A numerical heat transfer model and performance evaluation of coaxial geothermal heat exchanger under soil freezing conditions

Seasonal thermal energy storage can significantly contribute to district heating and cooling systems based on sustainable energy whenever there is a seasonal imbalance between energy generation and utilization. The seasonal cooling storage capacity of ground source heat pump (GSHP) systems can be enhanced by utilizing geothermal heat exchangers (GHEs) to extract latent heat from the soil moisture during winter. This study developed a numerical heat transfer model of coaxial geothermal heat exchanger (CGHE) under soil freezing condition which combines the finite difference method and the effective heat capacity method. Additionally, the Clausius-Clapeyron equation is employed to model changes in the soil moisture field. The results show that higher soil porosity and moisture content lead to increased errors, with a 20% porosity resulting in a 25% higher heat extraction rate in the soil frozen model compared to the non-frozen model. The freezing process first occurs from the borehole top to the bottom, with higher porosity slowing the temperature drop and moisture reduction. The study also compares the heat transfer performance and pressure losses of five common antifreeze fluids, identifying a 35% glycerol-water solution as the most effective and cost-efficient. Using the Taguchi experimental design method, the influence of nine factors on CGHE heat extraction rates is analyzed, revealing that initial soil temperature has the greatest impact, followed by inlet fluid temperature and surface convective heat transfer coefficient. By incorporating real climate data from five representative cities across China, the model's accuracy and practical applicability are significantly improved.