Experimental and numerical investigations on operation characteristics of seasonal borehole underground thermal energy storage

To investigate operation characteristics of seasonal borehole underground thermal energy storage (SBUTES) with different operational strategies, a model test platform with reduced size was established based on similarity principle. The test results show that the larger the start-stop time ratio, the smaller the average heat exchange rate per unit depth (HERPUD) of borehole, and the lower the energy storage efficiency, but the thermal energy storage and extraction capacity increase. For the time ratio of thermal energy storage to extraction (TROSE), the larger the TROSE, the larger the thermal imbalance rate of energy storage body, which will lead to the gradual increase or decrease of temperature of energy storage body and is not conducive to the operation of energy storage. Compared with the asynchronous mode of storage and extraction, the synchronous mode of storage and extraction can effectively reduce the average temperature of energy storage body and increase the heat exchange efficiency of boreholes during the thermal energy storage. The focused energy storage mode can improve the energy storage efficiency and soil temperature recovery rate, balance the thermal energy storage and extraction capacity, and thus the performance of SBUTES can be optimized. A 3-D CFD model of borehole energy storage was established to further find the influences of borehole layout forms, layout spacing and depths on characteristics of the SBUTES. It can be found that for the energy storage efficiency, the hexagonal layout is the highest, the rectangular layout is the lowest, and the circular layout is slightly higher than the square one. Under the same volume of boreholes group, increasing the spacing of inner boreholes can effectively alleviate thermal interference of inner boreholes during the thermal energy storage, and the heat exchange capacity undertaken by inner boreholes can be improved. But it is not conducive to thermal energy extraction. Although increasing borehole depth can increase thermal energy storage and extraction capacity and system operation efficiency, the increase degree is decreasing with the increase of borehole depth, and the greater the borehole depth, the smaller the influence of borehole depth on characteristics of SBUTES.