Design and evaluation of a hybrid solar dryer for postharvesting processing of parchment coffee

Due to the extended drying time open-sun and solar drying of coffee procedures undergo, the development of microorganisms, mycotoxins and molds threaten the product. Alternatives such as mechanical dryers are available, nevertheless, their running costs and setups are usually expensive and unaffordable for small-scale coffee growers. Therefore, this research aimed to design, build and evaluate a hybrid solar dryer which mixes solar and mechanical drying principles. It uses a traditional solar tunnel-type dryer as a base featuring a biomass burner which uses coffee trunks left from the yearly crop renovation as biofuel. A heat exchanger heats the drying air, afterwards blown into a plenum chamber that homogenizes the air's static pressure before crossing the coffee bed, ensuring an even moisture removal. Also, the hybrid unit includes a photovoltaic system to obtain a fully self-sufficient drying unit. The newly developed dryer was tested under three different configurations: Solar and mechanical day and night (C1), solar during the day and mechanical during the night (C2) and fully solar with non-mechanical aid (C3). The results displayed a notable drying time reduction in the three evaluated configurations: C1 reduced the drying time by 70.47%, C2 by 45.75% and C3 by 21.5%. Also, the predictive model for different plenum chamber heights was obtained through computational fluid dynamics simulations, where the ideal height was 0.25 m. A biomass consumption of 1.9 kg h−1 was registered. Also improved temperature and relative humidity profiles were achieved. Its design easily adapts to the existing tunnel and parabolic-type solar dryers.