Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 328

only breaks down at higher angles of attack. Thus, the maximal absorbable
air forces or the nominal capacity may be exceeded, overloading the wind energy
converter and particularly the generator. Another effect is the dynamic stall that
occurs for a short while over a certain angle range of the rotor rotation. The result
is a periodically dynamic stall due to a cyclical alternation of the angle of attack
(e.g. while performing one turn in the bottom boundary layer). Periodic air forces
may stir up the wind energy converter to such an extent that the rotor may burst.
Thus, rotor blades need to be dimensioned accordingly to prevent mechanical de-
terioration of the converter, and of the rotor blades in particular, within the course
of its technical service life (even intentional stall creating eddy currents to limit
capacities may exert considerable strain on the rotor blades). Due to the discussed,
not fully deterministic stall effect, generator capacities can only be maintained
within certain ranges.

Standstill Standstill Wind speed Wind speed

Below the rated
wind speed

Below the rated
wind speed

Above the rated
wind speed

Above the rated
wind speed

Discontinued flow Discontinued flow Laminar flow

(Phase II)

Laminar flow
(Phase II)

Discontinued flow
(Phase III)

Discontinued flow
(Phase III)

v Wi v Wi

v I v I

v I v I

F D F D

F L,t F L,t v Wi v Wi

F L,t F L,t

v Wi v Wi

α α

-v R -v R -v R -v R

α α

F L F L

F L,a F L,a α α F L F L

F L,a F L,a

Direction of rotation Direction of rotation Direction of rotation Direction of rotation

Fig. 7.19 Flow conditions present at a rotor blade profile of a stall-controlled wind energy
converter (for an explanation of symbols see text; phases refer to Fig. 7.17; see /7-5/)

The characteristic power curve of a stall-controlled wind energy converter is
shown in Fig. 7.20. For wind speeds a little above the nominal wind speed its
power yield considerably exceeds the installed nominal generator capacity (i.e.
planned generator overload up to approximately 110 %). At wind speeds within
the cut out wind speed range, electricity output, by contrast, is a little below the
nominal generator capacity.
Besides stall control, a so-called active stall control has established itself in the
marketplace which allows transferring the rotor into stall position by means of a
blade angle adjusting device (i.e. adjusting the blade angle β towards smaller an-
gles of attack). Active stall controls ensure a smoother shape of the characteristic
power curve similar to that of a pitch-controlled converter.