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Capturing the Wind: Power for the 21st Century
(Released June 2008)

 
  by Ethan Goffman  

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How Wind Turbines Work

Contents

The generation of wind energy—or transduction of wind energy into electrical energy—depends upon some kind of surface that is struck by wind and transformed into energy usable by humans. In other words, "a wind energy system transforms the kinetic energy of the wind into mechanical or electrical energy" (AWEA Basics). With more sophisticated technology, this transformation has become increasingly effective, making wind power far more useful than in even the recent past. According to Greenpeace, "a single wind turbine can produce 200 times more power than its equivalent two decades ago" (3).

3 bladed turbine
This 3 bladed wind turbine is the most common modern design because it minimizes forces related to fatigue
Today's wind turbines generally possess three blades rather than just two, since "experience shows that this configuration offers the best balance of efficiency, dynamic performance, and construction economics" (Marsh). Blades must be configured to take advantage of high wind speeds while avoiding being blown away. A modern windmill uses aerodynamic principles from sophisticated aircraft: "The two primary aerodynamic forces at work in wind-turbine rotors are lift, which acts perpendicular to the direction of wind flow; and drag, which acts parallel to the direction of wind flow" (Layton).

Wind turbines must be configured to operate in an unpredictable and highly variable environment, in order to survive major storms. As one analyst explains, "the big problem is always how best to stabilize rotor speed to maximize electricity production. . . . Its control systems must safeguard operation in conditions that range from a dead calm to gusts with directional changes and velocities that may occur only once a century" (Marsh). Wind turbines do this through changes in angle and through mechanisms that cause them to shut down under dangerous conditions. A sophisticated modern turbine "can twist its head on the support column (yaw control) to match wind direction and adjust the pitch of its blades to achieve desired speed and torque characteristics" (Welander).

anatomy of wind turbine
How Stuff Works
The blades are the most unique part of a structure that includes a hub, a nacelle for protection from bad weather, a generator (and usually a gearbox), a tower, and electronic equipment (AWEA Basics). Once the wind makes the blades turn, the next step is to change this energy to a form that can be used over long distances: electricity. How Stuff Works explains, "when the turbine blades capture wind energy and start moving, they spin a shaft that leads from the hub of the rotor to a generator," which converts it to electricity.

Energy from a wind turbine is measured in kilowatt hours, that is, the amount of energy it takes to power a thousand watts over an hour's time. As it's generated, wind power is usually fed directly into the grid in conjunction with energy from other sources, such as coal plants. At times, however, wind power is stored for future use.

Of the two basic designs of wind electric turbine, vertical and horizontal, horizontal is by far the more common (although experimental vertical turbines continue to be built). If proponents of a rival design are to be believed, electricity can be generated from wind even more cheaply by turbines that rotate about a vertical axis, like a playground roundabout. "TMA, a company based in Cheyenne, Wyoming, claims that its first vertical-axis wind turbine" will harvest "43-45% of the wind's available energy; conventional propeller-style turbines, in contrast, have efficiencies of 25-40%" (Economist).

Location, Size, and Efficiency

A few stand-alone wind turbines exist, useful for areas off the electrical grid. Still, wind turbines are most effective when grouped into large wind farms positioned so as to take maximum advantage of prevailing winds. These can also share much of the equipment needed to bring electricity onto the grid.

global wind map
Global wind map from Stanford University showing the best locations for wind farms
Because the wind is as unpredictable as the weather-indeed it is (one component of) the weather-it stops and starts and blows with variable force. Wind turbines therefore rarely run at 100% of their theoretical capacity. The amount of the theoretical maximum that is actually available as energy is referred to as capacity factor, which "compares the plant's actual production over a given period of time with the amount of power the plant would have produced if it had run at full capacity for the same amount of time" (AWEA FAQs 5).

While conventional power plants usually operate at from 40 to 80% capacity, wind power's capacity factor is generally only 25% to 40% (AWEA FAQs) (but of course the source of this energy is free). Advances in technology may be able to improve on this performance, but only up to the Betz limit of 59%; "an upper theoretical limit" to the amount of available energy that "a wind turbine can actually capture or convert to usable energy" (NRC 18).

Don Quixote would hardly recognize today's enormous windmills, which would likely make him flee in terror. When it comes to wind turbines size matters, for both the blades and the towers. In the past 25 years wind-turbines have become significantly larger and taller, "typically 60-90 meters high with a three-bladed roter 70-90 meters in diameter" (NRC 2). This significantly impacts the amount of wind harvested; "Wind at 50 meters will, on average, have twice the power density as wind measured at 10 meters" (Ewing).

Siting is also critical. The more wind that exists in an area the better, because "the kinetic energy of moving air that passes the rotor of a turbine is proportional to the cube of the wind speed. Hence, a doubling of the wind speed results in eight times more wind energy" (NRC 17). Location is crucial for approaching maximum capacity factor: "When located appropriately, a turbine should run 60 to 80% of the time. When sized appropriately for a given location, it will provide its full rated output at least 10% of the time" (Welander). This is a key reason for building windmills at sea, where winds tend to be both more powerful and more predictable. This benefit is offset somewhat, however, by the higher cost of constructing wind turbines offshore.

Go To Advantages and Disadvantages of Wind Energy

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