Wallonia has at least one pumped hydro that I know of. There were plants for a 'energy island' 300MW hydro plant at sea, near the large-scale wind farms. The i

Wallonia has at least one pumped hydro that I know of. There were plants for a 'energy island' 300MW hydro plant at sea, near the large-scale wind farms. The idea is the same, pump sea water up an artificial horse-shoe shaped island. A nice bonus is that you can play with the tides as well. The height difference does not have to be great, if you can compensate with a large surface area. As far as I know, those plans were shelved. A quick googling says that similar concepts are being considered for interconnected dutch/danish wind farms.