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