The success of wind power production depends on wind speed and the actual energy of the wind varies with a multiple power of the wind speed. Double the wind speed, and the wind energy increases more than eightfold.
A 2 kW wind turbine at a site with a wind speed of 12 mph may generate around 1300 kWh/year for a constant wind location, i.e. 150 watts for 8760 hours. At a wind speed of 19 mph, the output could rise to 6000 kWh/year and, at 23 mph, the annual output could be 15,000 kWh. Other factors limit turbine output at very high wind speeds. However, these figures show that good site selection is important for wind power project economics. If the wind only blows for part of the day or week then the number of kWh per year are reduced.
A windy site is starting point for any wind turbine project, but there are other factors too. Wind speed varies with height; the higher a turbine is raised above the ground, the better the wind regime. This benefits larger wind turbines that are placed on higher towers, but larger turbines tend to be more efficient anyway, so additional advantages accrue.
Depending on the efficiency of a wind turbine, there is a cut-off wind speed below which wind power generation is not considered economical. This figure depends on the efficiency of wind turbine design as well as on the turbine cost. With the turbines available at the moment, a wind speed as low as 10 mph is considered economically exploitable at an onshore site. Since offshore costs are higher, an offshore wind speed of 15 mph is needed to make a site economically attractive.
Once a potential site has been identified, it must be studied in more detail to confirm that it is suitable. Long- and short-term wind speed measurements will normally be needed to ascertain the wind regime. Figures for at least one full year will normally be required, longer if possible.
When wind passes over land, the unevenness of the ground and interference to wind flow from trees or undergrowth cause a significant amount of turbulence. Turbulent air creates an additional strain on a wind turbine blade, accelerating the onset of fatigue damage. In order to limit this damage as much as possible, wind turbines are normally placed on a tower, which is tall enough to raise the blades above this turbulent layer of air.
Most wind turbines have blades attached perpendicularly to a horizontal shaft and this arrangement imposes restrictions on the wind turbine design because the turbine must be raised on a tower for the blades to clear the ground and the turbulent layer of air next to it. Additionally, a yawing system is needed so that the rotor and generator enclosure can be rotated as the wind direction changes.
Alternatively, a vertical axis machine has all its weight supported by a ground-level bearing. However, the vertical configuration has not yet achieved significant commercial success.