Wind Energy Basics

Basic information on wind energy and wind power technology, resources, and issues of concern.

Wind Energy

Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetative cover. This wind flow, when "harvested" by modern wind turbines, can be used to generate electricity.

How Wind Energy Is Generated

The term wind energy describes the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity to power homes, businesses, schools, and other facilities.

Wind Turbines

Most wind turbines turn in the moving air like aircraft propeller blades, and power an electric generator that produces an electric current. A wind turbine is essentially the opposite of a fan: instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity.

See brief wind turbine animation on DOE Wind and Hydropower Technologies Web site.

Wind Turbine Types

Modern wind turbines fall into two basic groups; the horizontal-axis variety, like the traditional farm windmills used for pumping water, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor. Most large modern wind turbines are horizontal-axis turbines.

Horizontal-axis wind turbine Click to enlarge

Vertical-axis wind turbine Click to enlarge

Turbine Components

Horizontal turbine components include: blade or rotor, which converts the energy in the wind to rotational shaft energy; a drive train, usually including a gearbox and a generator; a tower that supports the rotor and drive train; and other equipment, including controls, electrical cables, ground support equipment, and interconnection equipment. See wind power animation on DOE Renewable Energy Web site Web site.

Horizontal wind turbine Click to enlarge

Turbine Configurations

Wind turbines are often grouped together into a single wind power plant, also known as a wind farm, and generate bulk electrical power. Electricity from these turbines is fed into a utility grid and distributed to customers, just as with conventional power plants. See Wind Energy Photos page for more wind farm photos.

Wind farm Click to enlarge

Wind Turbine Size and Power Ratings

Wind turbines are available in a variety of sizes, and therefore power ratings. Commercially available land-based turbines range in capacity from 0.25 kW to 2.5 MW, while offshore wind turbines may exceed 4.5 MW. When operating at full power, the smallest and the largest wind generators can supply enough electricity to power a few light bulbs and thousands of homes, respectively.

The capacity of a turbine is determined largely by its rotor diameter. Present-day technology can be divided into three broad size ranges, briefly characterized below:

Residential: rated capacity below 30 kW, rotor diameter of 4 to 43 ft, hub height of 60 to 120 ft. Intermediate: rated capacity between 30 and 500 kW, rotor diameter of 43 to 100 ft, hub height of 115 to 164 ft. Commercial: rated capacity between 500 kW and 4.5 MW, rotor diameter of 100 ft to more than 325 ft, hub height of 164 to more than 260 ft. Although wind turbine blades may appear to turn slowly, because most wind turbines have constant rotational speed, the speed with which the tip of the rotor blade moves through the air (the tip speed) may exceed 140 miles per hour, while at the center of the hub it is zero.

450 kW Wind Turbine Click to enlarge        1.5 MW Wind Turbines Click to enlarge

Rated capacity is a measure of electricity generation under ideal conditions. In the real world, the wind blows intermittently, at varying speeds. Wind speed at hub height is the main determinant of actual capacity.

See Wind Energy Photos page for more wind turbine photographs.

Wind Energy Resources in the United States

Wind energy is very abundant in many parts of the United States, including the Western Area Power Administration's Upper Great Plains Region (UGP). Wind resources are characterized by wind-power density classes, ranging from class 1 (the lowest) to class 7 (the highest). Good wind resources (e.g., class 3 and above, which have an average annual wind speed of at least 13 miles per hour) are found in many locations (see the Maps page for U.S. and UGP state wind resource maps). Wind speed is a critical feature of wind resources, because the energy in wind is proportional to the cube of the wind speed. In other words, a stronger wind means a lot more power.

Advantages and Disadvantages of Wind-Generated Electricity

A Renewable Non-Polluting Resource

Wind energy is a free, renewable resource, so no matter how much is used today, there will still be the same supply in the future. Wind energy is also a source of clean, non-polluting, electricity. Unlike conventional, fossil-fueled power plants, wind plants emit no air pollutants or greenhouse gases. According to the U.S. Department of Energy, in 1990, California's wind power plants offset the emission of more than 2.5 billion pounds of carbon dioxide, and 15 million pounds of other pollutants that would have otherwise been produced. It would take a forest of 90 million to 175 million trees to offset the same amount of carbon dioxide production.

Cost Issues

Even though the cost of wind power has decreased dramatically in the past 10 years, the technology requires a higher initial investment per MW of installed capacity than fossil-fueled generators. If wind generating systems are compared with fossil-fueled systems on a "life-cycle" cost basis (counting fuel and operating expenses for the life of the generator), however, wind costs are much more competitive relative to other generating technologies because there is no fuel to purchase and minimal operating expenses.

Environmental Concerns

Although wind power plants have less impact on the environment than fossil fuel power plants, there is concern over birds and bats being killed by flying into the towers or rotors, visual impacts, and the noise produced by the rotor blades. Other impacts are associated with construction and operation of transmission lines associated with wind energy development, trenching and burying of collector lines, and construction of access roads.

See the Wind Energy Development Environmental Concerns page for more information.

Supply and Transport Issues

The major challenge to using wind as a source of power is that it is intermittent and does not always blow when electricity is needed. Wind energy is not easily stored, and not all winds can be harnessed to meet the timing of electricity demands. Further, good wind sites are often located in remote locations far from areas of electric power demand (such as cities). Finally, wind resource development may compete with other uses for the land, and those alternative uses may be more highly valued by some than electricity generation. However, wind turbines can be located on land that is also used for grazing or even farming.

For More Information

Visit the Wind Energy Links page to access sites with more information on wind energy science and technology and wind energy development issues. In particular, the DOE Wind Energy Technologies page has good information on wind energy basics, and is the source for much of the information presented here. The American Wind Energy Association web site has an excellent "Wind Web Tutorial" with information about wind technology.