Analysis 01: OWF Effect on the Atmosphere - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 40

4.2.2.2 Analysis of Different Wind Speeds

A wind turbine must comply with several safety requirements defined by the
International Electronic Commission (IEC). The IEC Technical Committee 88 pre-
pares standards dealing with safety for wind turbine generator systems and pro-
duces standards for design and technical requirements. IEC defined four different
turbine classes, which are determined by three parameters, the average wind speed
at hub height, extreme 50-year gust, and turbulence (IEC 2005 ). These parameters,
depending on location, will define the type of wind turbine generator (WTG) in
connection with the size of wind turbine. Therefore, WTGs and rotors only operate
in a limited range of wind speed depending on turbine class. Thus, a wind wake will
be only produced at a certain wind speed range. And the wind speed is taken into
account with respect to power by the cube. Thereby, stronger wind speeds result in
higher power and a major energy transformation, which again results in a different
strength of wind wakes behind wind farms.
To estimate the effect of wind speeds on the wind wake, three wind cases are
analyzed. The simulation of these three wind cases differs by the input of geo-
strophic wind ug, which was set to ug

¼ 5 m/s (run UG5), ug ¼ 8 m/s (run UG8),

and ug 16 m/s (run UG16)( 3.2 ). The wind farm consists again of 12 turbines

being arranged over four grid boxes.
These results are illustrated in Fig. 4.3 . The percentage changes of the horizontal
wind field clearly show for all three wind speed cases the three zones of surge, wind
wake, and flanked flanks. The wind wakes are larger than 120 km and exceed the
model area, but the intensity of the wake is stronger with increasing wind speed.
Extreme values of minima occur within the wind farm. The stronger is the
prescribed wind field, the stronger is the wake. The run of UG5 shows a reduction
of 64 %, UG8 72 %, and UG16 77 %, compared to the reference run without OWF
impact (Fig. 4.3 a1-a3). The impact of the wind speed, respectively wind stress, on
the OWF wake is nearly linear. Figure 4.4 pictures that relation between the
prescribed geostrophic wind ug and the wake given by wind stress in N/m 2 . But
due to the small data set, a linear dependence cannot be generalized. Further
simulations with wind speeds between ug

¼

¼ ¼

8 m/s and ug 16 m/s would be

necessary for an approved statement.
In the case of UG16, the wake is less influenced by geostrophic force, compared
to the other two wind speed cases, whose wakes are deflected more to the West, due
to the stronger mean wind field (Fig. 4.3 a1-a3).
The OWF-induced increase at wake s flanks does not follow the positive link-

age. Here, the lowest increase of 9.01 % is given for UG8, the strongest with 9.77 %
for UG5, which is close to the case of UG16 showing a wind speed rise of 9.65 %;
see also Table 4.2 .
Due to these flanks, the change in wind speed along the cross section NW-SE is
not symmetric, as it is in case of the Brostr¨m method. Thus, the wake flanks do not
only vary in intensity; their dimension even differs. While the area of wind
reduction downstream the wind farm is quite constant in all three wind speed

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