Zhiwen Sun, Gangqiang Kong, Liang Chen, Ting Bao and Hanlong Liu (Hohai University, Tibet University, Chongqing University) report in Applied Thermal Engineering (DOI 10.1016/j.applthermaleng.2026.133362) on a ground-source heat pump whose ground loop uses energy piles and borehole heat exchangers side by side. The authors start from the point that it had not been clear how the two kinds of exchanger divide the thermal load or what governs the energy efficiency of such a system.
What was studied and how
The full-scale system consists of 20 energy piles and 6 borehole heat exchangers in cohesive soil. Measurements covered operation in winter and in summer. The authors quantified the seasonal thermal response, how the load was divided, the energy efficiency of the system and its sensitivity to operating parameters, and then evaluated the operating strategy.
Results
Load sharing followed a different pattern in each season. At the start of the heating season most of the heat was drawn by the energy piles (load-sharing index above 0.6), which the authors attribute to the thermal mass of the structure. Early in summer the deep borehole heat exchangers took over most of the heat rejection, as they are less sensitive to temperature swings near the surface. Temperature changes in the piles and the soil pointed to thermal hysteresis resulting from the thermal inertia of the cohesive soil.
Statistical analysis indicated that the frequency of the ground-side pump has the largest effect on changes in efficiency, while conditions inside the building govern thermal comfort. The proposed strategy reached a COP of 3.68 and an EER of 3.91, with the room temperature deviating by no more than 0.34°C.
