Aerodynamics and aeroelastics of wind turbines - Wind Power Generation and Wind Turbine Design - page 97

Figure 9 : Blade elements of a rotor.

L/D-Variation, B=3

0.6

0.55

0.5

0.45

0.4

Glauert Ideal
L/D = Inf
L/D = 160
L/D = 80
L/D = 40
L/D = 20

0.35

0.3

0 5 10 15 20

TSR

Figure 10: c P via TSR: B = 3, L / D varies.

Sørensen and Okulov [39,40] recently formulated a vortex theory for these
types of rotors (see Fig. 11).
To sum up: at the present time a limit of c P = 0.52 seems to have been reached
by modern turbines, which is only possible if specially designed airfoils are used.
A c P of 0.45 (typical for turbines in the early 1990s) is obtained using old profi les
from the aerospace industry (see Figs 3 and 6).