Tuesday, 27 September 2016
17:00 - 18:30 Annual energy production: improved estimates through advanced modelling
Resource assessment  
          

Advanced modelling methods are now standard in wind estimating modelling of annual energy production (AEP). In this session, speakers will present round robin tests of models for spatial variability of wind resource on projects using different modelling approaches. We will look at the consequences of including atmospheric stability in the calculation of AEP offshore and how you can measure wind profiles at heights of 100-200 metres by using LIDARs with emphasis on charactering extreme shear situations such as low-level jets causing extreme loads. Finally, we will hear about the variability of turbulence intensities measured offshore.

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Learning objectives

• Understand the different methods for modelling the spatial variability of wind resource at the project scale and the errors based on the different modelling approaches;
• How to incorporate stability in the AEP modelling in offshore wind farms;
• Understand the effect of using high resolution modelled wind climatologies combined with advanced boundary modelling on the estimation of the tall winds at 100-200 metres;
• How to characterise and define the extreme events such as low-level jets that can cause fatigue loads in a wind farm;
• Understand the variability of the turbulence intensities offshore measured from multiple measurement towers.

This session will be chaired by:

Peter Clive SgurrEnergy Ltd, United Kingdom
Co-authors:
Hannah Ferchland (1) ^F
(1) Sgurr Energy, Glasgow, United Kingdom

Low level jets, intermediate boundary layers and veer: accommodating real world wind shear in established procedures

Introduction

The variation of wind speed with height influences cyclical loads that result in component failure in offshore wind turbines. A key consideration is the anticipation of these loads and their mitigation in wind turbine design and control. Wind turbine design guidelines such as IEC 61400-1 and -3 assume a Normal Wind Profile (NWP) in which the variation in wind speed with height can be described using a simple power law. Measurements show that anomalous wind shear conditions that do not conform to this model occur and must be accounted for.

The variation of wind speed with height influences cyclical loads that result in component failure in offshore wind turbines. A key consideration is the anticipation of these loads and their mitigation in wind turbine design and control.


Approach

Lidar measurements are providing the necessary data for assessing wind shear and veer across the entire rotor disc in offshore wind power deployments. In the course of making these measurements it has become apparent that real wind conditions deviate from the assumptions made in wind turbine design guidelines in important respects.

Methods of characterising these phenomena are considered on the basis of lidar measurements, and the consequent loads, such as flap-wise blade root bending moment, are assessed.


Main body of abstract

Complex wind shear profiles that deviate from the NWP description have been observed and studied onshore. In particular, low level jets (LLJs) have been observed in the Great Plains of North America. This presentation discusses results obtained offshore in the North Sea in which similar LLJs are observed.

The results include data acquired during the lidar measurement campaign at Alpha Ventus Offshore Wind Farm from February 2013 to March 2014, during which three scanning lidars were installed on an offshore wind turbine, one on the transition piece and two on the nacelle. Additional measurements from other sites are discussed and a review of historical evidence is also presented.

Clear patterns were observed in the incidence of LLJs, including diurnal patterns and consistent heights at which the maximum wind speed occurred. The frequency of these events offshore was non-negligible.

The extreme wind shear conditions that occurs during these events represents a significant challenge when designing and controlling offshore wind turbines. The conditions that have been observed deviate significantly from the assumptions codified in wind turbine design guidelines and wind turbine control algorithms, such as the NWP.

These observations must be considered when seeking to identify measures that will ensure the productivity and longevity of offshore wind power assets.

It is necessary to assimilate these assessments in our procedures through continuity rather than crisis. To that end, procedures that establish equivalence between NWP and non-NWP profiles are discussed, such that non-NWP shear profiles can be assessed using existing methods.


Conclusion

Complex wind conditions that do not conform to the simplifying assumptions embodied in wind turbine design guidelines can give rise to load cases that are neglected in established load assessment procedures. In particular, load scenarios more severe than fatigue loads and more frequent than extreme loads can occur.

One possible example of this is Low Level Jets. Evidence regarding the characteristics, geographic and seasonal prevalence, and impact on wind power assets, of these wind shear anomalies is presented.



Learning objectives
1. Define and observe persistent extreme wind shear conditions that represent an elevated cyclical fatigue load

2. Adopt wind turbine control strategies that can mitigate the extreme wind shear that can occur under real world conditions, as measured using lidar and reported in this presentation

3. Relate the incidence of Low Level Jets to the atmospheric conditions, in particular stability conditions, which vary on a diurnal basis