Large‐scale, economic and efficient underground energy storage.

The conversion of the energy supply system towards renewable technologies is indispensable for climate and environmental protection. With the predominant use of inexhaustible but fluctuating renewable energy sources, energy storage capacities and physical networks are key infrastructure for sustainable functionality. Efficient pumped‐storage hydropower can meet these demands. However, suitable sites for conventional surface reservoirs are facing natural limits, and new storage capacities in ecologically sensitive mountainous valleys are increasingly difficult to license. To gap these aspects this article reflects the construction of fully underground pumped‐storage hydropower plants to cover multipurpose energy storage requirements. Thus, the surface reservoirs are substituted by underground storage caverns with less water volume required by equivalent energy storage capacity since high heads are utilized. Proximity to urban areas improves the construction and operation through various infrastructure and close connection to existing power grids, power plants and consumers. Valuable surface resources remain untouched and the closed water cycle ensures a constant storage volume and independent operation against external influences. Furthermore pumped‐storage is combined with seasonal underground thermal energy storage. This system leads the well‐proven hydraulic energy storage technology into a new energetic age and ensures the demand‐based and cross‐sectoral renewable energy supply. [ABSTRACT FROM AUTHOR]