Field test shows long medium-deep U-tube heat exchanger delivers up to 1,035 kW of geothermal heat

A field experiment in the Weihe Basin has measured how much heat a long medium-deep U-tube borehole heat exchanger can actually deliver, and a follow-up modelling study has identified which design and operating parameters matter most. The system, tested with a horizontal section 1,100 metres long, produced an average outlet water temperature of 21.2 °C and an average inlet–outlet temperature difference of 10.8 °C, with heat extraction rates ranging from 760.4 to 1034.7 kW during circulation testing.

The technology works differently from open geothermal wells. A medium-deep U-tube borehole heat exchanger extracts geothermal heat through a closed loop without withdrawing groundwater, which preserves the geothermal resource. According to the study, its performance is governed jointly by the length of the horizontal section and by operational, geological and structural parameters. The reported heat extraction rate was governed primarily by the circulation flow rate and the inlet water temperature, meaning operators can influence output directly through pumping choices.

To go beyond the single field test, the researchers built a three-dimensional heat transfer model and validated it against the measured data. The predicted outlet temperature deviated by less than 6% from observations, which the authors describe as sufficient to use the model for design exploration. With the validated model in hand, they carried out a multi-parameter, multi-objective optimization using the Taguchi method combined with matrix analysis, considering nine factors at three levels each.

The optimization produced a clear hierarchy of influence. Analysis of variance showed that the geothermal gradient, contributing 34.50%, and the inlet water temperature, contributing 33.27%, dominate the heat exchange per metre of borehole over a five-year horizon. For the heat pump coefficient of performance, a single factor stood out: inlet water temperature accounted for 81.85% of its variation. In contrast, the buried-pipe thermal conductivity dominated the long-term decay metrics, contributing 44.73% to the predicted reservoir temperature decay rate and 72.31% to the system heat extraction decay rate over five years.

Using the optimal parameter combination identified by the matrix analysis, the model predicted five-year indicators of 80.40 W per metre of heat exchange, a heat pump coefficient of performance of 6.25, a reservoir temperature decay of 3.15%, and a system heat extraction decay of 3.58%. These are model predictions for the optimized configuration rather than direct field measurements, and they describe performance over a five-year horizon under the assumed conditions of the simulation.

The study also compared the U-tube design with the medium-deep coaxial borehole heat exchanger, an alternative closed-loop configuration. The comparison revealed significant differences in geometric parameters, with the U-tube characterized by horizontal-section length and the coaxial design by completion depth, as well as in the importance of buried-pipe thermal conductivity. The authors present this as a quantitative basis for heat exchanger selection and parameter design in medium-deep geothermal engineering.

For practitioners, the practical implication is that choosing between these configurations involves different trade-offs. Where the U-tube’s long horizontal section can be installed, the findings suggest that inlet water temperature and the local geothermal gradient largely determine how much heat can be extracted per metre, while the thermal conductivity of the buried pipe is the key lever for limiting long-term decline in both reservoir temperature and system output. The open-access paper was published in the journal Geothermal Energy on 3 September 2026, and the published version is noted as subject to further editorial edits.

Subject of Research: Field testing and multi-parameter optimization of a medium-deep U-tube borehole heat exchanger for geothermal heat extraction

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

Article References: Liang, R., Bi, S., Ren, J., Wang, P., & Zhang, Q. (2026). Heat extraction performance for a long medium-deep U-tube borehole heat exchanger: field experiment and multi-parameter optimization. Geothermal Energy. https://doi.org/10.1186/s40517-026-00405-5

Image Credits: AI Generated

DOI: 10.1186/s40517-026-00405-5

Keywords: geothermal energy, borehole heat exchanger, U-tube, Weihe Basin, heat extraction rate, Taguchi method, analysis of variance, coefficient of performance, groundwater preservation, reservoir temperature decay, heat transfer modelling, medium-deep geothermal