Darrieus wind turbine
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The Darrieus wind turbine is a type of wind turbine used to generate electricity from the energy carried in the wind. Unlike the more common type of generator which uses a propeller, the Darrius generator rotates in the horizontal plane rather than the vertical one. The turbine consists of a number of aerofoils vertically mounted on a rotating shaft or framework. The aerofoils are arranged so that they are symmetrical and have zero rigging angle, that is, the angle that the aerofoils are set relative to the structure on which they are mounted. This arrangement is equally effective no matter from which direction the wind is blowing - contrast to the conventional type which must be rotated to face into the wind.
This design of wind turbine was patented by Georges Jean Marie Darrieus, a French aeronautical engineer, in 1931.
The Darrieus type is theoretically more efficient than the propeller type, but in practice this efficiency is rarely realised due to the physical stresses and limitations imposed by a practical design.
How it works
When the Darrieus rotor is rotating, the aerofoils are moving forward through the air in a circular path. Relative to the airflow due to the wind, this creates a small positive angle of attack to the airflow, thus generating lift. This lift causes a force to act on the aerofoil which assists it continuing to rotate in the direction it is already travelling. As the aerofoil moves around the back of the apparatus, the angle of attack changes to the opposite sign, but the generated lift is still positive and in the direction of rotation, because the wings are symmetrical and the rigging angle is zero. The rotor spins at a rate unrelated to the windspeed, and usually many times faster. The surplus energy arising from the lift may be extracted and converted into useful power using a generator.
When the rotor is stationary, no net rotational force arises, even if the wind speed rises quite high - the rotor must already be spinning to generate lift. Thus the design is not self starting.
Another problem arises because the majority of the mass of the rotating mechanism is at the periphery rather than at the hub, as it is with a propeller. This leads to very high centrifugal stresses on the mechanism, which must be much heavier and stronger to withstand them. One common approach to minimise this is to curve the wings into an "egg-beater" shape such that they are self supporting and do not require such heavy supports and mountings.
The advantages of the design are the fact it is not sensitive to wind direction, and the generator can be mounted in a fixed position on the ground.
External links
- How a Darrieus Machine works (http://purrfectcatshelter.org:8080/james/darrius/)
- Darrieus wind turbine analysis (http://windturbine-analysis.com/index-intro.htm)