Breezy does it: how wind turbines work

(GETTY IMAGES)

Wind farms generate electricity by capturing the kinetic energy of moving air.

The three blades of each turbine turn on a horizontal axis in the wind and the movement of this drive shaft spins an electricity generator, usually via a gearbox, so that power can be fed into the National Grid.

The wind farms being planned for sites off the coast of Britain will include some of the biggest wind generators ever built, with blades up to 60m long. These offshore wind generators will stand in water that is up to 30m deep and will be up to 220m tall from the base of their foundations to the tips of the turbine blades, about 40m taller than the "Gherkin" skyscraper in London.

Their immense size will make them more efficient at generating power but it also introduces huge engineering challenges in terms of manufacturing, installing and maintaining wind farms so far out to sea. Yet it is the remote locations of the farms that make them so attractive in terms of capturing the strongest prevailing winds and avoiding planning objections.

They will be designed to generate power over a wide range of wind speeds, cutting in when the wind blows at more than 7mph and shutting down in storms of 60mph winds, a measure designed to protect against potential damage to a generator's drive train, gearbox and structure. Maximum output is usually achieved when wind speeds reach about 31mph.

A key measure of power generation is "capacity factor", a measure of how much electricity is actually produced relative to the total that could be produced if the generator is working flat out. For wind generators it is estimated that the capacity factor will range from between 20 per cent and 40 per cent of full capacity, due primarily to the vagaries of the wind. Offshore wind farms are assumed to have a higher capacity factor because winds out at sea blow stronger, for longer, compared to wind on land.

Nevertheless, the great advantage of wind is that it is a free fuel, unlike gas, coal or oil. Britain is also a windy place, making supply relatively secure. It is estimated that within a year of operation, each offshore wind generator will have generated the energy used to make, install and maintain them during their 25-year lifetime.