe energy piles using energy injection in winter mode: A numerical investigation

Abstract

A numerical investigation was conducted on the pipe energy piles to ensure the long-term and efficient utilization of geothermal energy for regulating the building environment. Various phase change materials (PCMs) and the thermal conductivity of PCM were optimized to enhance the thermal performance of energy piles. Further, during the cooling period, different inlet temperatures and cooling durations were investigated to evaluate the long-term restoration performance of energy piles. The study concluded that a relatively high phase change temperature (16.3 °C) was preferable for mitigating cold accumulation in the phase change pipe energy pile and the surrounding soil. Moreover, the proper thermal conductivity (1.65 W/m·K) of PCM may be comparable to that of ordinary grout during continuous operation. Furthermore, the relative high inlet temperature and extended recovery time were preferred to enhance the restoration performance of the soil and energy pile during each cycle. Notably, after 30 days of operation, the heat transfer rate of energy pile accounted for 99.2 % of the initial level (78.4 W/m) in mode 3 (heating and cooling alternately for 12 h each) at an inlet temperature of 27.4 °C. Additionally, the soil temperatures nearly returned to their initial state (17.4 °C). In this study, with the optimal PCM parameter, the maximum soil temperature restoration rate of (xr = 0.28 m) reached 99.1 % by alternating heating 5 °C and cooling 27.4 °C in mode 3, demonstrating long-term and efficient thermal performance of energy pile.