Why wind turbine blades coated with ice can spin (slowly) but not throw ice

Posted on January 29, 2008. Filed under: Wind |

Mick Sagrillo, one of the nation’s leading authorities on wind energy, dispells a myth frequently repeated by wind energy opponents:

Wind turbine blades are actually a composite of two devices: a drag portion and a lift airfoil. If you look at any wind turbine blade, you’ll notice a large wide area near the butt of the blade that is greatly pitched relative to the rest of the blade. This is the “drag” portion of the blade, a scoop that gets “blown out of the way” by the wind. Its job is simply to get the blades moving from dead still. The wind just blows this part “out of the way,” and since that blade is attached to a hub, “out of the way” means in a circle. Once there is forward motion, lift can take over on the rest of the airfoil portion of the blade, and get the blades spinning up to their designed revolutions per minute.The drag portion is very inefficient when it’s matched to an alternator. But its job is not to drive the alternator, only to get motion started in the first place, as there can be no lift without some initial motion. Since it is such a very small part of the total swept area, while it does “drag” the lift portion of the airfoil down, it is almost an inconsequential part of the total swept area of the rotor. But it is needed to start motion because the airfoil portion cannot on its own. If they could, airplanes would not need to taxi down a runway, only jump into the air.Once there is any forward motion, the lift portion takes over and pulls the blades around the swept area, quickly and efficiently. And this part of the blade is what is doing the work to actually generate electricity.

Several winters ago, there was an icing event at Rosiere. Despite being covered with ice, most of the eight turbines were turning, albeit slowly, between 2 to 3 rpm. Turbines rotating at that speed are not producing electricity.

The ice that covered the Rosiere wind turbines covered the entire blade, both drag and lift part. But since the drag part is only a scoop that merely gets motion started, ice on that part is of some consequence, but very little. If you had ice covering your snow shovel, you could still shovel snow, albeit it would be more cumbersome.

However, the lift part is very different. The airfoil portion is a finely tuned device, and anything that alters the airfoil, dead bugs for example, greatly alters its ability to function as designed. This is what happens with ice coating jet wings, or the blades in Rosiere. In the case of the jet, the ice must be removed or it will not be able to “lift” off of the ground at the end of a runway.

So what happened in Rosiere was that the blowing wind pushed on the drag portion of the blades, getting them moving slowly, which is what is supposed to happen, but little else. With the coating of ice on the blades, the airfoil portion of the blades could not do its job, and the blades merely spun slowly by drag, about 2 rpm.

Blades spinning at 2 rpm cannot “throw” ice; they are moving too slowly. When ice melts from a blade moving that slowly, it falls straight to the ground, just as when ice is shed from power lines, utility transmission towers, trees, and buildings. If there is a strong wind during the ice shedding, the ice may be blown slightly downwind of the tower, but is too light to travel farther than about half the height from which it falls.

Note: Research and publication are funded in part by Focus on Energy’s Renewable Energy Program.


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