If you are into this thing and looking for an even more stupid idea to store energy, I present to you the StEnSEA [3]. Rolls right off the tongue, right? It is a hollow concrete sphere that is lowered to the bottom of the lake. Pumps then remove water from it, creating a vacuum. Letting the water back in and using the pumps as generators, the energy is reclaimed. Curiously absent from all documentation of this project is the amount of energy stored. I did some back of the envelope calculations a while back and it is 3.8kWh, for a multi-million-euro prototype!
[1] https://www.energyvault.com/ev1
[2] https://www.youtube.com/watch?v=iGGOjD_OtAM
[3] https://www.iee.fraunhofer.de/de/projekte/suche/2013/stensea...