Another thing to consider with the 'dig a hole' based pumped storage approach is that you can only 'suck' water about 15ft vertically (well technically you can'

Another thing to consider with the 'dig a hole' based pumped storage approach is that you can only 'suck' water about 15ft vertically (well technically you can't really suck water any appreciable distance, but the atmosphere can push it up about 15ft) so it means that your pumps need to be at the bottom of the hole. You might be able to get away with some sort of remote actuated pump head like that used in an oil well (energy for pumping provided by a reciprocating metal rod) but the efficiency of such schemes is never going to get anywhere near the 80% you were targeting. Better hope that the pumps don't fail while the hole is full!