Applications and improvements of continuous wave Doppler lidars for wind energy

Liqin Jin*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Abstract

A wind Doppler lidar is a powerful tool to study important phenomena in the atmospheric boundary layer and to remotely sense wind and turbulence, which is widely needed in wind energy. For several decades, Doppler lidars have been extensively applied to detect aircraft wake vortices and dangerous wind shears in airports, assess wind resources, test wind turbine performance, realize lidar-assisted turbine control, measure turbine wakes, study turbulence around a suspension bridge, and study the flow in the near-wake region of a tree.
Continuous-wave (CW) lidars, which are one type of wind Doppler lidar, are the subject of this Ph.D. project. CW Doppler lidars have a good spatial and temporal resolution within a range of several hundred meters. However, the eective probe volume (or measurement volume) of CW lidars increases as the 4th power of the focus distance. Consequently, the measurement accuracy deteriorates with distance. Besides, a single CW lidar measures the line-of-sight wind velocity as a weighted average by a Lorentzian function. The Lorentzian weighting function makes CW lidars more susceptible to moving objects with strong backscatter, for example, clouds, far away from the intended measurement point. This hinders the application of CW lidars in precisely measuring wind velocity and turbulence in some situations.
The objective of this Ph.D. study is to apply CW wind Doppler lidars to measure turbulent flow around a drone, to improve the spatial resolution of CW lidars, and to reduce the precipitation-induced uncertainties when estimating wind velocity by CW lidars.
This thesis starts by presenting the use of the WindScanner system consisting of three CW Doppler lidars to retrieve wind vectors around a drone, in order to validate computational-fluid-dynamics simulations and determine the optimal placement of an upstream-facing and drone-mounted sonic anemometer. The results are compared with those resulting from the simulation. It is the first study to use three CW lidars to investigate the turbulent three-dimensional flow around a drone.
Subsequently, a novel configuration is suggested to integrate two co-planar quarter-wave plates into a conventional CW lidar system, in order to compact the measurement volume and suppress the fat tails of the Lorentzian weighting function. This is based on the principle that only two laser beams with the same polarization direction can beat, which is used to detect the Doppler shift. Both theoretical and experimental results show that, in a proper configuration with quarter-wave plates, the probe length can be reduced by 10% and the fat tails of the weighting function can be suppressed by up to 80% to reduce cloud returns.
In moderate to heavy precipitation, rain droplets may deteriorate Doppler lidars’ accuracy for measuring wind velocity. Finally, this thesis demonstrates how the rain bias can be effectively reduced by normalizing the noise-flattened-3kHz Doppler spectra with their peak values before averaging down to 50 Hz. Over a three-hour wind velocity comparison with a reference sonic anemometer, a significant reduction of the bias is observed under moderate rain intensity.
This project provides potential methods to improve the spatial resolution of CW Doppler lidars and reduce the adverse impact of precipitation on the accuracy of wind velocity measurements.
Original languageEnglish
Place of PublicationRisø, Roskilde, Denmark
PublisherDTU Wind and Energy Systems
Number of pages136
Publication statusPublished - 2023

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