I always have been fascinated with free/cheap energy. They actually do a version of your pool thing with the lake I am buying property on in Virginia. They generate power when Smith Mountain Lake (dammed) goes into a smaller lake at times of peak demand, then pump back water at times of low demand. So it is almost a form of a battery. Of course there is loss, but it apparently is worth it to meet the demand.

I think one day they will master fusion in some form hopefully.