Windmills use wind power and therefore are a renewable source of energy. Windmills are placed in fields, where wind is constantly blowing. The windmills then harvest the wind. Windmills are used in several parts of California, including Altamont and Tehachapi. California contains the world's largest concentration of wind turbines. Not only do windmills not produce air pollution, they also do not contribute to global warming. In the long run they are less expensive to use and they are cost effective. Last but not least, there is an unlimited supply of wind pwer for windmills to use.