The most successful VAWT in history was that developed by FloWind in the early 1980s. Using what has become a rather conventional two-blade phi-configuration or “eggbeater” Darrieus, FloWind installed more than 500 turbines in California’s Altamont and Tehachapi passes.

FloWind’s turbines were large commercial wind turbines of the day. They built two models: a 17-meter and 19-meter version. The 17-meter turbine, for example, was about 17 times the size of the architecturally dramatic Quiet Revolution QR5.

Characteristic of FloWind’s marketing, and that of other Darrieus turbines of the day, including DAF-Indal, and VAWTPower, was the turbines’ high power ratings. FloWind’s 17-meter model was rated at 142 kW at a wind speed of 38 mph (17 m/s), their 19-meter model was rated at 250 kW at a wind speed of 44 mph (~20 m/s). For comparison, that’s a Force 8 or “fresh gale” on the Beaufort scale. Translation: That’s so windy no one in their right mind wants to be outside.

To understand these power ratings, it’s necessary to look at the area swept by the eggbeater-shaped rotor. FloWind’s 17-meter model swept 260 m2, equivalent to a conventional wind turbine 18 meters (60 ft) in diameter. The 19-meter model swept 340 m2, equivalent to a conventional wind turbine 21 meters (70 ft) in diameter. Today, turbines of this size are considered small commercial turbines.

For comparison, a conventional wind turbine 18 meters in diameter would typically be rated at 100 kW, and a 21-meter turbine would be rated at 150 kW. Thus, the FloWind turbines were overrated in comparison to their peers by at least 50%.

The high ratings of the FloWind machines translate into a specific capacity 546 W/m2 for the 17-meter model and an incredible 735 W/m2 for the 19-meter model.

Another way of saying this is that the 17-meter model had a specific area of 1.8 m2/kW and the 19-meter model a specific area of 1.4 m2/kW. Conventional turbines of the day swept 2.5 m2  for every kW of generator capacity.

That FloWind was greatly overstating the potential performance of its turbine was reflected in its average capacity factor, a measure of performance relative to the size of the turbine’s generator. The capacity factor of FloWind’s turbines never exceeded 12 percent on average and was often less than 10 percent at a time when conventional wind turbines were delivering twice that.

Why were FloWind’s power ratings so high? Wind turbines of that era were often sold to uninformed investors who compared wind turbine prices based on the cost per kilowatt of installed capacity. FloWind’s aggressive power ratings enabled them to charge far more for their turbines than they were worth. FloWind’s turbines were never truly in the 150 kW or 250 kW size class, but that’s what they charged their investors.

For more information see FloWind: The World’s Most Successful VAWT (Vertical Axis Wind Turbine).