Home Wind Turbines.. are They Affordable?

Small-scale wind turbines are a $156 million industry with impressive growth numbers; the 10,500 sold last year constituted a nearly 70 percent increase for the industry. The benefits of such home home wind systems are numerous: instant independence from the grid, payback periods that can be as little as five years (with an existing subsidy), and demonstrating that 20 percent wind power—the government’s stated goal for the country’s energy mix for 2030—can be achieved by any individual in less than a year. On the other hand, if you live in an urban or suburban area, wind turbines are likely to be more trouble than they are worth.

Up above the rooftops—and, often, treetops—wind flows are relatively unimpeded, according to the American Wind Energy Association’s small-wind advocate, Ron Stimmel. But “wind behaves very differently near built structures,” Stimmel says, because houses and other buildings cause a lot of turbulence at roof level. If you live in or near a metropolitan area, however, installing a tower is likely not an option. Either you won’t have the square footage or local regulations prohibit erecting tall structures. “This is why there’s a lot of interest in selling roof turbines,” Stimmel says. But vibrations, sound, price and the complexity of installation has left this market mostly untapped: Nearly 99 percent of small wind turbines installed today are attached to a tower, with just 1 percent constituting rooftop rigs. “It’s an appealing option,” Stimmel says, “but unless it’s done very carefully, there are a lot of obstacles.”

Imad Mahawili, a chemical engineer and long-time wind energy consultant, has designed a unique wind turbine that he says solves the biggest problems associated with rooftop systems. His rig is a wind turbine turned inside-out—the gear system typically sitting in the center of the blades has been removed and the whole thing is enclosed in a wheel lined with magnets. The result is a 95-pound system with 20 replaceable blades that can generate power with even 2-mph winds.

In a typical turbine, “It takes 7 to 8 mph to overcome the resistance of gears,” Mahawili says, “and the loss from gears on average at any speed costs about 20 to 25 percent in aerodynamic efficiency.” So Mahawili removed the gear system, replacing it with a simple hub and bearings. Then, “We looked at the wheel system, near the hub,” Mahawili says, and found that “for 1 rpm [revolution per minute], the velocity at the rim of the wheel is nine times faster.” In response, he placed the magnets that he uses to create electricity (in place of a gearbox) near the rim. Finally, Mahawili designed the turbine to have a high, 120-degree directional acceptance of wind—especially efficient for turbulent gusts—and made the blades so that they could be individually replaced if debris or storm winds break them. All of these design specifications “drive cost down, simplify the product and improve productivity,” he says.

Still, even a device this streamlined could find catching wind in a suburban neighborhood an arduous task. “If you have a solar panel and you put it under a maple tree, the sunlight will still come through, but [the tree is] blocking a lot of the resource,” Stimmel says. The same goes for wind whipping through obstacles in a neighborhood like tall houses and walls or barriers to nearby highways. “Computational fluid dynamics studies for individual rooftops might be necessary for each of these systems,” he says. And in a dense city, “the best rooftop system you can get is an array of solar panels. It’s simply a matter of physics.”

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