Investigating the Performance and Optimization of Solar Coffee Drying Technologies—A Systematic Review

Postharvest crop losses, dependence on fossil fuels, and associated environmental consequences have emerged as critical global challenges. Drying is a food preservation method that consumes about 12%–15% of agricultural processing energy. A greenhouse solar-based crop drying system uses solar energy to create controlled drying conditions, preserving crop quality and reducing drying time. Coffee, a cash crop and primary source of income for many developing countries, is facing considerable postharvest losses due to quality degradation from traditional drying methods. This review article identifies efficient and practical solar coffee dryers suitable for large-scale producers in rural areas. It also examines the parameters for evaluating the performance of coffee drying systems. The review also highlighted the postharvest coffee processing techniques and drying kinetics. Based on the systematic selection method among the studies collected from three databases, 36 existing papers are included for comprehensive review. All the reviewed papers reported that solar-based coffee dryers are promising alternative systems for coffee drying. The type of dryer, their configurations, air temperature, humidity, and airflow rate are the key factors that influence the coffee drying rate and the dryer’s overall performance. Among the classifications identified, direct-type greenhouse solar dryers are found to be scalable and cost-effective to use in developing countries. Furthermore, it was found that the design with an insulated north wall increased the air temperature for natural and forced convection modes by 1°C–6.7°C and 1°C–4.5°C, respectively.