[kwhitefoot]

> And most already build hydroelectricity facilities can be retrofitted (although at high cost) to become pumped-storage hydroelectricity facilities.

Really? You need a huge downstream reservoir to hold the water that you are going to pump to the upstream reservoir.

[ElectricalUnion]

What prevents you from digging a very deep downstream hole to store such water besides cost?

[kwhitefoot]

I think you need a bigger word than digging, it wouldn't be a gang of navvies with shovels. The cost would be enormous, there are better ways to spend the money: solar, wind, better electricity networks, DC interconnects.