Design of mixed-mode natural convection solar crop dryers: Application of principles and rules of thumb

A mixed-mode natural convection solar crop dryer (MNCSCD) designed and used for drying cassava and other crops in an enclosed structure is presented. A prototype of the dryer was constructed to specification and used in experimental drying tests. This paper outlines the systematic combination of the application of basic design concepts, and rules of thumb resulting from numerous and several years of experimental studies used and presents the results of calculations of the design parameters. A batch of cassava 160 kg by mass, having an initial moisture content of 67% wet basis from which 100 kg of water is required to be removed to have it dried to a desired moisture content of 17% wet basis, is used as the drying load in designing the dryer. A drying time of 30–36 h is assumed for the anticipated test location (Kumasi; 6.7°N,1.6°W) with an expected average solar irradiance of 400 W/m2 and ambient conditions of 25 °C and 77.8% relative humidity. A minimum of 42.4 m2 of solar collection area, according to the design, is required for an expected drying efficiency of 12.5%. Under average ambient conditions of 28.2 °C and 72.1% relative humidity with solar irradiance of 340.4 W/m2, a drying time of 35.5 h was realised and the drying efficiency was evaluated as 12.3% when tested under full designed load signifying that the design procedure proposed is sufficiently reliable.