This article might be of use: https://www.researchgate.net/publication/48693439_Large-Scale_Hydrogen_Underground_Storage_for_Securing_Future_Energy_Supplies htt

This article might be of use: https://www.researchgate.net/publication/48693439_Large-Scale_Hydrogen_Underground_Storage_for_Securing_Future_Energy_Supplies https://www.researchgate.net/publication/48693439_Large-Scal... Long term means weeks to months of storage. This deals mainly with large season shifts in energy availability. Other ideas only really make sense for hours or days worth of storage. The need for this type of energy storage is well beyond what is plausible with pumping water or compressed air.