Long-term energy performance of an energy pile heat pump system under cooling-dominated conditions

Energy pile (EP) heat pump systems utilize foundation piles as ground heat exchangers, providing efficient, low-carbon heating and cooling for buildings. Under cooling-dominated conditions, sustained net heat rejection causes heat accumulation in the ground and affects long-term energy efficiency, yet evidence from long-term field monitoring remains limited. This study investigates a heat pump system comprising 44 EPs at an industrial site using monitoring data collected from 2023 to 2025. Field measurements were used to examine building heating and cooling demand, EP outlet water temperature, and heat pump coefficient of performance (COP). Three-dimensional numerical simulations assessed how structural constraints and thermal balance interventions affect long-term system performance. The industrial thermal environment and internal heat sources increased the deviations from predicted building loads, with an annual thermal imbalance ratio exceeding 93%. Sustained net heat rejection caused operating water temperatures to rise each year, with peak EP inlet water temperatures exceeding 50 °C and the mean cooling season COP declining from 4.70 to 4.44. Long-term simulations compared the EP system constrained by the foundation layout with a borehole heat exchanger system. For the same vertical heat exchange length and thermal loads, the EP system showed greater thermal interference and heat accumulation. Thermal balance interventions can substantially improve the long-term performance of EP systems within the existing foundation layout.