Techno-economic performance analysis of deep borehole heat exchanger heating system towards transforming depleted deep boreholes

Developing geothermal energy utilization technology is critical for achieving a low-carbon, high-efficiency energy system, and carbon neutrality objectives. The deep borehole heat exchanger (DBHE) represents a viable solution for extracting geothermal energy to meet building heating needs, especially in coal mines or oil fields with abundant depleted deep boreholes. In this study, a more comprehensive three-dimensional numerical model was constructed, incorporating segmented design parameters of the DBHE's inner pipe. The model's validity was confirmed through comparison with field experiment monitoring data. Subsequently, a series of long-term simulations were conducted to assess heat extraction performance, elucidating the influence mechanisms and interactive effects of various inner pipe parameters.

Additionally, a thermal-economic analysis was performed from a system-level perspective to quantify and evaluate the impact of inner pipe parameters on the DBHE's heat extraction performance, including assessing the necessity of inner pipe insulation. Results indicate that, under the specified conditions, reducing the thermal conductivity of the inner pipe increases the outlet water temperature while extending the payback period. Furthermore, greater drilling depth and lower circulation flow rate enhance the effectiveness of inner pipe insulation in improving the DBHE's heat extraction capacity, whereas the diameter of the inner pipe exerts a limited effect. These findings provide valuable guidance for the system design of practical DBHE heating projects in areas with depleted deep boreholes, enabling informed selection of inner pipe parameters.