A simplified calculation method for clusters of coaxial borehole heat exchangers in seasonal thermal energy storage

To develop a high-efficiency energy system, a novel seasonal thermal energy storage (STES) system based on a cluster of coaxial borehole heat exchangers is proposed. The system actively harvests and stores solar radiation, air thermal energy, and other climatic resources, achieving spatiotemporal energy transfer characterized by “storing summer heat for winter use and winter cold for summer use.” To address the high computational cost and low efficiency associated with traditional three-dimensional numerical models, a simplified two-dimensional numerical model in a cylindrical coordinate system is developed. The model reduces computational complexity by transforming the densely arranged coaxial borehole cluster into an equivalent concentric-ring structure. The accuracy of the three-dimensional model is validated through sandbox experiments. Based on this, a comparative analysis between the two-dimensional and three-dimensional models is conducted in terms of heat storage capacity, temperature distribution, and computational efficiency. The results demonstrate that the proposed two-dimensional model maintains high accuracy, with a maximum relative deviation of less than 7%, while improving computational efficiency by approximately 172.9%. The method is suitable for rapid simulation and optimization design of large-scale, densely arranged borehole heat exchanger systems.