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This study attempts to obtain more detailed knowledge of failures of wind turbines (WTs) by using the German '250 MW Wind' test program database. Specific objectives are to show the reliability of some major components and to analyze how their design has advanced through time, what the main failures are, and which technologies have proven to work. Within the program, reports on operation and maintenance are analyzed with respect to WT type, size, and technologies used. This paper presents a comparison of component reliability through time, with respect to their technology. The results show significant differences in reliability for certain subcomponents depending on the size of the WT and especially on the type of power control. For instance, induction generators show half the annual failure rate compared to synchronous generators. The study also includes failures of other components that are affected or added due to the use of the components being analyzed. In general, the results show that failure rates of WTs decrease with time. Most of the data show a short period of 'early failures' and later a long period of 'random failures.' However, this is not the case for the megawatt class: As technology is introduced into the market, WTs show a longer early failure behavior, which has not yet become stable. Furthermore, large turbines, included in the database analyzed, show a significantly higher annual failure rate of components, per WT. This may be due to the immature technology of the WTs included in the database. [DOI: 10.1115/1.2936235] Keywords: reliability, maintenance, failures, wind turbines

This study describes a methodology for designing airfoils suitable to employ actuation in a wind energy environment. The novel airfoil sections are baptized wind energy actuated profiles (WAP). A genetic algorithm-based multi-objective airfoil optimizer is formulated by setting two cost functions: one cost function for wind energy performance and the other representing actuation suitability. The wind energy cost function compares the candidate airfoils' performance with 'reference' wind energy airfoils, considering a probabilistic approach to include the effects of turbulence and wind shear. The actuation suitability cost function is developed considering horizontal axis wind turbines active stall control, including two different control strategies designated by 'enhanced' and 'decreased' performance. Two different actuation types are considered, namely, boundary layer transpiration and dielectric barrier discharge plasma. Results show that using WAP airfoils provides much higher control efficiency than adding actuation on reference wind energy airfoils, without detrimental effects in non-actuated operation. The WAP sections yield an actuator employment efficiency that is two to four times larger than those obtained with reference wind energy airfoils, at equivalent wind energy performance. Regarding geometry, and compared with typical wind energy airfoils, WAP sections for decreased performance display an upper surface concave aft region, while for increased performance, a convex upper surface aft region is obtained. The present study emphasizes that there is much to gain in designing airfoils from the beginning to include actuation effects, especially compared with employing actuation on already existing airfoils. The results demonstrate the potential of including actuation effects in the airfoil design process, thus enabling novel horizontal axis wind turbines control strategies. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The 3D dynamics of the near wake of a vertical axis wind turbine is simulated by a numerical model based on a free vortex wake/panel method. The model is first verified with respect to discretization sensitivity, stability and robustness. Results show little sensitivity to the discretization, requiring a medium spatial and temporal grid to reach convergence. Stability issues are addressed, identifying dependency of the simulation parameters and validity of the results. The in-rotor flow shows little or no effect of instabilities and the model proves to be an effective analysis tool for the flow at the blade level. Wake dynamics, especially the tip vortex evolution, is more subject to stability issues which have to be taken into consideration. The numerical results are then validated with experimental data to assess accuracy. The model is able to capture all the important dynamics in the near wake of a vertical axis wind turbine with good quantitative evaluation of the flowfield. Discrepancies in the comparison are discussed and limitations in the use of the model are presented. [ABSTRACT FROM AUTHOR]