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.” It is named after the French engineer Georges Darrieus who patented the design in 1931. (It was manufactured by the US company FLoWind which went bankrupt in 1997). 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.
Darrieus wind turbines are commonly called “Eggbeater” turbines, because they look like a giant eggbeater. 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.
(1) The rotor shaft is vertical. Therefore it is possible to place the load, like a generator or a centrifugal pump at ground level. As the generator housing is not rotating, the cable to the load is not twisted and no brushes are requires for large twisting angles.
(2) The rotor can take wind from every direction.
(3) The visual acceptation for placing of the windmill on a building might be larger than for an horizontal axis windmill.
(4) Easily integrates into buildings.
(1) Difficult start unlike the Savonius wind turbine.
(2) Low efficiency.
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