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Woodhead Publishing Series in Energy Advances in Wind Turbine Blade Design and Materials Second Edition Edited by Povl Brøndsted Department of Wind Energy, Technical University of Denmark, Roskilde, Denmark Rogier Nijssen Inholland University of Applied Sciences, Alkmaar, The Netherlands Stergios Goutianos Department of Manufacturing and Civil Engineering, Norwegian University of Science and Technology, Gjøvik, Norway WoodheadPublishingisanimprintofElsevier 50HampshireStreet,5thFloor,Cambridge,MA 02139,UnitedStates TheBoulevard,LangfordLane,Kidlington,OX51GB,UnitedKingdom Copyright©2023 ElsevierLtd.Allrights reserved. 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Bacharoudis Offshore Renewable Energy Catapult, Blyth, United Kingdom Christian Bak Poul la Cour Tunnel, DTU Wind and Energy Systems, Technical University of Denmark, Lyngby, Denmark M.S. Borst Knowledge Centre WMC, Wieringerwerf, the Netherlands Kim Branner Department of Wind and Energy Systems, Technical University of Denmark, Lyngby, Denmark P. Brøndsted Technical University of Denmark, Lyngby, Denmark P.D. Clausen School of Engineering, University of Newcastle, Newcastle, NSW, Australia S.P. Evans School of Engineering, University of Newcastle, Newcastle, NSW, Australia J.J. Heijdra Knowledge Centre WMC, Wieringerwerf, the Netherlands J.G. Holierhoek Wind Energy, JEHO BV, Rotterdam, the Netherlands Find Mølholt Jensen Bladena, Taastrup, Denmark B. Kjærside Storm Aalborg University, Denmark D.J. Lekou Wind Energy Section, Centre for Renewable Energy Sources & Saving (CRES), Pikermi, Greece T. Løgstrup Andersen Technical University of Denmark, Lyngby, Denmark B. Madsen Technical University of Denmark, Lyngby, Denmark J.F. Mandell Montana State University, Bozeman, MT, United States xiii xiv Contributors D.A. Miller Montana State University, Bozeman, MT, United States Leon Mishnaevsky, Jr. Department of Wind Energy, Technical University of Denmark, Lyngby, Denmark R.P.L. Nijssen Knowledge Centre Wind Turbine Materials and Constructions, Wieringerwerf, the Netherlands D.D. Samborsky Montana State University, Bozeman, MT, United States Holger Söker UL Solutions, Wind Certification & Testing, UL International GmbH, Oldenburg, Germany W.A. Timmer Delft University of Technology, Delft, the Netherlands D.R.V. Van Delft Knowledge Centre WMC, Wieringerwerf, the Netherlands Anastasios P. Vassilopoulos Ecole Polytechnique Fédérale de Lausanne (EPFL) School of Architecture, Civil and Environmental Engineering (ENAC) Composite Construction Laboratory (CCLab), Lausanne, Switzerland D.H. Wood Schulich School of Engineering, University of Calgary, Calgary, AB, Canada 1 Introduction to wind turbine blade design Find Mølholt Jensen1, Kim Branner2 1BLADENA, TAASTRUP, DENMARK; 2DEPARTMENT OF WIND AND ENERGY SYSTEMS, TECHNICAL UNIVERSITY OF DENMARK, LYNGBY, DENMARK 1.1 Introduction Wind turbines have grown substantially in size over the years since commercial wind turbineswereintroduced around 1980. Aftera few years during which the focus was on increased reliability, we once again (2021) see growth in the size of wind turbines. Using normal scaling laws, the weight of wind turbine blades should increase with length to the power of three. However, historically, according to Fig. 1.1, blade weight has only increased to the power of 2.5, as blade manufacturers have successfully improvedtheaerodynamicperformanceandcontrolofthewindturbines,aswellasthe structural design, and have optimized the use of materials and process technology. FIGURE 1.1 Comparison of actual and theoretical blade weight scaling for increasing length. Data is obtained throughpublicinformationavailableonline. AdvancesinWindTurbineBladeDesignandMaterials.https://doi.org/10.1016/B978-0-08-103007-3.00009-4 3 Copyright©2023ElsevierLtd.Allrightsreserved. 4 Advances in Wind Turbine Blade Design and Materials Wind turbine blades are now so large that gravity and inertia loads have started to dominate more than the aerodynamic loads. It is therefore of increasing importance to reduce weight. ResearchcarriedoutattheDepartmentofWindandEnergySystemsattheTechnical UniversityofDenmark(DTUWind)onwindturbinebladeshasshownthattheclassical failuremechanismssuchasbuckling,materialfailure,etc.,arenotenoughtodetermine the design of the blades. Other failure mechanisms need to be taken into account. One mechanismwhichmayleadtofailureiscross-sectionalshear.Thismechanismhasbeen demonstrated in full-scale tests and is not covered by type certification tests. Another mechanismisthenonlinearout-of-planedeformationoftheload-carryingcaplaminate, which could be the reason for some adhesive joint failures experienced in wind turbine blades today. 1.1.1 State of the artdBlade design The design of a wind turbine blade is a compromise between aerodynamic and structural considerations. Aerodynamic considerations are usually dominating the design of theoutertwo-thirdsoftheblade,whilestructuralconsiderationsaremoreimportantfor the design of the inner one-third of the blade. Structurally the blade is typically hollow with the outer geometry formed by two shells:oneonthesuctionsideandoneonthepressureside.Oneormorestructuralwebs arefittedtojointhetwoshellstogetherandtotransfershearloads,seeFigs.1.2and1.3. A load-carrying box girder is used in older blade designs, however in the past 10e15 years it has been replaced by load-carrying shells, see Fig. 1.4. The load-carrying shells’ design has been the dominant design in the blade design process since its imple- mentation. Such designs correspond to blades with one, two or three webs including sparcaps.Inthelongitudinaldirection,thebladesaretaperedandtwisted.Thetapering is required to compromise for the increasing loads from tip to root as well as for cost- effective reasons. Additionally, the tapering is designed to ensure the optimal lift and reduced drag at the faster moving tip. FIGURE1.2 Sketchofabladeconcept.Blade designwithoneshearweb. Chapter 1 (cid:1) Introduction to wind turbine blade design 5 FIGURE1.3 Bladedesignswithtwoshearwebs. FIGURE1.4 Bladedesignwithaload-carryingshell. In Fig. 1.5, two older blade designs from different manufacturers (LM Wind Power andVestasWindSystems)areshown.InthebladefromLMWindPoweranuppershell, alowershellandtwowebsarebondedtogethertoformthebladestructureasshownin Fig. 1.5b. InFig.1.6rib/bulkheadsolutionsfromtwodifferentblademanufacturersareshown. Thebladedesignfrom1948,showninFig.1.6,wasusedina200-footdiameterwind turbine which was the first to implement ribs in a wind turbine blade. The blade was manufacturedbyplywoodwithribsofstainlesssteelandrevealsquiteafewsimilarities to an aircraft wing design. Current blade-manufacturing technology based on thermo- settingcompositesisnotsuitableforproducingrib-reinforcedbladesinaneconomically soundmanner,seeref(Rijswijk,2007;Rijswijketal.,2006).However,rib/bulkheadblade designusingathermoplasticwithareactiveprocessingtogetherwithvacuuminfusionis currently under development, see ref. (Rijswijk, 2007; Rijswijk et al., 2006). Today, there are limitations in the melt processing and consequently the size and the thickness is 6 Advances in Wind Turbine Blade Design and Materials (a) Vestas design (b) LM Wind Power design FIGURE1.5 Differentwindturbinebladedesigns.(a)Vestasdesign,(b)LMWindPowerdesign. FIGURE 1.6 Ribs/Bulkheads used in wind turbine blades. (a) Photo of a 200-foot diameter wind turbine from UnitedStatesPlywoodCorporation.(b)PhotoofaTvindwindturbineblade. limited.However,ifcurrentresearchinmaterialstechnologyissuccessful,ribs/bulkhead design could be reintroduced in the wind turbine blades, see ref (Joncas et al., 2004). Other types of internal structures are considered today. DTU Wind had designed, in 2010, a load-carrying box using different structural members inside the box girder. By means of these additional members the thickness of the load-carrying laminates was decreasedby40%andtheboxwaspreventedfromdistortinginthetransversedirection. With increased bending moments due to increase blade lengths, the blade root re- quiressomestructuralreinforcement.Therefore,thebladedesignershaveshiftedtoward using thicker airfoils on the inwards region of the blade. The same design does not continuetowardtherestofthebladefortworeasons.Firstly,applyinguniformlythicker airfoilsalongthebladewouldrequiremorematerial,thusincreasingtheweightfurther, andsecondly,byintroducingthickerairfoilsthedraglevelsofthebladewouldincrease, thus reducing the aerodynamic performance of the blade, leading to reduced annual energy production. Chapter 1 (cid:1) Introduction to wind turbine blade design 7 The need for thicker airfoils has made the industry to shift to different airfoil family thantheoneusedadecadeago.TheNACAairfoilshavebeenreplacedbymodernairfoil families,whichareknowntohavealargermaxliftcoefficientthantheNACA,thusbeing more suitable for thick airfoils (Fuglsang et al., 1998). In an attempt to minimize the structural challenge in the root-transition zone, the blade designers have adapted to a slenderer design in wind turbine blades, in order to makethebladestiffer.However,byincorporatingacylindricalshapetotheblade,more resistance is created than aerodynamic lift. A counter measure to this aerodynamic loss istofitaspoilertocreatealiftforcesimilartothatcreatedbyairfoils.Vortexgenerators comprising of multiple small fins and mounted at the lowest part of the airfoil can also boost blade performance. Furthermore, this slender blade profile has encouraged blade designers to draw bladeswithdecreasedmaxchordandthereforeincreasedmaxchordregions,inorderto mitigate the increased edgewise bending moments. Moreover, theblade designers haveadopted theinclusion of blunttrailingedge airfoils, also known as flatbacks. By incorporating flatback trailing edge, the blade is easier to transportandmanufacturewithoutsacrificingaerodynamicperformance(Kahnetal.,2008). Theincreasedflapwisebendingmomenthasintroducedtheissueoftowerclearance. Thebladeshavebecomesolongthatwhendeflectinginaflapwisedirection,theremust beenoughdistancebetweenthebladetipandthetower,sothat acatastrophiceventof collapse is avoided. For this reason, the blades are designed with a prebend. One recent innovative blade design is the swept blade, which deflects during oper- ation.Withsweep,adesignrotordiametercanincrease,capturingmorepower,withthe loadsremainingwithinlimits(Larwoodetal.,2014).Adisadvantageisthatasweptblade has more torsional loads. 1.2 Design principles and failure mechanisms In the following section a number of failure mechanisms will be presented. 1.2.1 Design principles Current wind turbine blades are generally not fully optimized with regards to structural strength. Therefore, large differences can be expected in the safety analysis against various types of failure modes. The safety against different failure modes including material strength for in-plane loads, material strength for out-of-plane loads, structural and buckling failures in blades is schematic illustrated in Fig. 1.7. While material in- plane strength is significantly high, as explained below, composite materials are weak with regards to out-of-plane strength, which is illustrated by the thickness of the respectivelinks.Bucklingandstructuralstrengthhavebeenseparatedeventhoughboth relatetothestructuralaspect.Thisisdoneinordertoillustratethedifferenceinunused capacity.