TWISTED SAVO I S TURBI EBASEDMARl E CURRE T E ERGYCONVE RSIO SYSTE M
by
©MD.IMTIAJHASSAN
A Thesissubmitted to the School of Graduate Studies in partial fulfillment of the requirementsforthe degree of
MASTEROF E GINEERIG Depart ment of Electrical and ComputerEngi neeri ng
Memorial Universityof Newfoundland SI.John's, Newfoundland
November,20II
...To my beloved parents, brother, sister and wife
ACKNOWLE DGEMENTS
Duringthecompletionof thisresearchwork,I have workedwithseveral personnelwhose contributionsto thiswork deserve special mention.Itisa great pleasurefor metoconvey my deepestgratitude to them.
Firstof all Iwouldliketo placemy specialgratitude to my supervisor Dr. Tariq Iqbal for hisvaluable advice,guidanceandconstantsupport from thevery beginningof the researchwork,to itsfinalstage.Hisexperiencein the fieldof Marine Current technology wasinvaluable,ashe providedme withsteadfast encouragementthroughvaluableideas that helpedmetogrowasa research student. He gave me thetoolsI neededtosolve the different problemsIfacedduringthisresearchwork, andhisexpertise,foreknowledge, and perceptionin thisfieldmadehim a greatsource for ideas.Itisan honor for metobea studentunderhis supervision.
Iwould liketo record mysinceregratitude to my cosupervisor Dr. MichaelJames Hincheyfor hisguidance,advice andsupervision.He alwaysguidedme on theright track inpursuingthisdegree.Hisunflinching support and guidance allowed meto complete thiswork,and it was apleasuretohavehimasmy co supervisor.
Thisresearchwork hasbeen supported by theAtlantic Innovation Fund and Seaformatics.
Iwould like to acknowledgethem and thankAndrew Cook and all theother members of Seaformatics group. I wouldalso liketo show my gratitudeto Dr. VlatimilMasekfor his encouragementand appreciation.
Iwould like to thankWallaceRobert ofSeaCraftInternational Limited,for buildingthe prototype of the twistedSavoniusturbine.Myspecialthanks to BrianPretty for his iii
support in instrumentation for theexperimentalset up in theFlumeTank and Wind Tunnel.I learnedmuchfromhim.
Iam indebtedtomylabmates andfriendswho always supported mebothdirectly and indirectly.Iamgrateful tomy friendsfor their emotionalsupportover this project. My special thanksto oneofmy very goodfriends, Md.NahidulIslamKhan, for hisconstant supportduringthistime.
Duringmy researchwork, Ialwaysget supportfrommyfamilymembers.Itis a great pleasure for me that Icanacknowledgethem.Iwould liketo thankmyfather Md.
HabiburRahman,mymother JebunNassa,mybrotherMd.IstiakHassan,my sister Tasnim Tabassum,and the rest of my family membersfor theirendless love andsupport.
My veryspecial thankstomy wife Most. Nusrat Jahanforherperpetualreinforcement.
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Tableof Contents
ABSTRACT . ... ... ..ii
ACKNOWLEDGEM ENT S . ... ...iii
Tableof Contents .
Listof Tables.. ... . . . . ... .. .. .. .... ... ... .... ... .. ... .. ... ....ix
List of Figures x
ListofSymbol s ... ... . .. .... .. .... .. . . ... ... .. .xiv
List of Appendi ces . ... ...xvii
.. 1
.. 1
.. 3
.. 5
.. 7
1.1 TheONSFI Project .
1.2 TheSeaformat icsPod .
1.3 ThesisOutline ..
Chapter I ..
Chapter2 .
2.1 Introdu ction... .. 7
2.2 Formationof Marin eCurrent s .. 7
2.3 Marin eCurrentEnergyCon versionDevice s... .. 10
2.3.1 RotatingDevi ces .. 11
2.3. 1. 1 Typesof Marine Curre nt Turbines 11
2.3.1.2 SupportStructure.. .. 12
2.3.1.3 Gearbox 12
2.3. 1.4 Generator ..
2.3.2 Reciprocating Devices ..
.. 13
.. 13
2.4 CurrentStatusof Marine CurrentEnergy Devices.. .13
2.5 Technology Challenges .
2.6 Conclusion
... 20 ...21
. 22
.. 22
.. 23 .. 25 .. 26
.. 27
.. 30
.. 35
.. 36
... 37 3.2 Introductionto CFD...
3.3 CFDvs ExperimentalSetup ..
3.4 Applicationof CFD . 3.5 ConservationLaws ..
3.6 Turbulent Hydrodynamics .
3.7 VolumeofFluid .
3.8 MovingBodies in Flow ..
3.9 Conclusion Chapter3....
3.1 Introduction ...
4.5 Experimental Setup in Flume Tank ..
4.5.1 Test Results inFlumeTank . 4.2 Twisted SavoniusTurbine ..
4.3 Material Selection...
4.4 Prototype of Twisted SavoniusTurbine . Chapter4 ....
4.1 Introduction...
.. 38
.. 38 .. 38
.. 45
.. 47
.. 48
.52 vi
4.6 Wind TunnelTest of theTwisted Savonius Turbine... ... 56 4.6. 1 TestResult s atWindTunne l...
4.7 Conclusion...
Chapter S ...
5. 1 Introdu ction ...
...59
. 61
. 62
.. 62
5.2 Comput at ional Fluid Dynamics andFlow-3D 62
5.3 Flow3DSimulationResult s 64
5.3.1 5.3.2 5.3.3 5.3.4 5.3.5
Simu lation Resultsof TwistedSavonius Rotorwith OverlapRatio 64 Simu lation Resu ltsof Twisted Savonius Rotorwith aZeroOverl apRat io70 Hor izont ally MountedPosition-Ifor theTwiste dSavoniusTurbi ne...72 Hori zontall y Mounted Position-2for Twisted Savon ius Turb ine .73
Simulationof a QuarterPitchedSavonius Turbine 75
5.4 Sensitivity ofMesh ... ... ... .... ...77 5.5 DragModel
5.6 Conclusion
... .... ... 78 ... ...80
Chapter6 ..
6.1 Introdu ction ....
...82
. 82
6.2 The Twisted Savoni usWater CurrentTurbineEmulator 82
6.3 Genera tor. ..
6.3.1 GeneratorLosses 6.4 Perm anentMagnet Generator ..
6.4.1 Cogging Torque ..
...84
.. 85
... 86 ... . 88 vii
6.7 Maximum Power Point TrackingAlgor ithm(M PPT)...
6.8 Prop osedMPPT Algor ithm..
6.4.1.1 Startingof aTurbine . 6.4.1.2 TurbineRunningConditi on . 6.4.2 Reduction of Coggin gTorqu e . 6.5 TwistedSavoniu sTest ingwith Gen erator. . 6.6 DC-DC Convert er s..
6.6.1 6.6 .2
BoostConv ert er .
Design of aBoost Converte r
...88
.. 89
.. 89
.. 90
.. 92
.. 96
...98 ..102 ...10 3
6.8.1 HardwareSetup ... .. 106
6.8.2 Result s...
6.9 Volta geSensor...
.. 107
.. 110
6.10 Current Sensor.. .. 111
6.11 Energy Storage ...
6.12 Conclusion
.. 112
... ... ...113
Chapter7 114
7.1 ProjectOver view ..
7.2 Resear chContribution. 7.3 Future Work Biblio graph y
Appendices .
.114 ... 115 ... ... ... ...116 ..118
.. 124
viii
Listof Tables
Table 3-1 Comparisonbetweencroand ExperimentalSetup 26 Table6-1Co mpo nentsfor building the boost converter 99
ix
List of Figures
Figur e1.1MarineCurrent EnergyConvers ionSystems 2
Figur e1.2Schem atic of Seaformatics .4
Figure 2.1 Therm ohalin e Circula tion [19]... .. 9
Figur e 2.2 SpringTide and NeapTide.. ..10
Figur e 2.3Seagen[30]... .. 15 Figur e 2.4 Evopodand Free Flow[31,32] ... ... .. 16 Figur e 2.5 PulseTidal and Open Centre [33,34]... .. 17 Figur e 2.6 LunarTidal Turbine andTidalFence DavisHydro[35,361 18
Figur e2.7Neptun eTidal andStingrayTida l [37,38J 18
Figur e 2.8 GorlovHelical TurbineandNereusand Solon[39,401 19
Figur e 3.1 CFD Workfl ow 24
Figure 3.22-DGrid .. 34
Figur e 3.3F Valuein CFD Grid...
Figur e 3.4Pressur e in a triangul arbubble
.. 36
... ... ... 37 Figur e 4. 1ConventionalSavoniu sTurbine (T hreeDimensionalCAD View) .40 Figure 4.2TwistedSavoniusTurbine (Three Dimension alCAD View ) .40 Figur e 4.3a)LeftView (b)Top View (remov ing theend plate) ofTwistedSavon ius Turbine r. ... ... ...41 Figure 4.4 PowercoefficientvsTSR Curve of DifferentTypesof Turbines [48 ]... ...42
Figur e4.5TopView of Twisted Savonius Rotor 44
Figur e4.6 TwistedSavoniu s with Ribs . 46
Figur e 4.7Protot ype oftheTw istedSavon iusTurb ine .47
Figure 4.8Testin g ofTwi sted Savonius inFlume Tank 49
Figur e 4.9Rot ary Enco der 50
Figure4.10Experime nta l Setup for TwistedSavonius .. 51
Figu re4.IIRPMat Different Curre nt Speeds... .. 52 Figure4.12 MaximumPo we r Outp utat Diffe ren t Curre nt Speed s 53 Figure4.13MaximumPo wer at Diffe ren tRPM....
Figur e 4.14 TurbineOutput vs CurrentSpeed Curve. Figu re 4.15 TurbineTor qu e vs CurrentSpeed Curve..
Figur e4.16Cp-A.Curveat Differe nt CurrentSpeed Figur e 4.17Cp-A.Curve ofthe TwistedSavonius
. 53
.. 54
.. 54
....55 ...56
Figur e 4.18TheTwistedSavoniusin the Wind Tunn el 57
Figur e 4. 19 Torqu e Arm usedin WindTun ne l Testin g 58
Figur e 4.20 Instrum ent ation intheWind Tunnel 59
Figur e 4.21 Cp-A.Curveofthe Twisted Turb ine at Wind Speed ofIO mls 60 Figur e 4.22 Cp-A.Curve ofthetw isted turbi neatwindspe edof13mls 60
Figur e 5.1 TwistedSavoniu sRo tor 64
Figure 5.2CrossSection alVie w oftheMeshBlock ..
Figur e5.3TimeFrameat 16.8 Second ...
.. 65
. 66
Figure5.4TimeFram eat 17.1 Second 66
Figur e5.5TimeFram eat 17.3 Second 67
Figur e5.6TimeFrame at 17.5 Second . .. 67
xi
Figur e5.7Angul arSpeedof the Turbineat T=0.5Nm Torque 68
Figur e5.8Cp-),Curve ofTwi sted Savonius Turbine 69
Figur e 5.9 TwistedTurbine withZeroOverlapRat io ...70
Figur e5.10MeshSlice withZeroOverlapRatio ..70
Figur e5.11 Cp- '-Curve ofTwisted Savoniu sTurbinewith Zero OverlapRatio .. ...7 1 Figur e 5. 12Twisted Savoniu s as aHor izon talAxisTurbine... .. 72 Figur e 5.13 TwistedSavoniu sTurbine withNoEnd Plates 73 Figur e 5.14 Hori zont all yMount ed Turbineat Position 2 ... 74
Figur e5.15Cp-XCurve forHo rizont all y Positioned 75
Figur e 5. 16 QuarterPitchTwisted Savonius Turbine 76
Figur e5.17Cp-XCurve of QuarterPitchTwistedSavoniu sTurbine 76
Figur e 5.18Angular Speed ofthe rotor . .. 77
Figur e5.19Schemati c ofBasic Savonius 78
Figur e5.20 Strip Mod el ofTw istedSavoniu s .. 79
Figur e5.21 Strip Mod elPo wer . .. 80
Figure 6.1MCECEmulat or. .. 83
Figur e 6.2Output(a) Volt age and(b) Cur re ntofGenerat or at DifferentRPM 83
Figur e 6.3 CrossSect ion of aPMG .. 88
Figur e 6.4TurbineCoupled withGenera to rin theWindTunnel 90
Figur e6.5Wind Speed vsVolt ageOutputCurve 91
Figure6.6Wind SpeedvsCurre nt OutputCurve ... ... ... ....91
Figur e 6.7 Wind Speed vsPo werOutpu tCurve 92
xii
Figur e 6.8Gain vsDutyRat io CurveforDifferent Converters 95
Figure6.9DC-DCSwitch ingMod e Converter System 95
Figur e 6. 10 BoostCon vert er... .. 96
Figure6.11 Waveform s forBoost Converter... .. 97
Figure6. 12 Voltage Curre ntWaveformsforDiscont inuousModeOperati on... .. 98 Figur e 6.13 Schem atic ofBoost Converter.
Figure6.14Propose d Boost Converter....
..99
.. 100
Figur e 6.15 Input vsOutput Voltageof the Boost Converte r 100 Figu re 6. 16 InputvsOutputCurrentofthe BoostConver ter 101 Figu re 6.17 Input Voltage vsOutputPower oftheConv ert er 101 Figu re 6. 18 Pertu rbati on andObservation Method ControlAction 103
Figure6. 19FlowChart ofMPPT Algorithm 105
Figu re 6.20 HardwareSetu p 106
Figur e 6.21Angular Speed fallsto 8 rad/sec .. 107
Figur e 6.22PWM toredu ce AngularSpeed .. 108
Figur e 6.23PWM SignalVariat ion at OptimumSpee d... .. 108 Figure6.24 Angul arSpeed Stabilizesaround theOptimum Speed .. .. 109
Figure6.25 PWM to increaseAngularSpeed .. ... 109
Figu re 6.26VoltageDivider Circuit. ..
Figure6.27CurrentSensor ..
.. 110
.. 111
xiii
Listof Symbols
AngularSpeed
A AspectRatio
Pavailable Available power inarea Asin water Body forceper unitmass G Complexfunction of velocitygradient
Densityof Fluid Diameter Dr Diameteroftheendplate
Effectiveviscosity III Extra viscosity
Force perunit area atapoint onthe surface GravitationalAcceleration H, Heightof therotor
Intensityof turbulence III Laminarviscosity
Localdissipation rate Overlapbetween twobuckets Overlap ratio Power captured bytheturbine
xiv
Cp Pow er coefficie nt Pressur e
R, Radius
Speed ofsoundinwater
As Sweptarea
Us Tip periph eral velocity Tip SpeedRatio (TSR) Velocity
U,V,W Velocity compo nentinx, y, andzdirectionrespectiv el y Volume of Fluid
ElectricalSymbols Capacitance Current
Duty ratio Efficiency
Inductance toff OFFperiod of theswitch ton ON period oftheswitch
Period of the controlsignal Power
Resistance Voltage
xvi
List of Appendices Appendix A .
Appendix B ..
Appendix C ..
.. 124
.. 132
. 159
Appendix D 165
AppendixE 167
xvii
Chapter 1
Introduction
1.1 The 0 SFI Project
The Ocean Network SeafloorInstrumentation(ONSFI)Project is a multid iscip linary resea rchanddevelop me ntprojectthataimstodesign , fabrica teandvalida teaproo f-o f- conceptseafloo rarrayof wireless marinesensors forusein monit orin g sea bed processes.
IthasfourR&Dstreams: marine senso rs; pow er generation;communi cationnetworkin g;
andfina l integrati on ofthe systemsintoa working prototype.Thisthesisis apart ofthe powergenerat ion unit.The objectiveof thiswork isto selfpowerthepods, utilizing a micro marine current energy conversio nsystem. Thiswillelim inatetheneedfor battery replacement,which can beveryexpe nsive and cause sensor down time.The proposed marine current energycon version system consists of a twistedSavoniu sTurbine, Permanent Magnet Generat or (PMG),Rectifier,Power Electro nicsConverter, and Battery.Theconceptisbasedon a twistedSavoniu sturbine,directlycoupled with a perma nentmagnetgenerator.Thesystem isintended to be deployedontheseabed ;so, simplicity androbustnessare the main focuspo ints.The compre hensive design of the syste m demand sless rotating partsinorde r tohavelessmaintenan ce and morereliabilit y, because,once thesyste misdepl oyedoffshore, maintenance isboth difficult and expensive.The blockdiagram oftheproposed syste m is given in Figur e1.1.
LCD Display
PIC16fB77
MICROCONTROLlLR
~ c;=J
·' 8"
::.g-;;g,~':
...~
-
Figure1.1MarineCurrentEnergyConversionSystems
Thetwisted Savoniusrotor conv ertsthemarine energ yintomechan ical energy.At the same time,PMG convertsthemechanicalenergy intoelectr icalenergyasit isdirectly coupledwith the rotor.The alternatingcurrent(AC) outputofthegenerato ristheinput of the rectifier.Recti ficat ionistheconvers ionof alterna tingcurre nt(AC)todirect curre nt (DC).Thisinvol ves adevice thatonlyallowsone- way flow of electro ns. The outputof the rectifierisfedto the boostconverter,whichisa step- up de-d eswitching conver ter with high efficiency.Alow costmicrocontro llerPICI 6 f877based Maximum Po werPo int Tracking (MPPT)algorithmhasbeen impleme nted.MPPT tracksthemaximum power po intby contro lling the widthofthePulseWidth Modul ation which is supp liedtothe MOSFET(sw itch). The outp utofthe converterisconnectedwithabatt erytostorethe chargesin it. The LCD display connected withthe microcont rollerdisplaysdiffere ntdata, suchasinputcurre nt,voltage andoutput voltage,curre nt,and RPMof theturbine shaft.
These data arealso displayed on theHyperTerminal of ahost computer duringtesting.
1.2 The Seaformatics Pod
A schema ticoftheSeaform aticspodis showninFigure 1.2.
POWER
SENSOR
···ll \
Figure1.2 SchemalicofSearorrrsncs
Accordin g10ProjectEngineer.AndrewCook.itis esnmated lhatlhe target is100 Walls of powerat1m1s currentspeed.Thisisthe target powerforthisthesis.If thisisnol reached. scalinglawsdevelopedin earnerwork(I)willbeused[0scale:[he power from Ihis work[0that req uiredbytheprototype .
1.3 ThesisOutline
Thisthesishas sevenchapte rs. Con ci seinform ation about thechapt er sis given belo w:
The First chapte rgives thetotal outline ofthisresearchwork.Itdiscu ssesthetot al proposed syste m inacompact way.
TheSecondchapte rdiscussesthe marine energy sourcesandcurre nt.The marin e energy cond it ion isdiscu ssed in this chapter in det ail along with causes of curre ntsand different types ofmarin e currentturbines.
TheThird chapter focuseson thetheorybehind CFDso ftware.Itdevelopsthe governing Partial Differential Equations or PDEsfor turbulentwake flows.Itfocuse s on the discretiz ati on of the PDEs and the treatme ntof movingbodies.
The Fourth chapter sho ws theexper ime nta lset up of the twistedSavonius rotor. The rotor has been tested in the FlumeTankat the MarineInst itute and the Wind Tunnel in the EngineeringBuild ing atMUN. All the experimental result sare presentedin thischapter.
The Fifth chapte rexplores thesimulat io n ofthetwistedSavon iusrotor in the CFD so ftwa re FLOW-3D.Differ en tsimulationresultsare present edin this chapte r.A simple strippower mod el of the turbine isalso developed.
The Sixth chapter provides discussion on the electrical system.This chapter deals with the generator, converter, battery, and MPPT algorithm.
The Seventh chapter presentsthe conclusions of the research work.Italso suggestions for future work and research challenges.
Chapter2
Marine Currents and Energy Conversion Technology
2.1 Introduction
Thischapter focuses on theformation ofthemarine currents. It briefl ydescrib es surface and deepwater curre ntsand their causes,alongwith thedifferentpartsneedtoimplement amar ine curre ntenergyconversionsyste m(MCECS).One of themajorparts of MCECS istheturbine.This chapter discusses aboutsome marin e curre ntturbinesin aconciseway fromcommercial viewpo int.
2.2 Formation of Marine Curren ts
The continuo us movem ent of waterin anocean is called acurrent. Current smay flow eitheronor far belowthe surfaceofthe ocean.Theynorm allymo ve ina specific direction , andaidsignifica ntlyin thecirculatio nofEarth's moi sture ,theresultant weath er, and waterpolluti on . As seawaterisnearlyincompre ssible, vertica lmoveme nts areassociated with regions of convergenceand divergence in thehorizont alflo wpatterns, The windstress actin gon the surface layer ofthe ocean induce smo vem entof that water andcausessurfacecurre nts.Thisis called the Ekman Layertranspor t.Insomecases, stro ngcurre nts are generatedbecause apersistent wind blow sinone direct ion fora long duration[2-11].The strengthanddepthofthe curre nt depend on the stre ngthand power of thewind.TheCoriolis force and the gravityforce have significa nt impact onsurface currents . As surfacecurrentstravelalong way, theCoriolisforcetriestodeflectthem,
and help sto createa circu larpattern [12]. Ho rizont al curre ntsvelocit iesvaryfro m a few cent imeters per second to as muc has 4 metersper second. Acharac teristicsurfacespeed is abo ut5to 50centime tersper second.Curre nts diminishin inten sity with increasing depth .
Otherthan surfacecurre nts, there is alsoanot hertypeof curre ntthat ispresen t indeep water.Thesecurrentsare driven by densitydiffere nces inwater,whic hinturn depend on the water'stemperatureandsalinity [13, 14].This is also kno wn asTher mohaline circ ulatio n(THC).Althoug hthishappens mostly duetotheden sitydiffer e nces,part iall y ther eis alsoa roleof gravity for deep water curre nts. Inthecase of cold region s,the surfacewaterdensity ishigher than deep water.Pressuregradientsatdepthsresulting from den sity gradientsof thesurface watercausescirculatio nrII].Basicall y, THC is drivenby a pushandpuJlsystem; interms of timescale, intheshort term itis pus hed becauseofden sity cha ngesin deep water, andif the den sit ydrop stoomuch ,thcn the deepwaterforma tionisno t possible.Butinthelo ngterm ,it ispulled becauseof the turbul entmixing of water;deep water densitydropsuntilnewdeep waterformatio nscan start.Atlanticwarme rsurfacecurrentsheatupthe Nort hAtlanticandNorthernEuro pe.
Thisis show ninFigure2.1by the red stream,and the bluestreamrepresentscold North Atlantic deep water.Thevolume transportofthe overturn ingcircu latio natthe Nort h Atlantic has beenestimatedfrom hydrographicsectiondata. It is17 Sv (ISv=106m3/s), with heattra nsport as 1.2 PW (I PW=1015W) [15-2 1].
Figure 2.1Thermohaline Circulation119 J
Energy for turbulent mixing of the water comes from thetides.Tides are the verticalrise and fallof water, and are one of the mainfacto rs in ocean currentcirculation.Tidesare generatedbecause of the gravitationalunraction forces oftheSunandMoon on theEarth, and the centrifugalforce caused by the rotation of Earth on its own axis around the Sun, and the moon's orbitaro undEarth. The tiderises until it reaches a maximum height, calledhigh tide or highwater, and then falls to a minimumlevel calledlow tide or low water. Earth rotates on its own axis and the moonrotatesaroundEarthapproximately every 24 hours lind 50 minutes. At the sub-lunar point.there willbe two hightides during theinterval.Onehigh tidehappens whenthe moon is overhead,and another high tide occurs 12 hours and 25 minuteslater.atthe antipode.There willbea low tidebetween each high tide [22].When Earth, the moon,and the Sunareinthe same line,the Sun and moon's attractive forces act in the same direction.So, the magnitudeof the force is higher
than theaverage value.The tides created for this forceare calledspringtides.Figure2.2 showsthese tides.While the moon isatthe rightanglewithEarth, and the Sun, the tractive forces of theSun act on moon's tractive forces atrightangle.Theresultant force i~called neapudesinFigure2.2.The Neap tides'magn itudeislessthanthe average.
Figure2.2SpringTide and NeapTide
2.3 ;\IarineCu r re n tEnergyConversion Devices
Nowadays,twoissuesare of major concern in thisworld:One isthaihumanbeingsare using limited fossil fuelin at anextensiverate, andsecond ly.aregenerating too much waste. Because of these issues.theenvironme ntalbalanceisindanger ofdisproportion.
Thatis why differe nt systemshave been developed to extractcleanenergy,and new and improved energymethodscontinue 10 be developed.Marine curre ntenergyconversion
10
_I
devi ces are usedtoharness energyfrom the ocea n. Marin e curre ntenergyconve rs io n de vicesmaybe eit herrotatingorreciproca ting [231.
2.3.1 Rotating Devices
Marinecurrentturbines (MCT)are used as rotating devices.A number of blades are connec ted tothehub, whichis called the rotor. Dependinguponthe axisof rotat ion ,the turbinesmaybe ofver tical or ho rizo ntal axes.
2.3.1.1 Types of Marine Current Turbines
Hydrod ynami c forcesgeneratedin the fluidflowacton theMCTs,tomo vetheblades of theturb inethat genera teselectrici ty.Theturbi nes maybeplaced onvertica lor hori zo ntal
• Hori zont alaxismari ne current turbines -Theseturbines rotatearound the hori zont alaxismo stlikel y paralleltothe curre nt flow .The applica tionscenario det ermin esthenumber of bladesof the turbines.Multi-bladedturbines develop higher sta rti ng torqu e,but thehyd rodynam ic efficie ncy islo wer.The turb ine blademaybe of constan tor variablepitc h.Evendiffer ent co ntro ltechniq uescan be app lied totheturbines, depend ingon the energyreq uire me ntsand itslocat ion [24] .
II
• Verticalaxismar ine curre nt turbin es -Theseturbine srotate aro und the vertica l axis,and mostlikel ytheserem ainperp endicul arto theflow.They alsocan be multi-bl aded .Startin g charac ter isticsare an important issu ewith thisturbine.
Savoniu s turbin esha ve ahigher starting torqu e,but Derriu sturbinesha ve alow er sta rting torqu e.Theturbineismount edon the syste m,and the genera to r is connec tedon the top or down the bott om of the sys te m. Thistype of turbine can bedragor liftbased .Som etim es a vertica laxis hybrid typeturbineismad ewith bothdragand lift typede vices,to satisfy the applicatio n requir em ent s[25 -27].
2.3.1.2 Support Structure
Supportforthe turbinesis a vitalissueforde velopin g suc h asyste m. Som etim esthe outputof the syste m isinfluen cedby its suppo rtstructure,andsomet imes the syste m has toexperi en ce very har sh environments under thesea, whilethe curre ntveloc ityistoo high.Gra vityStructur e,Mon opil e Struc ture, FloatingStructur e,and TripodStructur e are differ enttypes ofsuppo rtstruc ture[28].
2.3.1.3 Gearbox
Unde rtheocean thewater speedislo w.resultin gina slow turbine rota tio nalspee d.
Therefor e, a gear bo xisused whe nthe turbineis coupled with agenera to r.The gear- box co nve rts thehightorqu e and lo w angularspe edof theturbine tolo wtorqu e and high spee d, inorder to couple it withthegenera to r. The gear ratiodepend s on the syste m design;more o ver ,the goal is to always use gearswith the least amo unt of friction.
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2.3.1.4 Generator
Theturbine convertsoceanenergytomech an ical energ y, and thismechan ical energy is conve rted to electr ica lenergy by a generator . Basicall y, genera to rscan be class ifiedinto two groups:Alterna ting CurrentGenerator s and DirectCurrentGen er ator s.Many ofthe marine curren tenergyconvers ionsyste ms use perma ne nt magnet generato rs.
2.3.2 Reciprocating Devices
Apart from rotating de vices,ther eisanoth er kindoftool calledarec ipro catin gde vice.
Fluid flow crea tesahydrodynam iclift forceon thehydrof oilof thereciprocatin gdevi ces, whichcausesoscillat ion in them. Theyprod uce hightorquebut low spee d. Thesedev ice s arege nera lly hydrauli cpo wer take-off systems utilizin ghigh-pr essur eoscill atin g rams.
These rams pressur ize and tra nsfer thehighpress ureoil todrive ahydr auli cmot or ,which in turn drives anelectricgenera tor.Efficiencyis amajorissue forthiskindofde vice.If this syste m is sto pped, it ta kes a longtime tomake it dynamic again, becau seliftfor ceis requir ed on thefoil s[29J.
2.4 Current Status of Marine Current Energy Devices
There arc somecompa niesworking on marin e currentenerg y de vices.Anumber of them are com mercia liz ing their products,whileotherprod uctsarestill underde velopment.Day by day.thepopul arit y of mar ine currentdevices isincreasing, and Marin eCurrentEnergy Devicesaretakingpart tolarge-scale energy genera tion.
13
Seallow Project: Marine CurrentTurbineLtd.is oneoftheleadin g compan ies build ing marin e curre nt turbines.The world'sfirstever tidalturbinewastestedin 1994-5 in the Corr an Narrows,LochLinnhe,Scotl and.It couldgenera te 15kw and had adiam et er of 3.5 met er s.Thisturbineisno w in themuseum ofScotl and.The Seaflo w proj ect devel op edthe world's firsttidalturbine and themost powerful marinerene wabl e energy device in thesea.Theydid the designin g and build ing of a fullscaletidalturbin e that couldgenera te300 kw. Itwas installed3 km NE ofLynm outh ofno rthDevon coast in May 2003,and decommi ssio ned in October 2009.Recent ly,they installed a fullscale co mme rc ialsyste mknow nasSea gen,in May 2008.Seagen isthelargest and most po wer fultidal curre nt turbineintheworld . Its16met erdiam ete rtwin rotorssweepover 200sq uare met er s offlo w.Itcan achieveanefficiencyof48%overabroadrange of flo w velocit y.TheSeagenis anaxialflowpitchcontro lledrotor. It is ope ra tio nalin Strang fo rd Narr ows, Northern Irel and , witha ratedpowerof 1.2MWata curre ntveloc ityof 2.4mls.
Whenitexperie ncesahigh flow,Seagen'srotor blades can bepitchedtolimit thepo wer to apredetermin edrated power.Itcan even bepitched180°,soitcan run bidirectio nal (on the ebband floodtide).Incasesthatuseatidal curre nt turbinesystem,theturbinehas tobemount ed ona stro ng stru cture, becauseflowin gmarin e curre ntsgenera lly produce hugethru st ,typicall y in the order of 100ton s perMW, showi ng thatthemounting ofthe turbin eis alsoa very impo rtantissue. Seagenismount ed ona struc turesecure lyseatedon theseabed[30]. Figur e 2.3showstheSeagen.
14
Figur e 2.3Scagcn [30)
Evupod. Evopod isa device for generat ingelectricity fromfree flowi ng tidal strea ms, river estuaries,andoceancurrents.Itisfivebladed,having adiam eter of1.5 ITL and has a horizo ntalaxis of rotation.A IIIOth scale Evo pod(Figure 2.4) and itsmooringsystems were tested inrealtidalstream condi tionsofStrangford Narrows nearQueen ' s Univ~rsity,atBelfllst'sPona ferryMarineLabor atory.Testing of the 10thscaleunit started inJune2008 and continued into 2009.During 2010, the installation was extended to allow the device tobe grid-connected attheMarineLaboratory.Atwin-t urbine version of its Evopod semi-submergedtethered platform is under developmen t. At fullscale.the unit wouldbe finedwithtwin1.2M W rated generators.eachcoupled to a 16mdia meter three-bladed turbine.The unit wouldgenerate itscombine dratedoutputof 2.4MW in flowspeedsof 3.2m1sorabove [31)
15
Figure2.4 Evopod and FreeFlow (31.32]
Free FlowSystem:TheFree FlowKineticHydropowerSystemis ahorizontal axis turbine with three blades.It has a 4.68m diameterand wasdeveloped by Verdant Power Ltd. during2006-2008. Verdant Power successfully demonstrated this technologyin a tidalsettingthroughtheRITE Project inNewYork City'sEastRiver. VerdantPower Canadawillgenerateclean renewable energyfromthe naturalcurrentsofthe SI.
LawrenceRiver near Cornwall.Ontario.Ultimately.theproject couldgenerateupto15 MW of power locally.Figure 2.4 shows the free flow turbine[32/.
16
PulseTidal:Pulse GenerationLtd , (UK)has developed a reciprocating device (Figure 2.5)thatuses a hydraulic gencrmot.The device is currentlyin the desig n stage.In April 2008,permissio n was granted to deploythe prototype intheHumberEstuary in theUK 1331.
Figure 2.5Pulse Tidal andOpenCentre[33,341
OpenCentre Turbine:This system was developed byOpenHydroLtd. (Ireland).Ithas a 6 meterdta and anopen centre rotor and starer(Figure2.5).It has a horizontalaxisof rotationand was installedat EMECoffOrkneyin Scotland.The turbinewas connectedto the UKnational gridinMay 20081341.
Lunart:m.''1ll'TidalTu rh ine: Developed byLunar Energy Ltd.(UK)135], thedesignis still under development.The company has agreed10a 500 millionEurodeal to install300 turbinesoftthe coast ofKorea. The proposed diameteris11.5m (Figure2.6).
17
fB ·.' · "- --
- - II -~
- . . ; I, .. ---~:.. . . __
Figure2.6Lunar TidalTurbine and TidalFenceDavislIydro[35,361
TidalFencena~ isHydru Turbine:BlueEnergyLid.(Canada) hasdevelopedthis turbine (Figure2.6).Turbines arefilledinanarraykno wnas aTidal Fence.It is a four- bladed design and has a verticalallisofrotution. Itis stillinthe designstage(36 ].
Xeptu ne TidalStreamTurhlne: This device isno winthe designstage.Testing of this device is expected to commenceattheEMEC in 2011.It hastwinhorizontal axes. and three-bladed rotors(Figure 2.7).Aquamarine PowerLtd. (UK) is developing this turbine 137).
Figure 2.7Neptune Tidal and Stingray Tidal (37,38J 18
Stingray Tid alEnergyCon verter: This is a reciprocatingdeviceand utilizesa hydraulic generator (Figure2.7).A prototype was installed in theYe ll soundoff the Shetlandcoast in September2002,and was removed weeks later.EngineeringBusinessLtd. developedit [381
Oorlo vHelicalTurbin e:This turbineis 2.5m inhe ight and 1m in dia.Ithas a vertical axisof rotation and has twistedblades(Figure2.8). This device was installedintbe UldulomokStraitoff the coastofKorea ,Itwas developedby GCKTechnologyLtd (USA)139 j.
xereu s andSolon TidalTurhine:Nereuss dimension is12mx4mandSolonhas a diameter of 16m. Thcy bothhave a horizontalaxis of rotation.Ncreus isa robust turbine and Solonis a dueled deep water turbine (Figure 2.8).These were developedby Atlantis Resourse CorporationPTE Ltd. (Singapore),and weretested in 2008. Solonproducedin excess of 500kWin 8 knots of water flow duringtesting, representingover 50% throat efficiency1401
Figure2.8 GorlovHe lical Turbine andNcreus and Solon[39,401 19
2.5 TechnologyChallenges
There are many mo retech nological challenges indevel opin g a comp lete mar ine curren t energyconve rs ionsyste m:
• First, the main cha lle ngeisto synchronizethewho lesystemwith the ocean's environme nt. Indepthana lysisof resourceand devi ceinterac tion isreq uired to deliver the predicted design perfor mance.
• Turbu lenceandcavitat ionseffects;the effectsof increasi ng the size ofa scaled model and manu facturing methods are somecha lleng ing issues forsyste m design.
• Thereare somecomplexitiesto insta ll the syste m undern eath theoceaninahar sh enviro nme nt,suchas foundation or mooring issues, electr ica lconnectors,and sub mari necabling.
• Thesyste mwou ldbe functioning undernea th the ocea n'ssurf aceforalo ngtime.
This canbe addressed along withthe maint en ance, biofouling,coating,and sealing.
• Thecostofthe wholesystem and theexpectedoutputfromthe systemsho uld be balanced.
20
2.6 Conclusion
Newfoundland and Labr ado rmightbe ause fullocationformarine curre nt turbines.With properselectio noflocat ion s and techn olog y. mari neenergy canbeharne ssed on alarge scale.Inthis regard. optimiza tionofthe energy conversio nsystem is a cha llenge.The turbineis oneof the majordevices for MCECS.Wo rld w ide.man ycomp a nies are comme rci alizi ng differen tturbines. using man ydiffer ent conce pts tomeettheman y challenges ofMCECS.
21
Chapter 3
Computationa l FluidDynami cs Study
3.1 Introduction
This chapter discu ssesthetheor ybehind Comput ationalFluid Dynamic s (C FD).
beginning with thebasic workflow ofCFD.Conser vationlaw sare subsequentl y described,ina comprehensiveway.Asturbulentwakeis a commonpheno menonin under ocean flow . a synoptic discussio nhas been present edin this chapter. Non-linear,
transient,second-o rderdifferential equatio nsareappliedto describe fluid motion.
De finingaMesh isa decisive issuefor CFD analysis.CFDdoesthe calculationsto find out different unknownsin the grid nodes.Physical space isdividedinto discretepoint s, depend ingon thenumber of cellsin the space. Finermeshin thespac e doesthe calculat ions invery closepo ints,butit isnot always po ssibleto do thesimulatio ns with finer mesh,because oftime constrai ntsandcomputercapability. Mesh and numerical solution algorithms can be of differe nt types. Flow -3D hasbeen used as the Computati onalFluidDynamics softwar e packagein thiswork.Itworkson eitherfinite differen ceor finite volumealgorithms. The Volumeof Fluid(VO F) isappliedto find the surface ofthe fluid in Flow-3D .Generalmovingobjectswillalsobeexamined in this chap ter.
22
3.2 Introduction to CFD
Computationalcomplexityisthemain challenge in flow analysis thatlimitstheaccuracy of flow-relatedproblems.Turbulence,chemical reactions,heat transfer,masstransfer, and bubblesinthe fluidmake finding the solutions difficult.Hand calculationof enormous numbers of equations forfluid flowisquiteimpossible.Thisiswhy CFO has beenintroduced,to solve Ilow relatedproblems.CFOgives acomprehensive analysis of Ilowproblemsthrough properselectionof computationalparameters,grid,andresolution ofthe grid. It gives realistic resultswithin a veryshortspanof time, satisfying the cost constraints.Fora CFOanalysis,the wholeregionof interest isdivided intosmallcells.
Depending on theresolution of thegrid,theset ofdifferential equationsis solved for the Ilow.Calculationsaredone inevery smallplaceoftheregime, and the resultsare returnedtotheuser interface.Calculationsare donediscretely.A blockdiagram of CFO workllowis shown in Figure3.1.
23
Figure 3.1 CFD Workflow
First ofall,thephysical propertiesof thefluid mustberecognizedfrom a l1uidmechanics pointof viewfor a certain fluidproblem. The mathematicalequations arc writtenthat represent the physical propertiesofthefluidaccording to fluidmechanics.These equationsare usually a set of partialdifferentialequat ions.Usingthem,theNavier-Srokes Equationcanbe derived,and this is the governingequationof CFD.Tbe Never-Stokes Equation is analytical;it is understandableand can be solved on a piece of paper. Butfor a practicalapplication involving a wholeregime, numerous equationsmust beso lved to get the CFDsolution.Thus,handcalculations are notpossible,because of time constraint andcalculat io n complexity;theyhave to be doneusing computers.Therefore, these
FiniteVolume meth ods are some of the numerical discret ization meth od susedto discre tize the equatio ns.Thewholeprob lem dom ainisdivid ed intosma llgrids,or eleme nts,as discretization is based on them. Initial and bound ary condi tionsare very import antissues forafluid problem,and must bedefin edto getCFOresults.Then , the programis writte n to solvetheequat ions.Thetypicallangu ages areFortra nandC.The solutio n meth od can be eitherdirect or iterat ive.As CFOana lys isreq u iressolutio nofa largenumber ofdiffer e nt ial equations;therefore,workstatio nsor compute rswithvery goodconfig ura tio nsare requ ired to run the program.Ouringthe simulatio n,certa in contro l param eter s areused to control theconve rge nce,sta bility,andaccuracyofthe meth od. Afterfinis hi ngthesimulatio n, differe ntCFOsoftwarereprese ntssimu latio n resultsin their ownway.Some of themgive ana lysis result s from differentpoint s of view,aswellas excellentvisualizationsof theresults[41,42] .
3.3 CF D vsExperimentalSetu p
Itisno t possible to rep lacethe experimenta lsetu pcomp lete ly bythe CFO,because some times CFO inputsmay involveextensiveguess ing,andavailablecomp ute r capabilitymay be an issue.But,CFOgivesinsight into thelluid flowsthataretoo expensive,difficult or sometimesimpossibleforexperi me nta lsetups.The table shows the compa riso nof CFOandExperimentalsetup.
25
Table 3-1 Com par isonbetweenCFD andExperim enta lSetup
CFD Experimental Setup
Co st Cheap er Expen sive
Scale Any Small/Middle
Optimization Cost Lesser More
Informat ion All Measuredpoint
Safet y Yes Sometim esDangerous
3.4 Application of CFD
CFDhas awide rangeof applicatio ns.Itis extens ivelyutilizedin industries and research areas.Som e areasofCFDapplicationareas follows:
Aerospace Automoti ve Steel industr y Renewable energy
Sports Water and wastewater
Arc hitec ture Biomed icine
Chem icalProcessing Civil Engineerin g
Hydraulics Powergeneration
Turbomachinery Glassindustry Marineapplica tion Semicondu ctorIndustry Heat ventilationaircondition
26
3.5 Conser vationLaws
Thereare often too manygroupsin the fluid flow to follow.So,the Lagrangian Formulation isnot thatpract icalfrom a mathematicalpoint of view,as it focuse son a spec ific group of fluid particles.Mathematically, the Eulerian Formulat ionismuch more
practical, as it focuseson aspec ific regionof fluid.Using theTransportTheoremitis possibleto switchfro m theLagrangianFormu lat iontothe EulerianFormulation.V is an arbitrary specificgroupoffluidvolume andSisthesur face anywhere in the flow.dV is a differentialvolum ewithin Vvolumehavinga den sity of p thatgivesthemasspdV.vis the veloc ity oftluid.U,V,and Ware velocity compo nentsinx,y,andzdirecti on.v is representedas:
v= Ui+Vj +Wk
Accordingto the Conser vat io n of Mas s,the time rateof cha ngeofthemass ofthe group is zero.So,
DlDtfP dV=0
«o
UsingtheTransportTheo rem thisiswritten
f[ap/ at+V.(pv )IdV= 0
vro
(3. 1)
(3.2)
Accordin gtotheConser vat ionof Mass, thetermin thesq uarebracketsis zero.
Expandin gthisterm,itcan be written
ap/at+paUlax+pav/ay + paw/az+uap/Dx +vapta y +WDpta z =0 (3.3) Accordingtothedefinition of incom pressi b le fluid,thechange ofden sityis ze ro.So,
27
aU/a x+ay /ay+aw/az=0 Eq. 3.4is called the ContinuityEquation.
(3.4)
For a differentialvolumedY,themomentum would be pdYv.The time rate of changeof the momentumof the group isequal to the net forceacting on it,called Conservation of Momentum.So,
D/D dpvdY=f(JdS +fpbdY
vro sro Y(t)
(3.5)
Where(Jis force perunit area at a pointonthesurface.Surface forcescan bedueto pressure and viscous fraction.bis a vectorrepresenting thebody forceperunit massat anypoint within the volume.Body forces arebasicallydueto gravity.TheTransport Theoremgives
f[a(pv)/at+~. (pvv)1dY=fadS + fpb dY
vro S(t) Y(t)
Manipulationgives
f[Xi+Yj+ZkldY
=
0 Y(t)Mathematicsrequiresthat
x , o y=o z=o
Expansiongives Xmomentum
paUlat+P(UaUl ax+yaUlay+ WaUl az)=-ap/ax +a/ax(AlaUl ax+aYlay+aw/az])+a/a x(~l[aUl ax+aU/ax]) +a/ay(11lay/ax+aU/a y])+a/az(11raw/ax+aUla z])
(3.6)
(3.7)
(3.8)
(3.9a)
28
Y momentum
pav /at+ P(UoV/ox+vav /ay+ WaV/8z)= -oP/oy
+a/By(A.[oU/ax+av/ay+aW/8z])+%x(ulov/ax+oU/oy]) + o/ay(fl[av /ay+oV/oy])+%z(fl[oW/oy+ov/az))
Zmomentum
pow/at+ P(Uaw/ox+VoW/oy+waw/az)=-ap/az-pg
+%z(A.IoU/ax+av /ay+aw/az))+a/ax(fllaw/ox+aU/az]) +a/ay(fl[ov/az+aw /ay))+a/az(fl[aw/az+aw /azD
(3.9b)
(3.9c) Stokes'hypothesis statesA.=-2I3fl.A fluidlikewater isincompressible and has constant viscosity.Inthis case, Equation 3.9reducesto
Xmomentum
poU/at+p(UoU/ox+vaU/ay+WoU/8z) =-ap/ax
+I.l(au2/ax2+au2/By2+
au
2/8z 2) Ymo mentumpav /at+P(Uav/ox+vav /By+WaV/8z)=-ap/oy
+fl(av2/ax 2+aV2/By2+aV2faz2) Zmomentum
paw /at+P(Uaw/ax+vaw /ay+wow/az)=-oP/az-pg +(aw2/ox2+aw2/a/ +aw2/az2)
Thesearecalled NavierStokesequations,whichare thebasis ofmostCFD [43].
(3.lOa)
(3.lOb)
(3.IOc)
29
3.6 Turbulent Hydrodynami cs
Fluidparticle mo tio nhas ala minar or layered structure at lo w spee d .Influid, partic les can mo veas groupsin small spinning bodies. Theyare callededdies.Athigh speeds, ther eis a chao ticor random motion of theedd ies . Thisflo wpattern is called turbulent flo w.Aturbulent wake flow contains lot s of smalledd iesalongwithsome large edd ies.
Sma llereddies are carried alongby thelocalflo w and those are associatedwith turbulen ce.Butthelarge eddiesremain roughlyinplace and the fluid in the mswirls aroundandaro undorrecirculates roughlyaroundclosedorbits.Thesma lleredd ies remain neartothe wake boundarieswhereas large eddies re maininsidethe wakes. Smaller edd iesaregeneratedin regions where velocit y grad ientsare high,like atthe edgesofthe wakesor the boundarylayers close tothe struc tu res .Theyare dissipa tedin theregion where the veloc itygradientsare sma ll,suc has inshe ltere dareas like corners.
Conse rva tion laws are applicab lefor turbulentwake flo wsbut the y areso comp lex that ana lytica lsolut ionsarenot possible.That'swhyCFO hasbeende velop edbased on the conserva tio n lawsto servethe purpose.Smallereddiesareso small that theyrequirevery finegr idsandsma ll time stepsto followthesma lledd ies. Smull eddiesareI mm indiu.
So,for CFOitreq uiresa grid spacing less tha n0.1 mm to follow the edd ies.CFO wo rks ona setof Alge bra icEqua tions(AE) thatare conver tedfrom the govern ingequatio ns.
Enor mo us numbers of AEshave to be able tohandl eto follow the smalleddies.So, work able CFO isnotpossible,becausea computerca nnot handl e the hugenumber of AEs.Forexamp le, ifthe gridspac ing is 0.1 mm, a100 mX 100 m X 100mvolumeof water wouldneed106X 106X 106or1018gridpoints. Italsorequiressma ll time ste ps to 30
dothe simulations.But, currentlythere isno computer thatcan handle somany grid pointswithsmall time steps.The fluid appears more viscous thanit is when the small eddies inaturbulentflowdiffusemomentum.Models which accountforthisapparent increaseinviscosityare known as eddyviscosity models.Thistype ofmodelis obtained frommomentum equationsby a complex time averaging process.Thedevelopedmodel canestimate the variationof eddy viscositythroughoutthe flow.Now, it isnotnecessary to followthesmalleddiesbecause,according tothemodeling,they are already suppressed by eddy viscosity.Consideringthis,aworkableCFDispossible with larger gridspacing and timestepping. However,it requires102X 102X 102or106grid points whereaspreviouslyitrequired1018•Therefore,aworkableCFDis only possiblewitha turbulencemodel.Aswell,hydrodynamicflows areoften turbulentflows.The governing equationsfor flowwith thismodelcan be writtenas:
Momentum Equations:
p(aUla t+uaU/ax+vaU/ay+ waU/ az )+A= -ap/ax + [a/ax(~laU/a x)+a/ay(~IaU/ay)+a/az(~aU/az)1
p(av/at+uav/ax+vav/ay+wav/az)+B=-apla y +la/ax(~lav /ax)+alay(~lav /ay)+a/az(~laVlaz)I
p(aw /at+uaw/ax+vaw/ay+waw/az)+C=-ap/az-pg +[alax(~law /ax)+alay(~law /ay)+a/az(~law /az)1
(3.1Iu)
(3.l l b)
(3. l lc)
31
Where UV Ware thevelocitycomponentsin the x y zdirections,P ispressure,p isthe density of water,and 11 isits effectiveviscosity.
Conservationofmass is consideredas:
DP/Dt+p c2( DUlOx+DV/Dy+ DWIDz)= 0 (3.12)
Where, cisthe speedofsoundin water. Basically, water isincompressiblebut formass conversion CFDconsiders itas compressible.
Engineersareinterested increatingmodels that account forthediffusive characterof turbulenceratherthandetails of eddies.Apopularturbulentmodelisthek-smodel.Kis theintensity of theturbulence andeis itslocal dissipationrate.Governing equations for thismodel are:
Dk/at+UDk/Dx+VDk/Dy+ WDk/Dz= Tp- To +[DIDx(iliaDk/Dx)+ DIDy(IliaDk/Dy)+ DIDz(IliaDk/Dz)1
DelDt+ UDElDx+ VDElDy+WDEIDz=Dp-Do +[DIDx (Ill bDelDx)+ DIDy(IlibDElDy)+DIDz (u/bDElDz)1
Where
(3.13)
(3.14)
Tp = GIl ,/ p Dp=Tp C, El k To = Co e Do =C2e2Ik III=C3k2Ie 11=III+~ll
Where, Co=1.0,CJ=1.44,C2=1.92,C3=O.9,a=1.0,and b=1.3 areconstants based on data fromgeometricallysimpleexperiments,~lJisthelaminar viscosity, IIIis extra
32
viscosity due to eddymotionand G isa complexfunction of velocitygradient.In practice,the k-sequation s account for the convection, diffu sion ,production, and dissipation of turbulen ce.Equation 3.13 isan intensityequation where Tpisthe production termand Toisthedissipati onterm.Equation 3. 14 isa dissipationequation whereOpisthe productiontermand00isthedissipationterm.The time averaging process intro duces A,B, and C termsin the momentum eq uatio ns.They are complex funct ionofthe veloc itygradient. To simplifyconside ra tio nofthesharp no rmal gradie nts inveloci tya nd turbulencc ncarawa ll,specia lwall functi on s are used.
Fortheanalysi sinCFO, the flow field isdiscreti zedbyaCartes iansyste mofgrid Iincs.
Sma llvolumesorcellssurroundpointswheregridIincs cross.If there is anyfixedbody in the flow, then flow isnotallo wed in the cellscontainingpartsof fixed body. For general moving objectstheposition of the body keepschanging with time as the bodyis cha nging itspositi oncontinuou sly.Boundaryconditionsare establi shed,accordinglyflow enter and leavethc region of interes t. Actually.CFO workson the governing equation s of the flow. For CFO each governi ngeq uatio n isput into the form
aMlot = N (3. 15)
For CFO,eq. 3. 17 isthc template which isapplied to eachgoverning equation at each point in a grid.The equation s are integratednumeri call y acrossa time step.Thetime steppi ngeq uatio nis in the form of
M(t+Lit)=M(t) + Lit N(t) (3.16)
Wherethe vario us deri vat ivesinN are discre tize d usingfinitediffe ren ce approx imatio ns.
Alge braicequationsfor thesca lars P,F, k, andEat pointswheregridlines cross canbe
33
found by thediscretizationand equations for the velocitycomponents at staggered positionsbetween the gridpoints.
N
1 w P E
y 5
I -
6,-
Figure3.2 2-DGrid
Central differencesare used to discretizethe viscousterms in themomentumand turbulenceequations.Fo r a2Dgrid, likeFigure 3.2, if a diffusionterm in the governing equations wouldbe8U2'0)(2,thiscanbeapproximated by central differences like
Again,most ofthe equations haveco nvective terms.likeV(cUl o)().By usingcentral differencesit becomes
V(iJUlJx)<::Upl{U"+Up)/2-(Up+ Uw)/ 2j16x'" Up(UE-Uw)126 .... (3.17)
This treatmentofconvectivetermsusually leadsto numericalinstability.To avoid this CFDcodes use upwinddifference . That gives
U(DV lox )<::VI'[(Up -Uw)/6xj flowfromwest U(i' Ul ox )::=VI'I(V" -VI')16)(Jflow from east
(3.18a) (3.ISb) 34
Toensure numerical stability,a combinationofcentral and upwind differencesisusedfor theconvectiveterms.Sometimesupwinddifferencing generatesafalsediffusion.It makesthetluid appear more viscous. So, anotherscheme namedskew differencinghas beendevelopedto counteract it.Collocationor lumpingisused forthe T and 0 terms.To marchtheunknownsforwardin time,themomentum equationsare usedtoupdateU,V, and W;themass equation isusedtoupdatePandcorrect U,V,and W.The processis repeateduntilvelocitiesand pressure bothconverge. Theturbulence equationsare usedto updatek,ande[4 3].
3.7 Volumeof Fluid
The volumeoftluid conceptis usedtotrackwater surfaces. Aspecial functionF is known asthe volumeoftluid or VOF function whichisusedtolocatethe surfaceofthe water.Material volumeconsiderationsgive:
aF/a t+uaF/ax+vaF/a y+waF/az= 0
In CFD,forwaterF istaken tobeunity andfor air F istaken to be zero.
(3. 19)
35
F=O F=O.05 F=O.J.'_-
-r---_
F=O F=O.-l F=l F=l
Figure3.3F Valuein CFO Grid
Figure3.3showsa20 view of the gridinCFO. However,F has got different value in Figu re 3.3.IfF=I,thereis waterinthatgrid,and ifF=0,thereisair in that grid. ButF having a value in betw een0and)sig nifiesthewater surface.In the Figur e 3.3,valuesof F determin etheposition ofthe watersurface. TheVOF equation ,equation3.22,isused toupdateFand thelocation of thewatersurface[4 3 J.
3.8 MovingBodiesinFlow
CFO can handl e genera l mo ving objects.Itcan handle constant pressure bubblesin no w, suc has,the air bubblesbehi nd a bodyimpactingafluidsurface.A mo vingbod y can be lookedupon as abubblewith pressure that variesalong its surface. Thepre ssuremustbe suc h thatit producesa bubble thathasthesame shape asthebody.In fact,bubbles can be ofanyshape 146] .Figur e 3.4shows a triangula r bubblewherepressure acts frominsid e thebubble.
36
Figure 3.4Pressurein a triangularbubble
Flo w 3Dcan includemovingobjects.Itallow smor ethanone movingobjectina problem. Each roov'ingobject can have differentmovement.Theobjectmoves withsix degree offreedomorlhe movement of theobject can be user defined. In thiswork.ithas been worked wilhonlyone moving object thai moves wilhsix degree of freedom(.HI.
This chapter examines the theory forCFD.1be neXlchapterdiscusses the eJ.perlmcnts onthetw istedSavcnius turbine in Flow-3D.To findlhe fluW;! surface. !he F value is calculatedfor every cellinlhe problemdomain.Flow-3Dhas a featurevolumeoffluid for findingthePvafue.
37
Chapter 4
Experimentson theTwisted Savonius Turbine
4.1 Introdu ction
This chapter describe sthe experiments on the drag type device.the twisted Savonius turbine. which has been acknowledged to meet the design req uire me nts for the North
Atlantic Ocea n Current.Thischapteralso discusses theeq ua tions thatdeterminethe charac teris ticsof theturbine.Thefinalsectio nof this chapte r dealswith theturbine testing in theFlume tank.Theturbine has alsobeen testedina wind tunnel,to observeits behaviorin wind.Therefore, the resultsof theturbineinbothwaterandwindhave been studied.
4.2 TwistedSa voniusTurbine
The twisted Savoniusrotor is avertical axismachine with a high starting torque and a reasonable peak power output [44].The twistedSavonius rotor has an "S-shaped" cross- section. The concept of the Savoniusrotor is based on the principle developed by Flettner [45]. The power from the rotor isbasedon the difference in pressure across the blade retreating from,andadvancing into, the fluid.This is,in turn, relatedto the differencein thedrag coefficientsassociated withtheconvexand the concavesideofthe blades[46, 471. TheSavoniu s rotor has a simp le struc ture.Thou ghit has good starti ng charac teris t ics,itoperates atrelatively low speedsandhasthe ability to acceptfluidfrom any direction.Itshyd ro dyna mic efficiency is lower compared to othertypes ofturbines, 38
like Darrieusand propellerrotors.The drag force of fluid actingon itsblade isthe driving force for the twistedSavonius.However,at lowanglesof attack,lift forcealso contributesto torqueproduction [46). Hence,the rotor isnota pure drag machine,buta compoundmachine, andcango beyond the limitation of Cp of a primarily dragtype machine.Becauseofthe highstatic torque,sometimes Savoniusisused asthestarter with other typesof turbineshavinglowerstatic torque, liketheDarrieusSavoniusHybrid turbine. Though thestatictorqueishigh,it isnotuniform atdifferentrotorangles.At certain rotor angles,Savoniusrotorscannotstarton theirown, asthe coefficientof static torqueisnegative.Thetwisted Savonius designisveryconsistentinoperation,and also has a much higher averagepower outputcompared to conventionalones.Therefore,the twisted Savoniusisone of thebestrotorsfor generatingsmallscale power from a turbulent fluid flow. But theprocessof constructinga twisted Savoniusturbineis extremely complex,requiring expensive materialsand machinery, making the manufacturing costveryhigh.Thoughthe conventionalSavoniusturbine hasa very simpleshape,the twisted version hasa complex three dimensionalgeometryby comparison.The radius oftheturbine is squeezed asthe turbine istwisted,which occurs because of the geometric principlesof the blade,not justthe limitationsof the materials.
A greater angleof twistresultsin a greater potentialefficiency in operation. Kamij i, Kedar and Prabhu completedexperiments onahelical rotor(48).TheSavoniusrotorand thetwistedSavonius rotor both have thesamecrosssectionalshape,liketwo semicircles inan'Sshape' throughout the turbine body,but threedimensional viewsarecompletely different.Figure4.1showstheCAD view ofthe conventionalSavonius, and Figure 4.2
39
sbows (hethree dime nsiona lview ofthetwisted Savomu sturbine.Solidwo rks has been usedrodesign tbe turbme s
Figure 4.1Con\'enlionalSavoeiusTurbine(ThreeDimen sional CAD View)
Figure4.2Twis ted SavoniusTurbine (ThreeDimensionalCAD View )
11le twistedSavomus is SOl'l"Cwhatsimilar10 the:Savonius, but ilhasatwistof Iscfaklng ils verticalaxis.This is whyitchanges the direction of ils body shape in e\"CT)'singlesrep.
Figure~.3shows the: sidevjewand thetopview afterrerooyingthe: end plateof the turbine.
(.) (b)
Figure 4.3 a)LeftView (b) Top View(removmgtbeendplate)ofTwisted Savomus Turbine
A Germanph)sici~t,AlbertBctz,concluded in19 19lhal 00 windlurbine can convert more than 16127 (59.3%)of thekind ;; energyof lhewind intomechanicalenergy.
lurning a rotor. To thisday.thisisknown asthe BetzlimitorBetz'law.The theoreuc al maximum power efficiencyofany design of wind turbine is0.59.The conceptofthe Savonius rotor isbased on the principle developed byFlen ner.The coefficientof performanceis of theorderof 15%forSavonius[441.Experiment s onconventional Savoniusrotorswith anover lapratio of 0.15aooan aspectralloof 1.0 havebeen reponedtohavea maximumcoefficientofpower at 0.17 3(49J and 0.17.whentested in 41
an openjet wind[50].Inan effort toimprove the efficiency, minor changesare made in the shape of the conventional Savcnius rotors. and these rotors have been referred10as modi liedSavoniusrotors.AmodifiedSavoniusrotor with a shaft is reported!Ohave a maximumcoefficient of power of around0.32151].Figure4.4 shows the efficiency of differenttypesof turbines.
0.6
1 - - - - ----==========-1
Turbinetip speedratio
Figure 4,4Power coefficient vsTSR Curve ofDifferentTypes ofTurb ines [48]
If a turb ineis placed inthe water,the poweravailablein theareais p•••i1•b1e= O.5pA,vl
WhereP....I. b1.isPower availablein water (Watts)
(4.1)
42
p isDe nsity of water(kg/m3)
Asis(He ig ht xDiameter)=(HsxDs), isthe sweptarea ofSa voniu s rotor(m2) v=Velocity of water(m/s)
IfCp isthe power coefficient ofthe twisted Sa vo nius turb ine, thepo wer capt ured bythe turb ine is
P=PavailabltCp
Thetipperiph er al veloc ityoftherotor is Us= wxR Where , wi stheangul arvelocity o f Savoniu srotor R(=D/2)istheradius oftheSavoniu srotor No wtheTip SpeedRatio (TS R)of aSavon ius tur bineisdefin ed as TSR=).=
V
wR(4.2)
(4.3)
The aspectratio(A) represent sthe height(Hs) ofthe rotorrelativeto its diame ter(Ds) . Thisis also an importantcriterionfor the performance oftheSavo niusrotor:
H 04= - D
(4 .4)
The re are a numbe rof geometrical parametersthat affect the efficiencyofthe Savonius rotor.Amo ng these,the aspect ratioplaysan import antrole in thehydr od ynami c perf ormance.Values of Aaround4.0seem toleadto thebestpo wercoefficientfor a conve ntiona l Savonius roto r. As well, end plates lead to bett er hydrodynamic performances.Theinlluence of the diamete r Drof these end platesin Figure4.5relati ve tothediameterDofthe rotor has beenexperime nta llystudied. Thehigher valueof the 43
power coefficient is obtainedfor a value of Of around 10% morc than D. whatever the:
velocitycoefficient[521.Tbere is another geometricalfactorthe:called overlap ratio.Itis
o - ~
(4.5) wberee is overlap between two buckets.
Disrbe diarrerer of esch becket sbown in Figurc4.5.
Better efficiency can be achieved for~values of 0.2to 0.3.
Figure 4.5To p ViewofT...istedSavo nius Roto r
4.3 Mat erial Selection
In a marine environment,asub-marinestruc ture ha.to withstandsalty water , abras ive suspended particle s, and fouling growth.Therefore, it is challenging for asub-ma rine structure to be there for a long time,because of the aggressive undersea environment. The designers first considerproducing the required marine rotors in steel. However, achieving thenecessary compound-curvedprofile in steel is expensive.Moreover, steelis very heavy, proneto fatigue,and susceptibleto corrosionind uce dby saltwater. Because of thesedisad vanta ges, adec ision wasmadetouse compos ites instead .If plasticmateri als
areused, theyeasethe fatigueproblem, both thro ughtheir inhere ntfatigue toleran ce and reduced bladeweight. Temperature of the water is anotherissue. but the te mpera ture range in water ismuch more limited than atmospheremaking the material selection easier [53].In thiswork,fibergla sshasbeen usedfor building the twistedSavoniusturbine prototype.Fiberglassis afiberreinforced polymer made of a plasticmatrix reinforced by fine fibersof glass.Itis a lightweight and strong material. At the very beginning of the research work.it wasplanned to designa twisted rotor with ribs shown in Figure 4.6.But due to the complexity of this,the ribswere left out. The turbine itself has a complex geometry.Itwas impossibleto build thisturbine at the TechnicalDivisio n of Memorial Univers ity.The TechnicalDivisionasked for a hugeamount to get theturbine bui ltfrom someother place. andthisquote was toomuchto spe ndfor theSeaforma tics project.
45
Figure 4.6TwistedSavonius withRibs
As a result. buildinga prototype ofthe turbinein the rapid prototype mach ine was attemptedatMe morial University.Unfortunately, problems with the rapid prototype machine cccured.Itcould make50'1of the model perfectly.but afterthatmachine could nol remain in the proper track tobuild the model !()()%.according 10 the design.At this lime, SeacraftInternational was a\ked 10 build this lurbine.'They also askeda huge amounllo build the full size lurbine of I meierinheighl.Atla'I,~tr.Wa llaceRobert. of SeacraftImemat jonal, gave lhe pectorjpe of lhe turbine free of charge 10 perform lhe experiments onil.Tbe turbineusedinthis work is a handmade turbine. builtbyWallace Roben ,SeaCr aftInternat io na l.
4,4 I'rulul)'pt"ofTwi..ted Savoni us Turbine
Figure 4.7~howsthe prototype of the twistedSavoniusturbine,Iti~held vertically by using the framelhal can be sbownin Figure 4.7.Thedimen~ion~of the rotoran: 18 inches inheighl and6inchesin diameter.IIhas two e-ndplale-Sand docsnothave- any central shah.Itdoesnothave-any gap between theIwOsemicircular beckers.
Figure 4.7 PrototypeoftbeTwistedSavoniu s Turbine
47
Characterist icsforthe protot ype ofthetwistedSavoniu sturbine are asfollows:
Heig ht Dia meter
Overlap ratio Aspectrat io Swept Area
18inche sor 0.4572 meter 6 inches orO .1524meter
108 sq uare inche s or 0.07 sq uare meters
4.5 Experimental Setup inFlumeTank
Thetwisted Savoniusprototype wastested in theflumetankat theMarin eInst itute of MemorialUniversity of Newfoundland,Canada.Itcontains1.7 million litersof water in total.If the tankisfull of water,itisdivided into twosectio ns:theupper and thelow er sec t ion.Theupper sec tio nisthetest section and allowsobser vati onfrom above and from theside,and thelow er sec tionallowswater toflow.In the return sec t io n, there are three impellers,or pumps,that areusedto circ ulate the wateraround the tank.Eachpump is driven bya 125HP DC mot or .TheFlume tank in the Marine Institut eistheworld ' s large sttankofitstype .TheFlumeTank is 22.25 m long ,8 m wide and4m deep [54 ].
Wat er ve loc ity began from0.1mls.Itincrea sedby0.05mlsineverystep,and thetest result s we re recorded.0.98 mlsisthehighest curre ntspeed providedbythe Flume tank.
Figure 4.8sho ws the turbine when it is submerged verticallyin the water.
48
Figure 4.8 Tellting ofT... isted Savonius in Flume Tank
The turbine was hooked up ... ith the frame arxl a shaft ...as takenout to integrate the load cell arxlthe torque arm with it.The turbine was held withtwo bearings on the t...o ends with the frame. All theinstrume ntswere attached on the top of the turbine for data collection.The rotatingshaft of the turbine is connected to a magnelic brake.The brakeis impo sed onthe turbineby applying voltageacross it.Voltageis proponionalto the brake and once the brakeis applied to it, il tries10stop the turbine. Voltage is increased ontil theturbine is fullystopped. During thistime ,datais encoded by theload cell. Al50mm lo ng torquearmhas been usedin thisexperi me nt. The frame holding the turbinewas attached with theplatform above theFlumelank with aclamp. Angular speedis mea suredby theNikon rotaryencode r R X A IOOO-22-IAattachedon the topof the shaft. The encoderis shown inFigure 4.9.
49
Figure 4.9Rotary Encode r
Allthemstrurrera s,itlClud ing load cell,torquearm. androtary encoder,are connected ... ithueDAQsystemwheredata areprocessedand fedtothe lapto pby USB. Labvjew has beenused for the observing the data intheU!>Crinterface.Figure 4.10 sbows the C'Ofl1)Jele~tup for the twisted Savomes turbine in the flumeTank.
50
Figure: ·1.10 Experimental Setup for Twi ..led Savonius
"
4.5.1 TestResultsin Flume Tank
The turbine was tested in the Flume Tankat differentcurrentspeeds. Currentsvaried from 0.1m/s to 0.98mls.The turbine started to rotate whenthecurrent speed reached 0.4 m/s, The current speed in the tank wasconstant for different torque imposed onitby the attached magnetic brake.MATLAB has been usedto plot the data. Figure 4.1 1 shows the rpm of the turbine considering data at different current speeds (Appendix A).
CunenlSpeed-.d RPMcurveoflhe lu rbone
~120 of
r:
ct6lJ~ : /""' 1
Figure 4.1 I RPMat Different Current Speeds
Figure 4.12shows the maximum power available at the turbine shaft at different current speeds.This curve has beengenerated. consideringonly thepeakpo wer atcertai ncurre nt speeds.
52
6
5
/
3
/
2
/ /
1
/ /
f---~
~4 06 0.7 08
WaterSpeedin mls
Figure 4.12 MaximumPower OutputatDifferentCurrentSpeeds
Figure 4.13shows maximum power outputoftheturbine at differentrpmoftheturbine.
6
5
/
3
V
/ V
L-/:---' -r-::
100 120
Maximum RPM of turbine
Figure4.13Maximum Powerat DifferentRPM
53
In theexperiment.datais recordedfordifferentcurrem speeds.Ifall the dataare considered togetber,a curve can be generated using the bin method.Figure4.14~ws thecurve forthe power output oftheturbine vs currentspeed.This prolotype can generate morethan 2 watts at a current speed of0.98nYs, wbereastheoretically it was expected to generatemorethan3 wansatthat speed.Frcr o ncauses thisdifference.
I' l:
j:
!.
Figure 4.14 Turbine Output vsCurrentSpeed Curve
I
j"
Figure 4.1:lTurbineTorque vsCUITCnlSpeedCurve
Figure 4.15showsto rq ue of the lurbineatdiff~nlcurrento;p«'ds.TbeCp-J. curve for each individual current speed is shown inFigure 4.1b.The bouom curveis at0.0$ rrVsand graduallythe currentspeediocre ase s by0.1rrVs.Aftt'f0.0$ rrVs.the figure showscurves for0.5rrVs,0.6rrVs,0.7rrVs.0.8 rrVs,0.9rrVs.andO.91irrVs.
1 °
12-s "
~
!
OOBs
Figur e 4.16Cp-J.Curv eat DifferentCUlTCntSpeed
Figure 4.17sho ...stheCp- J. curveforthe twistedSavomu sconsideringdata at alllhe currentspeeds.The collec ted data hasbeen smoothed witha spanof 10%. A span of datu has beenconsidered and then smoo thed by a moving averagefille r. The twisted Savonius turbineshe ...s a peakefficiencyof 12.50%around the TipSpee d Ratio of 0.8
"
0. 1 6r--~-~'-~--~---,
014
012f ··· : ' - . " ' "
s J
0,1'S 0.03
~
~
ooof ···' !···:·j
Figure4.17Cp- ACurve of the Twisted Savonius
4.6 WindTun nelTestof the TwistedSavuniusTu r h ine
The twistedSavoniu sturbinehas also been testedin the windtunne l. For windtunnel testing,theturbine wasmount edon astructure witha plate of higherdiameterthanthe turbine.The plate of thestr ucture was glued to one end of the turbine. and the platewas directlyconnectedwith theshali. The turbine body was mounted vertically insidethe windtunnel.Other urrangemenrsforinstrumentation remain out of the tunnel.andthe structure was attached to thewindtunnel's wooden floor usingscrew s.Figure4.18 shows the turbine insidethe wind tunnel.
56
