The turbulent nature of the atmospheric boundary layer and its impact on the wind energy conversion process

Abstract

Wind turbines operate in the atmospheric boundary layer, where they are exposed to turbulent atmospheric flows. As the response time of wind turbines is typically in the range of seconds, they are affected by the small-scale intermittent properties of turbulent wind. Consequently, basic features that are known for small-scale homogeneous isotropic turbulence, in particular the well-known intermittency problem, have an important impact on the wind energy conversion process. We report on basic research results concerning the small-scale intermittent properties of atmospheric flows and their impact on the wind energy conversion process. The analysis of wind data shows strong intermittent statistics of wind fluctuations. To achieve numerical modeling, a data-driven superposition model is proposed. For the experimental reproduction and adjustment of intermittent flows, the so-called active grid setup is presented. Its ability to generate reproducible properties of atmospheric flows on the smaller scales of laboratory conditions of a wind tunnel is shown. As an application example, the response dynamics of different anemometer types are tested. To achieve proper understanding of the impact of intermittent turbulent inflow properties on wind turbines, we present methods of numerical and stochastic modeling, and compare the results with measurement data. As a summarizing result, we find that atmospheric turbulence imposes its intermittent features on the complete wind energy conversion process. Intermittent turbulence features are not only present in atmospheric wind but are also dominant in the loads on the turbine, i.e. rotor torque and thrust, and in the electrical power output signal. We conclude that profound knowledge of turbulent statistics and the application of suitable numerical as well as experimental methods is necessary to grasp these unique features and quantify their effects on all stages of wind energy conversion.

Keywords:

• Atmospheric turbulence
• wind energy
• intermittency
• active grid
• stochastic modeling

Acknowledgments

[Acknowledgements] The authors kindly acknowledge financial support by the German Environment Ministry, the German Academic Exchange Service and the Mexican National Council on Science and Technology, the Deutsche Forschungsgemeinschaft, the German Ministry for Education and Research, and the Ministry of Science and Culture of the German Federal State of Lower Saxony.

Notes

1. The so-called order parameters, following [38].

2. The results of the stochastic model are presented here for a dataset obtained from a numerical model, see details in [42]. Similar results were obtained for various measurement datasets, for which the results were surprisingly good, even though the noise by the turbulent inflow will not strictly fulfill the conditions of a Langevin noise.

3. It should be noted that not only the interaction of tower and rotor blades can generate periodic fluctuations of the order of 1 Hz. Additional contributions from the rotating rotor are not identified here.

4. Here it should be noted that in a typical measurement setup with 1 Hz sampling frequency this oscillation would not be visible, making the validation of such a model extension impossible in most cases.