Borehole thermal resistance of U-tube borehole heat exchanger

Thermal resistance of U-tube borehole heat exchangers (UBHE) is one of the factors that controls their design. Several equations are available in the literature to estimate the thermal resistance of UBHE. However, these equations produce different results and no definitive recommendation is available regarding their validity. The aim of this study is to assess numerically the validity of these equations and identify their recommended working conditions.