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P.-Å. Krogstad and M. S. Adaramola The Norwegian University of Science and Technology, Trondheim, Norway A model study of wind turbine interference Wind Power R&D seminar, Deep offshore wind Trondheim, 20-21 January 2011 1

A model study of wind turbine interference - Sintef

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Page 1: A model study of wind turbine interference - Sintef

P.-Å. Krogstad and M. S. AdaramolaThe Norwegian University of Science and Technology,

Trondheim, Norway

A model study of wind turbine interference

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011 1

Page 2: A model study of wind turbine interference - Sintef

Background

Model and measurements

Effect of turbine operating condition

Yaw effects

Conclusions

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011 2

Page 3: A model study of wind turbine interference - Sintef

Background

In wind parks, turbines interact!

Turbine interaction reduces power output and increases dynamic loads

Wake structure depends on turbine operating conditions. Is it always best to operate at turbine peak performance?

Wake may be deflected by yawing the turbine. How much power is gained or lost by yawing?

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011 3

Page 4: A model study of wind turbine interference - Sintef

Model turbine

Model turbine designed using standard Blade Element Momentum theory

Rotor diameter D=0.9m. Design tip speed ratio, λ=6

Wind tunnel test section: Crossection=2x2.7m, total length=12m

Power predictions performed with BEM and CFD (Fluent) software

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Main purpose of investigation:Measure turbine interaction under controlled laboratory conditions

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Page 5: A model study of wind turbine interference - Sintef

Model turbine

Gentle separation due to trailing edge ramp

Rapid transition on suction side due to small radius of curvature

Low sensitivity to surface roughness

Strong separation on lower side at negative angles of attack

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Airfoil: NREL S826 14% thickness

Characteristics:

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Page 6: A model study of wind turbine interference - Sintef

Model turbine

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

2D predictions of S826 performance

Fully turbulent XFOIL predictions agree well with k-ω SST

CL vs α

-0.5

0

0.5

1

1.5

2

-10 -5 0 5 10 15 20 25

XFOIL Ncrit=3.0XFOIL Ncrit=0.01 (turbulent)Fluent SST (turbulent)Fluent RNG (turbulent)

CL

α

-0.5

0

0.5

1

1.5

2

0 0.05 0.1 0.15 0.2 0.25

XFOIL Ncrit=3.0

XFOIL Ncrit=0.01 (turbulent)

Fluent SST (turbulent)

Fluent RNG (turbulent)

CL

CD

CL vs CD

6

Page 7: A model study of wind turbine interference - Sintef

Model turbine

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Standard Blade Element Momentum theorygives blade geometry

View in streamwise direction

View in plane of rotation

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Page 8: A model study of wind turbine interference - Sintef

Model turbine

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Model and measurement systems

Model in wind tunnelModel instrumentation

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Page 9: A model study of wind turbine interference - Sintef

Fluent predictions

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

3D CFD

9

100.000 cells used to describe the blade and nacelle surfaces3.5*106 grid points in 1/3 volume

Page 10: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Comparisons between predictions and measurements

Power coefficient vs tip speed ratio Thrust coefficient vs tip speed ratio

0

0.1

0.2

0.3

0.4

0.5

0 2 4 6 8 10 12

MeasuredBEMFluent

CP

λ

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2 4 6 8 10 12

MeasuredBEMFluent

CT

λ

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Page 11: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Measurements for 2 similar turbines(Simplified wind farm experiment)

Two in-line turbines Yawed upstream turbine

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Page 12: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Effect of distance between turbinesUpstream turbine operating at peak efficiency

Power coefficient, downstream turbine

Thrust coefficient, downstream turbine

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Page 13: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

At a given distance, the output from the downstream turbine depends on the operating point of the first

S/D=3

Power coefficient, downstream turbine

Thrust coefficient, downstream turbine

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Page 14: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Total output compared to two unobstructed turbinesS/D=3

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Page 15: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Effect of yawing upstream turbineUpstream turbine operating at peak efficiency

S/D=3

Power coefficient, downstream turbine

Power coefficient, downstream turbine, compared to single, non-yawed turbine

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Page 16: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Total output compared to two unobstructed turbinesUpstream turbine operating at peak efficiency

S/D=3

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Page 17: A model study of wind turbine interference - Sintef

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

ConclusionsWhen two wind turbines are placed in-line and both operated at best efficiency, the output of a turbine at S=3D is less than 60% of that upstream

The power reduction is influenced by the wake characteristics from the turbine upstream and therfore by its operating point

By reducing the power extracted from the first, the TOTAL output may be increased

Yawing a turbine reduces its power by cos3γ. But it also deflects the wake which increases the output further downstream

Two turbines operating in-line at best efficiency may increase the total output from about 69% of two unobstructed turbines at zero yaw, to 78% when the first is yawed 30 degrees. (Figures taken for S/D=3.)

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Page 18: A model study of wind turbine interference - Sintef

Model turbine

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Reynolds number dependence

Turbine was designed for λ = 6 and Uref = 10m/s

0

0.1

0.2

0.3

0.4

0.5

0 2 4 6 8 10 12

Uinf=7.2m/sUinf=8.2m/sUinf=9.3m/sUinf=10.3m/s

CP

λ

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12

Uinf=7.2m/sUinf=8.2m/sUinf=9.3m/sUinf=10.3m/s

CT

λ

Power coefficient vs tip speed ratio Thrust coefficient vs tip speed ratio

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Page 19: A model study of wind turbine interference - Sintef

Fluent predictions

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

3D CFD details

1/3 of the rotor including the nacelle was simulated.

CFD domain same as wind tunnel test section (-4.5D to 7.8D in streamwise direction, 2.9D in spanwise direction).

k-ω SST turbulence model with y+<5 for first grid point.

Structured boundary layer grid around blade up to 0.1c, tetrahedral grids used further out.

QUICK and SIMPLEC schemes used for convective and pressere terms.

100.000 cells used to describe the blade and nacelle surfaces, 3.5*106 grid points used.

4CPU PC parallel processing, ≈ 24 hours computing time per case

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Page 20: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

At design tip speed ratio (λ = 6) Flow almost two-dimensional

r/R=0.44

r/R=0.89

Flow mostly attached except at the trailing edge separation ramp

Angle of attack close to 7o over most of the blade

CL ~ 1.2

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Page 21: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Force distributions near design tip speed ratio

Good agreement between BEM and CFD

Tangential force Streamwise force

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Page 22: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

At low tip speed ratio (λ = 3) the blade operates in deep stall mode and the flow is highly three-dimensional.

BEM expected to fail severely

r/R=0.44 r/R=0.89

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Page 23: A model study of wind turbine interference - Sintef

Results

Wind Power R&D seminar, Deep offshore windTrondheim, 20-21 January 2011

Force distributions for λ = 3

Significant differences between BEM and CFD distributions.

(Still CP predictions virtually identical, but BEM CT severely under-estimated)

Tangential force Streamwise force

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