1. What Is Wind?

Wind is simply air in motion. It is caused by the uneven heating of the earth's surface by the sun. Since the earth's surface is made up of land, desert, water, and forest areas, the surface absorbs the sun's radiation differently.

During the day, air above the land heats more quickly than air above water. The hot air over the land expands and rises, and the heavier, cooler air over a body of water rushes in to take its place, creating local winds. At night, the winds are reversed because air cools more rapidly over land than over water.

Similarly, the large atmospheric winds that circle the earth are created because land near the equator is heated more by the sun than land near the North and South Poles.

Today people can use wind energy to produce electricity. Wind is called a renewable energy source because we will never run out of it.

2. History of Wind Machines

Throughout history people have harnessed the wind. Over 5,000 years ago, the ancient Egyptians used wind power to sail their ships on the Nile River. Later people built windmills to grind their grain. The earliest known windmills were in Persia (the area now occupied by Iran). The early windmills looked like large paddle wheels.

Centuries later, the people in Holland improved the windmill. They gave it propeller-type blades and made it so it could be turned to face the wind. Windmills helped Holland become one of the world's most industrialized countries by the 17th century.

American colonists used windmills to grind wheat and corn, to pump water, and to cut wood at sawmills.

In this century, people used windmills to generate electricity in rural areas that did not have electric service. When power lines began to transport electricity to rural areas in the 1930s, the electric windmills were used less and less.

Then in the early 1970s, oil shortages created an environment eager for alternative energy sources, paving the way for the re-entry of the electric windmill on the American landscape.

3. Today's Wind Machine

Today's wind machine is very different from yesterday's windmill. Along with the change in name have come changes in the use and technology of the windmill.

While yesterday's machines were used primarily to convert the wind's kinetic energy into mechanical power to grind grain or pump water, today's wind machines are used primarily to generate electricity.


Like old-fashioned windmills, today's wind machines still use blades to collect the wind's kinetic energy. Windmills work because they slow down the speed of the wind. The wind flows over the airfoil shaped blades causing lift, like the effect on airplane wings, causing them to turn. The blades are connected to a drive shaft that turns an electric generator to produce electricity.

Modern wind machines are still wrestling with the problem of what to do when the wind isn't blowing. Large turbines are connected to the utility power network-some other type of generator picks up the load when there is no wind. Small turbines are often connected to diesel/electric generators or sometimes have a battery to store the extra energy they collect when the wind is blowing hard.

Types of Wind Machines

Two types of wind machines are commonly used today:

  • horizontal, which has blades like airplane propellers; and

  • vertical, which looks like an egg-beater.

Horizontal-axis wind machines are used the most. They make up 95 percent of all wind machines. A typical horizontal wind machine stands as tall as a 10-story building and has two or three blades that span 60 feet across. The largest wind machines in the world have blades longer than a football field! Wind machines stand tall and wide to capture more wind.

Vertical-axis wind machines make up just five percent of the wind machines used today. The typical vertical wind machine stands 100 feet tall and 50 feet wide. The most popular vertical wind machine today is the Darrieus wind turbine, named after its inventor, J.G.S. Darrieus, a French engineer.

Which Type Is Better?

Each wind machine has its advantages and disadvantages. Horizontal-axis machines need a way to keep the rotor facing the wind. This is done with a tail on small machines. On large turbines, either the rotor is located down wind of the tower acting like a weather vane, or drive motors are used. Vertical-axis machines accept wind from any direction.

Both types of rotors are turned by air flowing over their wing shaped blades. Vertical axis blades lose energy as they turn out of the wind, while horizontal-axis blades work all the time. Also, at many sites, the wind increases as you go higher-above the ground, giving an advantage to tall horizontal-axis turbines. The small tower and ground mounted generators on vertical-axis turbines make them less costly and easier to maintain.

Wind Power Plants

Wind power plants, or wind farms as they are sometimes called, are clusters of wind machines used to produce electricity. A wind farm usually has hundreds of wind machines in all shapes and sizes.

Unlike coal or nuclear plants, most wind plants are not owned by public utility companies. Instead they are owned and operated by business people who sell the electricity produced on the wind farm to electric utilities. These private companies are known as Independent Power Producers.

Operating a wind power plant is not as simple as plunking down machines on a grassy field. Wind plant owners must carefully plan where to locate their machines. They must consider wind availability (how much the wind blows), local weather conditions, nearness to electrical transmission lines, and local zoning codes.

Wind plants also need a lot of land. One wind machine needs about two acres of land to call its own. A wind power plant takes up hundreds of acres. On the plus side, farmers can grow crops around the machines once they have been installed.

After a plant has been built, there are still maintenance costs. In some states, maintenance costs are offset by tax breaks given to power plants that use renewable energy sources. The Public Utility Regulatory Policies Act, or PURPA, also requires utility companies to purchase electricity from independent power producers at rates that are fair and nondiscriminatory.

4. Wind Resources and Energy Production

Where is the best place to build a wind plant? There are many good sites for wind plants in the United States including California, Alaska, Hawaii, the Great Plains, and mountainous regions. Scientists say there is enough wind in 37 states to produce electricity from the wind. Generally, an average wind speed of 14 mph is needed to convert wind energy into electricity economically. The average wind speed in the United States is 10 mph.

Scientists use an instrument called an anemometer to measure how fast the wind is blowing. An anemometer looks like a modern-style weather vane. It has three spokes with cups that spin on a revolving wheel when the wind blows. It is hooked up to a meter that tells the wind speed. (By the way, a weather vane tells you the direction of the wind, not the speed.)

As a rule, wind speed increases with altitude and over open areas with no wind breaks. Good sites for wind plants are the tops of smooth, rounded hills, open plains or shorelines, and mountain gaps that produce wind funneling. The three biggest wind plants in California are located at mountain gaps.

Wind speed varies throughout the country. It also varies from season to season. In Tehachapi, California, the wind blows more from April through October than it does in the winter. This is because of the extreme heating of the Mojave desert during the summer months. The hot desert air rises, and the cooler, denser air from the Pacific Ocean rushes through the Tehachapi mountain pass to take its place. In a state like Montana, on the other hand, the wind blows more during the winter.

By happy coincidence, these seasonal variations perfectly match the electricity demands of the regions. In California, people use more electricity during the summer when air conditioners are used for cooling. Conversely, more people use electricity in Montana during the winter heating months.

Wind Energy Production

How much energy can we can get from the wind? We will use two terms to describe wind energy production: efficiency and capacity factor.

Efficiency refers to how much useful energy (electricity, for example) we can get from an energy source. A 100 percent energy efficient machine would change all the energy put into the machine into useful energy. It would not waste any energy. (You should know there is no such thing as a 100 percent energy efficient machine. Some energy is always "lost" or wasted when one form of energy is converted to another. The "lost" energy is usually in the form of heat.)

How efficient are wind machines? Wind machines are just as efficient as coal plants. Wind plants convert 30 percent of the wind's kinetic energy into electricity. A coal-fired power plant converts about 30-35 percent of the heat energy in coal into electricity.

It is the capacity factor of wind plants that puts them a step behind other power plants. Capacity factor refers to the capability of a plant to produce energy. A plant with a 100 percent capacity rating would run all day, every day at full power. There would be no down time for repairs or refueling, an impossible dream for any plant.

Wind plants have about a 25 percent capacity rating because wind machines only run when the wind is blowing around nine mph or more. In comparison, coal plants typically have a 75 percent capacity rating since they can run day or night, during any season of the year.

One wind machine can produce 275-500 thousand kilowatt-hours (kWh) of electricity a year. That is enough electricity for about 50 homes per year.

In this country, wind machines produce about three billion kWh of energy a year. Wind energy provides 0. 12% of the nation's electricity, a very small amount. Still, that is enough electricity to serve more than 300,000 households, as many as in a city the size of San Francisco or Washington, D.C.

California produces more electricity from the wind than any other state. It produces 98 percent of the electricity generated from the wind in the United States. Some 16,000 wind machines produce more than one percent of California's electricity. (This is about half as much electricity as is produced by one nuclear power plant.) In the next 15 years, wind machines could produce five percent of California's electricity.

Why is California outproducing every other state? More than any other reason, wind energy has taken off in this state because of California's progressive state policies that support renewable energy sources. Other states have just as good wind resources as California.

The United States is the world's leading wind energy producer. The U.S. produces about half of the world's wind power. Other countries that have invested heavily in wind power research are Denmark, Japan, Germany, Sweden, The Netherlands, United Kingdom, and Italy.

What does the future look like for wind energy? Using a best-case scenario, the American Wind Energy Association (AWEA) estimates wind energy could produce more than 10 percent of the nation's electricity within the next 30 years.

So, wind energy may be an important alternative energy source in the future, but it will not be the sole answer to our energy problems. We will still need other energy sources to meet our growing demand for electricity.

5. Wind Energy Economics and the Environment

On the economic front, there is a lot of good news for wind energy. First, a wind plant is far less expensive to construct than a conventional energy plant. Wind plants can simply add wind machines as electricity demand increases.

Second, the cost of producing electricity from the wind has dropped dramatically in the last two decades. Electricity generated by the wind cost 30 cents per kWh in 1975, but now costs less than five cents per kWh. In comparison, new coal plants produce electricity at four cents per kWh.

Wind and the Environment

In the 1970s and 1980s, oil shocks and shortages pushed the development of alternative energy sources. In the 1990s, the push may come from something else, a renewed concern for the earth's environment.

Wind energy is clean. Wind machines produce no air or water pollution because no fuel is burned to generate electricity.

The only environmental drawbacks to wind energy may be a wind plant's effect on bird populations and its visual impact on the surrounding landscape. To some, the glistening blades of wind machines are an eyesore; to others, they're a beautiful alternative to smog-filled skies.






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