An experimental investigation of a greenhouse solar dryer integrated with a solar air heater and a rock bed thermal storage material

Abstract

Efficient postharvest curing of onion bulbs requires controlled thermal and humidity conditions to reduce moisture effectively while maintaining product quality, especially in rural areas with limited access to conventional energy. In this context, this study aims to experimentally evaluate the thermal performance of a modified Quonset-shaped solar greenhouse dryer covered with a polycarbonate sheet under active and passive modes at Bahir Dar Institute of Technology. To achieve this objective, the active system incorporated a solar air heater with a rock bed thermal storage unit, extended surfaces to enhance heat transfer, and a suction blower for forced convection. Performance was assessed by monitoring temperature, relative humidity, and air velocity from 9:00 AM to 5:00 PM using onion bulbs as the test crop. The results revealed that moisture content decreased from 83% to 77.2% (wet basis), with curing completed in 10 hours under active mode, 22 hours in passive mode, and 36 hours under open sun conditions. Zeolite desiccants at the air inlet reduced relative humidity to as low as 10%, enhancing drying efficiency. Maximum temperatures reached 61 °C in the greenhouse and 108 °C in the solar air heater. The solar air heater achieved an average efficiency of 53.78%, while the overall system efficiency reached 31.74%. All cured bulbs exhibited full neck tightness, confirming effective curing. These findings indicate that combining thermal storage, humidity control, and enhanced airflow significantly improves drying rate and product quality, offering a practical and sustainable solution for agricultural processing in rural areas, contributing to food security and clean energy access in line with the Sustainable Development Goals.