NREL Lamar Low Level Jet Project

Measurements needed for Wind Farms

In windy regions throughout the United States, modern wind turbine farms are being built to generate electricity. A significant wind resource resides in the western Great Plains of the United States. This region frequently has the strongest winds at night in the form of a low level jet (LLJ). A LLJ is a fast moving current of air that occurs one hundred to several hundred meters above the surface of the Earth. The LLJ can be several hundred meters thick with slower winds above and below the jet.

To take advantage of higher winds, turbine rotors are being installed at increasing heights. However, recent measurements have also shown a higher likelihood of encountering harmful turbulence at these heights. Wind shear in the stratified flows below the LLJ produces coherent (i.e., organized) turbulence which can damage or shut down a wind turbine rotor. Few observations are available to define the severity and frequency of this turbulence on the scale of the wind rotors.

The Lamar Low Level Jet Project

The National Renewable Energy Laboratory (NREL) Lamar Low Level Jet Project (LLLJP) is a first step in determining the frequency and severity of potentially damaging coherent turbulence at turbine rotor heights. The experiment location was a site where the nocturnal LLJ routinely occurs during the warm season. The experiment results will be used to design and deploy turbines.

The NREL/LLLJP site was at the Golden Prairie Wind Farm, under construction by General Electic, south of Lamar, CO. This will be the nation's third largest wind farm with a maximum capacity of 162 megawatts, providing enough electricity for the equivalent of about 160,000 households. For each of the 108 turbines, the rotor height will be 80 m, and the length of each of the three blades will be 40 m.

The experiment involved taking year-long high-frequency wind measurements with tower-mounted anemometers at heights of 50, 65, 80, and 120 m, and a Doppler acoustic wind profiler (sodar). The sodar was intended to take vertical wind profiles above the tower measurements, however, the sodar signal routinely disappeared at or below the LLJ height.

Lidar Measurements

The program ended in September 2003 with a two-week intensive measurement campaign using the NOAA ETL High-Resolution Doppler Lidar (HRDL) to map coherent turbulence structures associated with the LLJ. The purpose of the lidar measurements was to:

• characterize the strength and height of the LLJ,
• compare the lidar-determined LLJ height with the top of the sodar signal,
• compare the lidar and tower velocity measurements, and
• characterize the intensity and extent of the coherent turbulence structures in the LLJ.