Low cost Linear Collector - PSAstage-ste.psa.es/deliverables/STAGE-STE_WP6_D6.2_Low-cost collector.pdfthe idea itself is much older and goes back to ASDASDAS [ 1]. 2.1. Primary field

STAGE‐STE Grant agreement no: 609837 Scientific and Technological Alliance for Guaranteeing the European Excellence in Concentrating Solar Thermal Energy

WP6:InternationalCooperationActivities

WP6‐Leader:Dr.WernerPlatzer/FraunhoferISE/Germany

Low‐costLinearFresnelCollector

DeliverableD.6.2

19/06/2016

Author(s):WernerPlatzerFrankDinterFelipeCuevas

STAGE‐STE Grantagreementno:609837 19/06/2016DeliverableD.6.2Low‐costLinearFresnelCollector

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Content

1. Introduction............................................................................................................................................................3

2. Descriptionofmaincomponents...................................................................................................................5

2.1. Primaryfield..................................................................................................................................................5

2.2. Receiver...........................................................................................................................................................5

2.3. Absorber..........................................................................................................................................................6

2.4. Glasscoverorglassenvelope.................................................................................................................7

2.5. Secondarymirror.........................................................................................................................................7

2.6. Casing................................................................................................................................................................8

2.7. Structure..........................................................................................................................................................8

2.8. Trackingsystem...........................................................................................................................................9

3. QualitativedesignconsiderationsintheSouthAfricancontext....................................................10

4. Collectorcharacterization...............................................................................................................................11

5. Quantitiativecalculations...............................................................................................................................12

5.1. Heatlossofsingletubereceivercavity............................................................................................12

5.2. Resultsheatlosscalculationsfordifferentreceiverdesigns..................................................13

5.3. OpticalefficiencyofLFC..........................................................................................................................15

5.4. Casesconsidered........................................................................................................................................15

5.5. Resultsofthecollectorcalculations–CaseSteamproduction..............................................16

5.6. Resultsofthecollectorcalculations–CaseHotwaterproduction......................................20

6. Discussionoflow‐costconceptandconclusions..................................................................................24

7. Literature...............................................................................................................................................................26


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1. Introduction

InthisdeliverablethedesignconsiderationsforaLinearFresnelCollector(LFC)oflow‐costandhighlocalcontentfortheSouthAfricansituationshallbedescribed.Theapplicationconsideredinthiscaseisanindustrialapplication,wherehotwaterorsteamshallbeproducedbythecollector,andtypicalsolarfieldsizeisaround1000to2000m2aperture.Themaincharacteristicsofthecollectordesignare:‐low‐costinmaterialsandconstruction‐highlocalcontent‐usedforindustrialapplicationsupto250°C.ThecomponentsandmaterialsforthisLFCshouldcomemostlyfromthehostedcountry.Intheselectionsomeemphasishastobegiventothetrackingaccuracyoftheprimarymirroranditsdrivingconstructionandsimilartothecontrolsystemforthedrivingmechanism.Inthefollowingthedesigngoalsaredefinedandmotivated.Afterwardsthegoalsarebrokendownintoquantitativedesignspecificationsandrequirements.Fromthesespecificationsandrequirementsvariousprototypescanbedeveloped.Thesuccessfulcompletionoftheabovethreedevelopmentalstageswillculminateintothetestingandevaluationofthefinalprototype.Theoverarchingdesigngoalsaretocreateasolution,whichislow‐costwhilstmaximizingtheuseofSouthAfricanindustry.Withinthisscopetherearevariousothergoals,whichneedtobesuccessfullycompleted.ThesedesigngoalsareoutlinedinTable1.Table1.Designgoalsforlow‐costLinearFresnelCollector

Table2showshowthedesigngoalscanbefurthertranslatedintoasetofdesignrequirements.“NEED”arethemustsand“WANT”wouldbenicetoget.


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Table2.Designrequirementsforlow‐costLinearFresnelCollector


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2. Description of main components

LinearFresnelCollectorshavebeenextensivelystudiedsincemorethan20years,althoughtheideaitselfismucholderandgoesbacktoASDASDAS[1].

2.1. Primary field

Theprimaryfieldreflectsthe lightontothereceiver.Themirrorsare locatedatthe levelofthegroundandaredrivenbyatrackingsysteminordertoeffectivelyconcentratethelight.Themirrorsaremanufacturedtoachievehighopticalperformance.Highopticalperformancemeans solar reflectance value approaching unity, and a Gaussian beam spread due tononspecularity and slope errors of less than 2 mrad. Ideally, the reflector must be cheap,durableandrequireslowmaintenance.Itisexpectedalifecycleofthereflectorofatleast20years under the harsh conditions of the desert. State‐of‐the‐art reflectors have a specularreflectance of 93 to 94 percent and expected life of 20 to 25 years without excessivedegradation[2].The reflectance of glass mirrors is related to the content of iron oxide in the glass whichprotects the mirror surface (metallic Silver or Aluminum). A very high reflectance can beachievedbyusing low‐ironwhiteglass in themirrorproduction–whichabsorbsnegligibleenergy in the infrared spectrum‐ typically less than 1% for a 3‐4mm glass thickness. Inmirrorsithasbeentakenintoaccountthatthereflectedlightistransversingtheglasstwice,thustheabsorptionnearlydoubles.Forsolarapplicationsitisthereforedesirabletouseglasswith low‐iron content. Another possibility is to use thin sheet glass (with less about 1mmthickness) This glass is more fragile. Both improved glass products are generally moreexpensive on the market than the ordinary float glass used for mirrors. Thus the designquestioniswhetherthereductioninopticalperformanceofabout5‐10%(dependingonthemirrorglassthickness)iscompensatedbythelowercostofthecomponent.Otherconsiderationsarewhethermirrorsnotbasedonglasscanbeused.Aluminumsheetswithorwithougcoating,polymericfilmswithcoatings,andsteelsheetshavebeenproposed.InprinciplealuminumsheetscoatedwithmirrorcoatingsbasedonSilverarehighlyrefective,but also very sensitive against dust (scratching of surface), a destruction of the necessaryprotective layer and thus deterioration. Other aluminum based refelction devices have thelower reflection characteristics ofAluminum compared to Silver. Evenwithperfect surfacequality(polished)thereflectanceofthesemirrorsisbelow90%.

2.2. Receiver

Thereceiveriscomposedbytheabsorbersurface,thesecondaryconcentrator,theglassplateorenvelopeandthecasing.DifferentconfigurationshavebeenimplementedfortheFresnelcollector.Figure1showsthemostcommonconfigurations


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Figure1CommonreceiverconfigurationsusedinLinearFresnelcollectors.a:Secondaryconcentratorwithabsorbertubeglassplateonthebottom.b:Secondaryconcentratorwithabsorbertubeandglassplate.c:Rowoftubesinsideacavity.Althoughmulti‐tubereceivershaveanadvantagewhensingleendconnectionofa collectorstring to piping is considered, also the preheating and direct evaporation of water can bedaoneinawaytoutilizethefocalintensitydistribution,wewillnotconsiderthisoptioninthefollowing.Thereasonsare

Complexoperationwithdifferentexpansionduetothermalgradients Riskofill‐matchedflowthroughtheparallelreceiverpipes Collectorconsideredissmall,soone‐sidedconnectionsisnotparamount

2.3. Absorber

The energy reaching the absorbermust be efficiently conducted to the heat transfer fluid.Main energy lossmechanismsof the absorber are reflectance of incident light and thermallossbyconvectionandlongwavelengthradiation.Convectionslossesdependsmainlyonthetemperature of the absorber and the surrounding conditions, therefore the absorber isprotectedwithglassenvelopeorglassplate,asitisshowninFigure1.Theoperationtemperatureoftheabsorbersurfaceinsolarapplicationsvariesbetween40 and500 (313Kand773K).Theeffectivetemperatureofthesunisapproximately6000K.Theinfraredspectrumoverlapsslightlythesolarspectrum.Thereforeitispossibletodevelopselectivesurfacesthathavehighabsorptanceforradiationinthesolarenergyspectrumandlow emittance in the infrared spectrum. The ideal surface (Duffie, Beckman 2006) and itspropertiesarerepresentedinFigure2

Figure2:Relationbetweenabsorptionandreflectionatvariouswavelengthsforsolarabsorbers.Ownelaborationbasedon[3]


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Inthedesignconsiderationsitisimportantwhetheraselectiveabsorberwithlow‐emissivityisneededfortheapplicationconsidered.Ablacksurfaceismuchcheapertoproduce,andalsolocalmanufacturingofa low‐techvariant ispossible,whereas selective surfacesneedhigh‐volume production and specialized manufacturing processes (usually nowadays physicalvacuumdeposition ,asgalvanizationisconsideredasadirtyprocess).Thisconsiderationissupportedbytheobservationthattheaveragereceivertemperaturesaremuchlowerthanforcollectorsusedinsolarthermalelectricityproduction.Thereforethetemperaturedependentemissionofthermalpower(Stefan‐Boltzmannlaw)ismuchsmaller.

2.4. Glass cover or glass envelope

Theglassaimstoprotecttheabsorbersurfacefromdirectcontactwiththeenvironment.Theglassprotectioncanbeeitheraplateoranenvelope(seeFigure1).Intermsofheatloss,theprotection suppresses forced convection and shields the absorber from wind. In vacuumreceivers,thespacebetweenabsorberandglassprotectionisevacuated.Evacuatedreceivershavebeendevelopedonlywithcylindricalglassenvelopeandnotwithaglassplateduetothemechanicalstabilityproblem:afixedglassplatewithapressuredifferencebetweeninteriorand exterior of nearly one atmospherewould break. Evacuated tubes suppress convectioncompletely. An additionaly benefit is that the environment is harsh for the absorber tube(especiallytheselectiveabsorbercoating)andtheglassenvelopeprotectstheabsorbertubefromdustandhumidity.Althoughtherearenovelcoatingsthatarestablenotonlyinvacuum,it is better for the longterm performance if deposition of dust and corrosion is inhibited.However,theglassabsorbsandreflectsapartof theincidentradiation, thusdecreasingtheopticalefficiencyofthecollector.Whenglasscontainsironoxidewhichisthecaseinalmostanyflaotglassproductionduetotheuseofsandasrawmaterialcontainingiron, itabsorbsenergy in the infrared spectrum‐ typicallyabout5% fora3‐4mmglass thickness.For solarapplicationsitisthereforedesirabletouseglasswithlow‐ironcontent.Howeverthisglassisgenerallymoreexpensiveonthemarket,astherequirementsontherawmaterialaremorestrict,andsecondlytheproductionvolumeismuchsmallerthanfortheordinaryfloatglassmass product. Thus on question in the design could be whether the reduction in opticalperformanceof5%iscompensatedbythelowercostofthecomponent.

2.5. Secondary mirror

Thesecondarymirrorredirectsthelightthatdoesnothittheabsorberdirectlybacktoit.Itsdesignisoftenbasedonnon‐imagingopticsprinciples,asitisshowninFigure3

Figure3Representationoftheray‐edgeprincipleusedtodesignthesecondarymirror.LineFErepresentsthelightsource(primaryfield)forthedesignofthesecondarymirror.


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The‐edge ray principle for a Linear Fresnel collector considers the primary field as adistributed light source. Taking the target as a tube, the resulting shape is a compoundmacrofocalellipse[4].The properties for secondary reflectors are the similar to the ones for the primary field,although theoperation conditions aredifferent.The secondarymirror is exposed tohigherenergy fluxes compared to the primary mirrors. Therefore its materials must withstandhighertemperatures.Thedistributionofincidentradiationonthesecondaryisnotconstant,with the consequence of temperature gradients along its transversal plane. Under theseconditions, the secondary mirror must retain its shape and optical properties. Secondarymirrors aremade from coated aluminum, steel or silvered glass. The glass bending needsmore effort and leads to higher costs, but also the shape distrortions of bent aluminumorsteelsheetsshouldnotbeunderestimated.

2.6. Casing

The casing provides stiffness to the complete receiver including absorber tube and thesecondarymirror. It is the joint element between the structure and the components of thereceiver. In configuration a) in Figure 1 the space between the secondary mirror and thecasingcanbeinsulated.Iffoamisusedarigidsandwhichtypecompententcouldbeproduced.The insulationaimstodecreasetheheat loss,whichhasthe indirectconsequenceofhighertemperaturesoverthesecondarymirrorsurface.Insulationinconfigurationb)doesnothavesignificant effects in the heat loss. The design of the casing must be such that allows thereplacement of the absorber if it is needed and allows the expansion of the absorber andsecondarymirror.Atthesametime,protectsat leastonesideofthesecondarymirrorfromtheambientconditions.

2.7. Structure

The structure provides not only the support to all functional elements like mirrors andreceivers, it gives alsomore stiffness to the collector. It supports theprimary field and thereceiver.Therelativelocationbetweentheprimaryfieldandthereceiverisfundamentalforthe precise concentration of light and the mutual shading and blocking of rays byneighbouringmirror rows. . The frame can bemade of galvanized steel or aluminum. Theassemblymaybedonewithscrews,bolts,rivetsandweldingorbymoreadvancedandspecialconnection methods. The structure is connected to the ground by foundations. Usually,foundationsaremadefromconcrete[2].Butinsomecaseswithsuitablegroundearthpolesorscrewscanalsobeused.Thestructurehasaconsiderablecontributiontothecollectorweight.Ideallyithastobelightto reduce material cost as much as possible, but it should able to provide the sufficientstiffness to the collector. The shadow over the primary field has to beminimal. There arethreedesignsthathavedominatedthemarket.Figure4showsasketchofthesedesigns


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Figure4Frontviewofdesignoptionsforthecollectorstructure.Top)Supportofthereceiver,a:centralpylonandlateraltensors,b:twopylonsinangle(Ashape),c:twopylonsinrectangularshape.Bottom)Supportoftheprimaryfieldandconnectiontotheground,a:threefoundationslocatedattheextremes,b:twofoundationslocatedattheextremes,c:twofoundationslocatedsymmetrically.

2.8. Tracking system

Theprimaryfieldtracksthesuntoreflectthelightontothereceiver.Thetrackingsystemisinchargeofmovingthemirrorsoftheprimaryfield.Itisusuallymountedonabasestructure.Itmustbepreciseanddurableunderharshconditions.Theexpected life iscomparabletotheoneoftheprimaryfield(between20and25years).Ideallytheorientationofthecollectorisnorth‐southalongthelongitudinalplane.Inthiscase,thetrackingsystemfollowstheapparentmovementofthesunfromeasttowest.Theangularchange for all mirrors is the same, meaning that one device can move all mirrors in thetransversalplane.Thedesignprinciple is tohaveoneactuatorthatturnsallmirrors(inthetransversal plane) by means of a mechanical coupling. There are designs that use manyactuators, allowing more flexibility in the collector operation (e.g. some rows can bedefocusedtobettercontrol theoutlet temperature).Thedecision isacost‐benefit tradeoff.Figure5showsasketchoftheseoptions.Thetrackingsystemshouldbeabletobeadaptedtolatitude,toorientationofthecollector,andinsomecaseseventoatilt.

Figure5Sketchoftrackingsystems.a:Eachmirroriscoupledtooneactuator,allowingtotalcontrolovereachrow.b:Allmirrorscoupledtooneactuator.c:groupsofmirrorsconnectedtooneactuator


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3. Qualitative design considerations in the South African context

Thespecificationforthemirrorsizeisduetothemanufacturer’sproductioncapability(PFGBuildingGlassinSA).Theyareamanufactureroflowironglass,howeverthisglasscannotbeusedfortheFresnelasitispatternedglass.ThefloatglassinSouthAfricahasanironcontentof0.75%andisproducedwiththickness1.8‐12mm.Itshouldbeusedasclearvariaty,notgreenglass.TheproductioncapabilityhasaninfluenceontheLFCdimensions:Asthestocksizeisamultipleof610mmwetakethatmeasureasthebasisforourdesign.Themirrorsproducedcouldbe610mmwideand3210mmlongwithathicknessof3mm.Astandardmirrorof2440mmx3210mmcanbecutin4equalstrips.Themirrorcurvatureradiusisconstrainedbythemirrorsizeandthickness,butthe3mmglassshouldallowenoughcurvature.Inordertoensurealow‐costdesign,whichmaximizestheinputofSouthAfricanindustries,basiccomponentsandmaterialsarefavored.Asafirstpointofdeparture,themirrorsupportstructureisconsidered.Inordertoensurerigidity(littletorsionandbendingofconstruction),thestructure’smaterialmassshouldbeminimizedandthecrosssectionalmomentofinertiainthebendplanesimultaneouslymaximized.Forthesereasonsstandardtubing,whichisavailableinthehostingcountry,isconsidered.However,tofurtherreducethestructure’scomplexityaswellasthecomponentsrequiredtomatethemirrortotheframe,squareandrectangulartubingshouldalsobeselectedforinvestigation.Afurtherreasonforselectingsquaretubingisthatitislocallyproducedandeasytoworkwith.ThisensuresthatmanufacturingtimefortheLFCisreducedduetothedecreasedcomplexityofthecomponents.Theprimarygoalofthemirrorsupportstructureistoprovideasurfaceforthemirrortomatetoaplatformforanactuatortoprovidethenecessarymovementforsolartracking.ThemirrorsupportstructurewillberotatedastheLFCtracksthesunthroughouttheday.Itiscriticalthatthemirrormaintainsitsshapetoensureaccuratereflection.Therefore,squaretubingwithsupportribsinthezy‐planeisused,whichincreasesthematerialinthemirrorbendplane(zyplane)butimprovestherigidityandstillensuringalightweightdesign.InordertooptimizetherigidityandminimizetheweightofthemirrorsupportstructureadetailedFiniteElementMethodanalysisshouldbeconducted.Thiswillprovideinsightintowheretheframelacksrigidity.Thiscouldalsohelptoreducetheamountmaterial,whichwillleadtosavingsinweightandcost.Drivesystemshavetobeinvestigated,whichareabletofollowtherequirementsandhavenobacklashforperformancereasons.Forthisreasonthecouplingofthemotortothemirrorframeneedstobelookedintodetailtopreventoscillationofthereflectedlightonthereceiver.AcontroldeviceshouldhaveanactivecontrolstrategytoreducethetrackingerrorthroughoutthedaywillincreasetheperformanceoftheLFR.Anadditionalsunsensorcouldbeused.


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4. Collector characterization

Theenergyperformanceofline‐concentratingcollectorsisdescribedbythethermalbalancebetweenheatabsorbedandheatlost

(1 )

Theempiricalexpressionforcalculatingtheusefulheatapproximatelywithinaspecifiedtemperatureintervallisthefollowing

, ∙ , ∙ ∙ ∆ ∆ ∙ (2 )

where

istherateofheateffectivelytransferredtothefluid.(W) istherateofheatabsorbedbytheabsorber.(W) istherateofheatlostbytheabsorbertube.(W) , istheopticalefficiencyatnormalincidenceangle.(‐) , istheIncidenceAngleModifieratagivenzenithal( )andazimuthal

( angle.(‐) istheDirectNormalIncidenceradiationfromthesun. ⁄ is the reference area reflecting the DNI radiation onto the receiver. (m2)

(thereferencecanbechosenase.g.themirrorarea) isthelinearheatlosscoefficient. ⁄ ∆ isthetemperaturedifferencebetweenthefluidandtheambient.(K) isthebiquadraticheatlosscoefficient. ⁄ istheareaoftheabsorber.(m2)

Thisequationisapproximatebecauseitisnotusingaphysicalmodelofthedifferentopticaland thermal processes. The advantage is that for testing the empirical parameters can befitted to experimental results. Thus themodel can be used for representing collectors in aspecifiedintervall(givenbythetesting)veryprecisely.Thefirstpartoftherighthandsideinequation(2)representstheopticalgainsabsorbedontheabsorbertubesurface.Thisistheenergyavailabletobetransmittedtothethermalfluid.Thesecondpartoftheequationrepresentstheheatlosses,anddescribestheabsorbedenergynot transmitted to the fluid, as there are thermal losses from the absorber to thesurroundings.With this equation collectors can be described in annual performance calculations orsimulations. It is therefore important to estimate how design changesaffect the mainparameters,whicharetheopticallosses(opticalefficiencyandincidenceanglemodifier)andthermal losses (described by the two loss coefficients). Some quantitative estimationsthereforehavebeenproducedandwillbepresentedinthenextchapter.


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5. Quantitiative calculations

5.1. Heat loss of single tube receiver cavity

Inthefollowingquantitativecalculationswereperformedtogetanimpression,howdesignconsiderationscanaffectopticalandthermalperformance.Thecalculationsweremadeusingsimplifieddesigntoolsfortheopticalperformanceandforthethermallossofthereceiverunderdifferentoperationconditions.ThenthethermalperformanceofthecollectorforacompleteyearinSouthAfricanclimate(4diffrerentlocations)wereinvestigated.Table3:LocationsinSouthAfricawithyearlyaverageofambienttemperatureandyearlyDNILocation Latitude Tamb[°C] DNI[kWh/(m2a)]Upington ‐28.4 21.3 2863.3Bloemfontein ‐29.1 16.0 2606.3Polokwane ‐23.9 18.5 2301.4PortElizabeth ‐34.0 17‐4 1987.3Thethermalheat losscalculationswereperformedwithasimplifiedtool,whichusesaheatresistancemodelforthecavityreceiverwithasingletubeabsorbertube.Thecavityisclosedwithaflatcoverglasstoprotecttheabsorbertubeandthesecondarymirrorfromdust.Thesecondarymirrorisassumedtobeanideallyinsulatedwalli.e.adiabatic.Radiationheattransferismodelledwiththeaviewfactormethodassumingdiffusesurfaces.This might be not completely correct as the secondary mirror might also reflect partiallyspecular the thermal radiation. Usually also with glass surfaces the diffuse assumption isreasonably good. For the radiative transfer the three surfaces absorber tube, secondaryrelector and glass cover are taken into account. No further discretization has beenimplementedforthisstudy.Inadditionconvectiveheattransferfromtheabsorberpipetotheglass cover has been assumed. It ismodeledwith a heat transfer coefficient according to acorrelationofMorgan[5].Theconvectiveheattransferisfromabsorbertubetoglasscoverasthesecondaryisconsideredasadiabatic.Whensettingthisheattransfercoefficienttozerowe may also approximate a vacuum tube receiver. The results of the total radiative andconvectiveheattransferfromabsorbertubetotheglasscover(andthentotheambientair)dependontheassumedemissivitiesof theabsorber tube, reflectorandglasscover.For thelattertwoanemissivityof84%forglasswasassumed.Alsothetemperatureofabsorberandglasscoverinfluencetheradiativeheattransferduetothestrongnon‐linearityintheStefan‐Boltzmann law. The Nusselt number for the convective heat transfer depends on thetemperaturedifference absorber‐ glass cover and themean temperaturewhichdeterminestheconductivityoftheair.Alreadyasearlyas200XMertins[6]triedtoderiveanempiricalsimplifiedcorreclationforradiative and convective heat transfer in a single‐tube receiver and validated that withexperiments on different absorber tube diameters and different absorber emissivities.However the experimental data for this validation used only absorber tube diametersbetween13and17 cm.The standard commercial tubesnowadayshavediameters from25mm to about 90 mm. In the following graph we compare the theory of Mertins with oursimplifiedcalculation.ThecomparisonisverygoodfortheexperimentalconditionsclosetothoseconsideredbyMertins.Alsoothergeometrieswerecalculated.Allourresultsshowthat


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the simplified model predicts somewhat higher heat losses than the empirical formula byMertins.Thereforewecantakethatasaconservativeapproximation.

Figure6:ComparisonofreceiverheatlossfortemperaturesTinletbetween80°Cand240°CwithanfixedoutlettemperaturofTout=380°C.Thediameteroftheabsorbertubeis15cm,theemissivity15%,andtheaperturewidth/glasscoverwidthis62.7cm.

5.2. Results heat loss calculations for different receiver designs

Heatlosswascalculatedtemperaturedependentwiththemodel.Thenaquadraticfunctionsin the temperature difference between the mean of inlet and outlet temperature and theambienttemperaturehasbeenfittedtotheresults.InFigure7areceiverrwithnon‐selectiveblackabsorbertubeisconsidered.

Figure7:Temperature‐dependentheatlosspermeterreceiverlengthforair‐filledreceiver(aperturewidth0.3m,absorberdiameter70mm)forabsorberemissivity0.9.Resultingthermallosscoefficients(linearandquadraticterminT)areu0=1.16W/mKandu1=0.010W/mK2


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The following Figure 8 and Figure 9 show the heat losses for selective absorbers withemissivities20%and10%,whichgivestherangefordifferentproductsonthemarket.

Figure8:Temperature‐dependentheatlosspermeterreceiverlengthforair‐filledreceiver(aperturewidth0.3m,absorberdiameter70mm)forabsorberemissivity0.2.Resultingthermallosscoefficients(linearandquadraticterminT)areu0=0.80W/mKandu1=0.0036W/mK2

Figure9:Temperature‐dependentheatlosspermeterreceiverlengthforair‐filledreceiver(aperturewidth0.3m,absorberdiameter70mm)forabsorberemissivity0.1.Resultingthermallosscoefficients(linearandquadraticterminT)areu0=0.74W/mKandu1=0.00240W/mK2Inordertoestimatealsothelossforavacuumreceiver(whichistubular)veryroughlyalsothemodel of the receiver having a glass cover plate has been used with zero conductive‐convective losses–asthereisnocoductioninthecavitythedimensionofthecavitydonotreallymattermuch.


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Figure10:Temperature‐dependentheatlosspermeterreceiverlengthforevacuatedreceiver(aperturewidth0.3m,absorberdiameter70mm)forabsorberemissivity0.1.Resultingthermallosscoefficients(linearandquadraticterminT)areu0=0.095W/mKandu1=0.00122W/mK2

5.3. Optical efficiency of LFC

TheopticalmodelforthemirrorfieldofNmirrorswithwidthwandgapbetweenthemirrorsb is no raytracing model but only calculates for different incident angles the geometricalrelationsbetweenmirrors,receiverapertureatheightHandincomingsolarradiation.Thuscosine losses, blocking losses and shading losses are considered in the model. A possibleopticallossduetosurfaceinhomogeneities,shapeimperfectionsandnon‐optimizedfocusoftheprimarymirrorsisonlyconsideredinageneraliterceptfactorfIC.Thisfactorcanbevariedbutisconstantforallmirrorsinasimulation.

5.4. Cases considered

Inthecasestudiestwodistinguishedcaseswereconsideredandsimulatedoverayearusingdifferentmeteodata.Table4_Casessimulatedinthedesignstudy

Case Tout[°C]

Tin[°C]

A)Hotwatertemperaturelifting 90 70B)Directsteamproduction 210 80The locationsconsideredwithinSouthAfricashowawidevariationofDNI,andrange fromUpingtoninthedrydesertareaintheNorthwesternCaperegiontoPortElizabeth,amaritimelocation in the South‐West with considerably lower DNI, but still sufficient to considerconcentratingcollectors.Whereas the resource isoptimal forUpington, the locationofPortElizabethbeingmoreindustrialisprobablybestwithrespecttothedemand.TheideabehindthisselectionisthatforlowerDNIaconcentratingcollectorwithdifferentspecificationsanddesignmightbetheoptimalsolution,thusopeningthepossibilitiesforalargermarket.


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Table5:SelectedlocationswithcharacteristicmeteorologicalvaluesandlatitudeLocation simulated

DNI [kWh/m2a]

Ta [°C]

Lat [deg]

A) Upington 2863 21.3 ‐28.4

B) Bloemfontein 2606 16.0 ‐29.1

C) Polokwane 2301 18.5 ‐23.9

D) Port Elizabeth 1987 17.4 ‐34.0

5.5. Results of the collector calculations – Case Steam production

Thesimplifiedsimulationischaracterizingthecollectorateachhourintheyear, inaquasi‐staticcalculation foreachhourof theyear.Thusalso idealizeddailyprofiles foropticalandthermalperformancearegenerated.Inrealityconsideringwarm‐uptimesanddeadtimestheexact hourly profile might look differently, however it is considered that for the annualaverageperformanceincomparisonofdifferentdesignoptionsthisprecisionissufficient.Incomparingdifferentdesignoptionsonly relative rankingof options is important. Figure11showsanexampleforthehourlyresultsforonedayintheyear,whenthecollectorisorientedexactlyinNorth‐Southorientationwhichisalwaystakenforgrantedinthedesgnsimulations.

Figure11:ExampleofdailyenergyflowsinLFCThe result shows the symmetricprofileona clear summerday.Looking into thedetailsonmayalsorecognizetheloweropticalefficiencyinthemorningandeveninghours(mainlyduetocosinelosses).Asaselectiveabsorbertubeisassumedinthereceiver(novacuumreceiver,butair‐filled)thethermallossesarerelativelysmall.


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Figure12:AnnualsolarincidentbeamradiationDNI,absorbedenergyandthermaloutputforLFC7200m2,differentlocations,Case1steamproduction250°Cwithinlettemperature80°CCollectorparameters:receiverheightH=6m,numberofmirrorsN=12,widthw=0.6,period0.8mInFigure12theannualresultsofseveraldesignoptionsareplottedfortheCase1ofsteamproduction.Hereatotalmirrorsurfaceareaof7.2m2permcollectorlength(12mirrors)isconsidered.Theemissivityoftheabsorbertubeintheair‐filledreceiverisvaried.Thedifferencebetweenselectiveandnon‐selectiveabsorbersisrelativelysmallthoughvisible.Considerablylargerlossesareofopticalnature:duetocosinelosses,shadingandblockingtheabsorbedenergyismuchsmallerthantheDNImultipliedbythemirrorarea.PleasenotethatthisvaluecalculatedforaparabolictroughwouldbeavalueofDNImultipliedbymirrorarea–notbythesmalleraperturearea!


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Figure13:Annualaveragecollectorefficiency/utilizationfactorfordifferentabsorberemissivitiesandlocations,LFC1000m2,Case1steamproduction250°Cwithinlettemperature80°CCollectorparameters:receiverheightH=6m,numberofmirrorsN=12,widthw=0.6,period0.8mFigure13showsthatduetothehigherDNIinUpingtonandBloemfontein(andthuslongeroperationtimes)theannualutilizationishigherintheselocations.Forthesteamapplicationduetothehightemperaturesinthecollectorthereisacleardifferencebetweenselectiveabsorbersandblackabsorbersinthereceivercavity,asthermallossesareverydifferent.Neverthelesstheefficiencyvaluesarenotbadforapossiblelow‐costsolutionusingjustablackabsorber.Inordertoimprovetheopticalconcentrationavariationofmirrorrowswasconsidered.

Figure14:OptimizationofnumberofmirrorsforlocationPolokwaneforselectiveabsorberemissivity10%(red)andnon‐selectiveabsorberemissivity90%(blue)Case1steamproduction250°Cwithinlettemperature80°CCollectorparameters:receiverheightH=6m,widthw=0.6,period0.8m


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Figure14showsthatcollectorswithselectiveabsorbersneedlessmirrorsasthermallossesareinsignificant.Forcollectorswithblackabsorbersmoremirrorsandthushigheropticalconcentrationisneededtocompensateforthethermallosses.Aneconomicoptimizatiionthushastotakeintoaccountthecostofmoremirrorsversusthehighercostofaselectiveabsorber.Theresultisverysimilarinotherlocations.

Figure15:AnnulaaveragecollectorefficiencyorutilizationwithvariationofreceiverheightforlocationUpingtonandforselectiveabsorberemissivity10%Case1steamproduction250°Cwithinlettemperature80°CCollectorparameters:receivernumberofmirrorsN=16,widthw=0.6,period0.8mWhenmoremirrorrowsareaddedtoincreasetheopticalconcentrationtheshadingandblockinglossesforafixedreceiverheightincreasemoreandmore.Thusforlargercollectorwidthahigherreceivercouldbeinterestingtoimprovetheviewangles.Howeverthismeansalsolargerdistancebetweenmirrorandreceiver.Onlyiftheprecisionofthemirrorsisgoodenoughsothattheinterceptofthereflectedsolarreadiationwiththereceiverapertureisstillverylargethismakessense.Fortherangeofheightsconsideredwehavenotvariedtheinterceptfactor.Thisisanasssumptionswhichhastobecheckedwithraytracingindetail,whenarealcollectordesignwithrealmirrorsubstructuresareconsidered.WhenwetaketheoptimumnumberofmirrorsN=16asderivedfromFigure14Figure15wecanseeinFigure15thataheightH=6mseemstobeverygoodforthiscaseandapplication.Higherreceiverreducetheblockingandshadingofmirrors,thereforewiththeassumptionofconstantopticalinterceptevenforlargerdistancesresultsintheoreticallyinabetteropticalefficiency.Howeverthatcouldneedbetterqualitymirrors(lessshapedeviations)andthegainsmaybenegligibleforlargerheightsthanH=6m.ThusitissuggestedtouseH=6m.


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5.6. Results of the collector calculations – Case Hot water production

WhenwelookintothesecondCase2withlowertemperaturehotwaterpreheatingfrom70°Cto90°C,thenthethermallossesaregenerallysmaller,andthereforthequestionofabsorberemissivityislessimportant.Thiscanbeseeninthecomparisonofdifferentemissivityoptionsfortheabsorberforthefourlocationsconsidered(Figure16).

Figure16:Annualsolarincidentbeamradiation,absorbedenergyandthermaloutputforLFC7200m2,differentlocations,Case2hotwaterproduction90°Cwithinlettemperature70°CCollectorparameters:receiverheightH=4m,numberofmirrorsN=16,widthw=0.6,period0.8mWhencomparedtotheCase1theannualcollectorutilizationrate(averagedefficiency)issimilarinshape,howeverthedifferencebetweenselectiveandblackabsorbertubesismuchsmaller(Figure17).PleaseobservethatduetotheresultsofFigure14anumberof16mirrorsandareceiverheightof4mhasbeenchosenforthecomparison.Certainlyalargerreceiverheightwouldimprovetheresult,howeveronly4misusedinthiscaseasweassumethatthecollectorwillberathersmall,possiblyroof‐mountedforthishotwaterapplication.Itisatypicalapplicationintheuseofsolarheatforindustrialpurposes.Hencealowerreceiverheightiscertainlyeasierwhenconsideriingproduction,mountingandbuildingregulations.


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Figure17:Annualaveragecollectorefficiency/utilizationfactorfordifferentabsorberemissivitiesandlocations,LFC1000m2,Case2hotwaterproduction90°Cwithinlettemperature70°CCollectorparameters:receiverheightH=4m,numberofmirrorsN=16,widthw=0.6,period0.8mInthefollowingFigure18nowagainthenumberofmirrorsisoptimized.Andduetothelowertemperatureandlowerthermallossestheoptimalnumberissmallerthanforthesteamcase(Figure14).Twocurvesareshowninthisgraph.Oneisconsideringthecasewhereaconstantapertureforacertaincollectorrowhasbeenconsidered.Increasingthenumberofmirrorsreducesthuscollectorlengthwhichleadstohigherendlosses(redcurve).Alternativelythecollectorlengthcanbekeptconstantbutthenumberofcollectorrows(inlargerfields)canbereduced(bluecurve).Heretheendlossesarethesameandonlytheeffectofincreasedopticallossesduetoshadingandblockingareconsidered.


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Figure18:OptimizationofnumberofmirrorsforCase2lowtemperaturehotwaterLocation:Polokwane,receiverheightH=4mblackabsorbereps=0.9a)forconstantapertureandb)forconstantlength(samefieldsizeusinglesscollectorrows)Inthedesignconsiderationsnotonlythegeometricalvariationsandthereceivervariationshavetheirplace,butalsotheselctionofmaterialsformirrorsandcoverglasscanbeconsidered.Theuseofwhitesolarglassisabitmoreexpensive,butcertainlywouldbebeneficialaswellformirrors(higherreflectance)asforthereceivercover(transittance).Ontheotherhandinlocalmarketsonlyordinarygreenfloatglasscouldbeavailable.Thereforeweconsideredalsoinafewsimulationsanexchangeofmaterials.Table6:Resultsforsimulations,locationBloemfontein,N=12,H=4m,Emissivity0.1AnnualDNI kWh/m2aAnnualopticalefficiency AnnualthermalefficiencyAnnualcollectorefficiencyAnnualsolarelectricalefficiencyAnnualPBefficiency Annualnetefficiency Annualsolarincident MWhAnnualabsorbedenergy MWhAnnualthermalproduction MWhAnnualnetproduction MWhAnnualgrossproduction MWh As can be seen from Fehler!Verweisquellekonntenichtgefundenwerden.Table 6 thechangeinreflectanceisnotsopronouncedinthechangeofthermaloutput.Somewhatlargerinfluence has the change of transmittance fromwhite glass to ordinary green glass in the


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receivercover.Howeverifallmaterialsareusedinlowerqualityandalsotheinterceptfactorwouldbereduced,thenareductionofabout14%wouldbetheresult.


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6. Discussion of low-cost concept and conclusions

EnergygeneratedbyLinearFresnelCollectorscanbetransformedinelectricalenergyorusedtofeedathermalprocess.Forelectricalgeneration,steamathighpressureandtemperatureis needed (above 250 ºC), therefore the collector is assembledwith very accuratemirrors,highlytransparentglass(envelopeorglassplate)andanabsorberwithhighabsorptanceandminimizedthermallosses.Thesepropertiesincreasetheopticalefficiencyofthecollector.Asthetemperatureoftheabsorberrises,heatlossbeginstodominate.Todecreasetheheatloss,theabsorberiscoatedwithselectivecoatingstodecreasetheemittanceandconsequentlytheradiativeheatloss.Convectiveheatlossdependsonlyonthetemperaturedifferencebetweenthe absorber surface and the surroundingmedia, so it has been effectively suppressed byevacuatingtheclearancebetweentheabsorbertubeandtheglassenvelope.For thermal processes below 250°C on which this study is focused, the collector can bedesigned according to the target temperature. As the temperature level decreases,componentsof lessqualitymightbeused.Nonetheless, thereductionofefficiencyhastobecompensatedbythereductionincost.Formourresultsitisclearthatmeasureschangingtheopticalperformancearemuchmoredominantthanchangesinthethermalperformance.Wedescribedtheheatlosscomparingtheusefulheatfordifferentreceiverconfigurations.Ontheone hand side a configuration of a single tube receiverwith low emittance coating.On theotherhandthesecondconfigurationhasanabsorberwithhighemittancecoating(Paint)andaglassplateonthebottom.Thecostofthecomponentsdependsstronglyonthematerialsandthetechnologyneededtomanufacture it. Mass production tends to decrease the cost. Local manufacturing has apotentialofdecreasethecostsaswell.Itisfavourable,ifproductscanbeusedwhichareusedalso in other, preferably larger, markets. Thus, the economical feasibility of localmanufacturingdependson themarketandon thealreadyexisting industryavailable in thecountry.Ifthereisanindustrialsectorwhichhassynergieswithconcentratingsolarenergy,thepotentialofreducingthecostofthecomponentincreases.DesignpossibilitiestodecreasethecostofLinearFresnelcollectors As alternatives to highly efficient components as used in CSP‐collectors for electricitygeneration,weidentifythefollowingoptions

‐ White‐glass (transmittance 0.91 at normal incidence) or green‐glass plate(transmittance0.87atnormal incidence) toprotect theabsorber, insteadofawhite‐glassplatewithanti‐reflectingcoating(transmittance0.96atnormalincidence).

‐ Silvered green‐glass reflector (reflectance 0.86) instead of silvered white‐glassreflector(reflectance0.93‐0.94)

‐ Reflectorswithonefocallengthinsteadofeachrowwithoptimizedfocallength‐ Aluminumreflectorsforsecondaryconcentrator‐ Non‐selective black paint coated on steel tubes instead of vacuum tube receivers or

selectivelycoatedhighperformancesteelabsorbers[7]Inordertoestimatewhethercertaindesigndecisionsshouldbechosenornot,exactcostinformationonthealternativematerialsandcomponentsfromdifferentsourcesaswellasmanufacturingcostshavetobeknown.Withinthisgeneralstudythisdataarenotavailable.It


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wouldneedadetailedcommercialdevelopment,againwithcheckingalldesignoptionswithmoreadvancedtoolsthenusedhere.Onlysomegeneralissuesmaybediscussedlookingattheperformancedataandassumingsomecosts.Thefirstquestioniswhethernon‐selectiveabsorbertubescouldbeusedwithbenefitinsuchalowtomediumtemperatureapplication.Fromthesimulationsweseethatforexampleforthemediumtemperaturecase(Steam,250°C)thedifferenceofannualthermalproductionperunitlengthofthecollectorinUpingtonisnearly1200kWh/a.Thisreducedproductionoverthenominallifetimeofthecollectorof20‐25aleadstoreducedsavingswhichhavetobediscounted.OnemaycalculateforcertainfinancialparametersofaprojectthelevelizedcostofheatLCOHforbothcases,highperformancecollectorwithhigherinvestmentcost,andlowperformancecollectorwithlowercost.OnlyifthereductionofinvestmentperunitlengthofreceiverleadstolowerLCOHthisoptionshouldbechosen.Forthiseconomicanalysiswejustselectedsometypicalvaluestoshowthemagnitudeofnecessarycostreduction.Table7:EconomicparametersforsimpleLCOHcalculation Parameters Unit Value1 interestrate 0.082 lifespan year 253 fractionoftotalplantinvestmentcostusedas

anuualinsuranceratem 0.01

4 fractionoftotalplantinvestmentcostusedasoperationandmaintainance

0.02

6 factorrepresentingsurchargeforEPC,projectmanagementandrisk

0.2

7 costofsolarfieldperunitmirrorarea Euro/m2 2508 factorofplantavailability 0.96Weshowheresomeexamplesofcomparisonusingourdata.Weassumedinvestmentcostfortheadvancedhigh‐performancecollectorof250€/m2.WiththiscostassumptionswecalculatedaLCOH,andthenwedeterminedthemaximumallowablepriceperunitcollectorlengthforthelowcostcollectorprducingthesameLCOH:Collectortype Annualheat

[MWh]LCOH[€/MWh]

Coll.Cost[€/m]

Advancedcollector 8521 32.66 1800Low‐Costcollector 7335 32.66 1550Thismeansthatthesimplereceivertubewithblackpaintshouldbe250€/mcheaperinthiscaseasahigh‐performancereceiver.Thisisunlikelyasthecostevenforgoodvacuumreceiversareinthisrange,sothecostsavingswouldnotbepossibleevenifthecostoftheblacktubewouldbezero.Thisresultholdsalosforotherclimates.ForPortElizabethforexampletherequiredcostreductioninordertoreachtheLCOHof48.13€/MWhinthiscasewouldbe295€/m.Forthelowtemperatureapplicationsthispicturemightchangetosomeextent.Thesameanalysisshowsthatforthehotwaterproductiontherequiredcostreductionisbetween90


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and110€/m,avaluewhichmightbepossibletoachieve.Sointhiscasethedesignoptionusingablackpaintedabsorbertubeshouldbecheckedindetail.Uptonowwecalculatedonlyrequiredcostreductionifonecomponentisexchanged,andallotherdesignparameterswouldstayfixed..Howeveralsootherscenariosmightexist.Onemightsavecostforacheaperreceiver,butaddsomecostwithanadditionalmirrorrow.ThesequestionscanbeinprinciplebesolvedbyoptimizingtheLCOHinamulti‐dimensionalparameterfield.Alsocostinformationonindividualcomponents(eg.Inthiscasetheadditionofanaddintionalmirrorrow)hastobedefined.Ifweexchangeacomponentlikethemirrorglassorthecoverglassforthecollector,similarperformancereductionisobserved(seeTable6).Soforaexchangeofhighlyrefelctingmirrorswith91%averagereflectivitytomirrorswithlowerreflectanceacostreductionof15€perm2mirrorhastobereachedinordertoresultinthesameLCOH.Asthecoverglassismuchsmaller,andtheperformancereductionissimilartotheexchangeofmirrors,heretherequiredcostreductionforthecoverglasswouldbenearly290€perm2glass.Thisiscertainlynotreasonable.Theconsequenceisthatcheapermirrorsmightbeinteresting,especiallywheninacountrythemirrorproductionforhighlyreflectingmirrorsisnotexisting,andmirrorshavetobeimported,possiblywithimporttax.Forthecoverglass,whichisasmallspecialcomponenthowever,thebestoptionshouldbeused,becausetheperformancelosscannotbecompensated.Asafinalconclusionwehaveidentifiedforlow‐temperatureapplications(<100°C)thepossibilitytouseaverycheapblackabsorber.Howeverevenunderthisfavourableconditionsthelow‐costreceiverhastosaveabout90‐110€permininvestmentcost.Forboth,higherandlowtemperatureapplicationsdependingonthepricecomparisonofmirrormaterialswithdifferentquality,theperformancelossmightbeacceptableforapricedifferenceofabout15€/m2.

7. Literature

[1] Zhu,Guangdong;Wendelin,Tim;Wagner,MichaelJ.;Kutscher,Chuck(2014):History,currentstate,andfutureoflinearFresnelconcentratingsolarcollectors.In:SolarWorldCongress2001103,S.639–652.DOI:10.1016/j.solener.2013.05.021.

[2] Duffie,JohnA.;Beckman,WilliamA.(2006):Solarengineeringofthermalprocesses.3rded.Hoboken,N.J.:Wiley;[Chichester:JohnWiley.

[3] Chaves,Julio(2008):Introductiontononimagingoptics.BocaRaton,Fla.:CRC;London:Taylor&Francis[distributor](Opticalscienceandengineering,134).Availableonlineathttp://www.loc.gov/catdir/enhancements/fy0806/2007047468‐d.html.

[4] Heller,Peter;Häberle,Andreas;Malbranche,Philippe;Mal,Olivier;Cabeza,LuisaF.(2011):Scientificassessmentinsupportofthematerialsroadmapenablinglowcarbonenergytechnologies.Concentratingsolarpowertechnology.Luxembourg:PublicationsOffice(EUR(Luxembourg.Online),25171).

[5] V.T.Morgan(1975):Theoverallconvectiveheattransferfromsmoothcircularcylinder,in:Irvine,Hartnett(Eds.),AdvancesinHeatTransfer,AcademicPress,NewYork,1975,pp.199–264.

[6] Mertins,Max(2008):TechnischeundwirtschaftlicheAnalysevonhorizontalenFresnel‐Kollektoren,Karlsruhe,Univ.,Diss.,2008

[7] Platzer,W.J.(2009);. Linear Fresnel collectors as an emerging option for concentrating solar thermal power, in: Proc. ISES Solar World Congress. Johannesburg, 11th-14th October 2009. Pergamon Press.

Documents

Low cost Linear Collector - PSAstage-ste.psa.es/deliverables/STAGE-STE_WP6_D6.2_Low-cost collector.pdfthe idea itself is much older and goes back to ASDASDAS [ 1]. 2.1. Primary field