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VAWT is not only choice

blue-light / 2014-05-04
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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 wind 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 vertical axis wind turbine, 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 vertical axis wind turbine rotor that is less efficient than a horizontal axis rotor.

Anything with an airfoil, ideally, can be 59.3 percent efficient. In reality, a horizontal axis turbine operates somewhere around 35 percent. A vertical axis turbine is lower, maybe attaining 30 percent, which doesn’t sound like much, but other factors such as increased maintenance and lower energy production add to the difference.

The vertical axis wind turbine manufacturers claim there’s less wear involved because they don’t have to actively orient themselves to the direction of the wind. That’s actually untrue; it’s just the opposite. There are lots of claims relative to the fact that a vertical axis turbine can take winds from any direction, well, so can horizontal turbines. There’s actually more wear involved with the VAWT. I have a rubber ball on the end of rubber band. When I spin it around slowly, it makes a circle, and if I spin it fast, the rubber band stretches and it makes a bigger circle. That’s centrifugal force. So anything that spins and has mass has centrifugal forces working on it.

The interesting thing about horizontal axis turbines — and this is just a fluke of physics — has to do with the way the airfoil is designed for horizontals. The greatest stress on it is at the root of the blade near the hub, which happens to be the strongest part of the blade. On a vertical, if it’s the egg beater shape (sort of an oval), the greatest forces are at the center, not at the ends where it’s attached. If you make the blade straight it distributes the force more equally, which is where designs have gone recently, but there are some intrinsic problems with vertical axis design and centrifugal forces that really can’t be overcome. It’s just the nature of the design.

So there’s actually more wear and tear on a VAWT. The way they overcome that is by beefing up the design. This is why you don’t see vertical axis turbines commercially viable in the United States today. The manufacturers must use more materials, which means more labor. This also means that vertical technology weighs more. You have a bearing on each end that has to be supported, and it’s easy to do that at the bottom end but hard to do on the top. If you end up putting it on a tower, you need guyed cables that extend for long distances in order to clear the rotor.

Some inventors say that you can put it on a building or on the ground, which eliminates the guyed cable problem. Well, you can, but they need to go back and learn something about fluid dynamics. The reason turbines are on towers is because that’s where the wind resource is. Fuel increases as we increase our distance above the earth. Along the ground we have this zone of friction, and as you get away from the friction, you get faster moving air.

Consumers should take into your consideration that HAWT is also very effective, and on some aspect, it can be better. Consumers should not be misled by old idea any more. Wuhan Wellsee is factory producing wind turbines, including HAWT, it is very famous in China, consumers can trust.

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