Blade Element Calculations - Small Wind Turbines: Analysis, Design, and Application - page 93

Fig. 5.10 Lift:drag ratio for
the SD7062 aerofoil 80

Re = 1x10 5
Re = 2x10 5
Re = 3x10 5
Re = 4x10 5

60

40

20

0

-10 -5 0 5 10 15

Angle of attack (degrees)

Fig. 5.11 Predicted effect of
changing the aerofoil on the
power and thrust coefficients
for the measurements of
Anderson et al. [ 5 ]. Open
symbols show twice C P ,
closed symbols show C T

1

0.8

0.6

N ACA 4412
SD 7062

0.4

6 7 8 9 10 11 12 13 14

Tip speed ratio, λ

NACA 4412. The computed power and thrust coefficients are compared in
Fig. 5.11 to the computed results for the NAC 4412 section from Fig. 5.2 .As
expected, there is a modest improvement in C P that reflects the increase in the
lift:drag ratio for the SD7062 compared to the NACA 4412.

5.5 Maximising Power Extraction

In preparation for the more complete study in Chap. 7 , the optimising of blade
performance is now considered in terms of maximising C P . Recall from Chap. 2
that the Betz-Joukowsky limit requires C P B 16/27 = 0.593 for unshrouded tur-
bines, and from Chap. 1 , that the high-Re blade of the large Vestas V47 has
C P & 0.5 (when allowance is made for mechanical and electrical inefficiencies).
Furthermore, it is known that a finite N, and a finite l/d, reduce the maximum C P
from the Betz-Joukowsky limit. Thus it would appear that a maximum C P of just