Study on heat extraction capacity of deep coaxial borehole heat exchanger equipped with spiral-finned inner tube

Deep geothermal energy is a hotspot for geothermal development, and the use of deep coaxial borehole heat exchangers for heat extraction is a good way for this purpose. The heat extraction capacity of a deep coaxial borehole heat exchanger (DCBHE) for geothermal applications was investigated by both experimental and numerical methods in this study. Field tests were conducted to evaluate the heat transfer performance of the DCBHE, while COMSOL Multiphysics simulations were used to analyze the flow and heat transfer of spiral fins. The test results demonstrated that the heat extraction capacity decreased with increasing water inlet temperature for the same geothermal well water flow rate, and the heat extraction capacity increased with increasing water inlet flow rate for the same geothermal well inlet temperature. Spiral fins enhanced heat transfer by intensifying turbulence, improving Nusselt numbers by 11.8-101.6% across the range of fin spacings (113.19-345.58 mm) and by 15.4-101.6% across the range of heights (4-20 mm), while increasing f-factors by 139.4-539.0% and 211.7-545.1% respectively. The fin thickness had a negligible effect on the flow and heat transfer performance. An optimal geometry of fin spacing 172.79 mm, fin height 20 mm and fin thickness 2 mm could effectively improve overall performance. Operating at a flow rate of 15-20 m³/h achieved a better balance between enhanced heat transfer and manageable pressure losses. These results provided design guidelines for energy-efficient geothermal DCBHE systems.