Civil Engineering Reference
In-Depth Information
s forcing includes data of wind, pressure, temperature, humidity,
precipitation, and cloudiness with their origin of, on one hand, ECMWF (European
Centre for Medium-Range Weather Forecasts) data and, on the other hand, forcing
data also modeled by the atmosphere model METRAS.
HAMSOM
'
3.1.2 Mesoscale Transport and Stream Model
The
ME
soscale
TRA
nsport and
S
tream (METRAS) model was developed by
Schl¨nzen in 1988 and was complemented with a wind turbine parameterization
by Linde et al. (
2014
). Hence, METRAS has implemented wind turbines and so is
able to resolve changes due to wind turbines in the atmosphere. This study uses
METRAS data as a meteorological forcing, which was modeled in collaboration
with the Meteorological Institute (MI) of the University of Hamburg based on the
need for ocean analysis. The METRAS data are a courtesy of the MI of the
University of Hamburg, and the METRAS simulations were done by Marita
Linde, whose Ph.D. includes atmospheric changes in the north of Germany due
to OWFs.
Here, only wind turbine parameterization of METRAS is considered.
Wind Turbine Parameterization in METRAS
Facts about wind parameterization in METRAS are based on Linde et al. (2014) and
personal correspondence with M. Linde (Ph.D. candidate at MI since 2011).
METRAS uses the actuator disc concept (ADC) for its wind turbine parameter-
ization (Linde et al.
2014
). This concept is based on Betz (
1926a
) constitute the
rotor as an infinitesimal thin disc with a fixed rotor diameter and a midpoint at hub
height and position of wind turbine. A schematic illustration of the concept of the
ADC is shown in Fig.
3.1
after Betz (
1926b
), Mikkelsen (
2003
), and Linde
et al. (
2014
).
It is assumed that an air package contains kinetic energy depending on its
velocity. Far in front of a wind turbine, the air package is not influenced by the
wind turbine and has the velocity
v
1
. Because of the extraction of kinetic energy, the
flow velocity
v
2
is reduced behind a wind turbine. This increases the pressure right
in front of the rotor disc
A
0
(Fig.
3.1
). The parallel streamlines of laminar flow are
spreading up. An air package that passes a small area
A
1
far in front of the wind
turbine passes a larger area
A
2
far behind the wind turbine. The maximal thrust
T
max
is reached for
v
2
¼
0. Using these assumptions, a dimensionless thrust coefficient
c
T
can be formulated as the percentage of rotor thrust
T
0
to maximum thrust with the air
density
ρ
:
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