Experimenting and modeling thermal performance of ground heat exchanger under freezing soil conditions

Abstract

Many studies have investigated the thermal performance of ground heat exchangers (GHEs) under normal conditions with inlet temperatures above 0 °C, but the freezing soil condition has been absent. We conducted a three-month test to investigate the heat transfer of GHE with inlet water-glycol temperatures of -7~0 °C. An improved thermal resistance and capacity (RC) model was developed to investigate the heat transfer between vertical single U-tube GHEs and the frozen soil. After validating with experimental results and CFD simulations, the RC model was applied to analyze GHEs' thermal performance under different freezing soil conditions. It shows that the frozen soil increases GHE's heat transfer capacity by 30% and the freezing inlet temperature has limited impacts on temperature distribution around the exchanger (with < 4 m influence radius). The GHEs' heat transfer rate remained at 75~80 W/m throughout the three-month test, which is surprisingly high to ensure the normal operation of the ground source heat pump (GSHP). These findings can be references for designing and operating GSHP systems in cold and severe cold climate zones, and the RC model can be applied to analyze future GHEs performance with phase change processes.