Comparison and Analysis of Wind Turbine Wake Interactions

Wakes produced by wind turbines interact with other turbines and turbine wakes to create a unique wind farm atmospheric boundary layer. An improved understanding of characteristics of wakes and the unique boundary layer they create will inform design and operation of wind farms toward reducing the levelized cost of wind energy. We have analyzed single wakes and interactions of multiple wakes by use of nacelle data from a 200-turbine utility scale wind farm. These data included 10-min averages of the wind speed, wind direction, temperature, and power generated at each turbine considered in this study. Concurrent micrometeorological measurements at multiple locations within the wind farm provided data on near-surface temperature, humidity, wind speed and direction, turbulence and net radiation for calculations of surface and turbine-layer stability in the surrounding agricultural fields. We found that the wind speed drops significantly after the first turbine, and then gradually recovers to about 75% of its original speed while passing through the next four turbines within the first 1.5 km downwind. Power generated at downwind turbines follows a similar pattern: a second turbine located less than 4 rotor diameters downwind of the lead turbine generates just over 40% of the power generated by the lead turbine, and recovers down the line. Under certain conditions, this recovery may be less, or may generate wind speeds and power even greater than at the lead turbine. Results show that power and wind speed recover best under high wind conditions, in high turbulence, and under unstable surface conditions. For these results, wind speed and power recovery occur faster but with more variance than observed in a comparable study of an offshore wind farm (Barthalmie, et al, 2007). Studies of wind speed and power generated for wind directions slightly off the direction of the turbine line give some insight on lateral structure of the wake.