Environmental Engineering Reference
In-Depth Information
Fig. 5.9 Maximum lift:drag
ratio for some small wind
turbine aerofoils
120
SG 6043
MEL
08
1
100
SD 706
2
80
SG 6041
60
NACA 4412
40
20
0
1
2
3
4
5
6
7
Reynolds Number x10
-5
performance. This demonstrates the efficacy of using pitch adjustment to control
power output, as occurs on many large turbines. On the other hand, the C
P
dis-
tribution is flatter at the higher pitch, which suggests that low wind speed and
starting performance is improved by increasing the pitch. As we will see in
Chap. 6
, this is indeed the case.
5.4 Changing the Aerofoil
It has been mentioned previously that the NACA 4412 is an old aerofoil, and
therefore may not give as large a power output as the designer would like because
its maximum lift:drag ratio is lower than for more modern sections, as shown in
Fig.
5.9
. The SG6043 and the MEL 081 are particularly impressive, but unfortu-
nately, the co-ordinates for the latter are not freely available. Of those sections
plotted in Fig.
5.9
, the SD7062, Lyon et al. [
8
], has been used extensively by the
Wind Energy Group at Newcastle University. Its main advantage over the SG
sections described in
Chap. 4
is its increased thickness, nearly 14%, which gives
it extra strength to withstand the high centrifugal loads on small blades.
The available lift:drag data for this aerofoil are shown in Fig.
5.10
. Calculations
for the chord and twist distribution of the blade of Anderson et al. [
5
] are now
described. The changes to
power_calc.m
to use this, or any new, aerofoil are
trivial. They are:
%ReadIn 4412 % Read in Cl and Cd for the NACA 4412 section
ReadIn 7062 % Read in Cl and Cd for the SD7062 section
immediately after the chord and twist distribution are established and, of course, a
data file must be prepared for the lift and drag of the new section.
For brevity, ReadIn_7062.m is not listed here, but it will be easily under-
standable for any reader who has mastered the corresponding functions for the
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