Page 212 - Materials Chemistry, Second Edition

200 E. Martínez Cámara et al.

2.3 Rotor

The rotor converts wind energy into mechanical rotational movement. It comprises
the turbine blades and the hub (which joins the blades to the main shaft). The hub
is the center of the rotor. It is made of cast steel or iron. Depending on the type of
wind turbine, hubs may be connected to the low-speed shaft of the gearbox or
directly to the generator if the turbine has no gearbox. A high percentage of the
turbines installed today are still fitted with gearbox to raise the rotor revolutions to
the rated speed of the generator. However, gearboxes can be problematic, so there
is a tendency for modern multimegawatt turbines to be built without them in order
to reduce maintenance requirements.
Currently, most rotors have three blades and a horizontal shaft and are between
40 and 90 m in diameter. Conventional windmill rotors for pumping water use 16
or more blades and are made of metal, but three-blade rotors are more efficient for
generating power in large turbines; they also make for better weight distribution,
which allows more stable rotation.
Rotor blades are made mainly of fiberglass or carbon fiber reinforced with
plastic. In profile, they are similar to aircraft wings, and they work on the same
principles of thrust. The wind passing over the bottom of the blade generates high
pressure, while low pressure is generated at the top. This force, plus the force of
resistance, causes the rotor to rotate. Each manufacturer makes blades according to
its own concepts and characteristics.

2.4 Nacelle

The nacelle encases and supports all the machinery of the wind turbine and must
be able to turn in order to face the wind direction. It is therefore attached to the
tower by bearings. Nacelle designs vary from one manufacturer to another in line
with the designs of the turbines and the location of the powertrain components
(main shaft, gearbox, generator, coupling, and brake).
The gearbox converts the 18–50 rpm rotation of the rotor to the rated rotation
speed of the generator, i.e., approximately 1,750 rpm. The generator rotation speed
depends on the frequency of the electrical current and on how many pairs of poles
the machine has. Apart from enabling low rotor rotation speeds to be coupled to
high generator rotation speeds, the gearbox also enables the unit to withstand a
wide range of variations in wind speed.
The generator is what turns mechanical energy into electricity. Double-fed
asynchronous generators are the most widespread type in use on today’s high-
power wind turbines.
Along with the main components mentioned previously, it is also necessary to
install a coupling between the generator and the gearbox. Flexible couplings are
usually utilized for this purpose. Mechanical brakes are also fitted to the
powertrain.