Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 304

L/D ratio and corresponds to an inflow angle of about 0° for the profile under
consideration.

1.4 1.4 1,4 1,4 1,4

1.4 1.4 0.28 0.28 0.28 0.28

1.2 1.2 1,2 1,2 1,2

1.2 1.2 0.24 0.24 0.24 0.24

1,0 1,0 1,0

1.0 1.0

1.0 1.0

0.20 0.20 0.20 0.20

ε min ε min ε min

c l (α) c l (α)

0,8 0,8 0,8

0.8 0.8

0.8 0.8 0.16 0.16 0.16 0.16

0,6 0,6 0,6

0.6 0.6 0.12 0.12 0.12 0.12 0.6 0.6

0.4 0.4

0.08 0.08 0.08 0.08

0.4 0.4 0,4 0,4 0,4

c d (α) c d (α)

0.2 0.2 0.04 0.04 0.04 0.04

0.2 0.2 0,2 0,2 0,2

0.0 0.0

0.00 0.00 0.00 0.00

0.0 0.0 0,0 0,0 0,0

-0.2 -0.2 -0.04 -0.04 0.04 0.04

-0,2 -0,2 -0,2 -0.2 -0.2

0,0

-10 -10 -10 -10 -5 -5 -5 -5 0 0 0 0 5 5 5 5 10 10 10 10 15 15 15 15 20 20 20 20

0,0 0,0 0,2 0,2 0,2 0,4 0,4 0,4 0,6 0,6 0,6 0,8 0,8 0,8 1,0 1,0 1,0

Inflow angle ( α ) Inflow angle ( α )

0.0 0.0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1.0 1.0

Widerstandsbeiwert ( c w ) Widerstandsbeiwert ( c w ) Widerstandsbeiwert ( c w )

Fig. 7. 7 Airfoil lift and drag coefficient as a function of the angle of attack (left) and pro-
file polar curve with marked minimum profile lift/drag ratio (right) (see /7-23/)

Drag coefficient ( c d ) Drag coefficient ( c d ) Drag coefficient ( c d )

The rotor blade receives its driving force F T in circumferential (tangential) di-
rection from the lift and drag force component ( F L,t and F D,t ) (Fig. 7.5). The rotor
torque can be calculated with F T = F L,t - F D,t multiplied with the effective radius.
(Fig. 7.8).

F T F T

dr* dr*

r* r*

M/z M/z

R R

Fig. 7.8 Torque determination at a rotor blade (for an explanation of symbols see text)

By Equation (7.15) the drive torque M may be calculated for a given number of
rotor blades z , whereas R is the rotor radius and r a certain point within the radius.

R

∫ =

M = z F ( r *) r * dr *

(7.15)

T

r * 0

By means of the drive torque M and the number of rotor revolutions n rotor
power P Rot is eventually calculated according to Equation (7.16), whereby effi-
ciency η Rot represents twisting and friction losses in comparison to ideal rotor
power P Rot,th .