Comparative Study for Two Types of Solar Dryers (Modified-Quonset and Lean-to) for Drying Stevia Plants (Rebaudiana bertoni)

The main objectives of this research work were to design, construct, and operate two different active solar dryers (Modified-Quonset and Lean-to structural forms). Thermal performance analysis of the two solar dryers, drying characteristics of thin layer of Stevia plants, and comparing experimentally between them were also tested, examined and evaluated. Moreover, the effect of drying process on the three different physical and chemical components (pigments, primary components, and anti-oxidant) had measured before and after the drying process using the two different solar dryers. The experimental work carried out on the completed plants (comprehended stems, branches, and leaves) from 25^st to 28^th of September 2016. The obtained results revealed that, the completed Stevia plants dried from the initial moisture content of 73.56% w.b. to 7.55% w.b. within 36 hours with drying rate of 30.03 g/hr using the Modified-Quonset solar drier. While, using Lean-to solar drier, the initial moisture content reduced from 73.56% w.b. into 5.35% w.b. within 12 hour achieving drying rate of 90.9 g/hr. For the duration of the experimental work, the hourly average thermal efficiencies for the Modified-Quonset and Lean-to solar driers, respectively, were 59.57% and 61.91%. The diurnal average solar drying efficiencies for the two solar driers during this research work were 20.45% and 25.51%, respectively. Therefore, the drying efficiency of the Lean-to solar drier was greater than that of the Modified-Quonset solar drier by 24.74%. Results also revealed that, the quality of dried product using Lean-to solar drier is better than Modified-Quonset solar drier. The very important content in the Stevia plants as a glycoside species, is the total sugar, which on an average slightly reduced (23.35%) due to drying process using Lean-to solar driers as compared with the Modified-Quonset solar drier when the total sugar reduced by 29.44%.