Experimental investigation and energy analysis of a solar-assisted fluidized-bed dryer including solar water heater and solar-powered infrared lamp for paddy grains drying

During drying process of paddy grains, as one of the major operations of rice milling process, a considerable amount of fossil fuel is consumed. The air pollutants emitted by combustion of fossil fuels are great threat to human health which strongly contribute to environmental problems. In the present study, to reduce the share of fossil fuels in drying process of paddy grains, a new type solar-assisted fluidized-bed dryer was used with the ability of thermal energy recovery. The drying system mainly consisted of a solar water heater, a solar-powered infrared lamp, a gas water heater, and a desiccant wheel. Two general conditions were considered for conducting the experiments: Natural gas drying (NGD), and Solar-assisted drying (SAD). The results of evaluations showed that the highest total energy consumption of the dryer was equal to 1.163 kWh which was obtained in NGD test mode, while the lowest amount of total energy consumption was obtained in SAD mode which was 0.314 kWh. The specific energy consumption of the drying system in SAD and NGD modes was in the range of 8.30-22.12 and 16.73-32.62 kWh/kg water evaporated, respectively. The highest value of solar fraction index was equal to 0.538 which was obtained at drying air temperature of 45 degrees C, drying air velocity of 8 m/s, while the lowest solar fraction value (0.179) was attributed to drying air temperature of 35 degrees C and air velocity of 7 m/s. Although applying the infrared lamp caused the solar energy fraction to increase to 0.741, however, the use of infrared lamp in the fluidizing chamber did not have a significant effect on the dehydration rate of the product. At two levels of the applied drying temperatures (35 degrees C and 45 degrees C), the required thermal energy during the drying process was completely supplied only by solar energy and without the need for fossil fuel.