Heat extraction performance for a long medium-deep U-tube borehole heat exchanger: field experiment and multi-parameter optimization

The medium-deep U-tube borehole heat exchanger (MDUBHE) extracts geothermal heat via a closed loop without groundwater withdrawal, thereby preserving geothermal resources, with its performance jointly governed by horizontal-section length and operational, geological, and structural parameters. A field test was conducted on an MDUBHE with a 1100 m horizontal section in the Weihe Basin. Circulation test results gave an average outlet water temperature of 21.2 ℃ and an average inlet–outlet temperature difference of 10.8 ℃; the heat extraction rate ranged from 760.4 to 1034.7 kW, governed primarily by circulation flow rate and inlet water temperature. A three-dimensional heat transfer model was developed and validated, with predicted outlet temperature deviation below 6%. Using the validated model, a multi-parameter, multi-objective optimization was performed via the Taguchi method combined with matrix analysis, considering nine factors at three levels. Analysis of variance revealed that geothermal gradient (34.50%) and inlet water temperature (33.27%) dominate the heat exchange per meter q L−5a ; inlet water temperature contributes 81.85% to the heat pump coefficient of performance COP −5a ; and buried-pipe thermal conductivity dominates the reservoir temperature decay rate D soil−5a (44.73%) and system heat extraction decay rate R d−5a (72.31%). The optimal combination from matrix analysis predicted five-year indicators of q L−5a = 80.40 W·m − 1 , COP −5a = 6.25, D soil−5a = 3.15%, and R d−5a = 3.58%. Comparison with the medium-deep coaxial borehole heat exchanger (MDCBHE) revealed significant differences in geometric parameters (horizontal-section length versus completion depth) and buried-pipe thermal conductivity, providing a quantitative basis for heat exchanger selection and parameter design in medium-deep geothermal engineering.