Numerical investigation on heat extraction performance of a downhole heat exchanger geothermal system

•A 3D model couples flow and heat transfer processes of DHE, wellbore and reservoir.•The model is validated against experimental data with a maximum error of 8.3%.•The entire temperature and flow fields of DHE system is analyzed comprehensively.•Performances of single U-tube, double U-tube and spiral tube are compared.•Effects of key factors on heat extraction performance of DHE system are studied. The downhole heat exchanger (DHE) geothermal system is commonly used to exploit geothermal energy for space heating. In this paper, a 3D unsteady state numerical model is established to couple fluid flow and heat transfer processes of DHE system. The model is validated by field experimental data. Temperature and velocity fields are analyzed to understand thermal process of DHE system. Heat extraction performances of three different DHE structures, including single U-tube, double U-tube and spiral tube, are compared. Subsequently, cases are studied to investigate how key parameters affect DHE performance. Simulation results depict that spiral-tube has the best heat extraction performance. As working fluid mass flow rate rises, outlet temperature declines and thermal power increases. When inlet temperature ascends, outlet temperature rises while thermal power decreases. Effects of reservoir porosity and tube wall heat conductivity on DHE performance are minor. Higher subsurface water velocity and larger rock heat conductivity can improve DHE performance, but the former has a more significant influence. Besides, subsurface water flow direction has neglected influence on performances of single and double U-tube, but appreciable impact on that of spiral tube. Key findings of this work are beneficial for optimal design and optimization of DHE geothermal system.