Modern utility-scale wind turbines operate in a turbulent, thermally driven atmosphere where wind speed and air temperature vary with height. Turbines convert momentum extracted from the wind into electrical power, and so changes in the atmosphere across the turbine rotor disk influence the power produced by the turbine. Local terrain can also modify the flow, causing acceleration, deceleration, and turbulence. Previous studies have shown that although wind speed governs power production, atmospheric stability and turbulence also determine turbine efficiency. It is therefore essential to have detailed information about the inflow conditions when analyzing the performance of turbines.

To measure and characterize the inflow of the numerous utility-scale turbines on site, the National Renewable Energy Laboratory (NREL) has erected two 135-meter monitoring towers at the National Wind Technology Center (NWTC) near Boulder, Colorado. The towers are instrumented with multiple levels of three-dimensional sonic anemometers, cup and vanes, thermocouples and dewpoint temperature measurements. Together these instruments characterize the flow through the rotor disk, as well as turbulence, fluxes and local atmospheric stratification. The tall-tower data can be combined with observations from other towers at the NWTC, several remote sensing devices deployed in measurement campaigns, and turbine performance data to gain an unprecedented understanding of the interaction between the atmospheric boundary layer and wind turbines.

In this presentation we discuss our approach to acquiring and processing data from the tall towers, including system architecture, quality control, data display and archival approaches. We develop methods to analyze, visualize and quantify mean and turbulent flow fields over short time periods. We also show how the data from the tall towers compares to other sources of data at the NWTC. Early results suggest that thermal forcing plays a significant role in the wind conditions at the NWTC, leading to high or negative shear across the turbine rotor. We have also observed frequent and strong directional shear between the top and bottom of the blade. These data show what can be achieved when measurements extend across the turbine disk, rather than being limited to the lower half of the turbine disk. This data can also be useful for other applications, for example as boundary conditions for computational fluid dynamics simulations or as inputs to noise propagation models.