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Wind Tunnel at the End of the KELLYN S. BETTS
he popularity of wind energy has grown dramatically in the past few years, and the presence of a wind turbine in the world's largest and most expensive wind tunnel this year is one more indication that the technology has come of age. Although hardly typical of the advanced aircraft usually found at the National Full-Scale Aerodynamics Complex operated by the U.S. National Aeronautics and Space Administration (NASA), the turbine is one of Ae most technically complex machines for harnessine wind enerev ever madp Debate con-
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Wind power is the most promising and viable technology for reducing the environmental problems the world faces in the 21st century, said Lester Brown, president of WorldWatch Institute, at this year's annual State of the World conference. The technology is the world's fastest-growing energy source, according to the American Wind Energy Association (AWEA), which documented that the 3600 MW of new wind capacity installed worldwide in 1999 represented the largest international growth in wind capacity in a sinele vear (i) "In some countries wind ene gy g r o ^ T ^ e x c e e d * e ™ ° o n ο tn'eTo b X o ^ T a r k e t » saîd Ï a u Rav^president ofTe ASsEurapÎncouÎter" 3 0 6 A • JULY 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
Energy Association (EWEA). The technology's potential value is truly global in scope. In addition to reducing the pollution associated with generating energy and providing power to the 2 billion people in the developing world who lack electricity, wind turbines are being developed to serve extremely remote and hostile places, including small villages in Alaska and McMurdo Station in the South Pole. NASA is even designing a turbine for use on Mars (see sidebar on page 308A). Powering 10% of the world Wind energy could provide 10% of the world's electricity requirements—1.2 million MW—by 2020, cutting global carbon dioxide emissions by more than 10 billion metric tons in the process, according to a report released last October by EWEA, Greenpeace, and the Forum for Energy and Development (2). Denmark already generates more than 12% of its electricity from wind, and the rest of Europe is in line to meet the 10% targets by 2020, according to the British Wind Energy Association. A number of developing countries, especially China, are also embracing wind energy in a big way (see Figure 1). Few expect the United States to generate more than 5% of its electricity from wind by 2020, despite wind energy's ardent support by Bill Richardson, head of the U.S. Department of Energy (DOE), and the agency's estimates that wind could supply © 2000 American Chemical Society
A l t h o u g h t h e world's growing appetite for w i n d energy is largely being fueled by European technology, a crucial w i n d tunnel test being conducted in the United States w i l l benefit turbine developers everywhere.
Small is bountiful Although the general trend in wind energy is toward ever larger turbines, small turbines are also growing in popularity. For people in developing countries and remote locations, even relatively small amounts of wind-generated energy can have a powerful impact. The American Wind Energy Association estimates that the number of small turbines generating less than 100 kW of power grew by 35% in 1999. "Typically, wind energy is used to electrify a rural village that had no electricity," said Mike Bergey, president and cofounder of Bergey Windpower Co., in Norman, OK, a manufacturer of small wind turbines. Over the past 20 years, small turbines have benefited from a number of advances in technoloav he said such as the use of neodymium-ironboron magnets that shrink the size of the generator and People in this small Northeast Moroccan village rely on 10-kW wind turbine topumpdrinkingwaHergay s turbines are ter up from the ground found in 90 countries. Mexico, Brazil, Argentina, China and Indonesia are nations tha have invested significantly m small wind power he said. Well-designed projects can dramatically affect peoples lives he said For example Bangladesh s Grameen Bank, which was founded to make small loans to needy citizens, installed turbines atop rural cyclone shelters, allowing the structures to nurture small entrepreneurs who use the electricity to run sewing machines and stores when the weather is dry, Bergey said. In general, however, the world's population is increasing faster than rural villages are being electrified, so the number of people without access to electricity continues to rise, Bergey said. The problem is all the more challenging because the rural poor cannot generally afford wind-powered electricity, although Bergey stressed that wind is less expensive than diesel generators and the fuel needed to run them. Unfortunately, small wind power has yet to make inroads at the World Bank, despite Bergey's best efforts, so he is focusing much of his time on educating power planners in individual countries about how wind can provide an attractive option to traditional centralized power lines. Small turbines are also an important source of power in inhospitable regions like Antarctica. Northern Power Systems of Waitsfield, VT, has been operating tiny 3-kW turbines there for more than 15 years, including on the South Pole. Because of problems keeping gear oil functional at temperatures below -100 °F, these turbines use low-speed generators that require no gear boxes. Northern Power is currently pilot-testing a much larger 100-kW turbine that it developed with funding from NASA and the National Science Foundation for use on Mars, according to Lawrence Mott, the company's director of special projects. The company hopes the design will also be useful in providing power to small communities in remote locations, such as the small Alaskan village where the company plans to install a turbine by the end of this year.
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more than one-and-a-half times the United States' power needs (3). Although the installed base of wind power in the United States grew by over 40% last year, the country still generates less than 1% of its energy from wind, according to AWEA. The United States' long-term goals are simply less ambitious: The aim of the Wtad Powering America initiative that DOE kicked off last year is to generate 5% of the nation's power in 20 years. The worldwide demand for wind turbines is nonetheless burgeoning, and the world's 12 major wind turbine manufacturers, together with academia and research labs that support renewable energy technology, are actively trying to improve turbine design. Researchers from around the world agree that NASAs wind tunnel tests should provide valuable data to aid in that effort Every major wind laboratory—including those operated by Denmark, the Netherlands, the United Kingdom, Greece, and Japan—is participating in the tests, according to Dave Simms, a senior engineer with DOE's National Renewable Energy Laboratory (NREL) in Golden, CO, which is sponsoring what is officially known as the "Unsteady Aerodynamics Experiment" at NASA's Ames Research Center in Moffett Field, CA. European wind turbine manufacturers and researchers are counting on the data to help them double the power that can be generated from a turbine, leading to 5-MW and larger turbines. (A turbine's rating in megawatts or kilowatts indicates the maximum amount of power it can generate under optimal wind conditions.) These larger turbine designs will support the European goal of siting turbines offshore, where they can capture more wind and are not a blight on anyone's property, explained Arthouros Zeros EWEAs acting chiei executive Britain is schea-
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Rotating Eiffel towers The additional complication and cost of anchoring turbines offshore are expected to make operating larger 5-MW turbines more economical in that environment, said Herman Snel, aerodynamic cluster coordinator for Wind Energy at ECN, the Netherlands Energy Research Foundation. These turbines will be enormous—with 120-m blades the size of rotating Eiffel towers. Aerodynamics become increasingly important when designing such outsize Uirbmes, Snel said. The data from the tunnel tests should help clear up some fundamental uncertainties about the rotating aerodynamics around wind turbines which should hete resSers design more dependable' a n d U h Z m o r e
100 times more power per unit area When the NREL turbine was tested in NASA's 80-by120-ft wind tunnel this spring, the blade's response to a variety of wind conditions was evaluated by exposing it to gusts of up to 90 mph generated by six 40-ft fans. Many of the tests focused on how the tur-
bine functioned under unsteady or "stall" conditions, Simms said. Understanding what wind researchers call "stall behavior" is crucial because stall puts a brake on the lifting forces upon which most modern turbines rely. The main way wind generates motion is by harnessing either dragging or lifting forces, or both (4). The farm wind mills that were used to pump water and grind grains for centuries capitalized on the dragging force, but using lift increases turbine efficiency: Lift generates 100 times more power per unit of blade surface area, than drag (5) In fact wind researchers intentionallv design turbine blades so that they c i l act " o i l s According to the aerodynamic principles governing the behavior of devices that rely on lift—which includes helicopter blades and airplane w i n g s stall is caused when the forces that cause lift are interrupted. Although stalling can be catastrophic in a manned aircraft it is a benevolent force for wind turbine developers. "Stall is a passive way of controlling a wind turbine," Simms explained. "We want to take advantage of it" Stall provides wind turbines with a way to dump power at high wind speeds, Simms said. No matter what their size, all wind turbines that rely on lift forces must have a way to dump power because the power that can be captured from wind increases as the cube of the wind's speed. Too much power can cause turbines to self-destruct (4). Not surprisingly, turbines that rely on stall to control power are known as stall-controlled turbines. The other popular way of controlling power is via pitch control. which requires the turbine to change its blade angle with respect to the wind. Powering stall Largely because the simpler mechanics underlying stall-controlled turbine designs make them cheaper to build and operate, they were the predominant design until about five years ago, Simms said. On a stallcontrolled turbine, the blades can be rigidly fixed to the hub at a specific pitch angle and do not have to be moved, he explained. This eliminates the need for pitch bearings, pitch drive mechanisms (including motors or hydraulics), and pitch control systems (including computers and sensors). The stall control concept is also the more reliable concept—there is no need for control systems that can fail and pitch bearings that do fail, Snel said. He said that European turbine designers have begun favoring the pitch control approach for the very big wind turbines in recent years, but largely because they fear problems with stall control Because the aerodynamics of stall have not been well understood until now, the models used to design stall control systems often did not do a good job of predicting aerodynamic forces on the turbine, which can result in unexpected fatigue loading of the structure, Simms explained. To gather the needed data, the test wind turbine has pressure transducers mounted inside its 10-m blade to measure the forces generated by the air flowing over it. The most sensitive of these sensors is able to detect pressure variations of 0.007 psi. "It is ex-
FIGURE 1 Worldwide growth in wind energy During 1999, more than 3600 MW of new wind energy-generating capacity were installed worldwide, catapulting total capacity to approximately 13,400 MW.
Source: American Wind Energy Association.
tremely difficult to make these types of measurements, especially in the environment of a rotating wind turbine blade," Simms said. It took researchers many years to develop the specialized equipment and procedures needed to successfully make such accurate measurements (5). Separation causes stalls "The way the air flows over the rotor blade represents a very complicated fluid mechanics problem," explained Bob Thresher, director of DOE's National Wind Technology Center. "As the air flows over the curved surface of a wind turbine blade, it creates a suction. As long as the flow is nice and even and smooth, you get a nice suction that pulls the turbine blades through the air," he said, noting that this suction is what creates lift on an airplane wing. Unfortunately, winds are mercurial by nature and can easily jump from 10 to 30 mph in a second. When a gust of wind comes along, it causes the angle of the flow over the turbine to change When that happens, the forces on the blade drop instantly, resulting inwhawind technologists call a flow separation, Thresher explained. Thisseparationis what causes stall. What is so crucial about the wind generated in NASA's Ames tunnel is that it provides a steady flow of known velocity that can be controlled very precisely, Simms said. The ability to carefully control the turbine's orientation relative to the wind flow is also important because it allows the wind velocJULY 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 3 0 9 A
FIGURE 2 Wind energy investment: Much steadier in Europe than in the United States Many experts attribute the superior wind energy turbines developed in Europe to those nations' much more consistent investment in the technology.
Source: American Wind Energy Association.
ity to be constant across the entire rotor plane, he added. In thefield,the velocity is typically "sheared", which means that it is usually higher at the top of the rotor plane than at the bottom, he explained. Even a second of steady, predictable wind with no turbulence is extremely rare in the field, Thresher said. Even if wind technology researchers know the turbulence in outdoor wind, they do not have a good way to predict the force it will generate on the blade, Thresher continued. "We're not predicting the actual load that the turbine is experiencing very wellwe're in error by 20-30% sometimes," he said. This makes it hard for wind turbine developers to decide how big and thick their airfoils should be, he explained The goal is to keep the blades as light as possible, but they cannot be so small that they risk snapping because they are too easily overloaded Thresher said noting that understanding stall behavior is but one of the research objectives from the wind tunnel testing "It's a great opportunity to test our computer models," said Flemming Rasmussen, senior research scientist and head of the Aeroelastic Design Research Programme at Denmark's influential National Research Laboratory (Rise) in Roskilde. Researchers use computational fluid dynamics software programs to determine the forces on the wind turbine blades produced by the air flowing over them, and some of them are very complex he explained For example centrifugal forces act on the blade when it is rotating, which means that there is some flow outward but up until now this movement has not been capAs the turbine size grows for offshore designs, accurate predictions from computer models become increasingly important, Rasmussen and Snel agreed. Researchers are being forced to redesign rotor blades 3 1 0 A • JULY 1,2000/ENVIRONMENTAL SCIENCE & TECHNOLOGY/NEWS
to reduce their weight, which is bringing the designs closer to fatigue and load limits. For example, one option is to give the blades a thicker profile, but this is associated with a higher risk of certain types of vibrations, which can impair wind turbine operation, Snel said. "Dynamic stall determines the aerodynamic dampening of vibrations, so we need to know more precisely how that works," he explained. For all these reasons, NREL's measurements are coming at an opportune time, he said. Wind tunnel test damage To some observers, projects like the wind tunnel testing effort exemplify everything that is wrong with the American effort to build turbines. "Wind tunnel tests have done more damage to the development of commercial wind energy than they have helped it," claimed Paul Gipe, an independent wind energy advocate, author, and historian. "The American wind energy program is a farce," Gipe contended, although he said he was sympathetic with the need to secure funding that inspires labs like NREL to undertake efforts like wind tunnel testing. But he argues that U.S. researchers' focus on breakthroughswhich he equates to "always trying to hit a home run"—is "one of our cultural failings" There are very few design breakthroughs in wind technology, but the Americandesign effort for the past 30 years has been breakthrough-focused, he said. The European approach contrasts sharply with that of the United States, Gipe said. "The Europeans are taking a much more staid—some would say boring—approach to building machines that work reliably, like a farm tractor," he explained. One of the reasons that the Danes lead the world is because they're building wind turbines to meet the needs of the markets they serve he claimed. The users don't care if their wind turbines have the most sophisticated airfoils that U.S. manufacturers can createthey want something that works, he said. By way of illustration, Gipe noted that in the late 1970s, a group of Danish high school students built a 2-MW wind turbine—a size that no U.S. manufacturer has yet succeeded in developing commercially—that is still working todav. The blade did need to be replaced in 1994, he noted. Hampered by breakthrough focus The Americans have been hampered by their focus on breakthroughs, agreed EWEA's Zervos. He credits incremental improvements for increasing the generating capacity of European wind turbines by an order of magnitude over the last decade, from an average of 150-250 kW to the current 1.5-2.5 MW. A major breakthrough isn't needed even to develop the 5-MW turbines that manufacturers like Denmark's NEG Micron, Vestas, Nordex, and Bonus, and Germany's Entercon are currently envisaging for use offshore, Zervos, Snel, and Rasmussen concurred. Zervos did concede that the gearless machines initially developed by Entercon in 1993 resulted from a breakthrough in technology development But the three maintained that the U.S. program's breakthrough focus has not resulted in much success.
It is not fair to criticize the U.S. wind turbine development effort for its focus on breakthroughs because "we can't just take the European technology and apply it in this country," Simms countered. In Europe, people are willing to pay more for energy than in the United States, where wind energy's main competition is coal-derived energy that costs 3
