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Friday, 20 November 2015
09:30 - 11:00 LiDARs replacing meteorological masts
Resource assessment  
Onshore      Offshore    


Room: Montparnasse

Over the last five years the on and offshore wind industries have seen an increase in both acceptance of LiDAR measurements and commercial applications for LiDAR. It is through sharing the results of validation studies that uncertainties can be reduced and the full commercial value of this technology and its wide number of applications can be realized.

Learning objectives

  • Delegates will be able to describe the value of validation of floating against fixed LiDARs and defend why this practice is a suitable alternative to validation against meteorological masts
  • Delegates will be able to explain why LiDAR measurements are at least as good as meteorological mast measurements
  • Delegates will be able to identify two different approaches to using commercial LiDARs to measure turbulence intensity
  • Delegates will be able to explain why different measurement devices have different uncertainties levels
Lead Session Chair:
Breanne Gellatly, Axys, Italy
Ameya Sathe DTU Wind Energy, Denmark
Co-authors:
Andrea Vignaroli (1) F
(1) Technical University of Denmark, Roskilde, Denmark

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2015 and are published here unedited

Mr. Ameya Sathe has been working in the wind energy research for about 5 years. After finishing his Ph.D from Delft University in The Netherlands, he has been employed as a Postdoc at the Technical University of Denmark. His research focuses on making turbulence measurements from wind lidars feasible and more accurate. He investigates novel methods of estimating turbulence statistics from routine and advanced lidar measurements, and has a considerable experience in data analysis He also has a few publications in this field, and has recently authored an International Energy Agency expert report on Lidar Turbulence Measurements.

Abstract

Measuring Accurate Turbulence using Commercial Lidars

Introduction

Understanding and measuring atmospheric turbulence is vital to efficient harnessing of wind energy and to measuring the structural integrity of a wind turbine. Traditionally, meteorological mast (met-mast) anemometry has been used, which still is the most accurate way of measuring turbulence. However the problems of using tall met-masts, especially offshore are well known.

Lidars have the potential to counter the traditional disadvantages of the met-mast anemometry. Commercial wind lidars have been demonstrated to be accurate tools for measuring the mean wind speed and wind profile. They have been successfully employed in resource assessments and for performing power curve measurements. However, their accuracy in measuring atmospheric turbulence has been shown to be poor. This study aims at improving the accuracy in measuring turbulence using commercial lidars such that they could be used as standalone instruments in conjunction with the available atmospheric stability measurements.


Approach

1. The correction factors are estimated using a previously developed model called as Systematic Lidar Error in Measuring Turbulence (SLEMT), at different wind speeds, heights and atmospheric stabilities.
2. The correction factors are applied to the turbulence intensity (TI) measurements from two different WindCube lidars (lidar 1, v1 and lidar 2, v2) such that
a. The measurement period of lidar 1 was included within that for which the correction factors are obtained.
b. The measurement period of lidar 2 was outside that for which the correction factors are obtained.
3. The corrected and uncorrected TI measurements are compared with reference sonic anemometers.
4. Percentage improvement in the TI measurements after the application of the correction is estimated.


Main body of abstract

WindCube lidar operates in a Doppler Beam Swinging (DBS) mode, where the measured line-of-sight velocities are combined to obtain the components of a wind vector field. Previous research has demonstrated that in a DBS mode, large systematic errors (SE) are introduced in the TI estimates that are dependent on the turbulence structure in the atmosphere. If the systematic errors are defined as the ratio of TI estimated using lidars to that estimated using sonics (or any other reference), then it was observed that for both measurement periods, the variation of SE without applying any correction was between plus and minus 30 %.

After the application of SLEMT model correction factors, a significant improvement in the estimated TI from lidar measurements was observed. Consequently a significant reduction in SE was observed, where the variation of SE was between plus and minus 5 % only. For lidars 1 and 2, improvement in TI estimates were observed for up to 81 % and 72 % of the cases respectively. Thus for majority of the cases applying the SLEMT model correction demonstrates a clear advantage over using the measurements from the WindCube without any correction.


Conclusion

The approach used in this study shows a very good potential of using a commercial WindCube lidar as a standalone instrument for estimating turbulence. Offshore, the impact of this method would be even greater. Quantification of atmospheric stability is however required, which can either be carried out using a sonic anemometer mounted on a very short met-mast (about 10 m), or from routine meteorological measurements of wind speeds and temperatures at different heights. The study can easily be extended for any other commercial wind lidar (pulsed or continuous-wave).


Learning objectives
The goal of this study is to investigate a new method, whereby a commercial wind lidar can be used as a standalone instrument in estimating turbulence statistics.