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

Abstract

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 152mm, pipe diameters in the range of 30.4–45.6mm and borehole depths ranging from 30 to 200m. 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.