Wind is generated by atmospheric pressure differences, driven by solar power. Of the total of 175,000 TW of solar power reaching the earth, about 1200 TW (0.7%) are used to drive the atmospheric pressure system. This power generates a kinetic energy reservoir of 750 EJ with a turnover time of 7.4 days (Soerensen, 1979). This conversion process takes place mainly in the upper layers of the atmosphere, at around 12 km height (where the "jet streams" occur). If it is assumed that about 4.6% of the kinetic power is available in the lowest strata of the atmosphere, the world wind potential is on the order of 55 TW. Therefore it can be concluded that, purely on a theoretical basis and disregarding the mismatch between supply and demand, the wind could supply an amount of electrical energy equal to the present world electricity demand.
As a consequence of the linear relationship between wind speed and wind power (and hence energy), one should be careful in using average wind speed data (m/s) to derive wind power data (W/m2). Local geographical circumstances may lead to mesoscale wind structures, which have a much higher energy content than one would calculate from the most commonly used wind speed frequency distribution (Rayleigh). Making allowances for the increase of wind speed with height, it follows that the energy available at, say, 25 m varies from around 1.2 MWh/m2/a to around 5 MWh/m2/a in the windiest regions. Higher energy levels are possible if hilly sites are used or local topography funnels a prevailing wind through valleys.
In terms of capacity, wind energy is the most widely used renewable energy source. Today there are many wind farms that produce electricity. Wind energy is, in fact, an indirect activity of the sun. Its use as energy goes as far back as 4000 years, during the dawn of historical times. It was adored, like the sun, as a god. For the Greeks, wind was the god Aeolos, the "flying man." After this god's name, wind energy is sometimes referred to as Aeolian energy (Delyannis, 2003).
In the beginning, about 4000 years ago, wind energy was used for the propulsion of sailing ships. In antiquity, this was the only energy available to drive ships sailing in the Mediterranean Basin and other seas, and even today, it is used for sailing small leisure boats. At about the same period, windmills, which were used mainly to grind various crops, appeared (Kalogirou, 2005).
It is believed that the genesis of windmills, though not proven, lay in the prayer mills of Tibet. The oldest, very primitive windmills have been found at Neh, eastern Iran, and on the Afghanistan borders (Major, 1990). Many windmills have been found in Persia, India, Sumatra, and Bactria. It is believed, in general, that many of the windmills were constructed by the Greeks, who immigrated to Asia with the troops of Alexander the Great (Delyannis, 2003). The earliest written document about windmills is a Hindu book of about 400 B.C., called Arthasastra of Kantilys (Soerensen, 1995), in which there is a suggestion for the use of windmills to pump water. In Western Europe, windmills came later, during the 12th century, with the first written reference in the 1040-1180 A.D. time frame (Merriam, 1980).
The famous Swiss mathematician, Leonhard Euler, developed the wind wheel theory and related equations, which are, even today, the most important principles for turbogenerators. The ancestor of today's vertical axis wind turbines was developed by Darrieus (1931), but it took about 50 years to be commercialized, in the 1970s. Scientists in Denmark first installed wind turbines during the Second World War to increase the electrical capacity of their grid. They installed 200 kW Gedser mill turbines, which were in operation until the 1960s (Dodge and Thresler, 1989).
The theoretical maximum aerodynamic conversion efficiency of wind turbines from wind to mechanical power is 59%. However, the fundamental properties of even the most efficient of modern aerofoil sections, used for blades of large-and medium-size wind turbines, limit the peak achievable efficiency to around 48%. In practice, the need to economize on blade costs tends to lead to the construction of slender-bladed, fast-running wind turbines with peak efficiencies a little below the optimum, say 45%. The average year-round efficiency of most turbines is about half this figure. This is due to the need to shut down the wind turbine in low or high winds and limit the power once the rated level is reached. Further, a reduction of the average efficiency is caused by generator (and gearbox) losses and because the machine does not always operate at its optimum working point (Beurskens and Garrad, 1996).
Wind turbines represent a mature technology for power production, and they are commercially available on a wide range of nominal power. In spite of the technology's maturity, new control strategies and improved energy storage
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.