KD215_(Darreus_design_fails)_Kragten

KD215_(Darreus_design_fails)_Kragten - The Darrieus rotor,...

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The Darrieus rotor, a vertical axis wind turbine (VAWT) with only a few advantages and many disadvantages ing. A. Kragten December 2004 report KD 215 It is allowed to copy this report for private use and spread it through the internet. Kragten Design is not available for discussions about Darrieus rotors. Engineering office Kragten Design Populierenlaan 51 5492 SG Sint-Oedenrode tel. + fax: +31 413 475770 E-mail: kragten.design@tridodsl.nl
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Contains page 1 Introduction 3 2 Advantages and disadvantages 3 3 Retrospect 5 4 References 5 2
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1 Introduction This report is about a translation into English of my report KD 20 (ref. 1) written in 1997. The Darrieus rotor is a vertical axis wind turbine (VAWT) provided with two or more blades having an aerodynamic airfoil. The blades are normally bent into a chain line and are connected to the hub at the upper and lower side. However, also Darrieus rotors with straight blades (H-Darrieus) have been developed which therefore have large hubs provided with spokes. The energy is taken from the wind by a component of the lift force L working in the direction of rotation. The same principle is used for a horizontal axis wind turbine (HAWT). There are also VAWT’s for which the power is gained by the difference in drag D of the blade moving in the direction of the wind and the blade moving against the wind. A cup anemometer, used for measuring wind speeds, is an example of such a rotor. There are also VAWT’s, like the Savonius rotor, which are working on a combination of lift and drag. One of the advantages of using the lift principle is that much higher power coefficients C p can be realised than for windmills using the drag principle. Another advantage is that the tip speed ratio λ , can be much higher resulting in a much higher rpm for the same rotor diameter and in using much lesser material. Although the Darrieus rotor is working according to the lift principle, the maximum C p is a lot lower than for a well designed HAWT. The reason is that the blade of a Darrieus rotor has an acceptable angle of attack α , only during two parts of a revolution. Therefore the average drag over a whole revolution is rather large. In the past period in several countries research for Darrieus rotors is performed. In The
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KD215_(Darreus_design_fails)_Kragten - The Darrieus rotor,...

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