Offshore wind turbines built according to current standards may not be able to withstand the powerful gusts of a Category 5 hurricane, new University of Colorado Boulder-led research shows.
The study, which was conducted in collaboration with the National Center for Atmospheric Research in Boulder, Colorado, and the U.S. Department of Energy’s National Renewable Energy Laboratory in Golden, Colorado, highlights the limitations of current turbine design.
Offshore wind-energy development in the U.S. has ramped up in recent years, with projects either under consideration or already underway in most Atlantic coastal states from Maine to the Carolinas, as well as the West Coast and Great Lakes. The country’s first utility-scale offshore wind farm, consisting of five turbines, began commercial operation in December 2016 off the coast of Rhode Island.
Turbine design standards are governed by the International Electrotechnical Commission (IEC). For offshore turbines, no specific guidelines for hurricane-force winds exist, so the researchers set out to test the limits of the existing design standard.
Due to a lack of observational data across the height of a wind turbine, they instead used large-eddy simulations to create a powerful hurricane with a computer. These high-resolution simulations showed that under Category 5 conditions, mean wind speeds near the storm’s eyewall reached 90 meters-per-second, well in excess of the 50 meters-per-second threshold set by current standards.
“Wind speeds of this magnitude have been observed in hurricanes before, but in only a few cases, and these observations are often questioned because of the hazardous conditions and limitations of instruments,” said George Bryan, a co-author of the study. “By using large-eddy simulations, we are able to show how such winds can develop and where they occur within hurricanes.”
Additionally, current standards do not account for veer, a measure of the change in wind direction across a vertical span. In the simulation, wind direction changed by as much as 55 degrees between the tip of the rotor and its hub, creating a potentially dangerous strain on the blade.
The next stage of this work is to assess how often these extreme winds would impact an offshore wind farm on the Atlantic coast over the 20-to-30-year lifetime of a typical wind farm.
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