Offshore Wind Power Meteorology

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					Offshore Wind Power Meteorology

Bernhard Lange

ISET e.V., Königstor 59, 34119 Kassel, Germany; blange@iset.uni-


Wind farms built at offshore locations are likely to become an important
part of the electricity supply of the future. For an efficient development of
this energy source, in depth knowledge about the wind conditions at such
locations is therefore crucial. Offshore wind power meteorology aims to
provide this knowledge. This paper describes its scope and argues why it is
needed for the efficient development of offshore wind power.

1. Introduction

Wind power utilisation for electricity production has a huge resource and
has proven itself to be capable of producing a substantial share of the elec-
tricity consumption. It is growing rapidly and can be expected to contrib-
ute substantially to our energy need in the future (GWEC, 2005). The 'fuel'
of this electricity production is the wind. The wind is, on the other hand,
also the most important constraint for turbine design, as it creates the loads
the turbines have to withstand.
   Therefore, accurate knowledge about the wind is needed for planning,
design and operation of wind turbines. Some tasks where specific meteoro-
logical knowledge is essential are wind turbine design, resource assess-
ment, wind power forecasting, etc. Wind power meteorology has therefore
established itself as an important topic in applied meteorology (Petersen et
al., 1998). For wind power utilisation on land, substantial knowledge and
experience has been gained in the last decades, based on the detailed mete-
orological and climatological knowledge available. Offshore, the meteoro-
logical knowledge is less developed since there has been little need to
know the wind at heights of wind turbines over coastal waters and any
measurements at offshore locations are difficult and extremely expensive.
   The aim of this paper is to describe the scope of offshore wind power
meteorology and to argue why this topic should be given more attention
both from the meteorological point of view and from the wind power ap-
plication point of view. The paper is structured in three main sections: First
some particular problems of offshore measurements are discussed in sec-
tion 2. This is followed by a section giving examples for meteorological
effects specific for offshore conditions. Their importance for wind power
application is shown in section 4, followed by the conclusion.

2. Offshore wind measurements

In recent years, measurements with the aim to determine the wind condi-
tions for offshore wind power utilization have been erected at a number of
locations (Barthelmie et al., 2004). Offshore wind measurements are a
challenging task, not only since an offshore foundation and support struc-
ture for the mast are needed, but also because of the challenges to provide
an autonomous power supply and data transfer, the difficulties of mainte-
nance and repair in an offshore environment, etc. These difficulties lead to
high costs of offshore measurements and often lower data availability
compared to locations on land. Additionally, the flow distortion of the self
supporting mast usually requires a correction of the measured wind speeds
for wind profile measurements (Lange, 2004).
Two measurements, from which results are shown in this paper, are the
Rødsand field measurement in the Danish Balitc Sea and the FINO 1
measurement in the German Bight. The FINO 1 measurement platform
(Rakebrandt-Gräßner and Neumann, 2003) is located 45km north of the

    FINO 1 (100m)

                                                  Rødsand (50m)
Figure 1: The measurement sites Rødsand in Denmark and FINO 1 in Germany
island Borkum in the North Sea (see Figure 1). The height of the meas-
urement mast is 100m. The field measurement program Rødsand (Lange et
al., 2001) is situated about 11 km south of the island Lolland in Denmark
(see Figure 1) and includes a 50 m high meteorological mast.

3. Offshore meteorology

There are fundamental differences between the wind conditions over land
and offshore due to the influence of the surface on the flow. The most ob-
vious one is the roughness of the sea, which is very low, but also changes
due to the changing wave field (Lange et al., 2004b). The momentum
transfer between wind and water, governed by the sea surface roughness,
therefore depends on the wave field (see figure 2).
Stability effects due to the different thermal properties of water compared
to land have been shown to be very important (Barthelmie, 1999),(Lange
et al., 2004). Both the surface roughness and the surface temperature
change abruptly at the coastline, which leads to important transition effects
for wind blowing from land to the sea. Additionally, other effects like cur-
rents and tides influence the wind speed over water (Barthelmie, 2001).
   The dedicated meteorological measurements made in connection with
planned offshore wind power development helped to improve the knowl-
edge about the wind conditions relevant for offshore wind farm installa-
tions. One example is the vertical wind speed profile over coastal waters.
   The wind speed profile is commonly described by a logarithmic profile,
modified by Monin-Obukhov similarity theory for thermal stability. In fig-
ure 3 the prediction of Monin-Obukhov theory for the ratio of wind speeds
at 50m and 30m height versus stability is shown together with measured
results from the two sites Rødsand and FINO 1 (Lange, 2004). It can be

       Geostrophic wind
                                           Wind profile
     Atmospheric stratification

     Air temperature
                                                   Momentum transfer
                                                   Sea surface roughness
                       Water temperature

               Fetch                       Wave field

Figure 2: Sketch of influences on the wind field over coastal waters
seen that the Rødsand data show a larger wind speed ratio for near neutral
and stable conditions than expected from theory.
   A qualitative explanation of this result based on (Csanady, 1974) has
been developed (Lange et al., 2004): Rødsand is surrounded by land in all
directions with a distance to the coast of 10 to 100 km. When warm air is
advected from land over a colder sea, an internal boundary layer with sta-
ble stratification develops at the coastline. The heat flow through the stable
layer is small, and the air close to the water is cooled continuously from
the sea surface. It will eventually take the temperature of the sea and be-
come a well-mixed layer with near-neutral stratification. Higher up an in-
version develops with strongly stable stratification. In such a situation with
strong height inhomogeneity of atmospheric heat flux, Monin-Obukhov
theory must fail. At the FINO 1 site, the coastline is much further away for
almost all wind directions and this flow situation does not develop.
                                                     FINO 1 data
                                                     Roedsand data
           Wind speed ratio u(50)/u(30)

                                          1,15       Monin-Obukhov theory




                                              -0,2          -0,1             0,0      0,1
                                                          Stability parameter 10m/L

Figure 3: Comparison of measured (Rødsand and FINO 1) and theoretical (Monin-
Obukhov theory) dependence of the wind speed ratio at the heights 50 and 30 m
on atmospheric stability

4. Application to wind power utilisation

   For planning and operation of offshore wind farms, it is important to
take into account the specific conditions at offshore locations. As shown
above, the vertical wind speed profile can be modified significantly in the
coastal zone. A simple correction method has been proposed to evaluate
the magnitude of the effect for wind power applications (Lange et al.,
2004a). The effect of this correction on the profile can be seen in figure 4,
where different theoretical wind profiles are compared.


                Height [m]   100


                             40                      Stable
                                                     Stable & Inversion
                             20                      IEC design profile

                                   6    8       10        12        14
                                       Wind speed [m/s]

Figure 4: Comparison of different theoretical wind speed profiles

   The logarithmic profile expected for neutral stratification, a Monin-
Obukhov profile for stable stratification (L=200m) and a profile addition-
ally taking into account the effect of an inversion (h=200m) (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.

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 200m.
Since the interest in the wind conditions at these locations is new, the spe-
cific 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 condi-
tions can have important effects on wind power utilisation, e.g. for turbine
design and wind resource calculation. This leads to the conclusion that off-
shore 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 at-
mospheric flow over the sea.


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