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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.

D. Todd Griffith Sandia National Laboratories, United States
Co-authors:
D. Todd Griffith (1) F P Phillip Richards (1)
(1) Sandia National Laboratories, Albuquerque, United States

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Presenter's biography

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

Dr. D. Todd Griffith is a Principal Member of the Technical Staff in the Wind and Water Power Technologies Department at Sandia National Laboratories. He is the Technical Lead for Sandia’s Offshore Wind Energy Program. His research contributions include work in the areas of structural dynamics, field testing, large offshore rotor technology (Sandia 100-meter blade work, deep-water VAWTs), and structural health & prognostics management for wind energy systems. Prior to joining Sandia, he completed PhD work at Texas A&M University in Aerospace Engineering.

Abstract

Investigating the effects of flatback airfoils and blade slenderness on the design of large wind turbine blades

Introduction

Design and development of large blades is challenging due to economic, logistic, manufacturing, and technical barriers. Regarding the technical barriers alone, designs must satisfy deflection, buckling, fatigue, and stability requirements. This is a very challenging design problem that becomes more difficult to solve cost-effectively as blades grow longer. Sandia has been researching large blades for several years and identified several key design barriers for large blades including aeroelastic stability, panel buckling and gravitational fatigue loading. This paper seeks to present pathways to reverse the trends and achieve cost-effective, manufacturable, lightweight, and aeroelastically stable blades of length 100 meters and greater.

Approach

Sandia National Laboratories Wind Energy Department creates and evaluates innovative large blade concepts to promote designs that are more efficient aerodynamically, structurally, and economically. Recent work has focused on the development of a 100-meter blade for a 13.2 MW turbine and a series of large blade design studies. A link to the project website can be found in Reference 1. The current work builds upon the earlier design studies that produced a baseline all-glass design (Reference 2) and an updated carbon spar design (See References 3 and 4).

In this paper, we perform a series of parameter studies to evaluate technologies to address these technology barriers. The focus is on design using flatback airfoils through a design study of aerodynamic, structural, and manufacturing trade-offs. We begin with design studies using flatback airfoils for the Sandia 100-meter blades. In these studies we investigate the effect of new design tools which optimize for both the blade aerodynamic and structural requirements, although we baseline this approach with traditional aerodynamic only optimization of blade geometry to understand the potential benefits and shortcomings of our approach. Again, the focus is on design with flatback airfoils with design studies to examine aero-elastic tradeoffs associated with various levels of blade solidity. Detailed blade designs are produced with geometry and materials/layups designed to meet design loads requirements of international design standards as a means to demonstrate feasibility of the new designs.

In addition, the final paper will include studies for much longer blades in the 125- to 150-meter range. These studies of larger blades may reinforce the new design approach as structural requirements become equally important for ultra-large blades and should be included in the initial design process. The intent is to further examine the effects of various “innovations” to enable even larger blades to be fielded meeting the economics, manufacturing (Reference 5), weight, and aero-elastic stability targets outlined above.


Main body of abstract

A survey of blade weights includes the most recent data on new large blades reported by both industry and by researchers producing concept designs (Figure 1). References 6 and 7 describe important rotor projects that have recently produced large blade design concepts, included in the figure. High innovation projections (exponent of 2.2) of weight assumes that technical barriers can be overcome in design. The extent to which these barriers can be overcome while maintaining weight targets and delivering cost-effective future designs is a primary focus of this present work. The figure also shows the weights for the series of Sandia 100-meter blades (with 102.5 meter radius) with the SNL100-00 all-glass baseline (~114 tons) to the SNL100-01 carbon spar blade (~74 tons). The more recent advanced core (SNL100-02, ~60 tons) and (SNL100-03, projected ~49 tons) designs are also included to show the weight reduction pathway of this research, which are the subject of this present study.



Figure 1. Blade Mass Survey and Projections Versus Rotor Radius

The final paper will also include results to benchmark the structural and aerodynamic properties of various flatback airfoil concepts. The FB-series of flatbacks utilized in the Sandia BSDS (Blade System Design Study) blade will be benchmarked against more recent flatback concepts.

The National Wind Technology Center (NWTC) design code HARP_Opt (Horizontal Axis Rotor Performance Optimization) (Reference 8) was used to optimize the 100m blade geometry. HARP_Opt performs a dual-objective genetic algorithm optimization, where the objectives are annual energy production (AEP) and blade weight. The design variables are control points for the twist and chord profiles of the blade along with variables to determine airfoil placement. For the aerodynamic model, HARP_Opt uses WT_Perf, which is a blade-element momentum theory wind turbine analysis code. The airfoil data was provided to WT_Perf in the form of multiple Reynold's number data tables, with Reynold's numbers spanning the range of 7.5e5 to 20e6. Therefore it is assumed that high Reynold's number effects are properly accounted for within the aerodynamic model. HARP_Opt was integrated with the Sandia National Laboratories NuMAD toolbox, which allowed for a more realistic and accurate structural model.

A design change with the potential to further decrease the blade weight is the implementation of flatback airfoils. The thicknesses of the desired airfoils are approximately 63%, 55%, 43%, and 34%, and 27%. The r/R locations for each airfoil are approximately 14%, 17%, 21.5%, 27%, 45%, respectively. An 18% thick NACA airfoil is located at r/R 73.8%. The root chord of the earlier SNL100-00 and SNL100-01 blades was 5.86m. Initial design space exploration of the 100m blade aero/structural design revealed that a further decrease in blade solidity can result in a lower blade mass without a significant change in AEP. The root chord was reduced to 4.5m. These results were subjected to the "extreme gust with coherent direction change" or ECD design load case to ensure the dynamic deflections do not exceed the allowable level of 13.5m.

Since the multi-objective genetic algorithm is used, each aero/structural optimizer run produced a Pareto front of candidates. The candidate from each optimization that has the same AEP as the baseline design was chosen. The optimization results are summarized in Figures 2 and 3 in comparison to the baseline 100-meter design and initial optimizations of the baseline which incorporated sharp trailing edge DU foils in one case and flatbacks in another.



Figure 2. Chord and twist distributions for either DU series or flat-back airfoils.



Figure 3. Spar layer distributions for either DU series airfoils or flatbacks.

A second round of design optimization with flatback airfoils investigated the effect of spar cap width. The first (“rev1”) having a more slender planform than the second (“rev2”). Both have root diameter of 4.5 meter. Figure 4 compares the resulting chord schedules. The spar cap widths of 1200mm and 750mm were analyzed; the more slender blade "rev2" is associated with the larger spar cap width. Of course, both of these designs are significantly more slender than the initial Sandia 100-meter blades studies owing to the flatback airfoil choice. The effect of slenderness on the blade performance, in particular the structural performance (e.g. buckling, fatigue, deflection, etc.), will be a key focus of the final paper.



Figure 4. Chord (meters) Versus Span (meters) Comparison for Flatback Design Options


Conclusion

A series of design studies to investigate the effect of advanced core materials and flatback airfoils on blade weight and performance for large blades was performed using the Sandia 100-meter series of blade designs as a starting point. The initial or baseline design was a 100-meter all-glass blade. Subsequently, a carbon design study was performed to produce the SNL100-01 carbon design, which provided the starting point for these studies. This paper presents an overview of the Sandia Large Offshore Rotor project and reports on the recent design studies leading to the SNL100-02 and SNL100-03 100-meter blade designs. In the former, advanced core materials placed in the panels, trailing edge, and shear webs were evaluated to reduce weight while satisfying buckling requirements (SNL100-02). More recently, the focus has moved to blade geometry and airfoil effects through the use of flatback airfoils, in leading to the SNL100-03 design. This paper will include an overview of the major conclusions of these design studies with focus on pathways to enable cost-effective large blade technology that is light-weight and aeroelastically stable, although the major focus is on use of flatback airfoils and the effects of blade solidity on aerodynamic and structural performance of large blades.

One key goal is the reverse the trends of increasing blade weight and cost, including manufacturing cost, while mitigating key design drivers for large blades such as fatigue life, buckling capacity, and flutter speed. The hope is that a more slender design can alleviate the issues associated with these key large blade design drivers; however, the limit to slenderness needs to be further examined in general and in the context of flatback airfoils. The paper will include results using the established design models and tools for blades from 100 to 150 meters to examine effects of scale.



Learning objectives
Key learning objectives include an overview of large blade design drivers and the Sandia Large Offshore Rotor Project. Flatback airfoil studies are examined through blade aerodynamic and structural tradeoff design studies. In addition to the design tools, Sandia flutter prediction and manufacturing cost estimation tools will be utilized to examine key cost and stability issues.


References
1. “Offshore Wind: Sandia Large Rotor Development,” Sandia 100-meter Blade Research Website: http://largeoffshorerotor.sandia.gov, Last modified 18-February-2013; accessed 01-October-2013.
2. Griffith, D.T. and Ashwill, T.D., “The Sandia 100-meter All-glass Baseline Wind Turbine Blade: SNL100-00,” Sandia National Laboratories Technical Report, SAND2011-3779, June 2011.
3. Griffith, D.T., “The SNL100-01 Blade: Carbon Design Studies for the Sandia 100-meter Blade,” Sandia National Laboratories Technical Report, SAND2013-1178, February 2013.
4. Griffith, D.T., Resor, B.R., and W. Johanns, “Carbon Design Studies for Large Blades: Performance and Cost Tradeoffs for the Sandia 100-meter Wind Turbine Blade,” AIAA Structures, Structural Dynamics, and Materials Conference, Accepted and to be presented in Boston, MA, April 2013.
5. Griffith, D.T., Johanns, W., “Large Blade Manufacturing Cost Studies Using the Sandia Blade Manufacturing Cost Tool and Sandia 100-meter Blades,” Sandia National Laboratories Technical Report, SAND2013-2734, April 2013.
6. Bak, Christian et al. "Light Rotor: The 10-MW reference wind turbine". Proceedings of EWEA 2012 - European Wind Energy Conference & Exhibition. EWEA - The European Wind Energy Association. 2012.
7. Peeringa, J., Brood, R., Ceyhan, O., Engels, W., and Winkel, G., “Upwind 20MW Wind Turbine Pre-Design: Blade Design and Control,” Energy research Centre of the Netherlands (ECN) Technical Report, ECN-E—11-0017, December 2011.
8. NWTC Design Codes (HARP_Opt by Danny C. Sale). http://wind.nrel.gov/designcodes/simulators/HARP_Opt/. Last modified 28-Feb-2013; accessed 1-June-2013.

Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.