How reliable are wind turbines?

How do wind turbines work?

How much land is required for large wind plants?

What are the factors in the cost of electircity from wind turbines?

Wind turbine glossary

How reliable are wind turbines?
    Modern wind turbines can be extremely reliable — the percentage of times many systems are available to produce power often nears 99 percent.
    Another perspective is provided by comparisons with helicopters. The rotor blades must often be replaced after several hundred hours, while wind turbine blades commonly last 10 to 20 years or more. Because the wind turbines at the MEAN Wind Project at Kimball were manufactured with modern, durable, high-quality materials, their estimated life span is more than 20 years.

Wind turbine life and reliability
    Driving your car an average of 50 mph would require 2,000 hours of engine run time to go 100,000 miles.
    At an average in-town speed, which may actually be much lower than 50 mph, the engine may get 3,000 hours. During that time, you would need to change the oil 20 times, tune-up perhaps 10 times, change the timing belt once or twice and replace two sets of tires. Reduced to engine hours, that is about 27,000 hours of use.
    At a U.S. Department of Agriculture test site in Bushland, Tex., a 40-kilowatt turbine runs about 60 percent of the time (when the wind is high enough to make power). Running 60 percent of the time with 8,760 hours in a year, 3,000 hours of operation takes about seven months. The turbine is still running after 15 years of almost continuous operation. —Contributed by Eric Eggleston

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How do wind turbines work?
Aerodynamic operating principles of wind turbines
    According to the basic aerodynamic operating principles of a horizontal-axis wind turbine, wind passes over both surfaces of the airfoil-shaped blade. It passes more rapidly over the longer (upper) side of the airfoil, creating a lower-pressure area above the airfoil. The pressure differential between top and bottom surfaces results in a force, called aerodynamic lift. In an aircraft wing, this force causes the airfoil to rise, lifting the aircraft off the ground.
    Because the blades of a wind turbine are constrained to move in a plane with the hub as its center, the lift force causes rotation around the hub. In addition to lift force, a "drag" force, perpendicular to the lift force, impedes rotor rotation. A prime objective in wind turbine design is for the blade to have a relatively high lift-to-drag ratio. This ratio can be varied along the length of the blade to optimize the turbine's energy output at various wind speeds.

Basic principles of wind turbine power production
    The output of a wind turbine varies with the wind's speed through the rotor. The "rated wind speed" is the wind speed at which the "rated power" is achieved and generally corresponds to the point at which the conversion efficiency is near its maximum. In many systems, the power output above the rated wind speed is mechanically or electrically maintained at a constant level, allowing more stable system control.
   At lower wind speeds, the power output drops off sharply. This is explained by the Cubic Power Law, which states that power available in the wind increases eight times for every doubling of wind speed (and decreases eight times for every halving of wind speed).
    Using the power curve, it is possible to determine roughly how much power will be produced at the average or mean wind speed prevalent at a site. In the example above, the turbine would produce about 20 percent of its rated power at an average wind speed of 15 miles per hour (or 20 kilowatts if the turbine was rated at 100 kilowatts). This is somewhat lower than most modern wind turbines.

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How much land and wind resources do large
wind plants require?
   Each of the seven turbines in the MEAN Wind Project
at Kimball takes up, approximately, a mere 25 feet by
25 feet of land.  
   In fact, wind farms take up such a minimal area that to provide 20 percent of America's electricity, or 560,000 million kilowatt-hours per year, only 0.6 percent of the land of the lower 48 states would have to be developed with wind power plants, according to a study by Pacific Northwest Laboratory (PNL). Further, less than 5 percent of this land would be physically occupied by wind turbines, electrical equipment and access roads. Most existing land use, such as farming and ranching, would remain unaffected.
   The PNL study found that almost every region of the United States has some areas that contain good wind energy resources. In fact, the Northeast, Northwest, Southwest and Atlantic Coastal regions all contain significant wind energy resources. Moreover, some states, such as those that lie on the Great Plains from Texas to North Dakota, have a huge electricity-generating potential from the wind. The wind potential from each of these states far exceeds its current electricity consumption.
   Today's technology exploits high-wind locations — those in Wind Power Class 5 or greater — with average annual wind speeds of approximately 16 mph and higher at a height of 30 meters.

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What are the factors in the cost of electricity from wind turbines?
    The cost of electricity from utility-scale wind systems has dropped by more than 80 percent in the last 20 years.
    In the early 1980s, when the first utility-scale wind turbines were installed, wind-generated electricity cost as much as 30 cents per kilowatt-hour. Now, state-of-the-art wind power plants at excellent sites generate electricity at less than 5 cents per kilowatt-hour. Costs are continuing to decline as more, larger plants are built and advanced technology is introduced.
    Aside from actual cost, wind energy offers the following additional economic benefits, which make it even more competitive in the long term:
    
    * Greater fuel diversity and less dependence on
    fossil fuels, which are often subject to rapid price     fluctuations and supply problems. This is a significant     issue around the world today, with many countries     rushing to install gas-fired electric generating capacity     because of its low capital cost. As world gas demand     increases, the prospect of supply interruptions and     fluctuations will grow, making further reliance on it     unwise and increasing the value of diversity.
    
    * Greatly reduced environmental impacts per unit     of energy produced, compared with conventional     power plants. Environmental costs are becoming an     increasingly important factor in utility resource planning     decisions.
    
    * Long-term income to ranchers and farmers who own     the land on which wind farms are built.
   
    Selection of a suitable site is key to the economics of wind energy. The power available from the wind is a function of the CUBE of the wind speed, which means, all other things being equal, a turbine at a site with 5-meters-per-second (m/s) (11 mph) winds will produce nearly twice as much power as a turbine at a location where the wind averages 4 m/s (9 mph). In the electric power business, where technology options often hinge on very small economic differences, good wind resource assessment and siting is critical.
    In general, winds exceeding 5 m/s (11 mph) are required for cost-effective application of small grid-connected wind machines, while wind farms require wind speeds of 6 m/s (13 mph). For applications that are not grid-connected, of course, these requirements may vary, depending on the other power alternatives available and their costs.

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Wind turbine glossary

Anemometer Measures the wind speed and transmits wind speed data to the controller.

Blades Most turbines have two or three blades. Wind blowing over the blades causes them to "lift" and rotate.

Brake A disc brake that can be applied mechanically, electrically or hydraulically to stop the rotor in emergencies.

Controller Starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 65 mph. Turbines cannot operate at wind speeds above 65 mph because their generators could overheat.

Gear box Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1,200 to
1,500 rpm — the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.

Generator Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.

High-speed shaft Drives the generator.

Low-speed shaft The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.

Nacelle The rotor attaches to the nacelle, which sits atop the tower and includes the gear box, low- and high-speed shafts, generator, controller and brake. A cover protects the components inside the nacelle. Some nacelles are large enough for a technician to stand inside while working.

Pitch Blades are turned, or pitched, out of the wind to keep the rotor from turning in winds that are too high or too low to produce electricity.

Rotor The blades and the hub together are called the rotor.

Tower Towers can be made from tubular steel or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.

Wind direction "Upwind" turbines are designed to operate facing into the wind. Other turbines are designed to run "downwind," facing away from the wind.

Wind vane Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.

Yaw drive Keeps the rotor of upwind turbines facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive because the wind blows the rotor downwind.

Yaw motor Powers the yaw drive.

Source: U.S. Department of Energy

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