Optimization and evaluation of a semi-continuous solar dryer for cereals (Rice, etc)

Abstract

In this research a new approach for employing solar radiation as the main source of energy for paddy drying was introduced. The drying test rig was designed, fabricated and evaluated. The rough rice solar dryer was a cross flow and an active mixed-mode type with a new and an efficient timer assisted semi-continuous discharging system. The rig consists of six ordinary solar air heaters, an auxiliary electric heating channel, a drying chamber with an electrically rotary discharging valve and an air distributing system. The area of each collector was 2 m 2 (totally 12 m 2) and they were installed on a light frame tilted 45° towards the south. The drying system consisted of: an inlet bin, a drying chamber ended with a discharging valve, an outlet bin and a plenum chamber. The dryer bed, with 2 m 2 surface area, acted as a single glazed solar collector was also tilted 45° towards the south. At the bottom of dryer bed, an electro-mechanical rotary valve was installed which was controlled by a timer. The timer activated the rotary valve to operate once a while to discharge the dryer bed semi-continuously. Air mass flow rate was measured by an orifice plate, the temperatures were monitored by T type thermocouples, the solar insolation was recorded by a solarimeter, and the electrical energy consumed by the fan and the heating channel, was recorded by watt meters. To evaluate the drying system, a local variety of medium size kernel of rough rice was selected to be dried by the dryer. One of the objectives in this research was to evaluate the effect of mass flow rate and interval time of crop discharging on the rate of crop drying by the dryer. Two distinct factorial experiments were conducted in a completely randomized design with three replications for each treatment. The first experiment was conducted with two factors: mass flow rate (three levels), and discharge interval time (two levels). The second experiment was conducted with three factors: the moisture content of different locations on dryer bed (four levels), mass flow rate (three levels), and discharging interval time (two levels). The dryer capacity, the efficiency of collectors and the overall efficiency of the drying system were evaluated. According to Duncan’s multiple range test, the effect of mass flow rate of drying air, and the interval time of discharge on rate of drying, were significant at 0.01 level. The effect of air mass flow rate and discharging interval time were highly significant on rough rice moisture content along the dryer bed. The maximum overall efficiency of drying system was 21.24% (with average drying air temperature of 55°C) and the fraction of energy consumed by the auxiliary heating channel during the drying process compared with solar energy was only 6–8%. The maximum capacity of the dryer was about 132 kg of rough rice with initially 27% db down to 13% db final moisture content in 3 h of drying period (11–14 with 865 W/m 2 average incident solar radiation and average ambient temperature of 25°C).