Alive Foods

.You can either buy a dehydrator (we stock all the best makes) or learn how to make one yourself:

UNDERSTANDING SOLAR FOOD DRYERS

Roger G. Gregoire, P.E.

Technical Reviewers

Gary M. Flomenhoft

Jacques L. LeNormand

Published By

VOLUNTEERS IN TECHNICAL ASSISTANCE

1600 Wilson Boulevard, Suite 710, Arlington, Virginia 22209 USA

Telephone: (703) 276-1800 Fax: (703) 243-1865

E-Mail: vita@vita.org

Undestanding Solar Food Dryers

ISBN: 0-86619-215-8

[C] 1984, Volunteers in Technical Assistance

PREFACE

This paper is one of a series published by Volunteers in Technical

Assistance to provide an introduction to specific state-of-the-art

technologies of interest to people in developing countries.

The papers are intended to be used as guidelines to help

people choose technologies that are suitable to their situations.

They are not intended to provide construction or implementation

details. People are urged to contact VITA or a similar organization

for further information and technical assistance if they

find that a particular technology seems to meet their needs.

The papers in the series were written, reviewed, and illustrated

almost entirely by VITA Volunteer technical experts on a purely

voluntary basis. Some 500 volunteers were involved in the production

of the first 100 titles issued, contributing approximately

5,000 hours of their time. VITA staff included Leslie Gottschalk

and Maria Giannuzzi as editors, Julie Berman handling typesetting

and layout, and Margaret Crouch as project manager.

Roger G. Gregoire, P.E., the author of this VITA Technical Paper,

is a consultant in the areas of energy management engineering,

solar design and analysis, energy audits, energy management of

buildings, and alternative energy systems. He has published on

energy conservation, solar greenhouses and solar water heaters as

well as solar food dryers. Reviewers Gary M. Flomenhoft and

Jacques L. LeNormand are also experts in the area of solar food

dryers. Flomenhoft is a consultant in renewable energy and engineering

for the San Diego Center for Appropriate Technology. He

has also taught on energy conservation and solar technology.

LeNormand is Assistant Director at the Brace Research Institute,

Quebec, Canada, which does research in renewable energy. He has

supervised work with solar collectors, has trained people from

overseas in solar technologies, and has published widely on solar

and wind energy, and conservation.

VITA is a private, nonprofit organization that supports people

working on technical problems in developing countries. VITA offers

information and assistance aimed at helping individuals and

groups to select and implement technologies appropriate to their

situations. VITA maintains an international Inquiry Service, a

specialized documentation center, and a computerized roster of

volunteer technical consultants; manages long-term field projects;

and publishes a variety of technical manuals and papers.

UNDERSTANDING SOLAR FOOD DRYERS

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

I. INTRODUCTION

Dehydration, or drying, is a simple, low-cost way to preserve

food that might otherwise spoil. Drying removes water and thus

prevents fermentation or the growth of molds. It also slows the

chemical changes that take place naturally in foods, as when

fruit ripens. Surplus grain, vegetables, and fruit preserved by

drying can be stored for future use.

People have been drying food for thousands of years by placing

the food on mats in the sun. This simple method, however, allows

the food to be contaminated by dust, airborne molds and fungi,

insects, rodents, and other animals. Furthermore, open air drying

is often not possible in humid climates.

Solar food dryers represent a major improvement upon this ancient

method of dehydrating foods. Although solar dryers involve an

initial expense, they produce better looking, better tasting, and

more nutritious foods, enhancing both their food value and their

marketability. They also are faster, safer, and more efficient

than traditional sun drying techniques. An enclosed cabinet-style

solar dryer can produce high quality, dried foodstuffs in humid

climates as well as arid climates. It can also reduce the problem

of contamination. Drying is completed more quickly, so there is

less chance of spoilage. Fruits maintain a higher vitamin C

content. Because many solar dryers have no additional fuel cost,

this method of preserving food also conserves non-renewable

sources of energy.

In recent years, attempts have been made to develop solar dryers

that can be used in agricultural activities in developing countries.

Many of the dryers used for dehydrating foods are relatively

low-cost compared to systems used in developed countries.

This paper describes some of these dryers and discusses the

factors that must be considered in determining what kind of dryer

is best suited for a particular application.

THE DRYING PROCESS

Drying products makes them more stable and in the case of foods,

a llows them to be stored safely for long periods of time. Safe

storage requires protection from the growth of molds and other

fungi, the most difficult of the spoilage mechanisms to detect

and control. The types of loss generally caused by fungi are:

* Reduction in the germination rate of seed.

* Discoloration, which reduces value of foods for many purposes.

* Development of mustiness or other undesirable odors or

flavors.

* Chemical changes that render food undesirable or unfit

for processing.

* Production of toxic products, known as mycotoxins, some

of which can be harmful if consumed.

* Total spoilage and heating, which sometimes may continue

to the point of spontaneous combustion.

Drying Grains

At harvest, most grains contain more moisture than is safe for

prolonged storage, because many fungi grow rapidly in warm, moist

conditions. Thus, any grain stored for future use must be dried

shortly after harvest to prevent the growth of destructive fungi.

In general, grains will not be completely dried since they are

hygroscopic--that is, they absorb moisture from the air. The

higher the relative humidity of the surrounding air, the higher

the moisture content of the grain. Table 1 lists the moisture

content of various grains as a function of the relative humidity

of the surrounding air. At the same time, there is a minimum

level of relative humidity, below which the harmful fungi will

not thrive. Table 2 shows these minimum relative humidity levels

for common storage fungi. Proper drying lowers the moisture

content of grains below the minimum needed for the growth- of

fungi.

Table 1. Moisture Contents of Various Grains and Seeds in

Equilibrium with Different Relative Humidities at

25 to 30 [degrees] Centigrade

Wheat, Rice Sunflower

Humidity Corn, Sorghum (Percent) Soybeans (Percent)

(Percent) (Percent) Rough Polished (Percent) Seeds Meats

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 9.0

Source: ASHRAE Handbook and Product Director: 1977 Fundamentals

(New York: American Society of Heating, Refrigerating and

Air Conditioning Engineers, Inc., 1980), p. 10.2.

Table 2. Minimum Relative Humidity for the Growth of Common

Storage Fungi at Their Optimum Temperature for Growth

(26 to 30 [degrees] Centigrade)

Type of Minimum Relative Humidity

Fungus (Percent)

Aspergillus halophilicus 68

A. restrictus, Sporendonema 70

A. glaucus 73

A. candidus, A.ochraceus 80

A. flavus 85

Penicillium, depending on species 80 to 90

Source: ASHRAE Handbook and Product Directory: 1977 Fundamentals

(New York: American Society of Heating, Refrigerating and

Air Conditioning Engineers, Inc., 1980), p. 10.2.

Solar dryers use the energy of the sun to heat the air that flows

over the food in the dryer. As air is heated, its relative

humidity decreases and it is able to hold more moisture. Warm,

dry air flowing through the dryer carries away the moisture that

evaporates from the surfaces of the food.

As drying proceeds, the actual amount of moisture evaporated per

unit of time decreases. In the first phase of drying, the moisture

in the exterior surfaces of the food is evaporated. Then,

once the outer layer is dried, moisture from the innermost portion

of the material must travel to the surface in the second

phase of drying. Figure 1 shows the representative change in

evaporation rate for hygroscopic materials (including most foodstuffs)

commonly dried. During the second phase of the drying

process, overheating may occur because of the lessened cooling

effect resulting from the slower rate of moisture evaporation.

If the temperature is too high, the food will "case harden" or

form a hard shell that traps moisture inside. This can cause

deterioration of the food. To prevent overheating during this

portion of the drying cycle, increased airflows or less heat

collection may be desirable.

III. DESIGN VARIATIONS

SOLAR DRYER TYPES

Solar dryers fall into two broad categories: active and passive.

Passive dryers can be further divided into direct and indirect

models. A direct (passive) dryer is one in which the food is

directly exposed to the sun's rays. In an indirect dryer, the

sun's rays do not strike the food to be dried. A small solar

dryer can dry up to 300 pounds of food per month; a large dryer

can dry up to 6,000 pounds a month; and a very large system can

dry as much as 10,000 or more pounds a month. (Figures are based

on harvests in temperate climates.)

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

Passive dryers use only the natural movement of heated air. They

can be constructed easily with inexpensive, locally available

materials. Direct passive dryers are best used for drying small

batches of foodstuffs. Indirect dryers vary in size from small

home dryers to large-scale commercial units.

Active Dryers

Active dryers require an external means, like fans or pumps, for

moving the solar energy in the form of heated air from the collector

area to the drying beds. These dryers can be built in

almost any size, from very small to very large, but the larger

systems are the most economical.

Figure 3 is a schematic drawing showing the major components of

an active solar food dryer. Either air or liquid collectors can

be used to collect the sun's energy. The collectors should face

due south if you are in the northern hemisphere or due north if

you are in the southern hemisphere. At or near the equator, they

should also be adjusted east or west in the morning and afternoon,

respectively. The collectors should be positioned at an

appropriate angle to optimize solar energy collection for the

planned months of operation of the dryer. The collectors can be

adjacent to or somewhat remote from the solar dryer. However,

since it is more difficult to move air long distances, it is best

to position the collectors as near the dryer as possible.

The solar energy collected can be delivered as heat immediately

to the dryer air stream, or it can be stored for later use.

Storage systems are bulky and costly but are helpful in areas

where the percentage of sunshine is low and a guaranteed energy

source is required; or in carrying out round-the-clock drying.

In an active dryer, the solar-heated air flows through the solar

drying chamber in such a manner as to contact as much surface

area of the food as possible. The larger the ratio of food

surface area to volume, the quicker will be the evaporation of

moisture from the food. Thinly sliced foods are placed on drying

racks or on trays made of a screen or other material that allows

drying air to flow to all sides of the food. For grain products,

pipes with many holes are placed at the bottom of the drying bin

with grain piled on top. The heated air flows through the pipes

and is released upward to flow through the grain--carrying away

moisture as it flows.

Passive Dryers

Passive solar food dryers use natural means--radiation and

convection--to heat and move the air. The category of passive

dryers can be subdivided into direct and indirect types.

Direct Dryers. In a direct dryer, food is exposed directly to the

sun's rays. This type of dryer typically consists of a drying

chamber that is covered by transparent cover made of glass or

plastic. The drying chamber is a shallow, insulated box with

holes in it to allow air to enter and leave the box. The food is

placed on a perforated tray that allows the air to flow through

it and the food. Figure 4 shows a drawing of a simple direct

dryer. Solar radiation passes through the transparent cover and

is converted to low-grade heat when it strikes an opaque wall.

This low-grade heat is then trapped inside the box in what is

known as the "greenhouse effect." Simply stated, the short wavelength

solar radiation can penetrate the transparent cover. Once

converted to low-grade heat, the energy radiates on a long wavelength

that cannot pass back through the cover. Figure 5 shows

the greenhouse effect in a simplified schematic drawing.

Figures 6 and 7 show examples of simple, direct dryers that can

be used to dry small quantities of a wide variety of foods. The

drying chamber can be constructed of almost any material-- wood,

concrete, sheet metal, etc. The dryer should be 2 meters (6.5

feet) long by 1 meter (3.2 feet) wide and 23 to 30 centimeters

(9 to 12 inches) deep. The bottom and sides of the dryer

should be insulated, with 5 centimeters (2 inches) recommended.

Blackening the inside of the box will improve the dryer efficiency,

but be sure to use a non-toxic material and avoid lead-based

paints. Wood blackened by fire may be a safe and inexpensive

material to use.

The tray that holds the food must permit air to enter from below

and pass through to the food. A wire or plastic mesh or screen

will do nicely. Use the coarsest possible mesh that will support

the food without letting it fall through the holes. The larger

the holes in the mesh, the easier the air will circulate through

to the food. Air holes below the tray or mesh will bring in

outside air, which will carry away the moisture evaporated from

the food. As the air heats up in the dryer, its volume will

increase, so either more or larger holes will be required at the

top of the box to maintain maximum air flow.

Finally, tests of the hot box dryer shown in Figure 7 have determined

that the temperature within the dryer can be as much as

40 [degrees] Centigrade (104 [degrees] Fahrenheit) higher than the outside ambient

(surrounding) temperature.

Indirect Dryers. An indirect dryer is one in which the sun's rays

do not strike the food to be dried. In this system, drying is

achieved indirectly by using an air collector that channels hot

air into a separate drying chamber. Within the chamber, the food

is placed on mesh trays that are stacked vertically so that the

air flows through each one. Figure 8 shows an indirect passive

dryer. The solar collector can be of any size and should be

tilted toward the sun to optimize collection. By increasing the

collector size, more heat energy can be added to the air to

improve overall efficiency. Larger collector areas are helpful in

places with little solar energy, cool or cold climates, and

humid regions. Section V of this paper indicates climatic conditions

where larger collector areas might be more effective.

Tilting the collectors is more effective than placing them horizontally,

for two reasons. First, more solar energy can be collected

when the collector surface is more nearly perpendicular to

the sun's rays. Second, by tilting the collectors, the warmer,

less dense air rises naturally into the drying chamber. The

drying chamber should be placed on support legs, but it should

not be raised so high above the ground that it becomes difficult

to work with.

The base of the collector should be vented to allow the entrance

of air to be heated for drying. The vents should be evenly

spaced across the full width of the base of the collector to

prevent localized areas within the collector from overheating.

The vents should also be adjustable so that the air flow can be

matched with the operating conditions and/or needs. Solar radiation,

ambient air temperature, humidity level, drying chamber

temperature, and moisture level of the food being dried must all

be considered when regulating the flow of air.

The top of the collector should be completely open to the bottom

of the drying chamber. Once inside the drying chamber, the warmed

air will flow up through the stacked food trays. The drying trays

must fit snugly into the chamber so that the drying air is forced

through the mesh and food. Trays that do not fit properly will

create gaps around the edges, causing large volumes of warm air

to bypass the food, and preventing the dryer from removing moisture

evaporated from the food.

As the warm air flows through several layers of food on trays, it

becomes more moist. This moist air is vented out through a

chimney. The chimney increases the amount of air flowing through

the dryer by speeding up the flow of the exhaust air. Figure 8

shows a solar chimney with plastic film on the south-facing side.

As the warm, moist air flows through the solar chimney, the

additional solar energy entering the chimney warms the escaping

air further. This added heat makes the air less dense and causes

it to flow up through, and out of, the solar chimney at a faster

rate, thereby bringing in more fresh air into the collector.

SOLAR DRYER APPLICATIONS

Solar energy is used throughout the world to dry food products

too numerous to list completely. Listed below are a few representative

items to show the diversity to which the sun's energy

is put to use.

* grains * fruits

* meat * vegetables

* salt * fish

EQUIPMENT/MATERIALS NEEDED

The glazing materials used to cover direct dryers or as cover

plates on the collector portion of indirect dryers can be any

transparent or translucent material. Glass is probably the best

known material, but it is costly and breaks easily.

Rigid plastic materials are equal to glass for solar transmission

and can be much more durable against breakage. Fiberglass reinforced

polyester, acrylics, and polycarbonates will not break

easily in normal use and, depending on the material, may cost

less, ranging from US$11 to US$32 per square meter (US$1 to US$3

per square foot). However, these materials tend to degrade

somewhat with time, allowing less sunlight to pass through them.

Their useful life is estimated to be about 10 years. Acrylics and

polycarbonates may be more expensive than glass. Many of these

materials are also difficult to find in developing countries and

may need to be imported.

Thin plastic films are inexpensive and have good transmissivity

(the ability of a material to allow sunlight to pass through it),

but may degrade quickly, and are easily punctured and torn. The

cheapest film, polyethylene, may cost US$.50 per square meter

(US$.05 per square foot) and last less than one season--a little

more than a year if it is handled carefully. Ultraviolet-stabilized

polyethylene can last two to four years but will cost

three to five times as much. Tedlar and teflon films have long

useful lives (10 years or more), excellent transmissivity (allowing

92 percent or more of the solar energy to pass through) and

cost in the range of US$4 to US$8 per square meter (US$.40 to

US$.70 per square foot). These films are probably the best

choice if they can be protected from puncturing.

SKILLS NEEDED TO BUILD, OPERATE, AND MAINTAIN

Building a solar food dryer requires some carpentry skills. Mastering

the technique of drying comes from direct experience with

drying products rather than from reading about it. Maintaining a

solar food dryer requires only that an operator monitor the parts

periodically for wear and tear. For example, an operator should

make sure that the legs that support the drying chamber are not

loose, and that vents are not blocked. Plastic glazing material

should be checked to see if it turns cloudy, which will cause

less sunlight to pass through it.

COST/ECONOMICS

Cost comparisons between indirect and direct dryers are presented

in Table 3. Dryers 1, 2, 3, and 4 are indirect dryers, and

dryers 5 and 6 are direct dryers. The table shows the cost per

unit; more important, it compares the cost per drying tray and

the tray area for each dryer. Table 4 gives some values of

vitamin C retention for two products dried by indirect, direct,

and open air drying. Overall, it appears that indirect dryers are

more efficient and have higher vitamin retention than direct

dryers.

Table 3. Cost Comparisons

Tray Space Cost Per Unit Cost Per Unit

Type of Dryer (Square Meter) (U.S. Dollars) (U.S. Dollars)

Indirect dryer 1.12 65.00 58.04

Indirect dryer 1.49 90.00 60.40

Indirect dryer 1.30 75.00 57.69

Indirect dryer 3.16 115.00 36.39

Indirect dryer 2.88 175.00 60.76

Indirect dryer 1.21 50.00 41.32

Source: American Solar Energy Society, Inc., Progress in Passive

Solar Energy Systems (Boulder, Colorado: American Solar

Energy Society, Inc., 1983), p. 682.

Table 4. Vitamin C Retention

Type of Type of Percentage of

Dryer Food Vitamin C Retained

Indirect Cantaloupe 70.4

Indirect Cantaloupe 51.0

Direct Cantaloupe 53.6

Open sun Cantaloupe 39.5

Indirect Spinach 35.9

Direct Spinach 22.4

Source: American Solar Energy Society, Inc., Progress in Passive

Solar Energy Systems (Boulder, Colorado: American Solar

Energy Society, Inc., 1983), p. 682.

IV. COMPARING THE ALTERNATIVES

FOSSIL-FUEL DRYERS VERSUS SOLAR DRYERS

Conventionally fueled dryers are the primary alternative to solar

dryers. In conventional dryers, a fuel is burned to heat the

food-drying air. In some cases, the gaseous products of combustion

are mixed with the air to achieve the desired temperature.

Although these drying systems are used around the world with no

apparent problems, there is the possibility of a mechanical

malfunction, which might allow too much gas into the drying

stream. If this occurs, the food in the dryer can become contaminated.

The great advantage that conventional dryers have over solar

dryers is that drying can be carried out around-the-clock

for days on end, in any kind of weather. Unlike solar dryers,

conventional dryers are not subject to daily and seasonal variations

and other climatological factors. On the other hand, the

fuels burned in conventional dryers may present other problems:

Use of wood may contribute to problems of deforestation; coal may

cause pollution. Fossil fuels are becoming increasingly expensive

and are not always available.

ADVANTAGES OF SOLAR DRYERS

Solar dryers have the principal advantage of using solar energy--a

free, available, and limitless energy source that is also nonpolluting.

Drying most foods in sunny areas should not be a

problem. Most vegetables, for example, can be dried in 2-1/2 to

4 hours, at temperatures ranging from 43 to 63 [degrees] Centigrade (110

to 145 [degrees] Fahrenheit). Fruits take longer, from 4 to 6 hours, at

temperatures ranging from 43 to 66 [degrees] Centigrade (110 to 150 [degrees] Fahrenheit).

At this rate, it is possible to dry two batches of food

on a sunny day.

A solar food dryer improves upon the traditional open-air systems

in five important ways:

1. It is faster. Foods can be dried in a shorter amount of

time. Solar food dryers enhance drying times in two ways.

First, the translucent or transparent glazing over the

collection area traps heat inside the dryer, raising the

temperature of the air. Second, the capability of enlarging

the solar collection area allows for the concentration of

the sun's energy.

2. It is more efficient. Since foodstuffs can be dried more

quickly, less will be lost to spoilage immediately after

harvest. This is especially true of produce that requires

immediate drying--such as a grain with a high moisture

content. In this way, a larger percentage of food will be

available for human consumption. Also, less of the harvest

will be lost to marauding animals, vermin, and insects since

the food will be in an enclosed compartment.

3. It is safer. Since foodstuffs are dried in a controlled

environment, they are, less likely to be contaminated by

pests, and can be stored with less likelihood of the growth

of toxic fungi.

4. It is healthier. Drying foods at optimum temperatures and

in a shorter amount of time enables them to retain

more of their nutritional value--especially vitamin C. An

extra bonus is that foods will look and taste better, which

enhances their marketability.

5. It is cheaper. Using solar energy instead of conventional

fuels to dry products, or using a cheap supplementary supply

of solar heat in reducing conventional fuel demand can

result in a significant cost savings. Solar drying lowers

the costs of drying, improves the quality of products, and

reduces losses due to spoilage.

DISADVANTAGES OF SOLAR DRYERS

Solar dryers do have shortcomings. They are of little use during

cloudy weather. During fair weather they can work too well,

becoming so hot inside at midday as to damage the drying crop.

Only with close supervision can this be prevented. As temperatures

rise (determined with a thermometer or by experience), the

lower vents must be opened to allow greater airflow through the

dryer and to keep the temperatures down. Rice, for example, will

crack at temperatures above 50 [degrees] Centigrade; seed grains can be

dried at temperatures no higher than 40 to 45 [degrees] Centigrade.

V. CHOOSING THE TECHNOLOGY RIGHT FOR YOU

Four important questions must be answered before one decides to

build a solar food dryer. The brief discussion following each

question points out many factors that must be considered prior to

the construction of a solar food dryer. The questions are:

1. What food will the dryer be used for? Also, what quantities

of food will be dried?

Grains, fruits, and vegetables require different drying

techniques. Figure 9 shows a flow diagram that may be

helpful in defining the type of design. The safe storage of

the harvest is of prime concern to all. As soon as fresh

fruits and vegetables have been prepared (i.e., some may

need to be peeled, sliced, or blanched) for the drying

process, they must be dried immediately. Grains, too, have

only a limited time in which they must be dried to ensure

their storage. Rice in the husk, for example, will begin to

germinate within 48 hours if its moisture content is about

24 percent. Crops that must be dried immediately after they

are harvested may require the use of portable dryers, which

can be set up in the harvest field as needed. Permanent

dryers can be erected near preparation areas for fruits and

vegetables or centrally located for grain crops.

Some foods may lose much of their nutritional value, or

become discolored, if dried at too high a temperature or if

exposed to the direct rays of the sun. Using indirect

dryers can minimize the loss of vitamins, especially vitamin

Finally, the quantity of food to be dried, the capacity of

the dryer, the average time requried to dry one batch, and

the time available in which to dry the harvest must all be

considered in determining the number and size of the dryers

needed.

2. What are the climatic conditions during the harvest (and

drying) season?

Climatic conditions (solar radiation, rainfall, temperature,

humidity, wind, etc.) should be considered in determining

what kind of dryer is best suited for a particular application.

Figure 10 will help you to visualize the factors that must

be considered here. If the occurrence of sunshine is low--say,

50 percent or less--then it may be wise to add an

auxiliary heat source to enable drying to continue on cloudy

days or even through the night. Dry climates with hot or

moderate temperatures are well suited for solar food dryers.

Cold climates or humid climates pose the problem of making

it more difficult to obtain the necessary quantity of warm,

dry air to dry foods effectively before spoilage can occur.

Such weather conditions may limit the use of direct dryers

to preserving only small quantities of food that must be

dried in a short time (one or two days). Indirect dryers

have the advantage over direct dryers in that they are

capable of concentrating solar energy. Enlarging the collector

area and varying the airflow through the collector

enable indirect dryers to achieve near optimum conditions in

most climates.

3. Is the food to be stored for long periods or will it be

shipped to market for quick consumption?

The answer to this question determines the dryness required

of the finished product. Rice at harvest might typically

contain 24 percent moisture. If it is sold quickly, say, for

milling, it is fine as is. If, on the other hand, it is to

be stored for any length of time, it must be dried to only

12 to 14 percent moisture content. Thus, the dryness required

will determine how long and at what temperature the

food must remain in the dryer. The time required for the

food to remain in the dryer must be taken into account in

determining the number of dryers needed to dry the entire

harvest.

4. What materials are available to construct the dryer? Are the

materials available locally?

Masonry may be a good construction medium for permanent

dryers, where the food can be brought to the dryer.

If, however, the dryers are to be transported into the

fields, lightweight materials will be needed to make

the units portable. The availability of materials may

govern, in part, the placement of the food dryers.

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Symposium on Drying. Princeton, New Jersey: Science Press,

1978.

TECHNICAL ASSISTANCE ORGANIZATIONS

Brace Research Institute

McDonald Campus of McGill University

Ste. Anne de Bellavue 800

Quebec, Canada

The Institute has designed direct and indirect dryers and

has plans available.

New Mexico Solar Energy Association (NMSEA)

P.O. Box 2004

Santa Fe, New Mexico 87501 USA

NMSEA publishes detailed construction plans for a solar crop

dryer.

Volunteers in Technical Assistance (VITA)

1600 Wilson Boulevard, Suite 710

Arlington, Virginia 22209 USA

VITA's Solar Crop Dryer manual includes plans for a direct

and an indirect solar dryer.