Physics of Wind Parks - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 146

Fig. 6.9 Example for the dependence of the mean stability of the marine boundary layer air for
different wind directions from FINO1 data for the year 2005. The full line gives the annual mean
stability parameter h/L* (right-hand axis), the dashed lines give the annual mean minus and plus
one standard deviation of this stability parameter. The dotted line gives the number of 10 min
data per 10 degree wind direction interval (left-hand axis)
year per 10,000 km 2 . A recent report by the European Wind Energy Association
(EWEA 2007 ) identified that offshore North Sea wind parks with an area of
17,900 km 2 were needed to supply 180 GW, i.e. about 25 % of Europe's current
electricity needs. A scenario for 2020 foresees the installation of 40 GW, which
would require about 3,980 km 2 of wind parks. Should this scenario materialise,
one or more waterspouts within an offshore wind park would have to be expected
every other year.
6.6 Summary for Wind Parks
The roughness of the underlying surface on which large wind parks are erected
turns out to be a decisive parameter governing the efficiency of such parks. This
happens, because the ability of the atmosphere to supply momentum from the
undisturbed flow above depends on turbulence intensity, which increases with
increasing surface roughness. Therefore, in offshore wind parks, this supply is
much less than over land, where turbulence intensity is much higher. Thus, in
offshore wind parks, the spacing between the turbines in the park must be larger as
onshore. The gaps between adjacent offshore wind parks must be larger as well.
Another important governing parameter for the efficiency of wind parks is the
thermal stability of the atmosphere, because turbulence intensity is much higher