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THEHAWKEYESOLAR
COOKER
presentedbyTheUniversityofIowaCollegeofEngineeringinconjunction
withTheUniversityofCalifornia,BerkeleyandClimateHealers
Mechanical
EngineeringSenior
DesignProject-
FinalReport
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TableofContents
Introduction...........................................................................................................................2
Background&Planning..........................................................................................................2
Research.................................................................................................................................5
Constraints.............................................................................................................................5
ConceptualDesigns................................................................................................................6
PreliminaryDesign.................................................................................................................8
Collector...............................................................................................................................11
Design................................................................................................................................................11
Fabrication&Assembly.....................................................................................................................12
Recommendations.............................................................................................................................13
Storage.................................................................................................................................13
Medium.............................................................................................................................................. 13
Insulation...........................................................................................................................................13
InnerStorageBoxes...........................................................................................................................14
OuterStorageBox..............................................................................................................................15
CanArrays..........................................................................................................................................16
Delivery..............................................................................................................................................18
LightFunnel.......................................................................................................................................19
Recommendations.............................................................................................................................21
Mechanics............................................................................................................................22Design................................................................................................................................................22
Fabrication&Assembly.....................................................................................................................22
Recommendations.............................................................................................................................25
Modeling..............................................................................................................................25
Considerations...................................................................................................................................25
ProjectedPerformance......................................................................................................................29
FinalRecommendations.......................................................................................................33
Acknowledgements..............................................................................................................34
WorksCited..........................................................................................................................36
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TheHawkeyeSolarCooker
Introduction
Forthepastthreemonths,studentsattheUniversityofIowaCollegeofEngineeringhavebeen
workingtowardprovidingthenativevillagersofRajasthanIndiawiththemeanstocookfoodwithout
theuseoffirewood.Approximately2billionpeopleworldwideburn1.2billiontonsofwoodannually.
Theecologicalandsociologicalimpactsofthisstatisticgrowevenmoreapparentasoncefertileforests
swiftlyprogresstowardbarrenwastelands(ClimateHealers).
Figure1:BarrenhillsideinKarech,2002
TheHawkeyeSolarCookeristhefirststeptowardprovidingaviablerenewablemeansof
cooking,notjustinRajasthan,butinconsistentlywarmregionsaroundtheworld.Thebenefitsofsuch
adevicearemanyandtranscendthetechnicalachievementtoprovidehealth,social,andecological
benefitstoend-users.InRajasthan,wherecollectingfirewoodtraditionallyfallsonthematriarch,the
abilitytocookwithoutfirewoodcanpotentiallysavehoursofdailylaboroncedevotedtowood
harvesting.Asfamiliesoftencookindoors,smokeinhalationanditsrelatedmedicalconcernscanbe
avoidedandreforestationofaregioncanbegin(Rao).
Background&Planning
InDecemberof2010,nineUniversityofIowastudentsalongwithProf.H.S.Udaykumar
embarkedonatriptotheKarechandMewarregionsofRajasthan,India.Theirgoalwastocollect
culturalinformationregardingtheRajasthanivillagersaswellastoobservethecurrentimplementation
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ofsolarcookerspreviouslyprovidedbythenon-governmentalorganizations,ClimateHealersandthe
FoundationforEcologicalSecurity .
Theexistingsolarcookerswerefoundtobesuccessfulincookingroti(aflatbread)andother
commonIndiandishes.However,theirinabilitytoconformtotraditionalcookingandeatingpractices
haveledtotheirabandonment.Figure2depictsthe NamastesolarcookercurrentlydeployedinRajasthan.
Figure2:Namastesolarcookerbeingusedtocookroti
TheNamastecookerisdesignedtoreachthedesiredcookingtemperaturesduringpeakhours
ofthedaybyconcentratingsolarraysviaconcavereflectorsontothelowerportionofthecooking
surface.TherewereseveralkeyissueswithitsdesigncontributingtoitsfailurewiththeMewarand
Karechvillagers.1)Thecookerrequiredtheusertocookstandingup.Traditionally,mealsarecooked
andservedfromaseatedposition.2)Thesolarcookerpossessesnoformofenergystoragerequiring
theusertotaketimeawayfromtheirdailytaskstomonitorcookingprogress.3)Onasimilarnote,the
lackofstoragepreventsusersfromcookingatthetraditionalmealtimesofabout7:00amand7:00pm,
IndiaStandardTime.
Inadditiontoevaluatingissueswiththesolarcooker,theWinterimstudentsalsoconducted
surveystogainagreaterunderstandingofthecultureandlifestyleofRajasthanivillagers.Basedon
theirobservations,theMechanicalEngineeringDesignProject(MEDP)Teamwasprovidedwithvaluable
informationonwhichtoestablishconstraintsfortheproject.
TwomealsadayareservedinMewarandKarech;eachatthepreviouslymentiontimes.One
mealconsistsofroti(almostdaily),whiletheotheristypicallycomposedofvegetables.Theaverage
familysizeintheregionis6.2withover300familiesbetweenthetwovillages.Eachfamilyharvests
approximately5-10bundlesofwoodeachweek(abundleweightsabout20kg).Whatfirewoodthat
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isn’tusedissoldtomarket;incentivizingthecuttingofadditionalwoodfromthesurroundingforests
(UniversityofIowaIndiaWinterimTeam).
FromaculturalperspectivethevillagersofRajasthansubscribetoamatriarchalfamilysystem.
Thewomenperformnearlyallofthedailytasksincludingcooking,firewoodcollection,andchildcare.
Basedonthethisinformation,itwastheWinterimTeam’sobservationthatsuccessfuldeploymentoffuturesolarcookersintheregionrestsinthefulfillmentofstringentconstraintsimposedbytheprimary
users;thewomenofRajasthan(UniversityofIowaIndiaWinterimTeam).
Forplanningpurposes,thesemesterwasdividedinto2parts:1)adesignphase,and2)abuild
phase.Forthedesignphase,theclasswasdividedintofiveteams,eachwiththeirownteamleader.An
additionalcoordinatingteam,comprisedofteamleadersandthreeexecutiveofficers,wasusedasa
centralhubforcommunicationandcoordinationbetweengroups.Table1highlightstheteam
breakdownandorganizationalstructure.
Table1:MEDPOrganizationalStructure
Asthedesignandbuildprocessesprogressedtheseteamsmorphedtomeettheneedsof
project.WithinStage1,subgroupswereusedfortofacilitatethecreationofconceptualdesigns.Each
ofthese5subgroups(laterdesignated Designs1-5)consistedofmembersfromeachoftheprimary
teamslistedinTable1.Thisensuredthateachaspectofsolarcookerconstructionwaswellrepresented
ineachoftheconceptualdesigns.ThesesubgroupsreducedinnumberuntilthebuilddesignwasfinalizedbyProf.UdaykumarandDr.SaileshRao.
AsExecutiveDirectorofClimateHealers,Dr.Rao,wasoneoftheprimaryinstigatorsofthe
projectandprovidedvaluablefeedbackanddirectionregardingdesigns.Healsodirectedateamfrom
theUniversityofCaliforniaBerkeleychargedwiththecreationofaheatpipethatcouldbeeasily
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integratedintoMEDPsolarcooker.TheBerkeleyTeam’sreportisforthcomingandtheirslide
presentationcanbefoundinthesupplementalinformationprovidedwiththisreport.
Stage1:Design
Research
BetweenJanuary21andMarch11,theMEDPstudentsweredevotedtoresearchingsolar
cookersandsolarcookertechnology.Numerouspapers,casestudies,instructionmanuals,and
websiteswereexploredastheteamsworkedtogatherasmuchinformationaspossibleonsolar
cookers.
Inthebeginning,Prof.Udaykumarprovidedtheclasswithseveraldocumentsandresearcharticles
highlightingthebasicsofsolarcaptureandenergystoragemethodsandmaterials.Astheclassbecame
moreautonomousandresearchmoreextensive,designparadigmsandrestrictionsbegantofallinto
place.Throughoutthedesignprocess,therearoseseveralreoccurringquestionsgoverningthe
decisionsmade:
1. Tracking–willthecollectoractivelytrackthesunthroughoutthedayorpassivelycollectlightas
thesunprogressesthroughthesky?
2. CollectorShape–shouldthereflectorbeaparabolictrough,adualparabolictrough,ora
parabolicdish?
3. PhaseChangeMaterialvs.LatentHeatStorage–whichofthesemediumsshouldbeutilizedto
storetheenergycollectedduringtheday.4. DeliverySystems–shouldheatpipesorconductiveheatingbeemployedtodeliverheattothe
cookingsurface?
5. Interface–howwillthecookerinterfacewithRajastanihomes?
Thefinalsolarcookerdesignaddresseseachoftheseitems;though,itshouldbenotedeachdesign
selectionpresenteditsowntradeoffsaccordingtothe“ Cost,Quality,Time” paradigm.
Constraints
BasedontheresearchconductedbytheWinterimandMEDPTeams,thefollowingconstraintswere
imposedfortheproject:
1. Thecookerrequireda$250dollarfinalproductioncost.Forthisprototype,thecostswere
expectedlyhigher.Therefore,itwasdecidedthattheteamshouldevaluatetheirselectionof
materialsbasedonwhatwasconsideredcheapandavailableinRajasthan.Endingcost
projectionsweremadebasedonwhattheclassbelievedthecostwouldbetomassproducethe
cookersinIndia.
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2. Thegoaltemperatureforthecooksurfacewasbetween150-200degreesC.Thisisthecook
surfacetemperatureforthecookingofroti.
3. Thedesignneededtobecapableofstoring6kWhofenergytoaccountforlossesandtomeet
thetemperaturerequirementsatthetraditionalcookingtimes.
4. Thecookershouldbebuiltusinglocalbuildingmaterials.
5. Thedesignshouldberelativelylowmaintenance.Thevillagershouldbeabletosetthecooker
totheappropriateangleandleaveittocollectenergythroughouttheday.
6. Traditionalcookingpractices,suchasseatedcookingandcookingtimes(7:00amand7:00pm),
neededtobepreservedbythedesign.
7. Safetywasheldparamountwhendesigningacookermeanttooperateathightemperatures.
Itshouldbenotedthatwhilesizewasnotoriginallyofmajorconcern,itdidpresentitselfasthekey
flawinthefinaldesign.Theendingdimensionswerebasedontheassessmentofthemodelingteam
regardinghowmuchmaterialwouldberequiredtomeettheenergyneedsforcooking.Thisrequires
addressinginfuturedesigniterations.
ConceptualDesigns
Afteranappropriateamountofknowledgewasgained,itwasdecidedthattheclassshould
partakeinacharettetobeginpoolingdesignideasandsolutions.Fivedesignswereinitiallyconsidered.
Fromthesefivedesigns,3uniqueconceptualdesignsemerged.
Thefirstofthethreeconceptualdesignswasthe SchefflerSolarCooker .Thisdesigndrawsits
inspirationfromtheSchefflerSolarKitchensfoundthroughoutvariouspartsofIndiaandtheMiddle
East.FirstdevisedbyWolfgangScheffler,the SchefflerSolarCooker hasbeenproventostorebetween2
and4kWhofenergybyfocusinglightviaparabolicdishontoacollectorplatetypicallyplaced10–15
feetaway.
Figure3depictsthebasicpositioningandoverallconceptfortheSchefflercooker.Theprimary
issueswiththisdesignlayinitsmanufactureandassembly.Thetolerancesassociatedwiththe
constructionofthedishposedmanytechnicaldifficulties.Thedishneededtobeconstructedwith
extremeaccuracytoensuremaximumconcentrationoflightonthecollectorsurface.Itwas
questionablewhetherornotthestudentshadtheproperresources,training,andtimetocompletesuch
atask.Thestoragemethodwasalsodeemedextremelycostly.TheSchefflercookeruseslatentheat
forstorageintheformoffour100kilogramsteelcylinderssurroundedbyfiberglassinsulationwithina
steeldrum.Steelinthatamountcouldpotentiallycostbetween$500-$1000dollarsdependingonbulk
pricingandthetypeofsteelused.Thedesignalsoaddressestrackingthroughagearsystemthatadjusts
thereflector’spositionthroughouttheday.Itishighlyrecommendedthatadesignofthisnaturebe
revisitedinfutureiterations.
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Figure3:BasicSchefflerSolarKitchenConcept
Thesecondofthethreeconceptualdesignswasthe“RoseBud”solarcooker.Aptlynamedfor
itsshape(seeFigure4),thiscookeremployeesaseriesofflatreflectiveplatespositionedtocomposea
dish,thusincreasingtheamountoffocusedlight.Alensisusedatthebaseofthecookertofurther
focuslighttoapointandtosealtheinteriorfromheatlosses.Atthefocalpoint,temperatureshave
beenfoundtoreachupwardsof800degreesCinsomestudies.Thestoragemediumutilizedbythis
designwaslatentheat,thoughaparticularmaterialwasneverfullyinvestigated.Thecollectordishalso
restsonatrackthatallowsitrotatetofollowthesunthroughouttheday.
Figure4:"RoseBud"SolarCooker
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Thelastofconceptualdesignswastheparabolictrough.Thisdesigncanbeapproached
multipleways.Figure5showsareceiverpipefilledwithafluidthatisheatedbythereflectedlightrays.
Atroughsimilartothiswasutilizedinthefinaldesign.Thepreliminarydesignutilizesasecondary
reflectortofurtherconcentratelight,furtherdemonstratingthemanyvariationsoftroughdesign.
Figure5:ParabolicTroughDesignOverview
Theparabolictroughprovidesfortrackingitsorientation.Whenpositionedinaneastwestmanor,thetroughcollectslightthroughoutthedayasthesunmovesacrossthesky.Efficiencies,as
result,arelowerinthemorningandnightandpeakduringthedaywhenthemaximumlight
concentrationisachieved.Thisisapassivemethodofsolarcollection.
Anumberofstoragemediumscanbeutilizedwhenemployingtheparabolictrough.The
collectionmethodisprimarilywhatsetthisdesignapartfromthepreviouslymentioneddesigns.
PreliminaryDesign
Afterreviewingtheconceptualdesigns,Prof.UdaykumarandDr.Raodeterminedthatthefinal
solarcookerprototypeshouldincorporatethebestaspectsfromeachoftheconcepts.Intheinterestof
time,theconceptdesignswerecombinedintoapreliminarydesignbytheclasssupervisors.Thegoal
wastoquicklyselectadirectioninwhichtheclassshouldproceed.Preliminarydesignaspectswere
selectedbasedonwhatthesupervisorsbelievedtheclasscouldfeasiblycompletegiventhetimeand
resourcesremaininginthesemester.
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Figure6highlightstheoverallpreliminarydesignfortheHawkeyeSolarCooker.Aparabolic
troughdesignwaschosenforit'ssimpleconstructionandabilitytotrackwithlittletonoeffortonthe
partoftheuser.Thepreliminarydesignalsoutilizesasecondaryreflector,however,thisdesignaspect
waslaterscrappedinanattempttoreducetheoverallweightandcomplexityoftheassembly.
Figure6:PreliminaryDesignCADAssembly
Analuminumfinarraysurroundedbysandwasdecideduponforthestoragemedium.
Aluminumisreadilyavailableintheformofpopcansthatcanbecutandfashionedintofinarrays.Sand
isalsoreadilyavailablegiventheproximityofdesertsand.Togetherthesematerialspossessedthedesiredthermalpropertiesforsolarcooking.Theforeseeableissueinusingsandandaluminumstems
fromthequantityrequiredforsuccessfulenergystorage.
Figure7showsacross-sectionalviewoftheproposedassembly.Thecollectorrestsandrotates
aboutitsownframeworkwhilethestorageboxrestuponarailsystemdesignedtomovethecooking
apparatustoandfromthehome.Inordertotrackthesunyearroundusingatroughdesign,abi-weekly
adjustmentwasproposedandbuiltintothecollectorframework.Thepreliminarydesignalsoutilizeda
heatpipetobemanufacturedbytheBerkeleyTeamforheatdeliverytothecookingsurface.
Figure8showsacloserviewoffunnelassemblydesignedtopreventlightraysfromescaping
oncetheyhavebeendirectedtowardtheabsorberplate.Aglasslens/platewasusedtopreventthe
lossofradiativeheatoutoftheabsorberarea.Theabsorberplateisattachedtothefinarraytoaidin
thespreadingofheatthroughoutthestoragebox.
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Figure7:SideViewPreliminaryDesignOverview
Figure8:PreliminaryDesignInteriorOverview
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Stage2:FinalDesign&Construction
ThefollowingsectionshavebeencompiledbytheStage2buildteams(Table1)andhighlight
theirworkandrecommendationsmovingforward.Additionalsupplementalinformationisforthcoming
andcanbeacquiredbyaccessingtheMEDPGoogleGmail/GoogleDocsaccountwiththeusername
medp.uiowaandpasswordslowcooker .TheOrganizationalTeamisintheprocessofcompiling
informationcollectedoverthecourseofthesemester.
Collector
Design
Thedesignoftheoutwardfacingtroughsolarconcentratorincorporatedacompoundparabola
toallowfora16degreeacceptanceangle.Theshapeofthecollectorwasselectedtoreducethefocal
planeasclosetothestorageabsorberplateaspossiblewhileusingaslittlematerialaspossible.Fromthemodelingteam,itwasdeterminedthattheremustbeanapertureareaofapproximately2square
meterstoobtainthenecessaryamountofenergy.Thestorageteamalsoneededtohaveasshortofa
collectionlengthaspossiblesothattheenergycouldbeconcentratedtothecenterstorageboxes.With
theseconsiderations,theteamdeterminedthefollowingparametersoftheparabola:
y=ax^2+bx+c
Eq. 1
witha=0.035,b=0.3,c=1.7
Figure9:CollectorShapeCalculation
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Oncetheshapeoftheparabolawasdetermined,theteamimportedthepointsinto Autodesk
Inventor tocreatea3-Dmodel.Theparabolawasextrudedandmirroredtoobtaintwosides.Eachside
wasthenmoved7inchestowardsthecenterandrotatedinward8degrees.Thiscreatedafocalplane
justbelowthebottomopeningoftheparabola.
Fabrication&Assembly
Theteambeganbyacquiringthree4'x8'sheets,Mother’sPolishingCompound ,polishingpads,
andanindustrialpolisher.Thesheetswerepolishedbysandingwith600,1000andfinally2000grit
sandpaperinorderfromcoursetofineandthenusingthepolishingcompoundtoobtainafinalshine.
Forfuturereference,thismethodprovedtobeslowanddidn'tprovidethedesiredshine.Theteam
experimentedwithseveraldifferentpolishingoptionsusingthepolisherandanglegrinder.Thebest
combinationfoundwastousetheindustrialpolisherwithaclothfoampadstucktotheVelcro
attachment.
Thenextstepwastostartontheframingfortheparabola.Toaccomplishthis,thegroup
createdatemplateoutof1/4"plywood.Thetemplatewasmadebyplacingreferencepointsalongthe
plywoodsheetaccordingtotheparaboliccurvepreviouslyestablished.Afinishingnailwasputateach
referencepointandanaluminumdrywallrulerwasbentalongthereferencepointsestablishingthe
curve.Thecurvewasmarkedandcutoutwithajigsaw.Thistemplatewasthenusedtomarkallsix
verticalribs.Eachoftheverticalribswerecutwiththebandsaw.Thehorizontalribsweremeasured
fromtheassemblyCADdrawings(attachedinthesupplementalmaterial)andcutwithamitersaw.The
assemblywasputtogetherusing3"screws.
Thegroupthencreatedbracketsfortheinterfacepiecethatmaintainsareflectivesurfacealong
thedistancebetweencollectionandstorage.Thebracketswereconstructedfromsquarealuminumtubing.First,theteamcutoneendoffofthetubingusingabandsaw.Second,thebracketswerecutto
8"witha45degreecutononeendtoallowforrotation.Ahingewasmountedthetopofthebracket
using1/4-20nutsandbolts.Finally,asheetofpolishedaluminumwasadheredtothetopinterface
sectionusinganindustrialtape.Thebottomsectionwastheninstalledintothecavityofthetwo
channels.Stopswereusedateachendtocontainthebottominterfacesection.Similarly,anindustrial
tapewasusedtofastenthepolishedaluminumtothefrontofthesub-assembly.
Thecollectorframingwasconstructedusing2”x4”woodboards.Theoveralldimensionsofthe
framewere6’widex8’deepx9’tallwhenthecollectorwasattachedtotheframe.Quickanalysis
concludedthattheoveralldepthoftheframingneededtobeabout7.5feettoenablethecollectorandframingtowithstanda25MPHwindandnotbeblownover.Thisanalysiswasalsoconductedwith
“worstcasescenario”assumptionstoaccountforpotentialgustconditions.
Attachedtotheframingwasadetentplatethatenabledthecollectortobeadjustedfor
seasonalchangesinthesun’salignmentwiththecollector.Thedetentplatewasmadeusingan
aluminumplatewithgrovesfortwospring-actuatedpinstofollowin.Thespring-actuatedpinswere
attachedtothedetentplatesothattheseasonaladjustmentscouldbemadewithlittleeffortfromthe
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user.Everycoupleofweeksonepinwouldberetractedandotherpinwouldsnapintooneoffourteen
1/4“holesallowingforthecollectortobeadjusted3.6degrees.
Recommendations
InthefutureitwouldbeverybeneficialtodeterminehowtoMylarcouldbeeffectivelyusedin
thecollector.ThealuminumwasnotwellpolishedmeaningthereflectivitywasnotnearlythatofMylar.Iftheissuesofairbubblesandtearingcouldbesolved,Mylarwouldprovetobeamuchbetter
materialduetoitshighreflectivity.Also,itwouldbeveryusefultohaveray-tracingsoftwarethatallows
fortheimportingof3-DCADmodels.Thiswouldallowthetestingofmanydifferentreflectorswithout
construction.Finally,therewassomeconcernregardingthedetentplate.Eachangleshouldutilizedual
pinstosecurethecollectorratherthanthesinglepinsystemcurrentlyemployed.
Storage
Medium
Manythermalstoragemediumswereresearchedandconsideredduringthedesignprocess.
Constraintsonthermalstoragemediumswerecostandavailability.Ultimatelysandwasselected
becauseofitsavailabilityintheregion.Sandwasalsochosenasthethermalstoragemediumbecause
ofitshighenergycapacity,about0.83W/g*K,anditslowthermalconductivity,about.2W/m*K.This
allowssandtostorelargeamountsofheatwithoutlossesfromconduction.
Thetypeofsandusedintheprototypewasplaygroundsandofmediumcoarseness.Theamountof
sandneededtostoretheappropriateenergywascalculatedbasedonvolumeandmass.Theevening
storageunitwastightlypackedwith62.1kgofsandaroundthealuminumarrays(discusseslater).The
morningstorageunitrequired86.8kgofsand.Thetotalamountofsandusedwas148.7kg.Itshouldbe
notedthatthesandwasslightlymoistwhenpackedintothestorageunits,whichhaveaffectedtheperformanceofthestoragedevice.
Insulation
Manyinsulationsolutionswereevaluatedduringthedesignprocess.Ultimately,aninexpensive
insulationthatcoulddrasticallyreduceheatlosswhileresistingheatandflamewasdesired.These
requirementsledtotheselectionofricehullsforinsulationbetweentheinnerandouterboxes.Rice
hullshaveaverylowthermalconductivity(approximately0.07W/m*K)anddonotburninhighheat.
RicehullscouldalsobefoundlocallyinIndiaatverylowcost.TheIowaprototypereceivedtheirrice
hullsforfreethroughadonationfromRiceHarvestersInc.
Evenwithfavorableinsulationproperties,modelingcalculationssuggestedthatalargevolumeofrice
hullswouldstillberequired;anapproximatemassof168.3kgofricehullswasusedinthefinalbuild.
Theinnerstorageboxesweresurroundedby.254mofricehullsonallsixsideswithinthelargerouter
box.Thericehullswerenottightlypackedasitwassuggestedsmallairpocketsthroughouttherice
hullswouldprovidefavorableinsulationproperties.
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InnerStorageBoxes
Theinnerstorageboxesweredesignedtoholdalloftheenergycollectedfromtheabsorber
plateatthebottomofthelightfunnel.Twoinnerstorageboxeswerecreatedtoallowthecookerto
storesufficientenergyforeveningandmorningmeals.Theinnerstorageboxesconsistedofsandand
aluminumforastoragemedium.Thematerialpropertiesoftheseitems,whicharepreviously
mentioned,ideallycreatedahybridmaterialwhichcouldstoreanddeliverheatenergy.Thesizeofthe
morningandeveningstorageboxeswascalculatedbasedonthevolumeofsandandaluminumneeded
tostorethe3.5kWhand2.5kWh,respectively.Therefore,themorningstorageboxwaslargerthanthe
eveningstoragebox.Additionally,thesandtoaluminumratiowithintheboxeswasestimatedbasedon
theenergystoragelengthoftimeandenergydeliveryraterequiredfromeachbox.Themorning
storageboxcontaineda15.5:1sandtoaluminumratio,whiletheeveningboxcontaineda10.4:1ratio.
Figure10andFigure11showthemorningandeveningstorageboxesdimensionsinmm.
Figure10:InternalStorageBox,MorningCooking
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Figure11:InternalStorageBox,NightCookering
Theboxeswereconstructedoutof0.0127[m]thickcementboardandaluminumbrackets.
Durable,heatresistantcinderblockswereusedtosupporttheinternalstoragestructureswithinthe
outerstoragebox.
OuterStorageBox
Theouterbox,whichcontainsallofthecomponentsofthestoragedesign,wasconstructed
using0.0127mthickplywoodwith2”x4”woodforsupportframing.Theextraframingsupportwas
crucialbecauseofthestorageunit’smass.Woodwaschosenasthebuildingmaterialbecauseofits
considerabledurability,lowcost,availability,andmachinability.Thefinaldimensions(inmm)ofthe
outerboxareshowninFigure12.Theouterstorageboxwasalsopaintedblackwithweatherresistant
paint,thepurposeofthiscolorwastocollectasmuchradiantheataspossiblefromthesun.
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Figure12:OuterStorageBoxAssemblyDrawing
CanArrays
Twoseparatealuminumarrayswereconstructedandplacedwithinthesandheatstorage
mediumtoincreaseheattransferfromthecollectionplatetotheheatpipeconnectedtothecook
surface.Onearraydesignwasbuiltfornightstorageandaseparatedesignwasusedforthemorning
storage.Thenightstoragearraywasdesignedtoallowmaximumheattransfertothecooksurfaceby
usingamoreconcentratedaluminummatrixwhilethemorningarraywasdesignedtotransferheat
moreconservativelyinordertoreduceheatlossesfromtherelativelylongtermstoragebox.Thearrays
variedintheamountofaluminumused,theiroverallgeometry,andhowtheywerepositionedwithinthecementboardsandstorageboxes.Becauseofthedifficultyofmathematicallymodelingthe
complexgeometryrequiredforthearrays,twoarraydesignswerechosentobebuiltfortheprototype.
Botharraydesignsconsistoftwo1/4”thickaluminumplatesbentinsuchawaythatthereis
directcontactwiththeanodizedaluminumcollectorsurfacealongtheentirelengthofthestoragebox.
Countersunkboltspressfittheplatesintothecollectorsurfacetoensureproperthermalcontact.The
1/4”platesextendfromthecollectorsurfacetothebottomofthestoragebox.Aholewasdrilledinthe
centerofeachplatesothatalongboltcouldconnecttheplatestotherestofthearray.Thearrays
consistof4”x8”piecesofaluminumsheetingstackedalongthelongboltinsuchawaythatthe
aluminumwouldcreateasand-metalmatrixwithinthesand.Thismatrixallowsheatcollectedfromthe
collectionplatetobeconductedsufficientlytotheentirevolumeofsandinordertomaximizeheat
storagecapability.Theboltrunsacrosstheboxandconnectstoanotheraluminumplatewhichmates
withtheheatpipeusedfortransferringheattothecooksurface.Washerswereaffixedtoeachendof
thelongboltandtightenedsothatthearraywaspressurefittoreducethermalresistancesasheatis
transferredthroughthearray.
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Thearrayfornightstorageconsistsofacrossdesign.One4”x8”sheetwaspositionedvertically
andanotherhorizontallyformingacross.Aholewasdrilledinthecenterofthesheets,andtheywere
slidontothelongconnectingbolt.Betweeneachcross,a1”diameter,1/2”thickaluminumrodspacer
wasslidontothebolt.Thespacerprovidesadequatemeansforheattotransferhorizontallythrough
thearray,whilethecrossesprovideamethodofheattransferintheothertwodimensions.An
illustrationofthenightstoragearraycanbeseeninFigure13.Thisdesignconsistsofarobustamountof
aluminumtoreadilytransferheattothecooksurface.Duetothefactthatthenightstorageboxonly
needstostoreheatforashortamountoftime,heatlossescausedbytheadditionofthisarraywasn’t
aninhibitorinthedesign.
Figure13:NightStorageAluminumArray
Thearrayformorningstorageconsistsofafanneddesigninwhichastackofthe4”x8”sheets
werepressfittogetherinthecenterandthenfannedoutalongthelongsidetocreateafannedeffect
sothatifviewedfromtheside,itlookslikethespokesofawheel.Thisdesignconsistsoflessaluminum
inalessconcentratedhelpeliminateheatlossesandincreaseheatstoragepotential.Figuresofthe
morningstoragearraycanbeseeninFigure14.
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Figure14:MorningCookingAluminumArray
Delivery
TheHawkeyeSolarcookerwasfabricatedfortheeventualinsertionoftheheatpipesfromthe
teamatCaliforniaBerkeley.Becausesemesterschedulesbetweenthetwoteamsdidnotcoincide,the
MEDPTeamdidnotreceivethefinishedheatpipesintime.Theteamdecidedtousea1”solidaluminumrodtoconducttheheatuptothecookingsurfacethroughconvection.Onboththenighttime
andmorningaluminumconductingrods,an8”inchdiametercooktopwasattached.Theteamdecided
togowithan8”cooktoptotryandstayconsistentwiththesizethattheyareusedtocookingonin
ruralIndia.ThisisintendedtobeatemporaryfixuntiltheheatpipesarereceivedfromCalifornia
Berkeley.Atthetopoftheboxalargerthan1”diametersquarewascutoutsothehotaluminumrod
wouldnotcomeincontactwiththebox.Apieceoftilewascutandplacedaroundtherodtoensure
thatthealuminumwouldnotcomeincontactwiththewoodoftheouterbox.Thesolarcookerwas
fabricatedwithtwolidstocoverthecooksurfaceswhentheyarenotinuse.Theeveningcooklidwas
madesmallerthanthemorningcooklidbecauseitdidnotneedtoinsulateaswell.Itwasmadesmaller
tocutcostsandminimizeweight.Themorningcooklidwaslargerandhadmorericehusksinsideofittoinsulatebetter.Thelidswereattachedbyhingestothemainboxtoeasewithopeningandclosingof
them.Themorninglidwouldcoverthecookplateontherightwithmoreinsulation.Howeverthese
lidswereunnecessarybecausetheHawkeyeSolarCookerdidnotworkasplannedandthecooktops
nevergothot.
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LightFunnel
Thefunneldesignwasoneofthemorecomplicatedprocessespresentedtothestorageteam.
Thebeamoflighthadtotravelalongthefunnelwithoutincurringtomanylosesalongtheway.Someof
themainlosesthatwouldoccurwouldbeforthelighttobereflectedbackoutsidethefunnelsystem
intotheatmosphere.Fromtheonsetofthefinaldesignitwasdecidedthatthesolarreflectorwouldbe
thesamelengthasthetotalstoragebox.Thestorageboxwouldhavealloftheinsulation,morningand
eveningstorageunitsinside.Ideallytheapproachwouldbetofunnelthelightfromthefocuslinetothe
absorberplatesasiftheywereonecontinuousplate.Howevermorecomplexgeometrywas
introduced.
Figure15showsthegeometryfromasideprospectiveofthefunnelsystem.Thesolarcollector
andmodelingteamsfoundthattheidealangleofdeclinationwastobe27degreesfromthehorizontal
plane.ThiswasduetothelatitudeinRajasthan.Thiswasthebasepointforthefunnelsystembecause
theabsorberplatewastobeorientatedatthe27degreeinclination.Thiswouldbeidealforspringand
falldeclinationangles.Ideallytheabsorberwouldbeadjustedtothesameangletothatofthecollector
butthiswasjustnotfeasible.Themodelingteamdecidedthatanidealfunnelangletobeat75degreeanglefromtheabsorberplatetoavoidscatteringeffects.Howeverthissettheopeningofthecollector
tobesetat30.4cmwidethatneededtobeabsorbedatonly3.84cm.
Figure15:LIghtFunnelConstructionBlueprint
Withthesetwoanglessettheoptimaldistancefortheopeningofthecollectorcouldbe
determined.ItshouldbenotedthatFigure6isfromthesideprospectiveoftheentiresystem.The
dimensionswerecommunicatedbacktothecollectorteamsothattheadditionalspacecouldbe
accountedforinthedesignoftheinterfacesystem.Thedistancebetweenthestorageboxtothe
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collectorwasaround23.0cm.Ifthestorageboxdidnothavetwoseparateabsorberplatesthenthis
wouldhavebeentheonlyanglesthatwouldhavebeennecessary.Theabsorberplateshadlengthsof
53.4and37.4cmforthemorningandeveningstoragesrespectively.Thismeantthatonly55%ofthe
beamwouldbereflectedinanidealstatereflectedinthesideview.
Figure16showsthefocusbeamsastheyreachedthefunnelsystem.Noticehowthebeamsatthetwoendsoftheboxdonotmakeittothestoragedeviceatall.Thiswasduetothelengthofthe
absorberplatesbeingshorterthanlengthofthefocusbeam.Thenextconundrumwastryingtocollect
asmuchasthefocuslengthaspossible.Thismeantfortheanglesgoingouttowardsthesideofthebox
theangleswouldbelessthantheideal75degrees.Thepaththatwasdecideduponwastoaccept
thoselosesandgeometricallylinktheendsoftheabsorberplatesandlinkthemtotheendofthe
collector.Thiswassetforroughly45degrees
Figure16:LightFunnelwithFocalBeamsShown
Thenextproblemwastodecideonthefunnelanglesbetweenthetwostorageboxes.Theideal
storageinsulationdimensionsweretosetthedistancesbetweenthestorageboxesat14cmtoprevent
thermallosses.Thisinturncreatedanotherlevelofdifficultytoconnectthetwofunnelsystemto
acceptthefocusline.Figure17showsthedimensionsofthelargemorningfunnelpart.Noticethatthe
anglesatbothendsarenotthesame.Essentialtheangleattheleftsideofthemorningfunnelwould
matchthatoftherightsideofevenings.Howeverbotheroftheseanglesarestillnotattheideal75
degreedeclination.
FocalLines
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Figure17:MorningFunnelDimensions
Ingeneral,mostofthedistancesandanglesforthemorningstoragesystemwerealreadyset
becauseofothercorrespondingdimensions.Thislimitedanefficientdesignofthefunnelsystemfor
boththemorningandtheeveningsystems.Dimensionswerenottheonlyproblemforthefunnel
system.Designingformanufacturabilityprovedtobeahardertask.Originallyitwasthoughtthatthere
wouldbemorethanenoughofthepolishedaluminumforthefunnels.Budgetconstraintsandtime
overrodethedecisiontousealuminum,soaglassmirrorsystemwastobeused.Attachingthemirrors
tothesystemprovedtobemostdifficultbecausetherewerenotreadilyavailablebracketingsystemfor
attachingeverythinggiventhecomplexgeometry.
Recommendations
Muchwaslearnedthoughtheplanningandconstructionoftheinternalstorageboxes,light
funnelsystem,insulation,andexternalstoragebox.Thesizeandweightofthestoragesystemsmustbe
changedinfuturedesigns.Withthecurrentprototype,awomanwouldnotbeabletoeasilymaneuver
thestorageunitinandoutofthehousetwotimesaday.Therearecurrently148.9kgofsand,172kgof
ricehulls,andadditionalweightofaluminumandplywood.Infuturedesigns,changingthestorage
materialwoulddecreasetheweightandsizeoftheoverallunit.
Theglasselementsofthestorageunitcanberedesignedtoprovidegreaterstructuralstrength.
Thecurrentdesignofthelightfunnelwasnotsturdyenoughtosurvivethetransportationfromthe
constructionareatothetestinggrounds.Byusingastronger,thickerglassandintroducingback
supportswouldincreasethedurabilityofthelightfunnel.Withmoreaccuratecutsandpropertooling,
theywouldallowformoreprecisecutsandbetterconstruction.
` Computersoftwareshouldbeasignificantpartinthedesignofthestorageunit.Withthelackof
knowledgeofcomputeraidedengineeringanalysissoftware,thecurrentcanarraysanddeliverysystem
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havenotbeentestedormodeled.Withincreasedknowledgeofsuchcomputersoftware,thesesystems
couldbemodeledproperlyandmodifiedtomaximizeoutput.Withmoreefficientdesigns,theweight
issuewouldcontinuetobeaddressed,hopefullydecreasingthetotalweightandsizeoftheunit.From
thedatacollectedforthecomputeranalysis,fewerprototypeswouldhavetobeconstructedandthe
numberoftestswoulddecreaseallowingforquickerimplementationofthedevicetoIndiaandother
developingregions.
Mechanics
Design
Giventhatthecombinationofmaterialsforthecollectionandstoragedevicetotals
approximately800lbs,thetaskofthemechanicsgroupwastodevelopafeasiblewaytotransportthe
device,asawhole,betweentheindoorcookingandoutdoorcollectingpositions.Themostcostefficient
methodofmovingthecookerwastorestthecollectorandstoragedeviceonametalframewithfour
castors.Handleswerethenattachedtothesidesofthestorageassemblytoaidinthemovementofthecooker.Duetotimeconstraints,itwasdecidedthattheframesectionswouldbejoinedthrough
welding.AsteeltrackwasalsodesignedtomovethestoragecontainerintothehomesinIndiathough
duetobudgetconcernsthispiecewasneverfabricated.Thetrackutilizestworailsapproximately60½”
apartwithanopenC-channeldesigntopreventdebrisfrombuildingupinthechannel.Theoverall
lengthofthetrackisdependentonthedistancebetweenthecollectionsiteandthecookingsite,which
willvaryfromhometohome.
Fabrication&Assembly Theinitialdesignofthecookerassemblywasexpectedtototalapproximately600lbs,although
aftercontinuedoptimization,thefinalweightrosetoapproximately800lbs.Consideringthepossibility
offurtherincreasesinweight,thecartfortheassemblywasdesignedtowithstandapproximately1200
lbsofforce.Towithstandthisforce,2”x2”x3/16”steelangleironwasusedintheconstructionofthe
bottomframe.Thedimensionsoftheframeweredependentuponthefinalmeasurementsofthe
bottomofthestoragebox.Oncetheboxwasconstructed,themeasurementswerefoundtobe64¼”x
33¼”.Toprovideenoughclearanceforthecollector,thecartwasbuilttobe64½”x33½”withtwo
additional33½”angleironrunnersinthemiddletofurthersupporttheassembly.
Theouterpiecesofangleironwerefirstmeasuredtospecificationandthencutona45degree
angletocreateamiteredjoint.Thesemitercutsallowforeasierweldingandcleanerjoiningofthe
separatepieces.Atungsteninertgas(TIG)welderwasusedtojointhepiecesofangleirontogether.
WeldswereplacedontheinsidehorizontalandverticallengthsofthejointasseeninFigure18.
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Figure18:Miterwelds
Thetwocentersupportlengthsofangleironwerethenmeasuredtorunwidthwiseinsideofthe
outerframeactingasadditionalsupportforthecenterofthestoragecontainer.Thesesupportsensurethattheweightoftheinternalstoragecomponentsdonotwarptheboxthusweakeningtheplywood
exterior.Additionally,thesecenterrunnershelptorelieveaportionoftheweightontheperimeter
framethusreducingdeflectioninthesteel.Sinceangleironwasusedintheconstructionofthecenter
supportingbeams,partoftheverticalsectionoftheangleironwascuttoproducenotchjoints,allowing
thebeamstobejoinedtotheexistingframe.Additionalweldswereusedtojointhebeamstothe
frameasdepictedinFigure19.
Figure19:Centersupportwelds
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Steelcastorswereprocuredfortheframe.Uponfurtherinspectionofthecasters,itwasfound
thatthetopplateswerecomposedofgalvanizedsteel.Originally,itwasthoughtthatthecastorswould
beweldeddirectlytotheframe.Atthetemperaturesrequiredtoweldthecastorstotheframe,thezinc
inthegalvanizedprotectivecoatingwouldhavevaporizedthusposingaserioushealthrisk.To
circumventthishazardandstilleffectivelyjointhecastorstotheframe,intermediateplateswereused.
Four3”x5”steelplateswereweldedtothefourcornersoftheframe.Four5/16”holesweredrilledin
theseconnectingplates.Figure20illustratestheconnectingplatesweldedtotheprimarysupport
frame.
Figure20:Weldedcasterattachmentplate
Aftertheplateswereweldedtotheframe,adrillwasusedtoborethroughthesteelframe,
allowingthecastorstobesecuredusing5/16”bolts.Onceconnected,theboltswerefileddownonthe
twoswivelcastorstoallowforfullrotationofthewheels.Itisimportantheretonotetheswivelcasters
wereusedforprototypepurposesandwillreplacedwithstraightcastersinthefinaldesign.
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Figure21:Completedframewithcasters
Recommendations
Forshippingpurposes,itisrecommendedthattheframeberedesignedwithoutwelds.Ideally
theframewouldbolttogethermakingiteasiertoshipandeasiertoassembleinRajasthan.
Assumingthatthenewframereliesonboltconnectionsratherthanwelds,itwouldalsobe
prudenttogalvanizethesteel.Despitethefactthattheclimateinquestionissemi-arid,itisstill
importanttoensurethattheprimarysourceofmobilityintheassemblywillnotsuccumbtodegradation
viarust.Ifgalvanizedsteelisnotused,thenperiodiccheckupswillbenecessarybyindividuals
knowledgeableabouttheoperationofthedevice.
Modeling
Considerations
Thefirststepincreatinga0-Dmodelwastodevelopanequationthatwouldbestsimulatethe
solarcookerinitsprojectedworkingenvironment.Thestorageteamstartedwiththeoverallenergy
equation.Thisequationthenhadtobemodifiedtoincorporatethetotalenergystoredinthemorning
andnightcookingboxes.Equation2belowdepictstheresultofthemodifiedtotalenergyequation.
!!!
!"
!"= !!
!′′ !!"− !" !! − !
!−
!!
!!
Eq. 2
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Ontherightsideoftheequation,thefirsttermevaluatedwasthetotalsolarenergyentering
thethermalstoragebox;dependentonthesolarflux,thecollectorplatearea,andanefficiencyfactor.
Thenexttermaccountsforanylossesthroughthewallsofthestoragebox.Thiswasaccomplishedby
findingtheoverallheattransfercoefficient,orU,throughthewallsofthebox.Thelasttermrepresents
theenergyusedincookingwhichwasassumedtobeextractedoveratwohourtimeperiodpermeal,
twomealsaday.Intheequation,misthemassofthestoragebox,C pisthespecificheatofthestorage
box,!"
!"isthechangeintemperaturebetweentheinsideandtheoutsideoftheboxwithrespecttotime,
ηistheefficiencyofthecollector,q s’’isthesolarfluxfromthesun,A cristheareaofthecollector,Uis
theoverallheattransfercoefficientthroughthewallsofthebox,Aistheareaofthestoragebox
collector,Tsisthetemperatureofthestoragebox,T ∞isthetemperatureoftheambientairsurrounding
thebox,qcistheenergyusedtocook,andη cisthecookingefficiency.
The0-Dequation,afterestablishingallofitsnecessarycomponents,wasintegratedwith
respecttotimeandtemperaturetofindtheinternalstoragetemperatureatsixminutetimesteps.The
finalintegratedsolutionwasthentransferredintoMicrosoftExceltosolveoverthespecifiedtime
intervals.Thefirststepincalculatingtheenergyandtemperaturesavailableforcookingafteraperiodof
storagewastofindthesolarinfluxduringthecollectiontimeperiod.Thisinvolvedfindingtheanglesof
incidentlightfromthesunwhichchangesdailywiththeEarth'srotationandannuallywiththe
revolutionaroundthesun.Thetotaloffsetwascalculatedtobe23.44degreesbetweentheEarth's
rotationalaxisanditsorbitalplane(Wikipedia).Theangleandcompassorientationofthecollector
surfacealsohadtobetakenintoaccountaswellasthelatitudeofthedevice.Thesolarflux,orsolar
powerreceivedperarea,canbecalculatedbytakingthesolarconstantjustabovetheearth’s
atmosphereandthensubtractingofftheefficiencylossesduetoatmosphericeffects,whichispartiallya
functionofcookeraltitude,butisalsodependentuponlocalatmosphericeffectssuchassmog.Asmall
amountofbackgroundskyradiationalsoaddstothetotalsolarflux.Thistermwasnotcalculatedbut
rathertakenfromknownengineeringstandardstables.Thetotalenergyintotheapertureofthe
collectorwasthenreducedbyanefficiencyfactortoaccountforthescatteringoflightandtheoverall
designofthecollection,transfer,andabsorptioncomponents.
Forsimplicity,aNationalOceanicandAtmosphericAdmiration(NOAA)calculatorwas
downloadedofftheNOAAwebsiteandinserteddirectlyintotheExcelspreadsheetsusedforthe0-D
model.Thiscalculatorallowedtheusertoenterthedate,latitude,longitude,andtimezoneofinterest
andsubsequentlybeprovidedspecificsolarpositiondataonasix-minutetimebasisforagiven24-hour
period.Ofparticularinterestforthisprojectwerethehourangle,sundeclination,solarelevation
(correctedforatmosphericrefraction),andthesolarazimuthangleateachsix-minuteincrement.
Figure22showsapartialpictureoftheExcelspreadsheetusedtodeterminethesevalues.Withthis
informationthesolarpositioncouldbedeterminedtowithinareasonabledegreeofaccuracy.Solar
positionwasusedinconjunctionwithcollectororientationandtiltanglestocalculatetheangleof
incidencebetweenthesunandthecollector.
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Figure22:NOAASolarCalculator(partialview)displayingvariousrelevantsolar
anglesonasixminutetime-stepbasedon:latitude,longitude,time-zone,
anddate.
Thenextpieceofinformationrequiredwasthesolarfluxforthelocationatwhichthesolar
cookerwouldbeused.Ratherthancalculatethisfigurebasedonestimations,experimentaldatawas
sought.TheANSI/ASHRAEStandard93-2003wasusedtoacquiredataforboth27degreesnorthlatitude(Rajasthan,India)and40degreesnorthlatitude(IowaCity).The27degreesnorthdata,convertedto
metricequivalentsandshownbelowinFigure23matchedupwellwiththelocationsofinterestin
Rajasthan.IowaCityisactuallycloserto42degreesnorthratherthan40degreesnorth.Thedifference
wassmallenoughtonotcreatesignificanterrorsinthepreliminarymodel.Theinformationfoundin
thesestandardsshowedhourlyaveragesforthesolarfluxtoaflatsurfacenormaltotheincominglight
onamonthbymonthbasis(ANSI/ASHRAE).Inordertomakethisdatacompatiblewiththesixminute
incrementsusedforthesolarangledata,asixthdegreepolynomialapproximationwascreatedforeach
monthwhichgavethesolarfluxonacontinuouscurvewithintheperiodofsun-uptosun-down,an
exampleofwhichisgivenforthemonthofMayat40degreesnorthlatitude(Figure24).
Thetimeofdaycouldthenbeenteredintotheseequationstoprovidethesolarfluxforanytenthofan
hourpointoverthecourseofadayforanymonthoftheyear.Itisimportanttonotethatwhilethese
polynomialscorrectedfordailysteppinginthedata,thefluxusedinthefinalmodelwasstillbasedonly
onmonthlyaverages.Inotherwords,themodelfalselyshowssolarinsolationvaryingsignificantlyfrom
the30ththe1
stofeverymonth.Sincethefinaloutputofthismodelwasdesignedtolookmostlyatyear-
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longtrends,thiswasdeemedavalidapproach.Caremustbetakenwhendeterminingdatathatspan
betweentwomonths;mid-monthfindingsareconsideredtobemostaccurate.
Figure23:Clearskysolarinsolation[W/m^2]atdifferenttimesoftheyearfor27degreesN.latitude
Figure24:ANSI/ASHRAEcurveforMaysolarinsolationat40degreesN.latitude(orange)andthesixthdegreepolynomial
approximation(black)
Theangleofinsolationwilllessentheintensityoftheincominglightrays.Forexample,aone
metersquareareathatisreceivingperpendiculartothesunhavingafluxof500wattspersquaremeter
willhaveaninsolationof500watts.However,ifthatareais60degreesfromtheperpendicular,itsview
factorissuchthattheprojectedareanormaltotheincominglightwillbeonlyhalfofitsfullarea.This
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resultsinaninsolationvalueofonly250watts.Thusthemodelwassetuptoaccountfortheseangles
andreducetheincomingsolarfluxintothecollectorbasedontheaforementionedsolarangulardata
andthecollectororientationandtilt.Theorientationwasassumedtobeduesouth,andthetiltwas
modeledwithastepequationthatadjustedthecollectortiltby3.6degreeseverytwoweeksinorderto
compensateforseasonalshiftsinsolardeclination.
Withtheenergyintothecollectorinhand,inefficienciesfromthecollectoritselfcouldbeused
tofindfinalenergyintothestoragedevice.First,thereflectivityofthepolishedaluminumsurfaceswas
assumedtobe96%;therestwaslosttoscattering.Multiplyingtheinsolationdatabythereflectivity
raisedtothepowerofthenumberofaveragebounceslightraysmakeontheirwaytotheabsorption
plategavethetotalefficiencyofthecollector.Additionally,lossesduetotheglasscontainmentbarrier
andtheabsorberplateabsorptivitywerealsoconsidered.However,basedonalargenumberof
unknowns,especiallyconcerningtheraypathsthroughthedevice,aroughestimateof60%efficiency
wasusedinanattempttoaccountforadditionallosses.Thus,thefinalenergyintothedevicewasfound
bymultiplyingthesolarinsolationbytheapertureareaofthecollectorandbythe60%efficiency
estimateforeachtime-stepconsidered.
Next,thetotalcookingtimeforRotiwasdetermined.Atthebeginningitwasassumedthat
cookingrequired1000wattsofenergyfor2hoursatatime.Thiswasassumedforbothmorningand
nightcooking.ThisportionofthemodelwaslaterrefinedtoreflectBerkeley’sexperimentalresultson
roticooking.Throughtesting,BerkeleywasabletoprovidetheIowateamwithdatasuggesting550
wattsofenergyisusedtocookoneroti.Thisvaluewasthenimportedintoourmodeltoprovideuswith
evenmoreaccurateresults.
Materialpropertieswerealsoincorporatedintothemathematicalmodel.Storage,Collector,
andMechanicsteamsprovidedtheModelingteamwithmaterialdataandsizingforuseinthemodel.
OnceimportedtoExcel,theequationsprovidedanextremelydynamicsolver,allowingchangestoeasily
bemadetothevariousdesignparameters.Afterallvariableswereaccountedfor,Excelwasableto
produceweeklyandyearlygraphsoftemperatureandenergyinboththemorningandeveningcooking
storageboxes.
ProjectedPerformance
ThefollowingfiguresshowgraphsfortemperatureandtotalenergystoredforbothIndiaandIowa.
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Figure25:NightHeatEnergyStored,India
Figure26:MorningCookingTemperatures,India
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Figure27:Morningheatenergystored,India
Figure28:Nightcookingtemperatures,India
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Figure29:Weeklymorningtemperatures,Iowa
Figure30:Weeklynighttemperatures,Iowa
AfterviewingtheheatandtemperaturegraphsforIndia,itwasnotedthatthereissufficient
energystoredwithwhichtocookroti.Themorningcookinghadaslightlylowercooktemperaturedue
toheatlossesthroughoutthenight.Amorecompletepictureofhowthemodelprojectstemperatures
inthecookercanbefoundintheweeklyIowagraphs(Figure29andFigure30).Thesegraphsshowthat
a1-2daychargingperiodisrequiredbeforethecookerisreadyfordailyuse.Extractionofenergydueto
cookingcanbeseeninthesharptemperaturedropsfoundintheeveningandagainintheearlymorning
timeperiods.Theslowermoreexponentialdropsintemperaturearefromtheheatlossestothe
surroundings.
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Conclusions
FinalRecommendations
Ultimately,theclasswassuccessfulinbringingthefinaldesigntoprototype.However,thereismuch
worktobecompleted.FutureMEDPclassesshouldbetaskedtocorrectsomeofthefollowingissues
whileimprovinguponthebuildqualityofseveralkeycomponents.Theprimaryrecommendation
regardingthesolarcookerinclude:
1. Calibrationandconfirmationofthethermocoupleresults-Theinitialtestresultsyieldedvery
littleinformationandafullcalibrationofthethermocoupleswasnotperformedpriortothe
assemblyofthecooker.Beforeadditionaldataisgathered,calibrationofthethermocouples
mustbeperformed.
2. Isolationoftheinteriorlightfunnelfromtheexternalassembly-Theprimaryhousingandlight
funnelneedstobeassembledseparatelytopreventcrackingthatcanbecausedduringthemovingofthecooker.
3. Asetofexperimentalevaluationcriteriashouldbeestablished-severalsuggestionsforfuture
workinclude:
a. Gatherdataondifferentintervalstoobtainamorecomprehensivesetofperformance
curves
b. Gatherdataatseveralcollectoranglestoevaluateperformancelossbetweenangle
adjustments.
c. Preparehourlynote/evaluationsforcookerperformance.Whilethismightseem
excessive,notinglittlenuancesregardingshadowlocationandlightconcentration
throughoutthedaywillgoalongwaytoaidintheoptimizationofthecooker.
4. Determinetheoptimumreflectorcurvatureforpracticaluse.
5. ObtaintheheatpipefromtheBerkeleyTeamandincorporateitintothefinalsolarcooker
design.
6. Reducetheoverallweightofthesolarcookerthroughouttheoptimizationprocess.
Themajorissuesatthispointintheevaluationprocessdealwiththesolarcooker’sabilitytodirect
andfunnellighttoafocalpoint.Itwasnotedduringthetwo-daytestperiodthatlightconcentrations
wereoffcenterontheabsorberplate.Additionally,thereflectorsurfacesrequireadditionalpolishingto
increasereflectivity.
ItistheOrganizationTeam’ssuggestionthattestsberunoneachofthesubcomponentsseparately.
Buildingandtestingtheindividualpartsfirstwillshedmorelightonproperconstructionpracticesand
actualperformancevaluespriortofutureintegrationbetweencomponents.Inotherwords,theMEDP
groupsshouldgeteachofthecookercomponentsworkingseparatelypriortotryingtogetthemto
worktogether.
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Itisalsorecommendthattheclassbesplitintotwoteamtodividetheworkandprovidemore
studentswithdeeperinvolvement.Thirtystudentsononeprojectprovedtobeabitofalogistical
problemattimeswhenopinionsgotthebetterofengineeringreason.Additionally,thereweretimes
whensomestudentswerewaitingonothergroupstocompletework,whichshouldbeavoidedto
preventteamsfromloosinginterestintheproject.
Acknowledgements
TheMechanicalEngineeringDesignProjectTeamwouldliketogiveaspecialthankstoSailesh
RaoandH.S.Udaykumarforinspiringtheclasstoachievesomuchinsuchashorttime.Thedesign
processwaslongandarduous.InthewordsofProf.Udaykumar,"designismessy;thatisjustthenature
oftheprocess."Movingforward,thereismuchworktobedone.Futuregroupswillneedtostay
focusedandthinkoutsideoftheboxtosucceed.Thisproblemwon’tbesolvedovernight.Makeita
goaltostepforwardeachsemesterandeventuallywewillfindthesolution.
Thankyouallforyourcontributiontoourproject…
SteveStruckman UIEngineeringShopkeeper–Machining&
constructionaid
MargretEvans MIEDepartmentStaff–Budgetaid
JenniferRumping MIEDepartmentStaff–Clericalandofficeaid
RichHarden MIEDepartmentStaff–DasyLabaid
Dr.GaniGanapathi NASAEngineer–Insulationadvisor
ClimateHealers InformationandpublicityEngineeringforChange Informationandpublicity
UCBerkeleyTeam Advisementandheatpipedesign/construction
DFMLabTA’s Machiningandconstructionaid
TheMill Foodandmeetingroomrental
RiceHarvestersInc. Donationofthericehulls
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Andaveryspecialthankstothestudentsforstickingwithitthroughoutthesemester…
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WorksCited
1. UniversityofIowaIndiaWinterimTeam."IndiaWinterimReport."ClassProject.Universityof
Iowa,2011.
2. Wikipedia."Declination."10052011.10052011<http://en.wikipedia.org/wiki/Declination>.
3. ANSI/ASHRAE."TableE.1-E.8:SolarPositionandInsolationValues."Standard93-2003(2003).
4. ClimateHealers.152011<http://www.climatehealers.org/olddump/home.html>.
5. Rao,Dr.Sailesh.ClimateHealers:SolarCookersforRajasthanMechanicalEngineeringDesign
ProjectTeam.IowaCity,1122011.
***Additionalinformationwillbeprovidedintheforthcomingcoursepacketwhichwillincludeaddition
researcharticle,presentation,andthemasterExcelfileusedinproj ectcalculations.