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for unstable stratification than for stable stratification. Over the ocean stability
mainly depends on the type of thermal advection. Cold air advection over warmer
water usually leads to unstably stratified boundary layers and warm air advection
over cold water to stably stratified boundary layers. In the west wind belts of the
temperate latitudes, cold and warm air advection regimes are coupled to different
wind directions which correspond to the typical wind directions in the warm and
cold sectors of the moving depressions (see Fig.
6.9
for an example). As mean
turbine distances and gaps between entire parks can be smaller for unstable
stratification than for stable stratification, it might be advisable to make at least the
gaps between entire offshore parks wind direction-dependent having larger gaps in
the direction of flow that is connected to warm air advection.
The example in Fig.
6.9
shows unstable stratification for north-westerly and
northerly winds and stable stratification for south-westerly winds. In such a wind
regime, it might be advisable to have larger distances between the turbines and
between wind parks in the south-west to north-east direction, while shorter dis-
tances are possible in the north-west to south-east direction. The lower frame of
Fig.
6.3
shows that there is a factor of two in power reduction between h/L
*
=-0.3
and h/L
*
= 0.1, which are the typical mean stabilities in Fig.
6.9
. Therefore, the
analysis of the relation between average stability of the boundary layer and the wind
direction should be analysed during the siting procedure for offshore wind parks.
This advice does not apply to onshore wind parks, because here the atmospheric
stability mainly depends on cloudiness and time of the day, but not so much on wind
direction.
References
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466-477 (2003)
Barthelmie, R., Frandsen, S.T., Rethore, P.E., Jensen, L.: Analysis of atmospheric impacts on the
development of wind turbine wakes at the Nysted wind farm. Proc. Eur. Offshore Wind Conf.
2007, Berlin 4.-6.12.2007 (2007)
Barthelmie, R., Hansen O.F., Enevoldsen K., Højstrup J., Frandsen S., Pryor S., Larsen S.E.,
Motta M., and Sanderhoff P.: Ten Years of Meteorological Measurements for Offshore Wind
Farms. J. Sol. Energy Eng. 127, 170-176 (2005)
Barthelmie R.J., L.E. Jensen: Evaluation of wind farm efficiency and wind turbine wakes at the
Nysted offshore wind farm. Wind Energy 13, 573-586 (2010)
Barthelmie, R.J., S. Pryor, S. Frandsen, S. Larsen: Analytical Modelling of Large Wind Farm
Clusters.
Poster,
Proc.
EAWE
2004
Delft
(2004).
Bossanyi, E.A., Maclean C., Whittle G.E., Dunn P.D., Lipman N.H., Musgrove P.J.: The
Efficiency of Wind Turbine Clusters. Proc. Third Intern. Symp. Wind Energy Systems,
Lyngby (DK), August 26-29, 1980, 401-416 (1980)
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