Investigation of the heat transfer of a single U-tube borehole heat exchanger for medium-shallow geothermal energy

The full scientific utilization of geothermal energy is an indispensable part of realizing the "double carbon" goal, based on its remarkable characteristics of energy saving and environmental protection. The study and utilization of medium-shallow geothermal energy (150-1500 m) has attracted increasing interest. Medium-shallow borehole heat exchanger (MSBHE) can meet the demand of both building cooling and heating, with a greater heat exchange capacity, a lower initial investment, and a greater capacity to withstand imbalance between heat extraction and rejection, which combines the advantages of both shallow borehole heat exchanger (SBHE) and medium-deep borehole heat exchanger (MDBHE). The paper focuses on single U-tube MSBHE. A quasi-threedimensional heat transfer model of MSBHE is proposed, and then experiments are carried out to verify the rationality and correctness of the heat transfer model. Subsequently, calculations are carried out using the model to study heat extraction in winter and heat rejection in summer for the MSBHE. The variations in the inlet and outlet temperatures of the circulating fluid with operating time are also studied. Moreover, diagrams are presented to show the relationships between the nominal heat exchange capacity (NHEC) and key parameters, such as borehole depth, circulating fluid flow rate, space between the two branches of the U-tube, U-tube size, grout, geothermal heat flux and average annual atmospheric temperature, which are helpful in engineering practice.