A vertical axis wind turbine (VAWT) is a type of wind turbine that has its rotor shaft oriented vertically rather than horizontally, as in the case of traditional horizontal axis wind turbines (HAWT). VAWTs are relatively less common than HAWTs, but they have some advantages, such as the ability to operate in turbulent winds, and the fact that they do not require a yaw mechanism to keep the rotor facing the wind.
VAWTs can be classified into two main types based on their rotor design: drag-type and lift-type. Drag-type VAWTs, also known as Savonius turbines, have a rotor with a curved blade that resembles an S-shape, which generates torque by using the drag force of the wind. Lift-type VAWTs, on the other hand, have a rotor with blades that are designed to produce lift, similar to the wings of an airplane.
VAWTs have some limitations, such as lower efficiency compared to HAWTs, and the fact that they can generate more noise and vibration due to their proximity to the ground. However, they are still an interesting alternative for certain applications, such as urban and residential areas, where space and height limitations make the installation of HAWTs impractical.
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Darrieus Wind Turbine
Darrieus turbine has long, thin blades in the shape of loops connected to the top and bottom of the axle; it is often called an “eggbeater windmill.” The Darrieus turbine is characterized by its C-shaped rotor blades which give it its eggbeater appearance. It is normally built with two or three blades. They have good efficiency, but produce large torque ripple and cyclic stress on the tower, which contributes to poor reliability. Also, they generally require some external power source, or an additional savonius rotor, to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in a higher solidity for the rotor. Solidity is measured by blade area over the rotor area. Newer Darrieus type turbines are not help up by guy-wires but have an external superstructure connected to the top bearing.
One type of VAWT is the Darrieus wind turbine that uses the lift forces of the wind to rotate the aerofoils of the machine. The tip speed ratio (TSR) indicates the rotating velocity of the turbines to the velocity of the wind. In this case, the TSR has a higher value than 1, meaning that the velocity rotation here is greater than the velocity of wind and generates less torque. This makes Darrieus turbines excellent electricity generators. The turbine blades have to be reinforced in order to sustain the centrifugal forces generated during rotation, but the generator itself accepts a lower amount of force than the Savorius type. A drawback to the Darrieus wind turbines is the fact that they cannot start rotation on their own. A small motor, or another Savonius turbine, maybe needed to initiate rotation.
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Savonius wind turbine
The Savonius wind turbine is a
type of vertical-axis wind turbine invented by the Finnish engineer
sigurd Savonius in the 1920’s. It is one of the simplest wind turbine designs.
It consists of two to three “scoops” that employ a drag action to convert wind
energy into torque to drive a turbine. When looked at from above in
cross-section, a two scoop Savonius turbine looks like an S-shape. Due to the
curvature of the scoops, the turbine encounters less drag when moving against
the wind than with it, and this causes the spin in any wind regardless of
facing.
Drag type wind turbines such as
the Savonius turbine are less efficient at using the wind’s energy than
lift-type wind turbines, which are the ones commonly used in wind farms.
A Savonius is a drag type turbine, they are commonly used in cases of high reliability in many things such as ventilation and anemometers. Because they are a drag type turbine they are less efficiency than the common HAWT. Savonius are excellent in areas of turbulent wind and self starting.
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