Belgium is building an artificial island in the North Sea to store water pumped by wind turbines in periods of low electricity demand. When electricity is needed, the water will be released. - 

A big hurdle in wind energy is, how do you store it, for when it's not, well windy? Investors and countries are betting on different technologies, but Belgium has a unique approach – an artificial island shaped like a donut. Yep, a donut.

The donut island (ok that's not the official name) will be built in the North Sea, next to offshore wind turbines.

Here's the problem it's trying to solve: wind often blows at night, when people don't need energy. One way to store it for later, is to use the energy to pump water.

"To push it kind of uphill," says Sophia von Waldow at Bloomberg New Energy Finance. "And that's where you store it. And when you need the energy again, it's pushed downhill again and that's when it generates electricity."

That works, if you happen to have a hill, and a place for water above and below. Otherwise, "in the North Sea, one would have to create these conditions, artificially," Von Waldow says.

Enter donut island -- with the middle dug down so it's the deepest part of the ocean.

When turbines turn, energy pumps water up and out of the donut. Later on, when Belgium needs energy, the water falls back down, making hydropower.

One key point: the technology's from Denmark. So it's actually more danish than a donut.

Follow Scott Tong at @tongscott