Large-eddy simulation of multiple wakes in offshore wind · PDF fileLarge-eddy simulation of multiple wakes in offshore wind farms ... commercial offshore wind farm, in operation since

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Qualifizierung

Moses Kärn


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]


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


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


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


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


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


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


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Moses Kärn

Strong wind speed fluctuations in the turbulent

atmospheric boundary layer


Qualifizierung

Moses Kärn




Qualifizierung

Moses Kärn




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


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


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


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


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


Qualifizierung

Moses Kärn

Results – flow development within the wind farm

u_av /

u_inflow

No wake recovery behind 1st

turbine


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Moses Kärn

u_av /

u_inflow


turbine



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Moses Kärn

u_av /

u_inflow


turbine



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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



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Moses Kärn

u_av /

u_inflow

Slightly stronger wake deficit for neutral + stable case


1st turbine








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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


1st turbine








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Moses Kärn

TI_v in % TI_w in % TI_u in %



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Moses Kärn


Turbulence intensity is nearly unaltered behind the 1st turbine



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Moses Kärn



Continuous increase of TI up to the last turbine



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Moses Kärn



Continuous increase of TI up to the last turbine

Higher TIu in convective case, TIv and TIw similar for all cases



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


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)


Qualifizierung

Moses Kärn


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

Documents

Large-eddy simulation of multiple wakes in offshore wind · PDF fileLarge-eddy simulation of multiple wakes in offshore wind farms ... commercial offshore wind farm, in operation since