[kbenson]

What about gravity+water->electricity->hydrogen->methane->electricity (assuming storage/transportation of methane, so there's a point)?

[ars]

gravity+water meaning hydroelectric? We're kinda maxed out on that, plus if you wanted to store energy pumped water storage is better than methane (but there are not a lot of suitable sites).

https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...