Renewable energy storage in geological formations

With the transition to renewable energies and, above all, strongly fluctuating electricity from wind and solar energy, there will be a need for energy storage in the future. For central grid-scale storages, underground geological storage, similar to those already used for fossil fuels, is in the first place under review. Compressed Air Energy Storages have already been successfully used to provide minutes to hours reserve. For storage capacities in the day to week range, storage is required on a chemical rather than a mechanical basis, through either the conversion of electricity into pure hydrogen (H-2) or the generation of mixtures of natural gas and synthetic methane. The latter - the so-called power-to-gas option - allows the use of the existing gas infrastructure. A likely first choice for the storage of H-2 or H-2-SNG mixtures are man-made salt caverns. The suitability of porous rock storage (depleted hydrocarbon reservoirs or water-bearing reservoirs - aquifers) is still under investigation. Interest in porous rock storage options arises, inter alia, from the fact that many regions of Europe lack suitable salt deposits. Favorable salt deposits exist in the UK, notably in the Cheshire Basin to the west and in eastern England, with six salt cavern-hosted facilities operated as natural gas storages. In any case, underground gas storages are characterized by high safety and low environmental impact.