Direct solar dryers

These direct solar dryers are modeled after plans from a Volunteers in Technical Assistance (VITA) publication that looks to be from 1984.  The default models have been built with marine board (plywood) and either a polyethylene sheet or fiber glass roofing sheet for glazing.  We have also built and will test a model made with the same mud and sawdust brick material used to make rocket stoves.  It should also be possible to build the collector box out of sawn lumber. for reduced cost but increase labor for assembly.  

A summary of what I have learned to date is found in this presentation from the 8th ECHO East Africa Biennial Symposium in collaboration with Hope International Burundi which was held 25 to 27 February 2025.  There is also a pdf version if Google Slides is not working for you.  Some of this work is also summarized in ECHO Development Notes (EDN) #69.

Based on Arusha prices, the default 3 tray model materials cost about  112,000 TZS with a fiberglass glazing while the 4 tray model materials cost about 147,000 TZS.  

The food drying trays we have been using are about 79 cm  by 37 cm and each tray can hold about 1 to 1.5 kg of wet fruit like mango.  In a seaside location in Tanga (abundant sunshine but also high humidity), mangoes, tomatoes, and papayas have all been dried in about 1.5 days in this kind of dryer.  If the materials to be dried get several hours of good sun and form a dry skin on the surface, they can be left in the dryer over night to complete the drying the next day without .  Less moist materials like leaves or cooked beans may  dry in one day.  It may be possible to stack a second tray on top of each tray to double the drying area.  This will extend the drying time but should also increase capacity.

There is a Kenyan company marketing a manufactured (plastic molded) model of a direct dryer.  It goes by the  name Dehytray and is being sold online for $75 to  $100 USD depending on the quantity purchased.

I have also found references to a solar tunnel dryer which fits under the umbrella of a direct solar dryer.  In the document (with a backed up copy kept in case the website goes away).   It makes mention of large 20 m long tunnel dryers.   I first learned about the tunnel dryer formulation of the direct dryer from the Solare Bruecke website that has some other useful resources.  There is also a paper on using tunnels dryers to make fruit leather.