This is an excerpt from the Third Quarter 2011 edition of the Wind Program R&D Newsletter.

Invisible to the eye, wind wakes created by multimegawatt wind turbines can nevertheless strongly impact performance of other turbines downstream. That's why understanding atmospheric effects such as wind shifts and wake behavior can be critical to advancing turbine design and improving their siting within wind farms. While wakes have been studied before, as turbines grow in size—approximately doubling in height over the past five years—they present more complex challenges to researchers and operators.

To gain new insights into turbine wind wakes, the U.S. Department of Energy's Wind Program and the Renewable and Sustainable Energy Institute (RASEI) joined together to fund a high-tech study in April and May of 2011. A multi-organizational team of experts used precise instruments to create a detailed picture of the atmosphere surrounding large turbines. Among these instruments was the high-resolution Doppler LIDAR, developed at the National Oceanic and Atmospheric Administration (NOAA), and used for the first time to produce a three-dimensional portrait of atmospheric activity in the wake of a multimegawatt wind turbine. The LIDAR captures a slice of air up to 3,280 feet from the ground and 4.3 miles long (Figure 3). Researchers focused on a 2.3-megawatt turbine that rises 80 meters (328 feet) from its base to its hub. The turbine is installed at DOE's National Renewable Energy Laboratory's (NREL) National Wind Technology Center near Boulder, Colorado.

The interagency team—from NREL, the University of Colorado at Boulder, NOAA, and DOE's Lawrence Livermore National Laboratory (LLNL)—also deployed other high-resolution atmospheric instrumentation. Researchers gathered wind and turbulence data using commercial platforms, including a specialized laser called a Windcube LIDAR and a sonic detection and ranging (SODAR) system, the Second Wind Triton. These data were supplemented by the high-frequency sonic anemometers installed on two new 135-meter (440 feet) meteorological towers at NREL's National Wind Technology Center. Each of these instruments contributes a unique perspective on the dynamic turbine wake system, and these data will be incorporated into advanced computer models. New questions are being explored, such as how wakes in wind farms lower the velocities of downwind turbines, and how the lengths of wakes depend on how strong the wind is as it enters the turbine.

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

Findings will be reported in upcoming journal publications. Also, data will be shared with the international wake modeling community. NREL researchers are optimistic about the analyses. The knowledge gained from this research could lead to improved turbine design standards, increased productivity in large wind farms, and a lower cost of energy from wind power.

The National Renewable Energy Laboratory (NREL), located in Golden, Colorado, provides industry with the technical support it needs to develop advanced wind energy systems. NREL's research capabilities include design review and analysis; software development, modeling, and analysis; systems and controls analysis; turbine reliability and performance enhancement; certification and standards; utility integration assessment; wind resource assessment and mapping; technology market and economic assessment; workforce development; and outreach and education.

The Lawrence Livermore National Laboratory (LLNL) in Livermore, California, has a robust and growing program in wind power to help address the challenges in developing clean and renewable energy. Currently, a staff of nearly 20 scientists and engineers, drawn from programs in atmospheric science, engineering, and computation, are directly involved in wind power. The laboratory includes a 7,000-acre rural facility in the Altamont foothills that is being used for meteorological data acquisition and wind resource characterization.