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Testing Cheap Wind Power

A continuously variable transmission could lead to cheaper wind power–if it is rugged enough.
October 29, 2009

Federal stimulus funds awarded to a wind-energy research consortium led by Illinois Institute of Technology will accelerate testing of small wind turbines that could point the way towards more efficient utility-scale machines. The eight-kilowatt turbines, the product of Cedar Park, TX-based Viryd Technologies, use a mechanical approach–continuously variable transmission (CVT) technology–to convert fluctuating wind speeds into the precise stream of alternating current required by power grids. If it can replace the pricey power electronics that regulate power in most turbines today, the same technology could cut the cost of wind-power generation at any scale.

Black box: This eight-kilowatt wind turbine uses a continuously variable transmission–the small, silver-colored unit on the left below the rotor–to regulate its power. The turbine’s developer, Cedar Park, TX-based Viryd Technologies, claims that its use of mechanical instead of digital power regulation will cut manufacturing costs by 20 percent and boost power output.

The question is whether the CVT is tough enough. Viryd parent company Fallbrook Technologies has already commercialized its technology as a smooth-shifting alternative to gears and derailleurs in high-end bicycles and is working on larger vehicle applications. Wind power, however, is a particularly demanding application, according to Jason Cotrell, a senior engineer at the Department of Energy’s National Wind Technology Center in Golden, CO. “Wind turbines are subject to very high torque for 80,000 hours of operation, so it’s a very challenging environment,” Cotrell says. “CVTs tend to be complex, and we haven’t yet verified that they’re suitably robust.”

Most CVTs vary transmission ratios by sliding metal belts up and down a set of precision curved parts–a design that is expensive to implement at high torque. Fallbrook’s technology relies on comparatively simple parts, promising lower cost and greater durability, according to CTO Rob Smithson. “It’s basically a big ball bearing, which is a global commodity,” Smithson says.

The CVT transfers power between a set of rings–an input ring and an output ring–via a set of rolling balls sandwiched between them (seven or eight balls, each slightly smaller than a golf ball, in Viryd’s case). Tilting the balls’ axis of rotation causes the rings to travel different distances with each rotation of the balls. A pressurized transmission fluid keeps the balls and rings from chewing each other up in the process.

Viryd CEO John Langdon says that its turbine control system manages the balls’ tilt to spin the turbine’s rotor at the optimum frequency to maximize energy capture for a given wind speed, and to synchronize the AC power output from the turbine’s generator with the power grid. As a result, they use substantially less power electronics, and less sophisticated generators. He promises the turbines will be 20 percent less expensive than existing eight-kilowatt turbines, which currently cost about $40,000 installed.

The $8 million project, led by Illinois Institute of Technology’s Wanger Institute for Sustainable Energy Research, is one of several to test whether the cheaper turbines can endure. If these prototypes pass muster, Langdon’s plan is to install 50 more during the first half of next year for dealers and then to begin marketing the turbine to homeowners and small businesses in the second half of the year. The turbine is rated to generate about 10,000 kilowatt-hours of electricity annually, which is close to the average U.S. homeowner’s power budget. Langdon predicts a ready market, thanks to state and federal incentives.

Viryd’s eventual goal is to scale up to utility-scale wind farms. Scaling up the CVT technology to deliver on that promise is a matter of increasing the size and number of balls to handle the higher torque coming from the utility-scale machine’s larger blades, which can exceed 60 meters in length (15 times longer than the blades on Viryd’s eight-kilowatt turbine). A utility-scale turbine could require 12 half-meter-diameter balls, says Langdon.

At least one other startup is chasing the same opportunity–Israel’s IQwind. Last month IQwind signed up Spanish engine manufacturer Grupo Guascor to produce its variable-speed transmissions as a retrofit for 750-kilowatt wind turbines.

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