Thermal Performance Investigation of Borehole Heat Exchanger with Different U-tube Diameter and Borehole Parameters

Abstract

At present the U-tube borehole heat exchanger (BHE) used for ground-coupled heat pump (GCHP) system mostly applies 32mm as outer diameter. In order to compare the heat transfer capacity of this diameter with other nearby standard diameters, the thermal performance of U-tube BHE with four kinds of standard outer diameter were simulated by TRNSYS based on a GCHP system case in Tianjin, heat exchange rate and total BHEs number were calculated under different borehole depth and borehole diameter. The simulation result indicated that under the same borehole depth and borehole diameter, BHEs number was the fewest when the U-tube outer diameter was 25mm, however the cases used U-tube with 32mm outer diameter needed the most BHEs to meet the load demand. Furthermore, another single borehole simulation was conducted to show that thicker U-tube wall is the reason that U-tube BHE with 32mm outer diameter had relatively worse thermal performance due to its larger thermal resistance, which offset the positive effect of bigger heat exchange area.