Influence of heat storage on performance of multi-borehole mid-deep borehole heat exchangers

Storing heat underground is an effective method to solve the problem of apparent ground temperature decrease due to long-term operation of a multi-borehole mid-deep borehole heat exchanger (MMBHE), which will cause heating capacity decrease and even heating failure. To understand the influence of heat storage on the MMBHE operation performance, based on the proposed model considering heat storage and extraction, this research comprehensively analyzes the influence of heat storage on the MMBHE heat extract capacity, ground temperature distribution, fluid temperature variation along flow path. The influential factors analyzed include borehole layout, borehole amount, borehole spacing, and borehole depth. The results indicate that adopting heat storage in non-heating seasons significantly improves the ground temperature and the heat extraction capacity of the MMBHE. The MMBHE can extract more heat with heat storage than that without heat storage when the heat storage fluid temperature is higher than the critical heat storage temperature which is much less the borehole bottom temperature. As the case discussed in this paper, the mean heat extraction capacity with heat storage is 24.25 % higher than that without heat storage. As the borehole depth increases, there is a substantial increase in the heat extraction capacity of the MMBHE with no heat storage, and the heat storage effect is weakened when heat storage adopted in non-heating period. Less borehole number and larger borehole spacing make larger mean borehole heat extraction capacity of an MMBHE with no heat storage. However, when the borehole layout, borehole amount, borehole spacing, and borehole depth change, fluctuation of the mean borehole heat extraction capacity with heat storage is much smaller compared to those with no heat storage. When the borehole spacing increases from 25 m to 45 m with no heat storage, the mean heat extraction capacity of the MBHE is increased by 27.67 kW, while that is 11.88 kW with heat storage. This research can provide some inherent understanding of the heat storage on MMBHE performance and reference for designing MMBHE with heat storage systems.