A 3-Dimensional Numerical Thermal Analysis for the Configuration Effect of a Single and Double U-tube on the Borehole Performance

Abstract

Abstract The thermal design of a borehole is the most important task in the thermal performance evaluation of a geothermal coupled heat pump system. It demands an understanding of the U-tube orientation and configuration in the borehole. The COMSOL Multiphysics 5.4 software was utilized in a 3-dimensional numerical simulation model to investigate the thermal performance of a single and double U-tube embedded in the borehole. The water was chosen as a thermal fluid carrier and circulated in the borehole heat exchanger at a mass flow rate range of (0.14–0.34) kg/s and inlet temperature of (33) °C. The configurations of the double U-tubes were assigned as a parallel flow in a cross U-tube arrangement (PFCD), a parallel flow in a parallel double U-tube in situ (PFPD), a series flow circuiting in a cross double U-tube geometry (SFCD), and a series flow in a parallel double U-tube installation (SFPD). The steady-state numerical solutions were compared at fixed borehole configuration and fixed operating conditions. Results showed that the (PFPD), (PFCD), and (SFPD) U-tube configurations have achieved a higher heat load than that of the single U-tube ones by (16–19) %, (13–16) %, and (15–18) % respectively. They produced a higher heat load than that of the series flow in a cross double U-tube (SFCD) arrangement by (30–31) %, (27–30) %, and (29–31) % for the (PFPD), (PFCD) and (SFPD) configurations respectively.