Wind information

The power in the wind varies proportionally with the cube of the wind speed, which has important bearing on the design and citing of wind turbines. As a result, even a small increase in wind speed can substantially boost the power available in the wind. For example, a 25% increase in wind speed approximately corresponds to a doubling in the power contained in the wind, which illustrates the importance of accurate resource assessment to a project’s success.
Accurate assessment of the quality of the wind resource at a proposed project site is a critical first step to the success of that project. Quality can vary significantly from site to site. Obviously, some locations are windier than others; and even within a known wind resource area, the wind resource can vary with location and terrain. Evaluating wind resource quality is further complicated by the fact that for a given site, wind resources generally exhibit seasonal, diurnal, and hourly variations. Wind resource quality is characterized by wind speed and direction, the wind shear or variation of wind speed with elevation, and the intensity of turbulence.

Prior to final site selection, the wind resource is measured for an extended period of time, usually two to three years, to statistically quantify the resource. A meteorological tower or mast is erected at one or more locations to continuously measure wind speed, direction, temperature, and sometimes other weather parameters. The measurements are made at multiple elevations above the ground (typically 10, 30, and 60 meters) to allow the wind shear to be estimated. The resulting data are stored onsite by a data logger and periodically downloaded onsite or remotely by modem. Data are analyzed to resolve erroneous values and calculate average wind speeds, directions, and temperatures over annual, seasonal, monthly, and hourly time intervals. The information is often expressed in wind speed frequency distributions and wind roses, which graphically show the relative frequency of wind speed and direction and wind energy.

Wind turbines are designed to function within a wind speed window, which is defined by the “cut-in” and “cut-out” wind speeds. Below the cut-in wind speed, the energy in the wind is too low to be of use; once the wind reaches the cut-in speed, the turbine comes online and power output increases with wind speed up to the speed for which it is rated. The turbine produces its rated output at speeds between the rated wind speed and the cut-out speed—the speed at which the turbine shuts down to prevent mechanical damage.
Power output and stress on mechanical components at high wind speeds are controlled through active or passive yawing to track wind direction and stall or blade pitch regulation to control power output. Stall-regulated airfoils are designed to lose their lift at high wind speeds and are, therefore, self-regulating. Pitch-regulated turbines vary the pitch of the blade to reduce lift and shave off power in high winds. If the wind speed rises to a cut-out value, the blade feathers and the turbine stops turning to avoid excess loads on the rotor and other mechanical components.

Pitch-regulated blades also provide a means for optimizing the power output at lower wind speeds. Other power-reducing alternatives that have been employed include pitching only the blade tips, tip brakes, and ailerons.