Solar drying
Practical Action
During the falling-rate period, the rate of drying is largely controlled by the chemical composition
and structure of the food. Design of a dryer depends upon the drying rate curve of the material
to be dried but these curves are indicative only and depend upon the factors mentioned above.
The heat required to evaporate water is 2.26kJ/kg. Hence, approximately 250MJ (70kWh) of
energy are required to vaporise 100kg water. If the ambient air is dry enough, no heat input is
essential. The greatest potential for drying crops in a short time is when the ambient air is arid
and warm. If the air is warm then less air is needed. This temperature will itself depend mainly
on the air temperature but also on the amount of solar radiation received directly by the food
being dried.
Solar drying Operation
All dryers need ventilation to be able to dry crops effectively. Air movement can be by natural
convection or can be assisted using fans. Solar food drying can be used in most areas but how
quickly the food dries is affected by the variables indicated above, especially the amount of
sunlight and relative humidity. Typical drying times in solar dryers are from 1 to 3 days
depending on sun, air movement, humidity and the type of food to be dried. Most dryers are
black inside, either painted or with black polythene inserts to absorb as much solar radiation as
possible.
Cabinet dryers
Base
Door
Tray
Legs
Insulation
Vent holes
Figure 2: Section through the cabinet
dryer showing the flow of air in through
the vent holes in the underside past food
placed on the drying trays and out of the
holes at the top of the cabinet.
Figure 1: The Brace solar cabinet dryer
Tent dryers
The distinguishing feature of tent and
cabinet driers is that the drying
chamber and the collector are
combined. Such dryers provide
protection from dust, dirt, rain, wind,
and pests.
Clear polythene
Figure 3: Tent Dryer
2
Air inlet