Heat Transfer Performance Analysis of Medium–Deep Coaxial Borehole Heat Exchanger: Combination of Heat Extraction Operation Test and Numerical Simulation

Geothermal energy is a clean and renewable energy source that can effectively reduce carbon emissions. Coaxial borehole heat exchanger (CBHE) is a crucial component of the geothermal mining system. To understand the heat transfer mechanism of a CBHE and to design its construction, this paper analyzes the geotechnical thermal response characteristics of CBHE under constant power conditions. comsol was used to simulate the performance of the heat exchanger during intermittent operation in both summer and winter. The results indicate that the inlet water temperature and the thermal conductivity of the grouting material have a weak impact on the energy efficiency coefficient of the heat exchanger. The energy efficiency coefficient can increase by about 22% when the inner pipe's thermal conductivity is low. The temperature difference between the medium flowing into the inner pipe at the bottom of a shallow-cased buried pipe and the medium flowing out of the inner pipe defines the thermal short-circuit value. The influence on the thermal short-circuit value is on the order of the inner pipe thermal conductivity, grouting material thermal conductivity, and the inlet water temperature. The fluctuation of soil temperature in the summer is greater than that in the winter, and the soil temperature of shallow-buried pipes is easier to recover in the winter. The characteristic law obtained in this paper provides a valuable reference for the scientific design of CBHE.