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Renewable energy sources
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Wind energy

Wind results from the movement of air due to the differential heating of the earth by the sun. For example, during the day by the seaside the air above the land is warmed faster than the water in the sea and so air flows from the sea to the land creating sea breezes. At night however the air above the land cools faster than the water in the sea so the air flow is reversed creating land breezes.

The windmill in its simplest form comprises a set of sails which converts the momentum of the air it captures into rotational (mechanical) energy of a shaft. This rotating shaft can in turn be used to grind corn or pump water out of the ground. They are still in common usage in dry parts of the world so when ever the wind blows some water is pumped and stored in an adjacent dam for subsequent use. In Crete, for example, windmills have been in continuous use for almost 5000 years (Figure)

windmills

A more recent application of wind energy conversion is that for producing electricity in which the rotation of a set of blades is then converted into electrical energy by mounting an electric generator at the end of the shaft.

small turbines

The current production of wind turbines range from very small turbines of one metre diameter or less which can be used at home to very large machines with blade lengths up to 70 metres.

Since the 1990’s, communities and electricity companies have been using this technology to build arrays of wind turbines whose output can be fed into the grid for supplying electricity.

The conversion process

Starting in the 1950’s, the sails were replaced by rigid aerofoil shapes (like those of the wing of an aeroplane) that are much more efficient in capturing the energy of the wind by having a higher lift to drag ratio. The generation process is the same for all size wind turbines.

wind turbine

A wind turbine works the opposite of a fan. Instead of using electricity to make wind, a turbine uses wind to make electricity. The moving air (wind) turns the blades which are shaped so that their lift is greater than its drag thus inducing rotation in the shaft to which it is attached. This spins the shaft, which is connected to a generator in order to produce electricity. The electricity is sent through transmission and distribution lines to a substation, then on to homes, business and schools.



 

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