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L'ÉNERGIE EOLIENNE
Combien de pales pour une éolienne ?!

    In simple terms, a wind turbine rotor with few blades produces less torque but spins faster than a rotor with many blades. So the power output is similar in both cases.
    In theory, turbines extract the most energy from wind (estimated to be 59 per cent of the wind's total energy) when the wind speed ahead of the turbine is exactly three times the wind speed in its wake. Because each blade on a turbine rotor extracts energy and slows the wind by a certain amount per revolution, the multi-bladed rotor must rotate more slowly than one with fewer blades to maintain an optimum wind-speed ratio.
    If a rotor has a very high number of blades, aerodynamic interference between them will reduce power. However, it will produce very good torque at low wind speeds, hence the use of the many-bladed design for powering water pumps in rural areas.
    But for generating electricity, high speed and low torque are favoured: generators typically require high rotation speed, and low torque means that shafts and gearboxes can be of lighter construction. So a small number of blades is preferable. But because single-blade and two-blade rotors suffer harmonic problems ­ dangerous vibrations at certain resonant speeds ­ and are displeasing to the eye, three-bladed rotors are the ideal choice. They are well-balanced, aesthetically pleasing and offer a high enough speed of rotation. Further blades add to manufacturing costs and, if designed to maintain a high rotation speed, would have to be very slender and, as a consequence, structurally inefficient.
    => Les rotors à trois pales sont le choix idéal. Ils sont bien équilibrés, esthétiquement satisfaisants et permettent une assez haute de vitesse de rotation.
Source ADIT

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