The idea is to place the island a few kilometers off shore near a wind farm, according to Vande Lanotte’s office. When the wind farm produces excess energy for the local electricity grid, such as off-peak times in the overnight hours, the island will store the energy and release it later during peak times.

It would use the oldest and most cost-effective bulk energy storage there is: pumped hydro. During off-peak times, power from the turbines would pump water up 15 meters to a reservoir. To generate electricity during peak times, the water is released to turn a generator, according to a representative.
– A Manmade Island to Store Wind Energy | MIT Technology Review

Sounds crazy, but a neat idea.