The U-type medium-deep borehole ground heat exchanger BGHE is a new form of utilization of geothermal energy and there has been relatively little research on it. This paper introduces the structure and working principle of the BGHE. The energy control equations based on the characteristics of the heat transfer model are established creatively. The initial and boundary conditions of the circulating fluid and rock soil are determined. And the reliability of the simulation is verified through comparing the simulated results with the engineering data. Based on the above theoretical foundation, the effects of the circulating fluid flow rate, geothermal gradient and thermal conductivity of different soil layers on the nominal heat extraction and the outlet temperature of the U-type BGHE are examined. The temperature changes in the soil surrounding the vertical pipe and the temperature of circulating fluid at the inlet and outlet of the BGHE are analysed. The corresponding reasons of these changes are revealed. It is shown from simulation results that the circulating fluid flow rate has opposite influence on the nominal heat extraction and outlet temperature. The increasing terrestrial heat flow is conducive to heat transfer, but the reverse heat transfer occurs during the heat transfer process. The influence of the BGHE on the surrounding rock soil will hardly affect the continuous use of the buried ground heat exchanger. The results of the study are beneficial to better understand the heat transfer characteristics of the U-type medium-deep BGHE and its impact on the surrounding soil. It will promote the popularization and application of medium-deep geothermal energy.

Medium-shallow borehole ground heat exchangers (BGHEs) utilize a burial depth ranging from 200 to 600 m. The heat exchange capacity of a single medium-shallow BGHE is higher than that of a single shallow BGHE. Compared to medium-deep BGHEs, the cost of medium-shallow BGHEs is lower, and both heating and cooling can be achieved, while the former can only be used for heating. However, there is a relative lack of research on the heat transfer characteristics of medium-shallow BGHEs, especially on the influence of the working fluid type on the heat transfer performance of BGHEs. This study aimed to investigate the impact of different working fluids on the performance of medium-shallow BGHEs. First, a heat transfer model for medium-shallow BGHEs was established considering the ground temperature gradient and geothermal heat flow, and its accuracy was validated using experimental test data. Second, the model was used to compare and analyze the effects of various working fluids on the heat transfer performance, pressure loss, and potential environmental benefits of BGHEs. Based on economic analysis, CO₂ was determined to be the most suitable working fluid among the organic fluids considered. Finally, the influence of the number of boreholes and the type of working fluid on the heat transfer performance of borehole clusters consisting of 2 and 4 boreholes was analyzed using the superposition principle. The results indicated that CO₂ could provide the highest heat transfer among the various working fluids selected in this study, as its heat extraction and heat dissipation were approximately 15% and 12% higher than those achieved by water. Isobutane (R600a) achieved the highest net heat and emission reduction, surpassing water by 66.7% and 73.6%, respectively. Regarding the four boreholes, the outlet temperature of the BGHEs gradually decreased at the end of each heating season. After 10 years of operation, the value decreased by approximately 2 °C. The results in this paper provide a theoretical basis and technical guidance for the rational selection of working fluids and improvements in the heat transfer performance of BGHEs, which could promote the development and application of medium-shallow geothermal energy sources.