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Wednesday, 12 March 2014
09:00 - 10:30 Aerodynamics and rotor design
Science & Research  


Room: Llevant
Session description

The session is oriented to show recent computational and experimental findings on aerodynamic phenomena in horizontal (HAWT) and vertical access wind turbines (VAWT), as well as on new developments on system identification techniques related to the aeroelastic behaviour of wind turbine rotors and new aerodynamic design trends for very large wind turbines.

Learning objectives:

Delegates will learn about:

  • recent computational and experimental findings on aerodynamic phenomena in HAWT and VAWT
  • new developments on system identification techniques related to the aeroelastic behaviour of wind turbine rotors
  • innovative design trends for the aerodynamics of very large wind turbines
Lead Session Chair:
Alvaro Cuerva, Universidad Politécnica de Madrid, Spain

Co-chair(s):
Sandrine Aubrun, Univ. Orléans, PRISME Laboratory, France
Giorgos Seros CRES, Greece
Co-authors:
Panagiotis Chaviaropoulos (1) F P George Sieros (1)
(1) CRES, PIKERMI, Greece

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Presenter's biography

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

Giorgos Sieros is a mechanical engineer (NTUA, 1993) with a PhD in turbomachinery component design (NTUA, 2000). He has worked in numerous research projects related to the design of steam, gas and wind turbine components at NTUA and CRES. He has also taught courses on these subjects at the Technological Education Institutes of Athens and Chalkida.

Abstract

Design of low induction rotors for use in large offshore wind farms

Introduction

Recently, a new generation of multi-MW offshore rotors that are under development are deviating from the established design trends, displaying high tip-speed, low solidity and larger than expected rotor diameters. A study was undertaken to quantify the relative merits of this approach, associated to low induction – high swept area rotors. We are using BEM analysis of such solutions and comparing the results to those of a reference wind turbine at the 10MW scale designed for the InnWind.EU project, in order to identify the possibilities for a significant reduction of the cost of energy, especially in large offshore wind farms.

Approach

The purpose of the development of the Low Induction Rotor (LIR) is to examine the validity of the proposed shift in design patterns, and determine targets for the design of airfoil and planform shapes, compatible with the new LIR. The approach used for the quantification of the results includes the performance evaluation of a LIR against a reference rotor having similar aerodynamic design loads.
The reference rotor addresses a typical state-of-the-art pitching-variable speed HAWT design, and for a given rotor radius has been designed to maximize the energy capture by maximizing the power coefficient CP. Following classical BEM theory this happens for an axial induction value a=1/3 and corresponds to a TSR design value λ which gets larger (along with CP,MAX) as the aerodynamic performance of the blades k gets better (higher).
For the LIR, an additional degree of freedom is added in the design process, allowing for a different axial induction factor, which does not correspond to the theoretically optimum CP,MAX. The new design goal is to maximize energy capture, as expressed by C_P (λ,α)⋅R^2 , keeping the bending moment at the blade root the same as before, giving (in a simplified form) the following optimisation problem
(C_P (λ,α)⋅R^2)/(C_P0 (λ_0,α_0 )⋅R_0^2 )→max,subject to (C_(M(0)) (λ,α)⋅R^3)/(C_(M0(0)) (λ_0,α_0 )⋅R_0^3 )≅1
A solution to this problem is examined, through the evaluation of an alternative blade design, operating at a lower power coefficient, but with a larger radius and tip speed. Comparisons for design point operation and operation for the complete operating envelope were performed and the main results are given in the next section.


Main body of abstract

A detailed description of the reference rotor planform, airfoil geometry and performance is given in [1]. There are five primary profiles of different thickness (24.1%, 30.1%, 36%, 48% and 60%) comprising the adopted FFA-W3 family. The solution is very close to the optimum for the aerodynamic part of the blade, deviating only in the inner sections, where structural constrains pose limitations on twist and blade shape.
The design of a representative LIR is based on the choice of a different Λ=cCL/R distribution, corresponding to an axial induction factor of ~0.2, as opposed to the theoretical optimum used for the RWT. The LIR is designed to replace the reference blade on the same wind turbine, and thus the rotational speed schedule remain unchanged. In order to produce such a design, without compromising on chord length and blade thickness (as this would affect structural integrity), blade sections with reduced design lift should be used. The resulting planform for the LIR solution is shown in Figure 1, with an elongated blade, but similar chord and thickness values to the reference rotor. The observed change in pitch distribution is a direct result of the requirement for reduced loading, leading to different AOA at the sections.

The differing aerodynamic requirements for the sections indicate that the original high-lift airfoils would not be a good fit for the proposed design, as they would operate far from their optimum L/D. As maximum CL for the new design is lower, we are looking for an airfoil family that will have optimum k=CL/Cd at lower CL values. As a starting point we utilised symmetrical NACA63 based profiles, deriving the polar characteristics through XFOIL calculations. To avoid introducing a bias in favour of the LIR design, the calculated drag was increased in order to have the same k as the original airfoil family, indicating a similar level of performance. The resulting operating points for the sections are shown in Figure 2, where it is obvious that the design CL for the LIR is ~0.8, as opposed to 1.25 for the RWT. In Figure 3 the radial distribution of the local pressure, thrust coefficient and axial induction factor at rotor design conditions are presented. It can be seen that at the aerodynamic part of the blade (30%< x < 85%) all three distributions are almost flat. However, while in the RWT α approaches its theoretical optimum value of 0.33, in the revised design a much lower value of ~0.2 is used. At the same time the local power coefficient which exceeds 0.5 for the original design is now significantly reduced. The thrust coefficient level for the LIR has dropped from ~0.9 to ~0.6 for the larger part of the blade, as expected, a drop of almost 50%.


Details about the operating range of the reference rotor are given in [5]. The LIR follows the same rotational speed schedule, with a range of 6 – 9.6 RPM. The LIR reached the rated power at a reduced wind speed and in general produces greater power at speeds below rated (Figure 4). The result of this is an increase of ~3.5% in annual energy production for a Class I Rayleigh wind distribution

In order to get a better insight on the performance of the proposed rotor we present some details based on rotor power (P), thrust (T), torque (Q) and thrust bending moment M(r) (at a given radial position) and the relevant non-dimensional coefficients .The variation of these quantities for the full range of expected wind speeds (from cut-in to cut-off) is given in Figure 5, compared to the respective values for the RWT. The power coefficient maximum value is reduced from 0.47 to ~0.4 for the LIR design. A more drastic change is observed in the thrust and bending moment coefficients, where maximum values are almost halved in the new design. Dimensional thrust and bending moment levels are given in Figure 6, for the complete operating regime. A non-negligible decrease of the maximum thrust from ~1500 kN to ~1350 kN is observed for the LIR design, while the corresponding root bending moment (at r=0) is practically unaffected, as expected.



Conclusion

The concept of a Low Induction Rotor is evaluated in this work. The evaluation includes
• Initial theoretical investigation of the concept, demonstrating that a low induction high swept area rotor can capture more energy than a conventional design that aims at CP,MAX, without a penalty on the aerodynamic loading of the blades.
• Evaluation using a BEM model, resulting in a modified version of the reference wind turbine. Two variations on the design were used without noticeable differences in the results. The main characteristics of the resulting design are i) The axial induction factor is chosen to have a value of ~0.2 (as opposed to 0.33, typical for Cp_max designs), ii) CCL/R drops 50-60% but the original chord is more or less maintained for structural reasons, iii) Rotor radius is increased by 13%, iv) CL design (where L/D is max) has to come down to 0.80, meaning that a new family of airfoils is needed. Realistic specifications have been given, it remains to be seen how these can be realised.
• Comparison of key performance parameters to the RWT, including i)Energy capture, where a 3.5% improvement of AEP compared to the RWT is observed and ii) Loads, a 10% reduction in thrust is obtained, while retaining the same blade root bending moment

Earlier analysis has shown that due to their increased energy capture, at the single turbine but also at the wind farm level, and despite their extra blades cost (it is assumed proportional to the blade length) LIRS can reduce the levelised cost of energy of large offshore wind farms. The required performance of airfoils has also been shown, through the lift-drag curves used. Further work is required to determine if this kind of performance is attainable through optimised blade designs.



Learning objectives
Determine design patterns for large scale offshore wind turbines. Examine alternatives to the established design method and examine their potential for improvements in energy capture at the single turbine but also at the wind farm level.


References
[1] C. Bak, F. Zahle, R. Bitsche, T. Kim, A. Yde, L.C. Henriksen, P.B. Andersen, A. Natarajan, M.H. Hansen, “Design and performance of a 10 MW wind turbine”, submitted to J. Wind Energy
[2] Chaviaropoulos, T; H.J.M. Beurskens and S. Voutsinas, Moving towards large(r) high speed rotors – is that a good idea? Proc. Scientific Track, EWEA 2013 Conference, Vienna.
[3] V.A. Riziotis, P.K Chaviaropoulos and S.G. Voutsinas, "Development of a State-of-the-art Aeroelastic Simulator for Horizontal Axis Wind Turbines. Part 2: Aerodynamic Aspects and Applications", Journal Wind Engineering, Vol 20, No. 6, pp. 423-439, (1996)
[4] DTU Pitch / VS schedule, Innwind.EU Internal site
[5] Chaviaropoulos, P.K., Chortis, D., Lekou, D. Definition of the Reference Wind Turbine–Analysis of Rotor Design Parameters (D1.2.1, May 2013, Innwind.EU Internal site)
[6] Peter Jamieson, Innovation in Wind Turbine Design, A John Wiley & Sons, Ltd., Publication, ISBN 978-0-470-69981-2, 2011
[7] J. Jonkman, S. Butterfield, W. Musial, G. Scott, ”Definition of a 5-MW reference wind turbine for offshore system development” NREL/TP-500-38060, 2007

Acknowledgements
This work was partially funded from the InnWind.EU project under the context of the 7th Framework Programme (Energy)