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Can wind lidars measure turbulence?A. SatheJ. MannJ. GottschallM. Courtney
Acknowledgements – 1. EU FP6 Upwind Project2. EU FP7 SafeWind Project
Risø DTU, Danmarks Tekniske Universitet
What do we mean by turbulence?Estimation of Reynolds stress tensor,
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Why should we measure turbulence?
1. Design of wind turbines2. Power curve of a wind turbine3. Validation of wind profile models at greater heights, e.g. using momentum
flux measurementThe aim is to reduce the overall cost of wind energy!
Many other applications in fields other than wind energy, e.g. Bridge design
Risø DTU, Danmarks Tekniske Universitet
How can we measure turbulence?• Sonic anemometers (current industry standard)• Cup anemometers• Hot wire anemometers
So, what is the problem?
Remote sensors that do not need a tower1. Wind lidars2. Sodars3. Any other remote sensing instrument
All these instruments need a meteorological mast • They are very expensive, both onshore and particularly offshore• They have a fixed structure to the ground, and hence, cannot be moved
easily
What is the alternative?
Risø DTU, Danmarks Tekniske Universitet
What are the challenges for remote sensors?• They should measure exactly what they are supposed to measure – Mean
wind speeds, Turbulence etc.• They should be cheaper than or at least as expensive as the current
standard measurement systems
What are wind lidars?
rv
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W
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),)(()()(
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dnvnv fr
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r
Risø DTU, Danmarks Tekniske Universitet
What are the challenges for the measurement of turbulence?
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rv
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w
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Which wind lidars are currently available?• Windcube – a pulsed lidar developed by Leosphere • ZephIR – a continuous-wave (CW) lidar developed by Qinetiq, Natural
power
fds
Risø DTU, Danmarks Tekniske Universitet
How are the wind speeds retrieved?
Windcube ZephIRE
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sin2
sin2
,sincos
rWrEEW
rSrNNS
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rNv
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Risø DTU, Danmarks Tekniske Universitet
How accurate is the turbulence measured by lidars?
ZephIR
Risø DTU, Danmarks Tekniske Universitet
Can we understand why lidars do not measure turbulence precisely?
tensorvelocity spectral3D
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Point measurements
Risø DTU, Danmarks Tekniske Universitet
Can we understand why lidars do not measure turbulence precisely?
conjugate Complex *lidar respective the for function weightingingCorrespond X
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)(
)()()( *''
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Assumptions in modelling• The terrain is horizontally homogeneous• The spatial structure of turbulence flow is well described by the spectral
tens or model of Mann(1994)
Risø DTU, Danmarks Tekniske Universitet
Can we understand why lidars do not measure turbulence precisely?
• Only neutral conditions are shown• The model compares reasonably well with the measurements from both lidars• Large systematic errors are observed for both lidars in turbulence measurements
Model
Measurements
Ideal caseIdeal case
Variation with wind direction
ZephIR Windcube
Risø DTU, Danmarks Tekniske Universitet
How can we measure turbulence using lidars?Directly using the variances of line-of-sight velocities
sincossin2coscossin2cossinsin2
cossinsincossin''''2''
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uvuvr
• Avoids averaging due to conical scanning
• Averaging only along the beam direction
• In principle, only 6 beams are required to measure Reynolds stress tensor
Risø DTU, Danmarks Tekniske Universitet
How can we measure turbulence using lidars?
How did we arrive at this configuration?
beam vertical172,45 beams, 5
Assumptions in modelling• The terrain is horizontally homogeneous• The spatial structure of turbulence flow is well described by the spectral
tensor model of Mann(1994)
ss1 and of consistingMatrix
ancessight vari-of-linestresses, Reynolds
,1
M
SΣ
SMΣ
450
,3600
),,(
f
2SΣΣ
Risø DTU, Danmarks Tekniske Universitet
Comparing the 6 beam configuration with the conical scanning method
• For ZephIR, the improvement is not significant except for w variance• The line-of-sight averaging effect is quite significant, especially for the ZephIR.• For the Windcube, the variances using 6 beam is quite different from the conical scanning method• Better results with the Windcube for the conical scanning method is due to luck (contribution of the cross components). The errors are quite
large for different stability conditions using the conical scanning method as compared to the 6-beam method.
Conical scanning
6-beam
Conical scanning
6-beam
Ideal caseIdeal case
ZephIR Windcube
Risø DTU, Danmarks Tekniske Universitet
Conclusions• Using the conical scanning method, turbulence cannot be measured
precisely due to large systematic errors arising from spatial averaging• In principle, the 6 beam configuration reduces systematic errors due to
conical averaging. Windcube errors are reduced significantly• We need to somehow negate the line-of-sight averaging effect from both
lidars, especially the ZephIR
Future work• 6-beam experiment using a short-range windscanner developed at Risø
DTU• Mann et.al(2010) developed a spatial filter model for the ZephIR to negate
the line-of-sight averaging effect, but needs measurements. Averaged doppler spectra will be used instead of using the radial velocities derived from the individual doppler spectra.
Risø DTU, Danmarks Tekniske Universitet
Thank you