Bizzarrini, Nadia (2016) Aerodynamic design of stall regulated wind turbines to maximize annual energy yield. [Tesi di dottorato]

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Tipologia del documento:	Tesi di dottorato
Lingua:	English
Titolo:	Aerodynamic design of stall regulated wind turbines to maximize annual energy yield
Autori:	Autore Email Bizzarrini, Nadia nadia.bizzarrini@gmail.com
Data:	30 Marzo 2016
Numero di pagine:	143
Istituzione:	Università degli Studi di Napoli Federico II
Dipartimento:	Ingegneria Industriale
Scuola di dottorato:	Ingegneria industriale
Dottorato:	Ingegneria aerospaziale, navale e della qualità
Ciclo di dottorato:	28
Coordinatore del Corso di dottorato:	nome email De Luca, Luigi deluca@unina.it
Tutor:	nome email Coiro, Domenico [non definito]
Data:	30 Marzo 2016
Numero di pagine:	143
Parole chiave:	Aerodynamic design, wind turbines, stall control, rotational effects, stall induced vibrations
Settori scientifico-disciplinari del MIUR:	Area 09 - Ingegneria industriale e dell'informazione > ING-IND/03 - Meccanica del volo Area 09 - Ingegneria industriale e dell'informazione > ING-IND/06 - Fluidodinamica Area 09 - Ingegneria industriale e dell'informazione > ING-IND/09 - Sistemi per l'energia e l'ambiente Area 09 - Ingegneria industriale e dell'informazione > ING-IND/22 - Scienza e tecnologia dei materiali
Depositato il:	11 Apr 2016 09:34
Ultima modifica:	31 Ott 2016 10:05
URI:	http://www.fedoa.unina.it/id/eprint/10796

Abstract

Looking back in wind turbines history, pitch-regulated machines gradually substituted stall-regulated systems. In fact, the possibility to optimize the power production for each wind condition by regulating the pitch angle of the blade, proved to be a key feature to maximize the Annual Energy Production (AEP) of the wind turbines. Nowadays, all the modern MW-class wind turbines are “by default” pitch-regulated and several innovations are implemented by Industry to improve the pitch performance (e.g. individual pitch control IPC, fine regulation mechanisms/algorithms) and extract more power. In apparent contradiction with MW machines however, small and medium kW wind turbines are still largely stall-regulated machines. The reasons of this are easy to explain. In fact, the advantages of the pitch system come with some costs. The first is the direct cost of the pitch system and its maintenance. Secondly, the pitch system increases the weight of the machine and the general complexity of the system, together with the development costs and the issues related to the system robustness/reliability. Extra components, such onboard anemometers and pitch bearings, are necessary to operate the pitch of the blade correctly. All these costs and complications can be too much significant for small machines and it explains why a robust and easy-to-maintain solution is preferred even with some AEP sacrifice. However, from the design point of view, the stall-regulated machines still turn out to be a challenging task, especially concerning the aerodynamics of the blade that should ensure the power performance and it is also the only component to provide the machine control. In particular the aerodynamics of the airfoils along the blade plays a crucial role and, compared to pitch control wind turbines, more characteristics have to be considered and carefully treated. Regarding the AEP maximization, a single point optimization can be sufficient for pitch controlled wind turbines, that means optimization of the aerodynamic efficiency of the airfoils along the blade. Whereas, for stall regulated machines a multi-point optimization of blade and airfoils turns out to be necessary, because these work in a wide range of angles of attack depending on the wind speed. Furthermore, regarding the overall design of the rotor, it’s fundamental to take into account several difficulties related to stall and post-stall regimes, where the power peak of the wind turbine and, then, the power control are obtained. First of all, to predict turbine performances the Blade Element Momentum Theory (BEM) is widely used in preliminary design phases because of a reasonably velocity and accuracy compared to CFD or other methods, but it lacks of the same accuracy in these regions for reasons related to limitations in predicting airfoil performances, which are used by the BEM, and limitations of the theory itself. Regarding prediction of airfoil performances, both experimental and numerical analyses are more delicate in this zones and not always good predictions can be obtained. CFD and experimental analyses would often be necessary, with related high time costs. Experimental data of drag coefficient in the deep post-stall region are not accurate or even not available for most of the published airfoils, thus specific experimental tests would be needed to use these airfoils on a stall regulated wind turbine. Furthermore, in stall and post-stall regions the effects of blade rotation (also known as ‘stall delay’ or ‘centrifugal pumping’) have to be taken into account, especially in the inner part of the blade, but this issue, nowadays, is still to be investigated in depth. The main effect of blade rotation is a stall delay and higher stall and post stall lift coefficient values for the inner airfoils of the blade, with a consequent less power control of the turbine. Regarding the limitations of BEM Theory itself in stall and post stall regions, these are caused by the simplifying assumptions of the theory, which are the force, but also the problem of the theory at the same time. Finally, design challenges in the structural dynamics of the whole machine are peculiar of stall regulated wind turbines and are deeply depending on the airfoils characteristics and the blade shape. In particular, difficulties related to dynamic stall and, most importantly, to stall-induced vibrations have to be taken into account. The main problem is that a gentle slope of lift coefficient curve of the tip airfoils of the blade is necessary to reduce stall induced vibrations but at the same time it causes a less power control of the turbine. Finding a good compromise between vibrations problem and power control can be considered the main issue in the design of stall regulated wind turbines. The main objective of this work has been the study of all the issues briefly presented, to properly design an experimental stall regulated wind turbine, maximizing the Annual Energy Production and minimizing the rotor weight. In particular, the issue of stall induced vibrations turned out to be critical for this turbine because it is intended to produce energy at low wind speeds and needs slender and highly deformable blades. First of all, some fast methods and codes for a correct prediction of airfoils and turbine performances in a preliminary design have been investigated. For the airfoils performances, some corrections have been proposed and validated through a comparison with 3D-CFD results on a rotating blade. Furthermore, BEM shortcomings have been analyzed. The stall induced vibrations phenomenon and its sources have been investigated and some solutions have been tested. Finally, desirable airfoils characteristics have been individuated both for the AEP maximization and the compromise between stall induced vibrations and power control, and have been used to design an airfoil specifically suited to the new wind turbine. The final purpose of this work is to give suggestions for a reasonably fast and accurate predesign about computing methods and design choices for stall regulated wind turbines.

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