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A More Durable Wind Turbine

New design does away with the need for a complex gearbox.
December 4, 2009

A Canadian startup has developed a small prototype wind turbine that uses friction instead of a gearbox to convert wind energy into electricity. CWind, based in Owen Sound, Ontario, recently began work on a larger two-megawatt prototype. The company claims that its “friction drive” system is more efficient and reliable–and less costly to maintain–than conventional wind turbines, which are prone to expensive gearbox failures.

Wheels turning: The blades of CWind’s wind turbine move an internal flywheel and several shafts that attach to small generators within the nacelle. In the lower image, a rubber wheel rolls on the inside wall of a flywheel inside a 65-kilowatt prototype turbine.

The blades on most turbines use the wind to turn a drive shaft connected to a gearbox. The gearbox manages the rotation of a second shaft that connects to a large electrical generator. The gearbox is the heaviest piece of equipment in a wind turbine’s “nacelle” (the section at the top of the turbine tower). It’s also a piece that’s among the most vulnerable to failure. Sudden wind gusts put the gearbox under tremendous mechanical stress. Over time this can wear down or break the teeth off its metal gears.

CWind’s design does away with the gearbox completely. Instead, the drive shaft is connected directly to a large metal flywheel. Hugging the outside of the flywheel are eight smaller secondary shafts, each connected to a 250-kilowatt generator and each lined with several specially designed tires that grip the surface of the flywheel. As the flywheel spins, it engages the generators by turning these tire-lined shafts. “We’re using friction. It’s not mechanically hard-coupled,” says Na’al Nayef, a CWind engineer and co-inventor of the system.

Nayef says the system uses software to control the eight secondary shafts. The tires are also designed to temporarily slip if a wind gust causes the flywheel to suddenly speed up. This feature eases the impact on the generators. Each secondary shaft can also be disengaged from the flywheel if the wind slows down, in effect reducing friction and allowing shafts that are still connected to keep their generators operating at high capacity. Likewise, connecting more shafts, thus adding more friction when the wind increases, will engage idle generators. “We can operate the generators at optimal speed all the time,” says Nayef, adding that tests on the smaller, 65-kilowatt prototype show efficiency gains over standard wind turbines of up to 5 percent.

CWind founder Paul Merswolke first pursued the design seven years ago after watching a documentary on the London Eye, a 135-meter-tall Ferris wheel on the bank of the River Thames. He saw that simple truck tires were used as “friction rollers” to turn the Ferris wheel and concluded that the same approach could be adapted for wind turbines. Nayef was brought aboard to come up with a preliminary design, and in 2004 CWind approached energy engineering firm MPR Associates in Washington, DC, for help on building a prototype.

“We said, ‘No, we’re not convinced this makes sense,’ ” says Larry Cundy, director of development at MPR. But CWind convinced MPR to do some basic analysis of the design, and eventually the engineering firm agreed to build the prototype. “It’s a very novel application, quite frankly,” Cundy says. “It’s really a stroke of genius.”

Cundy says the biggest advantage of CWind’s design is that it’s easier and less costly to maintain over the lifetime of the equipment. When a gearbox on a conventional turbine fails, the turbine is knocked completely out of service. Getting a replacement gearbox takes a long time, and removing the massive device from the wind turbine’s nacelle requires a large crane and many days of work. Every day the turbine isn’t generating electricity for the grid amounts to lost revenue for the operator.

“On a friction-drive system with multiple tires, if you lose a tire, the others are still there,” says Cundy, adding that replacing tires is quick–roughly a day’s work–and that future designs will allow maintenance while the turbine is still operating. The same redundancy applies to the generators–if one fails, the others can still function. Cundy says that the small, off-the-shelf generators used in CWind’s design can be obtained quickly and are installed fairly easily with the help of a small crane built into the nacelle.

Nayef says that the tires used are designed to last for three years, and replacing all the tires used on a two-megawatt wind turbine is expected to cost $30,000–or nearly $200,000 over 20 years. By contrast, gearboxes have an average life of six years and cost about $600,000 to replace, or nearly $2 million over 20 years. “We’re going to be competitively priced with conventional gearbox wind turbines, yet we have the advantages of high availability, high efficiency, and all of the advantages that come with serviceability.”

Last month CWind signed a manufacturing agreement with global auto parts maker Linamar, which has committed its McLaren Performance engineering team (of Formula 1 racing fame) to producing the two-megawatt prototype. As part of the 10-year contract, Linamar will also manufacture market-ready turbines, likely beginning in 2011. Nayef says work is already under way on five-megawatt and 7.5-megawatt designs aimed at the offshore wind market as well as remote onshore sites where easy maintenance becomes a key selling feature.

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