Today’s large wind turbines sit on towers up to 300 feet above the ground, with the diameter of the rotor and blades reaching more than 250 feet. Lying on the ground, a three-bladed rotor can almost cover a football field! Wind turbines consist of three main parts: the tower, the blades, and a box behind the blades, called the nacelle. A nacelle is the size of a small school bus! The nacelle houses the generator, which transforms motion into electricity.
In a typical turbine design, rotor blades are attached to an axle that runs into a gearbox. The gearbox, or transmission, increases the speed of the blades’ rotation, from 18 revolutions per minute (RPM) up to 1,800 RPM. The fast spinning shaft turns inside the generator, producing AC (alternating current) electricity. Electricity must be produced at just the right frequency and voltage to be compatible with the utility grid.
As you can imagine, the speed of the wind hitting the rotors affects how much energy a turbine captures. Modern wind turbines are designed to work most efficiently at wind speeds between 15 and 35 MPH. Because the wind blows stronger than this some of the time, a wind turbine must adapt itself to the prevailing wind speed to operate most efficiently. There are two basic approaches used to control and protect a wind turbine: pitch-control and stall-control. In pitch-controlled turbines, an anemometer mounted atop the nacelle constantly checks the wind speed and sends signals to a pitch actuator, adjusting the angle of the blades to capture the energy from the wind most efficiently. On a stall-regulated wind turbine, the blades are locked in place and do not adjust during operation. Instead the blades are designed and shaped to increasingly “stall” the blade’s angle of attack with the wind to both maximize power output and protect the turbine from excessive wind speeds. There are relative advantages to both design approaches. A pitch-regulated turbine, for example, is generally considered to be slightly more efficient than a stall-regulated turbine. On the other hand, stall-regulated turbines are often considered more reliable because they do not have the same level of mechanical and operational complexity as pitch-regulated turbines.
The size of the rotor or swept area ultimately determines how much energy a wind turbine can harvest from the wind. That general concept is fairly simple. However, calculating the potential yield of a wind turbine does require a bit of geometry.
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You can calculate the area of a circle—or a wind turbine’s swept area—using the following formula: 
 = 3.14 (otherwise known as pi)
r = radius. The radius is half of the diameter of the circle. What’s the diameter? A straight line drawn through the center of the circle.
The raised number 2 signifies a particular kind of multiplication: squaring. To square a number, you multiply it by itself. For example, the square of 2 is 4 (2x2=4). |
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So what happens when you double the size of a rotor? Something amazing takes place. Instead of doubling the energy yield, doubling the rotor size quadruples the energy yield, generating 4 times as much electricity.
So that wind turbines can capture the wind from any direction, all modern wind turbines have computer– controlled yaw systems that automatically turn the turbine and align the blades into the wind.
Modern wind farms are groups of turbines installed in rows or arrays perpendicular to the prevailing wind direction. Wind farms can consist of a few up to several hundred turbines, depending on the size of the project and individual turbines. A 100 MW wind farm with between 65 and 150 turbines can provide enough electricity yearly to power as many as 45,000 houses. |
