Wind Energy

8 Power Performance via Nacelle Anemometry on Complex Terrain 45
V ∞ (m/s)
20
18
16
14
12
10
8
6
4
2
0 0 2 4 6 8 10 12
V nac
General correlation
Correlations by sectors
14 16 18 20
Fig. 8.1. Comparison between correlations by sectors and general correlation for
all valid sectors on correlations
8.5.3 Influence of the Terrain
The influence of the terrain was investigated by calculating correlations for
each valid sector of 10 ◦ , rather than using only one general correlation. Between
the sectors, significant differences were found (Fig. 8.1). Two power
curves were calculated using the correlations by sectors, or by taking the general
correlation. Once again, significative differences were observed, varying
between -10 and 10% (Fig. 8.2). However, in terms of the annual energy production
(AEP), the variations were reduced to 0.65%. This is explained by a
compensatory effect due to the fact that certain variations are positive and
other negative. Overall, the use of a general correlation does not generate major
errors over a long period, although involving significant differences between
specific sectors.
8.5.4 Numerical Validation
The correlation is largely influenced by the cylindrical section near the root
of the blade, and much less by the airfoils located at larger r/R. A common
hypothesis is to use a cylinder drag coefficient Cd =1.2. In reality, the Cd
is function of the Reynolds number, and has to be calculated. Also, another
correction has to be applied because of the finite length of the cylinder [3].
Another conclusion is that one of the major factor influencing the correlation
comes from the acceleration of the flow produced by the attachment unit of
the anemometer, as shown in Fig. 8.3a.
A U-beam has been added in the numerical simulation. The exact location
and caracteristics of the fixation systems are not know precisely. Thus, two