Introduction - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 2

Systematic electricity generation from the wind has been performed for more
than 20 years. In the early years, the turbines were small, rotor diameters being
much smaller than the vertical extent of the atmospheric surface layer. In those
times it was relatively easy to assess the local wind climate in order to calculate
turbine loads and energy yields. The knowledge of the frequency distribution of
the mean wind speed at hub height and the overall turbulence intensity was
sufficient to supply the necessary background information for the siting of single
turbines and small wind parks.
In the meantime, the size of turbines has increased. The hub height of multi-
MW turbines is often above the atmospheric surface layer and rotor diameters of
more than 100 m are frequently found. Offshore turbines with diameters of more
than 160 m and a power of 7 MW have already been designed and will be
deployed in the near future. This leads to much more complicated interactions
between the turbines and the lower atmosphere. Meteorological features which had
been considered as irrelevant for a long time are now becoming decisive for
planning and running single large turbines and increasingly larger wind parks. In
particular, vertical gradients in mean wind speed as well as in turbulence intensity
have to be known. Furthermore, the vertical range for which these wind parameters
must be obtained has now moved to heights which are hardly reachable by masts.
New measurement techniques are required to collect the necessary wind infor-
mation. This has led to a boom in surface-based remote sensing techniques (see
Emeis 2011 ). The economic success of wind turbines depends on a precisely
determined trade off between erection and operation costs and wind energy yields.
Each additional meter in hub height is only meaningful if the higher yields pay the
additional costs.
Additionally, especially in countries adjacent to the North Sea and the Baltic,
the main area for wind park development has moved from land to marine sites.
Here, offshore wind parks will probably deliver most of the wind energy in the
future. This means that wind parks are now erected in areas where many details of
the vertical structure of the atmospheric boundary layer are not sufficiently known.
Experimental data from the marine boundary layer are available—if any—for only
a shallow layer previously explored from buoys, ships, and oil racks. A few masts,
like the three German 100 m high FINO masts, have been erected recently in the
German Bight and the Baltic. They are presently delivering long-term information
on a deeper layer of the marine boundary layer for the first time.
This topic tries to analyse and summarize the now existing information of
atmospheric boundary layers—onshore and offshore—with respect to wind power
generation. The presentation will focus on the vertical profiles of wind and
turbulence. It tries to explain the physical processes behind the observable vertical
profiles. It will not display wind climatologies for certain regions of the world. The
analysis will include features like vertical profile laws beyond those power laws
which had been suitable for the surface layer assessment for a long time, insta-
tionary phenomena like nocturnal low-level jets, the wind-speed dependent
roughness and turbulence conditions in marine boundary layers, and the complex
wind-wakes interactions in larger wind parks.