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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.

Peter Eecen ECN, The Netherlands
Co-authors:
Francesco Grasso (1) F P
(1) ECN, Petten, The Netherlands

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Presenter's biography

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

MSc degree earned in 2005 in Aerospace Engineering at Univeristy of Napoli "Federico II". In 2008 PhD degree in Aerospace Engineering at University of Napoli "Federico II" after a visiting period at UCDavis.
Since 2009, Aerodynamicist scientist at ECN. Specialized design and testing of airfoils , blade design and numerical optimization.

Abstract

ECN airfoils for large offshore wind turbines: design and wind tunnel testing

Introduction

For very large offshore rotors, achieving high performance is mandatory but reduction in loads and mass is also attractive to reduce the costs.
The performance of the airfoils installed along the blades have a direct impact on the wind turbine performance. New airfoils tailored on specific requiremnets can help to obtain outstanding performance, while reducing loads and mass.
The present work is focused on the design of a new family of airfoils and wind tunnel testing of one of the airfoils. Also, the effects of new airfoils on the performance of very large offshore wind turbine are investigated.


Approach

Different methodologies can be used to design airfoils. In the present work, a methodology based on numerical optimization (MDO - multidisciplinary design optimization) has been applied to design the airfoils. This choice has several advantages. Among others, MDO offer the possibility to include in the design process considerations and requirements belonging to different disciplines and combine them in efficient way. In case of airfoils, the aerodynamic requirements are very important, but especially at the root, the structural properties of the geometries should also be evaluated. MDO makes possible to achieve the most convenient combination of such requirements.
Several airfoils are used on the same blade. To have a smooth blade surface, the airfoils have been designed as family. This means prescribing consistent requirements and avoid that the properties of the geometries are changing suddenly, leading to non uniform local performance of the blade. Also in this respect, MDO approach is very beneficial, since the same set of requirements can be prescribed during the design.
The accuracy of the results is a key point to obtain improved turbine performance in reality. To validate the numerical results, one of these geometries has been tested in wind tunnel for different Reynolds numbers in clean and rough conditions.
Finally, the performance of a new blade incorporating these airfoils has been calculated and compared to a reference rotor to assess the benefits of the new design. To have realistic data, the results of the wind tunnel tests have been used to correct the numerical predictions. The annual energy production has been considered as main performance indicator, but reduction in axial force and root bending moment has been also considered as target.


Main body of abstract

As mentioned, a numerical optimization based approach has been used in this work. In particular, a hybrid scheme has been implemented where genetic algorithms (GA) are combined with gradient based algorithms (GBA). GA is used in first instance to explore wide domain and then GBA is added to refine the solution. The shape of the airfoil has been parameterized through Bezier curves and the control points are the variables of the optimization problem . 15 degrees of freedom are active. The ECN tool RFOIL is evaluating the airfoil performance during the design, due to the very good accuracy and computational speed.
A family of 6 new airfoils (named ECN-G1-xx) has been developed with the percentage thickness ranging between 18 and 40 percent. The main objective of the design was to obtain airfoils with high aerodynamic efficiency (L/D), but beside this, also other considerations have been included during the development. In particular, concerning the lift coefficient (Cl) and the maximum lift coefficient (Clmax), those should also be high. High lift airfoils can lead to a reduction in chord distribution that is beneficial to limit the extreme loads in parking conditions and the load fluctuations in case of gust. On the other hand, abrupt stall should be avoided and a safety margin between design condition and separated flow region should be ensured to protect the airfoils from working in separated flow during gusts (that could lead to fatigue problems for the blade). In order to prevent abrupt stall, the location of the transition should move gradually when the angle of attack changes. Sometimes high efficiency airfoils have large extension of laminar flow; this helps to have low values of drag coefficient (Cd) but at the same time, can mean that the geometry is sensitive to the roughness so the performance can significantly decrease. and show the performance. Compared to existing airfoils in literature, the ECN airfoils exhibit good values of efficiency over a wider range of angle of attack, high lift characteristics and superior performance in rough conditions.
To validate these results and assess their reliability, a measurement campaign at TU-Delft low speed low turbulence wind tunnel (LSLTWT) has been performed on the 21% thick airfoil . The tests have been performed at 1 and 3 millions Reynolds number, in clean and rough conditions with zig-zag tape. The results of the tests showed a general good agreement of the numerical predictions with the experiments. Despite this, in one case, the stall is more abrupt than predicted . So it is still a key point where the accuracy of the code can be improved.
Based on the numerical predictions, corrected according to the experimental results, the effects of new airfoils on wind turbine performance have been investigated. The 10 MW reference wind turbine of the European project INNWIND.EU was selected as reference for the case study. The ECN tool BOT based on BEM theory has been used to adapt chord and twist of the new blade, while the blade length and other operative parameters (i.e. rotational speed, tip speed ratio) have been kept. Looking at the performance of the blade with new airfoils, the same annual energy production has been obtained, while the axial force and the root bending moment have been reduced by 5%.


Conclusion

The design of new airfoils for very large rotors is a viable solution to achieve high performance in terms of annual energy production. In the present work, a new family of advanced airfoils has been presented and the design criteria have been illustrated. Also, the performance in respect of existing geometries have been showed and compared. Starting from these results, an investigation about the impact of the airfoils on the performance of a 10 MW machine has been performed. The expected advantages due to airfoils high lift performance have been confirmed: the new airfoils can contribute to a significant reduction in loads, while the overall performance of the rotor is kept. As positive consequence, a reduction of mass of the blade and tower can be expected that would allow to reduce the cost of energy and the investment associated with the future developments.
In addition, the decrease in axial force and root bending moment, would allow to make the blade longer, so higher annual energy production, but with the same loads. This would be beneficial also at wind farm level where the overall performance could be improved.
Despite the encouraging results, improvements in modeling of the stall behavior are still needed. In fact, to validate the numerical predictions, wind tunnel measurements have been performed at TUDelft that resulted in sharp stall response of the airfoil, while overall RFOIL is able to describe correctly the airfoil behavior, also in fixed transition conditions. Corrections have been made to the numerical data during the evaluation of the blade performance.



Learning objectives
Several solutions can be used to support the development of new very large rotors to reduce the loads and the mass.
One of these is the adoption of ad hoc designed airfoils. The results showed that there are good potentialities and room for improvement



References
Grasso, F., “Usage of Numerical Optimization in Wind Turbine Airfoil Design”, AIAA, Journal of Aircraft, AIAA, Vol.48, No.1, Jan.-Feb. 2011, DOI: 10.2514/1.C031089.
B.M. Jones, Measurement of profile drag by the pitot-traverse method. ARC R&M No.1688, 1936
H.C. Garner, E.W.E. Rogers, W.E.A. Acum and E.C. Maskell, “Subsonic wind tunnel wall corrections”, AGARDograph 109, October 1966.
van Rooij, R.P.J.O.M., “Modification of the boundary layer calculation in RFOIL for improved airfoil stall prediction”, Report IW-96087R TU-Delft, the Netherlands, September 1996.
Abbott, I., Von Doenhoff, A., “Theory of Wing Sections”, Dover Publications, Inc., Dover edition, 1958.