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Wind: Background


Properties of Air

We are all familiar with the feeling of wind blowing. The air around us is constantly in motion, and though invisible, contains a stream of moving particles that flows and mixes above the Earth's surface. Scientists and engineers face great challenges in harnessing the wind for power.

The troposphere is the lowest portion of the Earth's atmosphere. It starts at the Earth's surface and goes up to a height of 7 to 20 kilometers. The air that makes up this layer is a mixture of atoms and molecules of different gases. About 99 percent of dry air is nitrogen (approximately 78%) and oxygen (approximately 21%). Carbon dioxide and argon make up most of the other one percent. Air often contains water vapor as well. The amount varies from close to zero to five percent over time and from one place to another.

atmospheric preasure
Atmospheric pressure (shown on the left)
decreases with altitude (shown on the right).
Source: NOAA

Because air consists of atoms and molecules of matter, it has mass. The density of air is the amount of mass in a given volume of air. This means that if there are more molecules of a given gas in a volume of air, the density is greater. If there are fewer molecules of the same gas, the density decreases.

The gravitational attraction between the Earth and the gases of the atmosphere pulls particles of gas toward the center of the Earth. As a result, gases at the top of the atmosphere press down on the gases below, causing the particles to press closer together and increasing the density of the air. Thus, the density of air increases as you get closer to the bottom of the atmosphere.

Air pressure is the weight of a column of air pushing down on an area. Air pressure at any point depends on the weight of the air above. Sea-level air has the weight of the whole atmosphere pressing on it. Therefore, air pressure is greatest at sea level. Air near the top of the atmosphere has less weight pressing on it, and therefore has lower air pressure. This means that, much like the density of air, air pressure decreases with increasing altitude.

A change in altitude is not the only cause of changes in air pressure and density. Temperature (or heat energy) affects air pressure and density as well. When air is heated, its molecules have more energy and move faster. As the molecules in the heated air move, they bump into each other and move farther apart. As a result, the volume increases and the air becomes less dense. When air is cooled, its molecules have less energy, move slower, and come closer together. As a result, the density of the air increases. There are fewer molecules exerting pressure in a given volume of warm air than there are in an equal volume of cool air. To put it another way, air with fewer molecules has less mass and exerts less pressure. This means that, in general, air pressure decreases as temperature increases.

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Wind

Air is a fluid and can move easily from place to place. Air tends to move from areas of high pressure to areas of low pressure. We call the movement of air wind. The greater the differences in air pressure between two points, the stronger the wind that blows between them.

Most winds are caused by the uneven heating of the Earth's surface by the Sun. When an area of the Earth's surface is heated by radiation from the Sun, some of the heat is transferred to the air above the surface. The heated air expands, i.e. the molecules move quickly and spread out, and the air becomes less dense. As the air becomes less dense, the air pressure decreases. If a nearby area is not heated as much, the air above will be cooler, its molecules will move slower and not spread out. The cooler air will be denser. The cooler, denser air has a higher air pressure because the pressure exerted from its greater mass. As a result, it flows underneath the warm, less dense air, and causes it to be displaced. The lighter and less dense air will be forced to rise, which forms winds.

Local winds are winds that blow over short distances. They are caused by the unequal heating of the Earth's surface within a small area. A good example of how local winds form occurs on land that is next to a large body of water. It takes more energy to warm up a body of water than it does to warm up an equal area of land. This means that as the Sun heats Earth's surface during the day, the land warms up (gains heat) faster than the water. The air over the land becomes warmer than the air over the water. The warm air expands and rises, creating a low-pressure area. The relatively heavier, cooler air blows landward from the water and moves underneath the warm air, creating wind. At night, the process is reversed and winds flow towards the water because the air cools more rapidly over land than over the water.

Winds diagram
Local winds are caused by the unequal heating of the
Earth's surface within a small area.
Source: NOAA

Monsoons are large scale seasonal wind patterns that blow over large regions. The most prominent examples occur in Africa and southern Asia. They are caused by large and consistent differences in air pressure between land and water that occur with the seasons. For example, in the summer in South and Southeast Asia, the land gradually gets warmer than the ocean. A large flow of air blows steadily inland from the ocean all summer, even at night. In the winter, the land cools and becomes colder than the ocean. The air blows steadily from the land to the ocean.

Global winds are winds that blow steadily in specific and consistent directions over thousands of kilometers. Much like local winds and monsoons, global winds are created by unequal heating of the Earth's surface. Near the equator, energy from the Sun strikes the Earth almost directly from overhead. The direct rays from the Sun heat the Earth's surface intensely. Warm air rises in this region, creating a continuous belt of low pressure. In contrast, the polar regions experience the same amount of energy form the Sun, but spread out over a larger area. This is because the sun's rays strike the Earth at a low angle and the heat is dispersed. As a result, temperatures near the poles are much lower than they are near the equator. The air is cold and dense, which causes it to sink, creating a region of high pressure.

Solar Radiation
Incoming solar radiation is not evenly distributed on the
Earth's surface.
Source: NASA

If Earth did not rotate, global winds would blow in a straight line from the poles (area of high pressure) toward the equator (area of low pressure). However, the Earth rotates from west to east underneath the moving winds, making it seem as if the winds curve. As a result, in the Northern Hemisphere, global winds blowing toward the north gradually turn toward the northeast. Global winds blowing toward the south gradually turn toward the southwest. In the Southern Hemisphere, a south wind becomes a southwest wind and a north wind becomes a northwest wind.

Global Winds diagram
Global winds follow a pattern over the Earth's surface.
Source: USGS

The temperature and air pressure differences between the equatorial and polar regions and the Earth's rotation cause global winds to follow a specific pattern. In the Northern Hemisphere, the trade winds blow between 30° north latitude and the equator from the northeast. In the Southern Hemisphere between 30° south latitude and the equator, the trade winds blow from the southeast. The prevailing westerlies blow from the southeast between 30° and 60° north latitudes and from the northwest between 30° and 60° south latitudes. The polar easterlies flow away from the poles to the west until they meet the prevailing westerlies at about 60° north and 60° south latitudes.

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Generating Electricity from Wind

Wind contains kinetic energy (energy of motion) that can be used to generate electricity. Today, wind power is being used on a large scale to produce electricity for communities as well as on a smaller scale in households. The most common method for creating electricity from wind is through a wind turbine.

three-bladed wind turbines
A row of three-bladed wind turbines.
Source: Wikipedia
Inside a wind turbine
Gearbox and generator in a wind turbine.
Source: Wikipedia

Wind turbines convert the moving energy of the wind to mechanical energy, which is then used to produce electrical energy. Most turbines consist of a set of blades. Wind flows over the blades and causes them to turn. The blades are connected to a rotating drive shaft. The mechanical energy of the drive shaft is transferred to a generator through a gearbox. The generator uses the mechanical energy of the drive shaft to move an electric conductor (typically coils of copper wire) through a magnetic field, which causes electrons to flow through the conductor, generating an electrical field.

There are two main designs of wind turbines. The most common is the horizontal-axis turbine. These typically have three blades that turn a horizontal drive shaft at the top of a tower. This turbine looks similar to an airplane propeller. In vertical-axis turbines the drive shaft is arranged vertically. Their blades go from top to bottom. These are much less commonly used.

Horizonatal and vertical axis turbines
Left image: Horizontal-axis turbines. Source: Wikipedia
Right image: Vertical-axis wind turbine. Source: Wikipedia
Commercial wind turbine
Large commercial wind turbines.
Source: Wikipedia

Wind turbines vary in size, depending on use. Generally, the electricity produced by a turbine is proportional to its size. For example, a household might use a small 100 kilowatt turbine with a blade length of one meter. Large commercial turbines can have capacities up to 5 million watts (5 megawatts) and blades between 40 and 50 meters that sit atop towers up to 80 meters tall. These turbines can meet the annual electricity needs of up to several thousand households.

In some places, wind power plants, or wind farms, are being built to generate electricity for large numbers of homes. A wind power plant has many commercial wind turbines scattered over a large area. The number of turbines can vary from a few dozen to several hundred. Wind farms can cover tens of km2 or, for the largest ones, more than 150 km2. The electricity produced by the turbines is supplied to a power grid, in the same way it is done with other kinds of power plants, such as those that are coal-fueled.


Roscoe Wind Farm
The Roscoe Wind Farm in Texas has more than 600 hundred wind turbines.
Source: Wikipedia

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Wind Energy Production

A number of factors are considered when planning to use wind power for electricity generation. A preferred location for a wind turbine is one where the wind is strong and constant. Generally, wind speed increases with altitude. Friction with the Earth's surface slows wind down while at higher altitudes, friction is less and wind speeds are greater. Other ground obstructions, such as trees and buildings, act as windbreaks and reduce the speed of surface winds. The best places to place wind turbines are exposed high areas, like the tops of smooth rounded hills or small mountains. Other good places are open plains or coastal areas, where consistent winds blow. Some turbines are even placed off the coast in the sea. Other factors which affect the installation of wind turbines are the cost of transporting the turbine to an area and assembling the turbine. It can cost several million dollars to purchase and install a commercial scale wind turbine.

offshore and open plains wind turbines
Left image: Offshore wind turbines.
Source: Wikipedia
Right image: Open plains are good locations for generating electricity from wind power.
Source: Wikipedia

Wind speeds varies across the United States according to physical geography. The following map shows typical wind velocities in various parts of the country. Some regions, such as the Rocky Mountains, the flat Midwest states, and certain coastal areas, have a higher potential for wind energy than others.

Today, the capacity of the tens of thousands of wind turbines installed across the United States is over 40,000 megawatts. Texas is the top state in the generation of electricity from wind turbines with a wind-power capacity of over 10,000 megawatts. The next top five states are Iowa, California, Minnesota, Washington, and Illinois. The electricity produced by all the turbines throughout the United States is enough to power the equivalent of over 9.7 million homes.

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Advantages and Disadvantages of Wind Energy

Wind energy has a number of benefits over other sources of energy. In certain places, such as sites at high altitudes where winds are strong and constant, wind is an almost unlimited energy resource. Wind turbines are a form of clean energy and do not emit pollutants. In fact, they reduce pollution when they replace electricity usually generated from fossil-fuels, e.g. a coal-fueled power plant. Another advantage is that wind turbines are relatively easy to build. On a large scale, a wind power plant can be built quickly and expanded as needed. Wind turbines, although tall, do not require large plots of land. This means that the land below the turbine can be used for other purposes, such as farming or cattle grazing.

Like all methods of energy production, wind power has its own drawbacks. Wind power is economical only in areas with consistent winds. Otherwise, when the wind becomes weak or stops blowing, people have to rely on backup systems for electricity, which include fossil-fuel power sources. Also, some people find wind turbines to be unattractive structures. They argue that large wind turbines are unpleasant additions to the open spaces. In addition, the moving blades of a wind turbine can be noisy. Finally, in some places, wind turbines can be harmful to bird populations. Turbines can interfere or even kill migrating birds.

wind turbines blocking view
For some, large wind turbines are unattractive structures.
Source: Wikipedia

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