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close this bookAppropriate Community Technology - A Training Manual (Peace Corps; 1982; 685 pages)
View the documentThe Farallones Institute Rural Center
View the documentCHP International, INC.
View the documentPreface
View the documentAcknowledgments
View the documentIntroduction
Open this folder and view contentsPhase I: Introduction to training
Open this folder and view contentsPhase II: Earthen construction and fuel-saving cookstoves
Open this folder and view contentsPhase III: Pedal/treadle power
Open this folder and view contentsPhase IV: Solar water heaters
close this folderPhase V: Solar agricultural dryers
View the documentPhase V Calendar
View the documentSession 1. Introduction to agricultural dryers
View the documentSession 2. Tour of solar dryers
View the documentSession 3. Solar agricultural dryer design procedures and rules of thumb
View the documentSession 4. Two-hour dryer construction
View the documentSession 5. Review of existing solar dryer plans
View the documentSession 6. Smoke testing solar dryers
View the documentSession 7. Introducing new technologies: solar dryers
View the documentSession 8. Design of solar agricultural dryers
View the documentSession 9. Site selection and preparation
View the documentSession 10. Construction of solar agricultural dryers
View the documentSession 11. Issues and methods in development and diffusion of appropriate technology
View the documentSession 12. Natural cooling
View the documentSession 13. Approaches to health systems
View the documentSession 14. Nutritional gardening
View the documentSession 15. Practical drying tips
View the documentSession 16. Cardiopulmonary resuscitation
View the documentSession 17. Dryer assessment and modification
View the documentSession 18. Introduction to cost benefit analysis (cba)
View the documentSession 19. Presentation of solar dryers
View the documentSession 20. Introduction to the final phase of the training program
Open this folder and view contentsPhase VI: Concluding the program: The energy fair
Open this folder and view contentsAppendices

Session 8. Design of solar agricultural dryers

Total time:

5 hours


* To identify and list criteria for assessing a solar dryer design


* To calculate the cost of a solar dryer


* To design a solar dryer


* To evaluate problem-solving techniques used in the design process


* Attachment V-8-A, "Solar Food Dehydration Guidelines"


* Attachment V-8-B, "Design Considerations for Solar Dryers"


* Attachment V-8-C, "Bibliography for Solar"


* Materials Price List (see Trainer Notes)


* Procedures for developing final solar dryer designs (see Trainer Notes)

Trainer Notes

Part III of Attachment V-8-B is a sample materials price list. It can be used to give participants the data they will need to complete the cost problems outlined in Step 5. However, we recommend that you develop and distribute a price list that is more specific to your training program and area.

See Trainer Notes, Step 6, for the procedures for developing final solar dryer designs.


Newsprint, felt-tip pens, notebooks, graph paper, pens, pencils, rulers and architectural scales


Step 1. (10 minutes)
Present the session objectives and outline the activities.

Step 2. (15 minutes)
Distribute and review Attachment V-8-A, "Solar Food Dehydration Guidelines." Encourage questions and comments.

Trainer Notes

Point out that this information is helpful in developing criteria for designing solar dryers.

Step 3. (20 minutes)
Have the participants form their construction groups and list on newsprint the criteria for assessing a solar dryer design.

Trainer Notes

Explain that there are many factors to be considered in developing the criteria lists.

Suggest the following considerations:

* Crop or crops to be dried
* Amounts to be dried
* Time of the year when harvest occurs
* Moisture of crop at harvest
* Acceptable moisture for storage
* Relative humidity of climatic zone
* Appropriate drying methods that retain nutrients and preserve freshness
* Design feasibility (Can it built within 30 hours and will it function efficiently?)
* Appropriate insulation where needed
* Proper aperture size and tilt
* Adequate venting area with room for adjustments
* Appropriate rack design for the specific crop to be dried
* Simplicity of construction (minimal need for high skill input)
* Type and availability of construction materials
* Durability
* Maintenance
* Percent of possible sunshine at harvest time

Have each group post their completed lists at the front of the room.

Step 4. (40 minutes)
Reconvene the groups and have each construction group present, discuss and modify their list.

Trainer Notes

Have each group explain their list, compare it with the other groups' lists and make any changes.

Each group should develop its own criteria list. However, it is probable that, after two or three groups have modified their lists, the lists will begin to be similar or the same. If this happens, point out that this activity is an example of a technique which can be used to help large groups reach a consensus.

Step 5. (35 minutes)
Distribute and review Attachments V-8-B, "Design Considerations for Solar Dryers, and V-8-C, "Bibliography of Solar Food Dehydration Publications."

Trainer Notes

When reviewing Part II of Attachment V-8-B ("Cost Analysis of Solar Dryers"), solve a sample cost problem with the participants. Then have them solve one on their own. Part III of Attachment 8-V-8 can be used as data for completing the problems If you have developed a price list that is more specific to your program, use it instead (see Trainer Notes under Resources).

Following this activity, allow participants the opportunity to take a short break.

Step 6. (15 minutes)
Post and explain the procedures and guidelines for developing the final solar dryer designs.

Trainer Notes

Post on newsprint the following procedures:

1. Develop preliminary designs (45 minutes)
2. Assess and modify preliminary designs (30 minutes)
3. Develop final designs (60 minutes)

Explain that in the next step, participants will return to their construction groups and follow the above procedures to develop their solar dryer designs. Stress the importance of keeping within the time limits for each procedure.

Explain the following guidelines:

* All designs should include materials lists, venting schemes, food racks, sketches, dimensions, etc.

* During the preliminary design period, groups should work together, sharing and discussing their ideas with one another. There should be an atmosphere of cooperation among the groups. They should work together to explore as many design ideas as possible and to help each other by offering constructive feedback.

* Each group should then work independently to assess its preliminary design.

* Each group should modify its preliminary design such that it meets the criteria developed earlier in the session.

* The final designs should be clear and easy to read.

Step 7. (2 hours, 15 minutes)
Have the participants return to their construction groups and develop their final solar dryer designs.

Trainer Notes

As the groups are working, circulate among them and offer help whenever it is necessary. Be careful not to intervene so much that you inhibit the participants' opportunity to solve their own problem creatively.

Approximately ten minutes before the end of the time period allotted for this step, visit each group and check their final designs for completeness.

Step 8. (25 minutes)
Reconvene the groups and have them discuss and evaluate the problem-solving techniques used in developing their designs.

Trainer Notes

Stimulate the discussion by asking the following questions:

* Was there a particular pattern to the problem-solving techniques used by your group?
* What technique was most effective for solving problems or reaching a consensus decision? Least effective?
* Do you think that your problem-solving skills were improved by this activity? How?
* What relevance do problem-solving skills have to your future work as Peace Corps Volunteers?
* Are you satisfied with you final designs?
* Do you feel that the design will work?
* Do you understand how the dryer will be constructed and how it will work?

Step 9. (5 minutes)
Conclude by reviewing the session objectives and reminding the participants that they will be building their solar dryers in the next session.







Herbs, garlic, rosehips, parsley

6 lbs/
2 days

42° C

60° C


Fruits, apricots apples, peaches

10 lbs/
2 days

42° C

62° C



12 lbs/
4 days

50º C

70º C


Grains, wheat, rice barley, corn

5 lbs/
3 days




Vegetables, okra, onions

7 lbs/
2 days




Carrots (A&E)

5 lbs/
3 days





7 lbs/
2 days




Greens, spinach collards, endive

3 lbs/
2 days




* i.e., whether food is exposed directly to sunlight or only to sun.


Batch #


Food Source

Dehydrator Location




Outside Temper.

Dryer Temp.

Outside Humidity

Sky Conditions

Tray Rotation




I. Design Recommendations for Solar Food Dryers

If you have no other design data to go by, provide approximately 3 times the area of solar collector as drying tray area. This will allow enough heat to be collected to provide sufficiently quick drying at the appropriate drying temperature.

Try to get an air flow rate of 1/3 to 1/2 m³ per minute per m³ of dryer volume. This amount of air circulation should be obtainable with natural circulation, assuming the dryer is not overloaded.

Grains dry quickly (1 or 2 sunny days or 3 or 4 partly-sunny days) when the thickness of the grain is not too great. Research has shown that thicknesses over l5 cm (6") of rice or corn requires a fan to force air through it. The thinner the layers of grain, the quicker the grain will dry.

Use a back-up heat source if the sun cannot be depended upon to shine for at least six straight hours. It is best to have the first drying day be a full, sunny day. If the first drying day is not sunny, it is best to provide back-up (supplemental) heat to assure a high-quality food.

Find the lifetime unit cost of the system, using the cost analysis formula given below. This figure will give the designer and the user a basis for comparison of several dryers or designs.

II. Cost Analysis of Solar Dryers

To find an appropriate cost-per-unit weight of a solar dryer, the following equation can be used:



Total Lifetime Cost


Initial cost of solar system + cost of repair and maintenance of solar system for the lifetime of the system, considering the inflation rate + initial cost of the back-up system + cost of back-up fuel during the lifetime of the system


Loading Density, given in weight per area of tray, i.e., lbs/ft, Kg/m², etc.


Area of trays, given in same unit as in L.D., i.e. ft², m², etc.


Number of loads per week, given in the conservative estimate of the number of times in a week the dryer will be loaded with fresh food.


Number of weeks per year the dryer will be used. This should be a conservative estimate of weeks per year that the dryer can possibly be used. This will depend upon the ripening of crops, the availability of crops, the climate, the weather, etc.


Number of years the dryer will be functional. Give the conservative estimate of the life of the system, considering glazing material, the frame material, exterior material, screens, etc.


Find the unit cost ($/kg) for a solar corn dryer assuming the following conditions:

The cost of the solar system is $ 30.00.
The back-up system costs $ 10.00.
The cost of repair and maintenance is 1X per year, with inflation at 15%.
The cost of the back-up fuel for the life of the system (10 years) is $ 100.00
The loading density is 8 kg/m².
The area of trays is 10 m².
1.5 loads can be put into the system every week.
The system is usable 30 weeks out of the year.
All materials should last 15 years.


T. L. C.=$30.00+$10.00+$100.00+$6.13=$146.13

III. Materials Price List (Typical)




Galvanized pipe per foot



Polyethylene pipe per foot






Hose bibs












Nipples per inch






5 gallon tank, steel


5 gallon tin can


16 gallon tank


17 gallon drum with lid


30 gallon tank





.40 per board foot


.75 per square foot


2.00 per quart

Plastic (black or clear)

.10 per square foot

Glass (cut your own!)

2.00 per square foot


.15 per pound


.10 per pound


.50 per pound


1. Proceedings of the Solar Dryinq Workshop, Manila, Philippines, October 18-21, 1978.

Available from the Bureau of Energy Development, Ministry of Energy, Fort Bonifacio, Makita, Metro Manila, Philippines.

The conference focused on solar food drying in the humid tropics regions of the world. Participants came from such countries as Indonesia, India, Malasia, Papua New Guinea, Philippines, Singapore and Thailand. It is a valuable publication for those who will be drying foods in the tropics.

2. Survey of Solar Agricultural Dryers, Technical Report T99, December 1975.

Available from Brace Research Institute, MacDonald College of McGill University, Ste. Anne de Bellevue, Quebec, Canada H9X 3M1.

The survey contains the history, use, plans and drawings of solar dryers from throughout the world. Especially of interest are the dryers which have been locally designed, built and used for a number of years. These dryers have proved to be useful and adaptable to those who have them. Much technical data and many drawings and plans.

3. Solar Food Dehydrator Plans ($6.50, plus shipping)

Available from Solar Survival, Cherry Hill Road, Harrisville, New Hampshire 03450 USA

The plans show how to build a very practical and flexible solar food dryer from common materials using simple tools. The plans include information on how to dry various fruits and vegetables, plus how to build and install a back-up heater for non-sunny periods.

4. How to Build a Solar Crop Dryer ($2.50, plus shipping)

Available from the New Mexico Solar Energy Association P. O. Box 2004, Santa Fe, NM 87501 USA

The booklet includes detailed, step-by-step drawings and procedures on how to build a solar dryer. The design was made to be used in sunny New Mexico (Latitude 35° North), so it may have limitations to use on a worldwide basis.

5. Drying Foods at Home (#2785) Free publication.

Available from Agricultural Sciences Publications, University of California, 1422 Harbor Way South, Richmond, California 94804 USA.

This booklet includes excellent information on drying fruits and vegetables at home. It focuses on the mechanics of drying, but does not cover solar drying to any extent. However, the other material in the booklet is very good.

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