Heat transfer numerical analysis 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. To enhance the heat storage capacity, the soil temperature can be reduced to below 0℃ in winter, and the time and intensity of direct water supply by using low-temperature soil can be increased in summer. It is necessary to consider the effect of phase change on geothermal heat exchange operation. The water in the soil releases latent heat after freezing, which can improve the heat performance of the ground heat exchanger. An innovative numerical heat transfer model inside a coaxial geothermal heat exchanger (CGHE) is presented, considering the freezing of ground moisture. The finite difference method is adopted to solve the temperature distribution, and the model's accuracy is validated with experimental data. Based on the theoretical analysis to study the thermodynamic parameters influence of antifreeze and soil on the freezing heat transfer performance of CGHE, compare the heat transfer model with frozen and unfrozen heat transfer to analyze the calculated errors. The results show that glycerin is most suitable for CGHE as a heat transfer medium. The release of latent heat during freezing augments the heat extraction rate, and the error range is 11.8%, so the relevant conclusions in this study can provide a reference for the design of CGHE under freezing heat transfer conditions.