In the United States, wind turbines are becoming a common sight. Since the turn of the century, total U.S. wind power capacity has increased more than 24-fold. Currently, there’s enough wind power capacity in the U.S. to generate enough electricity to power more than 15 million homes, helping pave the way to a clean energy future.
What is a Wind Turbine?
The concept of harnessing wind energy to generate mechanical power goes back for millennia. As early as 5000 B.C., Egyptians used wind energy to propel boats along the Nile River. American colonists relied on windmills to grind grain, pump water and cut wood at sawmills. Today’s wind turbines are the windmill’s modern equivalent -- converting the kinetic energy in wind into clean, renewable electricity.
How Does a Wind Turbine Work?
The majority of wind turbines consist of three blades mounted to a tower made from tubular steel. There are less common varieties with two blades, or with concrete or steel lattice towers. At 100 feet or more above the ground, the tower allows the turbine to take advantage of faster wind speeds found at higher altitudes.
Turbines catch the wind's energy with their propeller-like blades, which act much like an airplane wing. When the wind blows, a pocket of low-pressure air forms on one side of the blade. The low-pressure air pocket then pulls the blade toward it, causing the rotor to turn. This is called lift. The force of the lift is much stronger than the wind's force against the front side of the blade, which is called drag. The combination of lift and drag causes the rotor to spin like a propeller.
A series of gears increase the rotation of the rotor from about 18 revolutions a minute to roughly 1,800 revolutions per minute -- a speed that allows the turbine’s generator to produce AC electricity.
A streamlined enclosure called a nacelle houses key turbine components -- usually including the gears, rotor and generator -- are found within a housing called the nacelle. Sitting atop the turbine tower, some nacelles are large enough for a helicopter to land on.
Another key component is the turbine’s controller, that keeps the rotor speeds from exceeding 55 mph to avoid damage by high winds. An anemometer continuously measures wind speed and transmits the data to the controller. A brake, also housed in the nacelle, stops the rotor mechanically, electrically or hydraulically in emergencies. Explore the interactive graphic above to learn more about the mechanics of wind turbines.
Types of Wind Turbines
There are two basic types of wind turbines: those with a horizontal axis, and those with a a vertical axis.
The majority of wind turbines have a horizontal axis: a propeller-style design with blades that rotate around a horizontal axis. Horizontal axis turbines are either upwind (the wind hits the blades before the tower) or downwind (the wind hits the tower before the blades). Upwind turbines also include a yaw drive and motor -- components that turns the nacelle to keep the rotor facing the wind when its direction changes.
While there are several manufacturers of vertical axis wind turbines, they have not penetrated the utility scale market (100 kW capacity and larger) to the same degree as horizontal access turbines. Vertical axis turbines fall into two main designs:
- Drag-based, or Savonius, turbines generally have rotors with solid vanes that rotate about a vertical axis.
- Lift-based, or Darrieus, turbines have a tall, vertical airfoil style (some appear to have an eggbeater shape). The Windspire is a type of lift-based turbine that is undergoing independent testing at the National Renewable Energy Laboratory's National Wind Technology Center.
Wind Turbine Applications
Wind Turbines are used in a variety of applications – from harnessing offshore wind resources to generating electricity for a single home:
- Large wind turbines, most often used by utilities to provide power to a grid, range from 100 kilowatts to several megawatts. These utility-scale turbines are often grouped together in wind farms to produce large amounts of electricity. Wind farms can consist of a few or hundreds of turbines, providing enough power for tens of thousands of homes.
- Small wind turbines, up to 100 kilowatts, are typically close to where the generated electricity will be used, for example, near homes, telecommunications dishes or water pumping stations. Small turbines are sometimes connected to diesel generators, batteries and photovoltaic systems. These systems are called hybrid wind systems and are typically used in remote, off-grid locations, where a connection to the utility grid is not available.
- Offshore wind turbines are used in many countries to harness the energy of strong, consistent winds found off of coastlines. The technical resource potential of the winds above U.S. coastal waters is enough to provide more than 4,000 gigawatts of electricity, or approximately four times the generating capacity of the current U.S. electric power system. Although not all of these resources will be developed, this represents a major opportunity to provide power to highly populated coastal cities. To take advantage of America’s vast offshore wind resources, the Department is investing in three offshore wind demonstration projects designed to deploy offshore wind systems in federal and state waters by 2017.
Future of Wind Turbines
To ensure future growth of the U.S. wind industry, the Energy Department’s Wind Program works with industry partners to improve the reliability and efficiency of wind turbine technology, while also reducing costs. The program’s research efforts have helped to increase the average capacity factor (a measure of power plant productivity) from 22 percent for wind turbines installed before 1998 to more than 32 percent for turbines installed between 2006 and 2012. Wind energy costs have been reduced from more than 55 cents per kilowatt-hour (kWh) in 1980 to under 6 cents/kWh today in areas with good wind resources.
Wind turbines offer a unique opportunity to harness energy in areas where our country's populations need it most. This includes offshore wind's potential to provide power to population centers near coastlines, and land-based wind's ability to deliver electricity to rural communities with few other local sources of low carbon power.
The Energy Department continues working to deploy wind power in new areas on land and at sea and ensuring the stable, secure integration of this power into our nation's electrical grid.