April 27, 2011

Researchers To Study Wind Turbine Wakes

While wind turbines are an important source of renewable energy, they also produce wakes of invisible ripples that can affect the atmosphere and influence wind turbines downstream. Researchers are have now launched a study to make the ripples visible in order to study how they impact the atmosphere.

Julie Lundquist, assistant professor in the atmospheric and oceanic sciences department at University of Colorado Boulder, and researchers from the National Oceanic and Atmospheric Administration (NOAA), the National Renewable Energy Laboratory (NREL), and Lawrence Livermore National Laboratory, are hoping the study -- Turbine Wake and Inflow Case -- will improve energy production at wind farms across the country.

The National Wind Technology Center has several wind turbines set up in the Rocky Mountains that will be the center of the new study coming next month.

Scientists will also collect valuable data that will help validate the wind flow models developed at Livermore and other laboratories and universities.

"This study is part of a larger suite of observational and model development efforts under way at LLNL to help attain aggressive state and national targets for renewable energy deployment," said Jeff Mirocha of LLNL. "This field campaign dovetails with ongoing observational studies at our Site 300 that are focused on understanding the complex wind patterns occurring in hilly, coastally influenced locations, which is similar to much of California's wind resource."

"Today's massive wind turbines stretch into a complicated part of the atmosphere," said Lundquist, who also is a joint appointee at NREL. "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 prediction models to predict power generated by the wind, so wind farms can operate more efficiently while providing more power. They are looking at prediction time frames ranging from an hour to days ahead of time.

During the study, researchers will collect meteorological data for validation of turbine wake models in a range of atmospheric stability conditions, including wind speed, wind direction and streamwise variance profiles.

Bob Banta, atmospheric scientist with NOAA's Earth System Research Laboratory, has spent the last several years using a high-resolution, scanning Doppler lidar to make three-dimensional portraits of wind speeds and directions in the atmosphere.

Researchers aim to capture turbulence and other wake effects in a broad wedge of air up to 4.3 miles long and 3,280 feet high. The team will use the lidar to make a detailed look at the atmosphere both in front of and behind one of the large turbines on the NREL site -- a 2.3 megawatt tower that rises up to 492 feet at maximum blade height.

Researchers hope to capture the effects of ramp up and ramp down events, when winds suddenly gust high or die down, and they will gather data on what happens downstream when winds shift direction quickly.

"This generation of wind turbines is stretching up into a complicated part of the atmosphere," Lundquist said. "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."

"The wake effect has been modeled in wind tunnel studies and numerical models," said Banta, "but the atmosphere is different, it's more variable and complicated."

Researchers will also use a specialized laser called a Windcube lidar and a sonic detection and ranging system called a Triton sodar to measure wind and turbulence. NREL has also installed two meteorological towers, each 442 feet high, which will be used to measure air temperature, wind and turbulence.

"Even fluctuations in air temperature throughout the day can affect wind turbine wakes," said Lundquist. "The resulting changes in wake behavior can impact the productivity of wind farms with many rows of turbines, so it's important to observe them in detail and understand how to minimize their impacts."

According to the American Wind Energy Association, wind energy made up 2.3 percent of US electricity by the end of 2010, up from 1.8 percent a year ago.

Researchers have argued that to attain the Department of Energy goal of "20 percent by 2030," the turbulent lower atmosphere -- and its effects on turbines and turbine arrays -- must be better understood.

Members of the study include: Banta and Yelena Pichugina of NOAA; Lundquist of the University of Colorado at Boulder and NREL's National Wind Technology Center; Jeff Mirocha of LLNL; Matthew Aitken, Michael Rhodes, Brian Vanderwende, Robert Marshall, University of Colorado at Boulder graduate students; and Neil Kelley and Andrew Clifton of NREL's National Wind Technology Center.


Image Caption: Normally invisible, wind wind wakes take shape in the clouds behind the Horns Rev offshore wind farm west of Denmark. Credit: Vattenfall


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