Performance assessment and parameter optimization of a solar collector coupled deep borehole heat exchanger heating system with thermal energy storage

Deep borehole heat exchanger (DBHE) heating systems have gained increasing attention due to their high efficiency, long-term sustainability, and operational stability. However, traditional DBHE systems are primarily limited to serving as direct building heating systems through one-way heat extraction from the subsurface, lacking flexibility for integration. This study explores a novel hybrid configuration that incorporates thermal energy storage (TES) and solar collectors (SC) into a DBHE-based heating system. A detailed TRNSYS simulation model was developed for an office building in Xi’an, China, incorporating the full configurations of the DBHE, TES, and SC subsystems. Key design parameters, including the solar collector area and tilt angle, TES tank volume, and the heat pump’s storage temperature setpoint, were systematically optimized to minimize the system’s levelized cost of heating (LCOH). The results indicate that the DBHE–TES–SC system achieved improved heating performance with a more dynamic temperature profile during the heating season. The system also benefits from short-term thermal storage and peak-shaving strategies, offering increased operational flexibility. Over a 10-year operation period, although the coefficient of performance (COP) of the heat pump decreases from 5.63 to 5.14, the outlet temperature decay rate is only 6.5%, demonstrating superior long-term operational sustainability. From an economic perspective, the proposed system reduced annual operating costs by over 36%, shortened the payback period by 0.45 years, and achieved a lower LCOH than that of a conventional DBHE system. In addition, a preliminary case study is presented to explore the feasibility of deep borehole thermal energy storage (DBTES) for storing intermittent solar energy during the non-heating season. The results show that this strategy can further reduce the annual electricity consumption by an average of 2.47MWh, underscoring the need for a comprehensive analysis of DBTES in future research. These findings highlight the strong techno-economic potential of integrating TES and SC into DBHE systems. The proposed optimization method can serve as a reference for decision-makers in the geothermal community for building heating applications.