In the spring of 2014, Urban Green Energy (UGE) announced with much fanfare that they had completed third-party performance testing on a new product, the VisionAIR wind turbine. As press releases for new wind turbines go, it wasn’t as over-the-top as many. Nevertheless UGE, an importer of small Vertical-Axis Wind Turbines (VAWTs), crowed that the tests “confirmed that the turbine . . . is twice as efficient as several of its competitors.” Of course, UGE avoids listing these competitors so no one can challenge whether the statement is true or not.
UGE’s VisionAIR is a relatively small household-size wind turbine that sweeps 16.6 m². Competing turbines in this size class include Bergey Windpower’s Excel 6 a conventional upwind turbine intercepting nearly twice the area (30 m²) of UGE’s VAWT.
So, how does the VisionAIR VAWT stack up? Not so well. Despite UGE’s hype, Bergey’s turbine is twice as efficient as the VisionAIR at extracting the energy in the wind at the standard wind speed used to rate small wind turbines: 22% for the Bergey and only 11% for the VisionAIR.
Note: The following is an excerpt from my new book Wind Power: Renewable Energy for Home, Farm, & Community that will be available some time in late 2015. This excerpt is from the chapter devoted to vertical-axis & Darrieus wind turbines. I am posting this now because of an article on CleanTechnica about VisionAIR that was so over-the-top I nearly choked on my coffee.
Worse, UGE’s data confirms the fundamental disadvantage of most VAWTs, they are much more material intensive than conventional wind turbines. The relative mass of a wind turbine—a measure of its material intensity that includes the rotor and nacelle—is often used as a shorthand for its expected cost. The more the relative mass of a turbine, the more its likely cost. The specific mass of UGE’s VisionAIR is four times more than Bergey’s Excel 6.
But the poor comparison doesn’t end there. The test report also confirms that UGE’s VAWT—like so many other fixed-pitch VAWTs–“has no passive braking means.” That is, the only way to limit the rotor’s power under emergency conditions is to stop it with a brake, whether that brake is mechanical or by using the generator. The VAWT has no passive or aerodynamic means—other than stall—to limit the power of the rotor in high winds. And stall is not a reliable means for overspeed protection. In contrast, the rotor on the Bergey can passively furl toward the tail vane in strong winds, regulating the rotor’s power.
Disclosure: I sold Bergey wind turbines—a few anyway—in the early 1980s and I’ve featured them in my previous books.
Future UGE press releases may well note that though the VisionAIR only produces 1.5 kW at 11 m/s (25 mph), it will produce a peak power of 2.5 kW at 14 m/s (31 mph) or two-thirds more than at 11 m/s. Yet this only illustrates one of the design’s principle weaknesses compared to conventional wind turbines: there is no passive means to control the rotor’s power. Compare this with Bergey’s Excel 6. Bergey’s turbine will produce up to 6.5 kW at an equivalent wind speed—an increase of only one-fifth—indicating that the Bergey has an effective means of limiting the rotor’s power in strong winds. Passive furling of the rotor in high winds is a significant design advantage of the Bergey relative to the limited stall control in the VisionAIR VAWT.
To summarize UGE’s VAWT is half as efficient, is four times more material intensive, and has no effective means for limiting the rotor’s power in strong winds relative to a competitive conventional wind turbine.
