Numerical analysis of characteristics of a single U-tube downhole heat exchanger in the borehole for geothermal wells

Geothermal energy is one of the most promising renewable resources and an alternative to conventional fossil fuels. Downhole heat exchanger is the most common form used for exploitation. Based on the geological data of Xinji geothermal field, China, the entire flow field of a single U-tube downhole heat exchanger inside the borehole is analyzed comprehensively by a three-dimensional steady state model. Thermo-physical properties of working fluid are considered specifically. The performance of the exchanger is studied through investigation on the influences of mass flow rate, degree of depth, length of the U-tube and temperature difference between working fluid and geothermal field. Finally, conversion rates are calculated and comparison of influences between four parameters has been made quantitatively via grey correlation analysis. Simulation values are validated by results obtained from field tests. For geothermal fields with high temperature, it is better to apply a large volume of circulating water to obtain higher heat extraction rate. The length of the U-tube is the most significant impact factor on the conversion rate, with the following sequence being temperature difference, mass flow rate and degree of depth. Therefore, lengthening the U-tube is the most efficient and cost-effective way for the exploitation of geothermal energy. •The working fluid flow in the U-tube and geothermal fluid are coupled by a 3D model.•The entire flow field in the borehole is analyzed comprehensively.•The thermo-physical properties of the working fluid are considered emphatically.•Influences of key factors on the performance and conversion rate are investigated.•Effects are compared via gray correlation to obtain the most efficient way.