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Delegates are invited to meet and discuss with the poster presenters during the poster presentation sessions between 10:30-11:30 and 16:00-17:00 on Thursday, 19 November 2015.

Lead Session Chair:
Stephan Barth, ForWind - Center for Wind Energy Research, Germany
Scott Wylie ZephIR Lidar, United Kingdom
Co-authors:
Scott Wylie (1) F Michael Harris (1) Mark Pitter (1) John Medley (1) Chris Slinger (1) Muhammad Mangat (1)
(1) ZephIR Lidar, Ledbury, United Kingdom

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Presenter's biography

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

Scott has been involved with wind energy research and development for 8 years, and is currently working as a wind engineer at ZephIR Lidar. He studied Electronic & Electrical Engineering at Loughborough University and completed his PhD at the Centre for Renewable Energy Systems Technology (CREST). His PhD focused on the use of Computational Fluid Dynamics in modelling wind flow and forest interactions. Since then he has worked within the small to medium scale wind industry and his current role at ZephIR focuses on the development of lidar technologies for wind resource and energy assessment.


Poster

Poster Download poster (7.31 MB)

Abstract

Derivation of cup-equivalent turbulence from Doppler spectra obtained by scanning CW lidar

Introduction

The accurate derivation of wind turbulence parameters from remote sensing systems is a topic of active current research. With the increased use of lidar in the wind industry, it becomes crucial to provide turbulence intensity (TI) data that can be related to measurements obtained from cups and other devices. Different lidar scanning patterns and signal processing approaches have previously been investigated as a means to overcome some challenges that result from two issues: spatial averaging over the probe and scan volume, and contamination of the horizontal Doppler measurements by vertical flow components. Here we adopt a radically different approach to those previously used by scanning lidars, to derive a measure of TI that corresponds closely with measurements from a cup anemometer. In fact the method is potentially superior to a cup in that it allows measurement of TI at different points in space, thus providing a better representation of the turbulence characteristics over a particular site.

Approach

We have re-analysed the Doppler spectral (“raw”) data from standard ZephIR DM lidars that were mounted on the nacelles of large wind turbines, looking forwards into the wind with a conical scan pattern, and measuring at different ranges. The raw data gives a good representation of the distribution of line-of-sight wind speeds within the probe volume, weighted by the Lorentzian sensitivity function inherent to CW lidar. Previous work at DTU has shown that such data from a staring beam gives good agreement with the turbulence characteristics obtained with a sonic anemometer. Our work extends this method to allow its use with a scanning beam from a currently-available product; in doing so it also avoids any laser eye-safety issues related to a fixed staring beam. Spectra obtained at the same height as a cup mounted on an IEC-compliant mast have been used to derive the 10-minute TI via the standard deviation of the averaged spectrum over that period. Results have been compared to those from the reference cup.

Main body of abstract

Independent Doppler spectra are obtained as standard at 50Hz in a circular scan ensuring that measurements are made close to the same height as the reference cup twice every second. These parts of the scan are identified, and the corresponding raw data obtained at the cup height are amalgamated to provide averaged spectra. The left and right parts of the scan are separately analysed to give an indication of spatial variability of turbulence, which may additionally be a promising approach for wake detection. The 10-minute averaged spectra provide much information on the wind statistics, but here we simply calculate the standard deviation of the distribution, and divide by the mean speed to derive values for TI. Detailed investigation is ongoing to understand fully any impact of other experimental parameters such as scan angle (both 15 and 30 degree half cone angles were used), yaw misalignment, range and signal–to-noise ratio. Preliminary results indicate that the approach is robust to variation in these parameters.

Conclusion

The results show a good level of agreement between TI values measured by the lidar and the mast. This is partly a consequence of the largely horizontal measurement geometry which excludes vertical components that normally contaminate ground-based lidar measurements. In addition the spectral analysis approach largely eliminates the effect of spatial averaging. Work is continuing to understand the range dependence of the method, and to use the results to investigate the statistical properties of turbulence as the flow enters the turbine’s induction zone, right up to the rotor.


Learning objectives
To become aware of a new approach to lidar measurement of turbulence using existing products.

To understand that this approach overcomes some well-known problems to provide good agreement with standard anemometry

To realise the potential to extend the method and provide more representative site assessments of turbulence than a point sensor