Tim Streater explained on 24/05/2017 :
Maybe its cheaper just to flood old mines and use turbines to generate
power then pump them out when there is little demand. Is it any less
efficient than a bleedin big battery which needs to then be converted to AC
in any case?

Flood them with what?

Water?

I would not think the water capacity of a disused mine would be enough
to make it worth while.