Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 308

The various types of wind energy converters (WEC) can be subdivided into four
different groups (Fig. 7.10). Wind turbines operating according to the lift princi-
ple are sub-divided into horizontal axis converters (Fig. 7.10, e.g. one, two, or
three-blade rotors designed as fast speed or multiple blade converters) and vertical
axis machines (Fig. 7.10, e.g. Darrieus rotor or H rotor). Furthermore, there are
wind power stations concentrating the airflow (Fig. 7.10, e.g. shrouded wind tur-
bines), as well as wind power stations operating according to the drag principle
(Fig. 7.10, e.g. Savonius, cup anemometer).
7.2.2 System elements
Currently and in the near future, almost exclusively grid-connected horizontal
(three-blade rotors and, to a very limited extent, also two-blade rotors) hold a pre-
dominant market position. The principle plant design is illustrated in Fig. 7.11. A
grid-connected wind power station thus consists of rotor blades, rotor hub, gear-
box, if applicable, generator, tower, foundation and grid connection. Depending
on the respective wind energy converter type further components may be added.
Fig. 7.11 also shows the difference between converters with and without gearbox;
whereas converters equipped with a gearbox convert the rotor rotations into a
higher number of revolutions to apply standardised and less expensive generators,
for the gearless converters special generator types must be used that directly oper-
ate at a given number of revolutions.
In the past, wind energy converters have mainly been installed on the mainland.
However, as average offshore wind speeds are generally higher in comparison to
those achieved by mainland installations, offshore installation slowly starts to de-
velop. Since the converter design needs to withstand unfavourable environmental
conditions to prevent malfunction, the framework conditions place different re-
quirements on offshore plant technology. For instance, maintenance is more diffi-
cult and also more expensive. However, the development of offshore wind energy
converters is still very much in its infancy, so that final conclusions cannot yet be
drawn.
The following explanations thus focus primarily on mainland wind energy
converter technology. However, current developments of offshore wind energy
converter installations are also considered.
Rotor. The system component of a modern wind energy converter that transforms
the energy contained in the wind into mechanical rotations is referred to as rotor.
It consists of one or several rotor blades and the rotor hub (see Fig. 7.11). The ro-
tor blades extract part of the kinetic energy from the moving air masses according
to the lift principle. The current maximum efficiency of the kinetic energy of the
free flow in relation to the rotor surface (see Equation (7.8)) amounts to 50 %;
usually, the so-called aerodynamic efficiency of state-of-the-art rotors amounts to
between 42 and 48 % at the turbine design point.