How can a geothermal storage system be optimally integrated into a local district? A case study.

Achieving net-zero targets requires the phase-out of fossil-based heating. A major challenge is the seasonal mismatch between renewable heat supply and demand. District heating networks often dispose of excess heat in summer and rely on fossil backups in winter. Large-scale thermal energy storage offers a solution by storing surplus summer heat for use during winter, thus reducing the need for fossil fuels. This study investigates the feasibility of a large-scale thermal storage system at a power production site that supplies a large district heating network in the city of Bern, Switzerland. Specifically, the study examines the potential of a geothermal storage system to offset fossil fuel heat generation in winter by utilising heat stored during the summer months. Using a Python-based multi-energy system model, we simulate the optimal operation of the geothermal storage system with respect to cost and emissions, considering both supply and demand on an hourly basis over one year. Multi-objective optimisation is applied to generate a Pareto-optimal front. The results show that the geothermal storage system eliminates the requirement of 8 GWh of gas-powered heat supply and increases the waste heat utilisation by 20%, therefore lowering emissions. This effect is further increased when combined with an expansion of the district heating network, as individual, emission-heavy heaters are replaced by low-emission heat from the district heating network. The findings presented in this study can prove useful when evaluating similar systems across Switzerland.