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UNDERSTANDING SOLAR FOOD DRYERS I, b y Roger Gregoire, P.E. Technical Reviewers: Gary M. Flomenhoft Jacques L. LeNormand TPt15:11/84 ------ . -------

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UNDERSTANDING

SOLAR FOOD DRYERS I,

by

Roger G Gregoire, P.E.

Technical Reviewers:Gary M. Flomenhoft

Jacques L. LeNormand

. -------

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UNDERSTANDING SOLAR FOOD DRYERS

byRoger G. Gregcire, P.E.

Technical Reviewers:Gary M. Flomenhoft

Jacques L. LeNormand

Published by:

Volunteers in Technical Assistance (VITA)1815 North Lynn Street , Suite 200

Arlington, Virginia 22209 USATelephone 703/276-1800

cable VITAINCTelex 440192 VITAUI

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UNDERSTANDING SOLAR FOOD DRYERS

By VITA Volunteer Roger G. Gregoire, P . E ~

I . INTRODUCTION

Dehydration, or drying, is a simple,l o w - c o ~ :

way to preservfood that 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 f o o d s ~ as whef ru i t r ipens. Surplus grain, vegetables , and f ruI 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 funginsects , rodents, and other animals. Furthermore, open a ir dryini s often not possible in humid cl imates.

THE DRYING PROCESS

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 ter looking, bet te r t as t ing , anmore nutr i t ious foods, enhancing both their food value -and t h ~ imarketabi l i ty . They also are fas te r , safer , and more e f f i c i ~ than t radi t iona l sun drying techniques. An enclosed cabinet-s tyso lar dryer can produce high qual i ty , dried foodstuffs in 6umcl imates as well as arid cl imates. I t can also reduce the probl'eof-contamination. Drying is completed more quickly, so theie-l ess chance of spoilage. Frui ts maintain a higher v i t a r r i ~ n _-

content . Because many solar dryers have no addit ional fuel c-osth is method of preserving food also conserves n o n - r e n e w a b sources of energy.

In recent years , attempts have been made to develop solar dryetha t can be used i n a g r ic ul tu ra l ac t iv i t i es 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 h a t must be considered in de term ining what ~ i n d of dryis bes t suited for a par t icu lar 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 protect ion from the growth of moldsan'd

1

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fungi, theand control .

most dif f icu l t of the spoilage mechanisms to detectThe types of loss generally caused by fungi a r ~ :

• ~ e d u c t i o n in the germinat ion rate of seed.

• Discoloration, which reduces value of foods for manypur'poses.

• Development of mustiness or other undesirable odors orf lavors.

• Chemical changes that render food undesirable or uhfi tfor processing.

• Produc tion of toxic products, known as mycotoxins, someof which can be harmful i f consumed.

• Total spoilage and heating, which sometimes may c D n t i n u ~to the point of spontaneous combustion.

At harvest, most grains contain more moisture than is safe for-prolonged storage, because many fungi grow rapidly in warm, moistconditions. Thus, any grain stored for future use must be driedshortly af ter harvest to prevent the growth of destructive fUrigi;,In general, grains will not be completely dried since they arehygroscopic--that is , they absorb moisture . f ro m the air . .J 'hehigher the relat ive humidi ty of tne surrounding a i r , the higherthe moisture content of the grain. Table 1 l i s t s the moisturecontent of various grains as a function of the relat ive humidityof . the surrounding a i r . At the same time, there is a .minimurn

level of relat ive humidity, below which the harmful fungi willnot thrive. Table 2 shows these minimum relat ive humidity levelsfor . common storage fungi. Proper drying lowers the . moisturecontent of grains below the minimum needed for the growth orfungi.

Drying Grains

2

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PREFACE

This paper is one of a series published by Volunteers in Technical Assistance to provide an introduction 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 guid elin es to help

people choose technologies that are suitable to th eir s itu at io nsThey are not intended to provide construction or implementationdeta i ls . People are urged to contact VITA 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 w e r ( ~ writ ten, reviewed, and i l lus t ra tedalmost entirely by VITA Volunteer te chnic al exper ts on a purelyvoluntary basis. Some 500 volunteers were involved in the product ion of the f i r s t 100 t i t l e s issued, contributing approximately5,000 hours of their time. VITA staff 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 VITA Technical Paperis 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 M. 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. H

has also taught on energy conservation and solar technologyLeNormand is Assistant Director a t the Brace Research Ins t i tu teQuebec, 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.

VITA 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 i

situations.. VITA maintains an internat ionalInquiry

Service,special ized documentation center, and a computerized roster ovolunteer technical consultants: manages long-term f ie ld projec ts ; andpubl i shesa variety of technical manuals and papersFor more information about VITA services in general, or thtechnology presented in this paper, contact VITA a t 1815 NortLynn Stree t , Suite 200, Arlington, Virginia 22209 USA.

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Table 1. Moisture Contents of Various Grains and Seeds inEquilibrium with Different Relative Humidities a t25 to 30°Centigrade

- - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - -- - - - - - - - - - -_ . - - - - - - - - - - -Wheat, Rice Sunflower

Humidi ty Corn, Sorghum __ l . F ~ . I . ~ ~ n . t l ___ Soybeans _ l . F ~ L ~ ~ D . t l __l . F ~ . I . ~ ~ n . t l ___ l ~ ~ L ~ ~ n . t l __ ~ ~ ~ g b L - ~ ~ l j ~ b ~ g - - l . F ~ . I . ~ ~ n . t l - - ~ ~ ~ d ~ __ ~ ~ a . t ~

65 12.5 to 13.5 12.5 14.0 11.5 8. 5 5.0

70 13.5 to 14. 5 13.5 15.0 12.5 9. 5 6.0

75 14.5 to 15. 5 14.5 15. 5 13.5 10.5 7.0

80 15. 5 to 16.5 15.0 16.5 16.0 11.5 8.0

85 18.0 to 18.5 16.5 17.5 18.0 13.5

~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ - - - - - - - - ~ - ~ ~ - ~ - ~ Source: h ~ H E h ~ _ n a D ~ ~ Q Q ~ _ g n Q _ ~ . I . ~ d ~ ~ ~ _ D i r ~ ~ . t Q . I . ~ ~ - - l j l l - ~ ~ D ~ g I D ~ D . t gl

(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 T e m p e ~ a t u r e for Growth(26 to 30°Centigrade)

-----------------------------------------------------------------Tyre of r-1inimum Relative Humidity-

_______ r J J n g ~ ~ - - - - - - . . , . - - - - - - - - - . . , . - ~ - . . , . - - - - - - - - - - J ~ ~ . I . ~ ~ n . t l - - - - - - - - - - - -Aspersi l lus halophi l icus 6B

A. res t r i c tus , Sporendonerna 70

A. glaucus 73

A. candidus, A.ochraceus 80

A. flavus 85

Penici l l ium, depending on species 80 to 90

----------------------------------------------------------------Source: h ~ B R A ~ _ B a n ~ ~ ~ Q k _ ~ D ~ _ ~ L ~ ~ ~ t _ D i . I . ~ ~ Q L ~ ~ - - l j l 1 - E ~ n ~ a I D ~ D t g

(New York: American Society of Heating, Refrigerat ing andAir Conditioning Engineers, Inc . , 1980), p. 10.2.

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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. Warm,dry a ir flowing through the dryer car r ies away the moisture thatevaporates from the surfaces of the food.

As dry ing p roceeds, 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 1 shows the representative change inevaporation rate for hygroscopic materials (including .most food-s tuf fs ) commonly dried. During the second phase of the dryingprocess, overheating may occur because of th e less ened coolingeffect result ing from the slower rate of moisture evaporation.I f the temperature i s too high, the food will "case harden" orfon:! a hard shel l that 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 airflows or less heatcollect ion may be desirable .

phase 2

>-...

o

phase 1

lowMoisture Content

Figure 1. Drying Rate Versus Moisture Content

4

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I I I . DESIGN VARIATIONS

SOLAR DRYEN TYPES

Solar dryers fa l l into two broad categories : active 2r.d passive.Passive dryers can be fur ther divided into di rec t and indirectmodels. A di rec t (passive) d ry er is one in which the food isdi rec t ly exposed to the sun's rays. In an indi rec t dryer, thesun 's rays do not s t r ike the food to be dr ied. A small solar

dryer can dry up to 300 pounds of food perm o n t h ~

a large dryercan dry up to 6 ,0 00 pounds a m o n t h ~ and a very large system cand ry a s much as lO,OOC or more pounds a month. (Figures are basedon harvests in t e m p e ~ a t e cl imates .)

Figure 2 shows the breakdown, by type, of solar food dryers.

PASSIVE

SOLAR FOOD DRYERS,f

,

,ACTIVE

! DIRECT

I ii I

V E R Y ~ - - - - - - - - . ~ S M i : LLARGE

!t

I tiD' RECT

I•":RGC

Figure 2. Breakdown of Solar Food Dryers

Passive dryers use only the natural movement of heated a i r . Thetcan be constructed easi ly with inexpensive, local ly a v a i l a b l ~mater ia ls . Direct passive dryers are best used for drying smallbatches of foodstuffs . Indirect dryers vary in size from smallhome dryers to large-scale commercial units .

A c t i v ~ Dryers

Active d ry ers req uire an external means, l ike fans or pumps; formoving the solar energy in the form of heated a ir from the c o l ~: ~ c t o r area to the drying beds. These dryers can be bui l t inalmost any s ize , from very small to very large, but the large'rsystems are the most economical.

5

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Figure 3 is a schematic drawing showing the major components ofan ac tiv e so lar food dryer. Either a ir or l iquid collectors canbe used to col lec t the sun 's energy. The col lec tors £hould facedue south i f you are in the n orth ern hemisphere or d ~ north i fyou are in the southern hemisphere. At or near the equator, theyshould also be adjusted east or west in the morning ~ n af te r -noon, respect ively . The col lectors should be posit ioned a t anappropriate angle to optimize solar energy col lect ion for theplanned months of operation of the dryer. The col lectors can be

adjacent to or somewhat remote from the solar dryer. However,since i t is more d i f f i cu l t to move a ir long dis tances , i t is bestto posit ion the COllectors as near the dryer as possible.

!!

.-----...J11 ~ tlz

exhaust

stack

! ! \- - - - - - - - ~ - - - i food

; dryin

__ ~ f---i' j ~ r a y •

thermal

storage

<=-ni C : : I ~ _ heat exchanger ! !-------1,.., . 1..;1 - - / ~ .

. - - - - - - - - - : - . : ' - - - - - ~ _ t _ H + _ - : : : : : : . _ _ _ 'CS'; l e e : ~ is

a i r

optionala ir , i

in le t

Figure 3. Major Components of an Active Dryer

6

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The solar energy collected can be delivered 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-c lcck dry ing.

In an act ive dryer, the s o l a r - h e a ~ e d 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 ra t io of food

surface area to volume, the quicker will be the evaporationof

moisture 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 ll 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 Dryers

Passive solar food dryers use natural rneans--radiation and

convection--to heat and move the a i r . The category of passivedryers can be subdivided into direc t and indirect types.

Direct Dryers. In a direct dryer, food is exposed direct ly to thesun's rays. This type of dryer typic ally consis ts of a dryingchamber tha t is covered by t ransparent cover made of glass orplast 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 is

placed on a perforated tray that allows the a ir to flow throughi t and the food. Figure 4 shows a drawing of a simple direc tdryer. Solar radiat ion passes through the t ransparent cover and

is 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 effect ." Simply s tated, the short wavelength so la r ra dia tio n can penetrate the t ransparent cover. Onceconverted to low-grade heat , the energy radiates on a long wavelength tha t 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, direc t 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 -- wood,concrete, sh ee t meta l, etc . The dryer should be 2 meters (6.5

feet) long by I meter (3.2 feet) wide and 23 to 30 centimeters(9 to 12 inches) deep. The bottom and sides of the dryershould be insulated, with 5 centimeters (2 inches) recommended.Blackening the inside of the box wil l improve the dryer ef f i -ciency, but be sure to use a non-toxic material and avoid leadbased paints . Wood blackened by f i re may be a safe and inexpensive material to u s e ~

7

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:::esn/s

S L : ~ p

a ir eX;1cL;S'i..I \ holes

o

-------- -----""7"---------------<.._ .dryi:-,g food

: ra rsparent c o v e r - e i t ~ e rhorizontal or s l o ~ e a

insulated

5 i dewa i ! 5

and cottor .

1

~ ~ o : ~ ~ ~ ~ ~ ~ - h ai r i n l e t ~ ..,..:, . . . I ' t ;P(cbove ground .1 V ..v-=- _- - ~ - - : . . - . _ - - - - - - - - - -----.-----

Figure 4. Direct .Dryer

"'-- . \ Short. wavelength\ . so la r r a di a t i on

' \ pene t ra tes cover

' \ "-t r ansparen t \, '"cover \.

\ ,

Figure 5. Greenhouse Effect

8

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C""""'l:-------- P l a s t i c Film Cover

- . - ~ :---_.. _-, .. --_.

- - - - - -_ ..

Figure 6. VITA Solar Crop Dryer

Hole

\

..••.... T r an s p a r e n t

. Gla s s Cover/ ,..,.

\ \

\\

Sunl ight

. . ""'•, r j f 1· \ ) twa t ~ vafor

. B l a c k e n e d - - ~ : : , : : : · , : : · · : ; · : · : · ~ ~ : : ~ ~ = = ~ ~ ~ r = ~ 5 ~ ~Walls =======.====:===.==Z===-::/===Z:::/=::··. = = ! ~ i : : : e : : : : . . ,

I n su l a t ed S ide s I Food tol D ~ i n g Tray

and Bot tom be dr ied

F i g u r ~ 7. Brace Research I n s t i t u t e ' s Bot Box Dryer

9

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The t ray t ha t holds the food must permit a i r to enter from belowand pass through to the f00d. A wire or plas t ic mesh or screenwil l do nicely . Use the coarses t possible mesh that 'Hi l l support~ h foed without l e t t ing i t fa l l through the holes. The larger:he holes in the mesh, the eas ier the a ir wil l ci rcu la te throughto the food. Air holes below the t ray or mesh wkll bring inouts ide a i r , which wil l carry away the mois tu re evapora ted fromthe food. As the a ir heats up in the dryer, i t s volume wil lincrease, so ei ther more or larger holes will be required a t thetop of the box to maintain maximum a ir flow.

Final ly , t e s t s : the hot box dryer shown in Figure 7 have de-termined tha t t temperature within the dryer C3n be as much as40°Centigrade (_J4°Fahr enhe it ) h ighe r than the outside ambient( s u r ~ o u n d i n g ) t e m p e r a t u r e .Indi rec t Dryers. An ind i rec t dryer is one in which the s u 8 raysdo not s t r ike the food to be dr ied. In th is system, drying isachieved indirect ly by using an a ir co llec to r th at c h a n ~ e l s hota i r in to a separate drying chamber. Within the chamber, the foodi s placed on mesh t rays tha t are stacked ver t ica l ly so tha t the

a ir flows through each one. Figure 8 shows an ind i rec t passivedryer . The so la r c o l ~ e c t o r can be of any s ize and should bet i l ted towa rd the sun to optimize col lec t ion . By increasing theco llec to r s ize , more heat energy can be added to the a ir toimprove overa l l ef f ic iency . Larger collector areas are helpful inplaces with l i t t l e so lar energy, cool or cold cl imates , andhumid regions. Section V of th is paper indicates cl imatlc condi-t ions where l ~ ~ g e r collector areas might be more effec t ive .

Til t ing the col lec tors is more ef fec t ive than placiny them hori -zonta l ly , for two reasons. Fi rs t , more solar energy can be col-lected when the col lec tor surface is more nearly perpendicular tothe sun 's rays. Second, by t i l t ing the col lec tors , the warmer,l e s s dense a ir r i ses natural ly into the drying chamber. Thedrying chamber should be placed on support legs, but i t shouldnot be ra ised so high above the ground tha t i t becomes d i f f i cu l tto work with.

The base of the col lec tor S Jld be vented to allow the entranceof a i r to be heated for d: :ng. The vents should be evenlyspaced across the fu l l width of the base of the col lec tor toprevent local ized a re as w ith in the col lec tor from overheating.The vents should also be adjus table so tha t the a ir flow can be

matched with the o pe ra tin g conditions and/or needs. Solar radia-t ion, ambient a ir temperature, humidity l eve l , drying chambertemperature, and moisture level of the food being dried must a l lbe considered w h ~ n regulat ing the flow of a i r .

10

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a i r exhaust

B

sunl isht

cry:ng

t rays

./

\\ : raf"sca ref"!:

co·,er

a ir freer.

\ .

Figure 8. Indirect Dryer

11

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:.f"

! ..

• f ru i t s• ve'getables

• f i sh

used throughout the world to dry food produ9tlist' complete ly . Lis ted below a r e a . few "

to show the dive r s i ty to which the sun ' s

• gra ins

• meat• s a l t

Solar eher-y i stoo numer<:...iS to

sen ta t ive i temsi s put to use.

SOLAR DRYER APPLICATIONS

As the warm a i r f lows through severa l l ayers of food.on t r ays , i'.becomes more moist . This moist a i r i s vented out. through ~ a chimney.. The chimney increases the amount of a i r flowing, t h t ' ougthe dryer by speeding up the flow of the exhaust a i r •.. -F igqre : · 8shows a sola r 'chimney with p la s t i c f ilm on the south-facing side"As _the warm, moist a i r flows through the __ sola r c h i m n e y / t 9 ~adclit ional so l a r ene rgy enter ing the <:himney warms t h e e ~ c a p i . n a i r fu r the r . This added heat makes the a i r l e s s dense a n d c ~ u s ¢ it to flow up t .hrough, and out of , the solar, c h i m n e y a t a . f a s t r a te , thereby ' br ing ing in more f resh a ir in to the co l l ec to r . ':

EQulpMENT!MATEIUALS NEEDED

12

The glaz ing mater ia l s used to cover d i rec t dryers or as,pla te s on _ the col lec tor por t ion of ind i rec t dryers can bet ransparen t or t r ans lucen t mater ia l . Glass i s , probablyj :he ,known mater ial ' , but itis cost ly and breaks eas i ly .

Rigid p la s t i c mater ia l s a re equal to glass for so la r t ransmiss i ,o

and can be much more durable agains t breakage. Fiberglass t.ein,·'

forced polyes te r , ac ry l i c s , and polycarbonates wi l l not. ' b r e ~ ,

e a ' s i l y i n normal use and, depending on the mater ia l , - may. cosl e s s , ranging from USSl1 to USS32 per .square meter (USSl to USS. per sq·uare foo t ) . However, these mater ia l s tend •.... to d e ' g t a dsomewhat with time, allow ing l e s s sun ligh t to pass t h r c ~ g h ' t h e r n

-Their useful l i f e i s es t imated to be about 10 yea t s • Acrylic:sa 'npolycarbonates may bemor.e e 'xpensive than glass • . . Manyof": th'emater ia ls a re a lso d i f f i cu l t to f ind in developing c o u n t r i ~ s 'may need to be .impor t ed .

The top of the col lec tor should be completely open to the bottom, of the drying chamber. Once ins ide th e dry ing chamber , thewa rmeda·ir w' - - l f l owup through the stacked food t r ays . The drying t raysmust t sn !.lgly in to the chamber so tha t the dry ing a i r i s forcedthro\ . ' the mesh and food. Trays t ha t do not f i t pr(",perly wil l

crea\ - ; a p ~ around the edges, causing large volumespf warm a i rto bypass the food, and preventing the dryer from re-movi1'}g m 0 i s ~ - ture evaporated from the food.

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Thin plas t ic films are inexpensive and have good t r a n s m i s s i v i t (the abi l i ty 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 mete(USS.05 per square foot) and l a s t less 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 costhree . to five times as much. Tedlar and tef lon fi lms have longuseful l ives (10 years or more), excel len t t ransmiss iv ity (allowing 9 ; percent or more of the solar energy to pass through) ancost in the range of US$4 to US$8 per square meter ( U S $ ~ 4 0 toU5$.70 per square foot) . These films are probably the beschoice i f they can be protected from puncturing.

SKILLS NEEDED TO BUILD, OPERATE, AND MAINTAIN

Building a solar food dryer requires some carpentry skil ls . . M a ~ te r ing the technique of drying comes from direc t experience wi t.d ry ing p roducts rather than from reading about i t . Maintaining

solar food dryer requires only that an operator monitor the partperiodical ly for wear and tear . For example, an operator sh6ulQmake sure tha t the legs that support the drying c h a ~ b e r are neloose, and that vents are not blocked. Plast ic glazing materiashould be checked to see i f i t turns cloudy, which will causl ess sunl ight to pa.ss through i t .

COST/ECONOMICS

Cost comparisons between indirect and direc t dryers are presentein Tab:'e 3. Dryers 1, 2, 3, and 4 are indirect dryers , andryers Sand 6 are di rec t dryers . The table shows the cost peunit ; more important, i t compares the cost per drying t ray a l1the tray area fo r each dryer. Table 4 gives some values o

vitamin C retention for two products dried by indirect , direc tand open a i r drying. Overall , · i t appears th at in dire ct dryer:s armore ef f i c i en t and have higher vitamin retention than direcdryers .

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Table 3. Cost Comparisons

Indirect dryer 1.12 65.00 58.04

Indi rec t dryer 1.49 90.00 60.40 .

Indi rec t dryer 1.30 75.00 57.69

Indi rec t dryer 3.16 115.00 3.6.39

Indi rec t dryer 2.88 175.00 60.76

Indirect dryer 1.21 50.00 41.32

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - _ . _ - - - - - - ~ - - - - - - - -

- - - - - - - - - - - - - - - - - ~ - - - ~ - - - - - ~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ '

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Table 4. Vitamin C Retention

14

Source: American Solar Energy Society, Inc . , . f L . Q . 9 . r . e . s ~ _ . i D J . a . s . s . i y . e. . s ~ l . a . I J n . e . r ~ ~ . s . t . e I D . s (Boulder, Colorado: American SolarEnergy Society, Inc . , 1983), p. 682.

Indirect Cantaloupe 70.4

Indirect Cantaloupe 51.0

Direct Cantaloupe 53.6

Open sun Cantaloupe 39. 5

Indi rec t Spinach 35.9

Direct Spinach 22.4

Type of Type of Percen tage ofD . . FA V'+- . r [) t' A_ . r ~ ~ . r _ - - - - - - - - - - - - - - - - - ~ - - - . Q . Q ~ - - - - - - - - - - - - . l . J . r . a I D . l n ~ _ ~ ~ - .. a J . n ~ . ! o J , - _

Source: American Solar Energy Society, Inc . , J ? . r ~ i J . r ~ . s . s _ . i n _ J ? . a . s . s . i y . e. . s . Q l . a J : _ . E n ~ J : ~ - ~ . . s . t . e m . s (Boulde r, Colorado: American SolarEner gy Soc i e ty , Inc . , 1 983 ), p.682.

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IV . COMPARING THE ALTERNATIVES

FOSSIL-FUEL DRYBRS VERSUS SOLAR DRYERS

Conventionally fueled dryers are the primary a l t e r n a t i \ ; ( ~ 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 ir to achieve the desired temperatureAlthough these drying systems are used around the world with n

apparent problems, there i s the possibi 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 ~ n d , in any kind of weather. Unlike solar dryer$conventional dryers are not subject to daily and seasonal var iat ions and other cl imatological factors . On the other hand, thfuels burned in conventiona l dryers may present other problem

Use of wood may c on trib ute to problems of deforest i \ t ion;coal mcause pollut ion. Fossil fuels are becoming increasingly expensive and are not always avai lable .

ADVANTAGES OF SOLAR DRYERS

Solar dryers have the principal advantage of using solar energya fre e, a va ila ble , and l imi t less energy source tha t is also nopol! uting. Dry ing most foods in sunny areas should not be ..

problem. ~ 1 o s t vegetables, for example, can be dried in 2-1/2.4 hours, a t temperatures ranging from 43 to 63°Centigrade (lJ

t o l45°Fahrenheit ). Frui ts take longer, from 4to6 hours,temperatures ranging from 43 to 66°Centigrade (110 to 150°Fahrehei t) . At th i s ra te , i t i s possible to dry two batches of fooon a sunny day.

A solar food dryer improves upon the t rad i t ional open-air systemin f ive important ways:

1. I t i s fas te r . Foods can be dried in a shorter amounttime. Solar food dryers enhance dry ing times in two wayFirs t , the t ranslucent or t ransparent glazing over th

col lect ion area t raps heat inside the dryer, r.aising. ttemperature of the a i r . Second, the capabil i ty of enlarginthe so la r c ol le cti on area allows for the concentrationthe 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 is way, a larger percentage of food wil l beavail.able- for human consumption. Also, less o f ~ : h e 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 ikelihood of the growth

of toxic fungi.

4. I t i s healthier . Drying foods a t optimum temperatures andin a shorter amount of time enables them to retainmore of their nutr i t iona l value--especia l ly vitamin C. Anextra bonus is that foods wil l look and t as te bet te r , whichenhances the i 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 ignif icant cost savings. Solar drying lowersthe costs of drying, improves the quali ty of products, andreduces losse.s due to sp oilage .

DISADVANTAGES OF SOLAR DRYERS

Solar 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. As temperatures r ise (determined with a thermometer or by experience), the

lower vents must be opened to allow greater airflow through thedryer and to keep the temperatures down. Rice, for example, willcrack a t temperatures above 5 0 ° C e n t i g r a d e ~ seed grains can bedried a t temperatures no higher than 40 to 45°Centigrade.

v. CHOOSING THE TECHNOLOGY RIGHT FOR YOU

Four important questions must be answered before one decides tobuild a solar food dryer. The br ie f discussion following eachquest ion points out many factors tha t must be considered pr ior to

the constructionof

a solarfood

dryer. The quest ions are :1. What food wil l the dryer be used for? Also, what quant i t ies

of food wil l be dried?

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

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FRUITS AND

VEGETABLES

i,MODERATE TIME

TO DRYING

,r l--------- TYPE 0FFOO OS - - - - - ~GRAINS

"HORT TIMETO DRY ING ------------1

I

ji VITAMIN LOSS FROM

SUN EXPOSURE

IN-FIELD OR NEAR !P R E P ~ R A T I O ~ ~ R E A I

I I LO'"..,' HIGH

p ; ~ ; b , , , ..." ' C ' ~ - - - - - - j J ~ c , ".JRECTi J DRYEKS ORYEP,SI PORTABLE I

I DRYERS ~ I ! .I 1------__ SMALL ,Q U A N T I T Y ~

jLARGE

l- .-....;.. ~ ~ _ ~ : _ M A _ ~ _ n _ - _ - ~ - _ - ~ - ~ - _ - _ - _ ~ - ~ - _ - _ - _ - _ - _ - _ - _ - _ - _ - _ - _ ~ . Q _ U _ J I A t T I T ~

Figure 9.· Flow Diagram of Factors to Consider

in Selecting a Solar 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 ensurethe i 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 that must be dried immediately af te r theyare harvested may require the use of portable dryers, which

can be se t up in the harvest f ie ld as needed. Permanentdryers can be erected near preparation areas for f ru i t s andvegetables or centra l ly located for grain crops.

Some foods may lose much of the i r nutr i t ional value, orbecome discolored, i f dried a t too high a temperature or i fexposed to the d i rect rays of the sun.· Using indirectdryers can minimize the loss of vitamins, especially vitaminc.

Finally, 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 (and

drying) season?

Climatic conditio ns ( so la r radiat ion, ra infa l l , temperature,humidi ty , wind, e tc . ) should be considered in determiningwhat kind of dryer is best su ited for a part icular applicat ion.

Figure 10 wil l help you to visualize the factors tha t mustbe considered here. I f the occurrence of sunshine is low-say, 50 percent or less-- then i t may be wise to add anauxil iary heat source to enable drying to continue on cloudydays or even through the night . Dry climates with hot ormoderate temperatures are well suited for solar food dryers.

Cold climates or humid climates pose the problem of makingi t more d i f f i cu l t to obtain the necessary quanti ty of warm,dry a ir to dry foods effect ively before spoilage can occur.

Such weather condit ions may l imi t the use of di rec t dryersto preserving only small quant i t ies of food tha t must bedried in a shor t time (one or two days) . Indirect dryershave the advantage over direc t dryers in that they arecapable of concentrat ing solar energy. Enlarging the . collec tor area and varying the airflow through the collectorenable ind i rec t dryers to achieve near optimum conditions inmost cl imates .

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ADD AUXILIARY

HEAT SOURCE

CLIMATIC CONDITIONS

SOLAR AVAILABILITY

% SUNSHINE

rr---!-----lHIGH LO'''':

JT ~ , J R A T E Jo 1i ~ ~ ~ rSDRY rlUM1D DRY HUMID DRY HUMID

j l ~ ~ ! D D COLLEC!DR A R ~ Al - - - ~ iDIRECT INDIRECT

!MALL LARGE

Figure 10. Flow Diagram 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 this quest ion determines the dryG?ss requiredof the finished product. Rice a t harvest :.iic;h: typicallycontain 24 percent moisture. I f i t is sold q u i c ~ l y , say, formilling, i t i s f ine as is . I f , on the other hand, it is tobe stored for any length of time, i t must be dried to only12 to 14 percent moisture content. Thus, the dryness re-quired will determine how 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 entireharvest .

. What materials are avai lable to construct the dryer? Are them a t ~ r i a l s avai lable local ly?

Masonry may 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 ~ n i t s portable. The avai labi l i ty of materials maygovern, in par t , the placement of the food dryers.

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TECHNICAL ASSISTANCE ORGANIZATIONS

Brace ~ ~ ~ . : ~ ~ :ch I ~ s t i t ~ t au , . . . r j , : . . ' ,"; " "-' ,,-, s ;: ~ . - . l - ' , 1··.. L,;c.n-..: .• ..., ~ _ . I I . t - ' \ J 0 . . , ,-_,-,_.•

S;:e. l>r.E ;12 P e l . l . : : - , , ' L ~ e 800

di rec t ..

Q u e b ~ c ; \ ~ a n d d aThe Ins t i tu te has designed d i r e c ~ and indirect dryers andhas plans avai lab le .

New Mexico Solar Energy Association (NMSEA)P.O. Box 2004Santa Fe, New Mexico 87501 USA

NMS&A publishes detai led construct ion plans for a so lar c r o ~dryer.

Volun teers in Technical Assistance (VITA)1815 North Lynn Stree t , Suite 200Arlington, Virginia 22209 USA

VI TA I s ~ . Q l . a I _ . c L . Q . t L D I : t . e L man ual incl udes pI an s for aand an indirec t solar dryer.

. ~ ,

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BIB LIOGRAPHY

American Society of Heating, f<efr igerat ing, and Air··C.')nditioninEn gin ee r s. h . s J j . B ~ . t : _ . B . a n . Q . Q . Q . Q . K - _ ~ n . Q _ . P . r . Q . o J J ~ t _ ~ . i . r ~ ~ _ t _ Q . i : · : : -~ _ _.J..2]l_f'JJD~ ~ m ~ ~ t . a l ~ . New York, New York: American Society cE HeatingRefrlgera t ing, and Air-Conditioning Engineers.

Andrea, A. Louise. J d . e . b ; t . Q . . t : . a . t . i D . 9 _ f . Q . Q . d ~ . Boston, Massachusetts: Th

Cornhill Company, 1920.

Archuleta, R.; Berkey, J . ; and Williams, B. "Research on SolaFood Drying a t the University of California , Santa Cruz.

g . . t : . Q 9 . . t : . e ~ ~__.iD__ g . a ~ ~ . i Y . e _ ~ . Q l . a . . t : _ ] n ~ . t : . 9 ; t _ ~ ~ . t ~ I D ~ . Edited by J

Hayes and D. Andrejko. Boulder, Colorado: American SolaEnergy Society, Inc . , 1983, pp. 679-682.

DeLong, D. H . Q j ~ L . t . Q _ . Q . r y _ f . Q . Q . d . . s . Tucson, Arizona: H.P. Books, 1979160 pp.

Exell , R.B.B. "A Simple Solar Rice Dryer: Basic Design Theory.~ J J n ~ . Q . . t : l . d 4 (1980): 188.

Ginsburg, A.S., ed. l 1 . r ~ . i n _ . Q . r y . i n . 9 _ . a n . d _ . G . r ~ . i n _ . Q . . t : y ~ . . t : . s . WashingtonD.C.: The Israel Program for Scient i f ic Translat ions, 196U

Gregoire, R.G.; Slajda, Robert; and Winne, Mark. "A CommerciaScale Solar Food Dryer." Edited by B.H. Glenn and G.EFranta. . P . I . Q ~ . e ~ . Q . i D . 9 § _ . Q . f _ l ~ ' § l _ h D D J J . a l _ . M ~ . e . t . i n . g .Boulder, Coloado: American Solar Energy Society, Inc . , 1981.

Lindblad, C., and Druben, L. "Freparing Grain for S to rage ." Vo l

of ~ I D ~ l l _ f . a I I D _ § . . t : . a . i D _ ~ . t . Q . . t : . a . 9 ~ . Frepared for ACTION/Peace Corpand VITA. Manual No. 35E. Arlington, Virginia: VITA, 1977

McGill Universi ty. Brace Research Ins t i tu te . ~ _ ~ J J . t : Y ~ Y _ . Q . f _ ~ . Q l , g h - 9 . r . i ~ J J 1 . . t J J . r . a l _ . Q . r y ~ . r . s . Technical Report T99. Quebec, .CanadBrace Research Ins t i tu te , McGill Universi ty, 1975.

Ong, K. S. "Sol a r Dry ing Technology for Rur a1 Development. II Pa pepresented a t the Regiona l Confe rence on Technology for RcraDevelopment, Kuala Lumpur, Malaysia, 1978.

van Brakel, J . "Opinions About Select ion and Design of.Dryers.Edited by A.S. Mujumdar. 1 ' J . : . Q ~ ~ ~ g . i n - 9 ~ _ . Q . f _ . f ' . i . r ~ . t _ . I n . t ~ . ! n , g . t . i. s y l D . P . Q . s . i J J I r L . Q D _ ~ . r : i . i D g . Princeton, New Jersey: Science Pres1378.

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TECHNICAL ASSISTANCE ORGANIZATIONS

Brace Research Ins t i tu teMcDonald Campus of McGill UniversitySte . Anne de Bellavue 800Quebec, Canada

The Ins t i tu te has designed di rec t and indirect dryers andhas plans avai lab le .

New Mexico Solar Energy Association (NMSEA)P.O. Box 2004Santa Fe, New Mexico 87501 USA

NMSEA pUblishes detai led construct ion plans for a solar cropdryer.

Volunteers in Technical Assistance (VITA)1815 North Lynn Stree t , Suite 200Arlington, Virginia 22209 USA

VITAls ~ ~ l ~ ~ _ ~ ~ ~ ~ _ Q L ~ ~ ~ manual includes plans for aand an indirect solar dryer.

di rec t

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BIB LIOGRAPHY

American Society of 1eating , R efrigera ting , and Air-ConditioniEng in ee r s. h . . s . E . B h ~ _ B . a n . d 1 J . 9 . 9 ~ _ . a n . d _ . . p J : . 9 g . . l J ~ . t _ . Q . i I . e ~ . t . 9 I : l . ; ._ j ~ ] l _ f J J Q g I D . e n . t . a l ~ . New York, New York: American Society of HeatinRefrigerating, and Air-Conditioning Engineers.

Andrea, A. Louise. D ~ b ~ . d I . a . t . i n ~ f . 9 ~ g ~ . Boston, Massachusetts: TCornhill Company, 1920.

Archuleta, R.; Berkey, J.; and Williams, B. "Research on SolFood Drying a t the University of California , Santa Cruz~ I ~ 9 I ~ ~ ~ __ .in__ . . P ~ ~ ~ . i Y . e _ . . s . 9 1 . a J : _ £ D . e I g y _ ~ ~ . t . e I D ~ . Edited byReyes and D. Andrejko. Boulder, Colorado: American SolEnergy Society, Inc . , 19B3, pp. 6 ~ 9 - 6 8 2 .

DeLong, D• . H . Q J t L . t . Q _ . Q . . t : ~ _ f . Q . 9 . d ~ . Tucson, Arizona: H.P. Books; 197160 pp.

Exe11, R.H.B. "A Simple Solar Rice Dryer: Basic Design Theory~ . . l J n ~ . 9 J : l . d 4 (1980): 188.

Ginsburg, A.S., ed. ~ J : . a . i n _ D J : ~ . i n ~ . a n ~ _ ~ . r a . i n _ . Q . r ~ . e . I . s . WashingtoD.C.: The Israel Program for Scient i f ic Transla t ions , 196

Gregoire, R.G.; Slajda, Robert; and Winne, Marl<. "ACommercScale Solar Food Dryer." Edited by B.H. Glenn and GFranta. . F . r . Q . Q . e . e £ J . i D g . s _ . Q . f _ j . . 9 ~ j _ h n n J J . a l _ E ! . e . e . t . i D . 9 .Boulder, Colado: Am 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 ~ . I D . a l l _ f . a I . I r L . s ; I ~ . i n _ . . s . t . 9 I . a . 9 . e . Prepared for ACTION/Peace Coand VITA. Manual No. 35E. Arlington, Virginia: VITA, 19

McGill Universi ty. Brace Resea rch Ins t i tu te . h_..5JJI.Y.e:l_.9.f_..sQh 9 I . i ~ J J 1 . t J J I . a l _ . Q I : l . e I ~ . Technical 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. Mujumdar. . I ' I . Q . s ; . e . e g . i D 9 ~ _ . 9 . f _ 1 ' . i J : ~ . t _ . I n . t . e I D . a . t . i~ m . p . Q ~ j , J J n L . 9 D _ ~ J : : i . i n . 9 ' Pr inceton, New Jersey: Science Pre1978.