A new study has suggested that wind farm operators could come to better understand the amount of power being generated at any one time by looking to the stability of the atmosphere for clues.
The energy generated by a wind farm, as expected, is very much at the mercy of wind speeds. However, the shape of the wind – the turbulence or wind shear – can dramatically alter the amount of power being generated.
This variable level of power can be foreseen by looking at the stability of the atmosphere.
“The dependence of power on stability is clear, regardless of whether time periods are segregated by three-dimensional turbulence, turbulence intensity or wind shear,” said Sonia Wharton, Lawrence Livermore National Laboratory scientist who, along with her colleague Julie Lundquist of the University of Colorado at Boulder and the National Renewable Energy Laboratory, authored the study which appears in the January 12 edition of the journal Environmental Research Letters.
Together the pair examined a years’ worth of turbine-generated power data from upwind modern turbines (80 meters high) at a multi-megawatt wind farm on the West Coast of the U.S, segregated by atmospheric stability, in an attempt to determine the power performance of the wind farm.
They found that power generated at a set wind speed is higher under stable conditions and lower under strongly unsteady conditions, such as turbulence and wind shear — which is a difference in wind speed and direction over a relatively short distance in the atmosphere. The two variances play a large part in determining how much power a turbine generate can create over certain time scales. In fact, the average wind power output difference is as much as 15 percent less when the atmosphere is unstable.
Another result from their study was the fact that wind speed and power production varied by season as well as from day to night.
“We found that wind turbines experienced stable, near-neutral and unstable conditions during the spring and summer,” Wharton said. “But daytime hours were almost always unstable or neutral while nights were strongly stable.”
“This work highlights the benefit of observing complete profiles of wind speed and turbulence across the turbine rotor disk, often available only with remote sensing technology like SODAR or LIDAR (Laser Detection and Ranging,)” Lundquist said. “Wind energy resource assessment and power forecasting would profit from this increased accuracy.”