Cavern thermal energy storage: State of play and prospects

Cavern thermal energy storage (CTES) is a technological variant of underground thermal energy storage that relies on flooding of subsurface cavities or tunnels for long-term heat storage. Such installations are crucial, particularly for conserving excessive solar and waste heat from the warm season to be used during the cold season of the year. This review provides, for the first time, a comprehensive synopsis of different types of these large installations, which are most prominent in Scandinavian countries. It is revealed that CTES can be distinguished between those that are pre-designed and those that are re-engineered former infrastructures (commonly oil reserves), whereas single and multi-cavern systems exist. Based on existing and planned CTES implementations, characteristic design parameters are identified, and a structured insight into common criteria for an optimal layout is provided. While it is most cost-efficient to reuse existing facilities and apply storage volumes of far more than 100,000 m3, a perfect geometric layout needs to account for controlled thermal stratification, attuned aspect ratio, and optimal area-to-volume ratio. One crucial factor is the long-term geo-mechanical stability of the ambient rock mass, which ideally represents compact crystalline rock with negligible groundwater flow. Our study summarizes the findings of existing installations that may serve as blueprints for planning, constructing, operating, and monitoring new CTES, including hot water and pressurized storage concepts.