Enhanced heat extraction for coaxial medium-deep borehole heat exchangers by adding triangular fins on the outer tube wall

This study investigates the thermal performance of coaxial medium-deep borehole heat exchangers (CMDBHEs) with triangular fins added to the outer tube wall. The investigation is performed using ANSYS Fluent simulations. The results indicate that the outlet temperature and heat extraction rate increase with longer fins, more fins, and higher soil thermal conductivity but decrease with higher thermal conductivity of the inner pipe and grout cement. The relative improvement in heat extraction achieved by adding fins diminishes as borehole depth increases. After 120 days of continuous operation, heat extraction for a CMDBHE with 32 fins and a depth of 2000 m increases by 6.74 %–18.46 % compared to systems without fins. In cases where thermal conductivity increases with depth, layered models yield higher heat extraction than homogeneous models. However, when thermal conductivity fluctuates or remains uniform, both models yield nearly identical results. Over 25 days, under varying run-to-stop ratios, finned CMDBHEs achieve 11.46 %–13.87 % higher heat extraction rates. Combining fins with intermittent operation and improved inner-pipe insulation enhances heat extraction per unit runtime by 25.61 %–74.92 %. Additionally, adding fins to the lower section of the borehole pipe is more cost-effective than finning the entire length.