Many countries aim to achieve Net Zero emissions by the middle of this century requiring a dramatic increase in renewable energy uptake. However, unlike fossil fuels, renewable energy has challenges with seasonal intermittency, resulting in a lack of supply (when resource is scarce or demand is high) or a waste of excess heat or power (when resource is plentiful, but demand is low). As renewable energy implementation accelerates, there is an urgent need for developing reliable energy storage methodologies that better integrate low-carbon resources, and balance the distribution of energy networks.
Compared with traditional battery storage, underground energy storage has attracted relatively less attention. Underground spaces, including caverns, pores within reservoir rocks and aquifers, legacy mine shafts/workings, and tunnels can be effectively used to store different forms of energy (e.g. thermal, mechanical, and gas). Underground energy storage systems have great potential to provide a stable, green, and low-cost solution to balance energy supply and demand, and enhance renewable energy efficiency and utilisation: strategically contributing towards the green transition of energy sectors. They also provide comparatively large-scale storage capacity for minimal surface land take, so are more secure and face reduced competition from alternative and comparatively valuable land uses.
Underground energy storage (UES) systems include aquifer thermal energy storage, underground hydrogen storage, compressed air energy storage, underground pumped hydro storage, underground gravity energy storage, and other innovative approaches. Considerable progress has been made in these technologies in recent years; however, there are still engineering challenges and scientific questions to be solved in developing reliable and safe UES, such as the evolution of geological, geophysical, and geochemical properties during long-term energy storage and engineering disturbances, integrity and durability of underground energy storage structures, interaction between the engineered system and geological environments, and prediction and prevention of underground dynamic disasters like earthquakes during UES construction and operation.