Evaluation of the impact of the thermal shunt effect on the U-pipe ground borehole heat exchanger performance

A U-pipe ground borehole heat exchanger (BHE) is the most commonly used device for heat extraction from the ground. The amount of extracted heat depends on two interacting heat transfer processes - between the ground and the carrier fluid, and between the pipes. Heat flow between the U-pipe legs is called the thermal shunt effect (TSE). In this paper, a steady-state numerical model has been used to assess the contribution of TSE to the overall U-pipe performance. As the model requires borehole wall temperature as input, a CFD analysis has been performed to determine how this temperature is influenced by elimination of TSE. The analysis has shown that reducing pipe-to-pipe heat flow has no substantial impact on the borehole wall temperature. Fluid temperature profiles have been calculated for all possible pipe arrangements in a heat exchanger with the borehole diameter of 152 mm, pipe diameters in the range of 30.4-45.6 mm and borehole depths ranging from 30 to 200 m. Results indicate that thermal shunting becomes aggravated by decreasing pipe-to-pipe distance or increasing pipe diameters and borehole depths. The maximum loss of 35% in the U-pipe thermal performance has been observed in the range of investigated parameters, which proves the necessity of TSE elimination.