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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

José Francisco Herbert Acero Instituto Tecnológico y de Estudios Superiores de Monterrey (ITESM), Mexico
Co-authors:
José Francisco Herbert Acero (1) F P Oliver Probst (1) Manuel Valenzuela Rendón (5) Santos Méndez Díaz (2) Pierre-Elouan Réthoré (3) Krystel Kaliecta Castillo-Villar (4) Jaime Martínez-Lauranchet (1)
(1) Chair for Wind Energy, Department of Physics, Instituto Tecnológico y de Estudios Superiores de Monterrey (ITESM), Monterrey, Mexico (2) School of Mechanical and Electrical Engineering, Universidad Autónoma de Nuevo León, Monterrey, Mexico (3) Wind Energy Division, Risø National Laboratory for Sustainable Energy, Roskilde, Denmark (4) Department of Mechanical Engineering, The University of Texas at San Antonio, San Antonio, United States of America (5) Chair for Evolutionary Computation, Department of Computer Science, Instituto Tecnológico y de Estudios Superiores de Monterrey, Campus Monterrey. Eugenio Garza Sada 2501 Sur, C.P. 64849, Monterrey, N.L., México., Monterrey, Mexico

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Mr. José-Francisco Herbert-Acero has been working as a researcher in the wind energy field for more than 6 years. He is currently enrolled in the Sc.D. program at the Monterrey Institute of Technology (ITESM) in México. He got his bachelor degree in Engineering Physics from ITESM in 2009 as well as a double degree of specialization in energy and intelligent systems. During his studies, he has been involved in the development of large wind farm projects in México. His research is focused on wind farm performance modeling and optimization.

Abstract

Aerodynamic optimization of small wind turbine rotors based on NACA 4-digit airfoils through computational intelligence

Introduction

There has been an increased necessity for the development of small grid-independent energy applications [1] (e.g., domestic, street lighting, rural electrification) in which wind energy plays an important role. However, small-scale wind turbines (SSWT) show reduced efficiencies as compared to larger-scale wind turbines [2] since they operate in different inflow conditions where the effect of Reynolds number is significant. It is of capital interest to improve the SSWT efficiency so that they can be competitive with other renewable energy technologies. Therefore, improved SSWT aerodynamic technologies are required. This work contributes to this objective by evaluating a specific series of airfoils.

Approach

The theoretical analysis consisted of implementing a state-of-the-art Blade Element Momentum (BEM) model [3], incorporating tip and root Prandtl losses corrections, as an objective function from which the aerodynamic performance of the optimized rotors was calculated in terms of the power coefficient (Cp) and the thrust coefficient (Ct). The variables to be optimized were the geometric distributions of the chord, thickness and pitch angle as a function of the blade span position. Flexible analytical functions that depend on a reduced set of continuous parameters were developed to describe the geometric distributions. With the use of a metaheuristic algorithm, Simulated Annealing (SA) [4], it was possible to optimize the geometry of the rotor to achieve higher aerodynamic efficiencies. The experimental analysis consisted of constructing the optimized rotors with the use of rapid prototyping technologies (3-D printing). The optimized rotors were mounted in a brushless radial flux EMP6354-KV200 electric generator that was previously characterized using a customized electromechanical test bench and an electromechanical model [3]. The electric generator in turn was connected to an efficient AC/DC electric system that was monitored with the use of a data acquisition system (NI DAQ USB-6211) and a LabVIEW interface which reported the real-time operation of the wind turbine. The optimized variable-rotational-speed wind turbines were tested at the Center for Research and Innovation in Aerospace Engineering (CIIIA) closed-return wind tunnel [5]. Rotational speed was controlled by changing the effective electric load of the electric system. A complete characterization of the Cp-lambda and Ct-lambda curves, being lambda the tip-speed ratio, was performed by using different electric loads and different free wind speeds. Comparisons between theoretical BEM predictions and experimental results were performed as well as comparisons between optimized rotors.

Main body of abstract

The main use of SWT is to provide power to small electrical systems (e.g., systems for domestic use) or energization of rural zones where the energy supplier grid lines cannot reach. Given the inherent characteristics of the areas where the SWT are located, it is common to find low average wind speeds that do not show a well-defined distribution (e.g., different than the typical Weibull distribution found at sites where common large wind farms are constructed). Consequently, it is of interest to improve the aerodynamic efficiency of small wind energy systems. A variety of studies have been focused in describing/characterizing the aerodynamics of specific airfoils [6,7]. However, it is rare to find a complete characterization of an entire series of airfoils, including the NACA 4-digit airfoils. Additionally, there are only few studies that describe how different airfoils help to improve the aerodynamics of small wind turbine rotors [8,9]. The literature about which types of airfoils are best for the construction of efficient small wind turbines is limited. For large-scale wind turbines, it is necessary to balance five different targets for the optimal design of wind turbine blades: (1) the aerodynamic efficiency, (2) the total cost, (3) the structural design, (4) the total noise at operation and (5) control aspects. The Wind Turbine Blade Optimization (WTBO) problem [10-17] consist in the design and manufacture of WT blades that have best efficiency, low noise generation and a proper structural design to withstand the dynamic loads generated by the considered wind field. However, for small wind turbines, the main objectives are to increase the aerodynamic efficiency and reduce total costs. A formal study to identify the aerodynamic/geometric features that improve the aerodynamic efficiency is of practical and theoretical relevance. To gain insight on this matter, the present work focuses in the evaluation of the NACA 4-digit airfoils for the construction of optimized small wind turbine rotors. To achieve the goal, the present work extended and improved the state-of-the-art methodologies [10-17] for the construction of optimized rotors. As described in the approach section, optimized rotors of 0.18 m radius were constructed and tested at the CIIIA closed-return wind tunnel. The main results show that (1) the different optimized rotors, based on different NACA 4-digit airfoils, differ quite substantially in their aerodynamic efficiency. (2) Rotors based in commonly studied NACA 4-digit airfoils (e.g., NACA44XX) are not the best performer rotors, as shown in Figures 1 and 2. (3) The efficiency of the optimized rotors increased when considering NACA 4-digit base airfoils that have larger distances between the maximum camber point and the airfoil leading edge. In particular the optimized rotors based on the NACA [75,85,95]XX airfoils showed higher efficiency in contrast with other rotors. (4) The rotors based on NACA [75,85,95]XX airfoils tend to have lower cut-in wind speeds (up to 2.34 m/s) in contrast with other rotors having different NACA 4-digit profiles. (5) The rotors based on NACA [75,85,95]XX airfoils require less material during its construction. This is a direct consequence of its natural geometric asymmetry. The findings of this work have a strong impact in the small wind turbine rotor aerodynamics and increase the feasibility of using such optimized wind turbines in urban areas or in sites having low wind resource.

Conclusion

This research work studied the aerodynamic performance of the NACA 4-digit airfoils with the aim of identifying the best performer profiles in the construction of optimized small-scale wind turbine rotors. It was demonstrated that the steady state BEM model provides accurate predictions of the aerodynamic performance as long as the supplied airfoil aerodynamics information (lift and drag coefficient as a function of both the Reynolds number and angle-of-attack) is accurate. This work used XFOIL 6.96 for airfoil aerodynamics prediction complemented with Viterna-Corrigan extrapolation procedures [3]. BEM predictions fail to describe the actual wind turbine operation in stall regime. In stall regime, advanced Computational Fluid Dynamics (CFD) tools that solve the Reynolds Averaged Navier-Stokes (RANS) equations complemented with turbulence models may provide better predictions of the airfoil aerodynamics at the cost of increased computational requirements. Significant Reynolds number effects were observed in the rotor performance; at low Reynolds numbers the rotor efficiency can be 5% up to 10% lower depending on the operation regime of the wind turbine. Additionally, it was observed that certain asymmetric features in the profile geometry increases the aerodynamic efficiency of the wind turbine rotor. Three major improvements were achieved while using specific NACA 4-digit airfoils: (1) a reduction of the cut-in wind speed (up to 2.34 m/s in contrast with other NACA 4-digit airfoil based rotors), (2) increased aerodynamic efficiency (the maximum achieved efficiency was 46.7% at tip-speed ratios close to 4), and (3) a reduction on the amount of material used in the manufacturing process (in contrast with other with other NACA 4-digit airfoil based rotors). The work demonstrated that the NACA [75,85,95]XX airfoils provides suitable aerodynamic characteristics for the development of small-scale wind turbine rotors. It is important to note that the optimized rotors did not incorporate technologies such as spoilers, winglets, stall barriers, vortex generators or flaps. In case of incorporating these technologies, it may be possible to overcome the 50% aerodynamic efficiency. Research in the performance improvements by the inclusion of these technologies is currently underway. Additionally, other families of airfoils are being tested as well.


Learning objectives
(1) There are specific NACA 4-digit airfoils (e.g., NACA75XX) that outperform conventional airfoils (e.g., NACA44XX). (2) Computational intelligence metaheuristics are very useful for optimizing the performance of wind turbine rotors. (3) Rapid prototyping is a precise, cheaper and faster alternative for rotor manufacturing at a research level. New technologies (airfoil profiles, spoilers, winglets, stall barriers, vortex generators, flaps, etc.) can be tested using rapid prototyping technologies.


References
[1] Chávez R., Gómez H., Herbert J., Romo A., Probst O.: Mesoscale Modeling and Remote Sensing for Wind Energy Applications. Rev. Mex. Fis. S 59(2) (2013) 114.

[2] Wood D. Small Wind Turbines: Analysis, Design, and Application. Green energy and technology. Springer 2011, ISBN 9781849961752. http://books.google.dk/books?id=DDlMPmjUD9MC

[3] Martínez L. J. Aerodynamic and Electromechanic Model with Experimental Characterization for the Prediction of the Power Curve of the Bergey BWC XL.1 Wind Turbine. Master Thesis presented at Monterrey Institute of Technology (ITESM), December 2004.

[4] Gendreau M., Potvin J. Handbook of Metaheuristics. Volume 146 of International Series in Operations Research & Management Science, ISSN 0884-8289, ISBN 1441916652, 9781441916655, Springer, 2010. http://www.springer.com/business+%26+management/operations+research/book/978-1-4419-1663-1

[5] Center for Research and Innovation in Aerospace Engineering (CIIIA). School of Electric and Mechanical Engineering (FIME). Universidad Autónoma de Nuevo León (UANL). http://www.fime.uanl.mx/en/investigacion.php

[6] Abbott IH, Von Doenhoff AE.: Theory of Wing Sections. Dover Publishing, New York (1959).

[7] L. Loftin, Jr., and H. A. Smith.: Aerodynamic Characteristics of 15 NACA Airfoil Sections at Seven Reynolds Numbers from 0.7E6 to 9E6, NACA TN 1945, (1949).

[8] Selig Michel, S.: Wind tunnel aerodynamics tests of six airfoils for use on small wind turbines. ASME 986–1001, Vol. 126, (2004).

[9] Nicolette A. Cencelli. “Aerodynamic Optimization of Small Scale Wind Turbine Blade for Low Wind Speed Conditions”. Master Thesis, Stellenbosch University, South Africa, 2006.

[10] Richard W. Vesel. “Optimization of a Wind Turbine Rotor with Variable Airfoil Shape Via a Genetic Algorithm”. Bachelor Thesis, Aeronautical and Astronautical Engineering. The Ohio State University. 2009.

[11] Daniil Perfiliev. “Optimization of Wind Blade Design Including Its Energetic Characteristics”. Master Thesis, Lappeenranta University of Technology, Faculty of Technology, 2010.

[12] Joaquin R.R. and G. Kenway. ”Aerostructural Shape Optimization of Wind Turbine Blades Considering Site-Specific Winds”. University of Toronto, Institute of Aerospace Studies, 2009.

[13] R. S. Amano, R. J. Malloy. “CFD Analysis on Aerodynamic Design Optimization of Wind Turbine Rotor Blades”. World Academy of Science, Engineering and Technology 60 2009.

[14] M. Jureczko, A. Mezyk. “Optimisation of Wind Turbine Blades”. Journal of Materials Processing Technology 167 (2005) 463–471.

[15] M. J. Clifton, D. H. Wood. “Further Dual Purpose Evolutionary Optimization of Small Wind Turbine Blades”. Journal of Physics: Conference Series 75 (2007) 012017. doi: 10.1088/1742-6596/75/1/012017.

[16] A. Kusiak, H. Zheng. “Optimization of Wind Turbine Energy and Power Factor with an Evolutionary Computation Algorithm”. Journal of Energy 35 (2010) 1324–1332.

[17] Ki-Hak Lee. “Two-Step Optimization for Wind Turbine Blade with Probability Approach”. Journal of Solar Energy Engineering. August 2010, Vol. 132 / 034503-5.