Environmental Engineering Reference
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their optimum running position, once the turbine is operating. This form of pitch control does
not limit power, but aids in starting the rotor in low winds.
Another technique used by several small-scale turbine manufacturers for speed and
power control is furling , in which the rotor is turned partially out of the wind by yawing to
one side (called horizontal furling ) or pitching upward ( vertical furling ). It is possible to
accomplish this through the downwind force of aerodynamic thrust balanced by spring or
gravity forces.
Stall regulation of power is commonly used on rotors that do not feature full-span blade
pitch. For a set rotational speed and blade pitch angle, airfoil angle of attack and lift increase
as the wind speed increases, and the rotor produces more power. This continues until the
angle of attack reaches a point at which aerodynamic stall occurs, and power passively lev-
els off. Although stall regulation offers great control simplicity and reliability, it requires
significant aerodynamic modeling, design, and testing. Techniques developed in the early
980s [Viterna 98] allowed wind turbine designers to utilize stall regulation more effec-
tively. By 993, 60 percent of the operating wind turbines used stall regulation for power
control [Hansen 993]. It continues to be used today, predominantly in small- and medium-
scale wind turbines.
For overspeed control, tip brakes, buckets, and pitchable blade tips are sometimes used.
Two types of tip devices for stopping a wind turbine with fixed-pitch blades are illustrated
in Figure 4-. Tip brakes , Figure 4-(a), are flat metal or fiberglass plates attached to the
Figure 4-. Tip devices for overspeed control in the undeployed (left) and deployed
(right) positions. (a) Tip brakes (b) Pitchable tips.
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