Simultaneous Spectral Analysis of Wind Turbine Wakes at the Surface and Aloft in Different Stability Conditions

Wind energy is a popular clean energy source and wind turbines are now prolific in agricultural regions. Seven of the ten states with the largest agricultural productions are also in the top ten states with the largest installed wind energy capacity (US Department of Agriculture, American Wind Energy Association). In the Midwestern US, farmers supplement income with land leases to wind energy companies, who benefit from the strong wind resource. However, downwind of wind turbines at the height of the turbine blades the atmosphere experiences increased turbulence and decreased wind speeds in areas known as wakes. The effects of these wakes on the surface environment, where crops may be, are not yet well understood. If wakes reach the surface, then turbulent fluxes at the surface may change due to the enhanced turbulence. Such changes to turbulent transport of heat, momentum, water vapor, and CO₂ can influence crop respiration. Thus, improving our understanding of wake propagation from aloft to the surface is necessary for harmonious deployment of wind turbines near agriculture.

Prior studies applied wind-profiling lidars to study wind turbine wakes in different stability regimes or have analyzed surface flux station data when turbines are simply “on” or “off”. Using data from the 2011 Crop Wind Energy Experiment (CWEX-11), we use spectral analysis to relate, for the first time, the wind turbine wake aloft with surface wake effects in a range of atmospheric stability conditions. By combining surface flux station and lidar data, we can understand the net effect of turbine wakes and their propagation.

Our results show statistically significant wake effects at both the surface and aloft that are strongest when winds are fast and the atmosphere is stable. However, spectral characteristics of surface wake effects differ from those of wakes aloft. Wakes aloft manifest as a broad spectral increase in power (1 to 3600 seconds). In contrast, surface wake effects show an increase in power only from 0.05 to ~10 seconds but with every stronger increases at specific periods. The most prominent of such periods is around ~3 seconds and is related to the rotation rate of the turbine blades. These peaks occur at all measured distances behind the turbine (3.5 D, 9 D, and 14 D downwind, where D is the turbine rotor diameter of approximately 80 m). Interestingly, this peak (of over five orders of magnitude) emerges in both the cross-stream and vertical components of wind speed, but not the streamwise component of wind speed. Different surface wake effects in the streamwise component of wind speed do appear at 9 D and 14 D downwind. These different manifestations of wake effects imply that the role and occurrence of surface wake effects may be more complex than previously hypothesized. In this presentation, we further delve into the differences between surface and aloft wake effects and describe the physical mechanisms behind these differences, which could have implications for crops.