Flate Plate Collector (theory presentation).pdf

The presentation provides an introduction to flat plate collector; a type of solar collector.

1. Flat Plate Collectors
Lecture 6

2. Introduction
•Asolarcollectorisaspecialkindofheatexchangerthattransformssolarradiantenergyintoheat.
•Asolarcollectordiffersinseveralrespectsfrommoreconventionalheatexchangers.
•Flat-platecollectorscanbedesignedforapplicationsrequiringenergydeliveryatmoderatetemperatures,uptoperhaps100◦Cabove
ambienttemperature.
•Theyusebothbeamanddiffusesolarradiation,donotrequiretrackingofthesun,andrequirelittlemaintenance.`
•Theyaremechanicallysimplerthanconcentratingcollectors.Themajorapplicationsoftheseunitsareinsolarwaterheating,building
heating,airconditioning,andindustrialprocessheat.

3. Description of Flat Plate Collectors (FPC)
•Theimportantpartsofatypicalliquidheatingflat-platesolarcollector,‘‘black’’solarenergy-absorbingsurface
withmeansfortransferringtheabsorbedenergytoafluid,envelopestransparenttosolarradiationoverthesolar
absorbersurfacethatreduceconvectionandradiationlossestotheatmosphere,andbackinsulationtoreduce
conductionlosses.
•Flat-platecollectorsarealmostalwaysmountedinastationaryposition(e.g.,asanintegralpartofawallorroof
structure)

4. Basic Energy Equation
•Insteadystate,theperformanceofasolarcollectorisdescribedbyanenergybalance
thatindicatesthedistributionofincidentsolarenergyintousefulenergygain,
thermallosses,andopticallosses.
•Insteadystatetheusefulenergyoutputofacollectorofarea??????
??????isthedifference
betweentheabsorbedsolarradiationandthethermalloss:
•Here,Sisthetotalradiationabsorbedbythecollector,�
??????istheheatlosscoefficient,
�
�??????isthemeantemperatureoftheplateand�
??????istheambienttemperature.
•Theproblemwiththisequationisthatthemeanabsorberplatetemperatureis
difficulttocalculateormeasuresinceitisafunctionofthecollectordesign,the
incidentsolarradiation,andtheenteringfluidconditions.
•Ameasureofcollectorperformanceisthecollectionefficiency,definedastheratio
oftheusefulgainoversomespecifiedtimeperiodtotheincidentsolarenergyover
thesametimeperiod:
•Ifconditionsareconstantoveratimeperiod,theefficiencyreducesto

5. Heat Transfer Analysis of FPC
Assumptionsfortheanalysis
1.Performanceissteadystate.
2.Constructionisofsheetandparalleltubetype.
3.Theheaderscoverasmallareaofcollectorandcanbe
neglected.
4.Theheadersprovideuniformflowtotubes.
5.Thereisnoabsorptionofsolarenergybyacoverinsofarasit
affectslossesfromthecollector.
6.Heatflowthroughacoverisonedimensional.
7.Thereisanegligibletemperaturedropthroughacover.
8.Thecoversareopaquetoinfraredradiation.
9.Thereisone-dimensionalheatflowthroughbackinsulation.
10.Theskycanbeconsideredasablackbodyforlong-
wavelengthradiationatanequivalentskytemperature.
11.Temperaturegradientsaroundtubescanbeneglected.
12.Thetemperaturegradientsinthedirectionofflowand
betweenthetubescanbetreatedindependently.
13.Propertiesareindependentoftemperature.
14.Lossthroughfrontandbackaretothesameambient
temperature.
15.Dustanddirtonthecollectorarenegligible.
16.Shadingofthecollectorabsorberplateisnegligible.

6. •Atsometypicallocationontheplatewherethe
temperatureis�
�,solarenergyofamountSisabsorbed
bytheplate.
•ThisabsorbedenergySisdistributedtothermallosses
throughthetopandbottomandtousefulenergygain.
Heat Transfer Analysis of FPC
Heat
loss

7. •Thesteady-stateenergytransferbetweentheplateat�
�andthefirstcoverat�
??????1isthe
sameasbetweenanyothertwoadjacentcoversandisalsoequaltotheenergylosttothe
surroundingsfromthetopcover.
•Thelossthroughthetopper-unitareaisthenequaltotheheattransferfromtheabsorber
platetothefirstcover:
•whereℎ
??????,�−??????1istheconvectionheattransfercoefficientbetweentwoinclinedparallel
plates
•If the definition of the radiation heat transfer coefficient is used, the heat loss becomes
•The resistance �
3can then be expressed as
•Asimilarexpressioncanbewrittenfor�
2,theresistancebetweenthecovers.
Heat Transfer Analysis of FPC

8. •Theresistancetothesurroundings�
1isthengivenby
•whereℎ
??????,??????2−??????istheradiationheattransfercoefficientbetweencollector2andair.
•whereℎ
??????istheconvectiveheattransfercoefficientbetweencover2andair.
•Visthevelocityofwind.Forthistwo-coversystem,thetoplosscoefficientfromthe
collectorplatetotheambientis
•Beginningattheabsorberplate,anewtemperatureiscalculatedforthefirstcover.This
newfirstcovertemperatureisusedtofindthenextcovertemperature,andsoon.For
anytwoadjacentcoversorplate,thenewtemperatureofplateorcoverjcanbe
expressedintermsofthetemperatureofplateorcoverias
Heat Transfer Analysis of FPC
ℎ
??????=2.8+3.0�

9. NATURAL CONVECTION BETWEEN FLAT PARALLEL PLATES AND BETWEEN
CONCENTRIC CYLINDERS
•Therateofheattransferbetweentwoplatesinclinedatsomeangletothehorizonisofobviousimportanceintheperformance
offlat-platecollectors.
•Free-convectionheattransferdataareusuallycorrelatedintermsoftwoorthreedimensionlessparameters:theNusseltnumber
Nu,theRayleighnumberRa,andthePrandtlnumberPr
•TheNusselt,Rayleigh,andPrandtlnumbersaregivenby

10. •Inamorerecentexperimentalstudyusingair,Hollandsetal.(1976)givetherelationshipbetweentheNusseltnumberandRayleighnumberfor
tiltanglesfrom0to75◦as
•where the meaning of the + exponent is that only positive values of the terms in the square brackets are to be used (i.e., use zero if the term is
negative).
NATURAL CONVECTION BETWEEN FLAT PARALLEL PLATES AND BETWEEN
CONCENTRIC CYLINDERS

12. Heat Loss

13. Forthissingle-glass-coversystem,theoverallheatloss
coefficientwillbe
Theradiationcoefficientfromtheplatetothecoverℎ
??????,�−??????
is
Theradiationcoefficientforthecovertotheairℎ
??????,??????−??????is
givenas
??????��??????????????????�
??????=35
�
??????
The two radiation coefficients
Themeantemperaturebetweentheplateandthecoveris
�
??????=67.5
�
??????
The air properties are
??????=1.96×10
−5
Τ??????
2
�
??????=0.0293�/????????????
��=0.7
�
??????=340.5??????
The Rayleigh number is

14. The Nusselt will be given by
????????????=3.19
The convective heat transfer coefficient is
The heat loss coefficient will be given by

15. The cover temperature will be
With this new estimate of the cover temperature, the various heat transfer coefficients become
And the new heat loss coefficient will be
When the cover glass temperature is calculated with this new top loss coefficient, it is found to be 48.4
�
??????, which is essentially
equal to the estimate of 48.5
�
??????

16. COLLECTOR HEAT REMOVAL FACTOR AND FLOW FACTOR
•Itisconvenienttodefineaquantitythatrelatestheactualusefulenergygainofacollectortotheusefulgainifthe
wholecollectorsurfacewereatthefluidinlettemperature.
•Thisquantityiscalledthecollectorheatremovalfactor??????
??????.Inequationformitis
•Thecollectorheatremovalfactorcanbeexpressedas
•??????
′
is the collector efficiency factor
•This collector flow factor is a function of the single variable, the dimensionless collector capacitance rate
Also

17. •It is convenient to define the collector flow factor F as the ratio of FR to F
•Thequantity??????
??????isequivalenttotheeffectivenessofaconventionalheatexchanger,whichisdefinedastheratio
oftheactualheattransfertothemaximumpossibleheattransfer.
•The maximum possible useful energy gain (heat transfer) in a solar collector occurs when the whole collector is at
the inlet fluid temperature; heat losses to the surroundings are then at a minimum.
•The collector heat removal factor times this maximum possible useful energy gain is equal to the actual useful
energy gain �
??????.
COLLECTOR HEAT REMOVAL FACTOR AND FLOW FACTOR

18. Direct Active solar heating

19. Indirect solar passive heating system

20. Parabolic trough collector
•Formanyapplicationsitisdesirabletodeliverenergyattemperatureshigher
thanthosepossiblewithflat-platecollectors.
•Energydeliverytemperaturescanbeincreasedbydecreasingtheareafrom
whichheatlossesoccur.
•Thisisdonebyinterposinganopticaldevicebetweenthesourceofradiation
andtheenergy-absorbingsurface.
•Concentratorscanbereflectorsorrefractors,canbecylindricalorsurfacesof
revolution,andcanbecontinuousorsegmented.
•Toavoidconfusionofterminology,thewordcollectorwillbeappliedtothe
totalsystem,includingthereceiverandtheconcentrator.
•Thereceiveristhatelementofthesystemwheretheradiationisabsorbed
andconvertedtosomeotherenergyform;itincludestheabsorber,its
associatedcovers,andinsulation.
CONCENTRATION RATIO
•The ratio of the area of aperture to the area of the receiver.

21. Typical application of parabolic trough collector(linear)

22. Parabolic dish 3D collector

23. Solar tower spain

25. Crescent dunes solar tower