Experimental and numerical investigations on performance of seasonal borehole underground thermal energy storage system

To ensure energy grade and minimize energy loss in seasonal borehole underground thermal energy storage (SBUTES), a novel SBUTES system was designed and subjected to experimental testing and numerical simulation for its thermal charging process. The system incorporated a heat cascade design, dividing the circular arrangement of the underground thermal storage system into a high-grade energy reservoir (HGER) located in the core zone and a low-grade energy reservoir (LGER) located in the peripheral zone, operated in parallel. The experimental setup maintained an adequate soil temperature gradient, with higher temperature in the core region and lower temperature in the peripheral region compared to conventional borehole thermal energy storage (BTES) systems. Using a similar principle, a reduced-scale test bench was designed and constructed with dimensions of 1.5 m in height, a cross-section of 2.5 m, and an angle of 16°. The test bench was filled with sand and consisted of 8 vertical coaxial tubes with precisely positioned thermocouples. Predictions from the numerical model were compared and validated against experimental data obtained from a 72-hour heat transfer experiment. The results demonstrated that the experimental data exhibited a high degree of concordance with the numerical simulation data, with an average relative error of the buried pipe outlet temperature within 1.39% and a relative error of the soil temperature within 9.02%. Additionally, the numerical simulation compared the average temperature in the core area with LGER to that without LGER. The results demonstrated that the system equipped with LGER achieved higher average soil temperatures after the charging period. Moreover, during a 61-day heat storage period, this system exhibited a slower rate of decrease in average soil temperature, resulting in reduced heat loss. This configuration effectively minimized the temperature gradient between LGER, HGER and the external geotechnical environment, thereby mitigating energy loss from HGER. The numerical model of the test bench proved valuable in studying long-term heat transfer behavior of buried pipes under complex conditions at a relatively low cost and with controllable short-term operation times. It provided a solid foundation for further comprehensive investigations into the novel SBUTES system.