Thermal performance of borehole thermal energy storage for building energy systems in stratified ground with groundwater advection

Borehole thermal energy storage (BTES) is widely used in ground source heat pump systems (GSHP) to support building heating and cooling through seasonal thermal storage. However, geological stratification and groundwater flow can strongly affect thermal plume migration, inter-borehole thermal interference, and heat pump operation. This study develops a composite analytical framework to predict BTES thermal performance in stratified ground with groundwater advection. The framework is validated against four independent experimental and numerical benchmarks, yielding RMSE, MAE, and MAPE values spanning 0.163–0.68 °C, 0.003–0.50 °C, and 0.012%–2.02%, respectively. It is then applied to a 90-day heat injection process in a 63-m-deep, three-layered ground system, under L-shaped, linear, and rectangular layouts, each with nine boreholes. Under the baseline condition of 4-m spacing and a groundwater velocity of 5 × 10⁻⁷ m/s, the rectangular array shows the least variation in field-averaged outlet temperature with groundwater flow direction (0.06 °C), versus 0.09 °C for the L-shaped and 0.40 °C for the linear arrays. Extended analyses of spacing and velocity show that this robustness is not universal. At 8-m spacing, the L-shaped array becomes comparable to, or slightly more stable than, the rectangular array, while the linear array remains the most sensitive to flow direction. The COP analysis further indicates a trade-off between efficiency and stability, with COP fluctuations of 0.009, 0.014, and 0.070 for the rectangular, L-shaped, and linear arrays, respectively. The vertical thermal discontinuity is quantitatively explained by Peclet numbers, which range from 3.96 to 7.00 in groundwater-bearing layers but fall to zero above the groundwater table. These findings enable more reliable design of BTES for GSHP systems in stratified ground with groundwater advection.