Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 300

power coefficient of modern three-blade wind turbines nowadays amounts to val-
ues up to 0.47.

v

λ

u

(7.10)

v

Wi , Rot

Mechanical losses due to bearing and gear friction as well as all losses of the
electrical plant components are considered by the corresponding efficiency η mech.-
elec. , which generally amounts to about 90 % for the currently available plants. The
useful power of a wind power station P WEC can thus be calculated according to
Equation (7.11). P Wi describes the wind power and c p the power coefficient.

P = η c P

(7.11)

WEC p mech .− elec . Wi

7.1.2 Drag and lift principles

There are two different principles available for technically exploiting moving air-
flow by rotating wind energy converters, which can also be combined under cer-
tain conditions. Energy can be extracted from flowing air masses either by the lift
or drag method. In the following both principles are explained.

Lift principle. According to the lift principle, wind is deviated to generate pe-
ripheral force inside the rotor. For high-speed propeller-type converters, in most
cases rotor blades are evaluated according to the wing theory. If a rotor blade
(represented schematically as flat profile in Fig. 7.4) is hit symmetrically by air-
flow at velocity v Wi (angle of inflow α = 0), a force referred to as drag force F D is
built up in flow direction due to its shape and frictional drag (i.e. resistance prin-
ciple; Fig. 7.4, top, and Fig. 7.9). However, there will be little resistance force if
the rotor blades are of airflow-friendly design.
Only if the rotor blade is hit asymmetrically (i.e. if the rotor blade profile is in-
clined at a certain angle to the airflow; flow angle α > 0) the streamlines above
and below the profile have to cover different lengths (Fig. 7.4, centre). When con-
sidering the above framework conditions (i.e. α > 0) with regard to a laminar flow
pattern, free from losses (not subject to any swirls nor friction), the air particles
need to reunify after having passed the cross-section. Consequently, those air par-
ticles that need to cover longer distances (see Fig. 7.4, centre, on the topside of the
schematic profile exposed to airflow) need to move faster. Energy yield consid-
erations according to Equation (7.2) (i.e. according to Bernoulli´s law without
considering potential power ( ρ Wi v Wi 2 )/2 + p Wi = const. ) without power extraction
from wind reveal that the faster air particles generate less pressure than the slow
particles. There is thus higher pressure below (pressure side) the cross-section