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* Island planned for North Sea, 3 km off Belgian coast
* Would be doughnut-shaped with a reservoir in the middle
* Will take five years or more to build
BRUSSELS, Jan 17 (Reuters) - Belgium is planning to build a doughnut-shaped island in the North Sea that will store wind energy by pumping water out of a hollow in the middle, as it looks for ways to lessen its reliance on nuclear power.
One of the biggest problems with electricity is that it is difficult to store and the issue is exaggerated in the case of renewable energy from wind or sun because it is intermittent depending on the weather.
"We have a lot of energy from the wind mills and sometimes it just gets lost because there isn't enough demand for the electricity," said a spokeswoman for Belgium's North Sea minister Johan Vande Lanotte.
"This is a great solution," she said, adding she thought it could be the first of its kind.
Excess energy would be used to pump water out of the centre of the island, and then the water would be let back in through turbines when demand outpaces supply.
Vande Lanotte revealed the plans during a presentation at the Belgian port of Zeebrugge late on Wednesday.
Belgium plans a complete exit from nuclear power as soon as enough energy from alternative sources becomes available, but last year it postponed the closure of its oldest reactor for a decade over energy supply worries.
In 2011, around 57 percent of Belgium's energy came from nuclear power.
Belgium hopes eventually to generate 2,300 MW from its network of North Sea wind farms, which could replace a significant part of either of its two nuclear sites, Doel and Tihange, which each generate about 3,000 MW.
The island is still in the planning stages, but will be built out of sand 3 km off the Belgian coast near the town of Wenduine if it gets the final go-ahead.
The island, which would also work as an offshore substation to transform the voltage of the electricity generated by wind turbines, could take five or more years to plan and build.
It won't be until Elia, Belgium's power grid operator, strengthens the power lines leading out to the coast.
"One of the driving elements is the reinforcement of the grid onshore towards the coast," a spokeswoman for Elia said.
Belgium's nuclear power operator Electrabel, a division of France's GDF Suez, shut down two of its reactors last year while it investigates cracks found in the reactor casing. (Reporting By Ben Deighton; editing by James Jukwey)