There is an idea for a reverse reservoir: https://nl.m.wikipedia.org/wiki/Plan_Lievense https://nl.m.wikipedia.org/wiki/Plan_Lievense Make a dike ring in eg. t

There is an idea for a reverse reservoir: https://nl.m.wikipedia.org/wiki/Plan_Lievense https://nl.m.wikipedia.org/wiki/Plan_Lievense Make a dike ring in eg. the north sea, pump water out of the ring into the rest of the North Sea to store power, and vice versa to get it back. Easy peasy...