Comparative numerical investigation on hollow-finned borehole heat exchangers: Multi-criteria optimization and dimensionless sensitivity analysis

Ground-source heat pumps (GSHPs) harness geothermal energy efficiently but are constrained by high initial and drilling costs. To address these issues, this study explores the reduction of borehole depth and thermal resistance and optimizes the borehole heat exchangers (BHEs) design to enhance GSHP performance and reduce costs. Novel optimized-fin-shapes (rectangular, triangular, elliptical, and oval) are integrated with traditional circular U-shaped BHEs, and their performance is compared through multi-physics computational fluid dynamics (CFD) simulations. These simulations assess transient heat transfer and fluid flow, with results validated against experimental field data conducted by the authors’ group. Key performance parameters such as heat flux, BHE thermal resistance, heat-to-work ratio, and pressure drop are computed and analyzed for the investigated designs of hollow-finned borehole heat exchangers (HFBHEs). The study identifies the optimal hollow-finned (HF) shape of the different cross-sectional configurations of HFBHEs and compares it with the conventional circular finless BHE. Additionally, an optimization approach is formulated by using the dimensionless shape factor to determine the optimal length ratios of the different studied HFBHE designs that achieve the maximum heat transfer efficiency. The findings reveal that both rectangular and oval HFBHE configurations outperformed the other investigated HFBHE designs, enhancing the average heat transfer rate by up to 13.91 % and 13.10 %, respectively, compared to typical circular finless BHE. Shortening the borehole depth for oval HFBHEs can significantly enhance their performance compared to circular models, raising the maximum and average heat transfer rates by 43.11 % and 29.37 %, respectively. Optimal operating parameters are identified as a 0.60 kg/s mass flow rate, and 0.40 dimensionless shape factor. These findings provide invaluable insights into HF shapes, pipe cross-sectional configurations, and length ratios that affect the thermal performance of shallow U-tube BHEs in GSHPs.