Wind power. Devices that use wind power

Wind power is the energy associated with the air that moves over Earth's surface. The world's growing demand for energy threatens to exhaust the supply of such fuels as coal, oil, and natural gas. But wind is a renewable energy source that cannot be used up. In addition, wind provides clean, nonpolluting energy.

The kinetic energy (energy of movement) of wind throughout the world is estimated at more than 11 quadrillion kilowatt-hours per year. A kilowatt-hour is the amount of work done by 1 ,000 watts in one hour. If people could capture and use only 10 percent of the wind's kinetic energy, it would far exceed the world's yearly energy demand.

Devices that use wind power. People have harnessed the power of the wind to do work for thousands of years. For example, the Egyptians used wind-powered sailing ships as early as 2800 B.C.

Windmills probably originated in Persia, now Iran, during the A.D. 600's. These devices convert a portion of the kinetic energy of the flowing air into rotational kinetic energy that turns a shaft. Most early windmills used this rotational energy to operate machinery to grind grain. But a windmill can also be used to drive other devices, such as a pump to lift water from a well.

Today, modern devices called wind turbines use rotational kinetic energy to drive electric power generators. Wind power was first used to generate electric power in Denmark in the 1890's. Windmills generated electric power in some rural communities of the United States in the early 1900's. An oil shortage in the 1970's focused greater attention on the idea of using the energy of the wind to generate electric power. By the 1990's, there were about 25,000 wind turbines operating in the world. However, these devices produced only about 0.1 percent of the world's electric power.

Wind turbines are used in some parts of the world to generate electric current. The turbines shown here are among thousands set up in California’s Altamont Pass, east of San Francisco

Challenges of using wind power. When a windmill or wind turbine is used to extract wind power, only part of the kinetic energy of the wind is transferred to the blades of the device. Scientists believe it is possible to extract only about 59 percent of the wind's kinetic energy. Modern wind turbines convert only about 40 percent of the energy of the wind that strikes their blades into useful rotational energy.

A number of other factors limit the extraction of power from the wind. These factors include practical limits on the size of wind power devices and the availability of land upon which to erect such devices. Another issue involves environmental concerns. Some people oppose the spread of wind power devices, saying that they pose a danger to birds that fly into the moving blades. Others complain that the devices are unsightly and create too much noise.

Because wind speeds vary greatly over the earth's surface, some sites are more suitable than others for wind power installations. In addition, wind does not blow at a constant speed, and at times it dies down completely. Thus, to ensure a reliable supply of electric power, wind turbines must be combined with either an energy storage system or a backup generator that uses a different energy source.