Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 322

7.2.3 Energy conversion chain, losses and characteristic power curve

Energy conversion chain. Wind energy utilisation converts energy extracted
from moving air masses into electrical energy. Conversion usually involves sev-
eral steps as illustrated in Fig. 7.14.
As shown in the illustration, kinetic energy of moving air masses is first con-
verted into rotation (of the rotor) and thus into mechanical energy of the power
train. Power trains of conventional wind energy converters include an inserted
mechanical gearbox to increase the number of revolutions for the synchronous or
asynchronous generator which generally require a higher number of revolutions
than the rotor. Yet, there are also wind energy converters whose generator is
adapted to the feasible number of revolutions and which do not require any gear-
box (Fig. 7.14). Subsequently, the mechanical energy of the power train is con-
verted into electrical power by a mechanical-electric converter (generator). Since
the specifications of the generator outlet do not necessarily match the grid specifi-
cations, in most cases a second electric/electric converter is needed. The simplest
version consists of a transformer; however, an indirect grid connection by either a
direct current intermediate circuit or a direct converter is suitable.

Kinetic-
kinetic
converter
(rotor)

Kinetic-
kinetic
converter
(gearbox)

Kinetic-
electric
converter
(generator)

Electric-
electric
converter
(transfor.)

Grid

Kinetic
energy

Kinetic (rotational)
energy

Electrical
energy

within the
wind

within the
wind converter

within the converter
resp. within the grid

Fig. 7.14 Energy conversion chain of a wind energy converter (transfor. Transformator,
resp. respectively; see /7-1/)

Losses. The different conversion steps are illustrated in Fig. 7.14. According to
this they are subject to several loss mechanisms which may reduce the overall ef-
ficiency significantly in comparison to the Betz power coefficient of 59.3 %.
Commercially available wind energy converters only transform 30 to 45 % of the
energy contained in undisturbed wind into electric power. The difference between
the maximum physical efficiency and maximum values that can be achieved at
present is caused by a series of different and unavoidable losses which are inher-
ent to the commercially available wind energy converters and any other type of
energy conversion plant (Fig. 7.15).
The electric power output available at the generator outlet of a wind energy
converter is determined by the power quantity contained in the wind minus aero-
dynamic, mechanical and electric losses. Net energy yield may additionally be re-