Comprehensive simulation based thermal performance comparison between single and double U-tube borehole heat exchanger and sensitivity analysis

Abstract

In the ground coupled heat pump system that delivers space heating and cooling to the buildings, often questions are raised as to which borehole heat exchanger configuration (single or double U-tube) provides higher thermal performance. To answer this question, in this paper, detailed comparative transient heat transfer analysis between single and double U-tube borehole heat exchanger (BHE) with two independent circuits (that can operate with the same or different mass flowrate and inlet fluid temperature) is conducted. Validated numerical heat transfer model was applied to perform the dynamic simulation and analysis. The two borehole heat exchangers are compared from the perspectives of their thermal performance in the heating and cooling mode of operation. Detailed sensitivity analysis of the BHE has also been performed by investigating the impact of the important borehole parameters, thermal properties and working fluids on the thermal performance of the double U-tube BHE. The simulation result revealed that the heat injected and extracted by the double U-tube BHE at steady state is 21.8 (77%) and 10.2 W/m (71.8%) more than that of the single U-tube BHE respectively. The simulation results also indicate that the thermal effectiveness of the single and double U-tube BHE at steady state are 0.24 and 0.31 respectively. The single U-tube BHE showed higher borehole thermal resistance than that of the double U-tube BHE with values of 0.47 and 0.31 m.K/W, respectively. The results of the study also revealed that among the studied working fluids, CaCl2 (25%) solution showed better thermal performance than other anti-freeze solutions with thermal effectiveness of 35.6%. The simulation result of this work is crucial as an input for the optimization studies made to obtain a single set of optimal borehole paramaters and thermal properties that results in the best performance of BHE coupled to ground source heat pump system designed to provide space heating and cooling of buildings.