Comment by Pepe1vo

It is in fact possible to benefit from pumped hydro even if you have no significant height differences in a certain area. How? By creating a basin inside of an existing body of water and emptying/filling when there's over/under supply of electricity.

I remember reading a couple of years ago that there were plans to construct such a "valmeer" inside of the Ijsselmeer, but I can't find much about it now so no idea whether it's been canned or not.