Mike Courtney, Wind Energy Division, Risø DTU [email protected] · Mike Courtney, Wind Energy...

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Wind lidars – Not the final answer i l t i !in complex terrain!

Mike Courtney, Wind Energy Division, Risø DTU

[email protected]

I d R k 3Ice and Rocks 3

Zadar, May 6th 2010

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Risø DTU – Danish National Laboratory for Sustainable Energygy

DTU Th T h i l U i it f D k• DTU = The Technical University of Denmark

• Fuel Cells• Biomass• Biomass• Systems Analysis

• Wind Energy Division (150 employees)– Meteorology– Aeroelastics and Aerodynamics– Wind turbines

Wi d t– Wind energy systems– Test and Measurements

• Høvsøre Test Station for Large Wind Turbines

Risø DTU, Technical University of Denmark

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My backgroundy g

• Senior Scientist in Test and Measurements Program Wind Energy Measurements Program, Wind Energy Division, Risø DTU.

• 5 years lidar experience.

• Work package leader for– UpWind WP6 (Remote Sensing)

S f Wi d WP2 (R t S i )– SafeWind WP2 (Remote Sensing)

• Active in IEC 61400-12-1 revision

• Current mission: to get lidars working properly in wind energy.


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OverviewOverview

• Lidar state-of-the-art

• Problems in complex terrainProblems in complex terrain

• Lidar vs metmast

• Using lidars in complex terrain


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Lidar – state-of-the-art


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Basic measuring principle

Wind

φ

Vlos (line of sightVlos (line of sight velocity)Fb

Fs

F F 2 V / λFs- Fb= 2 Vlos/ λ


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Combining line-of-sight speeds to obtain the horizontal wind speed.p

Ideal, no assumptions needed

Practical, we need to assume that the flow is homogeneous


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How lidars work – conical scanning

The assumption is that the (mean) flow around the circumference at any height is uniformcircumference at any height is uniform.

100 m

Diameter = 100 m

Circumference = 314 m


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Different lidar types

Leosphere Natural Power WindCube™

Pulsed ZephIR™

ContinuousRange-gated

Simultaneous Focused

Sequentialheights

Fixed probe-

Sequential heights

Probe-length length

gf(H2)


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Newcomers

Galion Vindicator


Vindicator

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Good lidars aregetting accurategetting accurate- in flat terrain!

• Best lidars are within ±1.5% Best lidars are within ±1.5% of traceable cup (for the heights we can test).

• Very low noise

• Cup anemometer calibrationand cup-mast mountinguncertainties are the limitingconstraints for assessinglidar accuracy


lidar accuracy.

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Error standard deviation for different sensors - at 60 m height

Scintek 2004Cup to cup

80

p p

20

40

60

- 0.5 0 0.5 1Sodar Error@m�sDž 60 m

Wi d b 2009Zephir 2009Scintek mean -0.08, STDEV: 0.37

40

Windcube 2009Zephir 2009

250

300

10

20

30

50

100

150

200


- 0.5 0 0.5 1Lidar Error@m�sDž 60 m

Zephir mean -0.08, STDEV: 0.18- 0.5 0 0.5 1

Lidar Error@m�sDž 60 m

50

WC mean -0.08, STDEV: 0.09

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Turbulence sensed by a lidar – spatial attenuationspatial attenuation

probe volume

lens

Average over the probe volume: The laser intensity is distributed along the beam with a distribution that has its maximum at the focus point.

Average over the circular path: The laser beam scans conically measuring 50 radial

laser source

maximum at the focus point. Each radial speed is obtained from backscattered signals averaged over the probe volume.

speeds per rotation equally distributed over the circular path. The wind speed vector is calculated from the 50 (for a lidar scanning for 1second, 150 if it scans for 3 seconds) radial


if it scans for 3 seconds) radial speeds.

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The horizontal variance ’seen’ by the ylidar depends on the cone angle.


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The ratio also varies with height


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Problems in complex terrain


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The basic problem

The assumption is that the flow around the circumference at any height is uniform. y g

This is almost never true in complex terrain.


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Where most of the error comes from

The assumption is that w =w (and v =v )

w1

w2

w1=w2 (and v1=v2).

This will clearly not be the case if the flow is curved.v1

v2

The consequence is a BIG error in calculating v.

You might end up with 10You might end up with 10-20% error.


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If the flow can be modelled, the lidar errorscan be estimatedcan be estimated

M d l th fl ( d t d thi )• Model the flow (need to do this anyway).

• At the lidar sensing positions, calculate the x,y,z components of the flow.

• Calculate what the lidar would have sensed as radial speeds (geometry).

• From the 4 (or 50) radial speeds, calculate the ’horizontal’ speed the lidar would have reported.

• Relate the simulated lidar horizontal speed to the predicted horizontalspeed at the centre of the lidar (or somewhere else)speed at the centre of the lidar (or somewhere else).

• This is becoming established practice and lidar manufacturers offer packages to do this.


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Measurements by CENER – Paula Gomez


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Mast

Lidar


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Speed ratios and comparison to WEng

Horizontal wind speed ratio (79m)

1 3

1.4

700

800samples

experimental A': y=A'·x

wasp79 1º

1.1

1.2

1.3

500

600

700

ta

Num Data 79mValid sectors

0.9

1

1.1

Lida

r/cup

300

400

500

Num

ber o

f dat

0.7

0.8

0.9

100

200

300 N

0.6

0.7

0 50 100 150 200 250 300 350 400

Wind Dir (º)

0

100


Wind Dir ( )

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Turbulence comparisons in complex terrain

U_STD 79m

y = 0.8919x2

0.3U_STD 40m

y = 0.8898x2

0.3

R2 = 0.93260.2

D_l

idar

(-) R2 = 0.9316

0.2

D_l

idar

(-)

0

0.1ST

0

0.1ST

0 0.1 0.2 0.3STD_cups (-)

0 0.1 0.2 0.3STD_cups (-)

STDlid /STD 0 9STDlidar/STDcup~0.9Flat terrain results and

theoretical analysis (Risø): ~0.8


We can not predict this well yet!

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Doing true 3D turbulence measurements –Windscanner.dkWindscanner.dk


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Long range Windscanner for complex terrain


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Lidar vs metmast


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Lidar vs metmast - accuracy

• Whilst we ’calibrate’ lidars using cups, we can never do better than the cup accuracy.

• Many mast measurements are not particularly goody p y g―Mast shadow and flow distortion―Bad calibrations

• Even in complex terrain, cup and lidar errors may well becomparable.

• Lidar verification requirements are driving improvementsin cup calibration and mounting techniques.


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Lidar vs metmast – vertical range

• Large wind turbines require shear measurements over most of the rotor in order to fairly represent the incoming energy in the wind.

• Power curve measurements may require this in the future. Goodones certainly will !

• Wind resource measurements should be made in the same way(over a vertical range comparable to the rotor disk).

• Even with hub-height wind ressource estimates, an actual lidar h-h speed can be much more accurate than an extrapolatedvalue from a lower cup.

• Bottom line – going up in height is ’free’ for lidars, veryexpensive for cups.


p p

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Lidar vs metmast - priceLidar vs metmast price

• Assuming lidars can be used many times, their economicsappear promising, especially for replacing high masts.

• There are also ’hidden/forgotten’ lidar costsgPower supplyMaintenanceRepairs! (renting might be attractive)Repairs! (renting might be attractive)

• Well conducted lidar resource measurements shouldreduce the AEP uncertainty giving more subtle economicreduce the AEP uncertainty, giving more subtle economicbenefits.


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Lidars vs metmast - reliabilityLidars vs metmast reliability

• No contest!

• Lidars improving but there is still a way to go.


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Conclusion - Using lidars in complex terrain

• Need a mast as well?– No firm rules – maybe the banker’s consultant decides!– If yes, include some 3D sonic anemometers!– If no, do some good CFD modelling and predict the lidar errors

• Measure where the streamlines (tilt) changes least• Measure where the streamlines (tilt) changes least.– Avoid hill tops– Slopes are not a problem, changing slope is.

• Don’t use a CW lidar if there is a lot of low cloud

• Be careful interpreting the turbulence measurements – the (our!) uncertainty is quite high!uncertainty is quite high!

• Imperfect measurements are often better than no measurements if youknow their limitations.


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Thanks for listening!Thanks for listening!

[email protected]


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Mike Courtney, Wind Energy Division, Risø DTU [email protected] · Mike Courtney, Wind Energy...

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