Evaluation of a Solar-Biomass-Rock Bed Storage Drying System, and its Application for Chilli Drying

Abstract

A renewable energy-based air heating system that does not require a conventional auxiliary heater, but can still meet a daily load fraction exceeding 90% and supply hot air at a steady temperature and flow rate continuously for 24 hours a day, has been developed. It combines an unglazed transpired solar collector (UTC), rock bed, and a biomass gasifier stove with heat exchanger. The system utilises part of the rock bed to supply supplementary heat. The UTC supplies the required hot air during the day (to meet the load), and the stove-heat exchanger unit supplies hot air to the rock bed (to charge it), also during the daytime. The rock bed stores the thermal energy during the daytime, and supplies heat during off-sunshine hours — both during day and night. The system was evaluated by drying 22 kg of red chilli, using hot air at 60°C and 90 m³/h, from an initial moisture content of 76.7% (w.b) to 8.4% over 32.5 hours of continuous drying. The dryer contributed to a reduction of 66% in drying time compared to open sun drying. The temperature of hot air supplied was stable at 60±3°C for over 24 hours during the entire drying duration. Providing a load fraction of 91.6% during the 24-hour operation, the air heating system can successfully dry red chilli in a continuous drying operation, at required air temperature and flow rate, which can be maintained fairly constant.