Wind has been used as a source of energy for thousands of years. For much of the 20th century, however, it was largely replaced by coal, gas, and oil. Today, though, wind is making a comeback as a source of electricity and power.

In modern times, wind energy is produced with wind turbines. A wind turbine is a tall, tubular tower 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 wind turbine. Most have three large blades. These spin parallel to their towers, where the main rotor and generator are located.

Most HAWTs stand about 61 to 91 meters (200 to 300 feet) tall. Their 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 first determines which way the wind is blowing. It then turns the turbine to face the oncoming wind.

Vertical-axis wind turbines (VAWTs) have blades that rotate in complete circles around their tower. The main rotor and generator are located near the ground. VAWTs do not have to face the wind to create electricity. They can be much smaller than HAWTs. Often, they are 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 a controller that turns the turbine on when winds are blowing between 13 and 88 kilometers (eight and 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. These are likely to be spaced out over hundreds of acres.

Wind farms are often located on farmland. In the United States, they are found especially on farmland in Iowa, Nebraska, and Kansas.

Wind farms can also be located offshore. These turbines use the stronger winds that develop above the ocean.

Technology is also being developed to create wind farms at very high altitudes. Jetstreams are fast-moving winds that blow 9,753 meters (32,000 feet) above Earth's surface. Scientists are designing wind turbines that will be able to turn jet streams' energy into electricity. These turbines will be tied to the ground like a kite, but float thousands of meters in the air.

Wind can be difficult to predict. Both its speed and direction change often. Today, this unpredictability prevents wind power from becoming the main source of energy. However, it can be an excellent addition to traditional power sources.

Windmills and the Evolution of Wind Energy

Windmills are the ancestor of wind turbines. The world's first windmill was designed almost 2,000 years ago by the ancient Greek engineer Heron of Alexandria.

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

In both ancient and modern windmills, a drive shaft connects the blades to two large wheels, or millstones, on the floor of the windmill. The wind rotates the blades. The blades then rotate the drive shaft, and the drive shaft rotates the millstones. Grain 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 being used all around the world.

In the late 1800s, wind turbines were developed to generate electricity. They were used for that purpose in both Europe and North America.

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. Such fuels create a great deal of pollution, however. By the 1970s, many people had become interested in finding less-polluting sources of energy. The world's first wind farm was established during this time in the U.S. state of New Hampshire.

Today, there are wind farms in many parts of the world. The largest one in the United States is the Alta Wind Energy Center in Kern County, California. It has more than 300 turbines.

Advantages

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

• Wind cannot be used up.
• Wind is a clean source of energy. Turbines 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 found all over the planet. Turbines can be installed almost everywhere.

Challenges

There are also many challenges in using wind energy:

• Wind energy is cheap to produce once a wind farm has been built. However, the cost to build a wind farm is quite high.
• Wind farms need acres of land. In hilly areas, trees might need to be cut. Many kinds of birds and animals depend on the presence of trees.
• Wind turbines can kill bats and birds.
• Offshore wind farms may damage the ocean ecosystem. To install a wind turbine, the seafloor must be drilled into.
• Some people who live near wind farms complain about the noise. Others say the turbines are ugly.
• Locations that produce great amounts of wind energy are often far away from cities. Transmission lines have to be built to transfer the electricity long distances.

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