Wind turbines take the mechanical energy of the wind and use it to spin a mechanism (blades, a cage, or other design) that turns a generator, producing electricity.
We've taken advantage of windpower ever since someone realized, in about 3200 BC, that attaching a sail to a reed boat allowed them to sail upstream on the Nile River. The first windmills were built in Persia around AD 900, and the first electrical generating wind turbines were designed in the late 19th Century.
As of June 2012, wind power accounts for three percent of the US's total electrical generation, and an unknown but considerable amount of electrical power is conserved due to use of windmills rather than electric pumps for tapping groundwater. California, long a pioneer in wind power production, gets about five percent of its consumed power from wind. Texas and Iowa have outstripped California in installed wind generation capacity in the last few years. Unsurprisingly, the states that get the highest percentage of their power from wind are clustered on the windy northern plains: the Dakotas, Wyoming, Minnesota and Iowa. South Dakota leads the pack, getting more than a fifth of its power from wind.
Wind power is now economically competitive in many parts of the US, costing 5-8 cents per kilowatt hour.
Most wind installations are large, both because the newest turbines themselves are quite imposing -- towers up to 300 feet tall, and blades as long as 185 feet for the most common new designs -- and because turbine towers must be widely spaced to avoid turbulence from nearby towers. The scale of these installations has provoked opposition. Some opponents object to the change in quality of life for the turbines' neighbors, and cite the prevalence of "wind turbine syndrome," a suite of physical complaints similar to chronic stress symptoms. Others express concerns about impacts -- some of those impacts literal -- on wildlife. Wind turbines do kill significant numbers of birds, some of them large and rare. The turbines have also been implicated in bat deaths.
The scientific community has not reached a definitive verdict on the existence of wind turbine syndrome. A 2009 study found no evidence to support the existence of the syndrome. Critics have pointed out that that study was funded by wind industry trade associations. Regardless of the eventual scientific verdict, wind installations are unarguably industrial in nature, and it makes sense that local residents might incur stress when a new industrial facility is built near their homes.
Some wind turbine opponents cite the threat from "dirty electricity," a form of electromagnetic field created by wind generators and their associated transmission lines. There is no credible scientific evidence that "dirty electricity" poses a significant health hazard.
It's possible that some of the effects on wildlife may be addressed through design. The majority of commercial wind turbines are horizontal-axis turbines, with the familiar long blades that spin like a pinwheel. Studies indicate that this design is quite difficult for many birds to see.
The main alternative to horizontal-axis turbines is a vertical-axis configuration, in which windmill blades or scoops spin vertical shaft, which in turn drives a ground-mounted generator. The two main types of vertical-axis wind turbines are the Darrieus turbine, which in its basic form resembles a large egg beater, and the Savonius turbine, which uses semicylindrical scoops rather than airfoil blades. Vertical-axis wind turbines seem to be much more visible to birds, and would likely address the wildlife mortality issue. They're also much more suitable for use in crowded urban settings. However, vertical-axis wind turbines are also less efficient than their horizontal cousins at converting wind energy into electrical power. Additionally, torque creates more stress on vertical-axis turbines, which increases mechanical failures. Vertical-axis turbines must therefore either be made more heavily, further reducing efficiency, or be subject to frequent and costly repair.
It's possible that advances in design and materials science will allow creation of a vertical-axis turbine that addresses or minimizes the torque and wear problem and maximizes efficiency. If so, we may see a developing market in micro-wind generation, with smaller rotors taking up suitable windy spots in cities.
Until then, developers will continue to rely on horizontal-axis turbines, and safety issues will have to be addressed through consideration of siting. (Most wind experts agree that California's Altamont Pass wind facility, built in the 1970s in a major raptor migration route, was built in the wrong place.)