Design and simulation of the coffee drying unit using solar thermal energy and ambient air circulation

Sun drying is one of the most crucial and widely used techniques in coffee processing due to its effectiveness in inhibiting microbial spoilage and slowing enzymatic activity. However, several drawbacks exist, such as the prolonged too high surface temperature, and strong dependence on weather conditions. Despite these limitations, sun drying is preferred over mechanical drying because of its low cost and availability. High temperature drying can degrade the structure of green bean coffee and its overall quality. This study focuses on the design and simulation of a coffee drying greenhouse using a three-dimensional model developed in COMSOL Multiphysics and an experimental solar drying unit. The research aims to analyze airflow characteristics and temperature distribution within the greenhouse. Simulations examine airflow through inlets of varying areas (0.010, 0.025, and 0.050 m2 ) and different exhaust fan sizes (6-, 8-, and 10-inch), each with different volumetric flow rates (270, 540, and 900 m³/h), with sunlight transmitted only through the greenhouse roof. Results indicate that narrower inlet openings, together with larger ventilation fans, resulted in higher inlet air velocity. For larger fans, airflow recirculation became more noticeable due to the increased flow rate, leading to the formation of larger vortex zones, especially at the center of the drying house. Additionally, narrower inlet openings consistently resulted in higher internal temperatures for the same fan size. Simulation results show that with a 0.050 m² inlet area, the drying house maintained internal temperatures of 38.7–41.0 °C. Reducing the inlet area to 0.025 m² and 0.010 m² raised these temperatures to 39.9–43.2 °C and 42.4–45.3 °C, respectively. The smaller inlet area limited airflow and thus reduced convective heat transfer, causing more heat to be retained within the drying house. Consequently, the overall interior temperature increased. Moreover, different coffee processing stages require different temperature ranges, suggesting that operators should adjust the inlet area accordingly to maintain optimal conditions throughout the drying process.