An optimal flow rate design method for medium-deep coaxial borehole heat exchangers

Medium-deep geothermal energy utilization via coaxial borehole heat exchangers (CBHE) presents a promising approach for clean building heating. The design flow rate critically affects both heat transfer and ground-source pump power consumption, yet a comprehensive evaluation method is lacking. To address this, a validated CBHE heat transfer model based on Shenyang field experiments was developed, introducing net heat exchange intensity (q) as an integrated metric encompassing heat transfer rate, pump power consumption, and system coefficient of performance. Effects of pipe materials and diameters on optimal flow rate were examined. The optimal flow rates for SSVP, PPR, and PE-RT II are 19.0, 24.0, and 25.5 m3 h−1 at 2000 m. SSVP shows 6.2 % and 11.2 % higher q than PE-RT II at 2000 m and 3000 m, reflecting increasing performance differences beyond 2000 m. Reduced inner diameter amplifies pipe roughness effects on pump power. Regionally, the optimal flow rates at 2000 m in Harbin, Beijing, Shenyang, and Xi'an are 27.0, 26.0, 25.5, and 24.0 m3 h−1, with thermal conductivity strongly correlating with flow rate, explaining the distinct heat extraction and flow patterns between Shenyang and Xi'an. Furthermore, thermodynamic perfectibility is largely independent of pipe material. An initial 25 % improvement yields a more substantial reduction in optimal flow rate than a further increase from 25 % to 50 %. These findings support flow rate optimization in medium-deep geothermal systems.