Numerical analysis of borehole heat exchanger using a single shape-stabilized phase change material in heating and cooling seasons

Borehole heat exchanger (BHE) plays a key role in ground source heat pump (GHSP) and underground thermal energy storage for heating and cooling, which can backfill with phase change material (PCM) to enhance its thermal performance. In this study, a novel PCM-grouted BHE which can run in both heating and cooling seasons by using a single shape-stabilized PCM (SSPCM) is presented and investigated, and the phase change hysteresis (PCH) of SSPCM is detected and applied to achieve the all-seasons run. A heat transfer model considering PCH for U-type BHE was established to evaluate the thermal performance of the novel BHE. And four kinds of backfill materials were concerned and compared to investigate the effect of initial temperature, and pipe spacing on the performance of the BHE and its characteristics of intermittent operation. The results show that octadecane SSPCM with PCH showed the best comprehensive performance in the two seasons with an energy storage rate of 68.9 W/m in heating and 75.9 W/m in cooling, presenting 21.4 % higher than BHE with conventional backfill material. Mean COP during cooling can be improved by 3.61 % when the thermal conductivity increases from 0.5 W/(m center dot K) to 2 W/(m center dot K). It also reveals that the selection of PCM for different regions depends on the ground initial temperature.