INTRODUCTION: WHAT IS WIND ENERGY?
The word "windmill" for many people brings to mind the Netherlands, whose countryside for centuries has been dotted with thousands of windmills. Windmills represent an early technical skill or ingenuity (inventiveness) that seemed to be lost during the industrial revolution, when fossil fuels replaced wind and running water as the most widely used energy sources. Some people of the twenty-first century support a return to greater reliance on the wind that powers windmills, chiefly because wind power is clean and endlessly renewable.
The first written record of a windmill is in a Hindu book from about 400 BCE (before the common era). About four hundred years later, the Greek inventor Hero of Alexandria devised a wind-driven motor he used to provide air pressure to operate an organ. From about 400 CE (common era), there are references to prayer wheels driven by wind and water in the Buddhist countries of central Asia. These devices were handheld windmills that contained prayers and religious texts on rolls of thin paper wound around an axle. Individuals could access the prayers whenever they wanted (the thought was increasing the speed of the spinning prayer wheels strengthened the prayers). Early devices used the power of the wind, but it was not until much later that wind power was developed as a way to do work.
Some historians believe that the earliest true windmills—that is, windmills built to do work—were built in China two thousand years ago, but no records exist. The first recorded references to true windmills date from seventh-century Persia, later called Iran, particularly the province of Sijistan, which became Afghanistan. During the reign of the Muslim caliph 'Umar I (633-44), windmills were constructed primarily to obtain water for irrigating crops and grinding grain. These working windmills may have been imported into China from the Middle East by Genghis Khan (1162–1227), the Mongol conqueror of much of what is now Iran and Iraq (1216–23). The first reference to a Chinese windmill dates from the year 1219, when a statesman named Yehlu Chhu-Tshai documented construction of one. Windmills became widely used along the coasts of China during this period.
The design of these seventh-century windmills, some of which survive in Iran and Afghanistan, was the reverse of modern windmills. In modern windmills the axle is horizontal and is positioned at the top of the windmill. In early Middle Eastern windmills the blades that turned in the wind were enclosed in a chamber at the bottom of the windmill. The blades were attached to a vertical axle, which was attached to a millstone above. The early windmills, which are still used, could grind a ton of grain per day and generate about one-half the power of a small car.
Words to Know
- A device used to measure wind speed.
- Coriolis force
- The movement of air currents to the right or left caused by Earth's rotation.
- The slowing force of the wind as it strikes an object.
- One kilowatt of electricity consumed over a one-hour period.
- Kinetic energy
- The energy contained in a mass in motion.
- The aerodynamic force that operates perpendicular to the wind, owing to differences in air pressure on either side of a turbine blade.
- The part of a wind turbine that houses the gearbox, generator, and other components.
- The hub to which the blades of a wind turbine are connected; sometimes used to refer to the rotor itself and the blades as a single unit.
- The loss of lift that occurs when a wing presents too steep an angle to the wind and low pressure along the upper surface of the wing decreases.
- Wind farm
- A group of wind turbines that provides electricity for commercial uses.
Windmills in Europe
During the Crusades, which took place over a two-hundred-year period beginning in 1095, European conquerors of Palestine probably became familiar with Middle Eastern windmills and imported the technology back to Europe. The first documented reference to a European windmill dates to 1105 in France, the home of most of the early crusaders. A similar reference is made to a windmill in England in 1180. Both of these windmills were built to pump water to drain land.
For reasons that are unknown, the Europeans mounted the windmill blade on a horizontal axle rather than a vertical one. They may have adopted the design from water wheels, which by this time were being mounted on horizontal axles (poles around which an object rotates). Some of the windmills from this period were able to lift more than 16,000 gallons (60,566 liters) of water per hour, using augers (a type of screw) that raised the water from lower levels to higher levels, where the water could be sent into channels. The augers acted like spiral staircases that carried the water up as the windmills turned. These windmills were often arranged in what were called gangs, meaning that they were arranged in rows so that water could be drained in stages, especially from lower to higher levels.
Because much of the Netherlands is below sea level, the Dutch made extensive use of windmills to drain land and to grind grain. By the fourteenth century the Dutch had introduced or adopted a number of technologies, such as post mills and tower mills. The post mill consisted of a four-bladed mill mounted on a central vertical post or shaft. Wooden gears transferred the power of the shaft to a grindstone. The grindstone turned to make grain into flour. The tower mill, which originated along the Mediterranean seacoast in the thirteenth century, consisted of a post mill mounted on top of a multistory tower. This tower housed the grinding machinery and had rooms for grain storage and other milling functions as well as living quarters in the bottom story. The tower mill is the type most often seen in pictures of Dutch windmills.
A major concern of windmill operators was to make sure that the mill was positioned correctly in relation to the wind. This task was done with a large lever at the back of the windmill that was pushed to move the windmill blades toward the wind. The blades were made of lattice frames over which canvas sails were stretched. By 1600, windmills were in such widespread use in Holland that the bishop of Holland, seeing a chance to increase funds for the church, declared an annual tax on windmill owners.
Al-Dimashqi Describes a Windmill
In the thirteenth century, the Arab historian al-Dimashqi (1256–1327), described a windmill:
When building mills that rotate by the wind, they proceed as follows. They erect a high building, like a minaret, or they take the top of a high mountain or hill or a tower of a castle. They build one building on top of another. The upper structure contains the mill that turns and grinds, the lower one contains a wheel rotated by the enclosed wind. When the lower wheel turns, the mill stone above also turns…. Such mills are suitable on high castles and in regions which have no water, but have a lively movement of the air.
Also by that time the basic technology of windmills was in place. It remained for engineers and inventors to find ways to increase efficiency, primarily by coming up with new designs for windmill blades. Some of these designs included improvements in the blade's camber, or the outward curve of the blade from its leading edge (the edge first struck by the wind) to its trailing edge. Other experiments were conducted to find the best location for the blades spar, or the long piece of a blade; its center of gravity; and the correct amount of twist in the blade. One of the most prominent millwrights (mill builders) during the period, Jan Adriaanzoon Leeghwater (1575–1650), experimented with these matters. Largely through his efforts, about twenty-six lakes in the Netherlands were drained.
By the end of the nineteenth century, at least 30,000 windmills were operating in Europe. These windmills were used not only to pump water and grind grain but also to power sawmills and for other industrial uses, including processing agricultural products such as spices, cocoa, dyes, paints, and tobacco.
Windmills in North America
In the seventeenth and eighteenth centuries, the Dutch migrated to the American colonies in large numbers. They brought with them the technology for constructing windmills, and many Dutch-style windmills were built throughout New York and New England, where they worked well in the relatively gentle eastern winds.
In the nineteenth century, American settlers moved westward and onto the Great Plains. The settlers wanted to harness the power of the wind to irrigate the land and water their cattle. However, on the plains a fundamental design flaw in Dutch windmills became apparent: The slow-moving blades were too fragile for the strong winds that swept across the prairies in places such as Kansas and Nebraska. As soon as they were hit with high winds, these windmills fell apart.
Back in New England, a designer named Daniel Halladay (1826–?) patented a design that could withstand the high winds of the plains. His company, the Halladay Windmill Company, began building windmills with the new design in 1854. The chief improvement Halladay made was to use numerous blades, rather than the four blades that were common on New England windmills. The new windmills also had a tail that would orient them to the wind, and they had hinged blades that would fold up in high winds so that they would not fall apart. In 1857 Halladay's company began doing business as the U.S. Wind Engine and Pump Company.
In about 1870 windmill manufacturers made another improvement when they began using steel rather than wood in the manufacture of blades. These blades were stronger but also could be curved, making them much more efficient than the flat wooden blades in use up to this time. In 1886 the inventor Thomas Perry designed a more aerodynamic blade, a blade that gets the most power from the wind and a design that continues to be used in the early twenty-first century.
Halladay's company, along with numerous competitors, sold thousands of windmills. Many windmills were sold to farmers and ranchers, but another industry emerged as a major customer. The railroads needed large amounts of water for their steam engines at their many stops across the plains and on to the West Coast. Windmill-powered pumps pumped water into tanks at the side of the railroad tracks. Trains could stop at each tank and get water enough to continue the journey to the next tank.
What's in a Name?
One project that Jan Adriaanzoon Leeghwater started in Holland was a drainage plan to protect Amsterdam and Leiden from the Haarlem Meer, a lake that was growing each year and threatening to flood the cities. The project that he began in 1643 was so large that it was not completed until 1852. One of the three pumping stations still operating in the early twenty-first century was named after Leeghwater. The engineer's life course may have been set the day he was born. In Dutch "Leeghwater" means "empty water."
Another major improvement occurred in 1915, when the Aeromotor Company designed an enclosed, self-lubricating gearbox. Until then, the open gears of windmills had to be lubricated every week, often by horse-mounted cowboys who rode out with their saddlebags packed with bottles filled with oil. In windmills with the Aeromotor gearbox, the gears had to be oiled only about once a year.
About one million windmills made by about 300 companies were built in the United States between 1850 and 1970. Although most of these windmills were small, and used on family farms primarily to pump water, others were large, with blades up to 26 feet (8 meters) long. These were purchased mainly by the railroads for their system of track-side water tanks.
The next step in the development of wind energy was electrification. Until the late nineteenth century, all windmills produced only mechanical power for pumping or grinding. With the emergence of electricity, designers and engineers quickly recognized that windmills could be attached to electric generators and that the power they produced could be used for heating and lighting.
The first windmill used to generate electricity on a large scale was built in 1888 by Charles F. Brush (1849–1929) in Cleveland, Ohio. Its rotor, which consisted of 144 blades, was almost 56 feet (17 meters) in diameter. The rotor includes the hub and the blades that are attached to it. Brush's major technical challenge was to find a way for the windmill's rotor to produce the 500 revolutions per minute he needed for the generator to operate. Brush designed a step-up gearbox (a series of parts that transmitted motion from one part of the machinery to another) in a fifty-to-one ratio. This meant that for every turn of the rotor, the operational parts of the generator turned 50 times. During the 20 years it was in operation, the Brush machine produced about 12 kilowatts of power, which Brush stored in batteries in his nearby mansion.
From 1890 to 1930 the windmill industry in the United States boomed. Spurring the boom was the prominent place given to electric windmills at the World's Columbian Exposition in Chicago in 1893, where they were used to generate power to light the fairgrounds after dark. Electric lights were not common in 1893 homes; most still used gaslights. So people were amazed that a cheap source of power could make this new marvel available to them, even if they lived out in the country. However, the windmill industry soon collapsed after the U.S. Rural Electrification Administration, or REA, was established. This government program was one of many created to help the nation overcome the effects of the Great Depression (1929–1941). The REA provided partial federal funding for electricity to homes and farms in rural areas, much of it produced by hydroelectric dams. If these hard-to-reach places could now get inexpensive electrical service from the government, then they no longer needed windmill-generated power.
Decline and revival
From the 1930s to the 1970s in the United States coal and oil remained relatively inexpensive, and little interest was shown in harnessing the wind to meet the need for electricity. In Russia, however, a 100-kilowatt wind generator was built in Balaclava in 1931. Mounted on a tower 100 feet (33 meters) high, the rotor was 100 feet in diameter and produced power when the wind speed exceeded 25 miles (40 kilometers) per hour. The wind generator supplied this energy to a steam power station 20 miles (32 kilometers) away. The turbine did not last very long because the blades were made of old roofing metal and the gears were made of wood. During one year of operation, however, the wind generator produced 279,000 kilowatt-hours of power.
From the mid-1930s until 1970 commercial-sized wind generators were built in Denmark, England, Germany, and France. These countries were left with shortages of fossil fuels and most everything else because of the destruction left by World War II (1939–1945). The development of wind power in Europe filled some of the need for electricity that was not being filled by fossil fuels. In Denmark, for example, a 200-kilowatt wind generator was built and operated until the early 1960s. Denmark led the way in wind-power generation in terms of the percentage of electricity that was wind generated, about 20 percent.
Watts, Kilowatts, and Kilowatt-hours
Electric output is generally measured in watts, named after the Scottish inventor James Watt (1736–1819). A watt is 1/746th of one horsepower (the power of one horse pulling). Because 1 watt is a small amount, power is generally measured in kilowatts, or thousands of watts. Large power-generating stations often measure power output in megawatts, or millions of watts.
By itself a wattage figure does not indicate how much power is being consumed. A 100-watt lightbulb needs 100 watts to operate, but more power is consumed if the light is left on for an hour than if it is left on for a minute. The term "kilowatt-hour" takes into account the time dimension. If a 100-watt bulb is left burning for 10 hours, 1 kilowatt-hour of electricity has been consumed. A typical family in the United States uses about 10,000 kilowatt-hours of electricity each year.
Although Europe was leading the way, the largest commercial-grade wind generator was located on Grandpa's Knob, a 2,000-foot-high (610 meters) hill near Rutland, Vermont. It was called the Smith-Putnam wind turbine after its designer, Palmer C. Putnam, and the company that provided the money to build it, the S. Morgan Smith Company of New York. The generator was built over a two-year period beginning in 1939. The 175-foot-diameter (53 meters) rotor produced an enormous 1.25 megawatts of power during the four years it was in operation. The Smith-Putnam turbine stopped operating when metal fatigue caused some of the blades and bearings to break. Replacements could not be found because metals and other materials were being used by the military to build weapons to fight World War II. Although the Smith-Putnam turbine was not a long-term economic success, it was considered a technical success because it produced a lot of electrical power while it was working.
During the years following World War II, several wind energy designs were built and tested. In England the Enfield-Andreau wind turbine, built in St. Alban's in the 1950s, had a 79-foot (24 meters) rotor that produced 100 kilowatts of power. A unique feature of this turbine was that its hollow propeller blades acted as air pumps for transmitting power from the rotor to the generator.
In Denmark the Gedser wind turbine was built in 1957, and its 79-foot blades produced about 400,000 kilowatt-hours per year until the turbine was shut down in 1968. Also during the 1950s, two large machines were built in France. One produced 130 kilowatts and the other 300 kilowatts. In Germany the Hütters wind turbine achieved great efficiency by producing 100 kilowatts of power in only 18-mph (29 kph) winds. Earlier systems needed higher wind speeds.
During the 1970s it seemed as though the United States was ready to make the necessary investments to develop wind power. In 1973 the country was affected by the Arab oil embargo. Countries that normally sold oil to the United States were refusing to do so. This served as a warning to the nation that it was too dependent on foreign oil, which could be cut off at any moment. In 1974 the U.S. Federal Wind Energy Program was established. Over the next decade scientists from U.S. agencies such as the National Aeronautics and Space Administration (NASA) and the U.S. Department of Agriculture built and tested at least thirteen different wind turbine designs, ranging in output from 1 kilowatt to 3.2 megawatts. Major efforts were made to develop more efficient rotor designs. Many of these designs were successful, and engineers learned to design better ones.
The Coriolis force
The Coriolis (kawr-ee-OH-luhs) force, sometimes called the Coriolis effect, is named after the French mathematician Gaspard-Gustave de Coriolis (1792–1843). The principle behind the Coriolis force is that because Earth rotates, any movement in the Northern Hemisphere is diverted to the right, if observed from a fixed position on the ground. In the Southern Hemisphere, the movement is to the left. This means that wind tends to rotate counterclockwise around low-pressure areas in the Northern Hemisphere and clock-wise in the Southern Hemisphere.
The Coriolis force has a major effect on prevailing wind patterns throughout the world. As equatorial air heats, rises, and moves toward the poles, expansion of the air creates low pressure. Cooler air from the poles flows in behind the warmer air to equalize the pressure. At about 30 degrees latitude north and south, the Coriolis force prevents air from moving much farther toward the poles, because the warmer air encounters a high-pressure area of cooler, sinking air. Because of the diversion of the air caused by Earth's rotation, prevailing winds generally blow in the following directions:
|© 1997–2003 Danish Wind Industry Association. Reproduced by permission. Thomson Gale|
The Coriolis force does not explain wind direction in all places at all times. Local factors also determine the speed and direction of the wind. A good example is a sea breeze. Land masses warm faster in the sun than water does. This means that the air over land expands and rises faster than the air over the sea. As the land air rises,
However, by the late 1980s it was becoming more and more difficult to attract funding for wind energy efforts. Many people remained unconvinced that wind power could ever provide more than small the sea air flows in behind it, causing wind to blow onshore. At night, the process is reversed, and wind tends to blow offshore, that is, from land out to sea. Mountain ranges also play tricks with the wind, diverting it in different directions. amounts of electricity for local use. Since that time research on wind technology has been conducted in the United States largely by the National Wind Technology Center near Boulder, Colorado.
HOW WIND ENERGY WORKS
In everyday discussions of alternative forms of energy, most people make a distinction between wind power and solar power. From one point of view, however, this distinction is unnecessary because the wind that powers wind turbines is itself a form of solar power.
Earth absorbs overwhelming amounts of energy from the sun: 1.74 × 1017 kilowatt-hours, or 174,423,000,000,000 kilowatts every single hour of the day. Although the oceans and land masses absorb a great deal of this energy, much is absorbed by the atmosphere (the whole mass of air surrounding Earth).
The energy from the sun does not strike Earth evenly. Air around the equator absorbs more energy than the air above the poles. This difference causes air, a fluid much like water, to move in currents. Air, like any substance, expands when it is warmed and contracts when it is cooled. Warm air, because it is less dense than cool air, is lighter, so it rises, much like a less-dense piece of wood rises to the top of more-dense water. This effect can be seen by looking at the hot air above a fire, which seems to shimmer as it expands and moves upward, carrying smoke and ash with it. Cold air, because it shrinks, is denser than surrounding warm air, so it sinks. This property explains in part why a freezer generally operates more efficiently when it is placed at the bottom of a refrigerator rather than at the top and why the basement is generally colder than the upper levels of a house.
As warm air rises, colder, heavier air flows in to replace it, causing a current of air—in other words, wind. Earth's rotation also plays a role in wind production. If Earth did not rotate, air heated at the equator would rise only about 6 miles (10 kilometers) into the atmosphere and flow toward the North Pole and the South Pole, where it would cool, sink, and return to the equator. Earth's rotation allows winds to circulate in more or less predictable patterns across the Northern Hemisphere and Southern Hemisphere. These winds contain huge amounts of kinetic (kuh-NET-ik) energy, or the energy contained in any fluid body in motion. About two percent of the solar energy that strikes Earth is converted into wind. For various reasons, including the revolution of Earth and features of its terrain, some parts of Earth have more wind than others.
The southeastern United States has relatively little wind on a steady basis, so this region is generally not considered a good place to place wind turbines. In addition, the storminess in the Southeast would leave wind turbines vulnerable to damage from high winds, during hurricane season, for example. The Rocky Mountain states experience a great deal of wind on a consistent basis, making them better candidates for wind power. The best places to build the turbines are North Dakota, Texas, and Kansas, which by themselves could provide all of the electricity needed in the United States, according to a 1991 U.S. Department of Energy wind resource report.
According to the Battelle Pacific Northwest Laboratory, the top twenty states and the amount of wind power they could produce, measured in billions of kilowatt-hours per year, are as follows:
According to the American Wind Energy Association, by the end of 2004 wind facilities in thirty U.S. states were generating a total of 6,740 megawatts of electricity, enough to provide power for about 1.6 million homes.
The states leading the way were these:
California: 2,096 megawatts
Texas: 1,293 megawatts
Iowa: 632 megawatts
Minnesota: 615 megawatts
Wyoming: 285 megawatts
The largest wind farms, or large facilities with numerous turbines, operating in the United States were the following:
Stateline, Oregon-Washington: 300 megawatts
King Mountain, Texas: 278 megawatts
Storm Lake, Iowa: 193 megawatts
Colorado Green, Colorado: 162 megawatts
High Winds, California: 162 megawatts
The countries that led the world in wind power production in 2004 were as follows:
|World Leaders in Wind Capacity, December 2004|
|Country||Capacity in Megawatts|
|© 2004 American Wind Energy Association. Reproduced by permission. Thomson Gale.|
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CURRENT AND FUTURE TECHNOLOGY
Throughout the twentieth century, engineers experimented with various rotor designs. One was called the Darrieus windmill, named after the person who invented it in the 1920s. Rather than using blades that look like airplane propellers, the Darrieus windmill looks more like a giant eggbeater, with thin blades connected at the top and bottom of a vertical shaft. The Darrieus windmill has the advantage of working no matter which way the wind is blowing. In addition, generators can be mounted at the bottom rather than the top.
The most common type of windmill in the early twenty-first century was called the vertical-axis wind turbine, which had airplane propeller-type blades mounted at the top of a tall tower. This windmill, called the MOD-2, was designed by NASA. Each MOD-2 was mounted on a 200-foot-tall (61 meters) tower. The blades were up to 150 feet (46 meters) long. The MOD-2 could produce about 2,500 kilowatts of power in a 28-mph (45 kph) wind. Other wind turbine rotors may be larger, but their fundamental design owes much to the design of the MOD-2.
The technology of wind-power generation is well-developed. Although refinements in blade configuration and other factors probably can be made, the technology is cost-effective and sound. The major challenge for the future is harnessing the technology on a big enough scale to provide power to large numbers of users.
BENEFITS AND DRAWBACKS OF WIND ENERGY
The chief benefits of wind power are that it is clean, safe, and endlessly renewable. The fuel that powers wind turbines is free, so its price to utility companies does not vary. Wind power does have a number of drawbacks. Wind speed does not remain constant, so the supply of power may not always be the same as demand from consumers. Because many of the best locations for wind turbines are far from urban areas, there are problems with distributing the energy.
Environmental impact of wind energy
Wind power is clean and renewable, but it also raises environmental concerns. Wind power farms require large stretches of land or have to be placed in environmentally sensitive areas such as deserts or on ridgelines. Many people consider wind farms unsightly, a form of visual pollution. A major environmental concern is the effect of wind farms on patterns of bird migration. Many birds have been killed by flying into wind turbine blades.
Economic impact of wind energy
The cost of generating electricity with wind power has steadily decreased. Wind-power electricity can be generated for about four to six cents per kilowatt-hour, making wind power competitive with other forms of generation of electricity.
Societal impact of wind energy
The societal impact of wind power is similar to that of many other renewable fuels. About two billion people worldwide do not have electricity. Many of these people live in areas where connecting them to the power grid would be extremely expensive. Wind power may be an alternative way to provide power to these people, improving their quality of life.
Wind power also may reshape the way people think about electricity and their place in a nation's power distribution system. Most electric power is provided by huge facilities, which often are far from the consumer's home or business. Wind power, at least for the near future, is likely to be generated closer to home, in communities and even at the neighborhood level. As fossil fuels become increasingly more expensive and eventually are depleted, alternative energy, including wind, solar, tidal, and wave power generated locally, may contribute to a sense of people belonging to communities rather than to large, anonymous societies. Decisions about power supplies and distribution would be made close to home in response to local needs.
In the early twenty-first century, wind turbines are mainly used to produce electricity. Some turbines are on wind farms and contribute electricity to the power grid for commercial use. Remote areas also use turbines, providing electricity to small villages that are too far away from the transmission lines of the commercial areas. Turbines have other uses besides producing electricity, such as pumping water, and ice making. Near oceans there is some use of wind turbines to help remove the salt from the ocean water.
How wind turbines work
The technology of wind turbines is simple. Wind turbines capture the kinetic energy of wind with, in most cases, two or three blades shaped much like airplane propellers. These blades are attached to a tower that rises at least 100 feet (30 meters) above the ground. At this height air currents tend to be stronger but less turbulent than they are at ground level. When the wind strikes the blade, the angle and configuration of the blade form a pocket of low pressure on the downwind side of the blade. This low pressure sucks the blade into movement, causing the rotor to turn. Force is added by the high pressure on the upward side of the blade. In aerodynamic theory, this property is called lift. If the blade is designed correctly, lift is stronger than drag, or the slowing force exerted by the wind on the front of the blade.
In wind turbines lift and drag work together to make the entire mechanism spin like a propeller. In earlier windmills drag rather than lift was the force that turned the blades. The process is the opposite of that of a fan. With a fan electricity is used to make wind. With a wind turbine wind is used to produce electricity. The turning rotor of a wind turbine is connected to a shaft, which is connected to an electric generator. Power can be distributed to users over the electric grid in exactly the same way any other electric power is distributed.
The most important feature in the operation of wind turbines is lift. To achieve lift, wind turbine designers have borrowed technology from aircraft designers. In cross-section an airplane wing looks like an irregularly shaped teardrop. The shape is irregular because the wing's bottom is slightly flatter than the top, which is more curved. When a plane flies, its wings slice through the air, creating wind. Because of the curve of the upper surface of the wing, the air has to flow faster to get around the wing. At the same time, the air flows at a lower speed along the bottom surface of the wing. Because of the difference in speed, the air above the wing is less dense; that is, the air pressure is lower than the pressure of the air below the wing. This difference in pressure creates lift perpendicular to the direction of the moving air, allowing the plane to fly. The same principle applies to turbine blades.
Unlike airplane wings, wind turbine wings are almost always twisted. The reason they are twisted has to do with another aerodynamic principle, stall. When an airplane wing is tilted back, the wind continues to flow smoothly along the bottom surface, but along the top surface, because of the steeper angle presented to the wind, the air no longer sticks to the wing but swirls around in a circle above it. The result of this swirling is the loss of the low pressure along the upper surface of the wing. Without this low pressure, the plane has no lift and drops like a rock.
Unlike airplane wings, wind turbine blades are constantly rotating, and the speed of the rotation differs along the entire length of the blade. At the precise geometric center, the speed of rotation is zero. This speed steadily increases along the length of the blade until at the tip the blade can be moving hundreds of feet (meters) per second. This rotation changes the direction at which the wind hits the blade all along its length. In effect, the angle at which the wind hits the blade would be different at each point along the blade if the blade were not twisted. When the blade is twisted, the angle at which the wind hits the blade is the same at each point, and stall is eliminated under normal wind conditions. Excessively high wind speeds can damage rotors, however, so engineers have designed blades that stall when the wind is too strong, and the rotor stops spinning.
Wind turbines come in two configurations. One, called a vertical-axis turbine, looks much like an oversized eggbeater. The axis of the turbine is positioned vertically, and the blades are connected to the axis at the top and the bottom. This configuration has one primary advantage: The turbine does not have to be faced into or away from the wind, so it operates no matter which way the wind is blowing, and it does not have to be repositioned to accommodate changes in wind direction.
The other configuration, the horizontal-axis turbine, is much more commonly used. With this style, the axis is parallel to the ground on a tower, and the blades, which look like airplane propellers, are perpendicular to the axis. This type of wind turbine looks like a pinwheel.
The Mathematics of Wind Energy
Three factors determine how much energy the wind can transfer to a wind turbine: the density of the air, the area of the rotor, and the speed of the wind. The first factor is air density. Any moving body contains kinetic energy. The amount of this energy is proportional to the body's mass or weight. A truck hurtling down the road at 50 miles per hour (80 kilometers per hour) has more kinetic energy, and consumes more gasoline, than a subcompact car traveling at the same speed. With wind the amount of kinetic energy depends on the density of the air. Heavy air contains more energy than light air. When the atmospheric pressure is normal and the air temperature is 59°F (15°C), air weighs 1.225 kilograms per cubic meter (0.076 pounds per cubic foot). Humid, or damp, air is denser than dry air, so it weighs more. Air at high altitudes, such as in mountain regions, is less dense, so it is lighter.
The second factor that determines the amount of energy the wind can transfer to a wind turbine is the area of the rotor. The diameter of a 1,000-kilowatt wind turbine is 54 meters (177 feet). Rotor diameters can vary with designs, but this diameter is typical. The area over which a rotor of this size operates is 2,300 square meters (24,757 square feet). As the diameter of a rotor increases, the increase in the area it covers increases with the square of the diameter. Thus, doubling the size of a turbine allows it to receive four times as much energy, or 22 = 2 × 2.
The third factor that determines how much energy the wind can transfer to a wind turbine is the speed of the wind. The relation between wind speed and energy is cubic. In other words, when the speed of the wind doubles, the amount of energy increases eight times, or 23 = 2 × 2 × 2.
When the three factors are put together, the formula used to calculate the amount of wind energy available at a given site is P = 0.5 ρ v3 π r2 where P equals power measured in watts; ρ or the Greek letter rho (ROH), equals the density of dry air in kilograms per cubic meter (1.225); v equals the speed of the wind measured in meters per second; π, or the Greek letter pi (PYE), equals 3.14159; and r equals the radius, or half the diameter, of the rotor in meters.
A wind turbine has the following components:
- Rotor and blades. The rotor is the hub around which the blades are connected. Often, however, "rotor" is used to refer to the hub and the blades as a single unit. The rotor is the key component, because it translates the wind's kinetic energy into torque (TORK), or turning power.
- Nacelle (nuh-SELL), or the enclosure that houses the turbine's drive train, including the gearbox, the yaw mechanism, and the electric generator. The gearbox connects a low-speed shaft to a high-speed shaft. This mechanism can increase the speed of the shafts by a factor of as much as fifty to one, meaning that the high-speed shaft turns fifty times faster than the low-speed shaft. The yaw mechanism automatically senses the direction of the wind and rotates the rotor to keep it facing into the direction of the wind.
- Tower, or the support for the rotor and drive train.
- Electric equipment such as controls, cables, and an anemometer (an-uh-MAH-muh-tuhr)
Blades come in various sizes and have tended to grow over the years. In the early 1980s, a typical blade was likely to be 33 feet (10 meters) long, and such a wind turbine could generate about 45 megawatt-hours per year. By 1990 the typical blade measured 89 feet (27 meters) and could produce 550 kilowatt-hours per year. In the early twenty-first century blades as long as 233 feet (71 meters) can generate 5,600 megawatt-hours per year.
Building a wind turbine is far more than simply a matter of finding a field or mountaintop where the wind is blowing and plopping one down. Engineers give a great deal of attention to finding the proper site for a wind turbine. The main factor they consider is the average speed of the wind over an extended time. Using a device called a wind-cup anemometer, which looks like three or four ice-cream scoops arranged in pinwheel fashion, engineers take extensive measurements of wind speed over a long time.
Wind speed measurements have to be precise. If engineers overestimate the amount of wind, the power output of the turbine can be reduced considerably. If, for example, wind is believed to average 10 miles (16 kilometers) per hour but is only 9 miles (14 kilometers) per hour, the power output of the turbine is reduced 27 percent. If the wind speed is only 8 miles (13 kilometers) per hour, the power output is 41 percent less than expected. If the wind speed is higher than believed, power output increases. If the wind speed is 11 miles (18 kilometers) per hour, the power generated increases 33 percent. If the wind speed is much higher than expected, the equipment may be too small and too fragile for the site.
In addition to wind speed when looking for a place for a wind turbine, engineers consider factors such as wind hazards, characteristics of the land that affect wind speed, and the effects of one turbine on nearby turbines in wind farms. The following factors are important:
- Hill effect. When it approaches a hill, wind encounters high pressure because of the wind that has already built up against the hill. This compressed air rises and gains speed as it approaches the crest, or top, of the hill. Siting wind turbines on hilltops takes advantage of this increase in speed.
- Roughness, or the amount of friction that Earth's surface exerts on wind. Oceans have very little roughness. A city or a forest has a great deal of roughness, which slows the wind.
- Tunnel effect, or the increase in pressure air undergoes when it encounters a solid obstacle. The increased air pressure causes the wind to gain speed as it passes between, for example, rows of buildings in a city or between two mountains. Placing a wind turbine in a mountain pass can be a good way to take advantage of wind speeds that are higher than those of the surrounding air.
- Turbulence, or rapid changes in the speed and direction of the wind, often caused by the wind blowing over natural or artificial barriers. Turbulence causes not only fluctuations in the speed of the wind but also wear and tear on the turbine. Turbines are mounted on tall towers to avoid turbulence caused by ground obstacles.
- Variations in wind speed. During the day, winds usually blow faster than they do at night, because the sun heats the air, setting air currents in motion. In addition, wind speed can differ depending on the season of the year. This difference is a function of the sun, which heats different air masses around Earth at different rates, depending on the tilt of Earth toward or away from the sun.
- Wake. Energy cannot be created or destroyed. As wind passes over the blades of a turbine, the turbine seizes much of the energy and converts it into mechanical energy. The air coming out of the blade sweep has less energy because it has been slowed. The abrupt change in speed makes the wind turbulent, a phenomenon called wake. Because of wake, wind turbines in a wind farm are generally placed about three rotor diameters away from one another in the direction of the wind, so that the wake from one turbine does not interfere with the operation of the one behind it.
- Wind obstacles, such as trees, buildings, and rock formations. Any of these obstacles can reduce wind speed considerably and increase turbulence. Wind obstacles such as tall buildings cause wind shade, which can considerably reduce the speed of the wind and therefore the power output of a turbine.
- Wind shear, or differences in wind speeds at different heights. When a turbine blade is pointed straight upward, the speed of the wind hitting its tip can be, for example, 9 miles (14 kilometers) per hour, but when the blade is pointing straight downward, the speed of the wind hitting its tip can be 7 miles (11 kilometers) per hour. This difference places stress on the blades. Too much wind shear can cause the turbine to fail.
CURRENT AND POTENTIAL USES
The American Wind Energy Association predicted that in 2005 as much as 2,500 megawatts of new wind power capacity could be added in the United States, bringing the total to more than 9,000 megawatts. Worldwide, as of the end of 2003, about 39,000 megawatts of wind power were being generated, producing about 90 billion kilowatt-hours of power, enough for about nine million average American homes.
The power produced with wind energy is only a fraction of the potential. The U.S. Department of Energy says that, in theory, wind can provide the equivalent of 5,800 quadrillion British thermal units, or quads, of power each year, a number that is fifteen times the total world energy demand each year. Just a single quad has as much power as 45 million tons of coal or 172 million barrels of oil. In the United States, it is estimated that wind realistically could supply 20 percent of the nation's electricity requirements. In 2005 it was supplying about 0.4 percent. A goal is for the United States to generate 5 percent of its electricity from wind power by the year 2020.
An example of wind power in action in the United States is Spirit Lake, Iowa. At Spirit Lake, the elementary school has a 250-kilowatt wind turbine that provides 350,000 kilowatt-hours of electricity each year, more than the school needs. The rest of the power is fed into the local utility grid, earning the school $25,000 during its first five years. The school, however, is not fully dependent on the wind turbine. When the wind is not blowing, the school purchases electricity from the power company. Officials at Spirit Lake considered the system so successful that a second turbine, with a capacity of 750 kilowatts, was installed.
Commercial wind power usually is generated at wind farms rather than from single turbines. Wind farms consist of a group of turbines at the same site. The largest wind farm in the United States is the Stateline Wind Energy Center, located on the Vansycle Ridge, which runs along the Columbia River on the Washington-Oregon border. The ridge is an ideal site because of its consistent average winds of 16-18 miles (26-29 kilometers) per hour. The farm consists of 454 wind turbines, each 166 feet (51 meters) tall and at peak capacity generating 660 kilowatts of power. This wind farm provides power to about seventy thousand homes. Plans are to expand the farm so that it can produce 300 megawatts of power.
Benefits of wind turbines
Wind power has grown to be economically competitive with other forms of power. Although it costs more to generate 1 kilowatt of electricity by wind power than it does with coal- or oil-fired generators, the gap is closing. If 20 percent of a family's electricity were to come from wind power, the electric bill would be less than $2 higher per month. The cost of generating wind power has decreased 85 percent since 1980.
Wind power can be an alternative crop for farmers and ranchers. A small family farm in western Pennsylvania provides 5 percent of the power used at the University of Pennsylvania. Many farmers and ranchers are leasing their land to produce electricity. A farmer can be paid as much as $4,000 per wind turbine, and the farmer can continue to use the land for traditional farming. Wind turbines add to the local tax base. In Lamar, Colorado, wind-power generation added $32 million to the county tax base, providing money for schools and other local needs.
Wind turbines do not consume water, making them ideal for dry or drought-stricken areas. In contrast, conventional and nuclear power plants consume large amounts of water for cooling and other purposes. According to the California Energy Commission, the number of gallons of water consumed per kilowatt-hour by nuclear power plants is 0.62; by coal plants, 0.49; and by oil, 0.43. In contrast, wind-power turbines consume 0.001 gallons of water per kilowatt-hour.
Wind power is homegrown, unlike oil, which the United States and other countries have to import in large quantities from areas of the world that are often unstable. Not buying these fuels from abroad increases national security and improves the nation's balance of payments. Because wind is free, consumers are not at the mercy of frequently increasing fuel prices.
Wind power in inexhaustible and renewable, in contrast to fossil fuels, and it is clean. Wind power does not contribute to acid rain, smog, global warming, or mercury contamination. It does not release dangerous particles into the air. In 2000 the Harvard School of Public Health conducted a study on the health effects of two conventional power plants in Massachusetts. The researchers concluded that among the health effects of the plants' air pollution were 159 premature deaths, 1,710 emergency department visits, and 43,300 asthma attacks.
Wind energy is safe. Although the risk exists for industrial accidents in the construction of a wind turbine, the same can be said about the construction of any facility. The risk that the public will be harmed by a wind-power facility is nearly zero. With nuclear power the risk of catastrophe is ever present, and with fossil fuel plants, the danger from fire and explosions is high. There has been only one case of a person's being killed by a wind turbine: A skydiver sailed off course and fell into the rotating blades of a turbine.
Wind power has many uses. Small turbines can power schools, businesses, campuses, homes, farms, and ranches. They can be used in remote locations for telecommunications, ice making, and water pumping, eliminating the need for remote communities to run smoky and noisy diesel-powered generators. Turbines could benefit native communities in small, poorer nations.
Drawbacks of wind turbines
Wind turbines can be noisy, and engineers are working on ways to quiet the noise. The best method has been to reduce the thickness of the trailing edges of blades. Noise also has been reduced by placing turbines in an upwind rather than a downwind position. The wind hits the blades first, then the tower, rather than the other way around, eliminating the thumping sound that downwind designs make as the blade passes the wind shadow cast by the tower.
Wind turbine blades can cause shadow flicker as the blades rotate in the path of the sun's rays. The flickering of light and dark can be a minor annoyance for local residents when the sun is low in the sky. Most turbines are set back far enough away from homes and businesses so that shadow flicker is not a concern.
Wind farms require a fair amount of land, about 24 hectares (60 acres) per megawatt. However, the turbines themselves plus service roads occupy only about 1 hectare (3 acres) of the 24 hectares. Once the turbines have been built, farmers and ranchers can continue to use the land under them for traditional purposes. Land is difficult to find near larger cities. One solution to this problem is to place wind turbines in shallow waters offshore where possible.
Wind turbines are visible, contributing to visual or horizon pollution. Placing some wind turbines offshore can help lessen this problem. Some people consider wind turbines sleek and attractive, embodying a forward-looking concern for the environment. Wind turbines are no more visible than ski resorts, water towers, and junkyards.
The wind is intermittent, meaning that wind power has to be supplemented by other forms of power. Wind-power generation poses additional challenges for power-grid managers, who have to ensure that enough power is available to meet peak demand at all times, even when the wind is not blowing.
Not all areas of the United States, or any country, are suitable for wind-power generation. Wind towers and rotors can interfere with radar, posing a potential hazard for air travelers. They can also interfere with television and radio transmission, particularly if they are in the line of sight between the signal source and the receiver. Finally, wind turbines can be a hazard to birds, which sometimes fly into the rotors.
Environmental impact of wind turbines
The use of wind power benefits the environment, because this form of energy is clean and it does not consume water. It has been estimated that in 2004, existing wind power prevented the release of 10.6 million tons of carbon dioxide, 56,000 tons of sulfur dioxide, and 33,000 tons of nitrogen oxides. It also has been estimated that if only ten percent of wind potential were developed in the ten windiest U.S. states, total carbon dioxide emissions could be cut by one-third.
Wind power, however, can have harmful effects on the environment. Some environmentalists are concerned about soil erosion, particularly in desert regions, where a thin, fragile layer of topsoil would be disturbed in the construction of turbines, and in the eastern United States, where turbines would be built on mountain ridgelines. Good engineering practices could lessen these effects.
Another potential problem is the effects of wind farms on bird life. Although birds and bats sometimes fly into wind-turbine blades and are killed, this problem is site specific and has been exaggerated. In a study in California researchers concluded that in a total of ten thousand bird deaths, 5,500 birds were killed by flying into buildings and windows and that motor vehicles caused seven hundred deaths. Cats caused one thousand bird deaths. Wind turbines, in contrast, accounted for less than one in ten thousand bird deaths. Environmentalists are also concerned that wind farms with their service roads and transmission lines may break up the habitat of birds and other wildlife.
Economic impact of wind turbines
The chief economic impact of wind power is that the fuel is free, so it does not have to be mined, transported, stored, and purchased by utility companies. In the early 1980s, when the first large wind turbines were being installed, the electricity they generated cost about thirty cents per kilowatt-hour. At that time, wind power was not competitive with other forms of power because it was just getting its start at a commercial level.
As the scale of wind operations grew and the technologies used to exploit wind energy improved, wind power in the early twenty-first century cost about four to six cents per kilowatt-hour, making it competitive with traditional power sources. The cost of wind power tends to be higher in the eastern United States, where wind speeds are lower, wind farms are smaller, and the cost of construction is higher because most wind turbines are constructed on elevated ridgelines. The cost tends to be lower in the Great Plains, where wind speeds are higher, wind farms are larger, and the cost of construction is lower because of the flat terrain. To put the figure of four to six cents per kilowatt-hour in perspective, the cost of electricity per kilowatt-hour in some U.S. states in 2000, according to the Energy Information Administration, was as follows:
Hawaii, 14 cents
New York, 11.2 cents
Connecticut, 9.5 cents
California, 8.4 cents
Florida, 6.9 cents
Illinois, 6.6 cents
Colorado, 6.0 cents
Nebraska, 5.3 cents
Kentucky, 4.1 cents
Wind power provides jobs. Every megawatt of wind power provides about 4.8 job-years of employment. Wind power also provides exports. It is estimated that by the mid-2010s, 75,000 megawatts of new wind power will be installed worldwide at a cost of $75 billion. Countries with the industrial capacity to build wind turbines, such as the United States, could capture a share of that growing market, providing employment for thousands of people. Many farmers and ranchers earn money by leasing their land to wind-power companies. They receive as much as $3,000 to $4,000 per year for each wind turbine. Wind farms increase local tax bases, providing funds that counties can use to improve schools and providing other services to residents.
Wind power does not have the hidden costs of other energy sources. Hidden costs are those that society has to pay but that are not reflected in the price of the resource. Such costs include transportation and storage with their risk of causing polluting accidents, air and water pollution, and the health effects of pollution.
Societal impact of wind turbines
The effects of wind power on society are difficult to measure. Because the fuel is free, use of wind power would release billions of dollars that are currently spent mining, transporting, storing, and burning fossil fuels. However, the price of land near wind turbines often decreases, which is a concern to local land owners.
Most conventional power-generating plants, and even some alternative energy plants such as hydroelectric dams, are large facilities that often alter the course of rivers and other natural landscapes. Nuclear power and coal-fired generating plants are considered necessary, but they pose health and safety dangers particularly related to smoke and other emissions. Wind power, in contrast, is perhaps the most harmless form of power available. It consumes no fossil fuels or water, it poses no health risk and only the smallest safety risk, and as the technology develops, it is likely to be relatively inexpensive, especially as the cost of fossil fuels rises. Wind energy would provide the United States, or any nation, with at least some measure of energy independence, making the nation less reliant on the energy sources that come from other parts of the world. In many communities, using wind energy would bring power generation closer to home, so that cities, counties, and states would be responsible for their own power needs. Being responsible for their own power may contribute to a greater sense of community among local residents.
ISSUES, CHALLENGES, AND OBSTACLES
Three principal issues surrounding wind power continue to be discussed by policy makers and legislators: the renewables portfolio standard, the production tax credit, and net metering.
The renewables portfolio standard, or RPS, refers to proposals for laws that would require electric utility companies to provide a portion of the electricity from renewable sources such as wind power. The company could either produce the energy itself, or it could buy the energy from another company. Rather than buying the electricity, the company could also buy credits, which it could then trade or sell to other utility companies. In this way, company A might not provide any electricity at all from renewable sources, but company B, which bought A's credit, might provide twice as much as it otherwise would have. Thus, the purpose of the RPS is not to force any single company to provide energy from renewable sources but to force the industry as a whole to provide such electricity. Twelve states have an RPS in place, and various proposals have been made to enact RPS laws at the federal level.
A second issue is the production tax credit. As a way to encourage the development of wind power, the government gives wind energy producers a 1.8-cent tax credit for every kilowatt-hour they produce. This money can be subtracted directly from the company's income tax bill, making it less expensive for the company to produce energy and therefore making the energy less expensive to consumers. In this respect, the wind industry is no different from other energy industries, all of which receive help from the tax code so that they can keep down costs to consumers. The tax credit was enacted in 1992. In 2004 President George W. Bush (1946–) signed a two-year extension to expire at the end of 2005. The wind industry would like to see the tax credit extended beyond that date so that the industry can continue making investments in wind-power plants.
A third issue is called net metering or sometimes net billing. This term refers to laws that permit citizens with wind turbines to allow their electric meter to run backward when they are supplying excess power to the electric grid. For example, a rancher has a wind turbine that generates 200 kilowatt-hours of electricity. During the day, the turbine provides much of the electricity needed to run the ranch, but when the wind is not blowing, the rancher has to buy supplemental power from the utility company. At night the turbine generates excess electricity that the rancher can sell to the local utility company.
Under net metering laws, each excess kilowatt-hour the rancher supplies would offset each kilowatt-hour he buys from the utility, lowering the ranch's electric bill each month. Some utility companies argue that this practice is unfair, because they say they are being forced to buy power from the rancher at high retail rates rather than at the low wholesale rates at which they usually buy power. The wind-power industry has successfully argued in thirty-four states that the rancher and the utility are swapping power and that this is a standard practice among utility companies. Meanwhile, other states are considering enacting net metering laws.
For More Information
Burton, Tony, David Sharpe, Nick Jenkins, and Ervin Bossanyi. Wind Energy Handbook. New York: Wiley, 2001.
Manwell, J. F., J. G. McGowan, and A. L. Rogers. Wind Energy Explained. New York: Wiley, 2002.
National Renewable Energy Laboratory, U.S. Department of Energy. Wind Energy Information Guide. Honolulu, HI: University Press of the Pacific, 2005.
Linde, Paul. "Windmills: From Jiddah to Yorkshire." Saudi Aramco World (January/February 1980). This article can also be found online at http://www.saudiaramcoworld.com/issue/198001/windmills-from.jiddah.to.yorkshire.htm.
"Energy Efficiency and Renewable Energy: Wind." U.S. Department of Energy. http://www.eere.energy.gov/RE/wind.html (accessed on July 25, 2005).
"Guided Tour on Wind Energy." Danish Wind Industry Association. http://www.windpower.org/en/tour.htm (accessed on July 25, 2005).
"Wind Energy Tutorial." American Wind Energy Association. http://www.awea.org/faq/index.html (accessed on July 25, 2005).