Geoscience Reference
In-Depth Information
3.8 Summary for Flat Terrain
Today's wind turbines have hub heights well above the surface layer. Therefore,
the wind profiles describing the wind conditions can no longer be based purely on
the logarithmic laws ( 3.6 ) and ( 3.16 ) or the power law ( 3.22 ) valid for the surface
layer. The profile law of the Ekman layer ( 3.50 ) has to be considered for heights
above the surface layer. A combined profile for both layers such as those given in
( 3.65 ), ( 3.69 )or( 3.70 ) are probably the most suited laws to be used for load
assessment and power yield estimates. Equations ( 3.65 ), ( 3.69 ) and ( 3.70 ) apply
for the description of the vertical profile of the scale parameter of the Weibull
distribution as well.
The diurnal variation of wind speed in the layer above the surface layer is
different from the one in the surface layer. Here, in the Ekman layer, in roughly
one quarter of the nights in Central Europe, the night-time wind speed is higher
than the daytime wind speed. This phenomenon is called low-level jet. The vertical
profile of the shape parameter of the Weibull distribution has a maximum at the
top of the surface layer due to this phenomenon. Therefore, the relation ( 3.90 )
from Justus et al. ( 1978 ), which served rather well for the surface layer, is no
longer meaningful, but the relation ( 3.91 ) from Wieringa ( 1989 ) must be used.
Thermal winds ( Sect. 2.4 ) get some relevance in larger heights above ground
where the wind shear due to surface friction becomes small. Usually colder air
masses coincide with low-pressure areas and warmer air masses with high-pressure
areas in temperate latitudes. Therefore, thermal winds usually contribute to an
additional increase of wind speed with height.
As really large homogeneous surfaces are rather rare in densely populated areas
with frequent land-use changes, the features which come from the development of
internal boundary layers described in Sect. 3.5 have to be considered regularly.
For wind turbines erected in forests the special turbulence characteristics
addressed in Sect. 3.6 have to be taken into account over pervious forest crown
layers. Hub heights should be at least at three times the canopy height in order to
avoid enhanced turbulence over pervious forests crown layers. The quite large
displacement height deserves special attention for forest sites, because the wind
profile laws start from this height and not from the surface.
Typical urban features compared to rural areas are a higher wind shear at heights
of several hundreds of metres above ground, a larger increase of turbulence with
height especially at night, and a doubling of the turbulence intensity. The nocturnal
increase of the standard deviation of the vertical velocity component with height in
spring and summer is not just an urban feature but a feature which comes from the
interaction between rural and urban air flows. Low-level jets form over rural areas
and the additional surface friction due to cities is not sufficient to destroy them.
Thus, the higher mechanically-produced turbulence below low-level jets at heights
between 100 and 400 m above ground continues the higher thermally-produced
turbulence in the urban boundary layer below 100 m. It is obvious that urban areas
and forests (see Sect. 3.4 ) have mechanically some features in common (enhanced
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