- (Peace Corps, 1984, 175 p.)
Solar food dryers: design theory[edit | edit source]
OVERVIEW AND GOALS:
As there is no "ideal" Solar Food Dryer (SFD), it is important that the participants be able to design a dryer appropriate to a given situation. This session focuses on design principles of the dryers and includes a look at improved sun drying as a potential first step in introducing the dryers on a community level.
Review the history, advantages and disadvantages of both open-sun and solar drying as methods of preserving food
Review optimal conditions for drying
Discuss Solar Food Dryer design principles
Discuss how to correct potential problems with the dryers
"Brace Research Institute Dryer Survey" "Preserving Food by Drying" "Proceedings of Solar Dryer Workshop, Manila" "Drying Foods in the Tropics" "Small Farm Grain Storage"
A variety of dehydrated foods for sampling Newsprint, markers and easel Incense and matches Cardboard (combination model) Solar Food Dryer SFD Slide Show (available from Farallones Institute; see address in resource list) - showing a variety of SFD designs, slide projector and screen
Solar Food Dryer-Design Criteria (Handout 5A)
Solar Food Dryer Sizing: Rules of Thumb (Handout 5a)
Build cardboard SFD model.
Prepare slide show, pictures or drawings of a variety of SFD designs.
Arrange the dried foods in a way to make sampling by the group convenient.
PROCEDURES AND ACTIVITIES:
- (20 minutes) Introduction
Make the dehydrated food samples available to participants so they can learn about the wide variety of foods that may be dried, as well as observe their taste, texture and appearance. Follow the sampling with a discussion of people's reactions to the dried foods, both before and after sampling.
This activity may be done either at the beginning of the session or during a break half way through for a more efficient use of time.
- (25 minutes) Comparing Open-Sun and Solar Drying
Stimulate discussion regarding the difference between open-sun and solar drying, and what experience the participants have had with either method.
Briefly review the history of drying, emphasizing the fact that sun drying is one of the oldest methods of food preservation, while solar drying is a relatively new and developing technology.
Ask participants for their ideas regarding the advantages and disadvantages of both methods, and record the list on newsprint. Guide them in completing the list if necessary!
Discuss why dehydration is an effective method of preserving food. What actually happens in the drying process?
- (10 minutes) Optimal Drying Conditions
Guide the group to identify the optimal drying conditions: heat, air flow, and dry air. An analogy may be drawn between those and the most desirable conditions for quickly drying a load of wash hung out on the line.
- (25 minutes) Design Principles
Give examples of how the optimal drying conditions are achieved by the following design principles:
Greenhouse effect (heat) Absorbent colors (heat) Chimney effect (airflow) Low relative humidity/high temperature (dryness)
A drawing of a house with a shed roof, glazing only on one wall (the low wall facing the sun), dark interior walls, and high and low windows may be used to illustrate the four relationships. The smoke from burning incense may be used to illustrate the chimney effect.
Once the participants understand how to achieve the conditions for drying, using the example of the house, discuss the relationship between that example and "a small scale model" (or solar food dryer).
- (15 minutes) Components of a SFD
Use a SFD model to familiarize people with the names for different parts of the dryer. Attach a prepared label to each part as it is named to help in remembering the terms.
TRAINER '5 NOTE
At this point participants should have a basic understanding of the purpose or use of each part, but emphasize that the objective now is merely to identify the parts and that the use and design of each component will soon be discussed in detail.
The following parts should be included:
Vents (upper and lower)
- (15 minutes) Three types of SFD's
Using the SFD model, illustrate the direct, indirect and combination design possibilities. Ask people what the main advantages and disadvantages of each are so that they understand the application of the three different dryer types.
- (45 minutes) Design Criteria
Using the Handout on SFD Design Criteria (Handout 5A), discuss the guidelines for design and construction of each component. Through ongoing questioning, determine that the participants understand the reason for each criterion.
- (20 minutes) Sack-Up Heating
Ask for ideas of methods that might work for back-up heating during cloudy weather. This might include: - bringing drying trays inside near a stove; - placing the trays inside a stove with the door left partly open; using wood-fired dryers; - using electric dryers
Ask for comments about the advantages and disadvantages of each.
The extent to which back-up dryers are needed in each area of the country should be linked to the work done in session 2 "Identifying Community Needs and Resources: assessing the potential for solar drying..."
- (25 minutes) SFD Sizing: Rules of Thumb
Distribute and discuss SFD Sizing: Rules of Thumb Handout 5B. Give the group several related problems to solve to make sure everyone knows how to apply the rules.
10. (10 minutes) Trouble Shooting
As a final review of the design criteria, design principles and operation of the dryers, ask what factors might be responsible for problems such as: food rotting in the dryer, condensation under the glazing material, insect invasion and scorched food. Have participants suggest ways of solving each problem.
11. (20 minutes) Slide Show
Show either slides, drawings or pictures of a variety of dryer designs and have the group analyze them from different aspects such as the advantages and disadvantages of design, appropriateness of materials used, ease of construction and use.
SOLAR FOOD DRYER DESIGN CRITERIA
Make them strong enough to withstand vigorous cleaning as well as the weight of food.
Use a material that will allow good circulation of air through the tray.
Make them of a non-toxic material (don't use galvanized or aluminum tray material).
Design them to easily fit into the dryer.
Space them at least 5 cm. apart.
Maximize the difference in height between the inlet and cutlet vents.
Maximize the width of both vents. In a cabinet dryer, make both vents 1/3 the area of the back wall.
Insect-proof the vent openings with screen or netting.
Make a door over the bottom vent that hinges from the top side.
Make a door over the top vent that hinges from the bottom.
Make both doors sealable against night air.
Devise a way to maintain the vent doors open at different angles.
Should be of an appropriate size for its projected use.
Should be of a convenient height.
The floor and walls inside the drying chamber should be of a dark color.
Should be large enough for easy access to all the trays.
Should hinge either from the side or bottom
Should be insect-proof when closed.
Must be transparent.
May be two layers (with a 2-4 cm air space in between).
Should be considered as far as its: cost strength thickness ease of installation
Should have the correct compass orientation if built in place.
Should have the correct angle according to the time of year it will be used. (For summer: Latitude -109; winter: Latitude +30 Q; spring, fall and year round: Latitude +10 ÂºC.)
Inlet and outlet vents should be of the same size as the ones on the dryer.
Glazing angle may be different from the one on the dryer to maximize use throughout the year.
The absorber surface on the bottom of the preheater should be a dark color, and preferrably made of metal.
It can be removable, but should seal well to the bottom vent of the dryer.
SFD SIZING: RULES OF THUMB
- TRAY AREA SHOULD BE EQUAL TO MAXIMUM OF TWICE THE GLAZING AREA OF THE ENTIRE DRYER. Depending on the moisture content of the food being dried and the temperatures achieved, fewer trays may have to be used at times.
- DRYER CAPACITY - 5 TO 10 Kg OF FOOD/MÂ² OF TRAY AREA. This rule will give a very general idea of a dryer's capacity, and should be used according to the moisture content of the food being considered.
Example: Herbs could be figured at 10 Kg/MÂ²
Peaches at 5 Kg/MÂ²