Wind turbines

A wind turbine can be seen from a long way away, and with good reason. It is a machine of impressive dimensions and perched very high. The height is a consequence of technical progress that has been achieved in the last 10 years, improving the electricity generating power and the competitiveness of wind turbines.

The mast, generally made of metal, supports all the equipment necessary to produce electricity (pod + rotor). It is set into foundations implanted in the ground, a heavy concrete base that provides the anchorage and stability of the turbine. Today, the most powerful turbines have masts 80m high (exceptionally up to 100m). Why are they perched so high in the air? Because the wind blows more strongly above ground level, where its flow is not disturbed by obstacles in its path: such as hills and valleys, trees and houses. Power supplied by wind a wind turbine is proportional to the cube of the wind speed.

The rotor consists of a nose and a propeller - usually called turbine blades - constructed using technologies derived from aeronautical engineering. The propeller generally has three blades, more rarely two. Nowadays those blades are made of composite materials, which are light but at the same time ensure sufficient rigidity and resistance: polyester reinforced with fibreglass and/or carbon fibre. Their length can be as much as 30 to 55 metres; in other words, the diameter of the rotor is between 60 and 110m.Why such enormous dimensions? The power of a wind turbine is proportional to the surface area swept by its blades, and thus to the square of the rotor diameter. The rotor transforms the kinetic energy of the wind into mechanical energy.

The pod houses the equipment that produces electricity from the rotation of the axle of the rotor. This axle is usually known as the drive shaft. Electricity cables running down the interior of the turbine mast ensure the flow of electricity produced in the pod down to the ground. Wind turbines are also equipped with a high performance control system, based around a computer, which enables:
- orientation of the rotor perpendicular to the direction of the wind;
- modification of the angle of incidence of the blades relative to the wind so as to maximise energy recovery.

How does it work?

The wind makes the rotor turn. In the pod, the main drive shaft activates an alternator, which produces electricity. The rate of rotation of the rotor (12 to 15 revolutions/minute) must be increased by a speed multiplier to around 1500 rev/min, the speed necessary for satisfactory operation of the alternator. Electronic power converters match the frequency of current produced by the wind turbine to that of the electricity distribution network to which it is connected (50 Hz in Europe), whilst allowing the rotor of the turbine to turn at varying speeds according to the wind strength. The voltage produced by the alternator, of the order to 600 to 1000 Volts, is subsequently increased, by means of a power transformer situated in the pod or inside the mast, to a level of 20 000 or 30 000 volts. These voltage levels enable the electricity produced by each of the turbines in a turbine farm to be taken to a point of connection to the public distribution network (in France the EDF network). The voltage produced by a wind farm can then be transformed again according to the voltage of the connection to the public network. For wind farms of 10 to 15 MW, the voltage connection level in France is generally 20 000 volts. For farms with a larger capacity, the connection voltage level can increase to 60 - 90 000 volts, even to 225 000 volts.

A wind turbine needs a minimum wind speed of the order of 10 - 15 km/h to start turning and in winds over 90km/h it stops. Why do the turbines stop in strong winds? It is the result of a choice made by the engineers who design the turbines. In fact, the frequency of occurrence of winds with speeds over 90km/h is generally low (less than 1%). And if wind turbines functioned in these conditions, they would undergo major stresses, resulting in the premature wearing out of equipment. Taking account of the relatively small increases in production that can be obtained by operating in strong winds, the engineers prefer to stop the machines and wait for the return of more moderate and regular conditions! Whilst wind turbines do not work beyond wind speeds of 90km/h, their foundations are nevertheless designed to stand up to very much stronger winds … The power of a first-generation turbine is 1 to 1.5 MW, but turbines of the new generation reach 2 to 3 MW and manufacturers are already testing models of 5 MW!