Wind Power: How Wind Turbines Work

The technology of wind power is simple. Wind turbines capture the kinetic energy of wind with, in most cases, two or three blades shaped much like airplane propellers. These blades are attached to a tower that rises at least 100 feet (30 meters) above the ground. At this height air currents tend to be stronger but less turbulent than they are at ground level. When the wind strikes the blade, the angle and configuration of the blade form a pocket of low pressure on the downwind side of the blade. This low pressure sucks the blade into movement, causing the rotor to turn. Force is added by the high pressure on the upward side of the blade. In aerodynamic theory, this property is called lift. If the blade is designed correctly, lift is stronger than drag, or the slowing force exerted by the wind on the front of the blade.

In wind turbines lift and drag work together to make the entire mechanism spin like a propeller. The process is the opposite of that of a fan. With a fan electricity is used to make wind. With a wind turbine wind is used to produce electricity. The turning rotor of a wind turbine is connected to a shaft, which is connected to an electric generator. The generator creates electricity using much the same principle as the alternator on your car (depending on the turbine type). A magnetic rotor on the high speed shaft inside the generator housing spins inside loops of copper wire that are wound around an iron core. As the rotor spins around the inside of the core it creates "electromagnetic induction" through the coils that generates an electrical current. That current is then regulated and fed into the grid (or a residential grid connect system) so that it can be used in our homes or routed into a battery bank for storage. Where a battery bank is used, a regulator prevents overcharging.

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The most important feature in the operation of wind turbines is lift. To achieve lift, wind turbine designers have borrowed technology from aircraft designers. In cross-section an airplane wing looks like an irregularly shaped teardrop. The shape is irregular because the wing's bottom is slightly flatter than the top, which is more curved. When a plane flies, its wings slice through the air, creating wind. Because of the curve of the upper surface of the wing, the air has to flow faster to get around the wing. At the same time, the air flows at a lower speed along the bottom surface of the wing. Because of the difference in speed, the air above the wing is less dense; that is, the air pressure is lower than the pressure of the air below the wing. This difference in pressure creates lift perpendicular to the direction of the moving air, allowing the plane to fly. The same principle applies to turbine blades.

Unlike airplane wings, wind turbine wings are almost always twisted. The reason they are twisted has to do with another aerodynamic principle, stall. When an airplane wing is tilted back, the wind continues to flow smoothly along the bottom surface, but along the top surface, because of the steeper angle presented to the wind, the air no longer sticks to the wing but swirls around in a circle above it. The result of this swirling is the loss of the low pressure along the upper surface of the wing. Without this low pressure, the plane has no lift and drops like a rock.

Wind turbine blades are constantly rotating, and the speed of the rotation differs along the entire length of the blade. At the precise geometric center, the speed of rotation is zero. This speed steadily increases along the length of the blade until at the tip the blade can be moving hundreds of feet (meters) per second. This rotation changes the direction at which the wind hits the blade all along its length. In effect, the angle at which the wind hits the blade would be different at each point along the blade if the blade were not twisted. When the blade is twisted, the angle at which the wind hits the blade is the same at each point, and stall is eliminated under normal wind conditions. Excessively high wind speeds can damage rotors, however, so engineers have designed blades that stall when the wind is too strong, and the rotor stops spinning.

Wind turbines come in two configurations. One, called a verticalaxis turbine, looks much like an oversized eggbeater. The axis of the turbine is positioned vertically, and the blades are connected to the axis at the top and the bottom. This configuration has one primary advantage: The turbine does not have to be faced into or away from the wind, so it operates no matter which way the wind is blowing, and it does not have to be repositioned to accommodate changes in wind direction.

The other configuration, the horizontal-axis turbine, is much more commonly used. With this style, the axis is parallel to the ground on a tower, and the blades, which look like airplane propellers, are perpendicular to the axis. This type of wind turbine looks like a pinwheel.

Wind Power: How Wind Turbines Work copyright 2011