Turbulence and wind speed
investigations using a nacelle-
based Lidar scanner and a met
mastAndreas Rettenmeier 1
O. Bischoff 1, D. Schlipf 1, J. Anger 1, M. Hofsäß 1, P. W. Cheng 1
R. Wagner 2, M. Courtney 2, J. Mann 2
1 Stuttgart Wind Energy (SWE), University of Stuttgart2 DTU Wind Energy - Test and Measurements Section, Risø Campus
Table of contents
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• Motivation
• Experiment setup
• Results of the methods “Projection”
and “Estimation”
• Wind speed
• Turbulence
• Conclusions & Outlook
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Motivation
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• Lidar: remote sensing technique with high spatial and temporal resolution
• Nacelle-based measurement methods show a great potential on- and offshore
• Direct applications in wake wind field analysis, wind turbine control, power curve measurement and load estimation are shown and presented
Studies are necessary regarding• Estimation of an equivalent wind speed and turbulence intensity• Investigations concerning vertical shear & turbulence measurements• Best fit of measurement points
Idea: Measuring with a Lidar scanner horizontally in various points, comparison with mast mounted 3D-sonic anemometers
Lidar measurements at Risø-DTU test site: experiment setup
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• Using Nordtank turbine as platform (stopped, not yawing)
• Installed Lidar scanner points towards a met mast in 100m distance
• Met mast equipped in three heights with
• 3D-Sonic anemometers at16.5m, 34.5m, 52.5m height
• Cup Anemometers at18m. 36m, 54m height
• Two temperature sensors at 10m, 54m
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• SWE Lidar Scanner allows steering the laser beam in any direction• Proof-of-concept demonstrated in various measurement campaigns
on- and offshore• Square grid: 3 x 3, 9 measurement points• Two times crossing the center point within one run, ~1,9sec per run• Center points corresponds to “the region” of sonic anemometers
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Lidar measurements at Risø-DTU test site: Lidar Scanner
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How can Sonics and Lidar be compared?
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Problem: Different measurement principles• 3D wind vector (Sonic) ↔ line-of-sight wind speed (Lidar)• Point measurement (Sonic) ↔ Volume measurement (Lidar)
Possible solutions1. Sonic → Lidar
Reduction of the 3D vector to “line-of-sight” wind speed
2. Lidar → Sonic Reconstruction of the 3D vector from line-of-sight wind speed (depending on assumptions)
𝑣 𝑙𝑜𝑠 ,𝐿𝑖𝑑𝑎𝑟
𝑢𝑠𝑜𝑛𝑖𝑐
𝑣 𝑠𝑜𝑛𝑖𝑐
𝑤𝑠𝑜𝑛𝑖𝑐
Normed laser vector
Lidar
Sonic → Lidar Reduction of 3D vector to line-of-sight wind speed – in time domain
𝑣 𝑙𝑜𝑠 ,𝑆𝑜𝑛𝑖𝑐=𝑙𝑥𝑢𝑠𝑜𝑛𝑖𝑐+𝑙𝑦𝑣𝑠𝑜𝑛𝑖𝑐+𝑙𝑧𝑤𝑠𝑜𝑛𝑖𝑐
Good correlation of high resolution Lidar and Sonic data.
Differences due to point ↔ volume measurement?
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Transformation of u, v, w with the laser vector
Sonic → Lidar Reduction of 3D vector to line-of-sight wind speed – in frequency domain
Spatial filtering / volume measurement responsible for the differences in the spectra and the underestimation of turbulence intensity. (valid for the frontal wind direction at the same height)
𝑆𝑆𝑜𝑛𝑖𝑐𝑉𝑜𝑙𝑢𝑚𝑒(𝑘)=𝑆𝑆𝑜𝑛𝑖𝑐 (𝑘)∗|𝐺 (𝑘 )|2Spatial filtering:
;
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𝐺 (𝑘 )=ℱ {𝑔 (𝑎)}𝑔𝑎
Lidar → Sonic Reconstruction of 3D vector from the line-of-sight wind speed
Top viewLidar9
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Wind speed and directions
Possible solutions:
1. Projection for each point of the lidar measurements on the vector (assumptions v=0 and w=0)
2. Combining 2 or 3 points of the lidar
measurements to estimate the and x
y
Wind speed – u-component , v=0, w=0Comparison center points vs. mast mounted sonics
Center pointsvs. met mast
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location R² slope offset
top 0,995 0,991 -0,222
center 0,997 1,016 -0,167
bottom 0,998 1,026 0,004 [Fig
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Center height1540 10-min datasets Filter >4m/s
Outer points vs. met mast
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Wind speed – u-component , v=0, w=0 Comparison outer points vs. mast mounted sonics
location R² slope offset
top 0,998 1,027 0,040
center 0,997 1,004 -0,162
bottom 0,996 0,986 -0,170
Center height1540 10-min datasets Filter >4m/s
All points averaged
vs. met mast
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Wind speed – u-component , v=0, w=0all points averaged in each height vs. mast mounted sonics
location R² slope offset
top 0,998 1,026 0,029
center 0,997 1,010 -0,164
bottom 0,996 0,987 -0,186
Center height1540 10-min datasets Filter >4m/s
Turbulence investigations – u-component , v=0, w=0Comparison center points vs. mast mounted sonics
Center pointsvs. met mast
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location R² slope offset
top 0,873 0,882 -0,019
center 0,867 0,938 -0,020
bottom 0,820 0,878 -0,031
Center height99 10-min datasets Filter >4m/s260°> dir > 300°
Lidar → Sonic Reconstruction of 3D vector from line-of-sight wind speed
Good correlation of the reconstructed Lidar and Sonic data. Determination of wind direction possible
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Top line vs. met mast (52.5m)
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Wind speed – Estimation of u- and v- componentsComparison points of top line vs. mast mounted sonics
location R² slope offset
u-component 0,998 1,027 0,0328
v-component 0,980 1,037 0,332
Top height1540 10-min datasets Filter >4m/s
Conclusions & Outlook
• Successfully applied the "Projection" and "Estimation" methods to estimate the wind speed components u, v (,w) of the Lidar
• Good correlation between met mast and Lidar measurements• Further correction of the turbulence intensity measurements are
necessary to improve correlation
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Next Steps• Turbulence intensity: Taking
“spectral broadening” into account
• Evaluation of the ground-based measurements performed at Risø Campus further validation of Estimation approach
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Current research • Wake measurements at Risø- Campus with Nordtank turbine• Lidar assisted control demonstration at NREL, USA
Thank you for your attention!
www.uni-stuttgart.de/windenergie/LIDAR.html