5.2 TECHNICAL OUTLINE

5.2.1 Wind Turbines

5.2.1.1 Lift and drag

Wind turbine rotors come in a wide variety of types. Each one is designed to convert the aerodynamic effects of the wind into a torque turning a shaft. The Persian windmill

 

 

made the wind push a vane in the direction of the wind, using aerodynamic drag (fig 5.2), whereas propeller type machines use aerofoil shaped blades which generate lift forces rather like the wing of an aircraft. Drag may be easy to use, but is inefficient because most of the wind energy is lost in eddies from the edge of the blade or cup, whereas the lift effect from an aerofoil is capable of slowing the air down with much smaller loss effects and results in a more efficient turbine. Drag is very effectively used in the rotor of the cup anemometer for measuring wind speed, but almost all modern wind turbines use lift effect rotors to maximise their power output. For instance the rotor may be made like an aircraft propeller, with rotating blades shaped like aircraft wings attached to a central hub, but other designs such as the vertical axis Darrieus rotor (see 5.2.1.3 below) also depend on aerodynamic lift.

5.2.1.2 Horizontal axis rotors

 

Multibladed	Propeller types

These rotors (fig 5.3) work the opposite way to aircraft propellers, by using the thrust of the wind to generate power; but the air velocities are normally much smaller, so the rotors are much larger for a given power. The blade speeds are also correspondingly low, but are usually several times faster than the wind, in order to maximise the lift forces and minimise the blade area required.

The exception is the multibladed rotor used for wind pumps, for which high torque is more important than high power, and so a large area of blades rotating at a lower speed is suitable. In either case the blades are set at a certain 'pitch' angle (i.e. each blade is rotated about its own axis away from the plane of rotation), so that the aerodynamic forces are optimised. This angle may be fixed, in which case the rotor is 'stall regulated' since the effect of a high wind is to stall the lift effect; or it may be controlled by a mechanism like an aircraft propeller in order to benefit from every change of wind ('pitch regulated'). Typical diameters and capacities for propeller type rotors range from 50 cm (50W) all the way up to 100 m (3MW). Possible disadvantages are that they need to be turned to face the wind, and that their towers normally need to be heavy enough to support all necessary electrical or other machinery at the level of the rotor.