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.You can either buy a dehydrator (we stock all the best makes) or learn how to make one yourself:
UNDERSTANDING SOLAR FOOD DRYERS
By 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. |
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