In “Finding solutions to Achilles’ heel of renewable energy: intermittency“, Diana Lutz reports on a special issue (Feb. 2012) of Proceedings of the IEEE titled “The Intermittency Challenge: Masssive Energy Storage in a Sustainable Future”, edited by William F. Pickard. She writes
Most schemes for the energy future, including transnational grids, also will require massive energy storage, some scheme to transform surplus grid energy into a different but conveniently stored form and then back-converted and returned to the grid when electric power is needed. Pickard calls them “granaries for electricity.”
“The solution is to excavate an underground reservoir many hundreds of meters below surface level and to exchange water between it and a surface reservoir created immediately above it and diked using spoil from the excavation. This variation of hydro storage is called underground pumped hydro,” Pickard says.
Such a facility could be put almost anywhere that there was low-quality land underlain with competent rock — in industrial brownfields, for example.