Wind energy is created when moving air causes a wind turbine to rotate, powering a motor that generates electricity. The energy of the wind itself derives from differential heating of the air by the sun and the rotation of the Earth, combined with landscape features which shape the air flow. In this way, wind power is an indirect form of solar power.
There are two main forms used to collect wind energy, both of which use wind to cause spin in a wind turbine. The most common is the horizontal axis variety. This type uses large blades extending out from a central hub. The largest of these has a blade span longer than 300 feet and is over 20 stories high. The other type is the vertical axis variety, based on the Darrieus model, which looks superficially like an eggbeater.
The main advantage of wind energy is its non-polluting nature. The generation of power by a wind energy turbine produces no waste gases or other pollutants. However, depending on location, the huge, fast-moving blades can pose a substantial risk to birds and bats. Wind power generators are also expensive to build and maintain, although this is compensated for by the fact that they require no fuel. In addition, because it only works in strong wind, wind power is somewhat unpredictable.