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However the Wind Blows
February 7, 2012
As they say,
the devil is in the details. A
spherical cow might be good as a
zeroth-order approximation, but a more precise description requires a deeper analysis. As I reported in two previous articles (
Kind of a Drag, August 10, 2011 and
Bigger than a Butterfly, January 11, 2011), the performance of an isolated
wind turbine can't be simply scaled to predict the power available from a
wind farm.
Turbines upwind from others on a wind farm can affect the performance of downwind turbines. Even when there's considerable spacing of turbines,
turbulence effects are still seen. For example, the wind turbines of the
Horns Rev wind farm on the coast of
Denmark are spaced seven diameters apart, but the downstream turbines generate about a quarter less
power than the front row turbines.
Now, that's a turbine!
Note turbine technicians, properly tethered, as a size reference.
(LLNL image))
At first thought, such effects, although unwelcome, can be analyzed, and the overall performance of a wind farm can be predicted by
computer modeling. The problem with such models is that they make the assumption that the wind flow against the first row of turbines is a nice,
laminar flow. Scientists associated with
Lawrence Livermore National Laboratory (LLNL), the
Department of Atmospheric and Ocean Sciences at the
University of Colorado at Boulder, and the
National Renewable Energy Laboratory,
Golden, Colorado, have analyzed this assumption and found that it's not quite true.[1-2]
This isn't LLNL's first foray into wind turbine modeling. LLNL has been conducting research on wind turbines under the WindSENSE program of the
US Department of Energy's Office of Energy Efficiency and Renewable Energy. The objective of this program is the generation of reliable forecasting of wind energy production.[3] Wind energy is variable, so its contribution to the
electrical grid must be moderated by other power sources.
California, where LLNL is located, has a considerable wind turbine
installed base. California currently has more than three
gigawatts of installed capacity, and this capacity is growing. Wind farms in the
Tehachapi Pass presently produce 700 megawatts of power, but they are planned for a capacity increase to 3,000 megawatts. Power output from such sites can drop to nearly zero over the course of an hour.[3]
The research, just published in
Environmental Research Letters, studied how
atmospheric variability affects the mean wind speed, wind direction and turbulence across turbine rotor disks.[1] In order to simplify discussion, the authors lump all these factors into the term, wind profile shape. They found that the power dependence on wind quality could be predicted by turbulence, the spatial profile of turbulence, or
wind shear.[1]
Aeolus, Greek god of the winds.
(Photo by Ed Stevenhagen, via Wikimedia Commons))
The atmospheric data for this study were obtained over the course of a year from instruments on an eighty
meter tower upwind from eighty meter high wind turbines at an undisclosed
US West Coast wind farm.
Sonic Detection and Ranging (SODAR) instruments profiled the wind stream from the surface to 200 meters (656 feet). These data were compared just with respect to the upwind turbines to eliminate the affect of turbine wake.[2]
Wind shear, the difference in wind speed and direction with distance was found to have an affect on turbine performance, and strongly
convective atmospheric conditions lead to a 15% reduction in power.[1-2]
As in previous studies mentioned in this blog, the research team found that wind speed, and therefore power production, varied with
season, and from night to day. Wind speed is higher at night and during warmer seasons. They found that average power production was 43 percent of maximum capacity on
summer days, and it was 55% higher than that on summer nights.[2]
References:
- Sonia Wharton and Julie K. Lundquist, "Atmospheric stability affects wind turbine power collection," 2012 Environ. Res. Lett., vol. 7, no. 1 (March, 2012), Document No. 014005.
- Anne M Stark, "Power generation is blowing in the wind," Lawrence Livermore National Laboratory Press Release, January 17, 2012 .
- Anne M Stark, "Lawrence Livermore ramps up wind energy research," Lawrence Livermore National Laboratory Press Release, December 14, 2011.
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