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Wednesday, 12 March 2014
14:15 - 15:45 Remote sensing: From toys to tools?
Resource Assessment  


Room: Tramuntana
Session description

The use of remote sensing within the wind industry has developed significantly since these techniques were first adopted. The new opportunities to make measurements that have been made available have themselves influenced the aims and objectives of the measurements, as it has become possible to consider assessing aspects of wind that were previously overlooked due to an inability to acquire data with more limited instruments. This has led to an industry-wide learning process, as new applications have emerged in response to the measurement opportunities made available by remote sensing, and more effective methods for meeting existing requirements of measurement campaigns have been identified. This session provides an opportunity both to review industry progress in making the most of remote sensing and to look ahead to the possiblities that are now emerging.

Lead Session Chair:
Peter Clive, SgurrEnergy Ltd, United Kingdom
Hilbert David Senvion SE, Germany
Co-authors:
Hilbert David (1) F P Schmitt Carolin (2) Markus Weimbs (2)
(1) REpower Systems SE, Hamburg, Germany (2) juwi Energieprojekte GmbH, Woerrstadt, Germany

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

David Hilbert is responsible for Advanced Flow Modeling at the Wind + Site department of Senvion SE. After studies in Hamburg, Berkeley and Nantes he joined Senvion and has gained almost four years of rich experience with advanced modeling topics since then. His research interest is focused on CFD modeling, turbulence assessments and the specific challenges of forested sites.

Abstract

Evaluation of Lidar Measurements for Turbulence Assessment

Introduction

Studies on lidars and their comparison to traditional cup anemometry have historically focused on wind speed measurements. However, for site suitability assessment the site specific turbulence intensity is of equal interest and only a limited number of investigations have been performed on this matter. Turbulence measurements with lidars could not only replace mast measurements, but they also have a great potential of reducing uncertainties in the vertical extrapolation. This makes them particularly interesting when assessing higher hub heights, which are getting more and more common in the industry.

Approach

Twelve different measurement sites in different wind park locations are used to evaluate lidar turbulence measurements over periods of three to eight months. For all sites data from co-located measurement masts (reaching 60-100 m in height) with concurrent measurement periods are available for comparison purposes. Furthermore, analytical approaches for turbulence estimates are compared to measured data.

Main body of abstract

Accurate estimates of ambient turbulence are a pre-requisite for lowering uncertainties in the estimation of effective turbulences. Therefore, improved measurements of turbulence enable an optimal wind park layout and lower the overall project risks.
Whilst wind speed measurements with lidars can be deemed as fairly established in the industry by now, a large amount of uncertainty still exists when it comes to measuring turbulence intensities with the same devices: On the one hand, it can be argued that common lidars - such as ZephIR, WindCube, etc. - cannot measure turbulence in a scientifically precise manner (Sathe et al. 2011); on the other hand, it is claimed that lidars can still provide turbulence measurements of sufficient accuracy for wind energy purposes (e.g. Barker et al. 2012).
This study sheds light on two aspects of lidar measurements of turbulence:
First of all the turbulence measurements with lidars are compared to traditional cup anemometry in practical field measurement campaigns. It can be shown that in many, but not in all cases the accuracy of lidar turbulence measurements is acceptable for wind energy purposes and could be used for site suitability assessment.
Furthermore the vertical distribution of measured turbulence over height is evaluated against typical analytical approaches. It becomes apparent that traditional methods for vertical extrapolation of turbulence estimates show significant deviations when compared to measured data; this uncertainty could potentially be reduced by using turbulence measurements from lidars.


Conclusion

This paper adds to the industry knowledge on turbulence measurements with lidars and demonstrates their added value. For the sites under study, it can be shown that turbulence measurements with lidars have a good potential to be used for site suitability assessment. This is especially true for high hub heights, which might suffer from significant vertical extrapolation errors.


Learning objectives
The audience shall learn
- how lidar measurements of turbulence compare to mast measurements, for the sites under study: accuracy, deviations, ranges of uncertainty, distribution over height
- how lidars can help reducing the uncertainty in vertical extrapolation of turbulence estimates, compared to analytical methods
- about the applicability of lidar turbulence measurements in everyday usage