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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Remote sensing: From toys to tools?' taking place on Wednesday, 12 March 2014 at 14:15-15:45. The meet-the-authors will take place in the poster area.

Rozenn Wagner DTU Wind Energy, Denmark
Co-authors:

(1) DTU Wind Energy, Roskilde, Denmark

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Abstract

What makes a nacelle mounted lidar a tool for power performance measurement?

Introduction

Nacelle lidars are attractive for offshore measurements since they can provide measurements of the free wind speed in front of the turbine rotor without erecting a met mast, which significantly reduces the cost of the measurements. Nacelle mounted pulsed lidars with two lines of sight (LOS) have already been demonstrated to be suitable for use in power curve measurements. To be considered as a professional tool however, power performance measurements performed using these instruments require traceable calibrated measurements and the quantification of the wind speed measurement uncertainty. Here we present and demonstrate a procedure answering these needs.

Approach

This paper describes the derivation of the complete uncertainty budget of a power curve measured with a two beam nacelle lidar. It focuses on the main deviations from a power curve assessed with a mast mounted cup anemometer. The main differences between the two techniques are:
1) the derivation of the uncertainty in the wind speed measurement, which highlights the need for a traceable calibration method for the nacelle lidar;
2) the varying sensing height error as the nacelle and lidar tilt due to the rotor thrust induced bending of the tower.


Main body of abstract

A nacelle lidar went through a comprehensive calibration procedure. This calibration took place in two stages. First with the lidar on the ground, the tilt and roll readings of the inclinometers in the nacelle lidar were calibrated. Then the lidar was installed on a 9m high platform in order to calibrate the wind speed measurement. The lidar’s radial wind speed measurement along each LOS was compared to the wind speed measured by a calibrated sonic anemometer, projected along the LOS direction. The various sources of uncertainty in the lidar wind speed measurement have been thoroughly determined: uncertainty of the reference sonic anemometer, the horizontal and vertical positioning of the beam, lidar measurement mean deviation and standard uncertainty. The resulting uncertainty lies between 1% and 3% for the wind speed range between cut-in and rated wind speed.
Finally the lidar was mounted on the nacelle of a wind turbine in order to perform a power curve measurement. The wind speed was simultaneously measured with a mast-top mounted cup anemometer placed two rotor diameters upwind of the turbine. The wind speed uncertainty related to the lidar tilting was calculated based on the tilt angle uncertainty derived from the inclinometer calibration and an uncertainty was added when the lidar beam was sensing outside the height range demanded by the IEC standard (hub height ±2.5%). The resulting combined uncertainty in the power curve using the nacelle lidar was only 10kW larger on average than that obtained with the mast mounted cup anemometer.


Conclusion

A nacelle lidar can be a very good tool for power performance assessment, on the condition that the instrument is first calibrated and all the uncertainties are accounted for. Both a calibration procedure and power curve measurement procedure fulfilling these demands for a two beam nacelle lidar have been designed and tested. We have demonstrated that a nacelle lidar is a serious and highly competitive tool for performing power curve testing.


Learning objectives
The purpose of this presentation is to make the audience aware of the need for a thorough uncertainty evaluation in order to make a nacelle lidar a tool to be used for power performance measurement. The procedure presented here has been tested and is ready to be applied.