Numerical Analysis of Inlet-Outlet Leg Barriers in Vertical Borehole Heat Exchangers: A Strategy to Mitigate the Thermal Resistance

The efficiency of heat transfer through borehole heat exchangers is influenced by the thermal resistances of both the borehole and the surrounding soil. Optimizing these resistances can improve the heat transfer performance and reduce system costs. Soil thermal resistance is geographically specific and challenging to reduce, according to previous research. In contrast, borehole resistance can be minimized through practical approaches, such as increasing the thermal conductivity of the grout or adjusting the shank spacing in the U-tube configuration. Previous literature also suggests coaxial pipes as a more efficient design than single U-tube borehole heat exchanger. A novel approach involves inserting a physical barrier between the U-tube’s inlet and outlet legs to reduce the thermal short-circuiting and/or to improve the temperature distribution from the inlet leg in a U-tube borehole. A limited literature exists on the barrier technique and their contribution to reduce thermal resistance. The effects of two different barrier geometries of flat plate and U-shape of different materials, with various grout and soil thermal conductivities as well as shank spacing configurations have been considered in this study. Using FlexPDE software, the study numerically assesses thermal resistances through the borehole. The study focusses on the sole contribution of barrier in mitigating thermal resistance of U-tube borehole heat exchanger. The study suggests that the barrier technique is an effective solution for optimizing heat transfer through U-tube borehole heat exchangers, especially with reduced shank spacing and lower thermal conductivity soil. It can reduce the length of a U-tube borehole by up to 8.1 m/kW of heat transfer, offering a viable alternative to increasing shank spacing in the U-tube borehole or enhancing thermal conductivity of the grout. Moreover, under specific conditions of soil and grout with low to medium thermal conductivity, U-tube borehole heat exchanger with a barrier between the legs demonstrates a reduction of up to 43.4 m per kW heat transfer (22.7%) in overall length compared to coaxial pipes.