Investigation on the heat transfer characteristics of the U-tube and the coaxial tube medium-shallow borehole heat exchangers

The study and utilization of medium-shallow geothermal energy (200–1500 m) has attracted increasing interest. The medium-shallow borehole heat exchangers (MSBHEs) can meet the demand of both cooling and heating for the buildings. The MSBHE outperforms the SBHE in heat exchange capacity and stability against heat imbalances, while requiring lower initial investment than the MDBHE. This paper investigates the heat transfer characteristics of single U-tube (1U-tube), double U-tube (2U-tube) and coaxial tube MSBHEs. Firstly, a quasi-3-D heat transfer model for MSBHEs is proposed, with boundary conditions defined. Secondly, the models are validated through field experiments, and then applied to simulate seasonal thermal performance (winter heat extraction and summer heat rejection). Moreover, Parametric analyses further quantify how key factors—including design parameters (borehole depth, circulating fluid flow rate, U-tube leg spacing, tube size, grout properties), and environmental factors (geothermal heat flux, average annual atmospheric temperature)—affect the nominal heat transfer capacity (NHTC). Finally, optimal borehole depths for various operating conditions are identified, providing practical guidance for engineering applications. This study provides theoretical and technical guidance for MSBHE applications, advancing the utilization of medium-shallow geothermal energy.