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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.

Gabriele Bedon University of Padua, Italy
Co-authors:
Gabriele Bedon (1) P Uwe Schmidt Paulsen (2) F Helge Aagård Madsen (2) Federico Belloni (1) Marco Raciti Castelli (1) Ernesto Benini (1)
(1) University of Padua, Padova, Italy (2) Danmarks Tekniske Universitet, Kgs. Lyngby, Denmark

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Presenter's biography

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

Mr. 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.

Abstract

Aerodynamic benchmarking of DeepWind design

Introduction

The vertical axis wind turbine (VAWT) has been subjected to new attention, as the DeepWind concept represents a novel offshore floating vertical axis wind turbine solution for energy production. The aerodynamic (and structural) design needs to be carefully conducted in order to obtain an optimized configuration which encounter particular dynamics of such turbine and cost of energy. In the proposed work, different codes have been compared and different rotor configurations have been analysed with respect to the aerodynamic performances.

Approach

The aerodynamic design is analysed in the present work. In order to improve its aerodynamic performances, a very accurate simulation method needs to be adopted in order to obtain a reliable estimation of the real behaviour. In order to increase the accuracy of the results, three different codes were adopted and compared. The first code is a Double Disk Multiple Streamtube Blade Element Momentum (BEM) code based on Strickland and Paraschivoiu theories. This code is providing a good estimation for the aerodynamic performances but, on the other hand, the flow representation is very simplified. Moreover, the computational time required to obtain the estimation is really reduced. The second adopted code is an actuator cylinder based model, which constitute a very good and innovative model for the VAWT simulation. A circular path which represents the VAWT rotor airfoil is considered: radial directed volume forces are applied on the circular path of the VAWT rotor airfoil, determining an energy conversion. The last model is a vortex lifting line code. This model is providing a very good representation for the flow field and the transitional effects, since the wake is carefully modelled, allowing the estimation of the induced velocities. The main differences between the codes are discussed for further works. Furthermore, the different code results are validated against experimental results from turbines with different sizes and aerodynamic profiles, in order to prove their reliability and the performance prediction accuracy. Different rotor parameters are varied in order to find the optimal solutions with respect to the energy production. The optimization parameters are composed by the geometrical characteristics of the blade, considering the blade shape as fixed.

Main body of abstract

The aerodynamic benchmarking for the DeepWind rotor is conducted comparing different rotor geometries and solutions and keeping the comparison as fair as possible. The objective for the benchmarking is to find the most suitable configuration in order to maximize the power production at the design wind speed and the annual energy production, as well as to minimizing the cost of energy. A preliminary analysis for the manufacturing costs is also conducted, considering constraints for the blade shape set in order to limit the rotor production costs. Different parameters are considered for the benchmarking study. The DeepWind blade shape, obtained by means of a structural optimization on the static load, is characterized by a shape similar to the Troposkien shape but asymmetric between the top and bottom parts. The blade shape is considered as a fixed parameter in the optimization process and, since it is characterized by different radii, it will experience different tip speed ratios in the same operational condition. This leads to a complex optimization problem, which must be carefully analysed in order to find a suitable parameter set. The number of blades is varied from 1 to 4. In order to keep the comparison fair, the solidity among the different configurations is kept constant and, therefore, the chord length reduced. A second benchmarking campaign is conducted considering different blade profiles, either uniformly distributed along the blade and optimized with respect to the blade radial position. The considered profiles are both classical symmetrical and asymmetrical, particularly developed for the DeepWind project. Finally a non-uniform chord distribution is considered and evaluated. As previously highlighted, the blade shape given by the structural optimization provided a configuration characterized by blade elements operating at different tip speed ratios. It is therefore reasonable to assume that the different sections should be characterized by different geometries in order to maximize the performances. A variable chord distribution along the blade should be the optimal configuration, as highlighted by previous studies, but in order to keep the manufacturing costs lower, given also the considerable rotor size, only three different chords (as many as the different blade profiles inserted on the blade) are chosen. The aerodynamic performance still is not completely optimized but the result still provides an increased energy conversion with respect to the baseline case. The use of the three different codes allows the deep study of the rotor dynamics and wake creation and evolution. In particular, the force parameters are analysed with respect to the different blade section heights and rotor azimuthal positions, providing considerations not only linked to the energy production but also to the load distribution on the different sections of the blade, enabling further iterations in the structural optimization procedure. Moreover, given the validation results provided by the different codes, recommendation for their use, applications and validity are made. In particular, the BEM validity is largely affected by the considered aerodynamic database: different experimental and analytical sets from literature are analysed and adopted in the in-house code and the best performing is referenced for future use.

Conclusion

The different codes above presented are first validated with turbines of different size and geometrical characteristics in order to prove the reliability of the simulated results. After the validation process, the codes are adopted and compared in order to find the best rotor configuration to accomplish the target of 6 MW design power. The design simplicity, the performance parameters and the cost of energy are considered as optimization scopes in all the benchmarking activity. In every benchmarking campaign, the optimal configuration is found and discussed with respect to the other configurations, achieving the fairest comparison possible. A global simulation, which involves all the findings derived from the single optimization campaigns, is conducted, highlighting a considerable increase in the performances with respect to the considered baseline configuration. Both the rotor aerodynamics and wake formation are analysed and presented, in order to provide a reliable result to be compared between the different codes. The results deriving from the different optimization campaigns highlight configurations which are characterized by different manufacturing complexity but also different load distributions and blade structural constraints. These results will represent the input for the sub-sequential structural optimization iteration as well as the next operational and control policy investigation. The next step in the benchmarking activity will be the code improvement in order to consider the dynamics of this particular design. In fact, the floating rotor will be characterized by a pitching angle and therefore an incoming not planar wind speed: recommendations provided by the work conducted are given in order to extend the validity of the model.


Learning objectives
The delegates will learn the main issues involved in the aerodynamic and structural design for a floating wind turbine of considerable size. In particular, the application of three different codes as well as their comparison will be deeply showed and analysed. The advantages and disadvantages that have been experienced (and which represent a precious contribute that the delegates can consider in their design activity) are highlighted.


References
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