Evaluating Thermal Performance of Oval U-Tube for Ground-Source Heat Pump Systems from in Situ Measurements and Numerical Simulations

Abstract

A new-shaped U-tube with an oval cross-section has been developed to boost the heat transfer inside the borehole heat exchanger and to reduce the heat exchanger depth and the drilling cost. In addition, it could fit efficiently inside tight boreholes, so it is fitting for installing GSHPs at urban areas. This research evaluated the effectiveness in situ by thermal response tests at total 20 borehole heat exchangers in three sites, Japan. The in situ borehole thermal resistances were about 10–30% smaller with the oval U-tube than the circle U-tube. However, the spacers had an imperceptible impact probably because of the soil pressure to the U-tubes. Also, transient numerical CFD simulations were carried out to calculate the long-term performance of a household GSHP system with oval U-tubes. It indicated that the required lengths of a borehole heat exchanger could be reduced by about 14% in the fine soils and 15–19% in the coarse soils.