With the federal government planning to hold the largest sale of offshore wind farm leases in the nation's history, a new Cornell University study could help inform the development of offshore wind farms by providing detailed models characterizing the frequency, intensity and height of low-level jet streams over the U.S. Atlantic coastal zone.
The research, "Occurrence of Low-Level Jets Over the Eastern U.S. Coastal Zone at Heights Relevant to Wind Energy," published in the journal Energies. The study finds that jet streams do occur low enough to reach wind turbine rotor planes at planned wind farms offshore from the U.S. East Coast, according to co-author Jeanie Aird, doctoral student in the Barthelmie Wind Energy Laboratory at Cornell.
Low-level jets -- fast-moving currents of air in the lowest 2 kilometers of the atmosphere -- can have pros and cons for wind turbines. They usually result in an increase in wind speed which can improve turbine performance, such as power output, but they can also increase the loads on turbine blades and towers with higher wind shear and turbulence. Understanding their occurrence is important for planning wind turbine longevity and power production.
Aird analyzed two years of simulations from the Weather Research and Forecasting (WRF) model provided by Sara C. Pryor, professor in the Department of Earth and Atmospheric Sciences at Cornell. She found that low-level jets occur more frequently at the offshore wind energy lease areas south of Massachusetts than those in the New York Bight (stretching roughly from coastal New Jersey to eastern Long Island) and further south.
"Our study specifically focuses on low-level jets that occur with maxima at approximately 500 meters or below," Aird said, "since at lower heights they are more likely to interact with wind turbines."
Aird found that the low-level jets can occur at approximately 150 meters above sea level, which means that they are likely to interact with offshore wind turbines planned for the U.S. East Coast. For example, turbines selected for the Vineyard Wind farm off the coast of Massachusetts have a hub height of 140 meters.
"Our analysis provides maps of low-level jet occurrence showing these phenomena occur up to 12% of the time in the late spring and early summer when there are strong horizontal temperature gradients," Aird said. "We hope this research will assist offshore wind farm developers and we are working to develop a predictive method for low-level jet occurrence."
Prior to this research there was uncertainty about how frequently low-level jets would occur in this area, or their speeds and heights, due to a relative lack of wind climatology measurements for the East Coast. This study is unique in that the team utilized two years of high-resolution WRF data over a wide geographical domain, covering 13 of the 16 planned offshore lease areas to analyze the low-level jets.
Aird co-authored the paper with Pryor; Rebecca Barthelmie, professor in the Sibley School of Mechanical and Aerospace Engineering; and postdoctoral researcher Tristan Shepherd.
The research was funded by the U.S. Department of Energy, the National Science Foundation and the New York State Energy Research and Development Authority through the National Offshore Wind Research and Development consortium.
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