Simulation-based planning for cost-effective and energy-efficient large-scale seasonal thermal energy storage systems

Large-scale seasonal thermal energy storage (sTES) systems play a crucial role in the transformation of district heating systems towards sustainable and renewable-powered systems. However, planning and optimization of sTES remains challenging due to complex interactions between design, hydrogeological conditions, and economic feasibility. Thus, this study investigates buried tank thermal energy storage (TTES) using a three-dimensional multiphysics model in COMSOL Multiphysics® focusing on insulation configuration, groundwater interactions, and storage volume. Results show that upscaling reduces geotechnical costs by 40 % and increases energy efficiency by more than 15 % for volumes between 100,000 m 3 and 2,000,000 m 3 . Insulation distribution strongly influences performance: inhomogeneous layouts improve energy capacity efficiency by 2–3 % and reduce the levelized cost of stored heat by up to 4 €/MWh compared to homogeneous insulation. In contrast, when groundwater is absent, omitting insulation can be more cost-effective for medium-sized tanks (<500,000 m 3 ), highlighting the importance of site-specific planning. Overall, this study demonstrates how targeted insulation strategies and hydrogeological considerations can improve the techno-economic performance of TTES. The findings provide practical guidelines for designing cost-effective, energy-efficient storage systems and support the integration of renewables into future high-temperature district heating networks.