Wind energy is produced by the movement of air. This movement is converted into power for human use. Wind has been used as a source of energy for thousands of years but was replaced by coal, gas and oil for much of the 20th century. Today, wind is making a comeback as a source of electricity and power.

Wind energy is produced with wind turbines — tall, tubular towers with blades rotating at the top. When the wind turns the blades, the blades turn a generator and create electricity.

Horizontal-axis wind turbines (HAWTs) are the most familiar type of electricity-producing windmill. Most have three large blades that spin parallel to their towers, where the main rotor and generator are located.

Most HAWTs are painted white to help make them visible to low-flying aircraft. They stand about 61 to 91 meters (200 to 300 feet) tall, and the blades rotate at 10 to 20 rotations a minute.

The enormous, stiff blades on a HAWT usually face the wind. A wind vane or wind sensor determines which way the wind is blowing, and turns the turbine to face the oncoming wind.

Vertical-axis wind turbines (VAWTs) have varied, unusually shaped blades that rotate in complete circles around a tower. The main rotor and generator are located near the ground. VAWTs do not have to face the wind to generate electricity. Vertical-axis wind turbines can be much smaller than their horizontal counterparts. They are often installed on the roofs of buildings.

Turbines cannot operate at every wind speed. If winds are too strong, they can be damaged. Therefore, the turbine has an automatic controller that turns on when winds are blowing at ideal speeds for generating electricity. This speed is usually 13 to 88 kilometers (eight to 55 miles) per hour. If the winds become stronger than that, the controller turns the turbine off.

Wind Farms

To generate a large amount of electricity, wind turbines are often constructed in large groups called wind farms. Wind farms are made up of hundreds of turbines, spaced out over often hundreds of acres.

Wind farms are often located in agricultural areas, where the land between the turbines can still be used for farming. Grazing animals are unaffected by the large, slow-moving turbines. In the United States, the "Corn Belt" overlaps with the "Wind Belt," an area across the Midwest that is ideal for harvesting crops and wind. Wind turbines tower over farmland in Iowa, Nebraska, and Kansas.

Wind farms can also be located offshore. These turbines use the stronger, more predictable, and more frequent winds that develop above the ocean.

Technology is also being developed to create wind farms at extremely high altitudes. Jet streams are fast-moving winds that blow at elevations of 9753 meters (32,000 feet). Scientists are developing a wind turbine that would be tied to the ground like a kite, but float thousands of meters in the air to capture jet streams' energy for electricity.

Wind is inconsistent and can be difficult to predict. Its speed and direction change frequently, depending on other conditions, such as temperature, humidity and season. Today, this unpredictability makes wind power a poor candidate to be the primary source of energy. However, it can be an excellent addition to traditional power sources.

Windmills and the Evolution of Wind Energy

For thousands of years, people have harnessed the energy of the wind. They used wind to power boats and to cool homes. The ancient Greek engineer Heron of Alexandria designed the world's first windmill almost 2,000 years ago.

Windmills work similarly to wind turbines. In fact, the only difference between windmills and wind turbines is in how the energy they harness is used. Wind turbines generate electricity. Windmills were originally designed to grind grain and pump water.

In both ancient and modern windmills, a driveshaft connects the rotating blades to two large wheels, or millstones, on the floor of the windmill. The wind rotates the blades, the blades rotate the drive shaft, and the driveshaft rotates the millstones. Grain, such as barley, is poured into the hollow, rotating millstone and crushed into flour as the wheels grind together. Windpumps, or water-pumping windmills, operate similarly.

Windpumps have as many as a dozen rotating blades. Rotation of these blades causes a long rod to move up and down, and the motion of the rod raises and lowers a cylinder. During the down stroke the cylinder fills with water, and during the up stroke, the water is raised to a pipe or well. Wind pumps are still a familiar sight in many parts of the world.

Eventually, wind turbines were developed to generate electricity in Europe and North America. The first wind turbine generated electricity for the Maykirk, Scotland, home of inventor James Blyth in 1887.

However, wind energy fell out of favor in the 20th century. Fuels such as coal, oil, and natural gas were seen as more reliable sources of electricity and energy. By the 1970s, though, the growing environmental movement had led people to seek less-polluting sources of energy. The world's first wind farm, a cluster of 20 turbines, was established during this time in New Hampshire.

Today, wind farms are constructed in many areas. The United States has the largest capacity for wind energy in the world and has developed wind farms in the Midwest, deserts and foothills. The largest wind farm in the United States is the Alta Wind Energy Center in Kern County, California, which consists of more than 300 turbines.

Advantages

There are many advantages to using the wind's energy to create electricity.

• Wind occurs naturally and cannot be used up.
• Wind is a clean source of energy. Turbines have no emissions and do not pollute the air.
• Wind energy is cheap! In the United States, it costs between four cents and six cents per kilowatt-hour.
• Wind is generated all over the planet, and wind turbines can be installed economically almost everywhere.

Challenges

There are also many challenges in using wind energy:

• Even though wind energy is cheap, the initial cost to build a wind farm is quite high.
• Wind farms require acres of land and must compete with other uses. When planning a wind farm in a hilly area, where winds are steady and strong, trees might need to be cut. This likely destroys habitats of dozens of species.
• Wind turbines can kill bats and birds.
• Offshore wind farms might damage the marine ecosystem. The seafloor must be disturbed and drilled to install a wind turbine.
• Some residents who live near wind farms complain about the noise or appearance of the machinery.
• Locations that produce great amounts of wind energy are often in remote areas, far away from the cities and people who could use it. Transmission lines have to be built to transfer the electricity to the cities.

The biggest problem with wind energy is, of course, the wind itself. When the wind is not blowing, electricity cannot be generated.