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Delegates are invited to meet and discuss with the poster presenters during the poster presentation sessions between 10:30-11:30 and 16:00-17:00 on Thursday, 19 November 2015.

Lead Session Chair:
Stephan Barth, ForWind - Center for Wind Energy Research, Germany
Gabriele Bedon University of Padua, Italy
Co-authors:
Gabriele Bedon (1) F Stefano De Betta (1) Ernesto Benini (1) Valerio Focanti (2) Marco Profili (2) Moreno Rotondi (2)
(1) University of Padua, Padova, Italy (2) Briareo S.r.l., Ferentino, Italy

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2015 and are published here unedited

Gabriele Bedon is a Ph.D. Student in Energy Engineering at University of Padua, Italy. His main research topics involve the aerodynamic simulation with semi-analytical models of the Darrieus wind turbines and their airfoil optimization with advanced optimization algorithms. He graduated at University of Padua in Mechanical Engineering and at Denmark Technical University in M.Sc. in Engineering, Sustainable Energy. He is currently cooperating with both Universities to improve the design of several projects.


Poster

Poster Download poster (10.57 MB)

Abstract

Aerodynamic Optimization of a Vertical Axis Wind Turbine Equipped with a Two-Element Airfoil

Introduction

The aerodynamic optimization of Darrieus vertical axis wind turbines is a largely debated theme in the scientific literature due to the inherent complexity of the rotor unsteady behavior. Different approaches have been exploited, considering advanced optimization tools as genetic and evolutionary algorithms and simulation routines including algorithms based on the Blade Element-Momentum theory and Computational Fluid Dynamics (CFD) solvers for the Unsteady Reynolds Averaged Navier Stokes (URANS) equations. While semi-empirical algorithms are generally the best choice for the traditional geometries involving a single-airfoil blade, the simulation of more complex geometries requires advanced computational tools based on the CFD techniques. On the other hand, genetic algorithms perform an efficient optimization activity but involve a large number of fitness evaluations. In the present work, a Darrieus vertical axis whose blade is made up using a two-element (main and slat) airfoil is considered. The size and the position of the slat is optimized fixing a layout of the main airfoil.

Approach

A rotor equipped with a double airfoil blade, respectively a main and a slat airfoil, is considered. The size and position of the slat airfoil is varied considering four different decision variables: the position with respect to the quarter of chord, the distance between the two airfoils, the chord and the pitch angle. Starting from a baseline configuration, the slat position and geometry are optimized by considering a univariate optimization algorithm to improve the total rotor power production in a target tip speed ratio range. The rotor efficiency is numerically estimated by means of a CFD software implementing a URANS k-ω SST solver. The simulation is conducted considering a validated 2D mesh representing a large testing field, where the boundary conditions are imposed far enough not to influence the rotor aerodynamics.

Main body of abstract

The results for the optimization of the Darrieus rotor equipped with a slat airfoil are presented. The problem appears to be well defined with respect to the adopted optimization routine and decision variables. These independently provide the maximum performance for a given value, which was correctly included in its range of variation. The physical reason behind the optimal slat airfoil position are linked to the rotor aerodynamics, given the increase in the main airfoil operative range which is obtained by introducing the secondary airfoil. Moreover, the analysis of the parameters such as the slat airfoil pitch angle and chord length highlighted a variation in the optimal operating conditions in the considered range. Depending on the slat characteristics, in fact, the aerodynamic effect is enhanced for different angles of attack and, therefore, different tip speed ratios. The maximum performance is however achieved for a particular operating condition which is maintained along the optimization process.

Conclusion

The optimization activity successfully provided an optimal configuration for a Darrieus rotor whose blades are equipped with a slat airfoil. This airfoil presents an optimal chord length and pitch angle, whereas its position is fixed by the quarter chord and airfoil surface distances. The slat airfoil is located in the proximity of the leading edge and on the suction side, in order to widen the airfoil efficiency range and enhance the production in the upwind rotor section. The simulations are of particular interest because, beside the performance increase, show the variations of the optimal working conditions with respect to the different geometrical configurations.


Learning objectives
Delegates will understand the details of an optimization activity that can be applied in a wide range of aerodynamic studies, successfully increasing the aerodynamic performance by means of a reduced number of simulation and changes in the geometrical parameters. Moreover, the increased efficiency of a double airfoil blade, where a second airfoil acts as a slat for the main airfoil, is shown and demonstrated, proving that such a technology could improve the performance of the traditional Darrieus concept.