Experimental Investigation of an Appropriate Thin Layer Drying Model of an Integrated Greenhouse and Flat Plate for Solar Coffee Dryer

Abstract

The predominant practice of coffee drying in Ethiopia involves an open sun system with direct exposure to solar radiation. This type of drying system leads to spoilage and quality degradation, and it also requires a longer drying time. This study presents an experimental investigation of the effectiveness of various thin-layer drying models in the context of a solar coffee dryer integrated with a greenhouse and flat plate collector system. The primary objective was to identify an appropriate drying model that best fits in the experimental results in the integrated dryer. Experimental setups were designed to simulate real-world drying processes, and data were collected on solar Irradiation, temperature, and moisture content over time. Both the greenhouse and the flat plate solar heater are encased in polyethylene plastic and transparent glass, measuring 1 m by 2 m and 1.6 m by 5 m in length and width, respectively. Seven models were evaluated for their accuracy in representing the drying behavior of coffee. Statistical analyses were employed to assess model performance. The findings highlight the importance of selecting an appropriate drying model to optimize the efficiency of solar coffee drying systems, ultimately contributing to improved quality and reduced post-harvest losses in coffee production. The drying result shows the average temperature difference between the greenhouse and the surrounding air was 10.7 °C. At the latitude of 6.4126 N and longitude 38.3008° E, 50 kg of wet, freshly harvested coffee with an initial moisture content of 54.23% was added to the system. After 80 h of drying in the integrated solar dryer, 24.14 kg of moisture was removed, and it takes 130 h in the open sun. The daily average solar insolation for the test periods of November and December was 673.267 W/m². Finally, to perform statistical analysis. Newton models were found to be the best explanation of coffee behaviour in the integrated flat plate and greenhouse dryer systems with R² = 0.99 and Two-Term in the open sun model with R² = 0.979. These models were tested by comparing the maximum value of the coefficient of determination R² and the minimum value of reduced chi-square X² and root mean square error RMSE between experimental and predicted moisture ratios.