Mathematical Model and Experimental Analysis of a Solar Dryer with Parvati Geometry for Coffee Beans

This article reports a mathematical model with experimental measurements of a novel solar dryer named: Parvati Solar Dryer (PSD), which consists of a double angle linear collector and a cylindrical drying chamber with a translucent surface. Semi-theoretical and empirical thin-film models were evaluated to select the one with the best fit (higher coefficient of determination R2 and lower mean square error RMSE) in terms of the humidity ratio MR and taking into account that the drying chamber temperature does not exceed 35 ℃ to maintain the quality of the coffee. The linear collector has a mechanism that changes the reflection angles; therefore, the solar energy is captured over the focal zone where the drying chamber is located. Experimental tests were carried out using 25 kg of coffee beans of the Caturra Arabica variety. The initial coffee beans moisture content was dropped from 53 to 12.5% w.b. during a period of 33 days with 6 h of drying per day. The beans drying speed was 2.2 g/day with an average solar irradiance of 490.3 W/m2. The efficiency of the drying was 36.6%. It was found that the Hii model best fits the drying process with R^2 = 0.997 and RMSE = 0.0586. Finally, the average thermal diffusivity was 1.48 × 10^–10 m^2/s.