Calculation of atmospheric stability used for wake …€¦ · Calculation of atmospheric stability...

DTU Wind Energy Department of Wind Energy

Calculation of atmospheric stability used for wake analysis

Kurt S. Hansen & Søren Ott

DTU Wind Energy

[email protected]

VindkraftNet meeting, DONG Energy January 15, 2014 Kurt S. Hansen

2 DTU Wind Energy, Technical University of Denmark

Outline

1. Background & motivation;

2. Models and required signals;

3. Introduction to AMOK;

4. Examples based on Horns Rev and Lillgrund WF;

5. Horns Rev park efficiency;

6. Conclusion and future work;

7. Acknowledgement.



Short background (ksh)

• Wind & load measurements (winddata.com);

• Far wake analysis; 3 – 70D (WF SCADA data & Load measurements);

• Near wake analysis; 1 – 3D; (Horiz. Lidar measurements)



Motivation

Detailed wind farm analysis need a quantification of

Wind direction/spacing;

Wind speed;

Turbulence;

Stratification;



Models used to determine the atmosperic stratification

Richardson number Ri(h,Δu, Δθ,Ta) Reference: IEA Rec. #3, 1990;

Gradient method (Δu), not completed;

Sonic 3-D+Ta;

Stability based on Froude number, Reference: Javier Sanz Rodrigo/CENER;

Bulk-Ri(h,Δu, Δθ, Ta) method & zref/L = f(Rib); Reference: B.Lange, PhD Thesis;

Monin-Obukov theory (AMOK) zref/L≈g(Rib); Reference: Søren Ott et.al.;



Method: Bulk-Ri (B.Lange et.al.)

2)(

01.081.9

hU

T

hRih

a

b

tionstratificastableforRi

RiLz

tionstratificaunstableforRiLz

b

bref

bref

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Ref. The impact of turbulence intensity and atmospheric stability on power deficits due to wind turbine wakes at Horns Rev wind farm; Kurt S. Hansen et.al. WIND ENERGY 2011, we510



Method: Bulk-Ri (B.Lange et.al.)

Conclusion: Result was not robust, (Søren was not satified)!

Input: HR-M2 1999-2002

Ta: Absolute air temperature, h=55 m

Tw: Sea surface temperature, h=-4 m

Uh: Wind speed, h=62,

Output:

Monin-Obukhov length scale(zref/L)

Input: HR-M7 2005-2007

Ta: Absolute air temperature, h=55 m

Tw: Sea surface temperature, h=-4 m

Uh: Wind speed, h=20,

Output:

Monin-Obukhov length scale(zref/L)



3-D sonic anemometer

• Input signals from one 3-D sonic anemometer;

• Stab=zref/L

• Comment: Applicable for onshore sites – but not verified for offshore locations

''4.081.9/)273(L3* twuTa



Determination of offshore stratification - AMOK

• Theory: Monin – Obukhov

Characteristic temperature:

Friction velocity:

MO-length L:

AMOK is a tool for analysing of offshore met data

Input:

<U> <Dir> < Tair > < Twater > <time>

Output: 1/L, θ*, u* (S.Ott)

)/(/1 2

** uTgL a

** /´´ uw ½

* ´´ wuu



Constants

AMOK version 1.0.3 Output



Classification of atmosperic stability (offshore)

Class Obukhov length [m] Atmospheric stability class

cL=-3 -100 <=L <= -50 Very unstable (vu)

cL=-2 -200 <=L <= -100 Unstable (u)

cL=-1 -500 <= L <= -200 Near unstable (nu)

cL=0 |L|>500 Neutral (n)

cL=1 200 <= L <= 500 Near stable (ns)

cL=2 50 <= L <= 200 Stable (s)

cL=3 10<= L <= 50 Very stable (vs)

Method: AMOK

Input: - Absolute air temperature, h=13m

- Water temperature, h=-4m - Wind speed, h=15m

Output:

- Monin-Obukhov length (1/L)



Classification of stratification

Horns Rev- sectorwise atmospherie stability; 7 - 9 m/s (AMOK)

0%

20%

40%

60%

80%

100%

0 30 60 90 120 150 180 210 240 270 300 330

Flow sector - deg.

PD

F -

%

vs

s

ns

n

nu

u

vu

HR-M2:1999-2002



Stratification at Horns Rev 1999-2002

Wind speed (m/s), h= 62 m



Preliminary results



Distribution of turbulence intensity

Fitted with a LogNormal Distribution

NOT DE-TRENDED



Example: Lillgrund offshore wind farm

N

Mast



Classification of stratification at Lillgrund WF

Input signals: 0-360 deg

• Inflow wind speed, h=70m (derived from wt power/pitch values);

• Inflow wind direction, h=70m (derived from wind turbine yaw position);

• Air temperature, h=8 m;

• Water temperature measured at Drogden fyr (hourly), Δd>10km;

• AMOK version 1.03;



Classification of stratification at Lillgrund WF

Stratification@Lillgrund: U = 8 - 10 m/s (2000 hours)

0

5

10

15

20

25

30

-3 -2 -1 0 1 2 3

Stability classification

pd

f [%

]



Stable

Unstable



Input: HR-M6 ta: Air temperatur at height, h= 16m; U: Wind speed at height, h= 20m; ts: Sea surface temperature, h=-4m;

Output: Monin-Obukhov length scale

length(1/L); U*, θ*;

Mean turbulence intensity

HR-M7: 2005-2009



Quantification of power deficit – caused by stratification

• Extract power deficit for 0 – 360°

All observations - reference

Stable stratification (vs,s)

Neutral stratification (ns,n,nu)

Unstable stratification (u,vu)

• Demostration wind farm: Horns Rev (80x2 MW)

• Results for wind speed 8±1 m/s

• Reference wind speed ≈ ”free” wind turbines ~ 100%



Example: Horns Rev offshore wind farm: 80 x 2 MW, D=80m



Example of power deficit along wind turbine rows with 7D internal spacing.

Narrow

inflow

sector = 5 deg Narrow sector, Δ=5o

Large sector, Δ =30o



Efficiency polar

PRELIMINARY



Park efficiency (Δ AEP):

μi = AEP160MW/80*AEP2MW ; for vi=4±0.5, 5±0.5,..24±0.5 m/s

7-8%

Δ A

nnual Energ

y P

roduction

PRELIMINARY



Conclusion

• Robust classification of stratification for offshore locations has been established and demonstrated;

There is a lack of high quality measurement:

1) Meteorological flow reference measurements (speed & direction);

2) Wind Farm SCADA data;



Future work

• Validate and implement the AMOK classification for other offshore sites;

• Determine the dependence of structural loads due to atmospheric stability;

• Results from Horns Rev and other sites will be used in future IEA Wakebench model validations.



Acknowledgement

The research has been performed in part by EUDP 64010-0462 which is funded by the Danish Energy Agency.

We acknowledge Vattenfall AB and DONG Energy A/S for using the SCADA data from the Horns Rev offshore wind farm.

We acknowledge Vattenfall AB for using the SCADA data from the Lillgrund offshore wind farm.

Calculation of atmospheric stability used for wake …€¦ · Calculation of atmospheric stability...

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