Evaluating the Performance of Seasonal Borehole Thermal Storage in 5th Generation District Heating and Cooling Networks

The operating temperature range of borehole fields includes low-temperature levels that may be suitable for fifth-generation district heating and cooling (5GDHC) networks, while also enabling bidirectional heat exchange and seasonal thermal storage. However, their performance depends on network temperature management, the balance between annual heat extraction and injection, and local ground conditions. This study investigates how supply- and return-temperature set-points affect short-term heat-pump coefficient of performance (COP) through their influence on the borehole entering-fluid temperature; whether cooling-season charging improves winter COP and seasonal coefficient of performance (SCOP) while mitigating long-term thermal drift; and whether year-round bidirectional operation or explicit summer recharge better supports multi-year thermal stability. An integrated TRNSYS model combining a low-temperature network, building-level water-to-water heat pumps, and a composite borehole-field model was used to simulate hourly and multi-year operation under three ground configurations, including an idealised representation of groundwater-induced temperature stabilisation. The results show that the heat pumps meet building thermal demands within the 5GDHC operating range, achieving SCOP values between 3.41 and 3.88. Maintaining a higher borehole entering-fluid temperature through appropriate temperature and mass-flow control improves heat-pump COP. Cooling-season charging improves winter performance and mitigates thermal drift when sufficient injected heat is retained in the ground. Balanced thermal-demand operation therefore provides greater multi-year thermal stability than heating-dominated operation with limited recharge.