Historical evidence indicates that windmills were being used in Persia and China for water- pumping and milling grain over four thousand years ago. In South Africa most wind energy is used for water pumping.
 

The first windmill to produce electricity was built in Denmark in 1890.The first windmills which produced electricity were capable of producing only small amounts of electrical power. The extensive use of coal and oil to generate electricity reduced interest in wind power as a source of generating electricity.
 
 

During the early 1970’s when oil prices rose sharply and people began to question the world’s reliance on non-renewable fossil fuels, interest in wind power was again aroused. Wind energy programs were established in most
European nations and in the United States. Both private companies and governments have carried out research on wind-generators. Larger wind generators, some capable of producing megawatts of electric power, have been installed in a number of countries.

There are some areas in South Africa where the wind resource compares fairly well with regions of the world where wind energy is being successfully exploited.
Unfortunately many in land areas do not receive strong regular winds.

South Africa also has an abundance of cheap coal and hence relatively cheap grid electricity. This reduces the economic attractiveness of wind-generators as an option for electricity grid connection.

However, on a small scale wind is an attractive source of power, particularly in rural areas, which are distant from the electricity grid.

Power In The wind

Everyone realizes that there is more power in a strong wind than in light breeze. The available power in the wind is derived from the kinetic energy of air molecules and particles. The energy in wind is due to the kinetic energy of moving air.

This is given by;
Ke = 0.5 mv2 , where m is mass and v is velocity
 
 

It can be shown that the power in the wind is proportional to the cube of the wind speed.

P a v3
 
 

This means that small increases in wind speed produce large increases in available wind power.
 

By slowing the wind down. The kinetic energy in the wind is transferred to kinetic energy of the windmill.

Do you think all the energy all the energy could be extracted from the wind?
Unfortunately not. If the windmill absorbed all the energy, the velocity of the air molecules striking the blade would be to zero. However, after striking the blade the air molecules continue to move.

Scientist has shown that maximum percentage of the wind energy that can be extracted is 59.3%. This limit is known as the Betz limit.
 

Wind-Generators

A wind –generator is a combined windmill-generator system that is used to produce electricity. Many different types of wind-generators ranging in output from less than 1 Kw bined windmill-generator system that are used to produce electricity. Many different types of wind-generators ranging in output from less than 1 kW to 3 MW of electricity have been built. Diagrams of two different designs are shown below.
How does the generating capacity of large 3 MW wind-generator compares with the generating capacity of Eskom's coal fire power stations?

Eskom's coal-fired power stations generations generate up to 3000 MW electrical power.
In order to produce the same amount of power, one hundred 3MW wind-generators would have to be built.

What could be done to supply electricity when the wind drops or even stops?

The wind-generator could be connected to rechargeable battery storage system. When the wind blows the batteries is charged. When the wind stops the batteries provide electricity. A wind-generator could also be combined with a diesel generator. Using this combination, diesel can be saved.
If large wind-generators are connected to the national electricity grid, energy storage isn't necessary. The wind-generators contribute power whenever sufficient wind available.

Large wind-generators are connected to the national electricity grid. Wind-generators arranged together are called a wind farm or park. There are approximately 400 wind farms around the world, most of them in North America and Europe.

To date, the United States is in the forefront of the wind energy utilisation. Approximately 1400 wind-generators have been installed in California where generous government tax incentives increased the economic attractiveness of wind generated electricity.
 
 

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