In contrast to the windmills common in the nineteenth century, a modern power generating
wind turbine is designed to produce high quality, network frequency electricity whenever enough wind is available. Wind turbines can operate continuously, unattended and with low maintenance with some 120,000 hours of active operation in a design life of 20 years. By comparison, a typical car engine has a design lifetime of the order of 6,000 hours.
The rotors of modern wind turbines generally consist of three blades, with their speed and power controlled by either stall or pitch regulation. Stall regulation involves controlling the mechanical rotation of the blades, pitch regulation (now more commonly used) involves changing the angle of the blades themselves. Rotor blades are manufactured from composite materials using fibreglass and polyester or fibreglass and epoxy, sometimes in combination with wood and carbon.
Energy captured by the steadily rotating blades is transferred to an electrical generator via a gearbox and drive train. Alternatively, the generator can be coupled directly to the rotor in a