This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.
Actually, this scheme is currently employed by utilities, albeit usually not with seawater. The technique is called pumped storage hydro. Pumped storage hydro accounts for the vast majority of grid energy storage world-wide. Pumped storage hydro is used to by power companies to achieve various goals, e.g.:
flatten out load variations (as you suggested elsewhere in this thread)
provide "instant-on" reserve generation for voltage and frequency support
level out the fluctuating output of intermittent energy sources such as wind and solar (as you suggested above)
Wikipedia states that round-trip efficiency of pumped storage hydro can range between 70% and 87%, making it an economical solution in many cases.
A couple of obstacles to using pumped storage hydro are:
Certain topological/geographic features are needed to make PSH viable
Social and ecological concerns
Yes! Thank you.
I wonder why there aren't more of them, or bigger ones. The only seaside-cliff one listed on Wikipedia is the Okinawa Yanbaru station, completed in 1999, which only provides 30 MW.
Apparently the cost/demand situation isn't favorable.