Field test and geologic-thermal-economic analysis of medium-depth borehole heat exchanger

Medium-depth ground source heat pumps with borehole heat exchanger are pivotal in energy-saving and decarbonization. The heat transfer performance of borehole is important for design, with stratum thermal conductivity significantly influencing heat exchange, and economic considerations also act as a vital constraint on technological advancements. However, there is limited on-site testing in severe cold regions, especially in the Songliao Basin. Stratum thermal conductivity usually relies on empirical values, and its diverse impacts are not integrated into borehole heat transfer model. Additionally, previous studies optimized borehole design only from the thermal perspective. To address these gaps, we conducted an inaugural test in Shenyang. Geologic-based heat transfer model explores the geological influences, heat transfer performance, and heat extraction cost. At a depth of 2500 m, the formation's thermal conductivity is 2.45 W⋅m⁻1 K⁻1. The heat extraction, at a flow rate of 26.0 m3 h−1, achieves 352.3 kW with an inlet-outlet temperature differential of 11.7 °C. The effects of saturation, porosity, and lithology on stratum thermal conductivity were also explored. Furthermore, considering heat extraction cost, optimal depths for borehole utilizing PE-RT II and stainless-steel vacuum pipe are 2790 m (10.3 CNY·W−1) and 3140 m (10.2 CNY·W−1), with critical depths, 2910 m, governing the selection of economic inner pipe. This research establishes medium-depth ground source heat pumps as the preferred choice for low-carbon transformation in building heating, domestic hot water, and industrial and agricultural heat applications.