Research on Thermal Characterization of Soil Heat Transfer in Cross-Seasonal Borehole Thermal Storage Systems

The heat transfer process of the cross-seasonal borehole thermal energy storage (BTES) is mainly affected by the structural parameters, material thermal parameters and operating parameters of the pipes. In this paper, on the basis of validation with experiments, a numerical model was established using FLUENT to simulate the heat storage characteristics of the soil in Chongqing. It analyses the influence law of the number of borehole and heat storage boundary, fluid inlet flow velocity, temperature and soil type on the soil temperature field and heat injection. The results of the study show that: the heat storage boundary of 15.5 m is considered to be a reasonable boundary size, and the radius of heat diffusion at the end of the heat storage and transition period are 13.48 and 14.72 m. The average soil temperatures of sand, sandstone and clay were 19.89, 20.24 and 21.07 °C. Therefore, clay soil is more suitable as a heat storage medium and sandy soil is more suitable for use in areas where intensive heat transfer is required. When the inlet flow rate is 0.4, 0.5, 0.6 m/s, the enhancement effect on the soil heat storage performance is not obvious, so the recommended flow rate does not exceed 0.6 m/s. The increase in the inlet temperature can increase the amount of heat exchange, but the degree of enhancement decreases gradually.