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Wednesday, 12 March 2014
14:15 - 15:45 Remote sensing: From toys to tools?
Resource Assessment  


Room: Tramuntana
Session description

The use of remote sensing within the wind industry has developed significantly since these techniques were first adopted. The new opportunities to make measurements that have been made available have themselves influenced the aims and objectives of the measurements, as it has become possible to consider assessing aspects of wind that were previously overlooked due to an inability to acquire data with more limited instruments. This has led to an industry-wide learning process, as new applications have emerged in response to the measurement opportunities made available by remote sensing, and more effective methods for meeting existing requirements of measurement campaigns have been identified. This session provides an opportunity both to review industry progress in making the most of remote sensing and to look ahead to the possiblities that are now emerging.

Lead Session Chair:
Peter Clive, SgurrEnergy Ltd, United Kingdom
Juan José Trujillo University of Oldenburg, Germany
Co-authors:
Juan José Trujillo (1) F P Janna Kristina Seifert (1) Martin Kühn (1) David Schlipf (2) Ines Würth (2)
(1) University of Oldenburg, Oldenburg, Germany (2) University of Stuttgart, Stuttgart, Germany

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

After his studies in mechanical engineering in Colombia, Mr. Trujillo received a master degree in renewable energies at the University of Oldenburg in Germany. He has been doing research for almost ten years in the areas of engineering wake model development and validation at the universities of Stuttgart and Oldenburg. He has participated in several German research projects in the development and application of lidar techniques for wake flow measurement. Lately, he has been working on the application of long range wind scanners for measurement of wind flow in and around offshore wind farms.

Abstract

Measuring wind turbine yaw misalignment by near wake tracking

Introduction

Yaw misalignment of large wind turbines is conventionally measured based on nacelle wind direction vanes. These strive to measure the wake deviation angle of a yawed turbine in the immediate vicinity of the rotor. However several factors affect their accuracy making them not suited for very accurate yaw control or support of novel wake control techniques. An alternative approach is proposed to measure yaw misalignment directly by inspection of the near wake flow of the wind turbine. The proposed technique overcomes issues of conventional wind vanes and also of alternative procedures based on nacelle based lidars pointing upstream.


Approach

The path followed downstream by the near wake centre is assumed as a reliable indicator of yaw misalignment. Moreover, its shape clearly reveals the initial condition of wake deviation at the rotor. The method proposed aims at estimating this path by means of nacelle lidar measurements combined with a wake tracking procedure at selected downstream stations. The deviation angle at the rotor is estimated by a extrapolation of the discretely measured path backwards towards the rotor centre. A proof-of-concept has been performed on two multi-MW wind turbines.


Main body of abstract

A higher accuracy in the estimation of the wake deviation angle should be achieved by measuring the deviation integrally over the whole rotor. This is achievable partly with present technology with nacelle based scanning lidars. A "slice" parallel to the turbine rotor at a defined downstream distance reveals the wake deficit. The wake position in lateral and vertical direction with respect to the nacelle is estimated by wake tracking. This procedure is performed for different downstream distances. The wake offsets at each downstream position are assumed to define the near wake path. The path is then fully estimated by joining the measured wake offsets with a smooth function and the rotor centre position. The wake deviation is taken as the slope of the fitted function at the rotor centre.

The technique is applied in full field on two multi-MW wind turbines. Their near wake is measured with a lidar scanner equipped with a short range pulsed lidar. The system measures five slices downstream quasi-simultaneously with an update rate below ten seconds. Averages are performed over ten minutes in order to estimate the wake path. The results show that the robustness of wake tracking is less effective in the sections near to the rotor. However it is sufficient to construct an averaged wake path. The results are comparable to expectations from theoretical model and wind tunnel experiments from the literature. Moreover, the technique captures yaw misalignment dynamics in detail.


Conclusion

A novel approach to directly measure yaw misalignment is proposed. It relies on a new application of nacelle based lidar systems. The technique overcomes issues of standard procedures based on nacelle wind vanes. Testing on two different multi-MW wind turbines has shown the applicability of the technique.

The procedure is to the authors knowledge, the only possible technique to be applied in full field for testing and monitoring wake control reliably at present. The technique could be applied for diagnostic activities such as calibration of nacelle wind vanes and evaluation of conventional yaw misalignment control systems.



Learning objectives
First principles of wind turbine yaw misalignment are discussed. Based on that the attendee will learn about the limitations of wind vanes for yaw misalignment estimation. Additionally, she will understand how wake tracking works and how it can be used for yaw error estimation.

The attendee will get an overview the state-of-the-art of nacelle based wake measurements. along with discussion of the reliability of short range lidars for this type of measurements.