Study on heat transfer characteristics of directly buried casing energy storage body backfilled with phase change material

Seasonal Thermal Energy Storage (STES) effectively utilizes solar energy to perform “peak shaving and valley filling” in the energy supply. Borehole thermal energy storage (BTES) is one of the most important solutions for STES due to its low initial cost, adaptability, and eco-friendliness. This study analyzes the effects of borehole spacing, depth, number of boreholes, and energy storage body type on the performance of BTES when backfilled with different volume fractions of PCM. The results show that an optimal borehole spacing exists for the BTES to reach the highest energy storage efficiency, which reaches 77.17 % at a spacing of 4 m. Increasing the depth and number of boreholes can significantly improve the heat storage capacity and heat extraction capacity of BTES. Increasing the depth of boreholes from 200 m to 400 m increases the heat storage capacity by 80.88 % and the heat extraction capacity by 81.47 %; increasing the number of boreholes from 9 to 25 increases the heat storage capacity by 167.75 % and the heat extraction capacity by 167.98 %. For the sphere, hexagonal prism, and cuboid as energy storage bodies, their energy storage efficiencies are 59.97 %, 63.97 %, and 59.3 % respectively. Meanwhile, the corresponding exergy efficiencies are 48.74 %, 46.7 %, and 47.43 % respectively. This indicates that the sphere has the largest exergy efficiency, whereas the hexagonal prism, among these three, has the largest energy storage efficiency but the smallest exergy efficiency.