Heat extraction evaluation of series/parallel U-shaped wells in middle-deep geothermal exploitation

This paper introduces a novel physical model for series/parallel U-shaped wells for the first time and develops a new method for solving fluid temperature that is suitable for medium-deep U-shaped closed-loop geothermal system (U-CLGS). Additionally, an economic evaluation model is presented to compare the economic feasibility of single-pipe with series/parallel geothermal systems. The research findings indicate that: (1) The temperature calculation model demonstrates high accuracy, with the difference between its prediction results and experimental data being less than 3 %, confirming the reliability of the predictions. (2) Regarding economic efficiency, the unit investment for the parallel configuration is the highest, followed by the single-pipe, while the seriespipes configuration has the lowest investment. The economic benefit of parallel-pipes U-CLGS is 5.86 % greater than that of single-pipe U-CLGS. Under both technical and economic operational conditions, the parallel configuration is recommended. (3) In terms of operating parameters, optimizing the fluid inlet temperature is more critical than adjusting the circulating flow rate. According to the economic evaluation diagram constructed for the single-pipe U-shaped heat exchanger, the ideal operating condition is an inlet temperature of 5 degrees C and a flow rate of 86.24 m3/h. This research provides a theoretical foundation for engineers working with mediumdeep U-CLGS.