Impacts to wind turbines from hurricanes and the potential of turbines to mitigate wind speeds

For the first time in the history of the burgeoning U.S. wind industry, a wind farm got hit by a hurricane — and it was back producing power within days.

Harvey made landfall as a Category 4 hurricane with winds of more than 130 miles an hour last Friday about 20 miles from the Papalote Creek Wind Farm near Corpus Christi, Texas.

One section of the onshore wind farm was producing electricity on Thursday and the other is expected to be back online on Friday, according to its owner, German power company E.ON SE.

“Papalote actually survived really well,” said Patrick Woodson, chairman of E.ON’s North American operations. The delay in restarting was mostly because the power lines were damaged, he said.

Weather gauges suggest the wind farm didn’t take the brunt of the storm: they recorded sustained winds of 90 miles an hour, or the equivalent of a Category 1 hurricane on the Saffir-Simpson scale.

The wind farm has total of 196 turbines and can generate 380 megawatts, making it a fairly large power generator. The turbines were made by Vestas Wind Systems A/S and Siemens AG.

The storm was the first major test of how U.S. wind power installations, which now provide roughly 6% of the nation’s electricity, hold up in hurricane-force winds.

Justin Sharp, a consultant and chairman of the American Meteorological Society’s renewable energy committee, said he wasn’t surprised the wind farm survived. The turbines are designed to shut down and feather their blades when the wind gets too strong.

He said turbines would probably begin to fail when winds reached 140 miles an hour, with blades detaching and smashing into nearby towers. At those speeds, Mr. Sharp said, “all bets are off.”

A Category 4 storm, super typhoon Usagi, hit a Chinese wind farm in 2013 and did extensive damage.

Whether a wind farm can survive a hurricane isn’t a mere academic question. More wind farms are being built close to coastlines and offshore, where wind speeds tend to be strongest.

Mr. Sharp said he calculated that a wind farm build right on the Texas coast would have a 15% chance of facing a hurricane in a 20-year lifespan.

“What we learned from Harvey is that the design being used right now and the operating rules work very well,” said Julie Lundquist, a professor of atmospheric sciences at the University of Colorado Boulder. For a milder hurricane, this is a “success story.”

She was the co-author of a recent study that found offshore wind farms would likely not be able to withstand a direct hit from a Category 5 hurricane, the strongest. She and a student are now working to determine what the probability of an offshore wind farm facing such a storm would be.

The first offshore wind farm in the U.S. went into operation earlier this year off the coast of Rhode Island, but there has been significant offshore development in Europe’s North Sea.

Jacobson and Univ. of Delaware research in Nature Climate Change and conclusions that

  • Walls of wind turbines can dissipate outer rotational near-surface hurricane winds by 25-39 m/s (56-88 mph), or up to 50% and storm surge by 12-72%.
  • Turbines first see slower outer rotational winds, reducing these wind speeds, reducing wave heights, friction, and convergence to the center, increasing central pressure by up to ~16 hPa.
  • Replacing fossil fuels with offshore turbines reduces hurricane damage and the need for sea walls, but also air pollution and global warming and provides electric power with zero fuel cost.

According to the computer model, the reduced winds would in turn lower the height of ocean waves, reducing the winds that push water toward the coast as storm surge. The wind farm decreased storm surge — a key cause of hurricane flooding — by up to 34 percent for Hurricane Sandy and 79 percent for Hurricane Katrina.  Jacobson and study co-author Willett Kempton, professor in UD’s College of Earth, Ocean and Environment, weighed the costs and benefits of offshore wind farms as storm protection.  The net cost of offshore wind farms was found to be less than the net cost of generating electricity with fossil fuels. The calculations take into account savings from avoiding costs related to health issues, climate change and hurricane damage, and assume a mature offshore wind industry. In initial costs, it would be less expensive to build seawalls, but those would not reduce wind damage, would not produce electricity and would not avoid those other costs — thus the net cost of offshore wind would be less.  The study used very large wind farms, with tens of thousands of turbines, much larger than commercial wind farms today. However, sensitivity tests suggested benefits even for smaller numbers of turbines.

Normally invisible, wind wind wakes take shape in the turbulence induced clouds behind the Horns Rev offshore wind farm west of Denmark. Image: NOAA

But most wind turbines are not built to withstand a direct hit from the strongest hurricanes, according to a new study in Geophysical Research Letters that models the worst-scenarios caused by category-5 storms.

Large and fast changes in wind direction could be problematic too, based on Worsnop’s model. Wind turbines work best when facing directly into the wind, so turbine rotors swivel about the tower to maintain a wind-in-the-face orientation. The researchers found most turbines would not twist fast enough to respond.

“We are learning more about the anatomy of a hurricane, which is improving the design resilience of future wind turbines,” Walt Musial, an engineer at the National Renewable Energy Laboratory. “One of the benefits of this study is that you can get a much better global, spatial quantification of that veer — and that’s fabulous, that’s exactly what a wind turbine designer needs,” said Sandy Butterfield, chairman of the International Electrotechnical Commission Renewable Energy (IECRE), the organization that writes the standards for wind turbines and other renewable energy equipment.

The researchers behind the study are now guiding a revamp of turbine engineering standards. Musial said they may take three years to implement.

“The simulation is the best estimate we have. It’s more accurate than any other estimate for the kinds of winds that could really damage a wind turbine,” Butterfield, who was not involved in the study, said. “It’s going to help us update the standards to reflect wind turbine design criteria for hurricanes.”