Design, manufacturing, and performance evaluation of box-type direct solar dryer for onions ( <i>Allium cepa L.</i> )

Smallholder farmers in Ethiopia face significant postharvest losses of perishable crops like onions. Solar drying offers a promising solution, yet adoption remains limited. This study explores eco-friendly solar drying methods to extend the shelf life of onions. It focuses on the design, manufacture, and performance evaluation of a box-type direct solar dryer for drying onions. The dryer’s performance was assessed in terms of efficiency, drying rate, and uniformity of temperature distribution. Results showed a drying efficiency of 20%, compared to 16.4% for open-sun drying, with a mass flow rate of 2.84 kg/hr. The dryer reduced onion moisture content from 87% to 7.5% in 77 hours, achieving a drying rate of 0.024 kg/hr, twice that of open-sun drying (0.0124 kg/hr). The drying chamber maintained a temperature of 3–5 °C above ambient, ensuring consistent drying conditions, with an average heat energy transfer of 7265 J. Overall, the solar dryer dried onions 21% faster than open-sun methods, making them safer for storage and transport. Drying data were fitted to seven mathematical models, with the Page model providing the best fit based on maximum coefficient of determination (R2), minimum reduced chi-square (χ2), and root mean square error (RMSE). These findings highlight the potential of direct solar drying to improve postharvest onion preservation.