Experimental investigation on the performance of a borehole thermal energy storage system based on similarity and symmetry

Although Borehole Thermal Energy Storage (BTES) technology has achieved significant progress in feasibility and sustainable energy integration, high heat loss and long preheating periods still strongly restrict its charging and storage performance. This investigation designed and constructed a laboratory-scale BTES sandbox based on similarity and symmetry principles. Detailed monitoring of sand temperature data and boundary conditions validated the effectiveness of the symmetrically simplified similarity experiment. The data comprehensively reflected the change of underground temperature fields during the charging and seasonal storage phases. The reasons for the performance limitations of BTES are thereby analyzed. During the charging phase, the prototype BTES achieved its highest average charging rate of 220W within the first 21 days, and the temperature field stabilized after 192 days. In the storage phase, the exergy destruction losses of BTES primarily occurred at the junction between the core and peripheral zones, accounting for 56 % of the total losses. An effective method to enhance BTES performance is to reconfigure the borehole layout and operation, which improves the uniformity and continuity of the underground temperature field. The experimental measurement provides fundamental data for the integrated seasonal utilization of heating and cooling, which can be essential for innovative building energy systems.