Wind Energy

1 Offshore Wind Power Meteorology 5
The logarithmic profile expected for neutral stratification, a Monin–
Obukhov profile for stable stratification (L=200 m) and a profile additionally
taking into account the effect of an inversion (h=200 m) (Lange et al., 2004a).
Clearly, the wind speed gradient with height increases when the inversion is
included. The gradient is then larger than the gradient of the power law profile
used in the IEC guidelines (IEC-61400-1, 1998) for wind turbine design, which
do not take atmospheric stability into account. This means that the fatigue
loads on e.g. the blades will in these situations be larger than anticipated in
the design guidelines. Over land stability is always near neutral at high wind
speeds due to the low surface roughness. Over water, on the other hand, stable
stratification also occurs at higher wind speeds. Therefore, atmospheric
stability might have to be included in the description of the wind shear.
1.5 Conclusion
With the example of the vertical wind speed profile offshore it was shown that
specific meteorological conditions exist at the potential locations of offshore
wind farms, i.e. over coastal waters in heights of 20 to 200 m. Since the interest
in the wind conditions at these locations is new, the specific meteorological
knowledge still has to be improved. The behaviour of the atmospheric flow over
the sea differs from what is seen over land due to the different properties of the
water surface. The findings still have to be investigated further, but it is clear
that specifically offshore wind conditions can have important effects on wind
power utilization, e.g. for turbine design and wind resource calculation. This
leads to the conclusion that offshore wind power meteorology is an important
research field, which is needed for the efficient development of offshore wind
power and which has the potential to produce new meteorological knowledge
about the atmospheric flow over the sea.
References
1. Barthelmie RJ (1999) The effects of atmospheric stability on coastal wind climates.
Meteorological Applications 6(1): 39–47
2. Barthelmie RJ (2001) Evaluating the impact of wind induced roughness change
and tidal range on extrapolation of offshore vertical wind speed profiles. Wind
Energy 4: 99–105
3. Barthelmie RJ, Hansen O, Enevoldsen K, Motta M, Højstrup J, Frandsen S,
Pryor S, Larsen S, Sanderhoff P (2004) Ten years of measurements of offshore
wind farms – What have we learnt and where are the uncertainties? In: Proceedings
of the EWEA Special Topic Conference, Delft, The Netherlands
4. Csanady GT (1974) Equilibrium theory of the planetary boundary layer with
an inversion lid, Bound-Layer Meteor. 6: 63–79
5. GWEC Wind Force 12 (2005) A blueprint to achieve 12% of the world’s electricity
from wind power by 2020. (available from www.ewea.org)