Simulation-based design of world’s largest cavern thermal energy storage: Optimization of VARANTO

As the global energy transition advances, cavern thermal energy storage (CTES) emerges as a key technology for large-scale, seasonal thermal energy storage within sustainable district heating networks. Thanks to its capability for storing massive quantities of thermal energy, CTES systems can balance the temporal mismatch between renewable energy supply and heat demand, improve system flexibility and support decarbonization. However, the planning and design of such large, pressurized underground systems are complex, requiring detailed assessment of the coupled thermal-hydraulic-mechanical processes that might undermine both performance and structural stability.
This study presents a simulation-driven planning approach for the world’s largest CTES system (VARANTO) to be realized in three interconnected rock caverns. The system is designed to store pressurized water up to 140°C, introducing significant thermo-mechanical stresses and deformation risks within the surrounding rock mass. To address these challenges, a comprehensive model is developed in COMSOL Multiphysics® capturing transient heat transfer, groundwater interactions and stress evolution.
Simulation outcomes guide the optimization of CTES geometry, spatial configuration and orientation, ensuring maximum storage efficiency and long-term integrity. The results demonstrate how multiphysics modeling provides critical insight into thermal performance and geomechanical safety, offering a transferable framework for the design of next-generation underground energy storage systems.