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Union of Concerned Scientists http://www.ucsusa.org/clean_energy/renewable_energy_basics/how-wind-energy-works.html Contents 1. The History of Wind Power 2. The Wind Resource 3. The Mechanics of Wind Turbines 4. The Market for Wind 5. The Future of Wind Power Harnessing the wind is one of the cleanest, most sustainable ways to generate electricity. Wind power produces no toxic emissions and none of the heat trapping emissions that contribute to global warming. This, and the fact that wind power is one of the most abundant and increasingly cost-competitive energy resources, makes it a viable alternative to the fossil fuels that harm our health and threaten the environment. Wind energy is the fastest growing source of electricity in the world. Global installations in 2005 reached more than 11,500 megawatts (MW) - a 40.5 percent increase in annual additions compared with 2004 - representing $14 billion in new investments. [1] In the United States, a record 2,431 MW of wind power was installed in 2005, capable of producing enough electricity to power 650,000 typical homes.[2] Despite this rapid growth, wind power is still a relatively small part of our electricity supply - generating less than one percent of both the and global electricity mix. But thanks to its many benefits, and significantly reduced costs, wind power is poised to play a major role as we move toward a sustainable energy future. The History of Wind Power Wind power is both old and new. From the sailing ships of the ancient Greeks, to the grain mills of pre-industrial Holland, to the latest high-tech wind turbines rising over the Minnesota prairie, humans have used the power of the wind for millennia. In the United States, the original 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 as far back as the 1920s, and prototypes of larger machines were built in the 1940s. When the New Deal brought grid-connected electricity to the countryside, however, windmills lost out. Interest in wind power was reborn during the energy crises of the 1970s. Research by the U.S. Department of Energy (DOE) in the 1970s focused on large turbine designs, with funding going to major aerospace manufacturers. While these 2- and 3-MW machines proved mostly unsuccessful at the time, they did provide basic research on blade design and engineering principles. The modern wind era 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. After the tax credits expired in 1985, wind power continued to grow, although more slowly. Perhaps more important in slowing wind power's growth was the decline in fossil fuel prices that occurred in the mid-1980s. In the early 1990s, improvements in technology resulting in increased turbine reliability and lower costs of production provided another boost for wind development. In addition, concern about global warming and the first Gulf War lead Congress to pass the Energy Policy Act of 1992 - comprehensive energy legislation that included a new production tax credit for wind and biomass electricity. However, shortly thereafter, the electric 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 renewable energy technologies like wind. America's largest wind company, Kenetech, declared bankruptcy in 1995, a victim of the sudden slowdown. It wasn't until 1998 that the wind industry began to experience continuing growth in the United States, thanks in large part to federal tax incentives, state-level renewable energy requirements and incentives, and - beginning in 2001 - rising fossil fuel prices. In other parts of the world, particularly in Europe, wind has had more consistent, long-term support. As a result, European countries are currently capable of meeting more of their electricity demands through wind power with much less land area and resource potential compared with the United States. Denmark, for example, already meets about 20 percent of its electricity demand from wind power.[3] Wind generation also accounts for about six percent of the national power needs in Spain, and five percent in Germany.[4] Serious commitments to reducing global warming emissions, local development, and the determination to avoid fuel imports have been the primary drivers of wind power development in The Wind Resource The wind resource - how fast it blows, how often, and when - plays a significant role in its power generation cost. 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. Therefore, higher-speed winds are more easily and inexpensively captured. Wind speeds are divided into seven classes - with class one being the lowest, and class seven being the highest. A wind resource assessment evaluates the average wind speeds above a section of land (usually 50 meters high), and assigns that area a wind class. 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. Wind speeds in classes three (6.7 - 7.4 meters per second (m/s)) and above are typically needed to economically generate power. Ideally, a wind turbine should be matched to the speed and frequency of the resource to maximize power production. Classes of Wind Power Density at Heights of 10m and 50m (See original article for Wind Chart) Since the late 1990s, the DOE National Renewable Energy Laboratory (NREL) has been working with state governments to produce and validate high-resolution wind resource potential assessments on a state-by-state basis. This data is being used to gradually replace a less precise national wind resource assessment completed in 1991 by researchers at DOE's Pacific Northwest Laboratory. Combined, these resource assessments have found that the windy areas (class three and above) in the United States - if fully developed - could supply more than four times the nation's current electricity needs. Most of the windiest lands (class five and above) are located in regions far from population centers, in states like Montana, North Dakota, and Wyoming. Wind speeds of classes three and four are more common and more evenly distributed across the country. Ê Several factors can affect wind speed, and the ability of a turbine to generate more power. For example, wind speed increases as the height from the ground increases. If wind speed at 10 meters off the ground is six m/s, it will be about 7.5 m/s at a height of 50 meters. A 2003 Stanford University study examined wind speeds at higher elevations and found that as much as one quarter of the United States - including areas historically thought to have poor wind potential - is potentially suitable for providing affordable electric power from wind.[5] The rotors of the newest wind turbines can now reach heights up to 70 meters. In addition to height, the power in the wind varies with temperature and altitude, both of which affect the air density. Winter winds in Minnesota State will carry more power than summer winds of the same speed high in the passes of southern The more the wind blows, the more power will be produced by wind turbines. But, of course, the wind does not blow consistently all the time. The term used to describe this is "capacity factor," which is simply the amount of power a turbine actually produces over a period of time divided by the amount of power it could have produced if it had run at its full rated capacity over that time period. 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 seven m/s, a typical turbine will produce about 1,100 kilowatt-hours (kWh) per square meter of area per year. If the turbine has blades that are 40 meters long, for a total swept area of 5,029 square meters, the power output will be about 5.5 million kWh for the year. An increase in blade length, which in turn increases the swept area, can have a significant effect on the amount of power output from a wind turbine. Another factor in the cost of wind power is the distance of the turbines from transmission lines. Although some large windy areas, particularly in rural parts of the High Plains and Rocky Mountains, are too far from power lines to be cost effective today, the potential for these regions is still enormous. 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 Californi>, for example, high temperatures in the central valley and low coastal temperatures near San Francisco cause powerful winds to blow across the Altamont 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. The electric system is designed to handle unexpected swings in energy supply and demand, such as significant changes in consumer demand or even the failure of a large power plant or transmission line. 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 eight percent of its power from wind, without reliability problems. There are also several areas in Europe, including Spain, Germany, and Denmark, where wind power already supplies over 20 percent of the electricity with no adverse effects on the reliability of the system. Finally, several recent U.S. based utility studies confirm that a significant amount of wind energy can be integrated into the electricity grid without large cost or reliability impacts.[6] The Mechanics of Wind Turbines 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 generally 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, these small wind turbines have tail fans that keep them oriented into the wind. Large wind turbines, most often used by utilities to provide power to a grid, range from 250 kilowatts up to the enormous 3.5 to 5 MW machines that are being used offshore. Today, the average land-based wind turbines have a capacity of 1.5 MW.[7] Large turbines sit on towers that can be anywhere from 50 to 100 meters tall, and have blades that range from 30 to 50 meters long.[8] 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 tens of thousands of homes. 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. Ê 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 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. The Market for Wind Ê The cost of electricity from the wind has dropped from about 25 cents/kWh in 1981 to as low as 4-6 cents/kWh in recent years. Though wind turbine prices have increased some since 2005 (see below for more information), in areas with the best resources, wind power is cost competitive with new generation from coal and natural gas plants. DOE projects that wind power costs will continue to fall over the next 10 to 15 years. As wind power costs become more competitive, demand is growing exponentially all over the world. Global wind power capacity rose from just over 6,000 MW in 1996 to more than 59,000 MW by the end of 2005 - almost a ten-fold increase.[9] Growth has recently been most significant in Northern Europe, Spain, and India, but markets in Asia and the Pacific region are emerging as well.ÊÊÊ At the end of 2005, the U.S. wind power market reached more than 9,100 MW - providing enough power to serve the needs of 2.3 million homes.[10] The majority of this capacity is located in California, Texas, Iowa, and Minnesota, but there are wind power projects either in operation or under development in at least 36 states. Once the long-time global leader in wind power development, the U.S. currently ranks third - well behind Germany. Inconsistent growth over the last ten years is mostly to blame, thanks in large part to the on again, off again status of the federal Production Tax Credit (PTC). The PTC provides a 1.9-cent/kWh tax credit during the first 10 years of a wind energy facility's operation.[11] Despite being one of the primary drivers of wind development, the federal government has allowed the PTC to expire on three separate occasions since 1999. These lapses in the PTC have lead to a boom-bust cycle that has drastically slowed the wind power industry for many months at a time. For the first time, the PTC was extended prior to expiration as part of the Energy Policy Act of 2005, creating some short-term policy stability through 2007. Not surprisingly, the U.S. wind power market responded by installing 2,431 MW of wind power capacity in 2005, leading all countries and setting a new national record in the process.[12] Record growth is also projected for the U.S. market in both 2006 and 2007.ÊÊÊÊÊ State-level renewable electricity standards - a requirement placed on electric utilities to gradually increase their use of renewable energy sources over time - are also working as a primary driver of U.S. wind development. Nearly half of all wind power capacity built from 2001-2005 was attributable to state standards, according to the DOE's Lawrence Berkeley National Laboratory.[13] In addition to serving the near-term market, the 20 states (plus Washington, DC) with renewable electricity standards are also designed to stimulate significant new development for years to come. In addition to standards, other state level policies driving the U.S. wind power market include renewable electricity funds and various tax incentives. Finally, voluntary green power markets and utility "green pricing" programs have resulted in a smaller, but quickly expanding market for wind development. The DOE reports that through 2004, more than 2,000 MW of wind capacity was serving voluntary markets, with an additional 365 MW in the development stages.[14] The Future of Wind Power With increasingly competitive prices, growing environmental concerns, and the call to reduce dependence on foreign energy sources, a strong future for wind power seems certain. The World Wind Energy Association projects global wind capacity will double in size to over 120,000 MW by 2010, with much of the growth happening in the United States, India, and China.[15] Turbines are getting larger and more sophisticated, with land-based turbines now commonly in the 1-2 MW range, and offshore turbines in the 3-5 MW range. The next frontiers for the wind industry are deep-water offshore, and land-based systems capable of operating at lower wind speeds. Both technological advances will provide large areas for new development.ÊÊ As with any industry that experiences rapid growth, there will be occasional challenges along the way. For example, beginning in 2005, high demand, increased steel costs (the primary material used in turbine construction), increased profit margins, and certain warranty issues have lead to turbine shortages and higher prices.[16] There are also concerns about collisions with bird and bat species in a few locations. And the not-in-my-backyard (NIMBY) issue continues to slow development in some regions. But new manufacturing facilities, careful siting and management practices, and increased public understanding of the significant and diverse benefits of wind energy will help overcome these obstacles. During a February 2006 speech at NREL, President Bush stated that wind energy could provide as much as 20 percent of America's electricity needs. In response, the American Wind Energy Association and the DOE have teamed up to launch an initiative to develop an action plan for achieving this goal. Getting to that level will require a determined national effort, but it will result in less dependence on fossil fuels, significant reductions in global warming emissions, and improved air and water quality for future generations. Wind energy is ready to meet the challenge. Endnotes Ê [1] Global Wind Energy Council (GWEC). 2006. Record year for wind energy: Global wind power market increased by 40.5% in 2005. Online at http://www.gwec.net/index.php?id=30&no_cache=1&tx_ttnews%5Btt_news%5D=21&tx_ttnews%5BbackPid%5D=4&cHash=d0118b8972 Ê [2] American Wind Energy Association (AWEA). 2006. Windpower outlook 2006. Online at http://www.awea.org/pubs/documents/Outlook_2006.pdf Ê [3] European Wind Energy Association (EWEA). 2006. Europe's energy crisis - The no fuel solution. Online at ttp://www.ewea.org/fileadmin/ewea_documents/documents/publications/briefings/no_fuel_lo_res_72dpi.pdf Ê [4] European Wind Energy Association (EWEA). 2005. Wind power markets. Online at http://www.ewea.org/index.php?id=196. Ê [5] Archer, C.L., and M.Z. Jacobsen. 2003. Spatial and temporal distribution of U.S. winds and wind power at 80 m derived from measurements. Journal of Geophysical Research 108, doi:10.1029/2002JD002076,2003. Ê [6] DeMeo, E.A., M. Milligan, B. Parsons, and J.C. Smith. 2004. Wind power impacts on electric power system operating costs: Summary and perspective on work to date. Online at http://www.uwig.org/windpower2004.pdf Ê [7] American Wind Energy Association (AWEA). 2004. Wind power today. Online at http://www.awea.org/pubs/factsheets/WindPowerTodayFinal.pdf Ê [8] Ibid. Ê [9] Global Wind Energy Council (GWEC). 2006. Global wind 2005 report. Online at http://www.gwec.net/fileadmin/documents/Publications/GWEC-Global_Wind_05_Report_low_res_01.pdf Ê [10] European Wind Energy Association (EWEA). 2006. Focus on the U.S.: America's new horizon. Wind Directions, May/June 2006. Online at http://www.ewea.org/fileadmin/ewea_documents/documents/publications/WD/WDmay06_focus.pdf Ê [11] Ibid. Ê [12] Global Wind Energy Council (GWEC) and Greenpeace. 2005. Windforce 12. Online at http://www.greenpeace.org/international/press/reports/windforce-12-2005 Ê [13] Wiser, R. 2006. "Meeting Expectations: A Review of State Experience with RPS Policies," Lawrence Berkeley National Laboratory, presented at AWEA RPS Workshop. March. Ê [14] Bird, L., and B. Swezey. 2005. Green power marketing in the United States: A status report. NREL/TP-620-38994, eighth edition. Golden, CO: National Renewable Energy Laboratory. October. Online at http://www.eere.energy.gov/greenpower/resources/pdfs/38994.pdf Ê [15]World Wind Energy Association (WWEA). 2006. Worldwide wind energy boom in 2005:58.982 MW capacity installed. Online at http://www.wwindea.org/default.htm Ê [16]European Wind Energy Association (EWEA). 2006. Focus on the U.S.: America's new horizon. Wind Directions, May/June 2006. Online at http://www.ewea.org/fileadmin/ewea_documents/documents/publications/WD/WDmay06_focus.pdf |
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