What is a Wind Turbine
A wind turbine is a machine that uses rotating blades to convert the kinetic energy of wind into mechanical energy. If the mechanical energy is used directly by machinery, the machine is called a windmill. If the mechanical energy is converted into electricity, the machine is called a wind generator, wind power unit, or wind energy converter.
Wind turbines require locations with high, constant winds. Wind resource assessment allows it to be possible to estimate the amount of energy that wind turbines in a particular location will be capable of producing.
Often, Wind Power Density is used to calculate the effective wind in a familiar location. This allows data to be expressed in terms of the elevation above ground level or over a period of time. Wind velocity and mass is also taken into account. Color coded maps are used to describe “the mean annual power density at 50 meters.” These results are applied to an index developed by the National Renewable Energy Lab and are referred to as “NREL CLASS.” The larger the WPD calculation, the higher it is rated by class.
Wind turbines are separated into two types, based on the axis. Horizontal-axis turbines are much more common than vertical-axis turbines. Horizontal-axis wind turbines have the main rotor shaft and the electrical generator at the top of the tower. This must be pointed into the wind. Smaller turbines are pointed by a simple wind vane, while large turbines use solar sensors. Most of these solar sensors, also known as servo motors, have a gearbox, which increases the rotation of the blades as needed.
Wind turbines produce turbulence behind them, with the turbine usually pointed upwind of the tower. Because of this the turbine blades are made of stiff and strong material to prevent the blades from being pushed into the tower by high winds. The blades are also placed a safe distance from the tower, and are tilted slightly upward to avoid danger.
Downwind turbine machines have been developed, despite turbulence problems, because they don’t need an additional mechanism to keep them in line with the wind. In this situation the blades are allowed to bend, which reduces wind resistance. This often leads to fatigue failures.
There are many advantages to horizontal-axis wind turbines. The variable blade pitch allows for the angle of attack to be remotely adjusted for greater control. This allows for greater energy collection. The tall tower of the horizontal wind turbine allows for strong wind sites with rear shear. Horizontal wind turbines are also high in efficiency, since the blades move perpendicularly to the wind, allowing it to receive power through the entire rotation.
There are, however, disadvantages to horizontal-axis wind turbines. The tall towers and blades are so difficult to transport, that transportation can be up to 20% of the overall equipment cost. They are also difficult to install, requiring tall and expensive cranes. The construction of the tower is required to support all the blades, the gearbox, and the generator.