Hindawi Publishing Corporation Journal of Energy Volume 2014, Article ID 247287, 8 pages Research Article Effect of Glass Thickness on Performance of Flat Plate Solar Collectors for Fruits Drying Ramadhani Bakari,1 Rwaichi J. A. Minja,1 and Karoli N. Njau2 1 2 Department of Chemical and Mining Engineering, University of Dar es Salaam, P.O. Box 35131, Dar es Salaam, Tanzania The Nelson Mandela African Institute of Science and Technology, Arusha, Tanzania C
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  Research Article Effect of Glass Thickness on Performance of Flat Plate Solar Collectors for Fruits Drying  Ramadhani Bakari, 1 Rwaichi J. A. Minja, 1 and Karoli N. Njau 2 󰀱 Department of Chemical and Mining Engineering, University of Dar es Salaam, P.O. Box 󰀳󰀵󰀱󰀳󰀱, Dar es Salaam, anzania 󰀲 Te Nelson Mandela African Institute of Science and echnology, Arusha, anzania Correspondence should be addressed to Ramadhani Bakari; ramaringo󰀲󰀰󰀰󰀶 and Rwaichi J. A. Minja; 󰀲󰀹 August 󰀲󰀰󰀱󰀳; Revised 󰀲󰀱 January 󰀲󰀰󰀱󰀴; Accepted 󰀲󰀳 January 󰀲󰀰󰀱󰀴; Published 󰀱󰀲 March 󰀲󰀰󰀱󰀴Academic Editor: S Venkata MohanCopyright © 󰀲󰀰󰀱󰀴 Ramadhani Bakari et al. Tis is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the srcinal work is properly cited.Tis study aimed at investigating the effect o thickness o glazing material on the perormance o 󿬂at plate solar collectors.Perormance o solar collector is affected by glaze transmittance, absorptance, and re󿬂ectance which results into major heat lossesin the system. Four solar collector models with different glass thicknesses were designed, constructed, and experimentally testedor their perormances. Collectors were both oriented to northsouth direction and tilted to an angle o 󰀱󰀰 ∘ with the ground towardnorth direction. Te area o each collector model was 󰀰.󰀷󰀲m 2 with a depth o 󰀰.󰀱󰀵m. Low iron (extra clear) glass o thicknesses󰀳mm, 󰀴mm, 󰀵mm, and 󰀶mm was used as glazing materials. As a control, all collector perormances were analysed and comparedusing a glass o 󰀵mm thickness and then with glass o different thickness. Te results showed that change in glass thickness resultsinto variation in collector efficiency. Collector with 󰀴mm glass thick gave the best efficiency o 󰀳󰀵.󰀴% compared to 󰀲󰀷.󰀸% or 󰀶mmglass thick. However, the use o glass o 󰀴mm thick needs precautions in handling and during placement to the collector to avoidextra costs due to breakage. 1. Introduction In many countries, the use o solar drying systems or agri-cultural products to conserve vegetables, ruits, and othercrops has been shown to be practical, inexpensive, and envi-ronmentally sound approach [󰀱]. Solar dryers offer a cheaperand an alternative way o processing ruits and vegetables inclean and hygienic condition within international standards.Moreover, they save time, occupy less area, improve productquality, protect the environment, and provide better controlorequireddryingaircondition[󰀲].However,theavailability o good inormation is lacking in many countries, whereood-processingsystemsaremostlyneeded[󰀱,󰀳].Centrallyto that,indirectcabinetdryerwithorcedconvection󿬂owisoneo the best drying methods which can produce high-quality products and eliminate the risk o spoilage during drying [󰀴,󰀵].Althoughsolaraircollectorisaveryimportantcomponentin thesolar dryingsystem, ithasnotreceivedmuch attentionduring dryer design [󰀶]. In theory, the perormance o solar collector depends on climatic conditions and severaloperatingconditionsuchascollectororientation,thicknesso covermaterials,windspeed,collectorlength,collectordepth,andthetypeoabsorbermaterialused[󰀷–󰀱󰀱].Currently,these actors are not well considered during solar system design.Tereore,developmentoawell-perormedsolarcollectoriso signi󿬁cant economic importance in solar drying system.Tis study has dealt with the effect o glass thickness on solarcollector perormance. 2. Literature Review  󰀲.󰀱. Flat Plate Solar Collector.  Flat plate solar collectors arespecial kind o heat exchangers that transer heat energy rom incident solar radiation to the working 󿬂uid [󰀱󰀲–󰀱󰀴]. Tey perorm three unctions, absorbing solar radiation,converting it to heat energy, and transerring the energy to a working 󿬂uid passing through the collector duct [󰀱󰀵].Te main use o 󿬂at plate solar collectors includes space Hindawi Publishing CorporationJournal of Energy Volume 2014, Article ID 247287, 8 pages  󰀲 Journal o Energy heating and crop drying [󰀱󰀶, 󰀱󰀷]. Flat plate solar collector can heat working 󿬂uid to a temperature range o 󰀱󰀰–󰀵󰀰 ∘ C aboveambienttemperaturedependingonthedesign[󰀱󰀸].Terearethreeprincipalpartso󿬂atplatesolarcollector:absorberplatewhich absorbs solar radiation and transers it to the working󿬂uid, transparent cover which allows short wave radiation topass and prevents them rom exiting, and insulation whichresists back and rear side heat losses. Te most importantadvantages o these types o collectors include low construc-tioncostsandminimaleffectinpressuredrops.However,themain drawback o solar air collectors is the low heat transercoefficient between the absorber plate and the air stream duetopoorthermalconductivityandlowheatcapacityoair[󰀱󰀹]. 󰀲.󰀲. Glazing Materials.  Glazing is the top cover o a solarcollector. It perorms three major unctions in particular: tominimize convective and radiant heat loss rom absorber, totransmit the incident solar radiation to the absorber platewith minimum loss, and to protect the absorber plate romoutsideenvironment[󰀲󰀰,󰀲󰀱].Otherimportantcharacteristics o glazing materials are re󿬂ection (  ), absorption ( 󽠵 ), andtransmission ( 􍠵 ). In order to attain maximum efficiency,re󿬂ection and absorption should be as low as possible, whilsttransmission should be as high as possible [󰀲󰀲]. Tereore,actors or consideration in selecting the glazing materialsinclude strength o material, durability, nondegradability when exposed to the ultraviolet light (UV), and low costs.Usually the common materials used as glazing materials areglass and plastics.Glass is the principal material used to glaze solar col-lectors [󰀱󰀰, 󰀱󰀳, 󰀲󰀳, 󰀲󰀴]. Glass material has highly desirable property o transmitting as much as 󰀹󰀰% o the incomingshort-wave radiation, while virtually none o the long waveradiation emitted by the absorber plate can escape outwardsby transmission [󰀲󰀵]. o be speci󿬁c, glass cover or solarcollector normally should be at least 󰀰.󰀳󰀳cm thick [󰀲󰀶].Comparedtoglasscover,aplasticcoverpossesseshighshort-and long-wave transmittance and hence high perormance.Generally, the main advantages o plastics are resistance tobreakage, light weight, and low cost. However, plastics havebeen reported to have limited lie span due to the effecto UV radiation which reduces its transmissivity [󰀲󰀴]. Also,plastics are transparent to long-wavelength radiation and arethereore less effective in reducing radiated heat losses romtheabsorberplate.Inaddition,plasticscannotwithstandhightemperature encountered in collector especially when thecollector is idle [󰀲󰀶]. 󰀲.󰀳. In󿬂uence of Cover Material on the Collector Performance. Temajorheatlossesinthecollectorareromtherontcover(glass cover), since the sides and the back o the collectorare ofen adequately insulated [󰀲󰀴]. Tereore, accurate pre-diction o the thermal perormance o solar collector systemstronglydependsonhowtheglasscovermaterialisanalysed.Tough almost all the studies reported assume that glasscoveroasystemistransparentorthesolarrangeandopaqueortheinraredradiation[󰀲󰀷],onlyewstudieshavereportedon the in󿬂uence o thickness o glazing materials on theperormance o solar collector.Kalidasa et al., 󰀲󰀰󰀰󰀸, [󰀲󰀸] compared a 󰀳mm and 󰀶mmglass covers and reported that solar collector with glasscover o 󰀳mm thickness was more efficient compared to thecollector with 󰀶mm glass thickness. Vejen et al. [󰀲󰀹] suggestthat using a glass cover with better optical properties canimprove perormance o solar collector by 󰀶%. However, theauthor did not give the optimal thickness o glass that givesbest efficiency. 󰀲.󰀴. Heat ransfer in Glazing Material.  Energy absorbed by glass cover depends on temperature difference between glassand 󿬂uid, glass and plate, and glass and ambient: 󝠵 ⋅ 󽠵   = ℎ 󽠵  󐀨   −  󽠵 󐀩 + ℎ 􍠵  󐀨 󝠵  −  􍠵 󐀩 + ℎ ,  󐀨   −   󐀩+ ℎ ,󽠵  󐀨   −  󽠵 󐀩.  (󰀱)Te radiative heat transer coefficients rom the absorberto the glazing and rom the glazing to the ambient are,respectively, given by  ℎ ,  = 󐀨 2 +  2 󐀩󐀨   +   󐀩󐀨󐀨1/  󐀩 + 󐀨1/  󐀩 − 1󐀩 ,ℎ ,󽠵  = 󐀨 2 +  󽠵2 󐀩󐀨   +  󽠵 󐀩󐀨󐀨1/  󐀩 − 1󐀩 . (󰀲)Te convective heat transer coefficients or air 󿬂owingover the outside surace o the glass cover were proposed by Kumar and Mullick [󰀳󰀰]. Consider ℎ 󽠵  = 5.7 + 3.8  .  (󰀳)Upward heat losses are greatly in󿬂uenced by convectiveheat transer rom the upper outermost surace o a solarcollector. Tis wind induced convective heat transer hasgreater in󿬂uence on upward heat losses in case o singleglazed collectors 󰀲.󰀵. General Efficiency of Flat Plate Solar Collectors.  Tethermal efficiency o a collector is the ratio o the useulthermal energy to the total incident solar radiation averagedoverthesametimeinterval.Mathematically,theefficiency(  )o a collector is expressed as [󰀱󰀸, 󰀳󰀱]  =  useul energy solar energy available .  (󰀴)Useul energy or a solar thermal collector is the rateo thermal energy leaving the collector, usually described intermsotherateoenergybeingaddedtoaheattranser󿬂uidpassing through the receiver or absorber [󰀱󰀳, 󰀳󰀲]. Consider    =  ⋅    ⋅ 󰀨   −   󰀩.  (󰀵)Te area o the collector on which the solar irradianceallsiscalledtheapertureareaothecollector.Tereore,total  Journal o Energy 󰀳energy received by collector (optical energy captured) can bedescribed by   in  = 󝠵 ⋅ .  (󰀶)Accordingly, absorptance and transmittance are multipleeffects o optical energy capture and, thereore, these actorsindicate the percentage o the solar rays penetrating thetransparent cover o the collector and the percentage beingabsorbed [󰀳󰀲]. Consider  in  = 󽠵 ⋅ 􍠵 ⋅ 󝠵 ⋅ .  (󰀷)Terateouseulenergyothecollectorcanbeexpressedbyusingoverallheatlosscoefficientandthecollectortemper-ature as (Yogi and Jan, 󰀲󰀰󰀰󰀰)  ̇ useul  = ̇ in  − ̇ loss  = 󽠵 ⋅ 􍠵 ⋅ 󝠵 ⋅  −    ⋅    ⋅ 󰀨 󝠵  −  󽠵 󰀩. (󰀸)Since,itisdifficulttode󿬁nethecollectoraveragetemper-ature in (󰀴). It is convenient to de󿬁ne a quantity that relatestheactualuseulenergygainoacollectortotheuseulgaini thewholecollectorsuracewereatthe󿬂uidinlettemperature[󰀱󰀸]. Tis quantity is known as “the collector heat removalactor (   )” and is expressed by     = ̇ ⋅    ⋅ 󰀨   −   󰀩 ⋅ 􀁛󽠵 ⋅ 􍠵 ⋅ 󝠵 −    ⋅ 󰀨   −  󽠵 󰀩􀁝.  (󰀹)Finally, equation or efficiency o 󿬂at plate solar collectorcan be given by “ Hottel-Whillier-Bliss equation ” [󰀳󰀳]  =    ⋅ 󽠵 ⋅ 􍠵 −    ⋅    ⋅ 􀀨   −  󽠵 󝠵 󝠵 􀀩.  (󰀱󰀰)I it is assumed that  􍠵  and  󽠵  are constants or a givencollectorand󿬂owrate,thenthecollectorefficiencyisalinearunction o the three parameters de󿬁ning the operating con-dition: solar irradiance ( 󝠵 ), 󿬂uid inlet temperature (   ), andcollector outlet temperature (   ). Tus, the perormance o aFlat-Plate Collector can be approximated by experimentally measuring these three parameters, and the efficiency can becalculated by using [󰀱󰀳]  =  ⋅    ∗ 󰁛   −   󝠵 󰁝.  (󰀱󰀱) 3. Materials and Methods Four similar 󿬂at plate solar collectors were used in thisstudy. Glazing materials used or experiments were low iron(extra clear) glass o thicknesses 󰀳, 󰀴, 󰀵, and 󰀶mm. Collectorswere constructed by using  Pterocarpus  timber (Mninga)o thickness 󰀲 inch and black painted marine plywood asabsorbing materials. Additionally, speci󿬁cations o collectorswere collector length to width ratio equal to 󰀲 (length 󰀱.󰀲mand width 󰀰.󰀶m) and depth o 󰀰.󰀱󰀵m. Both collectors wereoriented to northsouth direction and tilted to an angle o 󰀱󰀰 ∘ F󰁩󰁧󰁵󰁲󰁥 󰀱: Solar collector models with 󰀳, 󰀴, 󰀵, and 󰀶mm glass thick,respectively. 2025303540455055606:00 8:00 10:00 12:00 14:00 16:00 18:00Time (hour)Collector-1Collector-2Collector-3Collector-4Ambient    T  e  m   p  e  r  a   t  u  r  e    (      ∘    C    ) F󰁩󰁧󰁵󰁲󰁥 󰀲: emperature pro󿬁le o collector with similar glazingthickness. withthegroundtowardnorthdirectionasshowninFigure 󰀱.Collectors outlet temperatures were measured by using XR󰀵-SE data logger connected with P󰀹󰀴󰀰 temperature sensors,whilst ambient temperatures were recorded by CEM D-󰀱󰀷󰀲temperature and humidity data logger. On the other hand,solar intensity and air 󿬂ow rate were, respectively, measuredby using PCE-SPM solar radiation meter and esto 󰀴󰀲󰀵 HotWire Termal Anemometer. Air 󿬂ow rate in each collectorwas controlled by extract ans o capacity 󰀱.󰀲󰀷m 3 /min.Efficiency o the collectors was established by testingeach collector with the same glass thickness (󰀵mm). Teduration or this experiment was 󰀵 days each or collectorwith similar glazing and with different glazing. ime o experiments was rom 󰀷:󰀳󰀰 to 󰀶:󰀰󰀰 p.m. with an interval o data sampling o 󰀱󰀰 minutes. Experiments were conducted atthe University o Dar es Salaam at the College o engineeringandechnology.Bothcollectormodelswereplaced ontopo blockQbuildingsituatedattheDepartmentoChemicalandMining Engineering.  󰀴 Journal o Energy  󰁡󰁢󰁬󰁥 󰀱: ANOVA or collector with similar glass thicknesses.Sum o squares d Mean square    Sig.Between groups 󰀱.󰀴󰀲󰀷 󰀳 󰀰.󰀴󰀷󰀶 󰀱.󰀲󰀸󰀹 󰀰.󰀳󰀲󰀳Within groups 󰀴.󰀴󰀲󰀸 󰀱󰀲 󰀰.󰀳󰀶󰀹otal 󰀵.󰀸󰀵󰀴 󰀱󰀵 0501001502002503003506:00 8:00 10:00 12:00 14:00 16:00 18:00    E  n  e  r  g  y  r  a   t  e    (   J   /  s    ) Time (hour)Collector 1Collector 2Collector 3Collector 4 F󰁩󰁧󰁵󰁲󰁥 󰀳: Energy pro󿬁le o collector with similar glazing thickness. 4. Results and Discussion 󰀴.󰀱. Collector with the Same Glass Tickness.  Te main objec-tive o this experiment was to 󿬁nd out i there is signi󿬁canceperormances difference between designed collector modelswith similar characteristics. Each collector model was testedor its perormance by using 󰀵mm glass thickness. 󰀴.󰀱.󰀱. emperature and Energy Pro󿬁le of Collectors with SameGlass Tickness.  Figure 󰀲 shows the variation o ambient andoutlet temperature o our collector models recorded rom󰀷:󰀳󰀰 a.m. to 󰀰󰀶:󰀰󰀰 p.m. on September 󰀱󰀲, 󰀲󰀰󰀱󰀱, while Figure 󰀳shows the rate o 󿬂ow o energy.From Figure 󰀲, it can be seen that there is no variation intemperature between collectors; however, temperature variesaccording to the 󿬂uctuationo solar intensity. Fluctuationso temperature during the morning are high when comparedto afernoon due to high clouds coverage which results inlow solar intensity reaching the earth. Similar characteristicswereobservedinenergypro󿬁leinFigure 󰀳.Teefficiencieso the solar collectors were evaluated by 󿬁nding the area underenergy curve. Statistical analysis o the thermal efficiency o solar air collectors with the same thickness glazing materialswas analysed with SPSS program with con󿬁dence intervalo 󰀹󰀵%. Efficiency means o collector models 󰀱, 󰀲, 󰀳, and 󰀴were󰀲󰀹.󰀶%,󰀲󰀹.󰀸%,󰀳󰀰.󰀳%,and󰀳󰀰.󰀳%,respectively.Aone-way between-subjects ANOVA (analysis o variance) was used tocomparetheefficiencies othecollectormodelsandreported 20253035404550556:00 8:00 10:00 12:00 14:00 16:00 18:00    T  e  m   p  e  r  a   t  u  r  e    (      ∘    C    ) AmbientTime ( 24 hrs)Glass,  3 mmGlass,  4 mmGlass,  5 mmGlass,  6 mm F󰁩󰁧󰁵󰁲󰁥 󰀴: emperature pro󿬁les o collector with different glassthicknesses on 󰀱󰀷/󰀱󰀰/󰀲󰀰󰀱󰀱. in able 󰀱. Te main objective was to determine i there is asigni󿬁cance difference between collectors’ efficiencies whenoperated with the same glazing materials.From able 󰀱, the signi󿬁cance value is 󰀰.󰀳󰀲󰀳 (  < 0.05 ).Tereore; it can be concluded that there were no statisti-cally signi󿬁cant differences between the means o collectorefficiency with the same glass thickness and that their minor variations were due to changes in environmental conditionsand not due to design variations. 󰀴.󰀲. Collectors with the Different Glass Ticknesses󰀴.󰀲.󰀱. emperature and Energy Pro󿬁le of Collectors with Dif- ferent Glass Ticknesses.  Figure 󰀴 shows temperature pro󿬁lestrends o ambient and outlet temperature or our collectormodels recorded rom󰀷:󰀳󰀰 a.m. to 󰀰󰀶:󰀰󰀰 p.m. on 󰀵/󰀱󰀰/󰀲󰀰󰀱󰀱. Itcan be seen that, during sun rise to noon, there were hightemperature 󿬂uctuations mostly due to variations in solarintensity and these 󿬂uxes appear to be steady rom noon tosunset. On other hand, solar collector delivers low tempera-ture rom morning to noon session, while solar intensity washigh as a consequence o low angle o incidence on collectorsuraces (Figure 󰀵). Das and Chakraverty [󰀳󰀴] reported the decrease in solar transmittance to the surace o the glassto be signi󿬁cance or angle between 󰀰 and 󰀶󰀰 ∘ . Similarcharacteristicsalsocanbeobservedduringafernoonsession.
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