Wind turbine wakes produce invisible ripples that can affect the atmosphere and influence downstream units. Recent computer research into turbine spacing indicates about 15 rotor diameters are sufficient to dissipate wake effects and maintain the output of downwind machines. Because more detail is needed, researchers have launched a study to make the ripples visible to observe their affect on the atmosphere.
Julie Lundquist, assistant professor in the atmospheric and oceanic sciences department at University of Colorado Boulder along with researchers from the National Oceanic and Atmospheric Administration (NOAA), the National Renewable Energy Laboratory (NREL), and Lawrence Livermore National Laboratory (LLNL), are conducting the study–Turbine Wake and Inflow Case–to improve energy production at wind farms across the country.
The National Wind Technology Center with several turbines near Boulder, provided equipment of the study. (The data collection phase completed recently and the analysis has begun.) The collected meteorological data included a range of atmospheric stability conditions, including wind speed, wind direction, and streamwise variance profiles.
The ultimate laboratory shows turbine wake extending back many rotor lengths. But air flow and patterns between the turbines and above them remain unknown.
The data will help validate wind-flow models developed at Livermore and elsewhere. “The study is part of a larger suite of observations and model-development efforts under way at LLNL to help hit aggressive state and national targets for renewable energy deployment,” says LLNL’s Jeff Mirocha. “This field campaign dovetails with ongoing observational studies at our Site 300 that focused on understanding complex wind patterns occurring in hilly, coastally influenced locations, which is similar to much of California’s wind resource.”
Today’s massive wind turbines reach into a complicated part of the atmosphere, Lundquist expalins. “If we can understand how gusts and rapid changes in wind direction affect turbine operations and how turbine wakes behave, we can improve design standards, increase efficiency, and reduce the cost of energy.”
LLNL has also been working on numerical weather models to predict power generated by wind. Researchers at the Lab are looking at prediction time frames ranging from an hour to days.
Analysts will examine turbulence and other wake effects in a broad wedge of air up to 4.3 miles long and 3,280 feet high. The team used a lidar (laser detection and ranging) unit for a detailed look at the atmosphere in front of and behind one of the large turbines on the NREL site – a 2.3 megawatt unit with a rotor that reaches up to 492 ft. A goal is to capture the effects of ramp up and ramp down events, when winds suddenly gust or die down, and what happens downstream when winds quickly shift direction.
“The wake effect has been modeled in wind tunnel studies and numerical models,” says Mirocha, “but the atmosphere is different. It’s more variable and complicated.”
The output from a lidar unit tracks time versus altitude, while the color scale indicates wind speeds.
Researchers used Windcube’s lidar and a Second Wind sonic detection and ranging system to measure wind and turbulence. NREL also installed two meteorological towers, each 442 ft high, to measure air temperature, wind, and turbulence. Data analysis will take several months.
WPE
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