Overview of Large-Scale Underground Energy Storage Technologies for Integration of Renewable Energies and Criteria for Reservoir Identification

The increasing integration of renewable energies in the electricity grid is expected to contribute considerably towards the European Union goals of energy and GHG emissions reduction. However, it also brings new challenges for the grid. Large-scale energy storage can provide means for a better integration of renewable energy sources, balancing supply and demand, increasing energy security, enhancing a better management of the grid and also allowing convergence towards a low carbon economy. One way to ensure large-scale energy storage is to use the storage capacity in underground reservoirs, since geological formations have the potential to store large volumes of fluids with minimal impact to environment and society. There are several technologies which can be viable options for underground energy storage, as well as several types of underground reservoirs can be considered. The underground energy storage technologies for renewable energy integration addressed in this article are: Compressed Air Energy Storage (CAES); Underground Pumped Hydro Storage (UPHS); Underground Thermal Energy Storage (UTES); Underground Gas Storage (UGS) and Underground Hydrogen Storage (UHS), both connected to Power-to-gas (P2G) systems. For these different types of underground energy storage technologies there are several suitable geological reservoirs, namely: depleted hydrocarbon reservoirs, porous aquifers, salt formations, engineered rock caverns in host rocks and abandoned mines. Specific site screening criteria are applicable to each of these reservoir types and technologies, determining the viability of the reservoir itself, and of the technology for that site. This paper presents a review of the criteria applied to identify suitable technology-reservoir couples.