Combined solar and membrane drying technologies for sustainable fruit preservation in low-income countries – prototype development, modelling, and testing

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ContentslistsavailableatScienceDirect

Solar Energy Advances

journalhomepage:www.elsevier.com/locate/seja

Combined solar and membrane drying technologies for sustainable fruit preservation in low-income countries – prototype development, modelling, and testing

Ricardo Bernardo

^∗

, Henrik Davidsson , Peter Samuelsson , Gustaf Bengtsson , Viktor Döhlen , Joakim Olsson, Randi Phinney, Pia Otte, Lucas Tivana, Martin Andersson, Marilyn Rayner

Energy and Building Design, Architecture and Built Environment, Sövegatan 24, P.O. Box 118, 221 00 Lund, Skåne, Sweden

a r t i c le i n f o

Keywords:

Solar drying Fruit preservation Membrane drying Low-income countries

a b s t r a ct

Thisinvestigationconsistedofdevelopingandevaluatingsolardryerstogetherwithsemi-permeablemembrane pouchesfordryingjuicyfruitsinlow-incometropicalcountries.

Twodesigniterationswerecarriedoutincludingprototypemodellingandtesting.Thelatestdevelopedsolar dryerswereapassiveandanactivesolardryer.Modellingwasinitiallycarriedoutmathematicallyusingan equationsolversoftwarefollowedbycomputationalﬂuiddynamics.Preliminarymeasurementswerecarriedout onasmall-scalesolardryer.Thereafter,full-scalemodelsweredevelopedandtested,bothinlaboratoryandin realconditionsinMozambique.

ResultsfrommodellingwerevalidatedagainstmeasurementsinlaboratoryinSwedenandﬁeldtrialsin Mozambique.PrototypebuildingandtestinginMozambiquewasundertakenincollaborationwithlocalfarmers andauniversity.Measurementresultsshowthatthedryershelptopreventmicrobialgrowththroughincreased temperatures.Thedryingﬂuxwasincreasedby50%forthepassive,andby100%fortheactivesolardryers comparedtotheambientcontrolsthatdidnotuseasolardryer.Thetotaldryingtimewasbelowfourdaysforall pouchesinthedryers.Theactivesolardryerwasshowntohavetheshortestdryingtimeandthehighestcapacity (morepouches)butalsothehighestcosts.

Mouldgrowthandjuicefermentationwereobservedoncontrolpouchesdryinginopenair.Theseproblems weresolvedwiththeuseofsolardryertechnology.However,somechallengeswiththemembranepouches requirefurtherdevelopmentincludingdegradationofthemembranewhenexposedtodirectsunlight.

Introduction Relevance

Enoughfoodisgrowninmanylow-incomecountriestosatisfythe needs of the population. One such country is Mozambique. Despite this,manypeoplestillgohungry.Onereasonforthisisbecauselarge amountsoffruit ripenduringaveryshortperiod, whichmeansthat muchof theuneatenfruitspoilsbefore reachingtheend consumers.

InMozambique,post-harvestlossesareestimatedtobe25-40%,with thisestimateonlyaccountingforharvestedfruits[1].Sincesomefruits arenever harvested,thetotalamountofspoiledfruitwouldbeeven greater.Canningandasepticprocessingaretwooptionsforpreserva- tion,buttheyareoftennotfeasibleunlesscarriedoutonalargescale.

Theyalsorequirelargeenergyinputs,largecapitalinvestmentsanda transportinfrastructure,whichareoftennotavailableinlow-income

∗Correspondingauthor.

E-mailaddress:Ricardo.Bernardo@ebd.lth.se(R.Bernardo).

countries.Severalstudieshaveidentiﬁedtheneedforasimpleandaf- fordablefruitprocessingtechnologythatcansafelypreservefruitsclose totheharvestpointwhenthefruitisripe[2–7].Solardryersthatcan beusedatlargescaleforalargevarietyofagriculturalproductswould greatlybeneﬁtsmallandmarginalfarmers[2].

Traditionalfruitdehydrationmethodsandsolarassisteddrying

Foodhasbeenpreservedwithdryinganddehydrationmethodsfor thousands of years. Reducingthe amountof available moisturein a producthastwoeﬀects:pathogenicandspoilagemicroorganismsareno longerabletogrow,andthenutritionalvalueofthefoodisconserved.

Existing small-scale dehydration methods include oven drying;

wood,charcoalordieselburningevaporators;osmoticdehydration;and open-airsundrying.Ovendryersrequireanexpensiveenergysource (i.e.electricityorgas)andcontributetoCO₂emissions.Wood,char- coal,ordieselburningevaporatorsalsocontributetothereleaseofCO₂

https://doi.org/10.1016/j.seja.2021.100006

Received19February2021;Receivedinrevisedform5October2021;Accepted6October2021 Availableonline14October2021

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Nomenclature

Aabs Collectorabsorberarea(=glassarea)m²

Abag TheeﬀectiveareaforevaporationofaSAP-pouch m²

Abags TotaleﬀectiveevaporationareaofSAP-pouchesper shelfm²

Aeff Effectivecrosssectionareaforthedryerairflowm² Ainlet Collectorinletaream²

Asides Collectorsideaream² aw Wateractivitynounits

CP,air SpeciﬁcheatcapacityforairJ/(kg•K) CP,H2O SpeciﬁcheatcapacityforsteamJ/(kg•K) DH Hydraulicdiameterofthecollectorm

Evap Total powerrequired forevaporation forall SAP- pouchesplacedonashelfinthedryerW

G TotalsolarirradiationW/m²

Ge Eﬀectivesolarirradiation(heatabsorbedbytheab- sorber)W/m²

hcon,in Heat transfer coeﬃcient for internal convection W/(m²•K)

hcon,out Heat transfer coeﬃcient for external convection W/(m²^•K)

hrad,in Heat transfer coeﬃcient for internal radiation W/(m²•K)

hrad,out Heat transfer coeﬃcient for external radiation W/(m²•K)

mh Watermassﬂowkg/s

mvap WatermassﬂowfromSAP-pouchkg/s n Numberofbagspershelfinthedryernounits Nu Nusselt number for uniform surface heat ﬂux no

units

Prair Prandtlnumberforairnounits Pw VapourpressureforwaterPa Pws WatersaturationpressurePa Rabs Absorberreﬂectancenounits Reair Reynoldsnumberforairnounits Rglass Glassreﬂectancenounits RH Relativehumiditynounits

RV SpeciﬁcgasconstantforwatervapourJ/(kg•K) S Numberofshelvesinthedryernounits Tabs AbsorbertemperatureinthecollectorK Tair MeantemperatureofTinandToutK Tamb AmbienttemperatureK

tbottom Bottominsulationthicknessm Tglass GlasstemperatureincollectorK Tin InlettemperatureforasectionK Tout OutlettemperatureforasectionK tside Sideinsulationthicknessm Tsky SkytemperatureK

Twall WalltemperatureofdryerK

Twall OuterwalltemperatureinthecollectorK Twall,in InnerwalltemperatureinthedryerK Twall,out OuterwalltemperatureinthedryerK uair Collectorinletairvelocitym/s

ubags TheairvelocitysurroundingtheSAP-pouchesm/s X Airconstant(𝜌air•CP,air•uair•Ainlet)W/K X Airconstant(𝜌air•CP,air•uair•Ainlet)W/K z Additionalheatgainsfortheouterwallsofthedryer

W/m²

𝛼abs Absorberabsorbancenounits

Δhvap,H2O EnthalpyofvaporizationforwaterJ/kg 𝜀abs Absorberemissivitynounits

𝜀glass Glassemissivitynounits

𝜀wall Collectorouterwallemissivitynounits 𝜀wall,dryer Dryerouterwallemissivitynounits 𝜆air AirthermalconductivityW/(m^•K)

𝜆bottom Collector bottom insulation thermal conductivity W/(m•K)

𝜆side Collector side insulation thermal conductivity W/(m•K)

𝜆wall DryerwallthermalconductivityW/(m•K) 𝜇air Airdynamicviscositykg/(m•s)

𝜌air Airdensitykg/m³

𝜎 Stefan-BoltzmannconstantW/(m²•K⁴) 𝜏glass Glasstransmissivitynounits

andhaveatimeand/oreconomicburden.Open-airsundryingoffood isapreservationmethodwidelyusedindevelopingcountriesaspartof localancienttraditionsbutalsohasasigniﬁcantlimitation.Itisnotsuit- ableforpreservingjuicyfruitsbecause1)itisdiﬃculttohandlelarge opentraysofliquidand2)juices/puréesdriedinopentraysattractdust, insectsandpestsandarethereforeeasilycontaminatedbymicroorgan- ismsandtoxinsasaresultoftheproductbeingdirectlyexposedtothe environment[4].

Inadditiontoopen-airsundrying,twosolarassistedfoodprocess- ingapplicationsaresolardryingandsolarcooking.Therearenumerous typesof solardryersrangingfromsimpler greenhousetypestocom- plexsolardryersthatuseheatexchangersandlargesolarstorages[2, 4,5,8].Itsapplicabilitygoes beyondfoodprocessingsuchasdrying cementmortarforbuildingapplications[9].Solardryersarenormally dividedintotwomaincategories:passiveornatural-circulationandac- tive or forcedconvection[8]. Since they aresusceptibleto weather changes,hybridizationisoftenneeded,i.e.,combinationwithbackup energysourcesthatusessolarenergytodecreaseitsconsumption.Are- centreviewarticleidentifiedfivecategoriesofhybridsolardryers:hy- bridthermalstorage-solardryer,hybridheatpump–solardryer,hybrid biomass–solardryer,hybridsolarwithnoveldryingtechniques,andhy- bridphotovoltaicsolardryers[7].Thesetypeofdryersprovidebetter controloverdryingconditionsandcanbeusedonawiderrangeofagri- culturalproducts[7].Efficiencyandcostvarysignificantlydepending onthetechnology[10].

Solarcookingisusedtopreparefoodinrealtime.Oneofthemost well-known solarcookingtechnologiesis theScheffler collectorthat consistsofaparabolicreflectorconcentratingsolarradiationtowards acookingpot[11].Thereflectorneedstobeconstantlyadjustedtofol- lowthesunduringthedayandwhenthesunisnotavailable,fuelis normallyusedinstead[12].

Eventhoughthesetechniqueshavebeendevelopedandtestedfor decades, thedevelopmentin solarfoodprocessing hasbeenlimited.

Twoofthemainreasonsarethenecessityofhybridization(combination withbackupfuel)andacceptancebytheend-users[13].Hybridization impliestechnicalcomplexityandhighinvestmentcosts,whicharema- jorhindrancesindevelopingcountries[4,12].Also,backupfuelsand infrastructurearenotavailableinmanycases.

Solardryingtechnologyismostcommonlyusedtofurtherimprove traditional drying techniqueswith fruits, such asopen-air drying of raisins,dates,ﬁgs,mangoes,etc[2–6].However,solardryingofalarge rangeofjuicyfruitssuchasoranges,tangerines,lemons,andgrapefruits, arenotcommonlydescribedinliterature.Furthermore,investigations ofsolardryersarefrequentlyeitherexperimental[2,6,14,15]orthe- oretical[16,17]lackinganintegratedapproachtothedesignprocess.

Additionally,literatureidentiﬁestheneedforfurtherresearchoneﬃ- cientdesignsatlowcosts[4,8,10].

Earlierstudiesontheimplementationofsolarenergytechnologies, suchassolarcookers,indevelopingcountries,haveshownthatthereare alsomultiplenon-technicalfactorsthatstronglyinﬂuencetheadoption ofthesetechnologies[18–20].Thesecanbe,forexample,aninappro-

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Fig.1.Left:schematicsofthesolarassistedpervaporationprocess.Middle:membranepouchesbeforedrying.Right:ﬁnalproduct(jam)afterdryingonthefarright.

priatetime-scheduleforcookingorchangesinthesocialorganization [21].Thequalityoftheﬁnalproductintermsofsensoryquality(taste, aroma,colourandacceptabilityofthedriedfruitproducts)andnutri- tionalcompositionofthedriedfruitproductsarealsoimportantfactors forlargeracceptanceinthemarket[6].

Solarassistedpervaporation(SAP)

SolarAssistedPervaporation(SAP)involvestheuseofapouchor sealedbagmadeofafood-gradebreathablemembrane(i.e.permeable towatervapourbutnotliquidwater)toconcentratefruitjuices/purées withthehelpofsolarradiationandambientair[22].TheSAPtech- niqueisrelatedtotheseparationprocesspervaporation,withthemass transportdrivenbythedifferenceinpartialpressureofwatervapourin thefruitjuice/puréetothepartialpressureofwatervapourinambient air[23,24].Thephenomenonof pervaporationwasfirstdocumented byKoberin1917[25].Iftherelativehumidity(RH)inthesurround- ingairisunsaturated,therewillbeachemicalpotentialgradientacross themembrane,allowingforthediffusionofwatervapourtooccur.The drivingforcecanbeincreasedevenfurtherbyheatingtheairaround thebag(e.g.withasolarcollector).Themembranepouchallowswater toevaporatebutprotectsthejuicefromcontamination.Fig.1illustrates theSAPprocessandtheproductbeforeandafterdrying.Iftheamountof waterinthejuicedecreasesbelowacertainlevel,thejuiceconcentrate becomesstoragestableforuptoayear,i.e.duetoalowwateractivity thatpreventsmicrobialgrowth[26,p.2].Thefinalproductcanthenbe consumedasjamorre-hydratedagainintonutritiousfruitjuices.

Althoughpromising,theSAPmethodusedinopenairdryingdoes notreachhighenoughtemperaturestodecreasetheriskofmicrobial growthintheproduct.Furthermore,thedryingtimeisrelativelylong.

Thesearecrucialfactorsforthesuccessfulpracticalimplementationof theprocess.Inshort,controllingthedryingparametersaroundandin- sidethepouches-airtemperature,relativehumidity,fruittemperature andairﬂow-iscriticaltomakethemethodsafeandpracticallyfeasible [26,27].

Purposeandscope

Themainpurposeoftheprojectbehindthisarticlewastoinvestigate thecombinationofsolarcollectortechnologywiththeSAPpouchesde- scribedabove,usingMozambiqueasacase-study.Theinvestigatedsolu- tionconsistedofadaptingandcombiningsolarcollectortechnologythat producessolarheatwithsemi-permeablemembranepouchesfordrying andthuspreservingandutilizingjuicyfruitsindevelopingcountriesto ahigherextentthantoday.Theﬁnalproductofthedryingisashelf- stablefruitmarmaladeorjuiceconcentratethatcanbelaterconsumed orsoldforuptooneyear.TheSAPpouchtechnologyisinnovativeand thereforenotasdevelopedassolardryers.Thisarticlecontributesto thetestingofSAPpouchesincombinationwithnewlydevelopedsolar dryers.

Theinvestigationfollowedanintegratedtransdisciplinaryresearch approachthatisabletoincludediversesetsofknowledgerelevantfor thedesignofuser-friendlysolardryers[28].Theprojectconsistedof aninterdisciplinaryresearchteamwithbackgroundsfromsolarenergy technology,foodtechnologyandsocialscienceswithpreviousresearch experienceinMozambique.Furthermore,theprojectworkedwithtwo farmerassociationsinaniterativeprocessthroughouttheresearchpro- cess.Theprojectincluded3phasesofﬁeldworkinInharrimedistrict between2016and2019.

Inthefirstphasetheprojectstartedbyassessinglocalfruitdrying traditionsandpracticesofthecommunities,existingknow-how,culture, androutines[2].Thisassessmentwascarriedoutbyengaginggroupsof farmersinparticipatoryexercisestoidentifytheirneedsandpreferences relatedtosolarfruitdrying[29].Furthermore,theimportanceofgender rolesandrelationshipswerealsoassessedinthescopeoftechnologyde- velopmentandimplementation[30].Inparallel,thefoodtechnologyin- vestigationfocusedonfactorsaffectingfoodsafetyriskandquality.This wascarriedoutbyevaluatingthedryingflux[27,p.1],[31]andmi- crobiologicalquality[26,p.2]ofthefruitjuicesinmembranepouches forseveraldryingconditions.Theseresultswerethenusedasinputfor thetechnologydevelopmentintheformofseveraldesigncriteriathat weretestedinphase2andthenmodifiedandimplementedinphase3.

This paper focuses on the solartechnology part of the research namelythedevelopment,modelling,andtestingofsolardryers,inlab- oratoryandinrealconditions.Tograspthecompletetechnologydevel- opmentandtesting,thecurrentpapercomprisesﬁndingsfromseveral mastertheses[32–35].

Methods

Sincethefarmersparticipatinginthisstudydidnothaveanyexpe- rienceonsolardrying,carefulselectionofthemethodologywasneeded forsuccessfuladoptionofthetechnology.Toavoida“technicalpush” theselectedworkflowwastostartwithasocialinvestigationincluding participatoryresearchexercises[29,30].Thisapproachallowsforan activeinvolvementofthefarmersinsteadoftreatingthemaspassivere- cipients[36].Togetherwithfoodtechnologyinvestigations,thedesign criteriaforthesolardryerswereestablished(Fig.2).Twodesigniter- ationsforsolardryerprototypeswerecarriedout.Foreachofthetwo designphases,modelsandmeasurementswereperformed.Modelling wascarriedoutmathematicallyandusingadvancedcomputationalfluid dynamictools.Resultsfrommodellingwerevalidatedagainstmeasure- mentsinlaboratoryinSwedenandinruralareasinMozambique.Proto- typebuildingandtestinginMozambiquewasundertakenincollabora- tionwithlocalfarmersandauniversity.Finally,theperformanceofthe solardryerswasevaluated.Resultsfromthefirstdesigninfluencedthe seconddesign.Fig.2illustratestheworkflowoftheentirestudy.The influenceofseveraldesigncriteriainthedryingwereanalysedduring thisstudy:direct/indirectsolarradiation;passiveoractivedrivensolar dryer;airflowandvelocity;temperaturelevels;dryingtimeforstorage

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Fig.2. Illustrationoftheworkﬂowofthewholeinvestigation.Thispaperfocusesonthesolartechnologyinvestigation.

Table1

Dimensionsoftheminiatureindirectsolardryerusedforpreliminarytests.

Collector Parts Dimensions (m)

Length, l Outer width, w Width absorber plate, w _a Width glass, w g

Height collector, h c

Distance absorber glass, h ag

Height heat storage, h hs

Thickness absorber, t a

Thickness glass, t _g

0.700 m 0.424 m 0.376 m 0.376 m 0.082 m 0.035 m 0.035 m 0.012 m 0.003 m

Drying cabinet parts Dimensions (m)

Height, h dc

Width, w dc

Depth, d dc

0.500 m 0.400 m 0.200 m

stableproductanddryingﬂux;materialsusedintheprototypes;costs fortwoprototypes.

Firstdesign

Prototypedesign

Preliminarytrialsofasmall-scalesolardryerweremade.Thegoal wastogetapracticalunderstandingofthedryingprocessofthemem- branepouchesinsideaconventionalsolardryer.Morespecifically,the small-scalesolardryerwasintendedtoidentifythemostrelevantpa- rametersinfluencingthedryingprocessofthepouches.Anindirectsolar dryerwaschosensinceitwasuncertainwhetherthemembranepouch wouldresistdirectsunlightandhightemperatures.Tosimplifythetest procedureattheverybeginningoftheinvestigation,waterwasused insidethepouchesinsteadoffruitjuice.Thesmall-scaleindirectsolar dryerandthemembranepouchesfilledwithwaterareillustratedinFig.

3andFig.4,respectively.Thecorrespondingdimensionsareillustrated inTable1.

Thesolardryerconsistsoftwomainparts:thesolarcollectoranda dryingcabinet.Thesolarcollectorconsistsofaglasspaneonthetop, insulationon thesidesandthebottomandanabsorberplateon top

ofthebottominsulationlayer.Theabsorberplateconsistsofasimple plywoodpaintedblack.Thedryingcabinetisinsulatedonthesidesand containsshelveswithpouches.Theairgetsheatedinthesolarcollector andrisesbynaturalconventionintothebottomofthedryingcabinet.

Attheoutletofthedryingcabinetmoistairisreleased.

Modelling

Thepreviouslydescribedsolardryerprototypewasmathematically modelledandanalysedusinganequationsolversoftware[37].Thegoal atthisstagewastobuildadeeperunderstandingofthecriticaldesign featuresofthesolardryertoconsiderwhenimprovingthedryingpro- cess.Basedonthismodel,thedesignofthesolardryerwasimproved.

AnillustrationofthemodelledsolardryerisshowninFig.5.Themod- elledenergyﬂowsandtemperaturesinthesolarcollectoranddrying cabinetareillustratedinFig.6andFig.7,respectively.

Tolimitthecomplexityofthenumericalequationssomeassumptions weremade.Themostimportantwere:thecollectoranddryerwereas- sumedinsteadystateforconstantsolarirradiation;noairleakages;the temperaturesofthepoucheswereassumedequaltothesurroundingair temperatureandthatthebagsdonotdryout(constantpartofthedry rate);theinnersidesofthecollectordonotreﬂectanyradiation;no

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Fig. 3. Miniature indirectsolar dryer used forpreliminary tests.

Fig.4. Membranepouchesﬁlledwithwaterforpreliminarytests.

heattransferwastakenintoconsiderationbetweenpouches;theglass doesnotabsorbanysunlight(𝛼glass=0).

The modelling procedure resulted in four energybalances tobe solvedforfourvariables.Theenergybalancescorrespondtotheglass coverinEquation1,theairﬂowinEquation2,theabsorberinEqua- tion3,andtheouterwallsinEquation4.Thevariablesare𝑇_𝑜𝑢𝑡,𝑇_{𝑔𝑙𝑎𝑠𝑠}, 𝑇_𝑎𝑏𝑠and𝑇_{𝑤𝑎𝑙𝑙}.Thewholeprocedureisdescribedindetailedin[38].

Equation1.Energybalanceoftheglasscover.

ℎ_{𝑐𝑜𝑛,𝑖𝑛}⋅𝐴_𝑎𝑏𝑠⋅(_𝑇

𝑖𝑛+𝑇_𝑜𝑢𝑡

2 −𝑇_{𝑔𝑙𝑎𝑠𝑠})

+ℎ_{𝑟𝑎𝑑,𝑖𝑛}⋅𝐴_𝑎𝑏𝑠⋅(

𝑇_𝑎𝑏𝑠−𝑇_{𝑔𝑙𝑎𝑠𝑠})

−ℎ_{𝑐𝑜𝑛,𝑜𝑢𝑡}⋅𝐴_𝑎𝑏𝑠⋅(

𝑇_{𝑔𝑙𝑎𝑠𝑠}−𝑇_𝑎𝑚𝑏)

−ℎ_{𝑟𝑎𝑑,𝑜𝑢𝑡}⋅𝐴_𝑎𝑏𝑠⋅(

𝑇_{𝑔𝑙𝑎𝑠𝑠}−𝑇_𝑠𝑘𝑦)

=0

Equation2.Energybalanceoftheairﬂow.

ℎ_{𝑐𝑜𝑛,𝑖𝑛}⋅𝐴_𝑎𝑏𝑠⋅(

𝑇_𝑎𝑏𝑠−^𝑇^𝑖𝑛⁺^𝑇^𝑜𝑢𝑡

2

)

+ℎ_{𝑐𝑜𝑛,𝑖𝑛}⋅𝐴_{𝑠𝑖𝑑𝑒𝑠}⋅(_𝑇

𝑎𝑏𝑠+𝑇_{𝑔𝑙𝑎𝑠𝑠} 2 −^𝑇^𝑖𝑛⁺^𝑇^𝑜𝑢𝑡

2

)

−ℎ_{𝑐𝑜𝑛,𝑖𝑛}⋅𝐴_𝑎𝑏𝑠

⋅(_𝑇

𝑖𝑛+𝑇_𝑜𝑢𝑡

2 −𝑇_{𝑔𝑙𝑎𝑠𝑠})

−(𝑇_𝑜𝑢𝑡−𝑇_𝑖𝑛)⋅𝜌_𝑎𝑖𝑟⋅𝐶𝑝_𝑎𝑖𝑟⋅𝑢_𝑎𝑖𝑟⋅𝐴_{𝑖𝑛𝑙𝑒𝑡}−(𝑇_𝑜𝑢𝑡−𝑇_𝑖𝑛)

⋅𝐶𝑝_𝐻2𝑂⋅𝑚_ℎ−ℎ_{𝑐𝑜𝑛,𝑜𝑢𝑡}⋅𝐴_𝑎𝑏𝑠⋅(

𝑇_{𝑔𝑙𝑎𝑠𝑠}−𝑇_𝑎𝑚𝑏)

=0 Equation3.Energybalanceoftheabsorber.

𝐺−ℎ_{𝑐𝑜𝑛,}_𝑖𝑛⋅(

𝑇_𝑎𝑏𝑠−^𝑇^𝑖𝑛⁺^𝑇^𝑜𝑢𝑡

2

)

−ℎ_{𝑟𝑎𝑑,𝑖𝑛}⋅(

𝑇_𝑎𝑏𝑠−𝑇_{𝑔𝑙𝑎𝑠𝑠})

−^𝜆_𝑡^{𝑏𝑜𝑡𝑡𝑜𝑚}

𝑏𝑜𝑡𝑡𝑜𝑚

⋅(

𝑇_𝑎𝑏𝑠−𝑇_{𝑤𝑎𝑙𝑙})

−^𝜆_𝑡^{𝑠𝑖𝑑𝑒}

𝑠𝑖𝑑𝑒 ⋅^𝐴_𝐴^{𝑠𝑖𝑑𝑒𝑠}

𝑎𝑏𝑠

⋅(_𝑇

𝑎𝑏𝑠+𝑇𝑔𝑙𝑎𝑠𝑠 2 −𝑇𝑤𝑎𝑙𝑙

)

−ℎ𝑐𝑜𝑛,𝑖𝑛⋅^𝐴_𝐴^{𝑠𝑖𝑑𝑒𝑠}

𝑎𝑏𝑠 ⋅(_𝑇

𝑎𝑏𝑠+𝑇𝑔𝑙𝑎𝑠𝑠 2 −^𝑇^𝑖𝑛⁺₂^𝑇^𝑜𝑢𝑡

)

=0

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Fig.5. Schematicﬁgureofthemodel.Thesolardryerisconsistingoftwoparts:

thesolarcollectorandthedryer.

Equation4.Energybalanceoftheouterwalls.

𝜆𝑏𝑜𝑡𝑡𝑜𝑚 𝑡𝑏𝑜𝑡𝑡𝑜𝑚 ⋅(

𝑇_𝑎𝑏𝑠−𝑇_{𝑤𝑎𝑙𝑙}) +^𝜆^{𝑠𝑖𝑑𝑒}

𝑡𝑠𝑖𝑑𝑒 ⋅^𝐴_𝐴^{𝑠𝑖𝑑𝑒𝑠}

𝑎𝑏𝑠 ⋅(_𝑇

𝑎𝑏𝑠+𝑇_{𝑔𝑙𝑎𝑠𝑠} 2 −𝑇_{𝑤𝑎𝑙𝑙})

−ℎ_{𝑐𝑜𝑛,𝑜𝑢𝑡}⋅(𝐴𝑎𝑏𝑠+𝐴𝑠𝑖𝑑𝑒𝑠)

𝐴_𝑎𝑏𝑠 ⋅(

𝑇_{𝑤𝑎𝑙𝑙}−𝑇_𝑎𝑚𝑏)

−ℎ_{𝑟𝑎𝑑,𝑜𝑢𝑡}1⋅(

−ℎ_{𝑟𝑎𝑑,𝑜𝑢𝑡}1⋅^𝐴₂_⋅𝐴^{𝑠𝑖𝑑𝑒𝑠}

𝑎𝑏𝑠⋅(

−ℎ_{𝑟𝑎𝑑,𝑜𝑢𝑡}2⋅^𝐴₂_⋅𝐴^{𝑠𝑖𝑑𝑒𝑠}

𝑎𝑏𝑠⋅(

𝑇_{𝑤𝑎𝑙𝑙}−𝑇_𝑠𝑘𝑦)

=0

Convectiveheattransfercalculationsfollowedmethodsandequa- tionsbased on well-established literature [39]. The convectionheat transfercoeﬃcientontheinsideofthesolarcollector,ℎ_{𝑐𝑜𝑛,𝑖𝑛},wascal- culatedusingEquation5.

Equation5.Nusseltnumber.

𝑁𝑢=ℎ_{𝑐𝑜𝑛,𝑖𝑛}⋅𝐷_𝐻 𝜆_𝑎𝑖𝑟

where𝐷_𝐻isthehydraulicdiametercalculatedusingEquation6.𝜆_𝑎𝑖𝑟 isthethermalconductivityforairandNuisthedimensionlessNusselt number.

Equation6.Hydraulicdiameter.

𝐷_𝐻=2⋅ℎ⋅𝑤 ℎ+𝑤

Fig.7. Schematicfigureforallenergyflowsandtemperaturesmodelledofthe dryingcabinet.Energybalancesfortheairflow,innerandouterwallofthecol- lectorweremodelledaccordingtothisfigure.Bluearrowsrepresentconvective heattransferandredarrowsradiativeheattransfer.Theyellowarrowsrepresent solarenergyabsorbedbythedryingcabinet.

whereℎand𝑤aretheheightandwidthofthecrosssectionfortheair passageinthesolarcollector.

The Nusselt number, 𝑁𝑢, is stronglydependant on theReynolds number, 𝑅𝑒_𝑎𝑖𝑟,calculatedusingEquation 7.Inthese calculationsthe ﬂowwasassumedtobefullydevelopedturbulentif𝑅𝑒_𝑎𝑖𝑟>3000.Oth- erwise,alaminarﬂowwasassumed.Moredetaileddescriptionsforthese calculationscanbefoundin[32].

Equation7.Reynoldsnumber.

𝑅𝑒_𝑎𝑖𝑟=𝜌_𝑎𝑖𝑟⋅𝑢_𝑎𝑖𝑟⋅𝐷_𝐻 𝜇_𝑎𝑖𝑟

where𝜌_𝑎𝑖𝑟isthedensityofair,⋅𝑢_𝑎𝑖𝑟istheairvelocityinsidethesolar collectorand𝜇_𝑎𝑖𝑟isthedynamicviscosityforair.

Resultsfromthesecalculationsledto𝑅𝑒𝑎𝑖𝑟 above4000[32].The airﬂowcanthereforebeconsideredtobeturbulent.𝑁𝑢couldtherefore becalculatedusingtheGnielinski’scorrelationandthefrictionfactorf accordingtoEquation8andEquation9.

Fig.6. Schematicfigureforallenergyflowsand temperaturesthatareincludedinthemodelof thecollector.Energybalancesfortheglass,air flow,absorberandouterwallofthecollectorwere modelledaccordingtothisfigure.Greenarrows representconvectiveheattransferandredarrows radiativeheattransfer.

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Fig.8. Illustrationofthemeasurementsetupoftheminiatureindirectsolar dryerusedforpreliminarytests.(colour)

Equation8.Gnielinski’scorrelationforthecalculationof Nusselts number.

𝑁𝑢=

𝑓 8⋅(

𝑅𝑒_𝑎𝑖𝑟−1000)

⋅𝑃𝑟_𝑎𝑖𝑟 1+12,7⋅(^𝑓₈)

1 2⋅

( 𝑃𝑟_𝑎𝑖𝑟²³ −1

)

Equation9.Frictionfactor.

𝑓=(0,790⋅ln( 𝑅𝑒_𝑎𝑖𝑟)

−1,64)⁻²

where𝑃𝑟_𝑎𝑖𝑟isthePrandtlnumberforair.

Withtheparametersusedinthisexampleℎ_{𝑐𝑜𝑛,𝑖𝑛}wascalculatedto be:

ℎ_{𝑐𝑜𝑛,𝑖𝑛}= 𝑁𝑢 ⋅ 𝜆𝑎𝑖𝑟

𝐷_𝐻 = 13.75 ⋅ 0.0263

0.324 =1.12 𝑊 𝑚²⋅𝐾

Aparametricanalysiswasperformedassumingthattheℎ𝑐𝑜𝑛,𝑖𝑛could beincreasedbyamultiplicationfactorintherange1to5betweenthe airandabsorber.ℎ_{𝑐𝑜𝑛,𝑖𝑛}betweenairandglazingwaskeptthesame.This wasassumedtobepossibleusingfinstoincreasetheconvectiveheat transferorbyanyothermeanspossible.This calculationshouldthus beviewedasaninvestigationtoseehowmuchanincreasedconvection heattransfercoefficientcouldaffectthetemperatureoftheoutletair fromthesolarcollector.Theconvectionheattransfercoefficientwas assumedtobe20W/(m²•K)ontheoutsideofthecollector[41].

Severalparametricstudieswerecarriedoutregardingthedesignof thesolardrying.Thebasecaseforthesimulationsisthefollowing.Col- lector:0.22mhigh;0.60mwide;1.70mlong.Sidesofcollector:1cm plywood.Bottomandsidesofcollector:1cmplywood+1cmStyro- foam.Dryer:0.60mhigh;0.60mwide;0.40mlong.Sidesofdryer:1 cmplywood+1cmStyrofoam.Sixshelves,10pouchespershelf.Ambi- entconditions:solarirradiation700W/m²;outdoortemperature25°C;

airvelocityincollector0.2m/s.

Prototypetesting

Basedontheknowledgegainedfromtheresultsofmodelling(shown furtheraheadinthisarticleunder‘Resultsandanalysis’)thedesignof thesolardryerwasimproved.Fig.8andTable2illustratetheexperi- mentalsetupofthesmall-scaleindirectsolardryerusedforpreliminary tests.Themosttrustworthyindicatorfoundtocharacterizetheperfor- manceofthedryingwasthedryingﬂuxexpressedinweightlossofthe pouchesperhour.However,openingthecabinettoweighthemem- branebagswoulddisruptthedryingconditions.Therefore,aloadcell wasmountedintothedryingcabinettoregularlymonitortheweight ofthepouches(illustratedbynumber7inFig.8).Thedryingﬂuxof

thepouchesinsidethedryerwascomparedtothatofacontrolpouch inopen-airdryingconditions.

Theinﬂuenceof varyingair velocitypassingthroughthepouches wasinvestigatedinpracticebydecreasingthecross-sectionareaofthe dryingcabinet(Fig.9).Thiswasdonebyinstallinganair-tightwall.

Seconddesign

Prototypedesign

Basedontheresultsfromthesmall-scalesolardryerdescribedunder

“Firstdesign” (furtheraheadinthepaperunder‘Resultsandanalysis’) thefollowingdesigncriteriaweresetfortheseconddesign(Table3).

Resultsfromtheﬁrstdesignshowedtheimportanceofmakinguseof directsolarradiation,toincreaseairvelocity,andtoachieveahigher heattransferbetweenabsorberandair.Hence,duringtheseconddesign stagetwosolardryerswerebuilt:onepassiveandoneactive(Fig.10).

Thepurposeofbuildingapassivedryerwastoachieveasimpleand cheapdesignwheretheairflowisthermallydriven.Thedryerwastilted tomaximizeincomingradiationandtoincreasethermallydrivenair flow.However,placingthepouchesonatiltedsurfacewouldposea problemforthedrying.Aswaterevaporatesfromthepouchesthere- mainingjuice flowstothebottomtoformthepouchintoateardrop shape.Thisdecreasessurfaceareacomparedtovolumewhichmakes drying moredifficult.Thisproblemwassolvedusinganextensionat theend ofthedryer intheform ofa horizontalplatformwherethe pouchescouldlayhorizontallyduringtheentiredryingprocess.

Thepurposeofbuildinganactivedryerwastoachievehigherper- formanceandcapacity(largernumberofpouches).Theactivedryer wasclosed,andairflowwasforcedbyphotovoltaicdrivenfans.Con- sequently,hightemperatureandairvelocitycouldbereachedsimulta- neously.Thefansforceacircularflowinsidethedryerandaroundall pouches.Theaxialfansaredrivenby12VDCcurrent,0.28A,andthe sizeis120×120×25mm.Theratedairflowis35l/s[40].Fourfansare poweredbyaphotovoltaicmoduleof20W_ppeakpower.Controllable sideopeningswereusedtoexchangetheairandregulatetherelative humidityinsidethedryer.Thefanspeedincreaseswithincreasedso- larradiationwhichprovidesacertaindegreeof‘self-regulation’tothe dryingprocess.Thepoucheswereplacedonagrateforsupportandto makesurethattheairflowsonbothsidesofthepouches.

Bothdryerswerebuiltinthesamewayandusingthesamelocally available materials, expect thephotovoltaic fansin the activedryer (Fig.11).Besidestheelementsillustratedintheﬁgure,anadditional blacknetwasaddedtothepassivedryerbetweentheabsorberandthe transparentcover.Theaimwastoforcetheairthroughitandincrease heattransfer.

Thetiltangleofthepassivesolardryerwaschosenconsideringthe solardistributionalongthedayduringtheharvestseason.Theharvest seasoninMozambiqueformostcitrusesisbetweenMarchtoJunewhen solaraltitudesarelowerinthesouthernhemisphere.Toachievemore balanced dryingconditions duringthedayredistributionof solarra- diationthroughoutthedayisdesired.Thetotalsolarenergyreceived throughoutthedayforseveralinclinationswasinvestigatedanda55°

tiltfromhorizontalwaschosen.

Modelling

Modellingofcomputationalﬂuiddynamics(CFD)wascarriedour usingthesoftwareCOMSOL5.2[41].CFDisthesimulationofﬂuids engineeringsystemsusingmodelling(mathematicalphysicalproblem formulation)andnumericalmethods(discretizationmethods,solvers, numericalparameters,andgridgenerations,etc.)Thiswascarriedout inparalleltothemeasurementsperformedontheseconddesign.The simulationsassumedsteadystateandconstantsolarradiation.Theplas- ticnetontopoftheabsorberwasneglectedandtheplasticcoverwas assumedtonotabsorbanyvisible radiation.Foramore detailedde- scriptionoftheCOMSOLsimulationssee[35].

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Table2

Measurementequipment,accuracy,andnomenclature.Theright-handcolumnwithnum- bersisconnectedtothepreviousﬁguretoillustratewherethemeasurementequipmentis placed.MeasurementequipmentfromVernierisused(ifnothingelseisstated).‘C_max’is themaximumcapacityoftheloadcell,15kg.

Measurement Equipment Abbreviation Accuracy Number in illustration Stainless steel temperature probe T _Ambient ± 0.4°C 11

Stainless steel temperature probe T C,In ± 0.4°C 1 Stainless steel temperature probe T C,Out ± 0.4°C 2 Surface temperature sensor T DC,Bottom ± 0.4°C 3 Surface temperature sensor T DC,Top ± 0.4°C 4 Surface temperature sensor T _DC,Bag ± 0.4°C 10

RH sensor RH _DC ± 2 % 5

RH sensor RH _Ambient 0.2 % 13

Pyranometer I W/m ²± 5 % 6

Scaime load cell V MassLoss ± 0.017 % Cmax 7

Anemometer v wind ± 0.15 ms ⁻¹ 8

Hand anemometer TSI 8330 v ± 0.05 ms ⁻¹ 9

Scale House – HS-3000 m _RB ± 2 g 12

Fig.9. Schematicillustrationofthecross-sectionmodiﬁcationinthe dryingcabinetshownbeforein(a)andafterin(b).

Table3

Summaryofthedesigncriteriaasaresultofthetestsfortheﬁrstdesignandtakingintoaccountthesocialandfoodtechnologyinvestigationscarried outinparallel.

Design Parameters Requirement

Direct/Indirect/Mixed Dryer Direct or Mixed

Passive/Active Dryer Both

Air ﬂow (Speed, characteristics, direction) Velocity: As high as possible, except if slowing the process

Characteristics: better turbulent (higher heat transfer) Direction: Parallel to the biggest dimension of the bag

Temperature Interval 50°C – 65°C

Drying time Two to three sunny days

Material No wood, has to be available in Mozambique

User friendliness As high as possible

Fig.10. Left:illustrationofthepassivesolardryer.Right:illustrationoftheactivesolardryer.

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Fig.11. Localavailablematerialsusedinbothsolar dryers.

Fig.12. Left:indoorsolarsimulatorinlaboratory.Right:levelofirradiationontheleaningcollectorinlaboratory.(colour)

Thesimulationmodelwasvalidatedagainstﬁeldmeasurementscar- ried out in Mozambique.Temperature sensors placed on theplastic cover,intheair,ontheabsorber,insidetheinsulationandatthebottom ofthecollectorwereusedforthevalidation.

Prototypetesting

LaboratoryinSweden

BeforeperformingtestsinMozambique,thepassiveandactiveso- lardryerprototypeswerebuiltandtestedinalaboratoryatLundUni- versityinSweden.Thelaboratoryisequippedwithafull-scaleartiﬁ- cialsolarsimulatorwhichallowsperformancetestingundercontrolled conditionssuchasradiation,roomtemperatureandrelativehumidity.

Fig.12showsthelevelsofincomingradiationontheleaningpartofthe passivedryer.Theaverageincomingradiationwas905W/m² which isclosetofullsolarradiation.However,asillustratedinFig.12,the variationoftheincomingradiationwassigniﬁcant,speciallyfarfrom itscentre.Forinstance,theincomingirradiationatthetopwasclose to400W/m²whileitshighestvaluewasapproximately1400W/m².A listofmeasuringequipment,theiraccuracy,range,andplacementare describedinTable4.Fig.13

FieldtestinginMozambique

AfterthetestsinlaboratoryinSweden,thepassiveandactivesolar dryerprototypeswerebuiltandtestedinrealconditionsinMozambique

(Fig.14).Ambientconditionsweremonitoredregularlywiththelogger:

temperature,relativehumidity,windspeedandsolarirradiation.The juiceinsidethepoucheswasmadefromlocaltangerinesfromthefarmer association collaboratingwiththeresearchgroup. Thesugarcontent ofthejuiceinfluencesnotonlythemicrobiologicalqualityofthefinal product[26]butcanalsoaffectthedryingprocess.Therefore,testswere performedwithandwithoutaddedsugar(i.e.,sucrose).Thepouches withaddedsugarwerepreparedsothat10%ofallweightwasadded sugarand90%juice.

Costestimatesandavailabilityofmaterials

Thecostestimatesforeachdryerwerebasedontheretailpriceof itsindividualcomponentsinlocalshopsinMozambique.Atthisstage, nocostwasattributedtolabour.Thedryersarethoughttobebuiltby farmingassociationsthatwillusethem.Also,availabilityofmaterials wasarequirementfortheprototypedesign.

Resultsandanalysis Firstdesign

Modelling

Resultsfrommodellingofthesmall-scaledryershowedthatitsper- formancecouldbesigniﬁcantlyimprovedsincethevastmajorityofin-

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Table4

Measuringequipment,accuracy,andrangeofeachdevice.

Device Measures Accuracy Range Location

Testo 435-2 Airspeed ± (0.3 m/s

+ 4 % of mv)

0 to 20 m/s Bag

Temperature ± 0.3°C -20 to 70°C Bag

RH ± 2 % 0 to 100 % Bag

Vernier LabQuest Temperature ± 0.5°C -40 to 135°C Ambient

RH ± 2 % 0 to 85°C Ambient

Irradiation ± 5 % 0 to 1100 W/m ² Ambient

Temperature ± 0.5°C -25 to 125°C Absorber

Scale Weight ± 1 g 0 to 3000 g Bag

Pyranometer (Vernier) Irradiation ± 1.29 % 0 to 1100 W/m ² Collector

Fig.13. SolardryersandcontrolpouchesinruralareainMozam- bique.

Fig.14. Estimatedheatbalanceofthesolarcollector.

comingsolarradiationwasestimatedtobelostbyheatlossesfromthe bottom,glass andsidesof thecollector(Fig.14).Heatfromtheab- sorberwasestimatedtobemainlytransferredtotheglassandbackside andfurtherlostintoambient.Consequently,only13%ofincomingsolar radiationwasestimatedtobetransferredtoairﬂowwhichmeantlow temperatureandhighrelativehumidity.

Severalparametricstudieswerecarriedouttoinvestigateincrease ofconvectiveheattransferoftheabsorber.Onepossibilitywastoadd convectivefinstotheabsorberorothertypeofsurface(Fig.15).The figureshowsaparametricanalysisoftheoutletairtemperaturefrom thesolarcollectorasafunctionoftheeffectiveconvectiveheattransfer betweentheairandabsorberincreasedbyamultiplicationfactorinthe

range1to5resultingfromthepossibilityofaddingﬁns.Theconvective heattransferwascalculatedaccordingtoEquation5toEquation9in themethodsection.Ahigherairvelocitycanbeachievedbydecreasing thecross-sectionheightofthesolarcollector(Fig.16).

Ahigherairtemperatureresultsinalowerrelativehumidityinside thedryer,whichincreasesthedryingflux.Ahigherairvelocityincreases thedryingfluxduetohigherheattransferbetweenthepouchandtheair andmakesthedryingalongthedryingcabinetmoreeven.Thepouches placedonthefirstshelf,closesttotheentrance,willbeexposedtoa lowerrelativehumiditythanthepouchesplacedfurtherupinthedryer [42].Thisoccursduetotheabsolutehumiditybeinghigherfurtherup inthedryeraswellasthetemperaturebeinglowersinceheatwillbe

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Fig.15. Parametricanalysisontheoutlettemperatureof thesolarcollectordependingonconvectiveheattransferbe- tweenairandabsorber.

Fig.16. Expectedinﬂuenceofcross-sectionheightonthe outlettemperatureofthesolarcollector.

absorbedbythepouchesandlostthroughthewalls.However,airtem- peratureandvelocityareinter-dependentwherea highervolumetric airﬂowresultsinlowertemperature.Abalancebetweenthesefactors isthereforeneeded.

Prototypetesting

Measurementresultsforthecontrolpouchshowedthatthedrying ﬂuxwas150g/h/m²intheshadeand372g/h/m²indirectsunlight showingtheimportanceofmakinguseofdirectradiation.

Fig.17illustrates6runsinthesmall-scalesolardryer.Duringrun4 to6thecross-sectionofthedryingcabinetwasdecreasedaccordingto Fig.9.Duringrun1to3,beforethecross-sectionwasreduced,therun withthehighestdryingfluxwasrun3with247g/h/m².Thisvalueis lowerthanthatofthecontrolpouchunderdirectsunlight,whichwas unexpected.However,thedryingfluxof run6,withasmallercross- sectionareaandthereforehigherairvelocity,was370g/h/m²which wasclosetothatofthecontrolpouchinopen-airdryingandunderdirect sunlight.Thedecreaseincross-sectioncausedanincreaseofairspeedof about44%andanincreaseofdryingfluxofapproximately33%while thetemperatureintervalformostoftherunswascomparable.

Basedontheresultsobtained,theairvelocityhadalargerimpact onthedryingfluxthanexpected.Whendryinglessjuicyandmorein- tactfruitsandvegetables,theimpactofairvelocityonthedryingflux isoftenlesssignificantduetothehighinternalmasstransferresistance

insidethefood.Forexample,in[4]itisstatedthat“… themosteffective factoronthedryingrateisthetemperatureoftheairinsidethecabinet;the effectofvariationofspeedofairinsidethedryingcabinetissmallandcan beneglected… ”;in[43]onecanread“...Studiesonthedryingoffruits andvegetablesindicatethattheairvelocityhaslittleinfluenceonthedrying kineticsofmostofthem...”;andin[44]“Theairdryingtemperatureisa principalfactorinfluencingthedryingkinetics… theairvelocityhadasmall impact onthe dryingkinetics”. Evenwiththeadditionalmasstransfer resistanceprovidedbythemembraneinthepouch,theexternalmass transferresistanceatthesurfaceofthepouchshowedtobehigherand thereforeadifferentdryingbehaviourwasobservedcomparedwiththe solardryingofmorein-tactfruitsandvegetables.

Aninterestingandimportantobservationwasthatatlowerairve- locitiesthepouchesinsidethedryerwerewetallaround.Dropletsof waterwereseendroppingfromthedrying cabinet.Furthermore,the temperatureofthepouchesinsidethedryerwasrelativelyhighandin- creasingduringthedryingprocess.Incomparison,thecontrolpouches inopen-airweredryandcolderduetoevaporativecoolingandlower ambientairtemperatures.

Fromtheanalysisoftheseresults,itseemedthatthehighertempera- tureandlowerrelativehumidityinsidethesolardryerwerenotenough toincreasethedryingﬂuxabovethelevelofthereferencebagplacedin ambientconditionswherewindanddirectsunlightwereavailable.The solardryerseemedtobemissingthecapacitytoremovethevapourfrom

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Fig.17. Acomparisonofthedryingfluxesand temperatures.Averagedryingfluxesandtem- peraturesareplottedagainstairvelocity.The horizontalbarsrepresentthedeviationofthe airvelocitywhiletheverticalbarsrepresentthe deviationofthetemperatureinthedryingcabi- net.Afirstorderlinearapproximationisdrawn fortherateofmassloss.

thesurfaceofthebagtocontinuethepervaporationprocessthroughthe membrane.Theairﬂowvelocitywaslowandseemedtobelimitingdry- ing.Therefore,thenextdesignofsolarfruitdryertookthisimportant outcomeintoconsideration.

Seconddesign

Modelling

Resultsshowthatmeasurementsandsimulationsaremainlyingood agreement.Thelargerdiﬀerencefortemperaturesontheplasticcover wasmostlikelyduetosensorsheatingupdirectlyfromthesun.This problemoccurredthroughouttheevaluationmeasurements.However, othermeasurementscarriedoutawayfromdirectsunlighthadsigniﬁ- cantlyhighercorrelationtothesimulationresults.

Themodelsgiveasufficientestimationofthetemperatureandrela- tivehumidityinadirectandinanindirectsolardryerwithamaximum relativeerrorof10%fortheoutletairtemperatureandapproximately 30%fortherelativehumidity.Guidelinestoimprovethedesignwere providedbythemodel.Anadequatebalancebetweentheairflowveloc- ity,therelativehumidityandtheconvectionprovidedontheabsorber plateandaroundthebagswasfoundtosignificantlyimprovethedrying flux.

Prototypetesting

LaboratoryinSweden

Theinﬂuenceinthedryingprocessofclosingtheactivesolardryer wasinvestigatedinlaboratorybymonitoringtemperatureandrelative humidityoftheairinside.Pouchescontainingorangejuicewereused tomimicrealconditions.ResultsareshowninFig.18.After4hours, whentheparameterswerestable, itwas observedthat thetempera- tureof theclosedactivedryerwas closetothatof whentheplastic coverwasopen.However,amuchhigherrelativehumiditywasmea- sured(35%comparedto10%).Furthermore,condensationwasseenin severalplacesinsidethedryer.Itwasthereforeconcludedthattheab- senceofopeningsdoesnotallowtherelativehumiditytobelowenough toavoidsaturation.Therefore,testinginMozambiquewascarriedout

withopeningsontheplasticcover.Resultsfromthedryingfluxinlabo- ratoryforbothsolardryersareshownfurtheraheadinTable6.Theair flowcouldnotbemeasuredintheactivedesignsolardryer.Thereason isthattobeabletomeasureanairflowusingananemometertheair flowdirectionmustbeknown.Astheflowisturbulentandchaoticthis measurementcouldnotbeperformed.

Measurementsoftheairtemperatureinsidethepassiveandactive solardryersduringclearskyconditionsalongthedayareillustratedin Fig.19.Theillustratedairtemperaturesweremeasuredinsidethesolar dryers.Theairtemperatureinsidethepassivedryerwasmeasuredat thewarmestpointlocatedattheoutletofthesolarcollectoratitstop.

Theairﬂowintheactivesolardryeristurbulentandthereforetheair temperatureisfairlyhomogeneousinsidethedrier.Asexpected,theair temperatureofthepassivedryerwasshowntobelessstableduetoits sensitivitytowindconditionswhiletheincomingsolarradiationwas higherduetoitsfavourableinclinationtowardsthesun.Asillustrated below,between11:00and14:00temperaturesintherangeof70°C-78°C and58°C-65°Cweremeasured,forthepassiveandactivesolardryer respectively,duringclearskyconditionsandforthatperiodoftheyear.

Suchresultsareconsistentwithlaboratorymeasurementsasillustrated inFig.18(left)fortheactivesolardryerandmeasurementsduringother daysasillustratedin[34].Itshouldbenotedthatthesharptemperature increaseoftheactivedryerat12:00isexplainedbyanair-leakagefrom theplasticcoverthatwascorrected.

Largepouchesofover1.5kilogramsofjuiceeachweredriedinthe solardryers.Thisrepresentsa330mlpotofjamofﬁnalproductforeach bag. Table5showsthedryingﬂuxfor bothsolardryersandcontrol pouchduringathree-dayperiod.Thepouchesusedforalldayswere thesame.

Duringthefirstdayasignificantincreaseindryingfluxeswasob- servedwhenthebagsareplacedinthesolardryerscomparedtowhen theyareplacedoutsideinambientconditionswasobserved.Thedrying fluxesfortheseconddaywerenothashighasduringthefirstday,but stillhighercomparedtothecontrolpouch.Finally,thedryingfluxesfor thethirddaywerelowerthanthesecondday.Thedryingfluxofthe

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Fig.18. Left:comparisonofrelativehumidityandtemperatureintheactivecollectorwhenplasticcoverwasopenedandclosed.Right:condensationontheplastic asaresultofnoairexchange.(colour)

Fig.19. Illustrationofairtemperaturesinsidethepassiveandactivesolar dryersandcorrespondingsolarradiationatthesurfaceofeachcollectordur- ingclearskyconditions.

Table5

Dryingﬂuxesforbothsolardryersandcontrolpouchduringthreedays,withandwithout addedsugar.

Day 1 Day 2 Day 3

Sugar No Sugar Sugar No Sugar Sugar No Sugar Active ( g/m ²/h ) 497 609 320 390 187 220 Passive ( g/m ²/h ) 465 457 325 354 216 287 Control ( g/m ²/h ) 293 311 246 264 192 227 Active (ratio to control) 1.70 1.96 1.30 1.48 0.97 0.97 Passive (ratio to control) 1.59 1.47 1.32 1.33 1.12 1.26

Control 1 1 1 1 1 1

pouchinsidetheactivedryerwasevenlowerthanthatofthecontrol pouch.Thismeansthatthedryingﬂuxdecreasesascumulativeweight lossincreasesasexpected,sincethemasstransferdrivingforcedecreases asthemoistureconcentrationinthejuicedecreasesandasthejuiceis turningintoamoreviscousproduct.Fortheactiveandpassivedryers, themoistureconcentrationdecreasedmorerapidlyexplainingwhythe dryingﬂuxesalsodecreasedsubstantiallyfromdays1to3.

Thepercentagecumulativeweightlossofpouchesinsidethepassive andactivesolardryersisshownin Fig.20 andFig.21,respectively.

Poucheswithandwithoutaddedsugarweretested andcomparedto controlpouches.Theminimumcumulativeweightlosswhenmicrobi- ologicalgrowthispreventedwascalculatedforthepoucheswithand withoutsugarandisrepresentedbythehorizontallines[26,p.2].These

linesrepresentthetargetfordrying.Thepoucheswereweighedatthe startandendofeachday,thisisshownbythecrossesintheﬁgures.

Thesuddenincreasefromendofonedaytostartthenextdayisdueto dryingduringnight.

Resultsshow thatpouchesdryfaster insidethe solardryers than theirrespectivecontrolpouches.Furthermore,poucheswithoutsugar dryfasterthantheoneswith,asexpected.Sugarwasnoticedtoslow downthedryingprocessbutthethresholdforcumulativeweightloss wheremicrobiologicalgrowthispreventedisalsolower.Consequently, pouches withandwithoutsugararethought tocomplete thedrying processapproximatelyatthesametime.Finally,resultsshowthatthe pouchesinsidetheactivecollectoralmostreachedthegoalforcumula- tiveweightlossduringthethree-dayperiod.

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Fig.20. Cumulativeweightlossintheactivesolardryerandcon- trolpouchforathree-dayperiod,withandwithoutaddedsugar.

(colour)

Fig.21. Cumulativeweightlossinthepassivesolardryerandcon- trolpouchforathree-dayperiod,withandwithoutaddedsugar.

(colour)

Insummary,themeandryingfluxforthepoucheswithoutsugarin- sidethepassiveandactivesolardryersincomparisonwiththecontrol pouches,bothinlaboratoryandinMozambique,areshowninTable6. Bothdryersconsistentlyachievedthetargettemperatureofatleast50°C duringmostofthedays,whichpreventsbacterialgrowthandlowersthe relativehumiditysignificantly.Anincreaseofthedryingfluxfromap- proximately50%(passivedryer)to100%(activedryer)wasmeasured inrealconditionsinMozambiquewhichmeansthatallpouchesinthe solardryerssignificantlyoutperformthepouchesthatdonotuseasolar dryer.Thetotaldryingtimewasbelowfourdaysforallbagsinthedry- ers,evenwhentheweatherwasnotoptimal.Theactivesolardryerwas showntohavetheshortestdryingtimeandthehighestcapacity(more pouches)butalsothehighestprice(Table7).

Itisimportanttonoticethatmoldgrowthontheoutsideandfer- mentationofthejuicewasconsistentlyobservedonthecontrolpouches dryingin openair.Thesefactorscompromisedrying withoutasolar

dryer.Furthermore,duringﬁeldmeasurementsinMozambiqueitwas discoveredthatthepouchmaterialdegradeswhenexposedtodirectso- larradiationbothwithandwithoutasolardryerand,asmanufactured atthetime,couldnotresistmorethan5daysofdrying.

Costestimatesandavailabilityofmaterials

Thematerialsusedtobuildthesolardryerswerebasedonlocalavail- ability,costsandinputsfromthefarmers[29].Thecostestimatefor eachsolardryerwasbasedontheretailpriceofitsindividualcompo- nentsinlocalshopsinMozambiqueispresentedinTable7.Asshown, theactivesolardryercostsapproximately2.4timesmorethanthepas- sivecollector.Thisismainlyduetothecostoffans(mostexpensive component) andsolarcellswhich,together,madeup61%oftheto- talprice.Themeandryingﬂuxisincreasedbyapproximately33%.This couldindicatethatthepassivecollectorismorecost-eﬀective.However,

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Table6

Comparisonofthemeandryingﬂuxesforthepassiveandactivesolardryersincomparisonwiththecontrol pouches,bothinlaboratoryandinMozambique,forthepoucheswithoutsugar.

Place of testing Type of drying method Mean drying ﬂux (g/m ²/h) Ratio (solar dryer / without)

Laboratory in Lund Solar passive dryer 678 1.65

Solar active dryer 924 2.25

Without solar dryer 411 1.00

Field tests in Mozambique

Solar passive dryer 457 1.47

Solar active dryer 609 1.96

Without solar dryer 311 1.00

Table7

Theretailpriceofallthecomponentsusedinbothdryers.(1MZN=0,0138589€,asof2017-08-08).

Product Size Unit price Qt. active Price active Qt. passive Price passive Metal sheet (mm) 800 ^∗3600 ^∗0.4 14.4 EUR 1 14.4 EUR 1 14.4 EUR

Net (m) 3 ^∗1 7.6 EUR 1 7.6 EUR 1 7.6 EUR

Styrofoam (cm) 100 ^∗200 ^∗2 12.4 EUR 1 12.4 EUR 1 12.4 EUR

Fillinf foam (ml) 750 5.3 EUR 2.5 13.3 EUR 2.5 13.3 EUR

Plastic sheet (m) 4 ^∗5 15.5 EUR 0.5 7.7 EUR 0.5 3.9 EUR Metal angles (cm) 8 pcs 10 ^∗10 0.5 EUR 8 4.0 EUR 1 4.0 EUR Plastic support (m) 10 ^∗2500 0.7 EUR 2 1.4 EUR 2 2.7 EUR Nuts and bolts (mm) 50 pcs, 6 ^∗75 6.5 EUR 1 6.5 EUR 1 6.5 EUR Wood for frame (mm) 38 ^∗38 ^∗5400 3.4 EUR 3 10.1 EUR 4 40.4 EUR Wood screws (mm) 50 pcs, 4 ^∗45 1.4 EUR 1 1.4 EUR 1 1.4 EUR

Fans (mm) 120 ^∗120 18.4 EUR 4 73.6 EUR 0 0 EUR

PV-panel (W) 20 47.8 EUR 1 47.8 EUR 0 0 EUR

Cables N/A N/A N/A 0 EUR 0 0 EUR

Total 200.3 EUR 78.6 EUR

theactivecollectorhasasigniﬁcantlylargercapacity,andthesolarcells couldpotentiallyalsobeusedforotherpurposesoutsidethejuicyfruit period.

Discussion

Theevaluatedsolardryerswerefoundtodecreasedryingtimeand increaseproductivity,toincreasefoodsafety,andtoprovideadditional protectionfromexternalenvironment.

However,animportantconstraintwasencountered.Thesuccessful- nessof theuse ofthepouch membraneis arequirementtosuccess- fullydryjuicyfruitsaccordingtotheinvestigatedconcept.Theobserved degradabilityofthepouchindirectsunlightmightbeanimportanthur- dleforsuccessfulimplementation.Thischallengeneedstobeovercome beforethepouchescanbecomesuitableforwidespreaduse.Moreover, moldgrowthontheoutsideandfermentationofthejuicewasobserved onthecontrolpouchesdrying inopenair.Openairsun dryingwith pouchesincreasestheriskofmoldgrowthontheoutsideofthepouch, whichfurtheremphasizestheusefulnessofasolardryer.Someopen-air samplesexperiencedmoldgrowthontheoutsideofthepouches,while allpassiveandactivesampleshadnovisiblemoldgrowthontheout- sidesofthepouches.Inanotherstudy,forthepoucheswithmoldonthe outside,thejuicewasnotcontaminatedwhichshowsthatthepouches havetheabilitytoactasabarriertomold[22].Despitethis,drying withasolardryerisstillpreferredoveropensundryingtoreducethe riskofanymoldgrowthduringtheprocessandalsodecreasethedrying time.Byusingsolardryerstoavoidmoldgrowth,thepersonhandling thepouchesalsoavoidstouchingmoldthatmayhaveaccumulatedon theoutsideofthepouch.Evenifapouchisnotused,theconceptofpro- tectingliquidfoodinacontainerisgoodtopreventasmuchmicrobial contaminationaspossible.

Concerningmeasurements,thehigheruncertaintyonmeasurements relatestoairﬂowvelocityonthepassivesolardryer.Thiswasmeasured attheoutletundercontrolledconditionsinlaboratoryinSweden.The measuredvaluedependsonﬂowcharacteristicsandhowfarthemea- surementistakenfromtheabsorberandsides.Approximately1m/scan beconsideredasareferencevalue.Astheaccuracyoftheanemometer

is±0.3m/s,therelativeerrorislarge.Ontheotherhand,largepouches ofover1500gramsofjuiceeachweredriedandsincetheaccuracyof thescaleis±1grams,thecorrespondentrelativeerrorinweightmea- surementsissmall.Hence,dryingﬂuxwasusedasakeyperformance indicatorforthedryingprocess.

Regardingfuturework,itisbelievedthatevenifthepouchisnot used thesolardryersmayhavepotential tobe used.Since theasso- ciationof farmersdonotcarryout anyfruitdryingtoday,thereisa largepotentialonsolarfruitdryingforallnon-juicyfruitsandherbs.

Furtherdevelopmentoftheconceptcouldfocusondesigningholistic solarphotovoltaicsystemsforactivesolarfooddryingandotheruses suchaslightingandcooking.Anotherpossibledevelopmentistodesign moreadvancedsolardryersthatincludethermalstorage,forexample.

Whenitcomestoprofitability,thecostestimatesshowasignificantly highercostfortheactivedryerbutalsoshorterdryingtime.Itbecomes thereforedifficulttoaccuratelyestimatethemostcost-effectivedesign.

Todoso,severalotherfactorsshouldbeaccountedforsuchasthecost ofbuildingthedryers,thequantityofjuicethatcanbedriedatonce, andtheselling priceoftheﬁnishedproduct.Regardingmodellingof thesolardryer,itshouldbenotedthatthecalculationsfortheNusselt numbercontainsapproximations.UsingtheGnielinski’scorrelationis suitableasitisvalidfor0,5<Pr<2000and3000<Re<5×10⁶.Noef- fectsduetoentrylengthwereincludedinthecalculations,whichcould leadtoalessaccurateestimationofNuwhichwouldbeinterestingto includeinfuturework.

Conclusions

Aholisticdevelopment,implementationandevaluationofsolardry- erscombinedwithsemi-permeablemembranepouchesfordryingjuicy fruitswascarriedout.Twoprototypedesigniterationswerecarriedout includingmodellingandtestinginMozambique.Thelatestversionof solardryerswereapassiveandanactivesolardryer.

Resultsfrommodellingshowedthattheaccuracywassuﬃcientto estimatelevelsoftemperatureandrelativehumiditywithamaximum relativeerrorof10%and30%,respectively.Themodelswereusedto improvethedesignofthedryersforafasterandsaferdryingprocess.