Wind turbine rotoreffective wind speed estimated by nacellemounted

Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars Dominique P Held 1, 2 Jakob Mann 1 1 Department of Wind Energy, Technical University of Denmark (DTU) Frederiksborgvej 399, 4000 Roskilde, Denmark 2 Windar Photonics A/S Helgeshøj Alle 16 -18, 2630 Taastrup, Denmark 19 th Coherent Laser Radar Conference June 18 – 21 2018 Okinawa, Japan

Outline 1. Motivation & Challenges 2. Experimental Setup 3. Estimation of rotor-effective wind speed 4. Theory 5. Results 6. Conclusion & Outlook 2 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Motivation & Challenges • Traditional wind turbine feed-back controller provide imperfect control • Nacelle mounted wind lidars can measure inflow • Additional information can be used to improve pitch control performance • Reduction of fatigue loads and small energy is expected From: Schlipf, D. et al (2010). Load analysis of look-ahead collective pitch control using LIDAR. In Proceedings of the 10 th German Wind Energy Conference (DEWEK), Bremen. 3 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Motivation & Challenges • Traditional wind turbine feed-back controller provide imperfect control • Nacelle mounted wind lidars can measure inflow • Additional information can be used to improve pitch control performance • Reduction of fatigue loads and small energy is expected How good can a lidar system estimate rotor-effective wind speeds? How well do models fit experimental results? 4 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Motivation & Challenges Adapted from: Schlipf, D. (2015). Lidar-Assisted Control Concepts for Wind Turbines. Ph. D thesis. Faculty of Aerospace Engineering and Geodesy, University of Stuttgart 5 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Experimental Setup • 2 commercial CW Doppler wind lidars from Windar Photonics A/S [1] – all-semiconductor light source (1550 nm) – Coherent homodyne detection 2 -beam 4 -beam 37 62 2. 1 6. 0 30 18 32 59 Time per beam per scan [s] 0. 5 0. 25 Period measured in 2016 Mar-May Oct-Dec • Heavily instrumented Vestas V 52 turbine (750 k. W) with 44 m hub height at the DTU Risø test site Met mast Vestas V 52 [1] Hu, Q. (2016). Semiconductor Laser Wind Lidar for Wind Turbine Control. Ph. D thesis. Department of Photonics Engineering, Technical University of Denmark 6 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

• Coherent detection utilizes the optical Doppler effect to determine the average aerosol speed within the probe volume • Measurement are limited to line-of-sight projection and low-pass filtered 7 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

• Incoming flow is affected by energy extraction of wind turbine (induction zone) • Induction correction is required for short-range lidars (within 1 -2 rotor diameter) • Analytical actuator disk model by Conway [1] Conway, J. T. (1995). Analytical solutions for the actuator disk with variable radial distribution of load, Journal of Fluid Mechanics 297, 327– 355 8 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

[1] Østergaard, K. Z. , et al (2007) Estimation of effective wind speed, Journal of Physics: Conference Series 75, 012082 9 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Theory - Coherence • Explicit solution for coherence can be found [1] Schlipf, D. (2013) Model of the Correlation between Lidar Systems and Wind Turbines for Lidar-Assisted Control, Journal of Atmospheric and Oceanic Technology 30, 2233 -2240 11 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Results – 10 Min mean values 12 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Results – Example of 4 -beam 1 Hz time series 13 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Coherence between lidar and turbine Mean squared error [(m/s)2] • 120 100 80 60 40 20 0 2 beam 14 4 beam DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Time-of-arrival analysis • [1] Moddemeijer, R. (1988) An information theoretical delay estimator, Ninth Symposium on Information Theory in the Benelux, 121 -128 15 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Conclusion & Outlook • Nacelle lidars can accurately estimate rotoreffective wind speeds • Adding additional beams decreases eddy sizes that can be resolved • Analytical models can predict performance and can be used for optimization • Experimental tests in complex terrain • Coherence model using different turbulence models (i. e. [1]) • Inclusion of – turbulence evolution [2] – convective processes [3] Thank you for your attention! Questions? [1] Mann, J. (1994) The spatial structure of neutral atmospheric surface-layer turbulence, Journal of Fluid Mechanics 273, 141 -168 [2] de Maré, M. et al (2016) On the Space-Time Structure of Sheared Turbulence, Boundary-Layer Meteorology 160 (3), 453 -474 [3] Chougule, A. et al (2018) Simplification and Validation of a Spectral-Tensor Model for Turbulence Including Atmospheric Stability, Boundary-Layer Meteorology 167, 371 -397 16 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Back-up Slides

[m/s] Diminishing utility of number of beams 18 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Theory - Coherence • 19 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Spatial filtering • 20 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Turbulence statistics • Using average spectra to retrieve turbulence statistics 21 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018

Induction Correction Induction influences ‣ Flow speed reduction ‣ Flow diversion Assumptions • Actuator disk to model rotor • Laminar and uniform inflow • Uniform and nonrotational disk loading Induction according to theory by Conway (1995) • Shed vorticity is advected in direction of inflow Conway, J. T. : 1995, Analytical solutions for the actuator disk with variable radial distribution of load, Journal of Fluid Mechanics 297, 327– 355. 22 DTU Wind Energy, Technical University of Denmark

2 -beam mounting tripod 23 DTU Wind Energy, Technical University of Denmark Wind turbine rotor-effective wind speed estimated by nacelle-mounted Doppler wind lidars 25 May 2018