Wind Turbines - Wind Energy Systems for Electric Power Generation - page 21

Fig. 2.13 Illustration of stall, active-stall and pitch effects

2.4.1.3 Other Power Limitation Concepts

Small wind turbines in the kW power-range sometimes use other concepts such
as passive pitch control, or a passive mechanism tilting the turbine in dependence
of wind exerted axial force and decreasing the swept area from circular to elliptic
(see 5.2).
Power and speed limitation can also be provided by concepts other than mani-
pulating the wind turbine. This may be an electrical or mechanical braking system.

2.4.2 Wind Classes

Wind energy systems can be assigned to different wind classes. The classes stan-
dardized by IEC are commonly used (see Table 2.1). The classes reflect the design
dependence on locations with strong or weak wind performance. Characteristic for
WES in classes of larger number (lower wind velocities) are larger rotor diameters
at same rated power, and often also a larger tower height. Reference values are the
average wind speed in hub height and an extremum which statistically happens as
10 min mean value only once in 50 years.
Note that in Germany there is also a classification in wind zones according to the
Deutsches Institut f ur Bautechnik (DIBT).

Table 2.1 IEC type classes
IEC wind class I II III IV

50 years extremum, m/s 50 42.5 37.5 30

Average wind velocity, m/s 10 8.5 7.5 6