Phased optimization of heat transfer performance for borehole heat exchanger in Karst regions: From geological screening to design parameter adjustment

Groundwater seepage in Karst regions enhances the thermal performance of borehole heat exchanger (BHE). However, systematic optimization is limited by the complex coupling between geological conditions and operating parameters. This study proposes a classified optimization framework to separate uncontrollable geological factors from controllable design variables. Comprehensive BHE performance evaluations are conducted using 3D transient numerical simulations, orthogonal experiments, and the response surface methodology (RSM). The research results are summarized as follows:(1) Regarding the environmental side, performance is dominated by the seepage velocity. Through the screening of optimal geological conditions, the heat transfer rate per unit length was increased by 27.76% under summer conditions and 28.86% under winter conditions compared to the control group. (2) Based on the screened optimal geological conditions, the user-side study was conducted. Through multi-objective optimization, the optimal combination of design parameters was determined: an inlet water temperature of 35 °C, a flow velocity of 0.43 m/s, and a wall thickness of 3 mm. This optimal parameter set yielded an additional performance gain of about 5%. The phased optimization strategy clarified the distinct effects of Karst seepage and design parameters. It provides a reliable basis for efficient design and sustainable operation of ground source heat pump systems.