A hybrid smart solar dryer (HSSD) based on indirect forced convection and a controlled auxiliary heating system was developed, fabricated, and tested to be convenient for sunny and cloudy weather conditions. The achievements of the developed dryer focus on controlling the temperature of the dryer, increasing the drying rate, reducing energy consumption, and providing high-quality products. The HSSD was tested and evaluated for drying basil and sage herbs at 30, 40, and 50°C. The results showed that the fresh basil and sage leaves of 1 kg with a moisture content of 84.7% and 75.53% (wet basis) were dried within 58, 46, 32 and 38, 28, and 20 h at 30, 40, and 50°C, respectively. Correspondingly, the traditional drying methods achieved 96 h outdoors and 144 h indoors at room temperature. The average of the fabricated flat-plate solar collector efficiency (thermal efficiency, ηfpsc ) was ranged from 49.18% ± 9.52% to 66.02% ± 2.8%. The highest drying rates were achieved with the HSSD method. Moreover, the HSSD method led to a remarkable saving in energy with values ranging from 25.54% to 77.1% versus the traditional drying methods. While the best quality in terms of essential oil content and microbial load for the dried basil and sage herbs was achieved by the HSSD at 40°C. Finally, the HSSD is a promising energy-efficient method where it can save 70% of energy consumption, thus reducing the carbon footprint of drying processes, and providing higher quality products compared to the conventional methods.
Keywords: drying; hybrid drying; medicinal and aromatic plants; solar energy.
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