The Path of Least Resistance

The Path of Least Resistance

Conventional motors account for 64% of the nation's energy use, according to the U.S. Department of Energy (DOE), Washington, D.C. Of that amount, half one-third of the total energy use in the United States is used by motors rated more than 1,000 horsepower. By virtue of their ability to carry current with almost zero resistive losses when used in industrial motor applications, high-temperature superconducting

Conventional motors account for 64% of the nation's energy use, according to the U.S. Department of Energy (DOE), Washington, D.C. Of that amount, half — one-third of the total energy use in the United States — is used by motors rated more than 1,000 horsepower. By virtue of their ability to carry current with almost zero resistive losses when used in industrial motor applications, high-temperature superconducting (HTS) materials offer the potential to double the power output and significantly reduce energy loss by as much as 50% in comparison to conventional motors of similar size with copper windings.

In addition to energy savings, there are also practical mechanical advantages to HTS motors: Besides lower noise emission, the iron teeth that conduct the magnetic field in conventional motors become redundant, resulting in a decrease in weight and volume. In fact, HTS materials are at their best when used in motors rated above 1,000 horsepower, where the additional costs of the superconducting technology can be offset by motor volume reduction and reduced energy-consumption costs, according the IEEE Transactions on Industry Applications, Vol 44, No. 5, September/October 2008 paper, “High-Temperature Superconducting Synchronous Motors: Economic Issues for Industrial Applications.” In the industrial arena, this could mean energy savings of up to $50,000 a year for a 5,000-horsepower motor that runs 24 hours a day, seven days a week.

Industrial motors aren't the only application that could benefit from widespread use of HTS technology. Superconducting wire in electric generators can increase machine efficiency beyond 99%, as well as decrease size by 50%, in comparison with generators wound with copper wire. A 1,000-megawatt superconducting generator could save as much as $4 million per year in reduced losses per generator. Industry analysts are estimating the potential worldwide market for superconducting generators in the next decade will reach between $23 billion and $30 billion.

Despite these advantages, HTS motors and generators are not yet available on a commercial scale. Granted, HTS is expected to be the future of large, industrial rotating machines, most likely replacing conventional motors and generators in the areas of electrical generating stations, petrochemical plants, wastewater treatment centers, and steel and paper mills where large pumps, fans, and compressors consume vast quantities of electricity. However, industry experts are reluctant to offer an estimated time line for their availability and adoption. “I've been saying HTS industrial motors will be available in five years for the last five years now,” says Rich Schiferl, director of advanced technology for the Baldor/Dodge/Reliance Advanced Technology Labs, Baldor Electric Co., Fort Smith, Ark., who, along with other scientists, has spent 21 years researching that application of superconductive materials to electric motors and generators.

So far, Schiferl's lab has been able to make several prototypes to test and demonstrate HTS large motor capabilities. Its largest one to date was a 1,600-horsepower model demonstrated in Cleveland in 2001. Other labs have created prototypes as large as 5,000 horsepower. “We demonstrate capabilities,” Schiferl says. “We understand how you build these machines and how you keep the rotor cold.”

All the same, development of large superconductor motors and generators for use outside the lab has been slow going. The major barrier that remains to commercial adoption is cost — for both the superconducting wire as well as the necessary cryogenic subsystems. “It's now a cost issue,” Schiferl says. “From our standpoint, it doesn't make sense to invest a lot of effort into developing a commercial motor unless we know that the costs are going to be met, so we're waiting for that to happen. We can't sell a superconductor motor that costs significantly more than a conventional motor; nobody would buy it.”

Experts working in the field of HTS generators agree. “We know how to go about making these generators,” says James Bray, a chief scientist at General Electric's Global Research Center, Schenectady, N.Y. “But if you want economically competitive machines, you'll need prices to go down more. You won't have widespread commercialization until the cost of wire and cryorefrigeration comes down.”

High-wire act

HTS wire is actually a thin tape coated with special materials and wrapped in coils. Since 2006, a second-generation (2G) wire that was expected to lower costs has been made in 100-meter lengths, but will need to be ramped up to 10 times the lengths and higher performance levels for use in industrial motors. “It's still not at the levels that we'd like it to be to make this a commercially viable technology for electric motors,” Schiferl says. “They need to take what they've been able to do in laboratory samples and scale it up into longer lengths and produce it at a lower cost.”

Under an agreement with SuperPower, Inc., Schenectady, N.Y., Baldor's Advanced Technology Lab is developing superconducting motors and generators using SuperPower's 2G HTS wire. According to Chuck Weber, director of HTS applications for the company, up-front prices aren't the only consideration related to the cost of HTS wire. Potential adoptees should also weigh the overall life cycle: energy and maintenance cost savings. “There may be a higher up-front cost, but how much is saved per year on a more efficient machine?” he asks.

Jason Fredette, director, Investor & Media Relations, American Superconductor Corp., a Devens, Mass.-based 2G HTS wire manufacturer, says that more industry involvement is the answer to bringing down the costs of wire for HTS machines. “We're willing to work with motor manufacturers and manufacturers of generators to move this along as quickly as possible,” he says. “We're now ramping up manufacturing, and the price of that wire is declining rapidly. But this tends to be a somewhat stodgy industry, and this is a real breakthrough technology. So we need to engage more of those companies for the commercialization of this.”

Temperatures rising

The other key to commercialization is lowering costs of the cryogenic system. “They also need to come down,” Schiferl says. “That's a cost component that's important for motors.”

Although superconductive materials were first discovered in 1911, they required a critically low temperature within the range of the cooling abilities of liquid helium (at 4.2 degrees Kelvin) — the record critical temperature for superconductors was 23 degrees Kelvin — effectively limiting their large-scale application in industrial electric motors. “You had to cool to just above absolute zero in order to make them act like a superconductor, and that's very expensive to do,” Fredette says.

In 1986, the discovery of a new class of ceramic superconducting-capable compounds (copper-oxides or “cuprates”) raised the critical temperature to above the boiling point of liquid nitrogen (77 degrees Kelvin or minus 321 degrees Fahrenheit) and was able to advance the technology to the current record HTS critical temperature of approximately 133.5 degrees Kelvin — 160 degrees Kelvin at high pressure (see A Time Line of Superconductor Technology, starting on page 30).

Therefore, when referring to superconductor technology, “high” and “low” are relative terms. Even at cryogenic levels, HTS machines operate at much higher temperatures than their low-temperature superconductor (LTS) counterparts. The cost to keep the superconductors cool drops with increasing operating temperature. This “higher” temperature permits use of more commonly available and less expensive liquid nitrogen as the cooling medium — versus the more costly helium gas or liquid neon needed to maintain LTS temperatures. Helium, which is more expensive but reaches lower temperatures, can also be used. By some estimates, HTS cooling costs may be less than 10% of LTS material cooling. “You still have to cool HTS machines,” Fredette says. “But you can do it with standard off-the-shelf refrigeration equipment.”

Nevertheless, the cost of the cryogenic subsystem is still another factor in the commercialization of HTS machines. “It depends on the application. If you look at the initial up-front cost of an HTS rotating machine versus a copper machine, you have to include the cryogenic system with the HTS machine. To overcome that cost, you need other savings,” Weber says. “If you look strictly at the energy savings, even if you cut the energy losses in half, it takes a number of years to recoup the cost of the cryogenics part of the system. Typically, it's three to five years for the larger machines.”

However, for some applications, price isn't a deal-breaker. Some customers may even be willing to pay a higher price for reductions in size and weight and increases in performance. “You go about designing superconductor machines based on what your customer wants,” Bray says. “Performance can be improved, as well as a reduction of footprint, size, weight, and noise.”

As an example, Bray cites an HTS generator his company recently created for the U.S. Air Force, which requested a reduction in size and weight. “They first wanted a portable generator,” he says. “They were also looking for efficiency in order to use the same amount of fuel for more usable electrical output.”

Wear and tear

The biggest difference in the commissioning and maintenance between a conventional motor or generator and an HTS version is the cryogenics subsystem. “The rotor for motors and generators made with superconductors would need to be cooled a lot more than the typical motor or generator, but that would be the only difference in terms of the system itself,” Fredette says.

Even that difference doesn't have to create a major change for plant maintenance staff. “We can define maintenance procedures that would not be any more complicated or more frequent than maintenance procedures they do today on bearings,” Schiferl says.

The same goes for HTS systems for generators. According to Bray, the fundamental electrical properties are the same as in conventional generators. “There's a schedule you need to follow to maintain the cryogenics, such as changing the filters, but HTS components are less likely to wear out from thermal cycling than in a conventional generator,” he says.

Operating HTS machines at cryogenic temperatures may require less maintenance than conventional machines and prolong the lifetime of equipment. Thermal cycling and high operating temperatures, which wears out insulation in conventional machines, is often blamed for the bulk of maintenance time and costs. This cycling doesn't occur as often with HTS machines, and there are no high operating temperatures as with conventional devices. “You go through only a moderate amount of thermal cycles in the lifetime of the HTS machine,” Weber says. “Once we cool it to operating temperature, it stays there until it needs to be shut down for maintenance procedures. The expected lifetime of the components is expected to be longer with fewer maintenance intervals.”

The current form for the delivery of a cryogenics subsystem is what Drew Hazelton, principal engineer at SuperPower, calls a “plug-and-play” package. Some of the components would need to be switched out periodically for maintenance, but that could be contracted out to a cryogenics provider. However, Hazelton doesn't see a potential problem if the maintenance staff chooses to perform the maintenance. “It's relatively straightforward to train someone to swap out certain components in the cryogenics,” he says.

Once commercialization is underway, companies that supply the cryogenic systems are prepared to work with clients either way, says Weber. “Clients could buy the equipment and do the training, or they can do what they call ‘supply cold' and guarantee you have a cold environment for the machine and take care of the maintenance issues,” he explains. “For the first system, a new user might want somebody with expertise to take on that responsibility, and then as they become familiar with it and get a second or third system, they might want to take it back in-house.”

With only lab prototypes to work on, this situation is entirely hypothetical. “But the reasoning is sound,” says Bray.

Alternative alternatives

The most difficult challenge to commercialization is overcoming a catch-22 situation: Companies must be willing to adopt HTS machines in order for them to have wider use, but many companies are reluctant to adopt the technology until there is more evidence of proven real-world applications — something that can't happen until more companies adopt the technology. “Copper motors have been working for 100 years now, so they're very well proven,” Fredette says. “We think superconductor motors are ready for prime time as well, but if you're a customer the fact that it's a relatively new technology can be a drawback.”

Therefore, early adopters may have to come from a market that has a long history of prototypes and has only recently become more popular for real-world applications: alternative energy. The use of superconductors could have a dramatic impact on reducing greenhouse gas emissions. Physicists in Finland have calculated that the European Union could reduce carbon dioxide emissions by up to 53 million tons if HTS machines were used in power plants. In addition, HTS generators and flywheels could be used to solve the intermittency problem present with several forms of renewable energy systems, such as wind and solar (see Super Flywheel on page 34). American Superconductor has developed a program with motor and generator manufacturer Teco-Westinghouse, Round Rock, Texas, to develop a 10 megawatt-class wind power generator that would sit inside the wind turbine. “We're definitely looking at the alternative-energy space,” Fredette says.

According to June 2002 estimates by Conectus, a consortium of European companies determined to use superconductivity, the worldwide market for superconductor products is projected to grow to $5 billion by the year 2010 and to $38 billion by 2020. LTSs are expected to continue to play a dominant role in well-established fields, with HTSs enabling the newer applications. Fredette encourages electrical engineers to pursue the development and use of HTS technology. “There's a great business opportunity here,” he says.

Sidebar: Super Flywheel

Boeing Phantom Works, Seal Beach, Calif., in conjunction with the U.S. Department of Energy's (DOE's) Energy Storage Program, Washington, D.C., is working on a superconducting flywheel electrical system. “The Boeing flywheel project started as part of the DOE's Superconductivity Partnership with Industry (SPI), but when it reached a certain degree of maturity, the energy storage program took it over,” says Imre Gyuk, program manager for energy storage research for the Energy Storage Program. “We have a lot of other projects in energy storage, and this happens to be one of the flywheel technologies we are pursuing. But this is a promising high-tech application of superconductivity.”

Decreases in the friction in the wheel bearings and air resistance on the flywheel create more efficient energy storage capability. With the development of efficient bearings based on HTS technology, losses can be reduced to less than 0.1% per hour. The flywheel is mounted in a vacuum vessel, fitted with either a vacuum pump or hermetically sealed, to reduce frictional losses. In addition, the HTS maintains a stable, self-correcting bearing for the rotating wheel. “You don't want to have a flywheel at 30,000 revolutions per minute bumping into the walls,” Gyuk says.

The key to a stable superconducting bearing is a strong response of the chilled superconductor to changes in the field pattern of a rotating permanent magnet assembly placed above the superconductor. Rotational motion takes place with very little drag because the magnet assembly provides what is essentially a “trough,” or circumferentially symmetrical field pattern. Unlike with the use of ordinary magnets, when the rotor moves away from its rotational center, restoring forces are generated by the super currents in the stationary HTS ring. “It's the difference between having a spinning top on a tabletop or having the spinning top inside a bowl,” Gyuk explains.” “The magnetic field essentially acts like a bowl. It's an elegant way of assuring stability.”

The primary applications for the HTS flywheel design is to increase electric utility efficiency and reliability in electric load leveling, as well as to be used for uninterruptible power systems (UPS) applications. Flywheels can eliminate both momentary voltage and frequency changes, as well as longer- and shorter-term power interruptions.

The solar industry is another potential application. The HTS flywheel can be used to combat the intermittency of the alternative energy. “You can put a certain amount of energy into the flywheel system,” Gyuk says. “When you have a cloud or dark skies, it can give back that energy, so it looks like there's no interruption in the performance of the solar system.”

Research into this area has been progressing steadily, according to Gyuk. The DOE is planning to attach and apply an HTS rotor to a working solar system. “We're thinking either Alaska or Southern California,” Gyuk says.

The HTS flywheel would augment the solar system, possibly along with a diesel generator. The current size of the flywheel would require a residential application, but in the future several HTS flywheels could be used for larger applications. “For one thing, you can gang these flywheels together,” Gyuk explains.

Currently, the HTS flywheel is in experimental phase only. The one challenge to commercialization is, of course, cost. “But that's partially because it's in the early stages of one-off built models,” Gyuk says. “If you turn these things out industrially, the cost could be lowered dramatically.”

TAGS: Design
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