Wind turbines harness the wind to produce energy by turning the blades around a hub which turns a shaft connected to a generator that provides electricity. The electricity generated is fed into the national electricity grid.

The blades (1) of a horizontal axis turbine as shown in Figure 1 have a similar profile to an aircraft wing. They are generally made of glass fibre or carbon fibre reinforced plastics.  Because the blades have a different profile either side, as with an aircraft wing, the air moves at different speeds across each side of the blade, causing a pressure difference across the each blade and therefore rotational movement of the rotor (2). The pitch of the blade (3) can be varied to optimise power output and also to stall the turbine should the wind speed be too high.  The turbine may then be stopped and held motionless through the use of the brake (4).  The control of the turbine in accordance with wind speed, and shutdown if necessary, is enabled through the measurement of wind speed by the anemometer (9) on top of the turbine.  In order for the turbine to be aligned with the wind, it needs to be made to rotate around the axis of the tower (15) or ‘yaw’.  The wind direction is detected by the wind vane (10) and via the controller (8) the yaw motor (14) is called to operate and turn the yaw drive (13) and thereby the top of the turbine.  The gearbox (6) facilitates an increase in rotational speed between the low speed shaft (5) and the high speed shaft (12) so that the rotational speed is more suited to electricity generation.  Gearboxes are sometimes avoided through the use of power electronics, which can ensure the right frequency of electrical supply.  The rotational kinetic energy of the shaft is converted into electricity in the generator (7) through rotating electromagnetic fields.  The rotating machinery is housed in the nacelle (11) of the turbine and the electricity generated is transmitted to the base of the turbine and through a transformer to convert to distribution voltage.

Geared systems versus direct drive systems

Gearboxes can be avoided through the use of direct drive systems with larger rotors.  These have more magnetic poles to give the required electrical output frequency at a lower rotational speed without the use of a gearbox.  Direct drive systems with larger rotors have been avoided in the past mainly for reasons of mass and cost, but the use of permanent magnets with new materials has made direct drive systems lighter than previously.  The advantages of direct drive systems are that they can allow greater efficiency and reliability.