Performance comprehensive evaluation of downhole heat exchanger geothermal systems with various configurations and working fluids

The downhole heat exchanger (DHE) geothermal system is suitable for exploitation of medium-low temperature geothermal resources. It has been widely used in space heating for residential and small-scale commercial buildings. The heat extraction performance of U-tube DHE system with various working fluids remains unclear. In this paper, several 3D unsteady-state fluid flow and heat transfer models considering various configurations were established and verified based on data of two field experiments. The simulation results and experiment data are in good agreement. Next, seven working fluids were selected and the datasets of real thermophysical parameters were used for simulation. Finally, a critical comparison of various working fluids for DHE systems with different configurations was carried out and the suitability of each working fluid under same condition was evaluated using the proposed comprehensive evaluation model. Study results show that when using the same working fluid, the heat extraction performance order of the four configurations of DHE systems is spiral tube > triple U-tube > double U-tube > single U-tube. Moreover, Isopentane performs best in single U-tube and Butane performs best in the other three. This paper provides different insights into medium-low temperature geothermal resources development and gives a good guidance for improving the heat extraction performance of DHE geothermal systems.