Martin Kühn

OC3—Benchmark Exercise of Aero-elastic Offshore Wind Turbine Codes

This paper introduces the work content and status of the first international investigation and verification of aero-elastic codes for offshore wind turbines as performed by the Offshore Code Comparison Collaboration(OC3) within the IEA Wind Annex XXIII - Subtask 2. An overview is given on the state-of-the-art of the concerned offshore wind turbine simulation codes. Exemplary results of benchmark simulations from the first phase of the project are presented and discussed while subsequent phases are introduced. Furthermore, the paper discusses areas where differences between the codes have been identified and the sources of those differences, such as the differing theories implemented into the individual codes. Finally, further research and code development needs are presented based on the latest findings from the current state of the project.

Wind velocity measurements using a pulsed LIDAR system: first results

Wind velocity measurements were taken using a Leosphere Windcube LIDAR system, which operates as a pulsed laser Doppler anemometer. Here we report on first results, which show typical characteristics of atmospheric wind velocities.

Atmospheric Impacts on Power Curves of Multi-Megawatt Offshore Wind Turbines

Power curves for offshore wind turbines within the German offshore wind farm alpha ventus were derived based on the IEC standard. Binning in groups of shear and turbulence intensity as measures of atmospheric stability were performed. The derived power curves show a strong dependency on these two parameters. Differences of up to 15% in power output between unstable and stable stratification in the non-wake case occur. For wind turbines within the wake of others the effects are even more pronounced. Here, the differences in power production between the stability classes approach 20%. This dependency of the power curves on stability can cause significant miscalculations of instantaneous power production, long-term energy yield and loads. Parameters other than the hub height wind speed are often not taken into account in state-of-the-art wind power forecasts. This can lead to substantial over- or underestimation of the resulting power.

Development of LiDAR measurements for the German Offshore Test Site

The paper introduces the content of the recently started joint research project Development of LiDAR measurements for the German Offshore Test Site which has the objective to support other research projects at the German offshore test site alpha ventus. The project has started before the erection of the offshore wind farm and one aim is to give recommendations concerning LiDAR technology useable for offshore measurement campaigns and data analysis. The work is organized in four work packages. The work package LiDAR technology deals with the specification, acquisition and calibration of a commercial LiDAR system for the measurement campaigns. Power curve measurements are dedicated to power curve assessment with ground-based LiDAR using standard statistical methods. Additionally, it deals with the development of new methods for the measurement of non-steady short-term power curves. Wind field research aims at the development of wake loading simulation methods of wind turbines and the exploration of loading control strategies and nacelle-based wind field measurement techniques. Finally, dissemination of results to the industry takes place in work package Technology transfer.

Comparing measurements of the horizontal wind speed of a 2D Multi-Lidar and a cup anemometer

Wind measurements of a 2D Multi-Lidar and a mast mounted cup anemometer are compared in this study. Average wind speed and direction as well as the turbulence intensity of the wind speed are considered. Data analysis is mainly performed using standard regression analysis on 10 minute average data and the calculation of the power spectral density. The results show a good agreement regarding wind speed and direction and the turbulence intensity of the horizontal wind.

Lidar Simulations to Study Measurements of Turbulence in Different Atmospheric Conditions

Modern lidar technology promises a fundamental enhancement of wind velocity measurements for site assessment. Previous studies have shown good agreements between lidars and mast mounted sonic anemometers concerning measurements of the 10 minute average horizontal wind velocity in flat terrain but have shown substantial differences concerning the measurement of turbulence intensity. One of the main reasons for poor turbulence measurements is assumed to lie in the scanning technique called VAD, applied by lidars. In contrast to a sonic anemometer, a VAD-scanning lidar senses the wind field at different positions along a circle. This is centred at the target point, and with a radius three orders of magnitude larger than the typical size of an anemometer. The resulting temporal and spatial averaging by the VAD scan influences the turbulence measurements. To understand the effects of different VAD scanning configurations and of the atmospheric condition on measuring turbulence, a numerical lidar scanner simulator was used. The influence of the VAD cone angle and the stability of the used LES generated wind fields were studied. The results show a high dependency on the used cone angle and the atmospheric stability.