Investigation on the heat transfer performance of a novel composite energy geo-structure with energy piles and boreholes

Ground source heat pumps (GSHPs) utilize geothermal energy for space heating/cooling with high energy efficiency and low environmental impact, making them a promising option for achieving carbon neutrality. Energy geo-structures, such as energy piles and boreholes, are essential components of the GSHP systems. However, the total heat transfer capacity of energy piles is limited by the number of building pile foundations, while boreholes require significant land occupation and drilling costs. To overcome the limitations of these two single-type energy geo-structures, a novel composite energy geo-structure consisting of energy piles and vertical boreholes is proposed. The minimum energy geo-structure group comprises two boreholes and two energy piles. A sandbox prototype of the composite energy geo-structure is established for experimental measurement and model validation. Additionally, a numerical model of the composite energy geo-structure is developed in COMSOL Multiphysics to investigate heat transfer performance under different influencing factors. Simulations indicate that the composite energy geo-structure exhibits higher heat transfer capacity, deeper heat injection depth, and a larger heat-affected zone in the soil. The central soil temperature of the composite energy geo-structure at a depth of 20 m is 0.8 °C higher than that of a single-type energy pile after one month of heat injection. Additionally, total heat transfer of the energy geo-structure can be increased from 4,110 W to 6,860 W, representing a 67% enhancement. Parametric studies show that heat transfer rate of the energy geo-structure is 26.78–77.71 W/m under different working conditions, soil types, and energy pile spacings. High inlet water temperature and velocity, high thermal conductivity soil, and large pile spacing are beneficial for improving the heat transfer rate of the composite energy geo-structure. Among them, soil type and energy pile spacing have more significant effects on long-term performance. The proposed composite energy geo-structure demonstrates better performance than conventional single-type energy piles or boreholes, contributing to its rational design and performance optimization, effectively promoting broader applications of GSHP systems.