Configuration selection and optimal flow design for medium-depth coaxial and U-type borehole heat exchangers

Coaxial and U-type borehole heat exchangers are widely used in medium-deep geothermal systems, but comparisons under equal drilling footage remain scarce. This study develops validated numerical models for a two-coaxial borehole system and a U-type borehole heat exchanger and evaluates their performance under a total drilling footage of 5000 m. Axial heat transfer intensity is introduced to characterize performance differences, while net heat exchange intensity is used for flow optimization. At identical flow conditions, system superiority varies with pipe configuration, flow regime, and geological conditions. The two-coaxial system with a stainless-steel vacuum pipe achieves 6–12% higher heat extraction than the U-type system but requires greater pump power, whereas with a PE-RT II pipe, heat extraction exceeds the U-type system only when total flow reaches a critical threshold of about 67.0 m³ h⁻¹. Under net heat exchange intensity-based optimal flow rates of 38.5 m³ h⁻¹ for the coaxial and 60.0 m³ h⁻¹ for the U-type, their heat extraction becomes comparable, whereas the U-type exhibits higher net heat exchange intensity above a critical value of 45.0 m³ h⁻¹. Both heat extraction-based and net heat exchange intensity-based critical flow rates vary among regions. Axial heat transfer analysis identifies upper 200 m of the U-type straight borehole as the key region limiting performance relative to the two-coaxial system. Regional results further indicate that lower stratum thermal conductivity and higher ground temperature reduce performance disparities between the two systems. These findings provide guidance for flow rate optimization and borehole configuration selection.