Generic Methodology for Field Calibration of Nacelle-Based Wind Lidars

Abstract

Nacelle-based Doppler wind lidars have shown promising capabilities to assess power performance, detect yaw misalignment or perform feed-forward control. The power curve application requires uncertainty assessment. Traceable measurements and uncertainties of nacelle-based wind lidars can be obtained through a methodology applicable to any type of existing and upcoming nacelle lidar technology. The generic methodology consists in calibrating all the inputs of the wind field reconstruction algorithms of a lidar. These inputs are the line-of-sight velocity and the beam position, provided by the geometry of the scanning trajectory and the lidar inclination. The line-of-sight velocity is calibrated in atmospheric conditions by comparing it to a reference quantity based on classic instrumentation such as cup anemometers and wind vanes. The generic methodology was tested on two commercially developed lidars, one continuous wave and one pulsed systems, and provides consistent calibration results: linear regressions show a difference of ∼ 0.5 % between the lidar-measured and reference line-of-sight velocities. A comprehensive uncertainty procedure propagates the reference uncertainty to the lidar measurements. At a coverage factor of two, the estimated line-of-sight velocity uncertainty ranges from 3.2 % at 3 m s −1 to 1.9 % at 16 m s −1 . Most of the line-of-sight velocity uncertainty originates from the reference: the cup anemometer uncertainty accounts for ∼ 90 % of the total uncertainty. The propagation of uncertainties to lidar-reconstructed wind characteristics can use analytical methods in simple cases, which we demonstrate through the example of a two-beam system. The newly developed calibration methodology allows robust evaluation of a nacelle lidar's performance and uncertainties to be established in order to further be used for various wind turbines' applications in confidence.

Wind turbine load validation using lidar‐based wind retrievals

“…Two forward‐looking lidars were mounted on the turbine nacelle: a five‐beam pulsed lidar provided by Avent technology and a continuous‐wave (CW) lidar from ZephIR . Detailed descriptions of the lidars, their technical specifications, and calibration procedures are given in Borraccino et al The pulsed lidar has five fixed beam directions, one central, and four outer beams arranged in a square pattern (see Figure , left) at 15° separation from the central beam. The measurements are performed by switching between the different beams, and LOS velocities for each direction are reported at a frequency of 0.2 Hz for 10 simultaneous ranges, which were configured from 49 to 281 m with equal increments of 23 m. The ZephIR Dual Mode (ZDM) lidar (Figure , right) has a single beam that follows a continuous circular motion thus drawing a conical shape in space, with a 15° cone angle.…”

Referee Comments for "Turbulence characterization from a forward-looking nacelle lidar"

“…The beam configuration is exactly as that in the right panel of Fig. 1 (and we will use the same beam numbering), with ϕ = 15.08 • and z R = 24.75 m (Borraccino et al, 2015). The lidar accumulated radial velocity spectra per beam position for 1 s before it moved to the next beam position; thus, radial velocity time series can be analyzed at 0.2 Hz.…”

“…A ZephIR dual-mode CW lidar (hereafter known as ZephIR) was also mounted on the nacelle of turbine 4. Five different ranges were considered (10, 30, 95, 120, and 235 m); for each range ≈ 50 azimuthal positions on the circle formed with a cone with ϕ = 15.05 • were measured during 1 s; the system averaged Doppler radial velocity spectra within azimuthal ranges of ≈ 7.38 • to get an estimate of the radial velocity per azimuth by computing the centroid of the average Doppler spectrum (Borraccino et al, 2015). The system also kept a record of each average Doppler radial velocity spectrum, which is used here to estimate the unfiltered variance.…”

A Metrological Characterization Approximation for the New Torque Measurement System in Wind Turbines Test Benches

“…Moreover, Foyer and Kock [3] studied several alternatives which are currently being used for measuring torque in nacelle test benches, concluding that all of them had several drawbacks, including poor accuracy values, and that none of the currently being used is able to measure torque in the MN•m range ensuring its traceability to international standards. When analyzing field calibrations, Borraccino et al [4] explained the importance of ensuring traceability of the measuring systems as it has a direct impact in power performance assessment.…”