Wind, which actually is also a form of solar energy, is generated by the uneven heating of the Earth and its atmosphere by the sun. As air is heated, it expands and rises because its density decreases. To replace the rising hot air, cooler air from elsewhere flows into the region, producing wind. Topographic features such as hills, mountains, valleys or large open spaces can also influence the speed, density, and direction of the wind.
The utilization of wind power, not unlike water power, dates back thousands of years. The Egyptians built and used sailing boats in 2000 bc, while later sailing ships made world exploration and travel possible, allowing Columbus to discover America and Ferdinand Magellan to complete the first voyage around the world. It was not until the end of the 19th century that wind, as the prime energy source for maritime transportation, declined rapidly with the development of steam-powered vessels. On land, windmills provided energy for milling and pumping of water in early Mediterranean and Eastern civilizations, as well as in medieval Europe. Windmills continued to play an important role well into the 20th century in the rural United States or Australia, providing energy for isolated farms, mostly to pump water for irrigation and livestock. In the late 19th century, windmills also began to be used to produce electricity, making it possible for farmers and ranchers far from electricity distribution lines to generate their own electricity and to operate small electrical devices such as a radio. Their use was however progressively eliminated before World War II by the availability of cheaper, more reliable, fossil fuel- or water-based electricity from central generating plants delivered through steadily extending electrical transmission grids. Until the early 1970s, the era of cheap oil and gas induced very little interest in electricity generated by wind, and it was only with the oil price shocks of the 1970s and early 1980s that efforts were made to develop and design new wind machines capable of producing electrical power efficiently at a competitive price. In the early 1980s, state subsidies, incentives and tax credits began to revitalize the wind energy industry. Almost all growth was concentrated in the United States, as well as Denmark. In 1985, the United States alone accounted for more than 90% of the installed generating capacity, with 1 GW, and had the world's largest windmill facility at the Altamont Pass, near San Francisco. With the expiration of tax incentives in the United States in 1985 and declining oil prices however, wind power development came to a standstill. Subsequently, thanks to improvements in turbine designs as well as the active promotion of wind energy, development was restarted in the 1990s and was this time led by European countries such as Germany, Denmark, and Spain. Over the past decade, the installed capacity has increased from 2500 MW in 1992 to over 47000 MW at the end of 2004, at an annual growth rate of almost 30% . Today, wind power is the fastest-growing renewable energy resource, and indeed the fastest growing of any energy source. In Denmark, wind produces 20% of the country's electricity needs. The spectacular increase in wind power capacity in recent years has placed Germany, with an installed capacity close to 17000 MW, meeting 7% of the national electricity needs, as the world leader in power production from wind, followed at a distance by Spain with an installed capacity of over 8000 MW . Almost three-fourths of the world's wind power capacity is now concentrated in the European Union, where wind met 2.4% of the electricity demand in 2003. Nine of the ten largest wind turbine manufacturers are based in Europe, and European companies control some 90% of the global wind energy market. Outside Europe, the United States and India are leading markets. Substantial development of wind energy is also expected in numerous countries, including Brazil, Canada, Australia, and China (Fig. 8.8).
When considering the installation of a wind-power generating facility, the single most important factor is the wind speed. As the power increases in relation to the cube of the speed, a doubling of the average wind speed induces a wind power increase by a factor of 8. Consequently, even small changes in wind speed can produce large changes in energy output. Therefore, the installation of new production capacity must be preceded by careful studies of wind flow conditions. Relatively constant winds, even with medium speeds, during extended periods of time are more suitable for reliable power generation than faster but more intermittent or unstable winds. Once the wind resource has been explored, wind turbines have to be constructed to transform the wind energy into useful electrical power. Modern wind turbines generally consist of a three-bladed rotor connected to an electric generator, able to operate over a large range of wind speeds and fixed on the top of a hub. In most cases clusters containing up to several hundreds of wind turbines are concentrated in so-called wind farms (Fig. 8.9). Over the past 20 years, the power of these turbines has increased dramatically as the rotor diameter increased from around 20 m and generating capacities of 20-60 kW, to more than 100 m and generating units over 2 MW in commercial applications, as well as prototypes close to 5 MW . Larger and taller turbines can also take better advantage of the fact that wind speed increases with increasing height above the ground level. These increases in size and technological know-how, coupled with the economy of scale from a fast-growing production, have greatly
reduced the cost of wind power to the point that some of the most productive onshore wind farms are approaching price competitiveness with energy from fossil fuels. The exploitable onshore wind resource for the European Union is conservatively estimated at 600 TWh and the offshore wind resource up to 3000 TWh . Taken together, this may represent more than the present total electricity consumption of the European Union. Onshore wind potential is being developed first because of obvious lower installation costs compared to offshore facilities.
To date, only Europe has installed wind capacity offshore, mainly in the North Sea, in relatively shallow waters and using large turbines to minimize the cost per unit of energy produced. However, because of the considerable potential of offshore facilities and often higher average wind speeds and low turbulence at those sites, projects are under way all around the World.
One major problem with wind-generated electricity is that the resource is highly variable, changing on a daily, seasonal, and annual basis. Therefore, power output prediction can be very challenging and additional generating capacity, as well as sufficient backup power, may be necessary in case of low wind speeds. Wind turbines being constructed in exposed, highly visible areas, also raise public resistance because of the visual impact of wind farms as well as noise levels, which should also be taken into account by building them far from populated areas and sensitive landscapes. Although only limited land is necessary for the installation of each turbine, they must be placed at sufficient distances in order not to interfere with each other, and this can result in extensive land requirement for their establishment. However, as the footprint of the wind tower is small, much of the land comprised in wind farms can be used for other purposes such as agriculture or grazing livestock. On the other hand, wind farms also represent other environmental hazards by killing birds and thus interfering with ecosystems. Nonetheless, the environmental impacts can be greatly reduced or eliminated by the construction of offshore wind farms.
Based on the large and widespread resources, costs, maturity of the technology and relatively limited environmental impacts, wind power has a promising future in supplying an increasing share of electricity in countries willing to invest in it. The absence of greenhouse gas and pollutant emissions is also a strong advocate for electricity generation from wind.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.