Impact of groundwater seepage on the thermal performance of pit thermal energy storage based on two-phase flow model of porous media

Pit thermal energy storage (PTES) plays a significant role in the field of building energy utilization. Due to the direct contact between PTES and soil as well as the widespread presence of groundwater, the hydrogeological environment must be considered for its impact on its thermal performance. In particular, groundwater seepage alters heat transfer characteristics between PTES and soil, thereby significantly impacting the thermal storage efficiency of the system. This study conceptualized the soil domain as unsaturated soil and developed a model based on two-phase flow in porous media to simulate thermo-hydro interactions between groundwater and the system. The model was validated against operational data from the Dronninglund's PTES. A parametric sensitivity analysis was conducted to elucidate the impact mechanisms of confined aquifer properties, including depth to the aquifer (6,9,14,25 m), thickness (10,15 m), flow velocity (5.4·10-6, 5.4·10-7, 1·10-7 m/s), and degree of saturation (0.3,0.45,0.9), on the thermal performance of PTES. Results indicate that the heat loss of PTES stabilized by fourth year under groundwater conditions, whereas stabilization occurs by seventh year without groundwater influence. Under dynamic groundwater conditions, the annual average thermal loss from the system increased by 69.24 % compared to scenarios without groundwater flow. The soil temperature field diffused along the groundwater flow direction, and the influence range of PTES expanded accordingly. Further analysis revealed differential impacts on PTES surfaces: thermal loss at the upstream surface increased by 131.9 % relative to static groundwater conditions, whereas the downstream surface exhibited only a 10.8 % increase. In addition, when the vertical separation between the aquifer and PTES base exceeded 10 m, thermal interaction attenuated significantly. These findings provide critical guidance for PTES site selection and engineering design.