Power from the Sun Chapter9Word格式.docx

Power from the Sun Chapter9Word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