3.3.4.3 DRYING OF AGRICULTURAL PRODUCTS

Using solar energy to dry crops is nothing new in the tropics. Many edible and even cash crops, such as cocoa and coffee beans, have been dried for decades on racks placed in the sun. In the case of a closed heated space, in which a damp agricultural crop has been stored, two things happen:
· The crops are warmed by the heat source.
· The surrounding air is also heated, absorbs moisture and rises, inducing more air.

Solar dryers can be divided into two categories:

1. Dryers in which the sunlight is directly employed; here the product itself absorbs most of the solar heat directly (fig. 3.11). These are further divided into three types:

Source CRES-TRASOL CD-Rom
Figure 3.11 Solar drier directly employed

2. Dryers in which the sunlight is employed indirectly (see fig. 3.12). In this method, the drying air is warmed in a space other than that where the product is stacked. The products are not then exposed to direct sunlight. Various sorts of construction are possible; this design can also be provided with powered fans in order to optimise the air circulation.

Source CRES-TRASOL CD-Rom
Figure 3.12 Solar drier indirectly employed