MILP framework for integrating seasonal thermal storage in energy hubs: economic and performance optimization

This thesis investigates the integration of Seasonal Thermal Energy Storage (STES) within a multi-energy optimization framework, using the REHO tool developed by the IPESE group at EPFL. The objective is to evaluate the techno-economic and environmental impact of STES in a fully electrified energy hub, assessing its ability to improve system efficiency, reduce emissions, and enhance energy independence. An initial optimization is conducted without considering temperature dynamics in the storage system, as commonly assumed in literature. However, a post-process analysis reveals that neglecting the storage outlet temperature leads to technically infeasible results, due to the temperature mismatch with the district heating network (DHN) requirements. To address this issue, a heat pump is introduced in a refined configuration to ensure operational feasibility. The analysis then progresses by introducing a minimum temperature constraint on the storage medium. Results show that enforcing this thermal limit improves the performance of the heat pump, significantly reducing electricity imports and operational expenditures. At the same time, however, maintaining the minimum storage temperature increases the required storage volume, leading to higher capital costs. Ultimately, the findings demonstrate that STES is an effective strategy to address seasonal energy imbalances and reduce grid dependence. Nonetheless, the minimum allowable storage temperature proves to be a critical parameter, influencing not only the energy performance of the system but also the design and dimensioning of key infrastructure components.