Seasonal geothermal energy storage and extraction using a deep borehole heat exchanger: An energy perspective analysis

Expanding the deep borehole heat exchanger (DBHE) to a dual-use scenario for heat extraction and storage can simultaneously enhance heat extraction performance and facilitate the integration of renewable energy. Understanding the spatio-temporal energy variations in the subsurface during heat extraction and storage is crucial for system design and optimizing the operation strategy. To visualize and quantify the energy migration process in the subsurface, a comprehensive numerical model and a novel energy analysis workflow were developed using the OpenGeoSys software. The model was first validated against monitoring data from a pilot project in Xi'an, China. Subsequently, the validated model was used to simulate additional scenarios involving heat extraction and storage. Energy analysis shows that, in the benchmark case, 60.2% of the extracted heat is supplied by the deep subsurface below 1500 m, while 60.8% of the injected heat is stored in the shallow subsurface above 1000 m after heat storage. However, the continuous thermal dissipation of shallow stored heat away from the DBHE limits its recovery, so more than half of the extracted heat is still supplied by the deep subsurface in the following heating season. Furthermore, results reveal that increasing both the heat storage temperature and flow rate allows a larger proportion of heat to be stored in the deeper formation. Furthermore, raising the inlet temperature is generally preferred over increasing the flow rate to enhance the heat storage capacity. After 10 years’ heat storage, the annual heat extraction power increases by 16.6% compared to the same scenario without heat storage. Although heat storage efficiency remains relatively low (no more than 15.6%), long-term heat storage can effectively alleviate subsurface temperature attenuation and result in a more stable outlet temperature (with only a 0.2% reduction). These findings provide valuable theoretical insights and practical guidance for enhancing the efficiency and sustainability of heat extraction-storage-coupled DBHE systems and for developing effective heat recovery strategies.