Turbulence and wind speed investigations using a nacelle-based Lidar scanner and a met mast

1. Turbulence and wind speed investigations using a nacelle-based Lidar scanner and a met mast Andreas Rettenmeier 1 O. Bischoff1, D. Schlipf1, J. Anger 1, M. Hofsäß1, P. W. Cheng1R. Wagner2, M. Courtney2, J. Mann2 1 Stuttgart Wind Energy (SWE), University of Stuttgart 2 DTU Wind Energy - Test and Measurements Section, Risø Campus

2. Table of contents • Motivation • Experiment setup • Results of the methods “Projection” and “Estimation” • Wind speed • Turbulence • Conclusions & Outlook [Fig. SWE / Risø-DTU] 2

3. Motivation • 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 3 3

4. Lidar measurements at Risø-DTU test site: experimentsetup • 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 [Fig. SWE, Risø-DTU] 4

5. Lidar measurements at Risø-DTU test site: Lidar Scanner • 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 [Fig. SWE, Risø-DTU] 5

6. How can Sonics and Lidar be compared? • Problem: Different measurement principles • 3D wind vector (Sonic) ↔ line-of-sight wind speed (Lidar) • Point measurement (Sonic) ↔ Volume measurement (Lidar) • Possible solutions • Sonic → LidarReduction of the 3D vector to “line-of-sight” wind speed • Lidar → Sonic Reconstruction of the 3D vector from line-of-sight wind speed (depending on assumptions) Lidar [Fig. SWE, Metek] Normedlaservector 6

7. Sonic → Lidar Reduction of 3D vector to line-of-sight wind speed – in time domain • Transformation of u, v, w with the laser vector • Good correlation of high resolution Lidar and Sonic data. • Differences due topoint ↔ volume measurement? [Fig. SWE, Risø-DTU] 7

8. Sonic → Lidar Reduction of 3D vector to line-of-sight wind speed – in frequency domain • Spatialfiltering: ; [Fig. SWE, Risø-DTU] • Spatialfiltering/ volumemeasurementresponsible for the differences in the spectra and the underestimation of turbulence intensity. (valid for the frontal wind direction at the same height) 8

9. Lidar → Sonic Reconstruction of 3D vector from the line-of-sight wind speed Wind speedanddirections Lidar • Possible solutions: • 1. Projection for each point of the lidar measurements onthevector (assumptions v=0 and w=0) • 2. Combining 2 or 3 points of the lidar measurements to estimate the and x [Fig. SWE] y Top view 9

10. Wind speed – u-component, v=0, w=0Comparisoncenterpoints vs. mastmountedsonics Center height1540 10-min datasetsFilter >4m/s Center pointsvs. met mast [Fig. SWE, Risø-DTU] 10

11. Wind speed – u-component , v=0, w=0 Comparisonouterpoints vs. mastmountedsonics Center height1540 10-min datasetsFilter >4m/s Outer points vs. met mast [Fig. SWE, Risø-DTU] 11

12. Wind speed – u-component , v=0, w=0all pointsaveraged in eachheight vs. mastmountedsonics Center height1540 10-min datasetsFilter >4m/s All points averaged vs. met mast [Fig. SWE, Risø-DTU] 12

13. Turbulenceinvestigations – u-component , v=0, w=0Comparison center points vs. mast mounted sonics Center height99 10-min datasetsFilter >4m/s260°> dir > 300° Center pointsvs. met mast [Fig. SWE, Risø-DTU] 13

14. Lidar → Sonic Reconstruction of 3D vector from line-of-sight wind speed [Fig. SWE, Risø-DTU] Good correlation of the reconstructed Lidar and Sonic data.  Determination of wind direction possible 14

15. Wind speed – Estimationof u- and v- componentsComparisonpointsof top line vs. mastmountedsonics Top height1540 10-min datasetsFilter >4m/s Top line vs. met mast (52.5m) [Fig. SWE, Risø-DTU] 15

16. 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 • Next Steps • Turbulence intensity: Taking “spectral broadening” into account • Evaluation of the ground-based measurements performed at Risø Campus  further validation of Estimation approach [Fig. SWE] 16

17. Thank you for your attention! • Currentresearch • Wake measurements at Risø- Campus withNordtankturbine • Lidar assisted control demonstration at NREL, USA [Fig. SWE] www.uni-stuttgart.de/windenergie/LIDAR.html 17