Wind power provides physics lessons


November 29th, 2010

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Wind power remains the goal of Nebraska scientists excited by its potential (Photo courtesy Harvey McDaniel, Wikimedia Commons)

Lincoln, NE – Nebraska’s one of the windiest states in the nation, and lots of people get blown over thinking about the money to be made converting that wind to electricity. As NET News’ Fred Knapp reports, the process of deciding where to do that can stretch the mind too.

Wind’s a serious business. A recent wind power convention in Kearney drew around 500 people; and vendors from all over set up booths in the convention hall. But amid all the grownup salesmanship, there was some kid-like enthusiasm, too.

Chad Johnson is a public relations education specialist with the Nebraska Public Power District. When he’s not hobnobbing with convention-goers, Johnson can be found talking to school kids about everything from electrical safety to renewable energy. Wherever he goes, he brings his knowledge of wind power physics. So when someone’s confused by a reference to the limits of wind power efficiency, Johnson is there with an explanation.

“That’s called the Betz limit,” he said, “and basically what that says is when you’ve got the maximum amount of force, because of the viscosity, the density of wind, the maximum amount that you could ever get out with the perfect turbine blade would be 59%.”

You may never have thought about wind having a density. Johnson explained the physics with a comparison.

“Force equals mass times acceleration. If I’ve got a 10 mph wind, and a 10mph flow of water, what’s going to have more power? Well, there’s a lot more mass in that 10 mile an hour water flow. So there’s a lot more force.”

Johnson said a flow of water can be over 90% efficient in being converted to electricity compared to wind’s 59%. And, Johnson said, the efficiency of wind also varies with elevation.

“The lower elevation, the better the density for wind turbines. At sea level, you got the most amount of air mass,” he said. “The air pressure is greatest at sea level …but it’s kind of that reverse concept, because you’re going higher up to get the better winds.”

But Johnson said there’s a practical limit to reaching higher for wind power.

“Typically the higher you go,” he said, “the stronger and more consistent the winds are. Ideal heights are way up in the stratosphere, because that’s where the winds blow all the time, in a straight line. We can’t get turbines that high.”

Johnson said wind speed is a lot more important than wind density for wind power developers. And Nebraska’s wind speeds make it one of the richest potential producers of wind power. At the same time other factors, like transmission bottlenecks and plenty of current supply, may push realizing that potential farther into the future.

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