Seasonal storage of district heating surplus heat through borehole thermal energy storage: a global sensitivity analysis

Seasonal storage of surplus heat in district heating networks is an effective strategy for decarbonizing the heating sector and facilitating the integration of new neighborhoods into existing systems. In this paper, a borehole thermal energy storage system integrated with a district heating network, and a heat pump was investigated, supported by site-specific data from the Nyhavna case study in Trondheim, Norway. An efficient simulation framework was developed to capture both short-term thermal fluctuations and long-term interactions within the borehole field, as well as the coupling among system components under realistic operating conditions. A global sensitivity analysis was performed across design and operational variables through a system-level perspective. Seven variables were sampled using Latin hypercube method to generate 150 simulation cases. The high thermal conductivity of the studied site was found to significantly influence the interaction among variables and performance metrics. The maximum achieved borehole storage efficiency, and the heat pump coefficient of performance were 0.80 and 6.23, respectively. Maximum charging temperature, borehole spacing, and total borehole length were identified as the most influential variables on system performance, while heat pump characteristics predominantly affected the discharging phase. Across all configurations, the total system energy demand ranged from 352 to 665 GWh, with peak electricity and district heating demands spanned 2.2 - 9.2 MW and 9.9 - 42.2 MW, respectively. These results highlighted how coordinated sizing of the borehole field and heat pump, together with effective waste heat utilization, can moderate peak loads and total energy requirements under realistic design constraints.