[bcrosby95]

AFAIK though only pumped water and batteries have been done at anything approaching utility scale. Anything else and you're probably looking at several decades to reach maturity, and then another several decades to roll it out.

[ncmncm]

All but chemical methods rely only on trivial physics used industrially billions of times worldwide every day. The only open question is which will have become cheapest, in each place, at the time that conditions become favorable to build it.

That time will arrive after we have brought enough renewable generating capacity online that storage would not need to be charged by burning fossil fuel.