Ah yes, the mythical Belgian donut island, from whence all Belgian donuts are harvested, skinned, stuffed with a creamy filling, and then exported to the rest of Europe. Or at least, that's what I thought it was, but donut island is in fact not mythical at all, and Belgium is going to build it as a giant and not edible battery for storing surplus wind energy.
Obviously, this picture is not of a real island, but it's a more or less accurate illustration of what Belgium is planning to build out of sand in the North Sea, about three kilometers off of the coast. It's a simple idea: you build an island in the ocean with a hole in the middle that goes down below sea level. When you've got spare wind energy, you use it to pump water out of the hole and into the ocean. To recover that energy, you let water back into the hole from the ocean through pipes and turbines, just like with a hydroelectric dam.
Belgium only produces about four percent of its energy from wind power, but that amount should quadruple by 2020. 57 percent of Belgian happy juice comes from nuclear power, but that's going to start getting phased out after cracks were found in two separate reactor casings. Probably a smart move.
If the Belgian donut island gets the final go-ahead, it's expected to take five years to construct, so we can expect it to start helping to regulate wind energy supply and demand in one of the tastiest ways ever roundabout 2018 or so.