Structural reuse of wind turbine blades through segmentation

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Delft University of Technology

Structural reuse of wind turbine blades through segmentation

Joustra, J.J.; Flipsen, Bas; Balkenende, A.R.

DOI

https://doi.org/10.1016/j.jcomc.2021.100137

Publication date

2021

Document Version

Final published version

Published in

Composites Part C

Citation (APA)

Joustra, J. J., Flipsen, B., & Balkenende, A. R. (2021). Structural reuse of wind turbine blades through

segmentation. Composites Part C, 5, [100137]. https://doi.org/10.1016/j.jcomc.2021.100137

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ContentslistsavailableatScienceDirect

Composites

Part

C:

Open

Access

journalhomepage:www.elsevier.com/locate/jcomc

Structural

reuse

of

wind

turbine

blades

through

segmentation

Jelle

Joustra

∗

_,

_Bas

_Flipsen

_,

_Ruud

_Balkenende

Faculty of Industrial Design Engineering, Delft University of Technology, Delft 2628 CE, the Netherlands

a r t i c l e

i n f o

Keywords: Composite materials Circular economy Recovery Structural reuse Design Design strategies Wind turbine blades

a b s t r a c t

Compositematerialsoffermanyadvantagesduringtheusephase,butrecoveryattheendofalifecycleremains achallenge.Structuralreuse,whereanendoflifeproductissegmentedintoconstructionelements,maybea promisingalternative.However,compositesareoftenusedinlarge,complexshapedproductswithoptimised materialcompositionsthatcomplicatereuse.Asystematicapproachisneededtoaddressthesechallengesand thescaleofprocessing.Weinvestigatedstructuralreusetakingwindturbinebladesasacaseproduct.Anew segmentationapproachwasdevelopedandappliedtoareferenceblademodel.Therecoveredconstruction ele-mentswerefoundtocomplytogeometricconstructionstandardsandtooutperformconventionalconstruction materialsonspecificflexuralstiffnessandflexuralstrength.Finally,weexploredthereuseoftheseconstruction elementsinpractice.Together,thesegmentationapproach,structuralanalysisandpracticalapplicationprovide insightsintodesignaspectsthatenablestructuralreuse.

1. Introduction

Compositematerials,knownfortheirlightweightproperties,are of-tenusedtomakeproductsmoresustainable.Lightweightdesignsreduce fuelconsumptionintransportapplications,andtherebyeﬀectively re-ducethecarbonfootprint[1].Lightweightingalsoallowseﬃcient ma-terialuse,andmakeslargespansinbuildingandarchitectural appli-cationspossible[2].Compositesareusedtomaximiseperformanceof these structures.However,whenthecomplete lifecycleistaken into account,theenvironmentaladvantageofusingcompositematerials be-comeslessevident[3,4].

ThelifecycleperspectiveiscentraltotheCircularEconomy(CE) con-cept.TheCEaimstopreserveresourcesbykeepingproductsand mate-rials‘intheloop’.Thiscanbedonethroughextendingproductlifetime andrecoveringproducts,componentsandmaterialswhentheyreach theirendofoperationallife[5].Maintainingproductintegrity,through e.g.reuse,repairorremanufacturing,isconsideredmostdesirable. Ma-terialintegrity,i.e.recyclingofmaterial,isanecessitywhenproducts cannolongerbekeptalive.Preferably,recyclingretainsmaterial prop-ertiesandavoidsdowngrading[6,7].

Compositematerialsenablelongproductlifespansandrequirelittle maintenance.Highqualityrepairscanbemadeinsitu:restoringoriginal strengthandappearance[1,8].Reuseatproductlevelismorediﬃcult becausethematerialcompositionisoftenoptimisedtoaspeciﬁc ap-plication.Thismaximisestheperformanceintheusephase,but compli-catesreuseinanothercontext.Forexample,windturbinebladescannot

Abbreviations: CE,CircularEconomy;EoL,EndofLife;GFRP,GlassFibreReinforcedPlastics;UD,Uni-Directional;DB,Double-Bias.

∗_{Corresponding}_author.

E-mail address: j.j.joustra@tudelft.nl(J.Joustra).

readilybeexchangedbetweenwindturbines.Consequently,material re-cyclingremainsastheonlyrecoveryoption.

Recycling composite materials is challenging due to the way in whichvariousmaterialsarestructurallycombinedatasub-millimetre scale.Thermosetresinsandglassfibres,GlassFibreReinforcedPolymers (GFRP),constitutethemajorityofcompositematerialsintoday’smarket [9].Forthesematerials,co-firinginacementkilnremainstheadvised recoveryroute[10].However,theenergygainislow,thematerialislost forfurtheruseandtheeconomicperspectiveislimited[11–13].Thus, muchofthematerialislandfilled,resultinginalossofmaterialsand value;andassuchlandfillingisatthebottomofthewastemanagement hierarchy.Topreventsuchloss,landfillingofcomposites hasalready beenprohibitedinanumberofcountries[14,15].

Therearevariousexplorationsintocircularsystemsforcomposite materials.Forexample,currentresearchprogramsinclude demonstra-torsforcircularcompositeproducts[16,17]andrecoveryofEndofLife (EoL)windturbineblades[18].Moreoveranumberofcompanieshave developednewreprocessingmethodstoclosethecompositematerial loop[19–22].Atagovernmentallevel,theincreaseduseofcomposite materialsislikelytoleadtonewpolicytargetingrecycling[23–25].In themeantime,compositeproductsnearingtheirEoLpresenta press-ingproblem.Currentrecyclingcapacityisinsuﬃcientwhileincoming decadesthecompositewastevolumewillincreasestrongly[26,27]. Bet-tersolutionstodealwithEoLcompositeproductsarethereforeurgently required.

Structuralreuse,alsoreferredtoasstructuralrecycling,isan attrac-tivealternativesolutionforEoLcompositematerials[28–32].Rather

https://doi.org/10.1016/j.jcomc.2021.100137

Received22September2020;Receivedinrevisedform1March2021;Accepted18March2021

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Fig. 1.Schematicrepresentationof NREL5MWblade, showingairfoilsandmainsections[36,40].

thanshreddingtheproductandattemptingtoseparatereinforcements fromthematrix,asisdoneincurrentrecyclingprocesses,thecomposite isreusedasastructuralmaterial.Comparedtocurrentrecycling prac-tices,structuralreuserequiresrelativelylittlereprocessingeﬀortand, toalargeextent,retainsthematerialquality[29].Assuchitisa com-pellingalternativeroutetorecapturevalueandextendthelifetimeof thematerial.Reusecanbedonebydirectlyharvestinglargepartsorby cuttingconstructionelementsfromtheEoLproduct.

Structuralreusehasbeendemonstratedforwindturbineblades,see e.g.[29]foranoverview.Bladesareinterestingobjectsforthisreuse ap-proachastheyretainhighstructuralquality,evenafter20yearsofuse. Moreover,bladesconsistofmultiplematerialsandlayuptypes,which canbereusedinmanydiﬀerentapplications.

Largestructuralpartshavebeenrepurposedforexampleforoutdoor applicationssuchasstreetfurnitureandaplayground[33].However thispracticeis regardedasbeingdiﬃculttoupscale.Thelarge size, complexshapeandcomplexmaterialcompositionallrestrictreuse op-portunities[34].Itisexpectedthatcutting-uptheselargestructuresinto practicalandusableconstructionelementslikebeamsandpanelswill diversifythepotentialapplications[28].

Inanearlierdesignstudyweexploredthereuseofconstruction ele-mentsfromawindturbineblade[35].Wefoundstructuralreusetobe feasible,butnewsegmentationapproachesneedtobeemployedtodeal withtheproduct’scomplexshapeandstructure.Inaddition,weexpect thattheyieldofreusableconstructionelementsfromabladecanbe higherand,withagoodpatterningapproach,thereuseprocesscanbe mademoreeﬃcient.Thisgaverisetothefollowingquestions concern-ingstructuralreuseofcompositeproduct,whichareaddressedinthis paper:

• Howtodetermineasegmentationpatterntoobtainreusable con-structionelements?

• Howtocomparestructuralquality ofrecovered construction ele-mentstoconventionalconstructionmaterials?

• Whichdesignaspectsenableorlimitstructuralreuse?

2. Materialsandmethods

Forthisstudy,wetookwindturbinesbladesascaseproductasthese representarealandpressingrecyclingproblem.Westudiedstructural reuseofcompositematerialsfromawindturbinebladeusingthe fol-lowingapproach.First,segmentationpatternsforconstructionelements weredeterminedbasedonthestructuralandgeometricspeciﬁcationsof thewindturbineblade.Then,thestructuralpropertiesofthese construc-tionelementswereevaluatedandcomparedtoconventional construc-tionmaterials.Totestthereuseapproach,arelativelysimpleproduct wasmadefromretrievedpanels.Observationsmadeduringthisprocess werethenrelatedtodesigninsights.

2.1. Materials

Areferenceblademodelwasusedtoanalysebladedesignandto determinesegmentationpatternsandstructuralpropertiesofrecovered materials.TheblademodelwasdevelopedbytheNationalRenewable EnergyLaboratory(NREL)andSandiaNationalLaboratories,basedon astudybytheDutchOffshoreWindEnergyConverter(DOWEC)project [36–38].Thismodelwasused,becauseincontrasttocommercialblades, thisbladewasdevelopedforresearchpurposesanditsspecificationsare publiclyavailable[39].Theblademeasures61.5minlengthandwas designedfora5MWturbine.Turbinesofthissizearefoundboth on-shoreandoffshore,whichmakesitrepresentativeforcurrent installa-tions[27,37].

The blade consists of three sections from root to tip: inboard, midspanandoutboard (Fig.1) [36]. Thelargestbendingmomentis exertedontheinboardsection,wherethebladeisjoinedtotheturbine axis.Thissectionstartsatrotorradiusr=1.8m(takinghub diame-terintoaccount).Itisaplaincylinderwithawallthicknessof61mm, madeof asolidglass fibrereinforcedepoxy laminatewitha triaxial layup.Themidspanstartsatr=10mandendsatr=54.5m.Theshells, madewithasandwichlayup,taperfrom100to25mmthickness.The sparcapstaperfrom48to20mm.ThelayupconsistsoftriaxialGFRP skins,foamcoreandglassfibreaswellascarbonfibreUD reinforce-ments.Themidspansectioncomprisessixairfoilprofiles,fiveofwhich wereselectedfromtheDelftUniversity(DU)systematicairfoilseries. Theaerodynamicprofiletaperstowardsthetiptomeetaerodynamic andstructuralrequirements.Theoutboard(tip)sectionhasarelatively flatairfoilprofilebecauseithastocopewithhighairspeeds.In com-mercialblades,thissectionisoftenpre-benttopreventcollisionwith thetowerwhenthebladedeflectsunderload.

Thestructuralandaerodynamicperformanceprimarily determine thedesign(Fig.2)[34,37].Thesparcaps(3)andshearwebs(4)make upthemainstructuralelementsofthebladeandfunctionasaboxbeam toprovidelongitudinalstiﬀness.Thepanelsoftheleadingedge(2)and trailingedge(5)givethebladeitsaerodynamicshape.Thetrailingedge hasadditionalreinforcements(6)toalleviateedge-wisebending mo-ments. Thebladetop andbottomshellareproducedseparately and joinedattheleadingedge(1)andtrailingedge(7),aswellasontop andbottomoftheshearwebs(4).Thepanels(2,5,6)andshearweb (4)aremadewithasandwichstructure.Sparcaps(3)aremadewitha monolithiccarbonﬁbrelaminateandcoveredwiththesameGFRPface laminateasthepanels.

Weretrievedmaterialpropertiesforcalculatingthestructural char-acteristicsofbladesegmentsfromtheoriginalbladedesign specifica-tions[37].Specificationsmissingfromthedesignreportwere supple-mentedwithvaluesfromequivalentmaterialsintheCESEdupacklevel 3database[41].Table1 liststhematerialsandspecifications.Density

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Fig.2. Cross-sectionalproﬁleofawindturbineblade, showingpartsandstructuraldesign.

Table1

Propertiesofmaterialsusedin5MWblade[37]usedtocalculatemechanicalpropertiesofrecoveredconstruction elements.Valuesmarked(a)aresupplementedfromCESdatabase,(b)calculatedfrommaterialdatasheets.

Material E-modulus Shear modulus Poisson’s ratio Density Tensile strength Compressive strength

[MPa] [MPa] [-] [Kg/mˆ3] [MPa] [MPa]

GFRP UD 41,800 2630 0.28 1920 972 702

GFRP Triax 27,700 7200 0.39 1850 700 292 a

GFRP DB 13,600 11,800 0.49 1780 144 a 213

Foam 256 22 0.3 200 5.6 a 4.4 a

CFRP UD 114,500 5990 0.27 1545 b 1546 1047

ofcarbonﬁbreUDwascalculatedusingfromthematerialdatasheets usingtheruleofmixtures[42–44].

2.2. Methods

Weinvestigatedstructuralreusebydevelopingasegmentation pat-tern,analysingthestructural performanceof theretrievedelements, andexploringtheirapplicationinpractice.Usingthesegmentation ap-proach,wecanexplorevariouscuttingpatternsandcalculatehow eﬀec-tivetheyareindeliveringreusableconstructionelements.Thestructural analysisallowedacomparisonoftheretrievedelementswith conven-tionalconstructionmaterials.Exploringtheapplicationofreused con-structionelementsgaveinsightintoitspracticalfeasibilityandtherole ofdesign.

2.2.1. Segmentationpatterns

TheNREL5MWmodelwasanalysedforrecoveryofconstruction el-ementsbyevaluatingthestructureandform.TheNuMADwindturbine bladedesigntool[45],wasusedtocalculatetheweightofindividual partslikeleadingedgepanelsandsparcaps.Thecalculatedproperties wereveriﬁedwithdistributedbladepropertiesprovidedbySandia[37]. Inaddition,aphysicaldecommissionedbladewasinspectedto investi-gatepracticalimplicationsofconstructionandrecoverywhichwerenot addressedinthedesignreport[37].

Thesuccessionofaerodynamicprofilesalongthebladelength indi-cateswherethecross-sectionalprofileisconstantorwhereshape tran-sitionsoccur.Changesinpitchandchordlengthindicatethetwistand taperingofthebladesurface.Thetwistisconstantforthemajorityofthe bladelength,butthecross-sectionalcurvatureneededtobecalculated. Wecalculatedtheclosestdistancebetweenapointandalineusing vectorcalculus[46].Thecalculationstartedwithasetof3points:start pointA, endpointBandintermediatepointC(Fig.3a).Here,length ABcorrespondstothesegmentwidthwandthemaximum perpendicu-lardistancefromlineABtoapointConthecurveABcorrespondsto segmentdeflectiond.Theobjectiveofthefunctionwastoachievethe largestpossiblesegmentwidthwforagivencurvatured/wordeflection

d(Fig.3b).

Weappliedtwosegmentationapproachestodeterminepanel seg-mentationpatternsbasedoncurvatureanddeflection(Table2).The firstapproachisgovernedbyd/wanddeliverspanelswithequal cur-vature[47].Thesecondapproachisgovernedbydanddeliverspanels withequalsegmentdeflection.Thecuttingpatternisalignedwiththe blade’slongitudinalaxis,perpendiculartotheairfoilsection.

Fig.3. Calculatingpaneldeﬂection d betweenpointsAandBontheairfoil proﬁle.

Table2

Boundaryconditionsfordimensionaldeviationofacurvedconstruction ele-ment,basedonNEN5461timberstandards[47].

Dimensional deviation NEN 5461 Curvature d/w Deﬂection d [m] Small d/w < 0.02 d/w < 0.02 d < 0.02 Medium 0.02 < d/w < 0.04 d/w < 0.04 d < 0.04 Large d/w > 0.04 d/w < 0.08 d < 0.08

The dimensional standards for construction timber wereused as boundaryconditionsasstandardshavenotyetbeenestablishedforthe recoveryofcompositematerials.Timberelementshapesdepend,like therecoveredcompositesegments,onraw materialshapeaswell as prospectiveapplicationareas.ThetolerancesaregiveninTable2.The goalofthesegmentationwastoobtainpanelswithawidthand curva-turesuitableforreuseasconstructionmaterial.

Thecross-sectionalsegmentshapealsodependsonthecutting an-gle,forexampleperpendiculartothelocalbladesurface,airfoilchord or panelchord.Althoughthisaffectsthecross-sectionalshapeof the resulting panels, calculation showsminimal effects oncutting losses (<<1wt%). Furthermore,thealignmentdoesnot affect segmentation patternsormaterialperformance,andisthereforenotfurtherdetailed intheseanalyses.Inpractice,thealignmentwilldependonthe process-ingcontext(i.e.cuttingtoolsandhandlingequipment)andintended panelreuseapplications.

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2.2.2. Structuralproperties

Thematerialpropertiesoftherecoveredelementswerecalculated andcomparedtoconventionalmaterials.Thegoalwastoevaluatethe material’sperformanceandidentifypotentialapplicationareas.The ma-terialswerecomparedusingthelevel3databaseofGrantaCESEdupack 2019[41].Propertiesoftherecoveredmaterialswerecalculatedusing theGrantaCESHybridSynthesizerandthebladedesignspecifications giveninTable1.Thesandwichmaterialmodelwasusedforallpartsand properties,exceptforthedensity(𝜌)andflexuralmodulus(Eflex)ofthe

trailingedgereinforcements.There,themultilayermodelwasusedto accountfortheadditionalUDlayers.Thecalculationusedadistributed loadconditionandasegmentlengthof4.1m,whichcorrespondstothe spacingbetweenconsecutiveaerodynamicproﬁlesintheblademodel.

Toevaluatestructuralqualityatendofuse,weconsideredarangeof materialproperties,ratherthanasinglevalue.Fortheminimumvalue, weassumedthebladescanstilloperateunderthedesignloadcaseatthe pointofdecommissioning.Forthemaximumvalueweusedtheoriginal designspecifications,whichincludedadditionalsafetyfactors.Thusto gettheminimumvalues,theoriginaldesignspecificationsforstiffness andstrengthweredividedbytheirrespectivesafetyfactors.Toreflect thebladedesignspecifications,weusedthesafetyfactorsasstatedinthe originaldesignreport:1.485forstiffnessand1.755forstrength[37]. Thematerialdensityremainsconstantalongtheproductlifespan.

Thecalculatedvalueswerethenplottedonmaterialpropertycharts toenablecomparisonwithconventionalconstructionmaterials.The ma-terialswerecomparedbasedondensity(𝜌),ﬂexuralmodulus(Eﬂex)and

ﬂexuralstrength (𝜎ﬂex)[44].Theseproperties combinedindicatethe

performanceof thematerialsforlightweight constructions loadedin bending[44].

2.2.3. Application

Toexploretheimplicationsofstructuralreuseinpracticeandthe roleofdesign,weconductedadesignstudyonadecommissionedwind turbineblade[35].Panelsfromabladewereusedtodesignasimple fur-nitureproduct,whichwassubsequentlybuiltandevaluated.Thestudy followedaresearchthroughdesignapproach[48,49]whichprovided richdataonrecovery,designandmanufacturing,aswellasonuser ac-ceptanceoftheresultingconstructionmaterials.Thedesignandreuseof thebladewereevaluatedusingapreliminarysetofdesignaspects[32]. Together,thesegmentationapproach,structuralanalysisandpractical applicationprovidedinsightsintodesignaspectsthatenablestructural reuse.

3. Resultsanddiscussion

Thestructuralreuseofcompositepartswasevaluatedusinga refer-encewindturbineblade.Thesegmentationapproachstartsbyassessing theproductshapeandstructure,followedbyamoredetailedapproach, whichtakeslocalcurvatureintoaccount.Thesegmentsarethen evalu-atedfortheirstructuralperformance.Thesegmentationandstructural evaluationprovideinsightintodesignaspectsthatfacilitatestructural reuse.

3.1. Segmentationpatterns

Theblademidspansection,whichcomprisesnearlythree-quartersof thebladelength,oﬀersthebestopportunitiestoretrievecontinuously shapedconstructionelements.Inthissection,allproﬁlesareselected from thesamesystematicairfoilseries.Thelineardecreaseof chord length(3cm/m)andtwistangle(0.25°/m)indicatecontinuous taper-ingandtwistofthebladetowardsitstip(Fig.4).Together,theseform factorsallowforsmoothshapetransitionsalongthebladelength.Thus, constructionelementsrecoveredfromthemidspansectionwillhave rel-ativelystraightshapes,despitetheiraerodynamicorigins.

Toﬁndthetypesofconstructionelementsandtheirproperties,the bladestructurewasreconstructedforSection8,asshowninFig.6 and Table3.Thissectionispositionedinthemiddleoftheblade,atradius r=28.15m(Fig.5).Here,airfoilDU25-A17isusedwithachordlength of4.01m.Twocompositelayupstructureswereused:asandwichlayup forthepanelsandshearwebs(2,4,5and6),andamonolithic lami-nateforthesparcaps(3)aswellasbondingareasatLeadingEdgeand TrailingEdge(1and8).Thisstructuraldesignprovidesastartingpoint forsegmentation.

Asastartingpointforthepatterning,thebladecross-sectioncanbe dividedintotwotypesofconstructionelements:panels(63wt%)and beams(33wt%).Panelscanberecoveredfromtheleadingedgepanels, shearwebs,trailingedgepanelsandreinforcedtrailingedge(2,4,5 and6).Beamsarefoundin thespar caps(3).Alternatively,thespar capsandshearwebscanberetrievedas-is,tobereusedasbox-beam. Inthisstudy,wechosetotakethemasseparateparts,asthispermitsa clearerstructuralcomparison.

Somecuttinglosseswilloccur,causedbytheadhesivebondingareas (3wt%),taperingofthepanelsduetodecreasingchordlengths(1wt%) andprocessing(minimal).Theadhesivebondsattheleadingedgeand trailingedge(1and8)obstructrecoveryofconstructionelements.These

Fig.4. Twistangle[°]andchordlength[m]of theairfoilproﬁlesalongthebladelength.

Table3

Parts,weight,weightpercentage,structureandconstructionelementtypesfoundinbladesegment8.

# Part Materials Mass [kg] Weight [wt. %] Element type

1 Leading Edge Solid GFRP, adhesive 17 2% None

2 Leading Edge Panels Sandwich 151 14% Panel

3 Spar caps Solid GFRP & CFRP 340 33% Beam

4 Shear webs Sandwich 132 13% Panel

5 Trailing Edge Panels Sandwich 304 29% Panel

6 Reinforced Trailing Edges Sandwich 73 7% Panel

7 Cutting losses (tapering) Sandwich 16 1% None

8 Trailing Edge Solid GFRP, adhesive 9 1% None

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Fig.5. Sketchofthebladeanditssections.Cross-Section8isusedforfurther analysis.

Fig.6. Structuralsegmentationoftheblade.

partshaveamixedmaterialscomposition,astrongcurvature,andthe structuretransitionsfromsandwichtomonolithic.Moreover,physical inspectionofadecommissionedbladerevealedpoorlydefinedbonding areasandabundantlyappliedadhesives,whichchallengesthe recov-eryofuniformmaterials.Taperingofthebladeresultsfromdecreasing airfoilchordlengthsandcausestriangularshapedoffcuts(7).Forthe weightcalculation,weassumedthesetobedeductedfromthetrailing edgereinforcements.Theprocessinglosseswerefoundtobenegligible; thewaterjetcutterusedintheapplicationtesthadajetdiameterof0.7 mm,andthusminimalcutlosses.Assuch,thesewerenotfurthertaken intoaccount.Still,evenafterexcludingthesebondingareasandoffcuts, 96%ofthissectioncouldbecutintoreusableconstructionelements.

However,itisnotrealistictoassumethat96%ofthecompleteblade canbereused;thisestimateisbasedonaprofileintheblademidspan, whichconstitutes58%ofthecompleteblademass.Thebladeroot(40 wt%)andtip(2wt%)cannotbedirectlysegmentedintoconstruction elements.Therootisacylinderwithanaveragediameterof4mand lengthof10m,itischallengingtocutduetoitsthickandsolidGFRP walls.Thebladetip ismadeofrelativelyflatairfoilsbutispre-bent topreventtowercollision.Thispre-bendaddstotheshapecomplexity andtherebycomplicatessegmentationandreuse.Thus,focusingonthe blademidspan,andaccountingforoffcuts,weexpectupto55wt%of thebladecanbesegmentedintoconstructionelements.

Theoriginaldesigndetermineswhatkindofconstructionelements can be recoveredintermsofsize, shapeandstructurallayup.These propertiesgraduallytapertowardsthetipofthebladewhichleadstoa largedistributionofpropertiesoftherecoverableconstructionelements. Inadditiontopropertiesimposedbytheoriginaldesign,thereuse ap-plicationcanalsopresentdesignrequirements.Theserequirementsare usuallydeﬁnedintermsofsize,mass,stiﬀnessandstrength, accompa-niedbytolerancesandsafetyfactors.Thesecanthenbeusedas bound-aryconditionsforthesegmentationpattern.Toextendreuse opportu-nitiesbeyondasingleproduct,weusedconstructionindustrystandards forthispurpose[47].

Thesparcaps(3)andshearwebs(4)arepositioneddirectlyabove and aside from the blade reference axis, which is aligned with the maximum airfoil thickness. Along the midspan, the spar caps have a constantwidth of 0.6m.The shapeof these elements is predom-inantly determined bybladetwist andlayup thickness.Abeam, cut from thespar cap willtwist 0.002m permetrelength, which corre-sponds toa“verysmall” dimensionaldeviation[47].Thus,beam el-ementscanberecovereddirectlyfromthesparcap.Thesurface pan-els(2,5and6)however,haveamorecomplexdouble-curvedshape andneedfurtherassessment.Table4 andTable5 showtheresultsfor segmentationusingthetwoboundaryconditions, basedoncurvature

Table4

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Table5

Segmentationpatternsformaximumpaneldeﬂectiond.

d/wanddeﬂectiond,fortolerancecriteriasmall,medium andlarge, respectively.

Table 4 shows the segmentation patterns and resulting segment widthsforbladeSection8withd/w<0.02,0.04and0.08respectively.

d/w<0.02resultsin13segments,ranginginsizefrom0.08to2.0m.The stronglycurvedleadingedgesectionisdividedinto9panelswithwidths under0.5m.Inthetrailingedgesection,weﬁndtwolargepanelsof1.5 and2.0mandtwosmallersegments.Thisclearlyshowshowcurvature aﬀectssegmentwidth:astrongcurvaturemakesfornarrowsegments.

d/w<0.04resultsin8segments,withfoursmallpanelsaround0.3m wide,twomedium-sizedof0.6and0.8andtwolargepanelsof1.8and 2.0m.Ford/w<0.8wefound2panelsofabout1.2mandthetrailing edgedividesintotwopanelsof2m.Unexpectedly,thispatternalso de-liveredthenarrowestsegmentfortheseboundaryconditions,just0.08 mwide.

Suchanarrow segmentis theresultofthechosenpatterning ap-proach.Theleadingedgepanelisnowdividedinapanelwitha curva-tureof0.08andasmall“leftover” piece.Thisindicatesanopportunity toimproveonthesegmentationpattern;bynotgoingforthemaximum possiblewidth,theleadingedgepanelcouldbedividedintotwoormore smallerpanelswithalowercurvature.Thisisshowninthecutting pat-ternsford/w<0.04andd/w0.02.

Aswastobeexpectedfrom theairfoilcurvature,thenarrow seg-mentsarefoundat theleadingedge,andthewidersegmentsatthe trailingedgepanels.Themaximumwidthremainedthesameforall tol-erancebounds,asthispartisconﬁnedbetweenthesparcapandtrailing edgebondingarea.Wherethelowertolerancecriterionresultsinaset ofdistributedwidths,theuppertolerancelevelresultsina segmenta-tionpatternthatalmostdirectlyfollowsfromremovingsparcapsand bondingareas.However,thisincreasedwidthcomesatacost;thewide

panelshavealargecurvaturewhichmaylimitapplicationareaswhere thepanelscanbereused.

The boundary condition on deﬂection d showed similar results. Table5showsthesegmentationpatternsforSection8withamaximum deﬂectiondof0.02,0.04and0.08m.d<0.02mresultedinapattern of11segments,mostofwhichwerebeloworjustabove0.5mwide. Thetrailingedgesectiondeliverstwowiderpanelsof1.4and1.8m.

d<0.04mresultedin7segmentswithwidthsalmostevenlydistributed between0.3and2.0m.Lastly,d<0.08malsohas7segments,twoof themarearound0.2mwide,twoaround1.1m,andthelargestat1.8 and2.0m.Hereagain,thelargesttoleranceboundresultedinthe nar-rowestsegment,just0.02mwide.Consideringthetaperingoftheblade andcuttingtolerances, anarrowstriplikethis islikely tobe lostin processing.

Thenarrowsegmentsfoundind/w<0.08andd<0.08mwereforthe samereason:theleadingedgepaneldeflectionwasjustoutsideofthe giventolerancebound,andwasthusdividedinapieceofmaximum width,andaverysmallremainder.Abetteroptionwouldbetodivide thepanelintotwoormoreelementswithalowerdeflection.Thusit maybe beneficial toapply multiple boundaryconditions onagiven cross-sectiontooptimisethecuttingpattern.

Thevaryingsegmentationpatternsprovideinsightsintherelation between boundaryconditionandpanelwidth. Thesegmentation ap-proachesshowthatthebladepanelscanbereusedwithinstandardized tolerances.However,italsoresultedinarangeofvariablepanelwidths andsomeun- orbarelyusablesegments.Thus,todeliverpractically reusablepanelstheboundaryconditionsneedtobereﬁned.Todoso, commonlyusedpanelwidthscanbeimposedasanadditionalboundary condition.Thesestandardwidthscanfunctionas‘bins’whencalculating theoptimalpattern.Combiningtheboundaryconditionsofcurvature

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andpanelsizewillresultinasegmentationpatternforconstruction el-ementswithstandardsizeandaccuracy.Theboundaryconditionscan thenbeused toexplorepatternstoﬁndone thatoptimallyusesthe availablematerialanddeliversreadilyreusableconstructionelements.

3.2. Structuralproperties

Withregardtothestructuralproperties,wefoundfoursandwich panellayupsandonesolidlaminatebeamintheblademidspan.Only theleadingedgepanelandtheshearwebhaveaconstantlayupalong theblade,theotherpartstapertowardsthebladetipbyreducingthe thicknessofthecorematerialandthenumberofplies.Table6 shows theresultingrangesforthickness,density,flexuralmodulusandflexural strengthofallbladeparts.Theminimumandmaximumthicknessare givenforeachpart,becausethethicknessofthecorematerial(foam) largelydeterminestheeffectivematerialproperties.

Thefollowingchartscomparethebladesegmentstoconventional constructionmaterials,usingtheequivalentmaterialproperties.Fig.7 setsouttheFlexuralmodulus(E_flex)tothedensity(𝜌).Thischartsshows thatthebladematerialshaveaflexuralmoduluscomparabletotimber. Fig.8plotstheFlexuralstrength(𝜎flex)versusdensity(𝜌).Noteworthyis

thatthebladeelementsarecharacterisedbytheirhighspeciﬁcstrength (𝜎ﬂex/𝜌).Thesparcapsarepositionedinthetop-middleinbothcharts,

indicatingarelativelyhighspeciﬁcstiﬀnessaswellasstrength com-paredtoconventionalmaterials.Boththepanelsandthebeamsdonot fullyoverlaponbothcharacteristicswithotherstructuralmaterials,thus directsubstitutionofothermaterialsbytheretrievedbladematerialsis notevident.

Usingmaterialindices,wecomparedtheperformanceofthe mate-rialinspeciﬁcfunctions:panelsandbeams.Theperformanceindices

forabeamandpanelofminimummass,loadedinbending,are𝐸13_∕𝜌 forstiﬀnesslimiteddesignand𝜎12∕𝜌 forstrengthlimiteddesign[44]. Theseindicesareplottedaslineswithrespectivelyslope3(Fig.7)and2 (Fig.8).Thelinesconnectmaterialsthathaveequalperformance regard-ingstiﬀnessandstrength,respectively.Materialsabovethelineexhibit betterperformance,whilematerialsbelowthelineperformlesswell.

Forstiﬀnesslimiteddesign(Fig.7)timberistheonlymaterialwitha materialindexsimilartotherecoveredbladesegments.Assuch,the re-trievedmaterialsoutperformallotherconventionalconstruction mate-rialsforlightweightconstructionsloadedinbending.Thisindicatesthat structurallyreusedcompositescanbeusedtosubstitutetimberpanels andbeamsinbending-dominatedstructures,andthattheywillenhance performancecomparedtoothermaterials.

Forthisreason,Ashby[44]andBeukers[2]arguefortheuseof com-positesinarchitecture.High-risebuildingsespeciallyrequirematerials with ahighstructuraleﬃciency, whichcomposites can fulﬁl. Archi-tectshavenowadoptedcompositepanelsforcladdingbuildingfaçades [8]andcompositesaregaininggroundininfrastructuralprojects,for ex-ampleinbridgesandlockdoors[50].However,thusfarthecostof com-positesincomparisontotoday’sbulkconstructionmaterialsremained prohibitive forlarge-scaleimplementation. Structuralreusehowever, hasthepotentialtolowerthematerialcostandassuchunlock compos-itesforapplicationinbuildingapplications.

Fig.8showsperformanceforstrengthlimiteddesign.Theblade pan-elsareabovetheperformanceindexline,thebeamsandtimberare in-tersectedbyitandallothermaterialsfallbelow.Thus,thebladepanels outperformallothermaterialsforconstructionsloadedinbending.The CFRPsparcapsperformequaltosometimbertypes.Thesparcap po-sition,tothetop-leftofGFRPmaterials,showsthehigherstrengthand lowerdensityofCFRPcomparedtoGFRP.

Table6

Propertiesofbladeparts,calculatedfrombladedesignspecifications,includingsafetyfactors,usingGranta CESEdupack2019.Theequivalentdensity,flexuralmodulus,andflexuralstrengthdependonthethickness ofthesandwich,whichisdominatedbycorematerialthickness.

Part Thickness Density 𝜌 Flexural modulus E ﬂex Flexural strength 𝜎ﬂex

[mm] [x10 2 kg/m 3 ] [Gpa] _[x102 MPa] Leading & Trailing edge panels 26 5.6 9.8 – 14.6 5.1 – 8.9

96 3.0 3.2 –4.7 1.6 – 2.8

Trailing edge reinforced panels 26 5.9 15.1 5.1 – 8.9

103 4.1 6.7 1.6 – 2.8

Shear web panels 54 3.2 2 – 3 2.8 – 4.9

Spar cap beams 20 16.5 37.1 – 64.9 7 – 11.7

48 16.1 52.2 – 99 8.1 – 13.3

Fig.7. Comparingconventionalconstructionmaterialsto retrievedbladeelementsonFlexuralmodulus(Eﬂex)vs.

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Fig.8. Comparingconventionalconstructionmaterialsto retrievedbladeelementsonFlexuralstrength(𝜎ﬂex)vs.

Density(𝜌).

Fig.9. Comparingthestiﬀnessandstrengthperformance ofconventionalconstructionmaterialstoretrievedblade elementsforaconstructionloadedinbending.

Fig.9 setsoutthematerialindicesforlightweightdesign;the verti-calaxisforstiffness-limiteddesign,andthehorizontalaxisfor strength-limiteddesign[44].Thebestperformance,minimum massfora pre-scribedstiffnessandstrength,isfoundwhenmaximisingtheindices;at thetop-rightofthechart.Eventhoughtimbercanachievehigher per-formanceforstiffnesslimiteddesigns,thebladepanelsexcelin strength-limiteddesigns.Allpanelsarefoundatthetop-rightofthechart,which indicatestheseprovidethebestcombinationofstiffnessandstrength forlightweight designs.Assuch, theretrievedconstructionelements outperformallothermaterialsforalightweightconstructionloadedin bending.

Theshapeandmaterialpropertiesoftheretrievedsegmentsaremost reminiscentofpanelsandbeamsfoundinconstruction,infrastructure and,insmallersizes,furniture.Indeed,theoccasionalapplicationsfor whichthesematerialshavebeenusedaremostlyfoundinthesesectors [28,29].This“occasional” applicationisattributabletotherestrictions imposedbytheoriginalsizeandshape.

Thesafetyfactorswereusedtocalculatetheexpectedrangeof mate-rialproperties.Thesewereusedontheassumptionthatthebladewould stillbeinsafeoperationatthepointofdecommissioning.Still,the ma-terialwillhavesuﬀeredfromfatigueandpotentialimpactdamage.This rangecouldbefurthernarroweddownthroughmodellingand

inspec-tion[51].However,determiningthedegradationofmaterialproperties throughfatigueandtheextentofdamageischallengingfor compos-ites[52,53].Insightsfromthefieldsoffatiguelifeprediction,structural healthmonitoringanddamageinspectioncouldimprovethedefinition ofpost-usematerialproperties.Also,identifiedweakenedareascould thenbeexcludedfromthecuttingpattern.

Usingcurvaturetodetermineasegmentationpatternandto evalu-atestructuralqualityusingmaterialindiceswerepromisingfirststeps that canbe developedfurther. Structuralanalysisshows thatthe re-trieved materialshaveexcellentproperties incomparison to conven-tionalconstructionmaterials.However,thestructuralshapeofthe com-positeproductisalreadydefined,whilethatofrawmaterialscanstillbe chosen.Inthestructuralcomparisoncharts,shapefactorscanbeusedto expandthematerialenvelopes[54],butsuchanextensivecomparison isoutsideofthispaper’sscope.Thedefinedshapesalsoaffectfinding therightapplicationfortheconstructionelements,aprocessthatneeds tobefurtherexplored.

3.3. Applications

Inadditiontothesizeandstructuralperformance,weneedto con-siderthepracticalusabilityofthematerial.Forreuseinpractice,the

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Fig.10. Picnictable,madeofrecoveredbladesegments.

constructionelementswillhavetobemachined(cuttodesiredsizeand shape),joinedandﬁnished.Wetestedstructuralreuseinpractice,by designingandbuildingasimplefurnitureproductfromawindturbine blade,retrievedfromarecyclingcompany(Figure10).

CuttingcuredGFRPlaminatesrequiresspecialisedequipment.When developingthefurniture,weconsultedanumberofworkshopsfor pro-ducingtheprototype,butnonewaswillingtoprocessthematerial.The mainobjectionswere:toolingdegradation,personalhealthandsafety, andcontaminatingthedustextractionsystemandmachinerywithGFRP residue.Thisreﬂectstheconcernsraisedonreprocessingwindturbine bladematerialinearlierstudies[28,29].Wefoundwaterjetcuttingto addresstheseconcerns;thereisnoriskoftoolingdegradation,anddust iscollectedinthewaterﬁltrationsystem.

The prototype was produced in two steps. First, the blade parts (sparcaps, shear websand panels)were separatedusing a portable waterjet cutterat therecycling company. Then,we hadthe compo-nentscutoutfromthepanelsusingaCNCwaterjetcuttingtable.These cutsweremadeperpendiculartothepanelchord,which resultedin near-rectangularpartcross-sections,especiallyforrelativelyﬂatpanels. Overall,wefoundthatwaterjetcuttingdeliveredgoodcuttingquality, highaccuracyandfewcuttinglosses(thejetdiameterwas0.7mm.).The piercingpointshowever,whereacuttingtrackstarts,neededtobe pre-drilled,topreventpressurebuild-upbetweencoreandbottomlaminate, andtherebydelaminationoftheGFRPfacesfromthecorematerial.

Threeconnectiontypeswereusedintheconstructionofthetable, wherespecialattentionwaspaidtopreventingwateringressduringuse andmaintainingstructuralintegrity.Slottedjointswereusedto min-imisescrewholes,andadhesivebondswereusedtoeliminatejointson exposed,horizontalsurfaces.Fasteners,placedonunexposedpositions, wereusedtofacilitatedis-andreassembly.

Thisconstructionwaschosenbecausethelargestthreattothe prod-uctinitsusephaseistheenvironmentalexposure.Thecorematerial ofthesesandwichpanelswasbalsawood,whichispronetomoisture degradation.Andthematrixmaterial,epoxy,issensitivetoUVageing. Sobyensuringnoholesonexposedsurfaces,theriskofwaterseepingin wasminimised.Acoatingwillshieldthematerialsfromhumidityand UVradiation.Theloadsduringusewillnotcauseanyproblems,because thesearewellbelowthoseinitsinitialapplicationinthewindturbine andthematerialsafetylimits.

Thepicnictablecanbe disassembledat theendofits usephase. Then,largepartscanbereused,becausethesearestillstructurallysound andlargelyunaﬀectedbythecuttingpattern,smallercomponentscan beprocessedintheGFRPrecyclingstream,asnoadditionalorforeign materialsareaddedorconnected. Fromthisapplication,welearned structuralreuseisfeasibleinpractice.Thebladedeliveredreusable con-structionmaterials,whichwereprocessablewithhighaccuracy,using therighttooling.

Prefab building could be another application areafor the recov-eredconstructionelements.Atypicaltimberframebuildingusestimber beamsandwoodpanels.These partsarefabricatedtospecifications, enablingquickassemblyon-site.Thesegmentationpatterncompliedto timberconstructionstandards,andpanelssizeswereinlinewithtrade standards.Thepanelscansubstitutethetimberaswellaswoodpanel partsgiventheirstructuralperformance.Theneedforspecialised cut-tingequipmentissolvedbypre-fabricatingpartsandeliminatingon-site rework.Thegainsmadeinweightcometothebenefitoftransportand installation.Prefabconstructionseeswide-spreaduseinvarious build-ingtypes,includingroofingandindustrialwarehouses,aswellaspublic buildingsandhousing.Thus,concerningdimensionalstandards, struc-turalquality,processingandscalability,weexpectprefabbuildingtobe apromisingsectorforreuseofbladesegments.

Thesegmentationpatterningmethod,structuralanalysisand appli-cationexampleprovideinsightsintowhatisneededforstructuralreuse. Thereuseofconstructionelementsreliesontheavailabilityof prod-uctspecifications.Determiningsegmentationpatternsrequiresdataon overalldimensionsand(aerodynamic)shapes.Thestructural compari-sonneedsmaterialsandlayupscheduleswhichalsoimplies identifica-tionoftheproductandcollaborationalongthevaluechain,toretrieve thisinformation.Thesefactorsmayseemevident,butinindustry prac-tice,intellectualproperty andcommercialconsiderationsmayprevail oversharinginformationamongstakeholders.Moreover,theusephase inflictswear,damageandfatigueontheproduct,introducingadditional uncertaintyaboutresidualmaterialqualityatendofuse.Additional in-formationonthematerialstate,e.g.through(embedded)monitoring, inspectionandcertificationmayreducetheseobstacles.

Thedesign ofwindturbine bladesis drivenbyaerodynamic and structuralperformance; thereisno roomfor designadjustmentsthat impairtheseelements.Thedesignaspectspresentedheredonotneed tohaveadverseeﬀectsonproductperformance,astheyarelargely con-textualandeasytoimplement.Thus,therecoverypotentialforasuch acomplexcompositeproductasablade,canbeimprovedbyrelatively simpleinterventions:

1) Documentingproductspeciﬁcations 2) Enablingtraceabilitythroughidentiﬁcation 3) Sharinginformationalongtheproductvaluechain

3.4. Futureresearch

Toimproveontheproposedapproach,furtherresearchcould ex-pandonadditionalproduct(sectors),moredetailedproductfeatures, boundaryconditionsanddesign forreuse.Othersectors,likemarine andaviation,alsousehighendcompositeparts,andfacesimilarend oflifechallenges.Furtherresearchcoulddetailstructuralreuseofparts fromthesesectors.Additionalproductfeaturestoincludecouldbe com-positelayupschedules,3Dshapes,connectionsandsub-assemblies.The segmentationboundaryconditionscouldalsobeexpandedtomeet re-quirementsforsuccessiveapplicationsmoreaccurately.Wherepossible, elementsoftheapproachcouldbeimplementedinthedesignofnew products,topreparethemforreuse.

Thepresentedstructuralreuseprocesscouldbe adaptedto varia-tionsinbladedesignandmaterialscomposition,whichvarypermodel andmanufacturer.Forexample,theNRELbladeusedsafetyfactorsfor onshore deploymentbased onIEC standards[55]. Thesefactorswill havetobe adaptedforanalysisofbladesusingotherstandards,such asDNVGL-ST-0376[56].Also,asbladesgetlarger,useofcarbonfibre reinforcementsincreasesinsparcapsandshells[34].Thisimprovesthe mechanicalperformanceoftherecoveredsegments,andtherebytheir economicvalue.Also,thesize,configurationandnumberofshearwebs mayincreasewithbladelength[57].Thisaffectstheboundary condi-tionsofthecuttingpattern(i.e.cuttinglinesalongthesparcaps)aswell asthereuseopportunities.Bank(2018)exploreddirectreuseofthese asdoorsandwindowframes[58].

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Asegmentedbladeconcept[59] mayfacilitatereprocessing,while atthesametimeintroducingbondingareaswhicharedifficulttoreuse. Anintegratedbladeconceptbycontrast,couldimprovethereuserateby eliminatingbondingareasattheleadingedgeandtrailingedge[60]. Al-thoughthepresentedsegmentationapproachcouldbeadoptedtomeet thesevariations,thisfurthersignifiestheneedforavailabledesign doc-umentationatthereusestage,inordertodefineoptimalsegmentation patterns.

Toimprovethereuserate,recoveryoftherootandtipsectionsneed tobefurtherinvestigated.Thiscouldbestructuralreuseasother con-structionelementtypesorapplications,whichareinlinewiththe ge-ometryandmaterialcharacteristics.Alternatively,otherrecoveryroutes mayneedtobefoundinthedomainofthermal,chemicalormechanical recycling.

Futureresearchcouldalsopursuemoreambitiousapplicationsfor structuralreuse.Thereusecaseinthisstudy,furniture,servedto evalu-atetheprocessandmanufacturinginpractice.Theexpectedloadswere wellwithinthematerialspeciﬁcations.Buildingontheinsightsofthe structuralpropertyevaluationandadditionalmaterialstestingor mod-ellingwillenablereuseatthematerialsfullpotential.

Structural reuse adds another use cycle, and thereby extends the material lifespan. This preserves energy and value embedded in the composite, and potentially substitutes use of virgin materi-als. To fully close the resource loop, recovery routes in terms of reuseorreprocessingoftheconstructionelementsneedtobefurther investigated.

4. Conclusion

Inthisstudy,weexploredasystematicapproachforstructuralreuse ofcomplexcompositematerials.Structuralreuseiscomplicatedbythe largesize,complexshapeandcomplexmaterialcompositionof compos-iteproducts.Toaddressthis,asystematicmethodwasneededtodeﬁne segmentationpatternsandevaluatestructuralquality.

Weproposedanapproachforthestructuralreuseofcomplex com-positeproductsthroughsegmentation,structuralanalysisandreuse ap-plications.Weappliedthis approachtoatypicalwindturbine blade madeofglass andcarbon fibrecomposite.Thesegmentationpattern showedthat highaccuracyconstructionelementscan be cut from a double-curvedproduct,insuchawaythat95%oftheanalysedblade section,and55%ofthecompletebladeareeligibleforreuse.The struc-turalanalysisrevealedgoodperformanceintermsofflexuralstiffness andflexuralstrengthin relationtoweight, withtheretrievedpanels exceedingtheperformanceofconventionalconstructionmaterialsfor constructionsloadedinbending.Reuseofbladepanelsinanother prod-uctshowedtheneedforspecialisedcuttingequipment,anddelivered accuratelydimensionedparts.

A numberof designaspects limit retrievalof construction mate-rialsfrom endof use windturbine blades.Converting alarge struc-ture into smaller, reusable segments,relies on availability of model detailstogenerate cuttingpatterns. Thematerials’fatiguebehaviour and(variationsin)operatingconditionscomplicatedetermining resid-ualstructuralquality,signifyingtheneedforstructuralhealth monitor-ing.Thesedesignaspectsaremostlycontextualinnature,andcouldbe addressedwithoutinfringingtheperformanceofthebladeinitsinitial use.

Tofacilitaterecoveryofconstructionmaterials,designers can ad-dressstructuralreusebydocumentingproductspeciﬁcationsand facil-itatingtraceability.Duringproductlife,reuseissupportedby collabo-rationalongthevaluechainandreducinguncertaintyaboutthe prod-uct’sstatethroughe.g.monitoringorinspection.Testingthestructural reuseapproachinafurnitureapplicationshoweditsfeasibility.In addi-tiontoidentifyingrelevantdesignaspects,thepresentedsegmentation andstructuralanalysisapproachbringsanewperspectivetostructurally reusingcompositeproducts.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingﬁnancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinﬂuence theworkreportedinthispaper.

Acknowledgments

ThisprojecthasreceivedfundingfromtheEuropeanUnion’sHorizon 2020researchandinnovationprogrammeundergrantagreementNo 730456.

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