Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 323

duced by the energy consumption of accessories such as the wind direction yaw
mechanism and the blade adjustment mechanism. The different loss mechanisms
are explained in the following.

2 - 5 %

40 - 50 % ca. 2 % ca. 2 % 1 - 2 %

2 - 10 %

Losses related to the energy contained in the undisturbed wind

Fig. 7.15 Energy flow through a wind energy converter (see /7-1/)

− Aerodynamic losses are attributable to the blade shape that is never optimal
with regard to the total surface covered by the rotor; aerodynamic losses are
considered by the real power coefficient (i.e. the power portion contained in the
airflow which can be extracted from the wind with regard to the ideal power
coefficient and the given losses of a determined wind energy converter). Power
coefficients are first and foremost determined by the number and the shape of
rotor blades (and thus by the blade tip speed/wind speed ratio) and thus differ
tremendously with regard to rotor designs. Fig. 7.16 shows the power coeffi-
cient c p in relation to the ratio of blade tip speed at the outer rotor edge to the
current wind speed λ . This ratio produces the characteristic curves c p ( λ ), typical
for wind rotors of different designs. The most important parameters are
summarised as follows:
• number of rotor blades,
• aerodynamic profile properties and
• twisting of rotor blades /7-3/, /7-4/, /7-5/.
Fig. 7.16 shows the tremendous variations of power coefficients of the illus-
trated rotor designs. The clear difference between the maximum power coeffi-
cient according to Betz and the maximum power coefficient of an ideal wind
turbine is due to twisting losses. Fig. 7.16 also illustrates the advantages of
high-speed propeller-type converters (i.e. wind energy converters of a large
number of revolutions and only a few rotor blades, i.e. one, two or three-blade
rotors) compared to low-speed converters whose twisting losses are considera-
bly higher (i.e. converters with a low number of revolutions and many blades,
i.e. Dutch windmills, American Western rotors). The respective maximum