an interesting way to store energy

For decades people have using excess energy to pump water to a higher reservoir and letting it run downwards spinning a turbine on the way to recover the energy when needed.

You can also reverse the process a bit and pump water out of a pool that sits in a body of water.  Usually this is avoided as the costs have always been assumed to be too high, but Belgium is going to try it.

I haven't thought about this variation since I was a student, but it may pair well with offshore wind.  It would be interesting to see what the projected cost per kWh is ....