Outer Range Wind Conditions in the Continental United States in 2010

Turbine power performance depends on climatic conditions, including wind speed, wind shear, wind veer, inflow angle, and turbulence intensity. The wind energy industry is developing a description of these conditions where they are sorted into the ‘inner range’ where turbine power performance is guaranteed, and the ‘outer range’, where performance may be reduced. The outer range is defined as those times when the wind shear exponent is outside the range 0.0 - 0.35, or when turbulence intensity is outside 5% - 20%. This research focuses on wind speed, shear, and veer. Vaisala, in collaboration with its customers and the National Renewable Energy Laboratory, has built a database of 200,000 hours of Vaisala Triton Wind Profiler data from 2010. These data include 49 locations in 26 states in the continental United States. The database is invaluable for research on seasonal and regional trends of wind conditions that can lead to wind farm underperformance. In this study, wind speed is calculated according to the proposed standard for power curves in IEC 61400-12-1, incorporating wind speed and wind direction across the rotor plane. Wind shear and veer are calculated using Theil-Sen regression, a robust statistical technique shown to be effective in estimating wind shear using remote sensing devices. At each site, every 10-minute measurement is evaluated, and the percentages of inner and outer range climatic conditions are presented. Within outer range conditions, turbine underperformance is estimated with higher granularity based on the wind conditions, and each site is scored with an aggregate loss factor. Results reveal interesting regional and seasonal patterns in inner/outer range frequencies of occurrence, as well as the potential scale of climate-driven variation in turbine performance.