The grain is, therefore,dried by a combination of both direct radiation with conduction of heat from the
top layer grains to the bottom ones and the convection of hot air from the solar air
heater entering the bottom layers and moving to the top ones [7]. In indirect-mode
dryers on the other hand, grain is dried by hot air alone from a solar air heater.
Solar dryers can be cost effective because relatively unskilled village artisans can
construct, operate and maintain the dryers at minimum cost and that cheap and locally
available materials can be used for the construction [2]. However, choosing the
geometry of the dryer and the materials for construction requires the use of physical
and biological sciences through computer simulations [6]. Simulation enables the
researcher to evaluate the performance of the solar dryer before actual production
of the prototype and experimentation is done. If simulation is combined with optimization,
the researcher is able to design a cost effective solar dryer, that is, a solar
dryer with minimum cost and maximum drying rate within given constraints [7].
Several studies have been carried out on the optimization of indirect solar drying
systems, both forced convection [8–10] and natural convection [11]. In these systems,
grain at the bottom of the bed dries rapidly while that at the top is still wet due to
condensation. Optimization of indirect natural convection systems has resulted in
long collectors with thin grain beds. Recent work on the simulation of a natural
convection mixed-mode solar dryer has included the effects of the drying chamber
transparent cover through a deep bed model that combines conduction and convection
in the grain bed [6]. Oversizing the drying chamber transparent cover will make the
system too expensive but with increased drying rate while undersizing reduces the
cost but causes a slow drying rate. Optimization of the collector and drying chamber
dimensions is, therefore, essential to make effective use of the mixed-mode solar
dryer.
The objective of this study is to carry out optimization of the mixed-mode and
indirect-mode natural convection solar dryers for maize and compare their performance.
The capacity chosen for this study is 90 kg since this is a standard unit of
measurement in the storage and marketing of maize.