Integrating seasonal borehole thermal energy storage into deep borehole ground source heat pump systems: Dynamic performance analysis under uncertainties

Integrating borehole thermal energy storage (BTES) into closed-loop medium-deep geothermal heating systems helps mitigate underground temperature decline and support sustainable operation. Existing research primarily focused on the heat storage performance of the deep borehole heat exchanger (DBHE) under deterministic conditions, neglecting the dynamic characteristics of the integrated system and multiple uncertainties. To address this gap, this study developed a solar-assisted deep borehole ground source heat pump system incorporating active heating, passive heating, and BTES, and investigated the long-term system performance and thermal storage characteristics under uncertainties. Firstly, a comprehensive DBHE model, considering heat extraction, storage, and recovery, was developed in TRNSYS. Subsequently, 20-year stochastic scenarios were generated considering uncertainty correlations. Long-term dynamic simulations were then performed to evaluate the system's behavior under the combined effects of borehole depth and solar heat generation capacity. The results revealed significant fluctuations in annual energy performance under uncertainties, in contrast to the relatively stable variations observed under deterministic conditions. Ignoring uncertainties in long-term simulations could underestimate the annual maximum heat storage rate by up to 18.2 %. The integration of deep BTES achieved up to 66.9 % energy savings for the system when borehole depth exceeded 2,500 m and solar collector area surpassed critical thresholds. A generally negative correlation was observed between the critical collector area and borehole depth. Additionally, the benefits of deep BTES on heat extraction performance were more pronounced at borehole depths of 2,000–2,200 m and 2,900–3,000 m, with heat extraction gains of up to 17.5 % per unit of energy charged. This study provides meaningful insights into the long-term operational characteristics of medium-deep geothermal systems with BTES under real-world conditions.