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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.

Ilmas Bayati Politecnico di Milano, Italy
Co-authors:
Ilmas Bayati (1) F P Marco Belloli (1) Stefano Foletti (1)
(1) Politecnico di Milano, Milan, Italy

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Presenter's biography

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

Ilmas Bayati obtained the MSc in Mechanical Engineering from Politecnico di Milano with a thesis titled “Design and validation of a Hardware-In-The-Loop experimental rig for wind tunnel tests on offshore wind turbines”. He has been awarded the prize “Bonfiglioli 2011” for the best italian thesis in the field of mechatronics. He's now a PhD candidate at the same university focusing on the hydro-aero-dynamics of wind turbines and sailing yachts, numerically and experimentally.He has been recently visiting the offshore team of NREL, working on the 2nd order hydrodinamics of Floating Offshore Wind Turbines

Abstract

Multi-objective design optimization of a high-performance active pitch-controlled H-Darrieus VAWT for urban installation

Introduction

This paper reports the design process of a high-performance H-Darrieius Vertical Axis Wind Turbine (VAWT) for urban installation. Among others, this type of wind-turbine was chosen for its potential improvements on items such as self-starting mode for standalone applications and active pitch control. A particular attention to the choice of materials and motor-reducer unit/blade was paid in order to optimize the overall efficiency.

Approach

This paper reports the design of a 3mx3m (diameter/height) pitch-controlled H-Darrieus VAWT. This wind turbine is a 3 straight-bladed machine whose blades are connected by their extremities to the shaft. A set of simmetric airfoil were adopted for the analysis. The NACA 0021 airfoil was finally chosen for its capability of matching different design requirements such as aerodinamic performance, maximization of the moment of inertia of the section, limitation of blade deflection due to centrifugal and variable loads, increment of blade natural frequencies. The pitch angle optimization of the airfoil pitch angle as function of the azimuth angle were calculated by implementing the double multiple stream tube (DMS) algorithm [1] integrathed with genetic algorithm, considering both independent tubes or whole tube for the maximization of the tangential force. A new manifacturing process for the realization of the blade is proposed. The blade skeleton is given by series of equidistant carbon-fiber NACA0021-shaped support structures installed on a carbon-fiber elliptical beam, being the main structural component of the blade. The choice of such elliptical shape is due to an efficient torque trasmition from the motor-redicer unit to the airfoil and then to the blade. NACA0021-shaped support structures are covered by a Clysar film, similarly to the America’s Cup 2013 wing sails technology. In order to set up a correct NACA0021 leading edge shape to the blade expanded polystirene (EPS) material was adopted for its advantage in terms of stiffness/mass ratio. Alpha-Beta theory for the choice of motor-reducer unit was implemented in order to maximize the efficiency of the actuation, also a bilancing beam was inserted inside the blade in order to minimize the eccentric masses with respect to the rotation axis.

Main body of abstract

The main limitations of lift vertical axis wind turbine is the inability of self-starting and the low efficiency for low Tip Speed Ratios. To overcome this issue, the ability of change is necessary[2],[3],[4]. The aim of this work is to present the design process of a H-Darrieus pitch-controlled vertical axis wind turbine (VAWT) and its new manifacturing items. The increment of the Cp coefficient at low TSR is evident by performing DMS algorithm combined with genetic algorithm in order to optimize the pitch angle as function of the azimutal angle. This is true for different simmetric NACA airfoils. The more the Tip Speed Ratio the more similar the behaviour between pitch-controlled and fixed pitched H-Darrieus in terms of Cp coefficient. This is due to the fact that in the former case tha angle of attack is set to be close to the optimal one, whereas in the latter case the fixed-pitch machine is set by nature to assume angles of attack that maximize the tangential force. The lift/drag ratio as function Tip Speed Ratio also indicates the higher efficiency of pitch-controlled machine compared to the traditional one. Since two different types of optimization alogorithms were performed in combination with Double Multiple Streamtube algorithm (DMS) to define the best pitch angle as function of azimutal angle, some consideration are reported on these algorithms: optimizing by independent sector favours the tangential forces within the upwind sector of the machine, whereas the optimization on the overall tube tends to limitate the differences between the forces of upwind and downwind sector.

The goal of such machine is to be selft-starting and rapidly reaching the optimal functioning point, with the lowest power consumption. The blade are mainly carbon-fiber made with a structural elliptical beam on which the carbon-fiber NACA0021 shaped airfoil structures are fixed. The process that led to the choice of such an airfoil deals with the optimization of aerodynamic and structural properties. In regards to the structural properties, the objective functions of the genetic optimization that were taken into account, consist in the optimization of the geometrical parameters of the section (airfoil) due to static and dynamic loads. Regarding the former the moment of inertia was considered as to be maximixed, whereas for the latter, the first natural frequency of the main carbon-fiber structural beam was taken as at least two times the first natural frequency of the maximal aerodynamic forces (given by the spectrum of the optimal-pitch output); the drag force was far below the lift force. A polyolephinic film (Clysar) was chosen to cover the whole blade providing the right airfoil NACA 0021. Also this covering film came out of an optimizing process that took into account the structural and thermal properties of the material in order to best matching the requirements of stiffness and lightness. Wind tunnel tests in extremen conditions (wind speed velocity and angle of attack) were performed in order to tests the material for this unconventional application. The optimization process for the choice of the three motor-reducer units to be installed at the bottom end of the blades was integrated with the alpha-beta approach that leads to an organic choice of optiale motor-reducer coupling among a wide variety of commercial items [5].


Conclusion

This paper reports the design of a 3mx3m (diameter/height) pitch-controlled H-Darrieus VAWT. A set of simmetric airfoil were adopted for the analysis. The NACA 0021 airfoil was finally chosen for its capability of matching different design requirements such as aerodinamic performance, maximization of the moment of inertia of the section, limitation of blade deflection due to centrifugal and variable loads, increment of blade natural frequencies. The pitch angle optimization of the airfoil pitch angle as function of the azimuth angle were calculated by implementing the double multiple stream tube (DMS) algorithm integrathed with genetic algorithms, considering both independent tubes or whole tube for the maximization of the tangential force. A new manifacturing process for the realization of the blade is proposed. The blade skeleton is given by series of equidistant carbon-fiber NACA0021-shaped support structures installed on a carbon-fiber elliptical beam, being the main structural component of the blade. The choice of such elliptical shape is due to an efficient torque trasmition from the motor-redicer unit to the airfoil and then to the blade. NACA0021-shaped support structures are covered by a Clysar film, similarly to the America’s Cup 2013 wing sails technology. In order to set up a correct NACA0021 leading edge shape to the blade expanded polystirene (EPS) material was adopted for its advantage in terms of stiffness/mass ratio. Alpha-Beta theory for the choice of motor-reducer unit was implemented in order to maximize the efficiency of the actuation, also a bilancing beam was inserted inside the blade in order to minimize the eccentric masses with respect to the rotation axis of the blade.


Learning objectives
This work presents a new H-Darrieus pitch-controlled vertical axis wind turbine concept for urban installations. The choice of design materials and the optimization process represent multi-objective exercises for sizing high performance VAWT


References
[1] I. Paraschivoiu and F. Delclaux, Double-multiple streamtube model with recent Improvements, Journal of Energy, vol. 7, no. 3, pp. 250–255, 1983.
[2] J.J.Miau, S.Y.Liang, R.M. YU, T.S.Leu, J.C.Cheng, S.J.Chen, Design and Test of a Vertical-Axis Wind Turbine with Pitch Control, Applied Mechanics and Materials Vol. 225 (2012) pp 338-343.
[3] A.J. Fiedler, S.Tullis, Blade Offset and Pitch Effects on a High Solidity Vertical Axis Wind Turbine, Wind Engineering Volume 33, NO. 3, 2009 P P 237–246.
[4] I.Paraschivoiu, O.Trifu, F.Saeed, H-Darrieus Wind Turbine with Blade Pitch Control, International Journal of Rotating Machinery,Volume 2009, Article ID 505343,7pages
[5] Giberti, H., Cinquemani, S., Legnani, G. A Practical Approach to the Selection of the Motor-Reducer Unit in Electric Drive Systems. In: Mechanics Based Design of Structures and Machines, vol. 39(3) (2011), pp. 303 - 319.