Modeling, environmental and economic analysis of drying of orange slice in an automatic indirect mixed mode solar dryer

Abstract

Abstract This study presents an indirect mixed-mode solar dryer (IMMSD) integrated with a photovoltaic system with an automatic, temperature-responsive control system that automatically switches between natural and forced convection, enhancing efficiency and reducing energy use. Unlike previous fixed systems, it prevents over-drying and spoilage. The cost-effective, solar-powered design suits off-grid communities. By integrating drying kinetics with economic and environmental assessments, the system supports sustainability goals. The study also examines how slice thicknesses and tray positions within the drying room affect orange drying kinetics. Then, the IMMSD was used to dry orange slices under real conditions at Aswan University, Egypt, in January 2025, aiming to evaluate performance across different slice thicknesses (4, 6, and 8 mm) and tray positions (lower, middle, and upper levels). The results demonstrated that orange slices with a 4 mm thickness dried on the lower tray reached the final moisture content (MC) more quickly than thicker slices (8 mm) dried on the middle and upper trays. The 4 mm slices achieved the lowest final MC at approximately 12.5%, with a 48% reduction in drying time compared to 8 mm slices on the upper tray. The effective moisture diffusivity (D eff ) ranged from 4.5 × 10⁻⁸ to 15 × 10⁻⁸ m²/s. Additionally, five semi-theoretical models—Midilli, Modified Midilli I and II, Aghbashlo, and Henderson–Pabis—provided the best fit for modeling the drying behavior of orange slices. The environmental analysis showed that the energy required for moisture evaporation ( $$\:{E}_{at}$$ ) increased with slice thickness, reaching 916.56, 1372.31, and 1839.76 kWh for 4, 6, and 8 mm slices, respectively. The 8 mm slices yielded the highest annual dried product output, net CO₂ reduction (90.72 tons), and the shortest energy payback time (0.85 years). Carbon credits ranged from 2179.29 to 4535.76 USD. Economically, the IMMSD required a low capital cost of 700 USD and annual costs of 996.12 USD, generating yearly savings of up to 14,015.9 USD, that reduced the payback period of investigation up to only about one month.