2.4 Mixed-type solar dryers
In this type of solar dryers the material to be dried is heated by two ways, through the direct absorption of solar radiation and the preheated air coming from the solar air heater. Bolaji and Olalusi [37] constructed a mixed-mode solar dryer for food preservation. They reported that the temperature rise inside the drying cabinet was up to 74% for about 3himmediately after 12.00 h (noon) The drying rate and system efficiency were 0.62kg/h and 57.5% respectively. The rapid rate of drying in the dryer reveals its ability to dry food items reasonably rapidly to a safe moisture level. Results showed also that during the test period. the temperatures inside the dryer and solar collector were much higher than ambient temperature during most hours of the day-light. Tripathy and Kumar [38] constructed a laboratory scale mixed-mode solar dryer consisting of an inclined flat plate solar collector connected in series to a drying chamber glazed at the top. They used the dryer to perform natural convection drying of potato cylinders of length 0.05 m and diameter 0.01 m and slices of diameter 0.05 m and thickness 0.01 m. Simate [39] designed, constructed and tested two different types of natural convection solar dryers. For the mixed-mode, the drying chamber cover was transparent whereas for the indirect mode it was opaque. Numerical calculations indicated that a shorter collector length should be used for the mixed-mode solar dryer (1.8 m) compared to the indirect mode dryer (3.34 m) of the same capacity (90kg) The quantity of dry grain obtained from the mixed-mode for the whole year is about 2.81 tones and was less than that from the indirect mode by 15%. Forson et al. [40] designed a mixed-mode natural convection solar dryer. They proposed a methodology combining principles/concepts and rules of thumb that enable the design of