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Mightiest Wind PUBLIC ACCESS

Engineers in the U.S. are Looking to Construct Wind Turbines Twice as Powerful as any Yet Built. The Key is a Motor That Relies on Superconducting Windings.

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Associate Editor

Mechanical Engineering 131(05), 28-31 (May 01, 2009) (4 pages) doi:10.1115/1.2009-MAY-2

Abstract

This article explores the increasing use of wind turbines for generating power. It also discusses that changing the economics of wind power can make it more practical for deep-ocean turbines to harness strong, steady offshore winds. Engineers around the globe are focusing on creating conventional motors that can improve performance of wind turbines. Companies have found that a direct-drive generator built with superconducting windings would produce twice as much power per volume as a conventional generator, with a small parasitic loss due to cryogenic cooling. The prospect of producing more power per tower, which would be the net effect of using 10 MW turbines, might enable more offshore wind projects to become economically feasible. Sinovel, a Chinese generator company, is already planning to build 5 MW machines using existing technology. Once 10 MW machines become available, it is conceivable that they would quickly adopt them for offshore installations. It would be a step toward clearing the coal-fed brown haze that envelopes much of East Asia.

Article

Wind turbines are obvious. A coal-fired power plant has its smokestack and many nuclear power plants use giant cooling towers, but depending on the weather conditions, one can’t count on seeing them at work. Gas turbines are even more discrete; many can be tucked into basements or parking garages.

It is hard to miss a wind turbine. The largest stand on 40-story towers, and when they are generating electricity, the rotors move about 10 turns a minute. The nacelles, which house the generating equipment, can weigh some 300 tons and are bigger than McMansions. And even if one wind turbine wasn’t obvious enough, they tend to stand in groups.

Changing the economics oftwnd power could make it more practical for deep-ocean turbines to harness Strong, steady offshore winds.

That bulk becomes a limiting factor to the power of a single turbine. Increase the power of the generator and the weight of the nacelle goes up accordingly. That necessitates a sturdier tower to put the nacelle atop, which increases the overall cost of the project. Thanks to the way these moving economic pieces shift in relation to one another, the most powerful single wind turbine in operation is rated at 5 MW, and new turbines typically run between 1 and 3 MW.

But engineers at American Superconductor in Devens, Mass., believe they have a technology that could change that High-temperature superconductors have had to bear the weight of expectations for some time. Superconducting magnets are particularly strong, and have been touted for use in levitating trains. The lack of electrical resistance in superconducting wire has led some to propose constructing transcontinental or even intercontinental transmission lines from them, or storing electricity in large loops of super-cooled wires.

Although superconductivity—a quantum state in which materials have no resistance to electrical current—was discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes, it was for a long time a curiosity only witnessed in cryogenic labs. Sure, one could eliminate electrical equation. The company, which specializes in wire made from material that is superconducting when cryogenically cooled, is designing a generator for a wind turbine that uses its wire in the coils. According to Dan McGahn, senior vice president and general manager for AMSC Superconductors, such generators could produce twice as much power, per weight, as the ones being used today.

This 36.5 MW electric motor uses superconducting wire in its windings. Designed for ship propulsion, the motor recently completed tests at a Navy facility in Philadelphia and is now ready for deployment.

Grahic Jump LocationThis 36.5 MW electric motor uses superconducting wire in its windings. Designed for ship propulsion, the motor recently completed tests at a Navy facility in Philadelphia and is now ready for deployment.

That means it would be economically feasible to put up wind turbines capable of generating 10 MW each. resistance in aluminum at 1.2 K (-272 °C) but the cost of maintaining that cryogenic environment tended to limit practical applications.

But in 1986, physicists at the IBM research lab in Geneva, Switzerland, discovered a material that could superconduct at 35 K, some 12 degrees warmer than the previous title holder and a higher temperature than many believed possible. In a flurry of activity in material science labs across the world, researchers cooked up recipes for other exotic ceramics in search of the mythic room-temperature superconductor. That goal remains elusive, but scientists have discovered some useful materials, many, including ones that go by the acronyms TBCCO, BCCCO and YBCO, are copper oxides.

What makes these materials particularly interesting is that they become superconductive at temperatures above 77 K (-196 °C), which is the boiling point of nitrogen. While liquid nitrogen is otherworldly cold, it is a fairly common industrial product that is used in everything from burning off warts to cooling computer equipment. Its higher boiling point and myriad uses enable liquid nitrogen to be fairly inexpensive. According to Brookhaven National Laboratory in New York, industrial quantities of liquid nitrogen can be had for as little as 6 cents a liter. Liquid helium, by contrast, runs over $3 a liter.

In spite of this advantage, high-temperature superconductors are ceramic materials, which are brittle and hard to work with. The most high-profile use of superconductive materials—the supercooled wires inside a magnetic resonance imager—use low-temperature niobium-alloy superconductors. But high-temperature superconductors are beginning to find applications.

These ribbons contain superconducting ceramic sandwiched between layers of metal. When cryo-genicatly cooled, these ribbons carry much more current than copper wires of the same size. A 10 MW generator made with coils of this material could weigh 60 percent less than conventional ones.

Grahic Jump LocationThese ribbons contain superconducting ceramic sandwiched between layers of metal. When cryo-genicatly cooled, these ribbons carry much more current than copper wires of the same size. A 10 MW generator made with coils of this material could weigh 60 percent less than conventional ones.

American Superconductor, the manufacturer of superconductor wire and associated products, can make more than 450 miles of high-temperature superconducting wire a year for applications such as power cables, fault current limiters, and motors. The wires consist of a rib bon of yttrium barium copper oxide, one of the first ceramics shown to superconduct above 77 K, sandwiched within layers of more ductile metals. The resultant wire can carry more than 150 times the current of a copper wire of identical thickness.

So what can you do with high-tech wire? Engineers at American Superconductor have been continually looking at ways to use their wire to make a new kind of motor.

“In 1990, American Superconductor built its first coils for a superconducting device,” said Bruce Gamble, director of engineering at AMSC. When researchers had before then built experimental rotating machines using low-temperature materials, the refrigeration demands were too great to make them workable. With high-temperature superconductors, Gamble said, “we were going to be able to build rotating machines without the same efficiency and reliability penalties.”

Indeed, Gamble said that as little as 0.1 percent of output goes to power the cryogenic system.

The company has developed some 10 prototype motors, each larger than the next. The concept was simple: with the superconducting coils carrying current without resistance, the motor could deliver more power in a smaller package than conventional motors. “While we can’t compete with permanent magnet or copper-wound machines with small size motors, they really pay off at high torque.”

The electric motor project has been tested as part of a military research contract. “Navy propulsion motors are some of the largest, highest torque motors that you’ll find—on the order of several millio'n foot-pounds of torque,” Gamble said. In January of this year AMSC and Northrop Grumman, its partner on the project, announced that a 36.5 MW superconducting ship motor they had delivered had successfully completed a full-power evaluation at a Navy test site in Philadelphia.

A similar machine, run backwards, could make for a highly efficient electrical generator. “Motors and generators are essentially all the. same product,” McGahn said. “The fundamental technology can be leveraged for both.” And thanks to other business relationships, executives at American Superconductor knew exactly where such generators could be used.

For years, a part of the company had built reactive power systems for wind farms. Those systems compensate for variations in the power output of the turbines— variations that can create large voltage spikes. Some wind farms compensate for this through use of banks of switched capacitors, which can absorb and discharge current when needed. The company’s dynamic volt-amperes reactive system tackles the problem through a set of power inverters (which don’t actually use superconducting parts).

The company also owns AMSC Windtec, an Austrian firm that designs complete wind turbines for more than a dozen companies worldwide. That work led to the realization that AMSC’s experimental motors could be deployed in wind turbines. Inside the enormous nacelles of wind turbines are giant gearworks and a generator rated in megawatts.

Beginning in 2007, the company, together with TECO-Westinghouse Motor Co., began investigating whether the investment in motors could reap benefits in the wind power industry. By some estimates, a direct-drive generator built with superconducting windings would produce twice as much power per volume as a conventional generator, with a small parasitic loss due to cryogenic cooling. That means that, in theory, a 5 MW wind turbine could be installed upon a much slimmer tower, reducing the cost of construction.

The economics could also run the other way, making it cost effective to build turbines generating as much as 10 MW. While the nacelle for such a turbine would be the same size as a 5 MW model, the other dimensions would be staggering: the rotor blades, for instance, would sweep a 550-foot diameter.

Ten-megawatt turbines are so much larger than what utilities are used to that the U.S. Department of Energy has teamed up with American Superconductor to study whether such machines are even practical. Under the terms of a cooperative research and development agreement announced by the National Wind Technology Center of the National Renewable Energy Laboratory, the company’s AMSC Windtec subsidiary will analyze the cost of a full 10 MW-class superconductor wind turbine. After this cost analysis, which will include the direct drive superconductor generator, blades, power electronics, tower—the works—the NWTC will evaluate the wind turbine’s economic viability, both in capital costs and in cost of electricity generated.

If the economics makes sense, 10 MW wind turbines could well change the face of renewable energy. For one, the wind is more reliable at higher altitudes, so it’s expected that a turbine placed upon a very tall tower would have a higher capacity factor than one on a shorter tower. What’s more, because the power of the wind scales m a non-linear fashion with the diameter of the rotors, a row of 10 MW turbines along a ridgeline would extract more energy than a larger number of smaller turbines.

“We think that by miniaturizing the drive train, it changes what you can do with the system,” McGahn said. “If we can develop a nacelle design that can produce double the power of a 5 MW system but be similar in size, you can leverage a lot of what is being developed for 5 megawatt systems. For instance, part of the concern with systems that size is doing the final fabrication, doing the installation, even moving it.”

Such mega-turbines could make the biggest mark offshore. The wind blowing across the ocean is generally steadier and more powerful than that seen over land; giant turbines built over the horizon from the cities of the northeastern United States would have to transmit their electricity at most a few hundred miles, in contrast to the 1,000 or so miles needed to import wind power from, say, the Great Plains.

The concept was simple: with the superconducting coils carrying current without resistance, the motor could deliver more power in a smaller package than conventional motors.

The drawback has been cost. Except in special instances where the seafloor is shallow enough to allow for easy installation, the expense involved in building and maintaining a wind turbine tower offshore drove up the projected cost of generated electricity. The prospect of producing more power per tower, which would be the net effect of using 10 MW turbines, might enable more offshore wind projects to become economically feasible.

McGahn said that it was unlikely that one would see nacelles with American Superconductor’s name on the side; instead, its AMSC Windtec subsidiary would license the technology and designs to other manufacturers. Companies like Hyundai and Sinovel, wind turbine generator manufacturers which American Superconductor already has partnerships with, might make the machines using materials and engineering supplied by AMSC.

Sinovel, a Chinese generator company, is already planning to build 5 MW machines using existing technology. Once 10 MW machines become available, it’s conceivable that they would quickly adopt them for offshore installations. It would be a step toward clearing the coal-fed brown haze that envelopes much of East Asia.

Is 10 MW the limit? McGahn said that if the infrastructure constraints could be overcome, wind turbines could in theory get even larger. “The ultimate limit,” he said, “is the length of blades you can support and how high up you can go.”

Copyright © 2009 by ASME
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