Solar Dryer

Hello Ndii! There are many alternatives for your ongoing project, most are very affordable as well: you can make a tunnel using nylon, or a box dryer using glass and wood. Also check: www.efn.org

This article by Barbara Kerr gives an overview of solar dryers, basic principles, different designs, and also tells how to pasteurize dried fruit, or to use the same process to kill weevils if the fruit becomes infested.

An enclosed design is most hygienic, keeping away animals and insects - of course the air intake and outflow must be screened and the box is built to be as airtight as possible when closed.

There are three examples of enclosed solar dryers with solar collector and chimney, shown in this video. The last two are "downdraft" solar dryer designs - which is recommended as the best design in all weather conditions. Updraft designs may not work in cloudy weather, taking longer to dry the fruit, or may overheat or burn the food on very hot days.

A schematic of a very simple downdraft box can be seen in pages of this book. The air is circulated by natural forces: warm air rises, moist air sinks.

The wood, screens, and glass or plexiglass needed to make these dryer designs would be more expensive than the simple screen tunnel covered with cloth, but they are durable, hygienic, and effective dryers, if the materials can be obtained.

You may also want to read this page - many of the links no longer work, but there are some contacts listed who would have, by now, a decade of relevant experience in Africa, and designs for community scale as well as family scale dryers.

A simple system that is inexpensive and quite efficient involves cutting the produce and laying it fleshy side up on an inclined wooden rack covered with bird wire mesh, and then placing a sheet of corrugated iron over it with about 200mm clearance. The iron is painted black, and most importantly the top of the sheet must be at least 300mm higher than the bottom (the rack also follows the "roof" contour). This will ensure that if there is no breeze, the hot air will form a convection current exiting at the high point. It is this air flow, or any other air flow, that stops the drying product from burning or 'cooking' too fast. In hot weather quite fast drying rates can be achieved. If insects or vermin could be a problem use flywire to enclose the sides and bottom, and slide the bird wire rack into it.

Solar drier is good for the work.A single glass covered box can build up to 100 degree centigrade temperature in the box. 1 SQUARE METER solar radiation is 1000 watts.THE DAILY AVERAGE HEAT AVAILABILITY IS 5000calories per sq meter.For adding temperature two more glasses need be placed over the first one with 1 cm thick boarder gasket of thermo cool or wood reaper.