A Review on Underground Hydrogen Storage: Insight into Geological Sites, Influencing Factors and Future Outlook

Without remorse, fossil fuels have made a huge contribution to global development in all of its forms. However, the recent scientific outlooks are currently shifting as more research is targeted towards promoting a carbon-free economy in addition to the use of electric power from renewable sources. While renewable energy sources may be a solution to the anthropogenic greenhouse gas (GHG) emissions from fossil fuel they are yet season-dependent faced with major atmospheric drawbacks which when combined with annually varying, but steady, energy demand, results in renewable energy excesses or deficits. Therefore, it is essential to devise a long-term storage medium to balance their intermittent demand and supply. Hydrogen (H2) as an energy vector has been suggested as a viable method of achieving the objectives of meeting the increasing global energy demand. However, successful implementation of a full-scale H2 economy requires large-scale H2 storage (as H2 is highly compressible). As such, storage of H2 in geological formations has been considered as a potential solution where it can be withdrawn again at the larger stage for utilization. Thus, in this review, we focus on the potential use of geological formations for large-scale underground hydrogen storage (UHS) where both conventional and non-conventional UHS options were examined in depth. Also, insights into some of the probable sites, and the related examined criteria for selection were highlighted. The hydrodynamics of UHS influencing factors (including solid, fluid, and solid–fluid interactions) are summarized exclusively. In addition, the economics and reaction perspectives inherent to UHS have been examined. The findings of this study show that UHS, like other storage systems, is still in its infancy. Further research and development are needed to address the significant hurdles and research gaps found, particularly in replaceable influencing parameters. As a result, this study is a valuable resource for UHS researchers.