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                        TECHNICAL PAPER #15
                      Roger G. Gregoire, P.E.
                        Technical Reviewers
                         Gary M. Flomenhoft
                        Jacques L. LeNormand
                            Published By  
                  1600 Wilson Boulevard, Suite 500
                    Arlington, Viginia 22209 USA
               Tel:  703/276-1800 * Fax:  703/243-1865
                     Internet:  pr-info@vita.0rg
                   Undestanding Solar Food Dryers     
                        ISBN:  0-86619-215-8                      
                [C] 1984, Volunteers in Technical Assistance    
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.
         By VITA Volunteer Roger G. Gregoire, P.E.
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.
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
     *   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
     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

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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.
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.

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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

28p06.gif (540x540)

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

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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

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the greenhouse effect in a simplified schematic drawing.
Figures 6 and 7 show examples of simple, direct dryers that can

28p090.gif (486x486)

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

28p09b.gif (540x540)

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

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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 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
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.
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 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
                 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.
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.
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.
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.
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

28p17.gif (600x600)

    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
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

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    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
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.
American Society of Heating, Refrigerating, and Air-Conditioning
     Engineers.   ASHRAE Handbook and Product Directory:   1977 Fundamentals.
     New York, New York:  American Society of Heating,
     Refrigerating, and Air-Conditioning Engineers.
Andrea, A. Louise.  Dehydrating Foods.  Boston, Massachusetts:  The
     Cornhill Company, 1920.
Archuleta, R.; Berkey, J.; and Williams, B. "Research on Solar
     Food Drying at the University of California, Santa Cruz."
     Progress in Passive Solar Energy Systems.  Edited by J.
     Hayes and D. Andrejko.  Boulder, Colorado:   American Solar
     Energy Society, Inc., 1983, pp. 679-682.
DeLong, D.  How to Dry Foods.  Tucson, Arizona:   H.P. Books, 1979,
     160 pp.
Exell, R.H.B.  "A Simple Solar Rice Dryer:  Basic Design Theory."
     Sunworld 4 (1980):  188.
Ginsburg, A.S., ed.  Grain Drying and Grain Dryers.  Washington,
     D.C.:   The Israel Program for Scientific Translations, 1960.
Gregoire, R.G.; Slajda, Robert; and Winne, Mark.   "A Commercial
     Scale Solar Food Dryer." Edited by B.H. Glenn and G.E.
     Franta.   Proceedings of 1981 Annual Meeting.   Boulder, Colorado:
     American Solar Energy Society, Inc., 1981.
Lindblad, C., and Druben, L. "Preparing Grain for Storage."  Vol.I
     of Small Farm Grain Storage.  Prepared for ACTION/Peace Corps
     and VITA.   Manual No. 35E.   Arlington, Virginia:  VITA, 1977.
McGill University.  Brace Research Institute.  A Survey of Solar
     Agricultural Dryers.  Technical Report T99.   Quebec, Canada:
     Brace Research Institute, McGill University, 1975.
Ong, K.S. "Solar Drying Technology for Rural Development." Paper
     presented at the Regional Conference on Technology for Rural
     Development, Kuala Lumpur, Malaysia, 1978.
van Brakel, J. "Opinions About Selection and Design of Dryers."
     Edited by A.S. Mujumdar.  Proceedings of First International
     Symposium on Drying.  Princeton, New Jersey:   Science Press,
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
Volunteers in Technical Assistance (VITA)
1815 North Lynn Street, Suite 200
Arlington, Virginia 22209 USA
     VITA's Solar Crop Dryer manual includes plans for a direct
     and an indirect solar dryer.