Operational performance and capacity optimization of a grid-interactive medium-deep borehole geothermal heat pump system with thermal energy storage

Medium-deep ground source heat pump systems (MDGSHPs) have significant potential for supporting the energy transition. However, integrated MDGSHPs have mainly been investigated as thermal supply systems, while their role as grid-interactive flexible loads remains insufficiently understood. In particular, the coordinated operation between heat extraction from a medium-deep coaxial borehole heat exchanger (MDCBHE) and sensible thermal storage in a water tank under time-of-use (TOU) tariffs has not been systematically clarified. To address this gap, this study develops a grid-interactive integrated MDGSHP system and establishes a multi-objective optimization framework for its analysis and design. The grid-interaction performance is evaluated at multiple timescales, and capacity optimization is conducted using surrogate models. The results show that coordinated heat storage and release under the TOU pricing strategy reduce quarterly electricity consumption during spike-price hours by 1904.00 kWh and increase the coefficient of system performance ( CSP ) by 5.37%. Response surface analysis reveals that the capacity of the ground source heat pump system (GSHPs) plays a dominant role in the trade-offs among heat extraction performance, operating cost, and energy efficiency. After optimization, the annual average specific heat extraction rate reaches 66.21 W/m, the daily operating cost is reduced by 58.49 CNY relative to the baseline system, and the CSP reaches 4.20. This study provides design and optimization guidance for integrated MDGSHPs with improved grid-interactive capability and operational flexibility.