Wind Energy

44 E. Bibor and C. Masson
was established. Treatment of data required important cares during the different
steps: determination of valid sectors and filtration of the data. Nacelle
anemometry’s correlation Vnac ⇔ V∞ was then calculated by synchronizing
the data from the WT and the reference tower.
8.4 Numerical Analysis
A numerical analysis as been performed with the objective of having a better
understanding of nacelle anemometry. In order to be as realistic as possible, a
turbulent flow was simulated. For that purpose, the Reynolds-Average Navier-
Stokes (RANS) equations were solved. The k − ɛ turbulence model was used
to close the system of equations. The rotor is represented as an actuator-disc
surface [2], on which external forces are prescribed according to the bladeelement-theory.
The commercial software FLUENT was chosen to perform this numerical
analysis. The rotor can be modelled by a pressure jump calculated from
the forces found previously. Also, a two-dimensional axisymmetric flow was
studied in order to benefit from the symmetrical geometry of the nacelle. The
size of the calculation domain was optimized so that the results were not influenced
by the boundary conditions, but also that the computing time was
optimum.
8.5 Results and Analysis
8.5.1 Comparaison with the Manufacturer
As explained in Sect. 8.3, a nacelle anemometry correlation Vnac ⇔ V∞ was
experimentally obtained: V∞ =0.953 + 0.791 Vnac. The latter was compared
with the correlation provided by the manufacturer: V∞ =0.536 + 0.723 Vnac.
For Vnac = 20 m/s, differences up to 1.59 m/s were measured. In order to
determine the validity of those results, power curves were constructed using
the two correlations. The use of the manufacturer’s correlation leads to unrealistic
results, with power coefficients (CP) higher than the theoretical limit
of 0.475 for this WT. These results confirm the inappropriateness in using a
common correlation for all WTs of the same type, with no consideration of
the type of terrain.
8.5.2 Influence on the Wind Turbine Control
The errors made in the estimation of V∞ have important consequences on
the performances of the WT. Indeed, the active control WT continuously
modifies his rotational speed in order to operate at the optimum tip speed
ratio λopti =7.2. An error in V∞ will automatically lead to an error in λ
calculated by the controller. Thus, the WT will always operate at CPs lower
than CP,max.