I.

Introduction

Wind Energy, energy contained in the force of the winds blowing across the earth’s surface. When harnessed, wind energy can be converted into mechanical energy for performing work such as pumping water, grinding grain, and milling lumber. By connecting a spinning rotor (an assembly of blades attached to a hub) to an electric generator, modern wind turbines convert wind energy, which turns the rotor, into electrical energy.

Wind is created when air that has been warmed over sun-heated land rises, leaving a vacuum in the space it once occupied. Cooler surrounding air then rushes in to fill the vacuum. This movement of rushing air is what we know as wind.

Egyptians may have been the first to capture wind energy when they sailed boats up the Nile River beginning around the 4th century bc. For centuries afterward, wind-powered sailing vessels plied the world's seas and oceans, serving as the principal form of commercial transport. Wind energy has been harnessed on land since the first windmill was developed by the ancient Persians in the 7th century ad. Windmills have since been used to mill grain, pump water, saw timber, and provide other forms of mechanical energy.

Because wind is a clean and renewable source of energy, modern wind turbines had been installed in 26 countries by 2007, including such nations as Germany, Denmark, India, China, and the United States, to supplement more traditional sources of electric power, such as burning coal. Design improvements such as more efficient rotor blades combined with an increase in the numbers of wind turbines installed, have helped increase the world’s wind energy generating capacity by nearly 150 percent since 1990. In 2006 the United States became the world’s third largest producer of energy from wind power, generating more than 11.5 megawatts of electricity.

II.

Evolution of the Windmill

The oldest known windmills were crude, simple devices used in the 7th century by the Persians (a region now occupied by Iran). Europeans made extensive use of the windmill beginning in the 12th century, providing mechanical energy for pumping water, sawing lumber, and grinding grain. In the United States, the windmill was used to pump water on homesteads across the American frontier. In the late 20th century, windmills were developed to convert wind energy into electric power.

A.

Ancient Persian Windmills

Early Persian windmills were crude devices consisting of a simple tower supporting an array of paddles made from bundled reeds. These paddles spun around a vertical axis, with a wall to protect the blades as they spun back into the direction of the wind. These early windmills were used for grinding grain.

B.

Traditional European Windmills

Traditional European windmills have been used for centuries on the lowlands of northern Europe. In fact, the term windmill derives from using these machines to grind, or mill, grain. The first windmills to appear in Europe were built during the 12th century in northwestern France and southern England. Use of the windmill subsequently spread into northern Belgium, Germany, and north to Denmark during the late 12th and 13th centuries.

Only with wind energy could Jan Leegwater and the Dutch engineers that followed him drain the wetland areas of the Netherlands and make them habitable. European windmills were also used for sawing timber, shredding tobacco, manufacturing paper, pressing flaxseed for oil, and grinding stone for paint dye. The 700 windmills erected in the Zaan district north of Amsterdam formed the core of what evolved into the center of Dutch manufacturing—an area that eventually helped launch the Industrial Revolution.

Europeans, unlike their Persian counterparts, developed windmills with rotors that turned around a horizontal axis. Typical European windmills used four blades, although some used five, and occasionally even six. The earliest European windmills placed the tower holding the windmill rotor on a vertical post. This allowed the entire windmill to turn and face the wind. Many of these short postmills, as they are called, are still standing in northern Europe.

Toward the end of the 14th century, these postmills evolved into the traditional European tower windmills—some with towers up to three stories high. The rotors of these windmills are attached to a rotating tower cap, allowing the windmill operator to point the rotor blades into the wind by turning the cap. Many of these European tower windmills contain two or three interior levels where goods milled or manufactured inside the windmill—including grain, lumber, paint, and tobacco—could also be stored.

European windmill performance increased greatly over the next 500 years. The typical windmill evolved into a tower built of wood, stone, or brick that supported a rotor with four cloth-covered blades that acted like sails. This rotor, spanning a diameter of 25 meters (80 feet), was capable of delivering 25 to 30 kilowatts of mechanical power. Technical innovations to the European windmill included the multi-blade fantail protruding behind the rotor to automatically keep the rotor pointing into the wind; air brakes; automatically adjusting slats on the blades (instead of cloth); and blades with airfoil-shaped leading edges that anticipated modern aircraft wings. During the zenith of the European windmill (which ended in the late 19th century when the steam engine came into widespread use), some 1,500 megawatts of power were being produced, a level not reached again until 1988.

C.

American Farm Windmill

Use of windmills began subsiding in Europe during the 19th century, but across the Atlantic Ocean, the American farm windmill was being used by homesteaders to settle the American frontier. The American farm windmill, ideally suited for pumping water from deep underground, became an integral part of agricultural communities across the American West. One historian credits the farm windmill—along with the Colt .45 revolver and the barbed-wire fence—with enabling European settlement of the American Great Plains.

A series of improvements were made during the 100-year reign of the American water-pumping windmill. Early farm windmills used blades made from simple wooden slats; American engineer Thomas Perry improved on these blades in the late 1800s. Using a steam-driven model, Perry conducted scientific tests that led to his invention of stamped sheet-metal “sails” that nearly doubled the rotor’s efficiency. Based on Perry’s improved rotor design, American businessman LaVerne Noyes built the world's most successful farm windmill, the Aermotor. The Aermotor was not the first windmill to use metal blades, but its stamped sheet-metal “sails” proved so efficient that they revolutionized the farm windmill and are still in use today.

The American water-pumping windmill is legendary for its reliability and efficiency, and is indispensable for its ability to pump water from deep under the plains. However, it only produces about one-tenth the power of an equivalent-size modern wind turbine. This poor efficiency is why the multi-blade farm windmill was not successfully adapted for generating electricity. Although the American farm windmill industry peaked in the early part of the 20th century, more than 1 million of these machines are still in use worldwide.

III.

The Modern Wind Turbine

During the 1930s, interest in powering electric lighting and appliances on homesteads across the Great Plains led to the development of small, battery-charging wind turbines. These so-called windchargers were early forerunners of the small two- and three-bladed wind turbines used today to provide electricity for remote residences, and to provide electricity to villages in developing countries.

The oil crisis of the 1970s spurred efforts into developing wind energy as an alternative source of electrical power. Many countries launched programs to develop modern wind turbines. While many of these programs failed, Denmark was successful in developing modern wind turbines. Countries such as the United States have adopted this technology to develop wind energy resources.

The modern wind turbine is the result of design and material advances made during the 1980s and 1990s, which have enabled wind turbines to become increasingly efficient. Today, wind turbines the same size as the traditional European windmill can generate 250 to 300 kilowatts of power—a nearly tenfold increase in efficiency. Awareness of global warming also spurred interest in developing wind energy during the late 20th and early 21st centuries. In many locations wind energy offered an attractive alternative to fossil fuels, which release carbon dioxide, a greenhouse gas, into the atmosphere.

A.

Wind Energy System Components

Modern wind energy systems consist of three basic components: a tower on which the wind turbine is mounted; a rotor that is turned by the wind; and the nacelle, which houses the equipment, including the generator, that converts the mechanical energy in the spinning rotor into electricity. The tower supporting the rotor and generator must be strong. Rotor blades need to be light and strong in order to be aerodynamically efficient and to withstand prolonged use in high winds.

A.1.

Tower

Improvements in structural design and construction materials have led to the construction of taller towers, allowing rotors to be mounted farther off the ground, where winds are typically stronger. Small wind turbines (less than 1 kilowatt) are installed on simple guyed (cable-anchored) poles ranging in height from 10 to 20 m (30 to 65 ft). Turbines from 1 to 30 kilowatts are installed on lattice or tubular towers ranging in height from 20 to 40 m (65 to 130 ft). Medium-size wind turbines are typically installed on tubular steel towers ranging in height from 25 to 50 m (80 to 165 ft). Wind turbine towers, which house the cables that conduct electricity from the generator through the base of the tower, can be constructed from metal, reinforced plastics, and concrete.

A.2.

Rotor

The rotor, which spins when driven by the wind, supports blades that are designed to capture kinetic energy in the wind. Nearly all modern wind turbines have rotors that spin about an axis parallel to the ground. The spinning rotor turns a shaft which converts the wind’s energy into mechanical power. In turn, the shaft drives the generator, which converts mechanical energy into electricity. Although some modern wind turbines have rotor blades made of composite wood, most modern wind turbine blades are made of fiberglass, a lightweight, strong material typically composed of polyester resins and glass fibers. Unlike the American farm windmill, contemporary wind turbines do not use blades made from aluminum or steel; aluminum is unable to withstand continuous stress from flexing in strong winds, and steel is too heavy. Small wind turbines (see Wind Turbine Size section below) typically use a tail vane to keep the rotor pointing into the wind. Most medium-size wind turbines use an electric motor to mechanically aim the rotor into the wind.

A.3.

Generators

The generator converts the mechanical energy of the spinning rotor into electricity (see Electric Motors and Generators). Most wind turbines use a generator and transmission in combination. Many of these wind turbines use two generators, a small generator for light winds and a large generator for strong winds. Other wind turbines use a single generator that contains dual electric windings. These dual electric windings accomplish the same task as the combination of a small and a large generator. Some wind turbines use another type of specially designed generator that is driven directly by the rotor without a transmission.

B.

Wind Turbine Size

Wind turbines can be arbitrarily divided into three classes: small, medium, and large. Small wind turbines are capable of generating between 50 watts and 60 kilowatts of power, and use rotors ranging in diameter from less than 1 to 15 m (3 to 50 ft). Small wind turbines are installed primarily in remote areas where power is needed but access to conventional sources of electricity is either too expensive or too unreliable. Some small turbines, known as micro-turbines, are so compact they can be carried to remote locations on horseback.

Most commercial wind machines are medium-size turbines. Medium-size turbines use rotors spanning diameters between 15 and 60 m (50 and 200 ft), and have a generating capacity ranging from 50-1,500 kilowatts. Most medium-size commercial turbines have a generating capacity in the range of 500 kilowatts to 750 kilowatts.

Large wind turbines are behemoths with rotors spanning diameters between 60 and 100 m (200 to 330 ft), and are capable of generating 2 to 3 megawatts of power. Because the cost-effectiveness of conventional coal-fired and oil-fired power plants increases with the size of the plants, it was originally thought that giant wind turbines would be more economical than smaller turbines. Various countries have attempted to develop commercial multi-megawatt wind turbines, but these machines have proved less economical and less reliable than medium-size turbines.

IV.

The Modern Wind Energy Industry

Many countries began exploring alternative sources of energy during the oil shortages of the 1970s. As improvements in wind energy technology have evolved, the modern wind energy industry has emerged. Concern about global warming and adoption of the Kyōto Protocol in 1997 also spurred interest in wind energy as an alternative to burning fossil fuels, which release greenhouse gases into the atmosphere. Increasingly, modern wind turbines produce electric power as efficiently as other power generation technologies. One of the major obstacles for developing wind energy is finding suitable terrain and wind conditions.

A.

Placement of Wind Turbines

Location is critical for maximizing the electricity wind turbines can produce. The amount of kinetic energy available in the wind is a cubic function of wind speed—that is, for every doubling of wind speed, there is a corresponding eight-fold increase in available energy. This exponential relationship between wind speed and wind energy makes location extremely important. A site with high average wind speeds can provide considerably more wind energy than a site with only slightly lower wind speeds.

For most continental locations, winds are strongest during winter and spring and weakest during summer and fall. Regional weather patterns and local topographic conditions can also cause wind patterns to vary. For example, wind speeds in California’s Altamont Pass are highest during summer months when temperature differences between the hot Central Valley and the cold waters of the Pacific Ocean are greatest. These high winds are created by cold Pacific Ocean air rushing in to fill the vacuum created by hot, rising air in the Central Valley.

B.

Wind Power Plants

Wind turbines can be installed in single units, in clusters of two to ten turbines, and in large arrays, called wind power plants or wind farms. Wind power plants can contain large numbers of wind turbines. California's Tehachapi Pass contains several wind power plants, each with more than 1,000 wind turbines.

Wind turbines aggregated into wind power plants are thought to generate electricity more economically than single turbines or those in clusters. It can be more cost-effective to operate and maintain large arrays of wind turbines; however, concentrating wind turbines can reduce individual turbine production when turbines upwind disrupt the airflow of turbines downwind.

The arrangement of wind turbines in a wind power plant is often determined by local geography. Wind power plants on flat terrain are often assembled in long parallel rows. One of the world's most visually pleasing wind power plants is Denmark’s Tændpibe-Velling Mærsk, a geometric array assembled in marching-band formation on the country’s Jutland Peninsula.

In hilly or mountainous terrain running perpendicular to prevailing winds, designers often line the ridgetops with long rows of wind turbines. This formation is used in several wind power plants in California's Altamont Pass. Wind turbines may also be placed in long single rows along other windy, exposed land features. Throughout the Netherlands, linear arrays are placed parallel to many of the country's dikes and drainage canals. Long rows of wind turbines are also located along harbor breakwaters at Ebeltoft in Denmark, Zeebrugge in Belgium, and Blyth Harbor in England.

Like conventional power plants, wind farms are an assembly of multiple independent generators—in this case, wind turbines. Although each wind turbine in a wind power plant operates independently, the turbines are typically connected to a central monitoring system. Whether power is produced from two turbines or from two thousand turbines, the power is aggregated and delivered to an electric utility network.

California is home to some of the largest arrays of wind turbines in the world. Wind power plants in the Altamont Pass contain a total of 6,000 wind turbines, and wind farms in the Tehachapi Pass comprise nearly 5,000 wind turbines. Wind farms near Palm Springs include some 3,000 wind turbines.

C.

Reliability of Wind Energy

Wind energy is a promising source of electrical power because it is a clean and renewable resource. However, because wind speeds vary by time of day, season, and even from one year to the next, wind energy is an intermittent resource. At windy sites it is common for wind turbines to operate 60 percent of the year. Even when operating, however, the wind may be insufficiently strong for wind turbines to generate at full capacity. Overall, turbines installed on windy sites operate at an average of 25 to 35 percent of full capacity. In comparison, coal-fired power plants usually operate at an average of 75 to 85 percent of full capacity.

The intermittent nature of wind energy does not affect consumers when wind turbines are tied to an electrical network, or power grid. Many wind turbines in North America, Europe, and parts of Asia are connected to large electricity networks. The effect of windless days can be offset by production from other power-generating sources, such as coal-fired plants and hydroelectric plants that are connected to the power grid (see Energy Supply, World). Such a system helps provide reliable power supplies to consumers. People located in remote sites that rely on electricity from wind turbines often use batteries or a backup generator to provide auxiliary power during extended periods without sufficient wind.

Operationally, modern wind turbines are as reliable as conventional power plants. Most commercial wind turbines are offline (down for maintenance or repair) less than 3 percent of the time. Wind turbines are also known for their longevity—many turbines have been generating electricity since the early 1980s. Many American farm windmills have been in continuous use for generations, while some traditional European windmills have been working for almost 300 years.

V.

Current Issues and the Future

With the growing worldwide demand for electric power and the rising concern about global warming, many experts believe that the use of wind energy will continue to increase. As wind power becomes an increasingly cost-effective source of electricity, the market for wind power should continue to expand. Some environmental and political factors, however, will also influence the growth of wind energy.

A.

Wind Energy and the Environment

Although wind energy is a relatively clean means of generating electricity, there are associated impacts. One of these is the potential of an array of turbines to alter the visual quality of the landscape, especially when located in a scenic area. In addition to aesthetic concerns, noise associated with spinning wind turbine rotors has generated complaints from the public, although recent technology has done much to address this issue.

B.

Politics of Wind Energy

In 2007 the worldwide wind energy industry was increasing generating capacity at a five-year compound annual growth rate of 26.3 percent, nearly twice the rate of the nuclear energy industry. The United States continued to be a leader in wind energy, adding more generating capacity in 2006 than any previous year. Much of this growth was spurred by state initiatives that required public utilities to obtain power from renewable resources. Germany and Spain continued to lead the world in wind energy generation in 2006. Other countries with significant new wind energy industries were Britain, Canada, Italy, Japan, and the Netherlands.