Wind turbines are machines that take the kinetic energy in wind and convert it into mechanical energy. If the energy is used directly by machinery, we usually call the machine a windmill. If the mechanical energy is not used directly but is converted to electricity, the machine is called a wind generator.
Different Types of Wind Turbines
Two types of wind turbines exist based upon the axis about which the turbine rotates. The most common turbine rotates around a horizontal axis while those that rotate around a vertical axis are used less frequently.
In addition to classification based on the location of the axis, wind turbines are also classified by the location where they will be used. Onshore, offset, and aerial wind turbines each have unique design characteristics. Wind turbines may also be used along with a solar collector in order to extract the energy from the sun, which rises through a large vertical solar updraft tower.
Horizontal Axis Wind Turbines
In a horizontal axis wind turbine, the main rotor shaft and electrical generator are located at the top of a tower where they must be pointed facing the direction of the wind. Small turbines usually use a simple wind vane, while larger turbines tend to use a wind sensor along with a servomotor. Most also include a gearbox for turning the slow rotation of the blades into quicker rotation, thus making them more suitable for generating electricity.
Because a tower creates turbulence, the turbine tends to be pointed upwind of the tower. The blades on a turbine are made stiff so that they will not be pushed into the tower by high winds. In addition, the blades are usually placed quite a distance in front of the tower and usually tilted slightly upward.
In spite of the problem with turbulence, downwind machines are in existence because they need no additional mechanism to keep them in line with the wind. In addition, in high winds the blades are allowed to bend, thus reducing their swept area and wind resistance. On the other hand, since turbulence leads to fatigue failures, most Horizontal Axis Wind Turbines are upwind machines due to the importance of reliability.
Vertical Axis Wind Turbines
The main rotor of a vertical axis wind turbine runs vertically. This type of turbine is advantageous in that the tower doesn’t need to support the generator and/or gearbox as they can be placed near the ground because the turbine doesn’t need to be pointed into the wind. On the other hand, a pulsating torque may be produced during each revolution along with drag that is created each time the blade rotates into the wind. Finally, because the vertical axis wind turbine cannot be mounted on a tower, it must operate within the much slower but more turbulent airflow near the ground. This causes less efficiency in the extraction process.
Although its design generally makes the horizontal axis wind turbine more efficient, there are times when the vertical axis wind turbine may be preferable since it doesn’t require the use of a tower. They are also easier to service and maintain since most of their moving parts are located near the ground. They are also advantageous in areas that do not allow the construction of high structures such as the type that would be required for a tower to operate a horizontal axis turbine.
A horizontal axis wind turbine has advantages of its own and is more widely used than vertical axis wind turbines. The fact that it is connected to a tower allows it to be placed offshore and on uneven ground. Its height also allows it to pick up the higher speed winds that surface above ground. Most importantly, they can be cheaper to operate because of their higher production volume, larger sizes, and their general factors relating to both capacity and efficiencies.