Investigating the Impact of Seasonal Heat Storage on the Thermal and Economic Performance of a Deep Borehole Heat Exchanger: A Numerical Simulation Study

Deep borehole heat exchanger (DBHE) is a clean and efficient technology that utilizes geothermal energy for building heating. However, continuous heat extraction from a DBHE system can lead to its performance decline over time. In this paper, the seasonal heat extraction and storage of a DBHE were simulated to assess the impact of seasonal heat storage schemes on its thermal and economic performance. The numerical model was constructed based on real project parameters and validated using monitoring data. Simulation results indicate that the extracted heat after storage increases linearly with the injected heat, enabling a straightforward estimation of the storage input to mitigate short-term thermal attenuation of DBHEs under varying storage durations. However, when the same amount of heat was injected annually, DBHE heat extraction still exhibited a declining trend from the third year, suggesting that short-term improvements in heat extraction could not be sustained in the long term. Furthermore, heat storage efficiency improves over time as the surrounding borehole temperature gradually increases, reaching more than 27% after 10 years for all storage scenarios. For the first time, an economic analysis was conducted for DBHE heat storage, revealing that when a solar supplemental heat system is applied, the levelized cost of heat (LCOH) is slightly higher than the base case without storage, except in cases where solar collector costs are excluded. Given the modest thermal and economic improvements, seasonal heat storage is recommended for DBHEs, especially when low-cost surplus heat is readily available.