Wind Power: Are Vertical Axis Turbines Better?
(Page 2 of 5)
February/March 2008
Alison Rogers Interview with Mick Sagrillo
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.
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The vertical axis 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.
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