A finite-volume method for full-scale simulations of coaxial borehole heat exchangers with different structural parameters, geological and operating conditions

Abstract The coaxial borehole heat exchangers (BHEs) are commonly used for efficient heat exchange between the heat carrier (usually water) and geothermal energy. Driven by the demand for the performance evaluation of extremely large length-diameter ratio BHEs under various working conditions in the practical engineering, this article presents a high-efficiency finite-volume computational model using variable-length control volumes to study the thermal behaviors of coaxial BHEs. The solution method is validated using data collected from three wells of 47.2 m, 879.6 m and 2,600.0 m depths. It is also used to guide the structural design and performance optimization of BHEs. The comparisons show that the present method can accurately predict the BHE performance with different structures, geological conditions and operating modes. With more reasonable thermal conductivity values, an approximate 100 kW deviation between the predicted and test heat exchange rates was avoided, and RMSE of the outlet temperature was improved from 4.0861 to 0.8220 for a 2,600 m BHE. The application of the proposed method in engineering projects indicates that setting the periodic operation interval of geothermal wells, according to the building types and heating demand, is an important factor to optimize the utilization of geothermal energy for space heating.