Many types of drying systems utilize both direct and indirect solar radiation. In these types of
systems, radiant energy from the sun falls directly onto the product being dried; however, in
addition, a preheater also is used to raise the air temperature, which in turn, accelerates the
drying rate. Acceleration of drying rate can occur in two ways: hot air can transfer some of its
heat to the product being dried, thus raising its vapour pressure causing a faster moisture loss;
or as temperature of air mass increases, the water-holding capacity also increases