There are two known solar drying techniques, direct drying and indirect drying. The most technical aspects are with indirect drying methods. These can be expensive to build and require motor driven fans to move the heated air. Direct drying is simple technology but the box design is important. Fruit-Full employs direct drying technology and has developed an industrial form of direct dryer using angle iron, plywood or cement board, table cloth plastic and insect screen for fruit support. The photograph shows our direct dryers which are quite simple.
Drying is of course a relatively simple concept. Heated air expands which leaves more room for water molecules to be taken up by molecules of expanded air. Moisture is at the surface of the cut fruits. By causing air to pass over these surfaces it also causes the moisture at the surface to be collected. To have the air move one must create convection currents in the box. The inside of the box is painted black so that heat is absorbed at the surface which then accumulates in the box. Hot air rises and this is important to the passive machine we create. The hot air seeks escape through the holes at the top of the box. Once escaped, it leaves a pressure (negative) in the box. This is felt by the air outside the box as a pressure differential. Air then enters at the bottom holes, is heated, completes the convection cycle and sets up patterns of air movement, which is the total function of the drying box.
The argument that is created between direct and indirect drying concerns the action of UV light on the surface of the fruits. UV is known to break down organic molecules. This is a known fact. (Check the action of sunlight on your skin, it will burn after much exposure). Indirect drying never allows fruits to see direct sunlight hence no UV exposure. Direct drying places the fruits in sunlight. The fruits then are subject to UV. However, any screen like a sheet of plastic acts as a filter to UV and also a reflector of UV until angles of incidence are too great for reflection. This happens at about 11am to 1pm. At those hours the fruit is indeed subject to filtered UV. The disturbance at the surface is so minimal that the dried fruit quality is not affected. Because that fact is true we offer our fruits as the high quality products they are.
Direct drying boxes allow us to take dryers to the source of fruits and thuse we don not ship water which reduces costs to you. Farmers can then learn food security techniques. We can build you solar drying boxes and will gladly enter negotiation with anyone interested. Just remember shipping would be a feature. Our design allows for local maintenance at relatively low cost which is another reason we have chosen direct drying.