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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
Klaus Vogstad Meventus, Norway
Co-authors:
klaus vogstad (1) F john amund lund (1) helena wickman (1) anne simonsen (1) kyle brennan (1) rebecka klintstrøm (1)
(1) meventus, kristiansand, Norway (2) meventus, kristiansand, Norway

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Presenter's biography

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

Klaus Vogstad has worked in the wind industry for over a decade. He has hold various positions in the wind industry serving as advisor, board member, manager, senior analyst, adjunct associate professor and entrepeneur. He headed the CoE Wind & Site group at Agder Energi, and is currently a Senior Analyst at Meventus. He is responsible for the development of ParkOptimizer and WindFarmDesigns.com - wind farm design tools used by the wind industry. Vogstad holds a MSc in mechanical engineering and a Ph.D in electrical engineering from NTNU.

Abstract

Turbulence measurements using scanning lidars: A new approach

Introduction

Can scanning lidars be used to improve on site turbulence measurements ? At present there are no practical precise methods to estimate turbulence at turbine locations in complex terrain. While standards focus on instrumentation uncertainty, the dominant uncertainties lies in the extrapolation of mast measuremnts on to turbine positions, hundreds of meters away from the mast. Turbulence models on the other hand are not better, displaying uncertainties in the order of 30% (Bechman et al, 2011). While lidars have been used to mimic met masts to conform to industry standards, they should be used to the full potential of their capabilities on their own premises.

This study uses a the StreamLiner scanning lidar in highly complex terrain to measure the line-of-sight turbulence at 1 Hz resolution compared against a 3D sonic mast in order to assess turbulence.

Approach

1. A lidar campaign using Halo Photonix Streamliner was deployed on a site in highly complex terrain.
2. The lidar was set up in staring mode at 1s resolution, pointing towards a 100m sonic mast about 354m away, to compare measurements over a one month period.
3. The lidar and sonic measurements were projected to line-of-sight for comparison and validation.
4. Based on the sonic 3D measurements, lidar line-of-sight measurements are related to the 3D sonic measurements
5 .The resulting lidar measurements are compared to the alternatives of mast extrapolation, turbulence models and the use of VAD lidars.


Main body of abstract

Cup anemometry and sonic anemometers are the industry standards for turbulence measurements. It is however not practically feasible to measure at every potential turbine location, and extrapolation methods are applied assess turbulence at potential turbine locations. The extrapolation procedure introduces uncertainties larger than the instrumentation uncertainties of lidars, especially in complex terrain. Turbulence models such as the Mann model is not really valid in complex terrain, and turbulence estimates from CFD models show uncertainties in the range of 30% according to the Bolund blind test (Bechman et al. 2011). Lidar technologies offer new possibilities for measurements, but have so far been considered too uncertain for turbulence measurements (Mann et al. 2012). Development of new methods for lidar measurements may however provide better alternatives than the current industry practice.

As part of the research project “Lidar in complex flow” we have verified the Halo Photonics Streamliner against a 100m 3D sonic anemometer in highly complex terrain. The lidar was placed 354 meter apart from the mast. Using staring mode at 1 Hz, we measured the line-of-sight wind speed and turbulence over a one month period. The sonic measurements were projected onto the line-of-sight (los) lidar measurements and compared.
The 1 Hz measurement agreements shows excellent agreements with the Sonic anemometer, standard deviation > 0.975. The 10 min turbulence intensity showed good agreements, as well as turbulence intensities over shorter time intervals.
Relating the los turbulence to the other two turbulence components, there is a good correspondence between the los and the three-dimensional turbulence components, but may be location dependent.
For practical purposes, however, the approach is considered to have a sufficiently low degree of uncertainty to be able to exclude and/or approve turbine locations for excessive turbulence in agreement with the the IEC recommendations.

References
Bechman et. al, 2011: The Bolund experiment Part II: Blind comparison of Microscale flow models. Boundary-Layuer Meteorol 141.2(2011). Web
Mann et al, 2002: Lidar turbulence measurements for wind energy. Springeer Proc. in physics. Progress in Turbulence and Wind Energy IV

Conclusion

The approach of measuring direct line of sight turbulence at high resolution using scanning lidars is a promising approach to the existing methods of mast extrapolation or turbulence models.

This method significantly reduces uncertainty for turbulence assessment at turbine positions compared to current practice of mast extrapolation.

Further investigations are needed to assess the sensitivity of relating los measurements to 3D turbulence components.


Learning objectives
The goal of this study is to investigate a new method to assess the turbulence on complex sites using scanning lindars in a new way.