Investigation of a mixed-mode solar dryer assisted with an air recycling system and phase change material unit for coffee beans drying: An experimental study

This study investigates a novel mixed-mode solar dryer designed to improve thermal efficiency in drying processes. The dryer incorporates a drying chamber connected to evacuated tube solar collectors, an air recirculation system, and phase change materials (PCM). The performance of the solar dryer was evaluated using coffee beans, with air recycling ratios of 100 %, 50 %, and 0 %, alongside PCM utilization. Results indicated that the 100 % recycling ratio raised the drying chamber temperature by 4°C–10 °C compared to the other ratios. With the assistance of PCM and an air recirculation system, the final moisture content of the coffee beans reached 12.23 % and 10.30 % for 100 % and 50 % recirculation ratios within 9 h to the Indonesian National Standard. Combining air recycling and PCM enhanced dryer efficiency while reducing energy consumption. The highest average solar dryer efficiency and specific moisture extraction ratio were recorded at the 100 % ratio with PCM, reaching 18.51 % and 0.282 kg/kWh, respectively, with the lowest specific energy consumption of 3.887 kWh/kg. The fastest payback period for this dryer is 1.56 years, yielding annual savings of 5496.32 USD over its 20-year at a 50 % ratio with PCM. Hence, this mixed-mode solar dryer has proven excellent thermal performance and economic feasibility.