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How Wind Energy Works

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 American Wind Energy Association
 Windpower Monthly
 Nat'l Wind Technology Center
 Danish Wind Turbine Mfr. Assoc.

Wind energy is the fastest growing source of electricity in the world. Between 1998 and 1999, over a billion dollars worth of wind power equipment was installed in the US, and over $2.5 billion worldwide.

Still, wind is a relatively small part of our electricity supply. Wind power is often the least expensive form of renewable power -- in fact, in some cases it is the cheapest form of any kind of power. This, and the fact that it produces no emissions of any kind and no dangerous waste, make wind power a very promising choice for new power generation.

The History of Wind Power

Wind power is both old and new. From the trireme sailing ships of the ancient Greeks, to the grain mills of preindustrial Holland, to the latest high-tech wind turbines rising over the Minnesota prairie, humans have used the power of the wind for millennia.

Clear Lake wind farm in Iowa (photo copyright 1999 Ben Paulos) In America, the heyday of wind was between 1870 and 1930, when thousands of farmers across the country used wind to pump water. Small electric wind turbines were used in rural areas of the United States as far back as the 1920s, and prototypes of larger machines were built in the '40s. When the New Deal brought grid-connected electricity to the countryside, however, windmills lost out.

Interest in wind power was reborn in the energy crises of the 1970s. Research by the Department of Energy in the 1970s focused on extremely large machines, with funding going to major aerospace manufacturers. While these 2- and 3-megawatt machines were mostly unsuccessful, they did provide basic research on blade design and engineering principles.

The modern wind era really began in California in the 1980s. Between 1981 and 1986, small companies and entrepreneurs installed 15,000 medium-sized turbines, providing enough power for every resident of San Francisco. Pushed by the high cost of fossil fuels, a moratorium on nuclear power, and concern about environmental degradation, the state provided tax incentives to promote wind power. These, combined with federal tax incentives, helped the wind industry take off.

To be sure, some of the wind developments were little more than tax shelters for wealthy investors. But after the tax credits expired in 1985, wind power continued to grow, although more slowly. Nevertheless, more than half of the turbines in operation as of 1995 were installed after the credits expired. Perhaps more important in slowing windpower's growth was the decline in fossil fuel prices that occurred in the mid-'80s.

In the early 1990s, wind power looked ready for another takeoff, with improvements in turbine technology. Concern about global warming and the recent Gulf War inspired Congress to pass the Energy Policy Act of 1992, which granted a new production credit for wind and biomass electricity. Shortly after this, the utility industry began to anticipate a massive restructuring, where power suppliers would become competitors rather than protected monopolies. Investment in new power plants of all kinds fell drastically, especially for capital-intensive renewables like wind. Only since 1998 has the wind industry rebounded in the US, thanks mostly to a number of state mandates.

In other parts of the world, this hasn't been the case. In percentage terms, wind power is currently the fastest-growing source of energy in the world. Serious commitments to carbon dioxide reductions have promoted wind power in Europe, while the ability to avoid constant imports of fuel has appealed to developing nations like India. As global warming concern continues to grow and the American utility sector settles down, wind power can be expected to expand.

The Wind Resource

In 1991, researchers at DOE's Pacific Northwest Lab investigated the national potential for wind power. Dividing wind speeds into seven classes, with 7 being the highest, they found that land with wind speeds of classes 5 through 7 could supply more than 20 percent of the nation's electricity demand. Most of these regions were far from population centers, though, in Montana, North Dakota, and Wyoming. Wind speeds of classes 3 and 4 are more common and more evenly distributed, and could provide between 2 and 6 times current US consumption.

The amount of power actually produced from wind is a function of price. A 1993 study by the Union of Concerned Scientists, Powering the Midwest, looked in detail at the potential for renewables in 12 Midwestern states. Researchers calculated the cost of wind power for each state, and determined a cost-effective potential. If electricity was sold at 4.5 cents per kilowatt-hour, for example, the regional potential for cost-effective wind power was about 7 percent of current total generation. But if the market would support a price of 5.0 cents per kWh, the potential bloomed to 177 percent of current generation. Add another penny to the price, and the potential skyrockets to more than 14 times current levels.

The price of power generation from wind is mostly a function of the wind resource -- how fast it blows, how often, and when. Higher-speed winds are more easily and inexpensively captured. The power output from a wind turbine rises as a cube of wind speed. In other words, if wind speed doubles, the power output increases eight times. Also, wind speed increases as the height from the ground increases. For example, if wind speed at 10 meters off the ground is 6 m/s, it will be about 7.5 m/s at a height of 50 meters. Finally, the power in the wind varies with temperature and altitude, both of which affect the air density. Winter winds in Minnesota will carry more power than summer winds of the same speed high in the passes of southern California.

On the other hand, wind turbines operate over a limited range of wind speeds. If the wind is too slow, they won't be able to turn, and if too fast, they shut down to avoid being damaged. Ideally, a wind turbine should be matched to the speed and frequency of the resource to maximize power production.

The more the wind blows, the more power will be produced by wind turbines. The term used to describe this is "capacity factor," which is simply the percentage of power a turbine produces compared to the most it could produce if it were always spinning. Consequently, turbines of different sizes may have different capacity factors yet produce the same amount of power.

A more precise measurement of output is the "specific yield." This measures the annual energy output per square meter of area swept by the turbine blades as they rotate. Overall, wind turbines capture between 20 and 40 percent of the energy in the wind. So at a site with average wind speeds of 7 meters per second, a typical turbine will produce about 1,100 kWh per square meter of area per year. If the turbine has blades that are 35 meters long, for a total swept area of 1,000 square meters, the power output will be about 1.1 million kWh for the year.

Another factor in the cost of wind power is the distance of the turbines from transmission lines. In Powering the Midwest, UCS used geographic information system, or GIS, software to map out areas where favorable winds were near sufficient transmission lines. Although some large windy areas in North and South Dakota were too far from power lines to be cost effective, the potential for those states was still enormous. In other states, such as Minnesota and Iowa, the promise of wind power is being realized, as utilities are beginning to offer wind power to their customers.

A final consideration for a wind resource is the seasonal and daily variation in wind speed. If the wind blows during periods of peak power demand, power from a wind farm will be valued more highly than if it blows in off-peak periods. In California, for example, high temperatures in the central valley and low coastal temperatures near San Francisco cause powerful winds to blow across the Altamont Pass in the summer, a period of high power demand.

While skeptics consider the variability of wind to be a fatal problem, it is, in fact, not so serious. In a large utility system, the variations in power output from wind turbines are absorbed in the constant variation in electrical demand. Other generators, like gas or hydroelectric turbines, "follow the load," matching power generation to demand from second to second. Pacific Gas and Electric, which buys the power from the Altamont wind turbines, obtains up to 7 percent of its power from wind, without reliability problems. The Electric Power Research Institute has calculated that Hawaii could get as much as 20 percent of its power from wind without problems.

The Mechanics of Wind Turbines

Lake Benton wind farm in Minnesota Modern electric wind turbines come in a few different styles and many different sizes, depending on their use. The most common style, large or small, is the "horizontal axis design" (with the axis of the blades horizontal to the ground). On this turbine, two or three blades spin upwind of the tower that it sits on.

Small wind turbines are used for providing power off the grid, ranging from very small, 250-watt turbines designed for charging up batteries on a sailboat, to 50-kilowatt turbines that power dairy farms and remote villages. Like old farm windmills, they have tail fans that keep them oriented into the wind.

Large wind turbines, used by utilities to provide power to a grid, range from 250 kilowatts up to the enormous multimegawatt machines that are being tested in Europe. Large turbines sit on towers that are up to 60 meters tall, and have blades that range from 30 to 50 meters long. Utility-scale turbines are usually placed in groups or rows to take advantage of prime windy spots. Wind "farms" like these can consist of a few or hundreds of turbines, providing enough power for whole towns.

From the outside, horizontal axis wind turbines consist of three big parts: the tower, the blades, and a box behind the blades, called the nacelle. Inside the nacelle is where most of the action takes place, where motion is turned into electricity. Large turbines don't have tail fans; instead they have hydraulic controls that orient the blades into the wind.

In the most typical design, the blades are attached to an axle that runs into a gearbox. The gearbox, or transmission, steps up the speed of the rotation, from about 50 rpm up to 1,800 rpm. The faster spinning shaft spins inside the generator, producing AC electricity. Electricity must be produced at just the right frequency and voltage to be compatible with a utility grid. Since the wind speed varies, the speed of the generator could vary, producing fluctuations in the electricity. One solution to this problem is to have constant speed turbines, where the blades adjust, by turning slightly to the side, to slow down when wind speeds gust. Another solution is to use variable-speed turbines, where the blades and generator change speeds with the wind, and sophisticated power controls fix the fluctuations of the electrical output. A third approach, adopted by only one company so far, is to use low-speed generators. Germany's Enercon turbines have a direct drive that skips the step-up gearbox.

An advantage that variable-speed turbines have over constant-speed turbines is that they can operate in a wider range of wind speeds. All turbines have upper and lower limits to the wind speed they can handle: if the wind is too slow, there's not enough power to turn the blades; if it's too fast, there's the danger of damage to the equipment. The "cut in" and "cut out" speeds of turbines can affect the amount of time the turbines operate and thus their power output.

A less successful turbine design is the Darrieus turbine, or "egg beater." The axis of the blades is vertical, like a spinning top, and the gearbox and generator sit on the ground. Darrieus turbines are easy to maintain, since the mechanicals are on the ground, and they don't have to turn to face changing wind direction. But they have suffered from a variety of problems that have limited their use. The first is that the power of the wind increases with height from the ground, and Darrieus blades are low to the ground. Also, the blades have been made with aluminum, which is weakened by stress. Because the blades are weak, they can't capture the higher-powered, higher-speed winds.

The Market for Wind

As the cost and price of wind power fall, demand for it is growing exponentially all over the world. The cost of electricity from the wind has fallen from about 25 cents per kilowatt-hour in 1981 to less than 4 cents now in some situations. With the help of a 1.5 cent per kWh tax credit from the federal government, wind power is comparable in price to new generation from coal and oil plants and sometimes cheaper; only gas turbines and energy-efficiency improvements are less expensive.

Global wind power capacity rose from less than 2,000 megawatts in 1990 to over 6,200 megawatts in 1996. In 1997, an additional 1500 MW were added, an increase of almost 25 percent. In 1998, capacity grew another 15% to 8719 MW, enough to power 2.3 million typical American homes. By mid-1999, world capacity passed the 10,000 MW mark. Growth has been especially rapid in Northern Europe, Spain and India.

The American wind market picked up in 1998 after a number of poor years. Uncertainties with electric utility restructuring caused the market for wind power in the United States to stall out, just as it appeared to be taking off. America's largest wind company, Kenetech, declared bankruptcy in 1995, a victim of the sudden slowdown. But thanks to state mandates and a tax credit for wind power, significant new wind farms in Iowa, Minnesota, Texas and Wyoming came online in early 1999. Green power markets and utility "green pricing" programs have resulted in smaller numbers of new turbines going up in California, Colorado, Wisconsin and elsewhere.

Installed Wind Energy Capacity
(in megawatts, MW)
Country 1996
1998 Net
Germany 1,545 2,080 2,583 503
USA 1,590 1,592 1,946 354
Denmark 857 1,116 1,380 264
India 816 950 968 18
Spain 249 512 660 148
The Netherlands 299 325 359 34
UK 270 320 331 11
China 79 166 190 24
Sweden 105 117 148 31
Italy - 100 154 54
Rest of World 234 274 489 215
Total 6,097 7,592 8,719 1127

Source: Windpower Monthly.

The Future of Wind Power

With costs continuing to fall and environmental concerns continuing to rise, a strong future for wind power seems almost assured. Turbines are getting increasingly large, with turbines of over 1 megawatt now common, and of up to 5 megawatts in the development stage. The offshore market in Northern Europe is driving this development, as are simple economies of scale for larger turbines.

A recent report by BTM Consult, a Danish wind energy consulting firm, predicts the world wind capacity to triple to over 31,000 megawatts by the year 2003, with much of the growth in Europe, especially Spain. US wind capacity is expected to double to over 4000 MW. By 2008, world capacity could reach 73,000 MW, with over 13,000 MW in the US.

But that growth will hinge on a number of factors, most importantly what rules are established for electric utility restructuring and whether the US will take meaningful action to reduce global warming emissions. Pro-environment restructuring policies, such as a Renewables Portfolio Standard, will mean continued strong growth for the wind industry.

If wind costs continue to fall as some in the industry predict, the future growth of wind power may not depend so much on environmental policies. Wind is already competitive in many situations. With a little more drop, wind may simply become the cheapest power source available. And the environmental benefits will be a bonus.

Further Reading

Michael Brower, Cool Energy: Renewable Solutions to Environmental Problems, MIT Press, 1992. Available from UCS.

Union of Concerned Scientists, Powering the Midwest, 1993.

Robert Righter, Wind Energy in America: A History, University of Oklahoma Press, 1996.

Paul Gipe, Wind Energy Comes of Age, John Wiley and Sons, 1995.

American Wind Energy Association, Wind Energy Weekly, available via e-mail from Tom Gray, and archived at the Solstice web site.