iec meeting iec61400-1, ed. 3 - wakes - wind power · pdf fileiec meeting iec61400-1, ed. 3 -...

IEC meeting IEC61400-1, ed. 3 -WAKESRisø Wednesday the 13/1-2010

Sten Frandsen + Morten Nielsen

Risø DTU

Risø DTU, Danmarks Tekniske Universitet

Topics re. wake load modelling

•The general idea

•Old and new IEC wake loads

•IEC-old, verification, uncertainties

•More elaborate model? More parameters?

•Simulations, meandering? Averaging time?

•WAsPEng implementation

•Conclusion


THE GENERAL IDEA


General response-modelling

(*),),(

efUe fwwuu

),(U is the vertical mean shear of the flow

),(Uuu is standard deviation of wind speed fluctuations

),(Uww is the wake-induced mean flow speed deficit

),(Uff is a frequency-scale of turbulence.

Dynamics of the structure:

Flow input to model, through load p:

X


How does reality look?The usual picture:

0.00

0.02

0.04

0.06

0.08

0 45 90 135 180 225 270 315 360

flp [

arb

. u

nit

]

0.00

0.02

0.04

0.06

0.08

0 45 90 135 180 225 270 315 360

Wind direction [deg]

flp [

arb

. u

nit

]


0.00

0.40

0.80

1.20

1.60

250 270 290 310 330 350

Vindretning [deg]

Tu

rbu

len

s/la

st

Turbulens

Last

Wake loading rather complicated, geometricaland aerodynamically. Nevertheless:

SMW

SMS

Vindeby experiment designedto make plots like this

X=8.5D


Change input parameter so that output becomes “right”

0

10

20

30

40

50

60

0 45 90 135 180 225 270 315 360

Wind direction

Eq

uiv

. lo

ad

(k

Nm

)

W4-2

E5-2


OLD AND NEW IEC WAKELOAD MODELS


IEC amendment, “wake changes”:

For fatigue:

For extremes:

The option of just using wake-turbulence has beenremoved


Resulting differences

-5

0

5

10

15

20

0 5 10 15y

-ln(-

ln(F

(y))

)

F0

Fw

FU

FU,ass

F 0

F w

F U

F U,ass

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20

Wind speed [m/s]

C_

T

C_T,stall

C_T, pitch

C_T-m1

C_T-m2

CT

CT – stall

CT – pitch

CT – mod1

CT – mod2

U [m/s]

CT - fatigue Extremes


IEC-OLD, VERIFICATION+UNCERTAINTIES


Equivalent load in 1, 2 and multiple wake:

0

10

20

30

40

50

60

0 45 90 135 180 225 270 315 360

Wind direction

Eq

uiv

. lo

ad

(k

Nm

)

W4-2

E5-2

Met mast

Wind turbine

Instrumented wt

N

..makes no difference


Equivalent (wake) load – how does it come out?

X=8.5D

Wind speeds 8-9 m/s, estimated CT=0.88, I0=0.077. Vindeby data.

0

10

20

30

40

50

60

70

230 240 250 260 270 280 290

wind direction [deg]

Eq

uiv

. w

idth

[k

Nm

]

4W_flp_ew

5E_flp_ew

Model


e [k

Nm

]

e4W-flp

e5E-flp

MET

Wind speeds 13-14 m/s, estimated CT=0.64, I0=0.075. Vindeby data.

0

10

20

30

40

50

60

70

230 240 250 260 270 280 290

Wind direction [m/s]

Eq

uiv

. w

idth

[m

/s]

4W_flp_ew

5E_flp_ew

Model

e [k

Nm

]

e4W-flp

e5E-flp

MET


A full scale experiment, 1992Wind farm designed for wake measurements


X= 7D X=5D X=9.5D

5D

9.5D7D


X=2D X=2.4D

standard deviation of flap-wise bending moment on Nordex turbine, based on 80sec time series. Wind speed 6-9 m/s. From Risø Test station, Vølund (1991).

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

-40 -30 -20 -10 0 10 20 30 40

p_w-02

p_w-03

p_w-04

p_w-05

p_w-06

M_w

Mod

standard deviation of power. Wind speed 6-8 m/s. From Middelgrunden 2001


Middelgrunden (Enevoldsen & Stiesdal 2002)


Normalisedequivalent loadsfor characteristicloads on a windturbine, multible-wake case; turbine separation 8.5D.

How do different cross-sectional forcesreact to wake conditions?


What parameters must be expected to influence fatigue loading?

•Turbulence: u v w

•Vertical mean wind speed shear

•Horizontal mean wind shear

• Flow inclination

• Scales of turbulence


Wake turbulence as experiencedby downwind turbine

I 0

I add

Wake deficit as experienced by downwind turbine

U

Ub D

D0


Scale of turbulence in free flow

Measured PFD of length-scale of turbulence. From Petersen et al (1998).

Scales in wakereported to be1-5 timesless thanin free flow


Wake-scale: Høvsøre Test Site – Risø DTU

0

50

100

150

200

250

300

350

320 340 360 380 400


Le

ng

th s

ca

le L

u [m

]

160 m

100 m

60 m


Is wake-turbulence just…turbulence?

Also analytical considerationsregarding narrow-band randomprocesses lead to a fixed ratio eand y, Crandall and Mark (1963)..

IeeU yuy: fixedFor

Ratio of equivalent load tostandard deviation of wind speedas a function of the wind speedfor.


Experimental sensitivity factors, Vindeby

Sensitivity coefficient for turbulence, u, as function of wind direction – to the leftfree-flow and to the right full wakecondition.

Sensitivity coefficients for vertical, free-flow shear , and wake deficit, w. Smooth lines are visual fits.


Model vs. data

Prediction of wake loads Site/wind

turbine

Regu-

lation

Mea-

sure

s

low winds high winds

Alsvik/Danwin stall std 9.5 fits** fits**

Vindeby/Bobus stall e 8.5 fits over-predicts*

Alsvik/Danwin stall std 7.0 over-predicts* over-predicts**

Alsvik/Danwin stall std 5.0 over-predicts* over-predicts**

Kappel/Vestas pitch e 3.7 fits over-predicts

Kegnæs/Bonus stall e 2.5 fits over-predicts

Risø/Nordex stall/pi std 2.0 fits --

Middelg./Bonus stall*

pow 2.4 fits

Oak C./NEGM stall e


Contemporary uncertainty in predicting fatigue?

•Numerical aeroelastic modelling uncertainty: 20-30%? –according to benchmark

•Uncertainty in modelling external conditions: 10-20%? -present story)

•Material models and testing (SN curves): 10-20-30%?

•Field testing - verification: 10% would be very good

•TOTAL, in terms of standard deviation:

20-30% would probably be very optimistic


MORE ELABORATE MODEL? MORE PARAMETERS?


Alternative possibilities

•Simulations

•Meandering – simulations

•More variables


Fatigue -close spacing(Middelgrunden)

Model: U=6m/s

0.00

0.02

0.04

0.06

0.08

0 45 90 135 180 225 270 315 360

W ind direction [deg]

Measured

Modelled

Model: U=8m/s

0.00

0.02

0.04

0.06

0.08

0 45 90 135 180 225 270 315 360

W ind direction [deg]Model: U=10m/s

0.00

0.02

0.04

0.06

0.08

0 45 90 135 180 225 270 315 360

W ind direction [deg]


Ekstremes(Middelgrunden)

0.0

0.1

0.2

0.3

0.4

0 45 90 135 180 225 270 315 360

Wind direction [deg]m

ax-m

in [

arb

. unit]

Modelled

Measured

6m/s <U < 9m/s

0.00

0.02

0.04

0.06

0.08

0.10

0.12

125 150 175 200 225 250


Mto

wer

[ar

b.u

nit]

9m/s <U < 12m/s

0.00

0.02

0.04

0.06

0.08

0.10

0.12

125 150 175 200 225 250



WASP ENGINEERING


WASP Engineering script for site assessment


Windfarm Assessment Tool• IEC 61400-1 site assessment by

• WAsP/WEng results

• Effective Turbulence Intensity(IEC 61400-1, Annex D)

• Complex terrain factors

• Wind-sector management

• Effect on Effective Turbulence Intensity

• Effect on Annual Energy Production

• Grid curtailment effect on AEP

• Support for IEC 61400-12-1 in-situ power performance measurements

• Obstacle Assessment (IEC 61400-12-1, Annex A)

• Terrain Assessment (IEC 61400-12-1, Annex B)


Conditional wind-direction distribution

1

0

,

,

j j j

N

i i ii

p u A k fp u pp u

p u p u A k f

WAsP sector-wise wind climates

• Frequency f j• Weibull scale parameter A j

• Weibull shape parameter k j

2

eff actual

0

1

( ) | |m

m

I u I u p u d

1

, exp

j

j

k

kj

j j j

j j

k up u A k u A

A Alocal site

but wake model TI depends

on flow at upwind sites

Flow corrections


Matching IEC and WASP Engineering turbulence

1 1

5 32

1 1

4

1 6

f S f f L u

f L u

21

1 0

1 sinc 2T

T S d

Correction for 10-min sampling

Correction for bias from neutral stability results (under development)


IEC 61400-1 (Ed.3) Annex D

Regular array

Windfarm Assessment Tool

Irregular array

Enhanced background turbulence

2 212 wI I I I

1 2

0.36

1 0.2w

T

Id d C 21

sec max sec2

0.36

1 0.2w

T

Id N C

mteetra


Model improvements?

•Include explicitly half-wake

•Must work in all wf-configurations• Single-wake, multiple-wakes, right combination of

wakes, right backgeround turbulence etc.

•Models for both fatigue and extreme loading

•Work for all wind turbine components

•Must be verified

iec meeting iec61400-1, ed. 3 - wakes - wind power · pdf fileiec meeting iec61400-1, ed. 3 -...

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