Wind speed

Map of available wind power over the United States. Color codes indicate wind power density class.

As a general rule, wind generators are practical where the average wind speed is 10 mph (16 km/h or 4.5 m/s) or greater. An 'ideal' location would have a near constant flow of non-turbulent wind throughout the year with a minimum likelihood of sudden powerful bursts of wind. A vitally important factor of turbine siting is also access to local demand or transmission capacity.

Usually sites are preselected on basis of a wind atlas, and validated with wind measurements. Meteorological wind data alone is usually not sufficient for accurate siting of a large wind power project. Collection of site specific data for wind speed and direction is crucial to determining site potential. To collect wind data a meteorological tower is installed with instruments at various heights along the tower. All towers include anemometers to determine the wind speed and wind vanes to determine the direction. The towers generally vary in height from 30 to 60 meters. The towers primarily are guyed steel-pipe structures which are left to collect data for one to two years and then disassembled. Data is collected by a data logging device which stores and transmits data for analysis. Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can sometimes double energy production.

Altitude

The wind blows faster at higher altitudes because of the reduced influence of drag of the surface and lower air viscosity. The increase in velocity with altitude is most dramatic near the surface and is affected by topography, surface roughness, and upwind obstacles such as trees or buildings. Typically, the increase of wind speeds with increasing height follows a wind profile power law, which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%.

Wind park effect

Wind farms have many turbines and each extracts some of the energy of the wind. Where land area is sufficient, turbines are spaced three to five rotor diameters apart perpendicular to the prevailing wind, and five to ten rotor diameters apart in the direction of the prevailing wind, to minimize efficiency loss. The "wind park effect" loss can be as low as 2% of the combined nameplate rating of the turbines.

Environmental and esthetic impacts

See Wind power environmental effects for a discussion of environmental impacts of wind power. Near-shore and certain inland wind sites may have significant esthetic impact, since the turbines are visible for great distances. Wind farm siting must also consider impacts on wildlife, including migratory animals. Wind project proponents may face opposition from area residents concerned about sound level, light flicker, appearance, and the other impacts of wind turbine placement.