TECHNICAL PAPER #15
UNDERSTANDING SOLAR FOOD DRYERS
Roger G. Gregoire, P.E.
Gary M. Flomenhoft
Jacques L. LeNormand
1600 Wilson Boulevard, Suite 500
Arlington, Viginia 22209 USA
703/276-1800 * Fax: 703/243-1865
Undestanding Solar Food Dryers
1984, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
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
They are not intended to provide construction or
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
almost entirely by VITA Volunteer technical experts on a purely
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
is a consultant in the areas of energy management
solar design and analysis, energy audits, energy management
buildings, and alternative energy systems.
He has published on
energy conservation, solar greenhouses and solar water
well as solar food dryers.
Reviewers Gary M. Flomenhoft and
Jacques L. LeNormand are also experts in the area of solar
is a consultant in renewable energy and engineering
for the San Diego Center for Appropriate Technology.
has also taught on energy conservation and solar technology.
LeNormand is Assistant Director at the Brace Research
Quebec, Canada, which does research in renewable
energy. He has
supervised work with solar collectors, has trained people
overseas in solar technologies, and has published widely on
and wind energy, and conservation.
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries.
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
UNDERSTANDING SOLAR FOOD DRYERS
Volunteer Roger G. Gregoire, P.E.
Dehydration, or drying, is a simple, low-cost way to
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
Surplus grain, vegetables, and fruit preserved by
drying can be stored for future use.
People have been drying food for thousands of years by
the food on mats in the sun.
This simple method, however, allows
the food to be contaminated by dust, airborne molds and
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
method of dehydrating foods.
Although solar dryers involve an
initial expense, they produce better looking, better
more nutritious foods, enhancing both their food value and
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
climates as well as arid climates.
It can also reduce the problem
Drying is completed more quickly, so there is
less chance of spoilage.
Fruits maintain a higher vitamin C
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
that can be used in agricultural activities in developing
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
is best suited for a particular application.
THE DRYING PROCESS
Drying products makes them more stable and in the case of
a llows them to be stored safely for long periods of
storage requires protection from the growth of molds and
fungi, the most difficult of the spoilage mechanisms to
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
Production of toxic products, known as
of which can
be harmful if consumed.
Total spoilage and heating, which sometimes
to the point
of spontaneous combustion.
At harvest, most grains contain more moisture than is safe
prolonged storage, because many fungi grow rapidly in warm,
any grain stored for future use must be dried
shortly after harvest to prevent the growth of destructive
In general, grains will not be completely dried since they
hygroscopic--that is, they absorb moisture from the
higher the relative humidity of the surrounding air, the
the moisture content of the grain.
Table 1 lists the moisture
content of various grains as a function of the relative
of the surrounding air.
At the same time, there is a minimum
level of relative humidity, below which the harmful fungi
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-
Moisture Contents of Various Grains and
Equilibrium with Different Relative Humidities at
30 [degrees] Centigrade
12.5 to 13.5
13.5 to 14.5
14.5 to 15.5
15.5 to 16.5
18.0 to 18.5
Handbook and Product Director: 1977
American Society of Heating, Refrigerating and
Conditioning Engineers, Inc., 1980), p. 10.2.
Minimum Relative Humidity for the Growth of
Fungi at Their Optimum Temperature for Growth
30 [degrees] Centigrade)
depending on species 80 to 90
Handbook and Product Directory: 1977
York: American Society of Heating,
Conditioning Engineers, Inc., 1980), p. 10.2.
Solar dryers use the energy of the sun to heat the air that
over the food in the dryer.
As air is heated, its relative
humidity decreases and it is able to hold more
dry air flowing through the dryer carries away the moisture
evaporates from the surfaces of the food.
As drying proceeds, the actual amount of moisture evaporated
unit of time decreases.
In the first phase of drying, the moisture
in the exterior surfaces of the food is evaporated.
once the outer layer is dried, moisture from the innermost
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
During the second phase of the drying
process, overheating may occur because of the lessened
effect resulting from the slower rate of moisture
If the temperature is too high, the food will "case
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 DRYER TYPES
Solar dryers fall into two broad categories:
active and passive.
Passive dryers can be further divided into direct and
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
can dry up to 6,000 pounds a month; and a very large system
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
can be constructed easily with inexpensive, locally
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 require an external means, like fans or pumps,
moving the solar energy in the form of heated air from the
area to the drying beds.
These dryers can be built in
almost any size, from very small to very large, but the
systems are the most economical.
Figure 3 is a schematic drawing showing the major components
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
you are in the southern hemisphere.
At or near the equator, they
should also be adjusted east or west in the morning and
collectors should be positioned at an
appropriate angle to optimize solar energy collection for
planned months of operation of the dryer.
The collectors can be
adjacent to or somewhat remote from the solar dryer.
since it is more difficult to move air long distances, it is
to position the collectors as near the dryer as possible.
The solar energy collected can be delivered as heat
to the dryer air stream, or it can be stored for later use.
Storage systems are bulky and costly but are helpful in
where the percentage of sunshine is low and a guaranteed
source is required; or in carrying out round-the-clock
In an active dryer, the solar-heated air flows through the
drying chamber in such a manner as to contact as much
area of the food as possible.
The larger the ratio of food
surface area to volume, the quicker will be the evaporation
moisture from the food.
Thinly sliced foods are placed on drying
racks or on trays made of a screen or other material that
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
with grain piled on top.
The heated air flows through the pipes
and is released upward to flow through the grain--carrying
moisture as it flows.
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
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
it and the food.
Figure 4 shows a drawing of a simple direct
radiation passes through the transparent cover and
is converted to low-grade heat when it strikes an opaque
This low-grade heat is then trapped inside the box in what
known as the "greenhouse effect." Simply stated,
the short wavelength
solar radiation can penetrate the transparent cover.
converted to low-grade heat, the energy radiates on a long
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
be used to dry small quantities of a wide variety of
drying chamber can be constructed of almost any material--
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
(9 to 12 inches) deep.
The bottom and sides of the dryer
should be insulated, with 5 centimeters (2 inches)
Blackening the inside of the box will improve the dryer
but be sure to use a non-toxic material and avoid lead-based
blackened by fire may be a safe and inexpensive
material to use.
The tray that holds the food must permit air to enter from
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 holes in the mesh, the easier the air will circulate
to the food. Air
holes below the tray or mesh will bring in
outside air, which will carry away the moisture evaporated
the food. As the air
heats up in the dryer, its volume will
increase, so either more or larger holes will be required at
top of the box to maintain maximum air flow.
Finally, tests of the hot box dryer shown in Figure 7 have
that the temperature within the dryer can be as much as
40 [degrees] Centigrade (104 [degrees] Fahrenheit) higher
than the outside ambient
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
air into a separate drying chamber.
Within the chamber, the food
is placed on mesh trays that are stacked vertically so that
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
Section V of this paper indicates climatic conditions
where larger collector areas might be more effective.
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Tilting the collectors is more effective than placing them
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.
drying chamber should be placed on support legs, but it
not be raised so high above the ground that it becomes
to work with.
The base of the collector should be vented to allow the
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
The vents should also be adjustable so that the air flow can
matched with the operating conditions and/or needs.
ambient air temperature, humidity level, drying chamber
temperature, and moisture level of the food being dried must
be considered when regulating the flow of air.
The top of the collector should be completely open to the
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
through the mesh and food.
Trays that do not fit properly will
create gaps around the edges, causing large volumes of warm
to bypass the food, and preventing the dryer from removing
evaporated from the food.
As the warm air flows through several layers of food on
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.
shows a solar chimney with plastic film on the south-facing
As the warm, moist air flows through the solar chimney, the
additional solar energy entering the chimney warms the
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
rate, thereby bringing in more fresh air into the collector.
SOLAR DRYER APPLICATIONS
Solar energy is used throughout the world to dry food
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.
The glazing materials used to cover direct dryers or as
plates on the collector portion of indirect dryers can be
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.
polyester, acrylics, and polycarbonates will not break
easily in normal use and, depending on the material, may
less, ranging from US$11 to US$32 per square meter (US$1 to
per square foot).
However, these materials tend to degrade
somewhat with time, allowing less sunlight to pass through
Their useful life is estimated to be about 10 years.
polycarbonates may be more expensive than glass.
Many of these
materials are also difficult to find in developing countries
may need to be imported.
Thin plastic films are inexpensive and have good
(the ability of a material to allow sunlight to pass through
but may degrade quickly, and are easily punctured and
cheapest film, polyethylene, may cost US$.50 per square
(US$.05 per square foot) and last less than one season--a
more than a year if it is handled carefully.
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
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
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
the technique of drying comes from direct experience with
drying products rather than from reading about it.
solar food dryer requires only that an operator monitor the
periodically for wear and tear.
For example, an operator should
make sure that the legs that support the drying chamber are
loose, and that vents are not blocked.
Plastic glazing material
should be checked to see if it turns cloudy, which will
less sunlight to pass through it.
Cost comparisons between indirect and direct dryers are
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
the tray area for each dryer.
Table 4 gives some values of
vitamin C retention for two products dried by indirect,
and open air drying.
Overall, it appears that indirect dryers are
more efficient and have higher vitamin retention than direct
Tray Space Cost Per
Unit Cost Per Unit
Type of Dryer
(Square Meter) (U.S.
Dollars) (U.S. Dollars)
Solar Energy Society, Inc., Progress in Passive
Systems (Boulder, Colorado: American
Society, Inc., 1983), p. 682.
Vitamin C Retention
Type of Percentage
Vitamin C Retained
Solar Energy Society, Inc., Progress in Passive
Systems (Boulder, Colorado: American
Society, Inc., 1983), p. 682.
IV. COMPARING THE
FOSSIL-FUEL DRYERS VERSUS SOLAR DRYERS
Conventionally fueled dryers are the primary alternative to
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
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
and other climatological factors.
On the other hand, the
fuels burned in conventional dryers may present other
Use of wood may contribute to problems of deforestation;
Fossil fuels are becoming increasingly expensive
and are not always available.
ADVANTAGES OF SOLAR DRYERS
Solar dryers have the principal advantage of using solar
free, available, and limitless energy source that is also
Drying most foods in sunny areas should not be a
vegetables, for example, can be dried in 2-1/2 to
4 hours, at temperatures ranging from 43 to 63 [degrees]
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
in five important ways:
1. It is
faster. Foods can be dried in a shorter
Solar food dryers enhance drying times in
translucent or transparent glazing over the
traps heat inside the dryer, raising the
temperature of the
air. Second, the capability of
collection area allows for the concentration of
the sun's energy.
2. It is more
efficient. Since foodstuffs can be
will be lost to spoilage immediately after
This is especially true of produce that requires
drying--such as a grain with a high moisture
In this way, a larger percentage of food
human consumption. Also, less of the
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
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
in a shorter
amount of time enables them to retain
more of their
nutritional value--especially vitamin C.
extra bonus is
that foods will look and taste better, which
5. It is
cheaper. Using solar energy instead of
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
the costs of
drying, improves the quality of products, and
due to spoilage.
DISADVANTAGES OF SOLAR DRYERS
Solar dryers do have shortcomings.
They are of little use during
During fair weather they can work too well,
becoming so hot inside at midday as to damage the drying
Only with close supervision can this be prevented.
rise (determined with a thermometer or by experience), the
lower vents must be opened to allow greater airflow through
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]
V. CHOOSING THE
TECHNOLOGY RIGHT FOR YOU
Four important questions must be answered before one decides
build a solar food dryer.
The brief discussion following each
question points out many factors that must be considered
the construction of a solar food dryer.
The questions are:
1. What food will
the dryer be used for? Also, what
of food will be
and vegetables require different drying
Figure 9 shows a flow diagram that may be
defining the type of design. The safe
the harvest is of
prime concern to all. As soon as fresh
vegetables have been prepared (i.e., some may
need to be
peeled, sliced, or blanched) for the drying
must be dried immediately. Grains, too,
only a limited
time in which they must be dried to ensure
storage. Rice in the husk, for example,
will begin to
48 hours if its moisture content is about
Crops that must be dried immediately after
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
centrally located for grain crops.
Some foods may
lose much of their nutritional value, or
discolored, if dried at too high a temperature or if
exposed to the
direct rays of the sun. Using indirect
minimize the loss of vitamins, especially vitamin
quantity of food to be dried, the capacity of
the dryer, the
average time requried to dry one batch, and
available in which to dry the harvest must all be
determining the number and size of the dryers
2. What are the
climatic conditions during the harvest (and
conditions (solar radiation, rainfall, temperature,
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
here. If the occurrence of sunshine is
50 percent or
less--then it may be wise to add an
source to enable drying to continue on
days or even
through the night. Dry climates with
temperatures are well suited for solar
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.
conditions may limit the use of direct dryers
only small quantities of food that must be
dried in a short
time (one or two days). Indirect dryers
advantage over direct dryers in that they are
concentrating solar energy. Enlarging
area and varying
the airflow through the collector
dryers to achieve near optimum conditions in
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
percent moisture. If it is sold
quickly, say, for
milling, it is
fine as is. If, on the other hand, it
be stored for any
length of time, it must be dried to only
12 to 14 percent
moisture content. Thus, the dryness
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
number of dryers needed to dry the entire
4. What materials
are available to construct the dryer?
be a good construction medium for permanent
where the food can be brought to the dryer.
the dryers are to be transported into the
lightweight materials will be needed to make
portable. The availability of materials
govern, in part, the placement of the
American Society of Heating, Refrigerating, and
ASHRAE Handbook and Product Directory:
New York, New
York: American Society of Heating,
and Air-Conditioning Engineers.
Andrea, A. Louise.
Dehydrating Foods. Boston,
Archuleta, R.; Berkey, J.; and Williams, B. "Research
Food Drying at
the University of California, Santa Cruz."
Passive Solar Energy Systems. Edited by
Hayes and D.
Andrejko. Boulder, Colorado:
Inc., 1983, pp. 679-682.
DeLong, D. How to Dry
Foods. Tucson, Arizona:
H.P. Books, 1979,
"A Simple Solar Rice Dryer:
Basic Design Theory."
Ginsburg, A.S., ed.
Grain Drying and Grain Dryers.
The Israel Program for Scientific
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Gregoire, R.G.; Slajda, Robert; and Winne, Mark.
Scale Solar Food
Dryer." Edited by B.H. Glenn and G.E.
Proceedings of 1981 Annual Meeting.
Energy Society, Inc., 1981.
Lindblad, C., and Druben, L. "Preparing Grain for
of Small Farm
Grain Storage. Prepared for
Manual No. 35E.
Brace Research Institute. A
Survey of Solar
Dryers. Technical Report T99.
Institute, McGill University, 1975.
Ong, K.S. "Solar Drying Technology for Rural
presented at the
Regional Conference on Technology for Rural
Kuala Lumpur, Malaysia, 1978.
van Brakel, J. "Opinions About Selection and Design of
Edited by A.S.
Mujumdar. Proceedings of First
Drying. Princeton, New Jersey:
TECHNICAL ASSISTANCE ORGANIZATIONS
Brace Research Institute
McDonald Campus of McGill University
Ste. Anne de Bellavue 800
has designed direct and indirect dryers and
New Mexico Solar Energy Association (NMSEA)
P.O. Box 2004
Santa Fe, New Mexico 87501 USA
detailed construction plans for a solar crop
Volunteers in Technical Assistance (VITA)
1815 North Lynn Street, Suite 200
Arlington, Virginia 22209 USA
Crop Dryer manual includes plans for a direct
and an indirect