Twisters vs. turbines: What if a tornado swept through a wind farm?

By Trish Choate

Sunday, May 9, 2010

WASHINGTON, D.C. — Turbines are popping up all over tornado alley — nearly 1,400 in windswept Nolan County alone.

Every year, about 1,000 twisters crop up across the country.

But no one knows what would happen if a high-intensity tornado struck a turbine

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A typical machine’s rotor weighs 47 tons and could nearly reach across the length of a football field.

“Can a turbine blade come off the turbine and fly in the air, and how far will it go? We don’t have an answer to that right now,” said Texas Tech University professor Kishor Mehta, an expert in windstorm structural damage.

But we need to know, Mehta said.

It’s worth noting turbines are designed to shut down in high winds, partly as a safety measure. And so far luck and Mother Nature have cooperated. West Texas wind farm owners NextEra Energy Resources, Iberdrola Renewables and Duke Energy have heard of a turbine-tornado close encounter.

“Not to say that it won’t happen in the future, but to date, we have not had an issue,” said Steve Stengel, Florida-based spokesman for NextEra Energy Resources. NextEra owns Horse Hollow with 421 turbines in Taylor and Nolan counties, as well as Capricorn Ridge with 407 turbines in Sterling and Coke counties

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The two wind farms can produce enough electricity to power more than 440,600 homes.

A widely connected engineer with the American Wind Energy Association came up dry, too.

“I have never heard of a weather-related failure of a wind turbine that was operating properly,” said Minneapolis, Minn.-based John Dunlop, AWEA project engineer.

That’s not to say twisters haven’t menaced turbines.

Amateur storm chaser Wesley Luginbyhil was on a dirt road near U.S. 183 in Oklahoma when he captured images of a vortex on May 23, 2008. The twister was close to a wind farm near Fort Supply in northwest Oklahoma.

“Had that tornado come down about five minutes before, it would have gone right though that wind farm,” said the 25-year-old resident of Borger, in the Texas Panhandle.

Nolan County Judge Tim Fambrough has experience with tornadoes and turbines.

Fambrough lives in a congressional district ranked first in wind power nationwide three years in a row by AWEA.

And he was living in Sweetwater on April 19, 1986, when one person was killed when a tornado struck the city.

That was bad, but it was worse April 10, 1979, when he was living in Wichita Falls.

On “Terrible Tuesday,” a mile-wide twister struck the city, killing 42 people and leaving thousands homeless.

Fambrough, whose home wasn’t damaged, worked for the electric company.

“I was the only manager in town, and I had a lot to do,” he said

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His opinion about tornadoes and turbines might not be scientific, but it can’t be ignored.

“I’m sure if one came through with the force of the one in Wichita Falls or the one here in Sweetwater, it would demolish the wind generators in its path,” Fambrough said.

But Fambrough, also the local emergency management coordinator, has never heard of a tornado-turbine encounter in the area.

Neither has Rick Perry, emergency management coordinator for San Angelo and Tom Green County.

“I haven’t seen it come through on our Texas Division of Emergency Management website that we monitor for different things that are happening around the state,” Perry said

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Of an estimated 1,000 tornadoes occurring annually, only about 10 percent rank high on the Enhanced Fujita scale, Mehta said.

The EF scale is based on the aftermath from tornadoes and measures wind damage.

An EF-4 tornado packs estimated winds of 166 mph to as much as 200 mph.

An EF-5 is anything more than that.

“There’s not too many things built by man that could withstand a direct hit from a tornado,” said Dennis Hechel, an engineer with Viryd Technologies, a small startup, and a wind-energy class instructor at Austin Community College.

Houses aren’t built to withstand severe

tornadoes, either, Hechel said.

“If we did, we’d all be living in bunkers with no windows and a thick steel door,” he said.

Wind turbines are typically designed to hold up to wind loads similar to buildings and other structures, said Dave Simms, testing and operations manager at the National Renewable Energy Laboratory’s National Wind Technology Center in Golden, Colo

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For instance, a local jurisdiction might call for a structure to be able to withstand 120 mph sustained winds and 140 mph 3-second gusts, Simms said.

“Codes usually do not require structures be designed to survive tornado conditions, probably because it would be too costly,” Simms said.

Turbines survive winds of about 50 mph or so by shutting down, he said.

Infrequent higher winds don’t generate enough revenue to justify the cost of fortifying a turbine to keep operating in them, he said.

Mitsubishi Power Systems Americas Inc. produces a machine designed to survive winds of about 156 mph, said Brian Li, a Mitsubishi engineer in Newport Beach, Calif.

Located on wind farms in the Corpus Christi area, they’re made to withstand tropical cyclones like those that hit Japan, said Jonathan Wang, a Mitsubishi spokesman.

“Tornadoes are probably a different beast from a hurricane or a typhoon,” Wang said.

“Hurricane” and “typhoon” are regional names for a tropical cyclone.

Indeed, the unpredictable, violent vortex of a tornado makes it a different beast, a tough one to study.

“We don’t know exactly how to simulate tornadic winds,” said Mehta, former director of the Wind Science and Engineering Research Center at Texas Tech.

Texas Tech researchers have built a simulator that can create a vortex about three to four feet in diameter, he said. They will be able to put models of structures like wind turbines in the simulator to investigate a tornado’s impact.

“It took us about two years to build this, what we call a vortex lab, and it is just starting to do some research,” he said.

Andrew Swift, director of the center, said scientists are first studying the effect of more common weather events on turbines such as the low-level jet.

This river of wind comes down close to the ground on summer evenings, Swift said.

“The answers are fuzzy because we have never done any research on this particular topic because the wind farms are new,” Swift said.

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