Impact of Atmospheric Stability on Turbine Rotor-Plane Wind Profiles and Power at Wind Power Project Sites

Ground-based remote sensing using SoDAR or LiDAR is becoming an increasingly accepted and even necessary part of wind resource assessment due to the increasing size of turbines. These techniques provide complete wind profiles through the entire layer of air in which a wind turbine rotor operates. They are also a rich source of statistics on the time-varying turbulence profile, and by extension, atmospheric stability. As turbines become larger remote sensing has the capacity to offer insight into turbine performance under real-world conditions. A recent study (Wharton and Lundquist 2012) has suggested that wind turbine power production is sensitive to a number of things beyond the hub height wind speed, including: U-equivalent or a rotor area weighted average wind speed, turbulence across the rotor, and atmospheric stability. That study and earlier studies (e.g. Moore and Bailey, 2008) have shown that sodar turbulence parameters can be used as surrogate measures of stability. The sodar turbulent kinetic energy (TKE) varies diurnally and seasonally in accord with changes in atmospheric stability (Figure 1), supporting the notion that it can be used as a surrogate for stability. The surrogate measures of stability are indirectly related to the ratio of power in the wind computed using the rotor equivalent wind speed to that computed with the hub height wind speed. The present paper examines a variety of rotor plane profile metrics derived from long-term (~ one year) remote sensing measurements at two proposed wind energy project locations. Both projects are on sites with moderate wind speeds, which would be particularly sensitive to the effects of atmospheric stability on the wind profile, in part because of the planned use of a larger rotor sweep. The two projects represent contrasting wind speed characteristics and shear profiles. An additional comparison will be shown with statistics from a third site in Kansas, where the wind resource is more robust.