Development and Testing of a Wind Simulator at an Operating Wind ...

Development and Testing of a Wind Simulator at an Operating Wind Farm

Bob Conzemius1

Hao Lu2

Leonardo Chamorro2

Yu-Ting Wu2

Fernando Porte-Agel2

1WindLogics Inc.2University of Minnesota, Saint Anthony Falls Laboratory

2Copyright 2010, WindLogics Inc. All Rights Reserved

Acknowledgements


GoalDevelop large eddy simulation capability for an operating wind farm.Determine turbine wake impacts using SCADA (via pi system) data.Take advantage of all available measurements to assess dependency on atmospheric conditions that affect the dispersal of wakes.Simulate wakes with LES and compare with observations.


Mower County Wind Farm with SODARs


Close-up View of Southwest Segment

Two prevailing wind directions: south and northwest

Description of the code – governing eqns, SGS model, boundary conditions, and wind‐turbine model

SGS model for fluxes (τij and qj )

Model for wind-turbine forces ( Fi )Subgrid-scale (SGS) modeling:

Averaging: • Over horizontal planes (Porté-Agel et al, 2000; Porté-Agel, 2004)• Local (Basu and Porté-Agel, 2006)• Lagrangian (Bou-Zeid et al, 2005; Stoll and Porté-Agel, 2006)

• Scale-dependent dynamic models

LZ

Wind‐turbine force ( Fi ) parameterizations using actuator line model (ALM) (Sørensenand Shen 2002)Turbine: SWT‐2.3‐93; airfoil ‐ NACA63xxx; tower height=80m; rotor diameter D=93m;constant rotation rate 16rpm

Wall stress at bottom: (Stoll and Porté-Agel, 2006, etc. )


Wind Tunnel Observations Agree with LES


Large Eddy SimulationsSelect a case with good sodar data representative of upstream conditions and also measuring turbine wakes (November 22, 2009 has neutral conditions).Pre-run the LES with upstream conditions, save output, and use it as inlet condition on run with turbines


Large Eddy Simulations (November 22, 2009 case)

Turbine wakes appear to dissipate rapidly in turbulent conditions

4/9/2010 Simulation setup: neutral ABL case, z0=0.3[m], u*=0.63[m/s]; Lx=2100[m] (including buffer zone), Ly=1350[m], Lz=300[m]; resolution: Nx=360, Ny=288, Nz=64 (each rotor plane covers ~20 grid points)

Time‐averaged (over one hour) contours along the indicated planes: (a) x‐component velocity u on the x‐z plane through the axis of the 3rd turbine; (b) y‐component velocity v on the x‐z plane through the axis of the 3rd turbine; (c) turbulence intensity (σu)2 on the x‐z plane through the axis of the 1st turbine; and (d) turbulence intensity (σu)2on the x‐z plane through the axis of the 3rd turbine

(a)

(b)

(c)

(d)

9/22/2009 Simulation: Lx=2400m (including buffer zone), Ly=1200m, Lz=300m; Nx=128, Ny=128, Nz=32; (each rotor plane covers 10 grid points); incoming flow: neutral ABL, z0=0.2m, u*=0.65m/s


LES Comparison with SODAR Profile

50

100

150

200

4 5 6 7 8 9 10 11 12

Sodar 172 from field Sodar 169 from field Log-law fit (u*=0.63,z0=0.3) Sodar 172 from LES Sodar 169 from LES

U [m/s]

z [m

]

Simulation has more velocity deficit than sodardata.

Turbine parameters may need to be re-tuned.

Simulating more cases will provide more generality to the results


Analysis TechniqueWind angle definitions:

Orientation is looking directly at turbine whose wake is being analyzedZero angle is wind direction straight from waking turbine, not NorthWind direction coming FROM the left of the zero angle is negative

Sample all data from time period when SODARs are deployedData filtered to remove points outside slope of power curve of the “free” turbine (otherwise wake power deficits may be less):

Blade pitch angle is greater than zero (nearing top of power curve)Active power less than 150 kW, turbine not spinning or unavailable

Turbines analyzed in pairs, one “waked”, one “unwaked”.Plot ratio waked:unwaked wind speeds and power vs. direction


Stratify According to Vertical Temperature Gradient


Stratify According to Vertical Temperature GradientCompute average of nacelle ambient temperature among all turbines in farm (80-meter temperature)Compute average of the two SODAR temperature readings (2-meter temperature)Resulting vertical gradient is (Nacelle-SODAR)deg_C/78mUse the following table for stability:

Temperature Gradient (degC/100 m) Stability Class< -1.7 A,B-1.7 ≤ X < -1.5 C-1.5 ≤ X < -0.55 D-0.55 ≤ X < 1.5 E1.5 ≤ X < 100 F


Turbine 42 waked by 41: Nacelle Wind Speed

41 w

aked

(40)

Mai

n 41

to 4

2 w

ake

WT4

3 w

akin

g W

T42

WT4

2 w

akin

g W

T41


Turbine 42 waked by 41: Active Power

Wakes are somewhat difficult to see in many cases. There’s a lot of apparent randomness in the data.

41 waked by 40

Mai

n 41

to 4

2 w

ake

WT4

3 w

akin

g W

T42

42 w

akes

41


Stability Class F

41 is waked (40)

Mai

n 41

to 4

2 w

ake

WT4

3 w

akin

g W

T42

42 w

akes

41


SODAR profile (Bud) at 80 m AGL, All StabilitiesView: looking from Triton 169 to T41


SODAR profile (Bud) at 80 m AGL, F StabilityView: looking from Triton 169 to T41


SODAR Cross-Sections May Show RotationShading is wind speed ratio (waked:freestream), Rd = Rotor radiusContours are vertical velocity (m/s), horizontal line = hub heightTriton 169, looking toward WT41, all data, X/Rd=8.5

Wake is fairly asymmetric:More deficit to the left of centerUpward vertical velocity there

This SODAR may have a bias toward negative vertical velocity.

X/Rd=15.2


SODAR Cross-SectionSODAR 169 Looking toward WT42: also has asymmetry

X/Rd=6.1 (WT42); X/Rd=14.8 (WT43)


Triton 172 and WT39X/Rd = 5.5


Triton 172 and WT40X/Rd=5.5 (WT40); X/Rd=11.8 (WT41)


Calculate Speed Deficits from SODAR DataWake center determined from wind farm geometry

Directions and distances to nearby turbines are known.SODAR data consistent with directions to nearby turbines?

Wind speed data are composited in 5-degree binsIdentify the minimum in wind speed ratio (waked:unwaked)

Take +/- 15 degrees from turbine directionFind minimum in ratio within this interval

Separate by atmospheric stability category


SODAR-Measured Wind Speed Ratios

Wind speed ratios:Triton172/Triton169Triton169/Triton172

Directions to various turbines are indicated by radials


SODAR-Measured Wakes: Discrete Wake Profiles

Wind speed ratios:Triton172/Triton169Triton169/Triton172


Minimum Wind Speed Ratios in Wake

39,4

0; T

riton

172

42, Triton 169

41, T

riton

169

41, T

riton

172

39, Triton 169Data align better by distance if wake minimum is used, but still not perfect function of distance, and all data must be averaged.


Cumulative Wakes: Looking Down a Row of Turbines


Cumulative Wakes: Looking Down Row 32-33

Class F Stability

Red line is average in 10-degree bins



Class F Stability



Calculate Average Speed/Power DeficitsUse F-class stability to identify wake center

Sample +/-90 degrees from 0 angleUse finite bin width (in this example, 10 degrees)Use 30-degree wide, centered filter (3 bins in this example)Find minimum filtered point in sample

Average over 30-degree widthTake average of minimum point, +/- 15 degreesDeficit is relative to the average of all samples outside of this area but still in the +/-90-degree sampled anglesDeficit is calculated separately for all stability categories

Westernmost turbine in this example is upwind


Power Deficits: Mower County Turbines 32-38


Conclusions and Future WorkBoth SCADA and SODAR data can be useful to measure wind turbine wakes, but compositing data is required.Temperature stratification is important– DON’T ASSUME NEUTRAL STABILITY!!SODAR profiles can reveal details of turbine wake, such as torque effects and wind shear effects, when QA/QC is good.Large eddy simulations show promise for simulating wind farm wake interactions

It is a challenge to get the incoming flow correct for case studies.Pre-simulation of upstream conditions (including fully-developed turbulence) must be conducted.Must simulate many cases before results can be generalized.

Second phase of field measurements now underway.

Development and Testing of a Wind Simulator at an Operating Wind ...

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