Horizontal vs Vertical Axis Wind Turbines
I found this on Roger Hamilton's newsletter.
"Worldwide interest in renewable energy options has given rise to a rash of new wind turbine designs. Some of the most recent models on the market are vertical axis wind turbines (VAWTs), which manufacturers claim are quiet, efficient, economical and perfect for residential energy production, especially in urban environments.
We asked Mick Sagrillo, veteran residential wind power authority, to answer our questions about this technology and its future in the realm of alternative energy.
First of all, how does a vertical axis wind turbine work?
There are actually two different designs out there. One’s called a Savonius rotor, which essentially, if you take a 55-gallon drum and cut it in half, then offset the two halves and put them on a shaft that rotates, you’ve built a Savonius rotor. It’s similar to an anemometer. A lot of what we see today are Savonius rotors. They’re very crude, very low-tech, very inefficient. We’re talking about something that operates in the, say, 5 to 10 percent efficiency range. People have been able to tweak the efficiency rate — ideally, they might be as high as 15 percent.
Then there’s the Darrieus model — the type that resembles an egg beater. Essentially, you have two vertically oriented blades revolving around a vertical shaft. But the Darrieus models use an airfoil design. A wind turbine airfoil works in the same way as an airplane wing. An airfoil has a flat side and a curved side. The result of air passing over the two sides is a force known as “lift.” When an airplane speeds down a runway, air passing on both sides of the airfoil wings, the lift force literally lifts the airplane into the air. This will continue as long as there is forward motion over the airfoil to generate the required lift.
A wind turbine uses this same principle, but instead of flying up and away, the airfoils are secured to a hub, which in turn is attached to a generator shaft. The air passing over the airfoils (wind turbine blades) are converted into rotational momentum which spins the generator.
On the Darrieus rotor, since the airfoils are the same as horizontal axis turbines, they will operate at the same efficiency. However, the difference occurs because a horizontal axis turbine's swept area always faces the wind. But with a VAWT, the swept area is a cylinder perpendicular to air flow. As such, part of the “swept area” is working, while part is simply being blown around, not at an optimal angle to generate lift. This results in a VAWT rotor that is less efficient than a horizontal axis rotor"
Read more: http://www.motherearthnews.com/Renewable-Energy/2008-02-01/Wind-Power-Horizontal-and-Vertical-Axis-Wind-Turbines.aspx#ixzz1rnBDSPHO