Power from the Sun Chapter9Word文档格式.docx
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Theresultinghardwareistermedthecollectorsubsystem.
Thischapterexaminesthebasicopticalandthermalconsiderationsthatinfluencereceiverdesignandwillemphasizethermalreceiversratherthanphotovoltaicreceivers.
Alsodiscussedhereisaninterestingtypeofconcentratorcalledacompoundparabolicconcentrator(CPC).
Thisisanon-imagingconcentratorthatconcentrateslightraysthatarenotnecessarilyparallelnoralignedwiththeaxisoftheconcentrator.
∙Tocompletethissectionwedescribeengineeringprototypeconcentratorsthathavebeenconstructedandtested.
Parabolicconcentratorsthatarenotcommercialproductswerechosenfordiscussion.
Thisallowsfreediscussionwithoutconcernforrevealingproprietaryinformation.
Inaddition,theprototypeconcentratorsdiscussedarerepresentativeoftheparabolicconcentratorsunderdevelopmentforcommercialuse,andconsiderabledesigninformationisavailable.
Performancedatafromsomeearlyprototypesarepresented.
Thedevelopmentincludesthefollowingtopics:
∙ReceiverDesign
oReceiverSize
oReceiverHeatLoss
oReceiverSizeOptimization
∙CompoundParabolicConcentrators(CPC)
∙PrototypeParabolicTroughs
oSandiaPerformancePrototypeTrough
∙PrototypeParabolicDishes
oShenandoahDish
oJPLPDC1
∙OtherConcentratorConcepts
oFixed-MirrorSolarCollector(FMSC)
oMovingReflectorStationaryReceiver(SLATS)
oFixed-MirrorDistributedFocus(FMDF)(sphericalbowl)
∙PrototypePerformanceComparisons
Specialnotetothereader:
Theprototypehardwaredescribedinthesectionsbelowrepresentsthestate-of-the-artinthe1970´
sandearly1980´
s.
Forupdatesoncurrentstatusofsolarconcentratorhardware,thereaderisreferredtothewebsiteofTheSunLab(combinedeffortsofSandiaNationalLabsandtheNationalRenewableEnergyLaboratorywebsite:
http:
//www.energylan.sandia.gov/sunlab/overview.htm
andtheInternationalEnergyAgencywebsite:
//www.solarpaces.org/csp_technology.htm.
Readersarealsoencouragedtoaccessthewebsitesofdifferenthardwaremanufacturers.
9.1
ReceiverDesign
Thejobofthereceiveristoabsorbasmuchoftheconcentratedsolarfluxaspossible,andconvertitintousableenergy(usuallythermalenergy).
Onceconvertedintothermalenergy,thisheatistransferredintoafluidofsometype(liquidorgas),thattakestheheatawayfromthereceivertobeusedbythespecificapplication.
Thusfarwehaveconcentratedourattentiononreflectionofincidentsolarenergyandnotbeenconcernedwiththegeometryofthereceiver.
Therearebasicallytwodifferenttypesofreceivers-theomnidirectionalreceiverandthefocalplanereceiver.
Ratherthandealincompletegeneralityandtalkaboutthemanypossibletypesofreceiversthatcouldfallintothesetwocategories,wediscussonlytwowidelyusedreceivers,thelinearomnidirectionalreceiverandthepointcavityreceiver.
Thiswillnotartificiallylimittheapplicabilityofthedevelopmentofthefollowingparagraphsbutwillprovideanicefocustothediscussion.
Figure9.1isasphotographofalinearomnidirectionalreceiverusedwithparabolictroughs.
Itconsistsofasteeltube(usuallywithaselectivecoating;
seeChapter8)surroundedbyaglassenvelopetoreduceconvectionheatlosses.
Asthename´
omnidirectional´
implies,thereceivercanacceptopticalinputfromanydirection.
Figure9.1
Linearomnidirectionalreceiver,(a)photographofoperationalreceiver;
(b)sketchofreceiverassemblycross-section.
CourtesyofSandiaNationalLaboratories.
Figure9.2isasketchofacavityreceiver.
Thisisclearlynotanomnidirectionalreceiversincethelightmustenterthroughthecavityaperture(justinfrontoftheinnershieldforthisreceiver)tobeabsorbedonthecavitywalls(coiledtubesinthiscase).
Figure9.2
Cavity(focalplane)receiver.
Typically,theplaneofthecavityapertureisplacednearthefocusoftheparabolaandnormaltotheaxisoftheparabola.
Thussuchareceiverissometimescalledafocalplanereceiver.
Althoughthecavitycouldbelinearandthususedwithaparabolictrough,acavityreceiverismostcommonlyusedwithparabolicdishes.
Figure9.3isaphotographofthissameparabolicdishcavityreceiver.
Figure9.3
PhotographlookingintothecavityapertureofthereceiverofFigure9.2.CourtesyofSandiaNationalLaboratories
9.1.1
ReceiverSize
OmnidirectionalReceivers-TheappropriatesizeforanomnidirectionalreceiverwasdevelopedinChapter8.
ThediameterofatubereceiverisΔrasdefinedinEquation(8.44)(and2r1
asshowninFigure9.1b).
Areceiverofthissizeinterceptsallreflectedradiationwithinthestatisticalerrorlimitsdefinedbyn.
Thisequationisreproducedhereasanaidtothereader.
(8.44)
wherepistheparabolicradius,nthenumberofstandarddeviations(i.e.definingthepercentofreflectedenergyintercepted),andσtot
theweightedstandarddeviationofthe
beamspreadangleforallconcentratorerrors,asdevelopedinSection8.4anddefinedbyEquation(8.43).
Aswillbedescribedbelow,thevalueofn(i.e.thenumberofstandarddeviationsofbeamspreadinterceptedbyareceiverofsizeΔr),isdeterminedinanoptimizationprocessbasedonbalancingtheamountofinterceptedradiationandamountofheatlossfromthereceiver.
Putinsimplifiedterms,alargerreceiverwillcapturemorereflectedsolarradiation,butwillloosemoreheatduetoradiationandconduction.
CavityReceivers-Theappropriatesizeofthecavityopening(i.e.itsaperture)isdeterminedusingthesameopticalprinciplesusedinthedevelopmentofEquation(8.44)butthenprojectingthereflectedimageontothefocalplanewherethereceiveraperturewillbelocated.
IfthebeamspreadduetoerrorsissmallinFigure9.4,theanglesα
andβ
areapproximately90degrees.
Thustheprojectionoftheimagewidthontothefocalplaneis
(9.1)
SubstitutionintoEquation(8.44)yields
(9.2)
Figure9.4
Sizingofcavityapertureconsideringbeamspreadingduetoerrors.
SelectionofConcentratorRimAngle-Itisinterestingtostudytheimpactofreceivertypeonthepreferredconcentratorrimangle.
Thewholeideaofaconcentratoristoreflectthelightenergyincidentonthecollectorapertureontoassmallareceiveraspossibleinordertominimizeheatloss.
Figure9.5isaplotoftherelativeconcentrationratiosforbothcavitiesandomnidirectionalreceiversasafunctionofrimangle.
TheconcentrationratioforthetwoconceptsistheratioofthecollectorapertureareadividedbytheareaoftheimageatthereceiverasdefinedbyEquations(8.44)and(9.2),respectively.
Notethatthecurvefortheomnidirectionalreceiverincreasesuniformlyupto90degrees,whereasthecurveforthefocalplanereceiverincreasesuptoarimangleofabout45degreesandthendecreasesbecauseofthecosineψ
terminthedenominatorsinEquations(9.9)and(9.10).
Figure9.5
Variationofgeometricconcentrationratiowithrimangle.
Theimpactofthisphenomenonisthatmostconcentratorswithanomnidirectionalreceiverhaverimanglesnear90degrees.
Ontheotherhand,concentratorswithfocalplanereceivershaverimanglesnear45degrees.
Thecurvesshowonlytrendsforeachreceivertype,andtheirmagnituderelativetoeachotherasshowninFigure9.5isnotcorrect.
9.1.2
ReceiverHeatLoss
LinearOmnidirectionalReceivers-TheheatlossratefromalinearomnidirectionalreceiverofthetypeshowninFigure9.1isequaltotheheatlossratefromtheoutsidesurfaceoftheglasstube.
Thiscanbecalculatedasthesumoftheconvectiontotheenvironmentfromtheglassenvelopeplustheradiationfromtheglassenvelopetothesurroundings.
(9.3)
where:
hg=convectiveheat-transfercoefficientatoutsidesurfaceofglass
envelope(W/m2°
C)
Ag=outsidesurfaceofglassenvelope(m2)
Tg=outsidesurfacetemperatureofglassenvelope(K)
Ta=ambienttemperature(K)
σΒ
=Stefan-Boltzmannconstant(5.6696×
10-8W/m2K4)
ε
g=emittanceoftheglass
Fga=radiationshapefactor
Ts=skytemperature(K)(typicallyassumedtobe6Kelvinslowerthanambienttemperature)(Treadwell,1976)
Ifallthevariablescanbeeval