Understanding Solar Food Dryers 27pages

8/13/2019 Understanding Solar Food Dryers 27pages

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UN ERST N INGSOL R FOO RYERS

Roger Gregoire P E

Technical Reviewers:ary FlomenhoftJacques eNormand


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UNDERSTANDING SOLAR FOOD DRYERS

byRoger G. Gregcire P.E.

Technical Reviewers:Gary FlomenhoftJacques L LeNormand

Published by:Volunteers in Technical Assistance VITA1815 North Lynn Street Suite 2Arlington Virginia 222 9 USATelephone 703/276 1800cable VITAINCTelex 44 92 VITAUI


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UNDERST NDING SOL R FOOD DRYERSBy VITA Volunteer Roger G. Gregoire,

I . INTRODU TIONDehydration, or drying, is a simple, way to preservfood tha t might otherwise spoi l . Drying removes water and -thuprevents fermentation or the growth of molds. I t also slows thchemical changes that take place natural ly in as whef ru i t r ipens. Surplus grain, vegetables , and f ru I t pre erved bdrying can be stored for future use.People have been drying food for thousands of years by placinthe food on mats in the sun. This simple method, however, allowthe food to be contaminated by dust , airborne molds and funginsec ts rodents, and other animals. Furthermore, open a ir dryini s often not possible in humid climates.

THE DRYING PRO ESS

Solar food dryers represent a major improvement upon th is ancienmethod of dehydrating foods. Although solar dryers involve ai n i t i a l expense, they produce bet te r looking, b et te r ta st in g anmore nutr i t ious foods, enhancing both the i r food value -andmarketabi l i ty . They also are fas ter safer and morethan t radi t ional sun drying techniques. n enclosed cabinet-s tyso lar dryer can produce high qual i ty dried foodstuffs in 6umclimates as well as arid cl imates . I t can also reduce the probl eof-contamination. Drying is completed more quickly, so theie-l e ss chance of spoilage. Frui ts maintain a higher _content. Because many sola r dryers have no addit ional fuel c-osth is method of preserving food also conservessources of energy.In recent years , attempts have been made to develop solar dryetha t can be used i n a g r ic ul tura l ac t iv i t i e s in developing cout r i e s . . Many of the dryers used for dehydratin g foods are relat ive ly low cos t compared to systems used in developed countriesThis paper describes some of these dryers, and discusses thfactors t ha t must be considered in de term ining what of dryis bes t suited for a par t icula r appl ica t ion.

Drying products makes them more s table and in the case ofallows them to be stored safely for Long periods of time .storage requires protection from the growth of moldsan d


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fungi theand c o n t ro l . most di f f icul t of the spoilage mechanisms to d e t e c tThe types of los s generally caused by fungi in the germination r a t e of seed. D is co lo r atio n which reduces value of foods for manypur -poses. Development of mustiness or other undesirable odors orfl a v o rs. Chemical changes t h a t render food undesirable or u h f i tfor processing. Pro duction of toxic products known as mycotoxins someof which can be harmful i f consumed. Total spoilage and heating w hi ch s om et im es m yto the p o in t of sp ontaneo us combustion.

At h a rv e st most g r ain s contain more m oisture than is safe forprolonged st o ra g e because m ny fungi grow rapidly i n warm moistco n d itio n s . Thus any g ra in sto red for future use must be dr ieds hor tly af ter ha rve s t to prevent the growth of d e st ru c t i v e fUrigi;In g en er al g ra i n s w i l l not be completely d r ied since they areh y g r o s co p ic- - th at is t h e y absorb moisture . from the ai r . J h ehigher the r e l a t i v e humidi ty of tne surrounding a i r the hi ghe rthe moisture c ont e nt of the g ra i n . Table 1 l i s t s the moisturecontent of various g r ain s as a function o f the r e l a t i v e humidityof . the surrounding a i r t the same time th er e is a .minimurnl e v e l of r e l a t i v e humidity below which the harmful fungi w i l lnot t h r i v e . Table 2 shows these minimum r e l a t i v e humidity levelsfor . common storage fungi. Proper drying lowers the moisturecontent of g ra i n s below the minimum needed for the growth orfungi.

Drying Grains


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PREF CEThis paper i s one of a series published by V olunteers in Technical Assistance to provide an i nt ro du ctio n to specific s ta te ofthe ar t technologies of in teres t to people in developing count r ies . The papers are intended to be used as g uid elin es to helppeople choose technologies that are suitable to their s i tuat ions.They are not intended to provide construction or implementationdetai ls . People are urged to contact VIT or a similar organizat ion for further information and technical assistance i f theyfind that a part icular technology seems to meet their needs.The papers in the series wri t ten reviewed and i l lus t ra tedalmost entirely by VIT Volunteer te chnic al e xper ts on a purelyvoluntary basis. Some 5 volunteers were involved in the product ion of the f i r s t 1 t i t l e s issued contributing approximately5 000 hours of their time. VIT s ta f f included Leslie Gottschalkand Maria Giannuzzi as editors Jul ie Berman handling typesett ingand layout and Margare t Crouch as project manager.Roger G Gregoire P E. the author of this VIT Technical Paperi s a consul tant in the areas of energy management engineeringsolar design and analysis energy audits energy management ObUildings and al ternat ive energy systems. He has pUblished oenergy conservation solar greenhouses and solar water heaters a well as solar food dryers. Reviewers Gary Flomenhoft andJacques L LeNormand are also experts in the area of solar fooddryers. Flomenhoft is a consultant in renewable energy and engineering for the San Diego Center for Appropriate Technology. Hhas also taught on energy conservation and solar technologyLeNormand is Assistant Director a t the Brace Research Inst i tuteQuebec Canada which does research in renewable energy. He hasupervised work with solar col lectors has t rained people. fromoverseas in solar technologies and has published widely on solaand wind energy and conservation.VIT is a private nonprofi t organizat ion that supports peoplworking on technical problems in developing countries.. VITA offers information and assistance aimed a t helping individuals angroups to selec t and implement technologies appropriate to the isituations.. VIT maintains an international Inquiry Servicespecial ized documentation center and a computerized roster ovolunteer technical consultants: manages long term f ie ld projects ; andpubl i sh e sa variety of technical manuals and papersFor more information about VIT services in general or thtechnology presented in this paper contact VIT a t 1815 NortLynn Stree t Suite 200 Arlington Virginia 222 9 US


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Table 1. Moisture Contents of Various Grains and Seeds inEquilibrium with Different Relative Humidities a t25 to 30Centigrade _ .Wheat Rice SunflowerHumidi ty Corn Sorghum __ ___ Soybeans __ __

65 12.5 to 13.5 12.5 14.0 11.5 8. 5 5.070 13.5 to 14. 5 13.5 15.0 12.5 9. 5 6.075 14.5 to 15. 5 14.5 15. 5 13.5 10.5 7.080 15. 5 to 16.5 15.0 16.5 16.0 11.5 8.085 18.0 to 18.5 16.5 17.5 18.0 13.5

Source:New York: American Society of Heating Refrigerat ing ahdAir Conditioning Engineers Inc . 1980 p. 10.2.

Table 2. Minimum Relative Humidity for the Growth of CommonStorage Fungi a t Their Optimum for Growth26 to 30Centigrade-----------------------------------------------------------------yr of r 1inimum Relative Humidity-_______

Aspersi l lus halophi l icus 6BA. res t r i c tus Sporendonerna 70A. glaucus 73A. candidus A.ochraceus 80A. flavus 85Penici l l ium depending on species 80 to 90

Source: New York: American Society of Heating Refrigerat ing andAir Conditioning Engineers Inc . 1980 p. 10.2.3


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Solar dryers use the energy of the sun to heat the a ir that flowsover the food in the dryer. As a ir is heated, i t s relat ivehumidity decreases and i t is able to hold more moisture. Warmdry a ir flowing through the dryer carr ies away the moisture thatevaporates from the surfaces of the food.As d ry ing p ro ceed s, the actual amount of moisture evaporated perunit of time decreases. In the f i r s t phase of drying, the mois-ture in the exter ior surfaces of the food 1s evaporated. Then,once the outer layer is dried, moisture from the innermost por-t ion of the material must t ravel to the surface in the secondphase of drying. Figure shows the representat ive change inevaporation rate for hygroscopic materials (including most food-s tuffs commonly dried. During the second phase of the dryingprocess, overheating may occur because of th e less en ed coolingeffec t result ing from the slower ra te of moisture evaporation. f the temperature i s too high, the food wil l case harden orfon: a hard shel l tha t t raps moisture ins ide. This can caus edeter iorat ion of the food. To prevent overheating during th isportion of the drying cycle, increased airf lows or less heatcol lect ion may be desirable .

phase 2

ophase 1

lowMoisture Content

Figure 1. Drying Rate Versus Moistu re Con tent4


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I I I . DESIGN VARIATIONSSOLAR DRYEN TYPESSo lar d r y e r s fa l l i n t o two broad c a t e g o r i e s : a c t i v e 2r.d p assiv e.Passi ve d r y e r s can be f u r t h e r d iv id ed i n t o di rec t and indirectmodels. A di rec t p assive) d ry e r is one in which the food isdirect ly exposed to the s u n s r a y s . In an i n d i r e c t d r y e r , thes u n s rays do n o t s tr ike the food to d r i e d . A small s o l a rd r y e r can dry up to 300 pounds of food per a l a r g e dryercan dry up to 6 , 0 0 0 pounds a and a very l a r g e system cand r y a s much a s lO OOC or more pounds a month. Figures a re basedon h a r v e s t s in c l i ma t e s . )Fi gure 2 shows th e breakdown, by t ype, of s o l a r food d r y e r s .

PASSIVE

SOLAR FOO DRYERSf

ACTIVE

DIRECTI ii IL RGE

tI tiD RECTI RGC

Fi gure 2. Breakdown of S o l a r Food DryersPassi ve d r y e r s use only the n a t u r a l movement of heated a i r . Thetcan be c o n s t r u c t e d e a s i l y with i nexpensi ve, local lym a t e r i a l s . D i r e c t p assiv e d ry e rs a re b e s t used fo r drying smallb atch es of f o o d s t u f f s . Indirect dryers vary in s i z e from smallhome d r y e r s to la rge sca le commercial u n i t s .

DryersActive d r y e r s r e q u i r e an e x t e r n a l means, l ike fans o r pumps; formoving th e s o l a r energy in the form of heated a ir from thearea to th e drying beds. These d r y e r s can be bui l t inalmos t any s ize from very small to very l a r g e , but th e large rsystems a r e th e most economical.

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Fig u re 3 is schematic drawing showing the major components ofan a c t i v e s o l a r food d r y e r . Either ir or l i q u i d c o l l e c t o r s canbe used to col lec t th e s u n s energy. The col lectors hould facedue so u th i f you a re in th e northern hem isphere or n o rth i fyou a r e in the so u th ern hemisphere. At or near the e q u a t o r theyshould a l s o be a d j u s t e d e a s t or west in the morning f te rnoon r e s p e c t i v e l y . The col lec tors should be p o s i t i o n e d t ana ppropri a t e angl e to opt i mi ze s o l a r energy c o l l e c t i o n for th eplanned months of o p e r a t i o n of th e d r y e r . The col lectors can bea d j a c e n t to or somewhat remote from th e s o l a r d r y e r . Howevers i n c e i t is more di f f i cu l t to move ir long di s t a nc e s i t is b e s tto p o s i t i o n th e C O llecto rs as near th e d ry er as p o s s i b l e .

. ...J11 tlz

e x h a u s ts t a c k

\ food; dry in

_ _ f i jthermals t o rag e

= n heat exchanger 1.. . 1 ;1 .. :CS is i r

o p t i o n al ir in le t

Fig u re 3 . Major Components of an Active Dryer6


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The solar energy col lected can be del ivered as heat immediatelyto the dryer a ir stream, or i t can be stored for la ter use.Storage systems are bulky and costly but are helpful in areaswhere the percentage of sunshine is low and a guaranteed energysource is required; or in carrying out round-the-clcck drying.In an act ive dryer, the a ir flows through the solardrying chamber in such a manner as to contact as much surfacearea of the food as possible. The larger the rat io of foodsurface area to volume, the quicker will be the evaporation ofmoisture from the food. Thinly s l iced foods are placed on dryingracks or on t rays made of a screen or other material that allowsdrying a ir to flow to a l l sides of the food. For grain products,pipes with many holes are placed a t the bottom of the drying binwith grain piled on top. The heated a ir flows through the pipesand is released upward to flow through the grain--carrying awaymoisture as i t flows.Passive DryersPassive solar food dryers use natural rneans--radiation andconvection--to heat and move the a i r . The category of passivedryers can be subdivided into direct and indirect types.Direct Dryers. In a direct dryer, food is exposed direct ly to thesun s rays. This type of dryer t yp ical ly consi st s of a dryingchamber that is covered by t ransparent cover made of glass orplas t ic . The drying chamber is a shallow, insulated box withholes in i t to allow a ir to enter and leave the box. The food isplaced on a perf ora te d tray that allows the a ir to flow throughi t and the food. Figure 4 shows a drawing of a simple directdryer. Solar radiat ion passes through the t ransparent cover andis converted to low-grade heat when i t s t r ikes an opaque wall.This low-grade heat is then trapped inside the box in what isknown as the greenhouse effec t . Simply s tated, the short wave-length so la r ra dia tio n can penetrate the t ransparent cover. Onceconverted to low-grade heat , the energy radiates on a long wave-length that cannot pass back through the cover. Figure 5 showsthe greenhouse effec t in a simplified schematic drawing.Figures 6 and 7 show examples of simple, direct dryers that canbe used to dry small quanti t ies of a wide variety of foods. Thedrying chamber can be constructed of almost any materia l -- woodconcrete, sh ee t meta l, etc . The dryer should be 2 meters (6.5feet) long by I meter (3.2 feet) wide and to 30 centimeters(9 to 12 inches) deep. The bottom and sides of the dryershould be insulated, with 5 cent imeters 2 inches) recommended.Blackening the inside of the box wil l improve the dryer e f f iciency, but be sure to use a non-toxic material and avoid lead-based paints . Wood blackened by f i re may be a safe and inexpen-sive material to


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:::esn/s

a ir eX;1cL;S i .. holeso

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C l:-------- P l a s t i c Film Cove r: _ _ .. --_.

- - - - - - _

Figure VITA Solar Crop Dryer

Hole\ T r a n s p a r e n t G l a s s Cover

/ ,..,.\ \ \\ Sunl ight . , r j f 1 \ tw vafor

Walls =======.====:=== ==Z=== ::/===Z:::/=::I n s u l a t e d S i d e s Food T r a yand B o t t o m be d r i e d

7 Brace Research I n s t i t u t e s Bot Box Dryer9


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The t ray t h a t holds the food must permit a i r to enter from belowand pass through t o the f00d. A wire or p l a s t i c mesh or screenw i l l do nicely. Use the coarsest possible mesh that Hi l l supportfoed without l e t t i n g i t f a l l through the holes . The larger:he holes in the mesh, the e a s i e r the a i r wil l c i r c u l a t e throughto the food. Air holes below the tray or mesh wkll bring inouts ide a i r which wil l carry away the mo is tu re evapo ra ted fromthe food. s the a i r heats up in the dryer , i t s volume wil lincrease, so e i t h e r more or larger holes wil l be required a t thetop of the box to maintain maximum a i r flow.F i n a l l y t e s t s : the hot box dryer shown in Figure have de-termined t h a t temperature within the dryer C n be as much as40Centigrade _ J4Fah renheit) h ighe r than the outside ambientI n d i r e c t Dryers. i n d i r e c t dryer i s one in which the 8 raysdo not s t r i k e the food to be dried. In t h i s system, drying i sachieved i n d i r e c t l y by using an a i r c o l l e c t o r t h a t hota i r i n t o a separate drying chamber. Within the chamber, the foodi s placed on mesh t rays t h a t are stacked v e r t i c a l l y so t h a t thea i r flows through each one. Figure 8 shows an i n d i r e c t passivedryer . The s o l a r can be of any s i z e and should bet i l ted towa rd the sun to optimize c o l l e c t i o n . increasing thec o ll e c t o r s iz e more heat energy can be added to the a i r toimprove overal l e f f i c i e n c y . Larger c o l l e c t o r areas are helpful inplaces with l i t t l e solar energy, cool or cold cl imates , andhumid regions. Sect ion V of t h i s paper indicates cl imatlc condi-t i o n s where c o l l e c t o r areas might be more e f f e c t i v e .T i l t i n g the c o ll e c t o rs i s more e f f e c t i v e than placiny them hori-z o n t a l l y for two reasons. F i r s t more solar energy can be c o l -lec ted when the c o l l e c t o r surface i s more nearly perpendicular tothe s u n s rays . Second, by t i l t i n g the c o l l e c t o r s the warmer,l e s s dense a i r r i s e s n a t u r a l l y into the drying chamber. Thedrying chamber should be placed on support l egs , but i t shouldnot be raised so high above the ground t h a t i t becomes d i f f i c u l tto work with .The base of the c o l l e c t o r S Jld be vented to allow the entranceof a i r to be heated for d: :ng. The vents should be evenlyspaced across the f u l l width of the base of the c o l l e c t o r toprevent l o c a l i z e d areas within the c o l l e c t o r from overheat ing.The vents should a l s o be adjus table so t h a t the a i r flow can bematched with the operating condit ions and/or needs. Solar radia-t i o n ambient a i r temperature, humidity l e v e l drying chambertemperature, and moisture level of the food being dried must a l lbe considered regulat ing the flow of a i r .

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r exhaust

sunl s t

cry ngt rays

: r f sc ref :co er

r fr r

Figure Indirect Dryer11


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: f

f r u i t s ve'getables f i s h

used throughout the world t o dry food produ9tlist completely . Lis ted below a r e a . few t o show the d i v e r s i t y t o which the s u n s

g r a i n s meat s a l t

Solar eher-y i stoo numer


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Thin p l a s t i c films are inexpensive and have goodthe a b i l i t y of a material to allow sunlight to pass through i t but may degrade quickly, and are eas i ly punctured and torn. Thcheapest film, polyethylene, may cost US .50 per square meteUSS.05 per square foot) and l a s t l e s s than one season--a l i t t l emore than a year i f i t i s handled careful ly . Ultraviolet--stab il ized polyethylene can l a s t two to four years but will cost h r e e . to five t imes as much. Tedlar and tef lon fi lms have longuseful l i v e s (10 years or more), excel len t t ransmiss iv ity allowing percent or more of the solar energy to pass through) ancost in the range of US 4 to US 8 per square meter toU5 .70 per square foot) . These films are probably the beschoice i f they can be protected from puncturing.SKILLS NEEDED TO BUILD, OPERATE N MAINTAINBuilding a solar food dryer requires some carpentry skil ls . .t e r ing the technique of drying comes from d i r e c t experience wi t.d ry ing p roducts rather than from reading about i t . Maintainingsolar food dryer requires only that an operator monitor the partperiodical ly for wear and t e a r . For example, an operator sh6ulQmake sure t h a t the legs tha t support the drying are neloose, and t h a t vents are not blocked. Plast ic glazing materiashould be checked to see i f i t turns cloudy, which will causl e s s sunlight to pa.ss through i t .COST/ECONOMICSCost comparisons between indi rec t and d i r e c t dryers are presentein Tab: e 3. Dryers 1, 2, 3, and 4 are indirect dryers , andryers Sand 6 are d i r e c t dryers . The table shows the cost peunit ; more important, i t compares the cost p e r drying t r a y a the tray area f o r each dryer. Table 4 gives some values ovitamin C retention for two products dried by i n d i r e c t , d i r e c tand open a i r drying. Overall , i t appears th at in direc t dryer:s armore e f f i c i e n t and have higher vitamin retention than d i r e cdryers .

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Table 3. Cost Comparisons

Indirect dryer 1.12 65.00 58.04Indirect dryer 1 49 90.00 60.40 Ind i rec t dryer 1.30 75.00 57.69Indirect dryer 3.16 115.00 3.6.39Indirect dryer 2.88 175.00 60.76Indirect dryer 1.21 50.00 41.32

Table 4. Vitamin C Retention

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Source: American Solar Energy Society, IncBoulder, Colorado: American SolarEnergy Society, Inc 1983 , p. 682.

Indirect Cantaloupe 70.4Indirect Cantaloupe 51.0Direct Cantaloupe 53.6Open sun Cantaloupe 39. 5Indirect Spinach 35.9Direct Spinach 22.4

Type of Type of Percen tage ofD V - _

Source: American Solar Energy Society, IncBoulder , Colorado: American SolarEner gy Soc i e ty , Inc 9 , p 82.


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IV COMPARING THE ALTERNATIVESFOSSIL FUEL DRYBRS VERSUS SOLAR DRYERSConventionally fueled dryers are the primary to soladryers . In conventional dryers , a fuel is burned to heat thfood-drying a i r In some cases, the gaseous products of combut ion are mixed with the a i r to achieve the desired temperatureAlthough these drying systems are used around the world with napparent problems, there i s the poss ibi l i ty of a mechanicamalfunction, which might allow too much gas in to the dryinstream. I f th is occurs, the food in the dryer can become contamnated.The great advantage tha t conventional dryers have over soladryers is tha t drying can be carried out around-the--c ocfor days on in any kind of weather. Unlike solar dryer conventional dryers are not subject to dai ly and seasonal var iat ions and other climatological factors . On the other hand, thfuels burned in conventional dryers may present other problemUse of wood may c on trib ute to problems of defores t i \ t ion;coa l mcause pollut ion. Fossil fuels are becoming increasingly expensive and are not always avai lable .ADVANTAGES OF SOLAR DRYERSSolar dryers have the principal advantage of using solar energya fre e, a va ila ble , and l imi t l es s energy source tha t is also nopol uting. Dry ing most foods in sunny areas should not be problem. vegetables , for example, can be dried in 2-1/2.4 hours, a t temperatures ranging from to 63Centigrade (lJt o l45 Fah renheit ). Frui t s take longer , from to hours,temperatures ranging from to 66Centigrade 110 to 150Fahrehei t ) . At th is ra te i t i s possible to dry two batches of fooon a sunny day.A solar food dryer improves upon the t radi t ional open-air systemin five important ways:1. I t i s fas te r Foods can be dried in a shor ter amounttime. Solar food dryers enhance dry ing times in two wayFi rs t the t ranslucent or transparent glazing over th

col lect ion area t raps heat inside the dryer, r.aising. ttemperature of the a i r Second, the capabi l i ty of enlarginthe so la r c ol le cti on area allows for the concentrat ionthe sun 's energy.2. I t i s more ef f ic ien t Since foodstuffs can be dried moquickly, less will be los t to spoilage immediatelyaft: :ha :vest. This i s especial ly t rue of produce tha t requir

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immediate drying--such as a grain with a high moisturecontent . In th i s way, a larger percentage of food wil l beavail.able- for human consumption. Also, less harvestwil l be los t to marauding animals, vermin, and insects sincethe food wil l be in an enclosed compartment.3. I t i s safer Since foodstuffs are dried in a controlledenvironment, they are less lik ely to be. contaminated bypests , and can be stored with less l ikel ihood of the growth

of toxic fungi.4. I t i s heal th ier Drying foods a t optimum temperatures andin a shorter amount of time enables them to retainmore of their nut r i t ional value--especia l ly vitamin C nextra bonus is tha t foods wil l look and tas te bet ter whichenhances thei r marketabi l i ty .5. I t i s cheaper. Using solar energy instead of conventionalfuels to dry products, or using a cheap supplementary supplyof so lar heat in reducing conventional fuel demand canresul t in a s igni f icant cost savings. Solar drying lowersthe costs of drying, improves the qual i ty of products, andreduces losse.s due to sp oilage .DIS DV NT GES OF SOL R DRYERSSolar dryers do have shortcomings. They are of l i t t l e use duringcloudy weather. During fa i r weather they can work too well,becoming so hot inside a t midday as to damage the drying crop.Only with c lo se supe rv is ion can th is be prevented. s tempera-tures r ise determined with a thermometer or by ex perien ce , thelower vents must be opened to allow greater airflow through thedryer and to keep the temperatures down Rice, for example, willcrack a t temperatures above seed grains can bedried a t temperatures no higher than 4 to 45Centigrade.v CHOOSING THE TECHNOLOGY RIGHT FOR YFour important questions must be answered before one decides tobuild a solar food dryer. The br ief discussion following eachquest ion points out many factors that must be considered pr ior tothe construction of a solar food dryer. The quest ions are :1. What food wil l the dryer be used for? Also, what quant i t iesof food wi l l be dried?

Grains, f ru i t s and vegetables require dif ferent dryingtechniques. Figure 9 shows a flow diagram that may behelpful in defin ing the type of design. The sa fe storage of16


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FRUITS ANDVEGET LES

iMODER TE TIME

TO DRYING

r TYPE 0FFOO OSGR INS

HORT TIMETO DRY ING Iji VIT MIN LOSS FROMSUN EXPOSURE

IN-FIELD OR NE R I I LO .., HIGH JRECT

i J DRYEKS ORYEP SI PORT LE II DRYERS .I SM LL jl L RGE . ....;..Figure Flow Diagram of actors to Considerin Selecting a olar Food Dryer


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the harvest i s of prime concern to a l l . As soon as freshf ru i t s and vegetables have been prepared i . e . , some mayneed to be peeled, s l iced, or blanched) for che dryingpro ecs, they must be dried immediately. Grains, too, haveonly a l imited time in which they must be dried to ensurethei r storage. Rice in the husk, for example, wil l begin togerminate within 48 hours i f i t s moisture content is about24 percent . Crops tha t must be dried immediately af ter theyare harvested may require the use of por table d ryer s, whichcan be se t up in the harvest f ield as needed. Permanentdryers can be erected near p repa ra tion a reas for f ru i t s andvegetables or central ly located for grain crops.Some foods may lose much of thei r nut r i t iona l value, orbecome discolored, i f dried a t too high a temperature or i fexposed to the rect rays of the sun. Using indi rec tdryers can minimize the loss of vitamins, especial ly vitaminc.Final ly , the quanti ty of food to be dried, the capacity ofthe dryer , the average time requried to dry one ba.tch, andthe time avai lable in which to dry the harvest must a l l beconsidered in determining the number and size of the dryersneeded.

2. What are the climatic conditions during the harvest anddrying) season?Climatic co nd ition s so la r radia t ion, ra infa l l , temperature,humidi ty , wind, etc . should be considered in determiningwhat kind of dryer is best suited for a part icular applica-t ion.Figure 10 wil l help you to visual ize the factors tha t mustbe considered here. I f the occurrence of sunshine is low--say, percent or less-- then i t may be wise to add anauxi l iary heat source to enable drying to continue on cloudydays or even through the night . Dry cl imates with hot ormoderate temperatures are well suited for solar food dryers .Cold cl imates or humid climates pose the problem of makingi t more di f f i cu l t to obtain the necessary quanti ty of warmdry a i r to dry foods effect ively befo re spo ilage can occur.Such weather condit ions may l imi t the use of direct dryersto preserving only small quanti t ies of food that must bedried in a short time one or two days) . Indi rec t dryershave the advantage over direct dryers in tha t they arecapable of concentrat ing solar energy. Enlarging the . col-lec tor area and varying the airflow through the collectorenable indi rect dryers to achieve near optimum conditions inmost climates.

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DD UXILI RHEAT SOURCE

CLIM TIC CONDITIONS

SOL R V IL BILITY SUNSHINE

rr lHIGH LO

o DRY rlUM D DRY HUMID DRY HUMIDj COLLEC DRDIRECT INDIRECTMALL LARGE

Figure 1 low iagram of Climatic Conditions

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. Is the food to be stored for long periods or will i t beshipped to market for quick consumption?The answer to th is quest ion de te rmines th e dryG?ss requiredof the finished product. Rice a t harvest : iic;h: typicallycontain 24 percent mois tu re . I f i t is sold say formill ing i t i s fine as is I f on the other hand is tobe stored for any length of time i t must be dried to only 2 to 14 percent moisture content. Thus the dryness required wil l determine ow long and a t what temperature thefood must remain in the dryer. The time required for thefood to remain in the dryer must be taken into account indetermining the number of dryers needed to dry the ent i reharvest .

. What materials are avai lable to construct the dryer? Are theava ilable lo ca ll y?Masonry m y be a good construct ion medium for permanentdryers where the food can be brought to the dryer.I f however the dryers are to be transported into thef ie lds l ightweight materials will be needed to makethe portable. The avai labi l i ty of materials m ygovern in par t the placement of the food dryers .

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TECHNICAL ASSISTANCE ORGANIZATIONSBrace :chu , . . . r j , : . . , ; - ,,-, s ;: 1.. L ;c.n ..: 0 , ,-_,-,_S;:e l r E ;12 800

di rec t ..

Q The Ins t i tu te has designed and indirect dryers andhas plans avai lab le .New Mexico Solar Energy Association (NMSEA)P.O. Box 2004Santa Fe, New Mexico 87501 USANMS A publishes detai led construct ion plans for a so lardryer.Volun teers in Technical Assistance VITA)1815 North Lynn Stree t Suite 200Arlington, Virginia 22209 USAVI TA I s man ual incl udes pI an s for aand an indi rect solar dryer.


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BIB LIOGRAPHY

American Society of Heating, f


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TECHNICAL ASSISTANCE ORGANIZATIONSB rac e R esearch ns t i tu teMcDonald Campus of McGill UniversitySte Anne de Bellavue 800Quebec CanadaThe nst i tu te has designed di rec t and indirect dryers andhas plans avai lab leNew Mexico Solar Energy Association NMSEAP O Box 2004Santa Fe New Mexico 87501 USANMSEA pUblishes detai led construct ion plans for a solar cropdryerV olunteers in Technical Assistance VITA1815 North Lynn Stree t Suite 200Arlington Virginia 22209 USAVITAls manual includes plans for aand an indirect solar dryer di rec t


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BIB LIOGRAPHYAmerican Society of 1eating , R efrigera ting , and Air-ConditioniEng in e e r s. __New York, New York: American Society of HeatinRefrigerating, and Air-Conditioning Engineers.Andrea, A Louise. Boston, Massachusetts: TCornhill Company 1920.Archuleta, R.; Berkey, J and Williams, B Research on SolFood Drying a t the University of Cali fornia , Santa Cruz__ in__ Edited byReyes and D Andrejko. Boulder, Colorado: American SolEnergy Society, Inc 19B3, pp.DeLong D Tucson, Arizona: H.P. Books; 197160 pp.Exe11, R.H.B. A Simple Solar Rice Dryer: Basic Design Theory4 (1980): 188.Ginsburg, A.S., ed. WashingtoD.C.: The Is rae l Program for Scient i f ic Transla t ions, 196Gregoire, R.G.; Slajda, Robert; and Winne, Marl< ACommercScale Solar Food Dryer. Edited by B H Glenn and GFranta. Boulder, Colado: e r ic an Sol a r En e r gy So c i e ty , Inc 1 9 81 Lindblad, C., and Druben, L npreparing Grain for Storage. voof Prepared for ACTION/Peace Coand VITA Manual No 35E. Arlington, Virginia: VITA 19McGill Universi ty. Brace Resea rch Ins t i tu te h_ 5JJI Y e:l_ 9 f_ sQTechnical Report T99 Quebec, CanaBrace Research Ins t i tu te McGill University, 1975.Ong K S. Solar Dryins Technology for Rural Development. It Pa presented a t the Regional Conference on Tecb ,ology for RuDevelopment, Kuala Lumpur Malaysia, 1978.van Brakel , J Opinions About Selection and Design of DryerEdited by A S MujumdarPr inceton, New Jersey: Science Pre1978.

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Understanding Solar Food Dryers 27pages