ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Large-eddy simulation of multiple wakes in
offshore wind farms
Björn Witha
Gerald Steinfeld, Martin Dörenkämper, Detlev Heinemann
ForWind – Center for Wind Energy Research
Carl von Ossietzky University Oldenburg, Germany
Contact: [email protected]
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Motivation
Europe’s offshore wind energy capacity is expected to grow up to
a total of 150 GW until 2030
Most of the wind farms will be located in clusters, mainly in the
Southern North Sea and Western Baltic Sea -> high density of
wind farms
Reduced power output in the wake of upstream turbines and
farms and increased loads due to enhanced turbulence intensity
Atmospheric boundary conditions affect wake characteristics,
especially stability modification of wind profile and turbulence
intensity
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Motivation
Wake simulations are a key task for energy yield predictions and
design of new wind farms
So far, wakes have been studied mostly with simple engineering or
RANS models
Current HPC clusters allow the application of LES models for wake
simulations, not only for single turbines but also for wind farms
With turbulence resolving LES, it is possible to study the interaction
between the turbulent atmospheric boundary layer and the
turbulent wakes as well as the interaction between the individual
wakes in a wind farm
The flow around the rotor blades has to be parameterized
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
The offshore wind farm “EnBW Baltic 1”
First German offshore wind farm in
the Baltic Sea and first German
commercial offshore wind farm, in
operation since 2010
16 km north of the Darß-Zingst
peninsula
21 turbines (2.3 MW each)
Hub height: 67 m
Rotor diameter: d = 93 m
Arranged in a triangle:
3000 m x 4580 m x 6563 m
Unique shape allows studying
wake situations from single to
sixfold wakes at the same time
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
EnBW Baltic 1 - realization in the model
LES model PALM (Raasch & Schröter, 2001)
Precursor run with periodic boundary conditions to
generate atmospheric turbulence
Main run initialized with results of the precursor run,
non-periodic boundary conditions and turbulence
recycling
z0 = 0.0005 m, strong capping inversion between
500 and 600 m height
A flow from the north (ug ≈ 15 m/s) has been simulated for three
different atmospheric stabilities:
convective neutral stable
surface heat flux 0.03 K m/s 0 K m/s -0.005 K m/s
model resolution 6 m 6 m 4 m
grid points (main run) 5120 x 1536 x 128 3072 x 1536 x 128 4096 x 2048 x 192
CPU time (main run) 10 h on 2048 CPUs 7 h on 2048 CPUs 27 h on 2048 CPUs
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Turbine parameterization: actuator disk model after
Jimenez et al. (2007) and Calaf et al. (2010):
Thrust uniformly distributed over rotor area
Rotation effects neglected
Reference velocity:
Jimenez et al.: velocity averaged over
actuator disk
Calaf et al.: velocity of the undisturbed flow
(axial induction factor included)
CT depends on wind speed (similar to thrust
curve, linear interpolation):
CT = 0.99 for u < 8 m/s
CT ≈ 0.35 for u = 13-14 m/s
F = − 0.5 C t A uref2
uref = ud
uref = ud ⋅ ( 1
1− a)
rotor area =
homogeneous
momentum
sink
CT
u
EnBW Baltic 1 - realization in the model
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Precursor runs
Roll convection in the
convective case
Streaks in the neutral and
stable cases
Flow field is not really
homogeneous, inflow will be
different for the different
turbine rows (but realistic)
convective
12.2 x 6.1 km2
neutral
2.3 x 2.3 km2
stable
2 x 2 km2
u
u_av
w
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Strong wind speed fluctuations in the turbulent
atmospheric boundary layer
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Strong wind speed fluctuations in the turbulent
atmospheric boundary layer
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Strong wind speed fluctuations in the turbulent
atmospheric boundary layer
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Results – wind farm flow (neutral case)
u_av /
u_inflow
neutral Normalization with inflow
velocity for each y-point
to eliminate effects of
convective rolls
Full multiwake situation
Larger wake deficits
behind 2nd turbines, but
nearly constant for
subsequent turbines
Significant wakes up to
40 d downstream of last
turbines
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Results – neutral case – single wake
u_av /
u_inflow
TI_v in %
Single wake: minimum
velocity ≈ 70% of inflow
velocity
Turbulence intensity
barely enhanced
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Results – neutral case – 4-fold wake
u_av /
u_inflow
TI_v in %
Stronger wake deficit behind
2nd and subsequent turbines
(~ 50 % of inflow velocity)
due to lacking wake
recovery and higher CT
Strongly enhanced turbulence
behind 2nd and subsequent
turbines
faster wake recovery
no further increase of wake
deficit for subsequent turbines
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Results – neutral case – 6-fold wake
u_av /
u_inflow
TI_v in %
Stronger wake deficit behind
2nd and subsequent turbines
(~ 50 % of inflow velocity)
due to lacking wake
recovery and higher CT
Strongly enhanced turbulence
behind 2nd and subsequent
turbines
faster wake recovery
no further increase of wake
deficit for subsequent turbines
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Results – effect of stability
u_av /
u_inflow TI_v in % convective
neutral
stable
Weaker deficit in the
convective case,
slightly stronger deficit
in the stable case
Wider wake in
convective case
Wake turbulence
similar for all cases,
background
turbulence higher in
convective case
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Results – flow development within the wind farm
u_av /
u_inflow
No wake recovery behind 1st
turbine
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
u_av /
u_inflow
No wake recovery behind 1st
turbine
Results – flow development within the wind farm
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
u_av /
u_inflow
No wake recovery behind 1st
turbine
Results – flow development within the wind farm
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
u_av /
u_inflow
Little wake recovery behind
1st turbine
Significant increase of wake
deficit from single to double
wake but constant (or slightly
weakening) wake deficit
behind 3rd to 6th turbine
Results – flow development within the wind farm
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
u_av /
u_inflow
Slightly stronger wake deficit for neutral + stable case
Little wake recovery behind
1st turbine
Significant increase of wake
deficit from single to double
wake but constant (or slightly
weakening) wake deficit
behind 3rd to 6th turbine
Results – flow development within the wind farm
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
u_av /
u_inflow
Slightly stronger wake deficit for neutral + stable case
90% recovery about 20-25 d downstream of last turbine, 95 % recovery > 40 d,
full recovery > 80 d downstream of last turbine
Little wake recovery behind
1st turbine
Significant increase of wake
deficit from single to double
wake but constant (or slightly
weakening) wake deficit
behind 3rd to 6th turbine
Results – flow development within the wind farm
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
TI_v in % TI_w in % TI_u in %
Results – flow development within the wind farm
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
TI_v in % TI_w in % TI_u in %
Turbulence intensity is nearly unaltered behind the 1st turbine
Results – flow development within the wind farm
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
TI_v in % TI_w in % TI_u in %
Turbulence intensity is nearly unaltered behind the 1st turbine
Continuous increase of TI up to the last turbine
Results – flow development within the wind farm
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
TI_v in % TI_w in % TI_u in %
Turbulence intensity is nearly unaltered behind the 1st turbine
Continuous increase of TI up to the last turbine
Higher TIu in convective case, TIv and TIw similar for all cases
Results – flow development within the wind farm
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Summary
LES results reveal strong velocity fluctuations in the turbulent atmospheric
boundary layer to be encountered by the wind turbines
First LES results of the offshore wind farm “EnBW Baltic 1” for different stabilities
show a clear development of the flow within the wind farm:
Significantly increasing deficit from single to double wake
Little wake recovery behind 1st turbine due to low turbulence intensity and
small distance between turbines
Slightly weakening deficit behind subsequent turbines
Pronounced wake recovery due to strongly enhanced turbulence intensity,
increasing for subsequent turbines
Stability affects the wind farm flow
Stronger wake deficits for neutral and stable stratification compared to
convective boundary layer, wider wake in convective case
Wind farm wake lasts up to 80 d downstream of last turbine
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
Outlook
Repeat study for different wind direction with larger distances
between turbines
Implementation of enhanced actuator disk model (non-uniformly
loading, rotational effects, turbine towers)
Validation with measurements (within the wind farm, from FINO 2
and from LIDAR campaign)
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
ForWind - Geschäftsstelle
Qualifizierung
Moses Kärn
The presented work was carried out in the framework of the project
Baltic 1, funded by the Federal Ministry for the Environment, Nature
Conservation and Nuclear Safety.
Computer resources have been partly provided by the North German
Supercomputing Alliance (HLRN)
Acknowledgement
Top Related