Why offshore wind turbines can't handle the toughest hurricanes
Offshore wind developments are rapidly expanding. 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.
Researchers predict new offshore turbines would face hurricane wind gusts of more than 223 miles per hour — but the turbines can only manage gusts of 156 miles per hour based on current engineering standards. Part of the problem: Offshore turbine designs often draw from onshore wind turbines in Europe, where hurricane conditions are essentially nonexistent.
"We need to make sure offshore wind energy is successful the first time around," said Rochelle Worsnop, doctoral candidate at the University of Colorado Boulder, who spearheaded the project. "We believe that this research can help guide those standards to help turbines placed in hurricane prone regions survive these major hurricanes."
Offshore wind energy development is growing along U.S. coasts. The first U.S. commercial offshore wind farm went into operation in December, and many more are on the horizon. Offshore wind energy generation could expand the nation's energy supply with potential to provide 160,000 jobs and low-cost energy for millions of Americans, according to a government report.
Worsnop and her colleagues started this project by looking into where hurricane winds cross paths with offshore wind farms. At first, getting this kind of data proved nearly impossible.
Hurricanes that come within striking distance of offshore wind turbines are infrequent. Plus, at the moment, offshore wind developments are few and far between. Most wind measurements she could find in public databases were recorded too high above the water or too far from shore to reflect what a wind turbine might experience.
So, Worsnop's team member — George Bryan of the National Center for Atmospheric Research — recommended she use a computer simulation driven by hurricane data from the last 15 years. Bryan used this high-resolution model to recreate the worst of the worst — a category-5 hurricane eyewall, where winds can exceed 220 miles per hour — to see how wind turbines would hold up. The team also investigated how wind characteristics, such as changes in direction and turbulence, might affect turbines.
Researchers found the extreme wind speeds they modeled would cause structural damage to wind turbines and possible failure of turbine parts. When wind speeds from typhoon Usagi in southern China exceeded turbine specifications in 2013, for instance, blades bent and towers toppled over.
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 and a senior author on the study, told NewsHour via email.
Their model also predicted wind direction changes up to 55 degrees between the ground and the tip of a blade — a measurement called veer. As a result, these category-5 winds could bend a turbine blade in one direction — say, at the tip — as it simultaneously applies stress on another portion, causing the blade to malfunction or break.
"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."