Alternative Energy Sources

Although today considered an alternative energy source, wind power is one of the earliest forms of energy harvested by humans. Wind is caused by the uneven heating of Earth's surface, and its energy is equivalent to about 2% of the solar energy reaching the planet. The amount of energy theoretically available from wind is thus, very great, although it would be neither practical, wise, nor necessary to intercept more than a tiny percentage of the world's total windflow.

Wind is usually harvested by windmills, which may either supply mechanical energy directly to machinery or drive generators to produce electricity. (Energy must be carefully distinguished from electricity; electricity is not a source of energy, but a form of it. In processes that burn chemical or nuclear fuel to generate electricity, more energy is lost as low-grade heat than is delivered as electricity; a windmill, likewise, supplies less usable energy when it is used to generate electricity than when it is used to do mechanical work. Electricity has the positive qualities of being transmissible over long distances via powerlines and of being useful for many applications—lighting, motors, electronics, and so on—at its points of end-use.) The kinetic energy of wind is proportional to its velocity, so the ideal location for a windmill generator is in a place with constant and relatively fast winds and no obstacles such as tall buildings or trees. An efficient windmill can produce 175 watts of electricity per square meter of propeller-blade area at a height of 75 ft (25 m). The estimated cost of generating one kilowatt-hour (the amount of energy consumed by ten 100-watt light bulbs in one hour) by wind power is about eight cents, as compared to five cents for typical hydropower and 15 cents for nuclear power. California leads the United States in utilization of wind power, producing approximately 1.3% of its electric usage in 2000 from wind, enough to light San Francisco. Denmark leads the world in this respect, presently obtaining 21% of its electricity from windmills (and six more percent from other renewable sources).