Experimental and analytical study on the performance of an efficient octagonal greenhouse solar chimney as a solar dryer

Agricultural product drying is one of the most widely used preservation techniques for reducing post-harvest losses and extending shelf life. However, conventional solar dryers often suffer from non uniform airflow distribution, thermal instability, airflow stagnation zones, and limited drying efficiency under varying climatic conditions. To address these limitations, this study experimentally and analytically investigates the performance of a novel octagonal greenhouse solar dryer integrated with a solar chimney under different seasonal operating conditions. Unlike conventional rectangular and trapezoidal configurations commonly reported in the literature, the proposed octagonal geometry was designed to enhance airflow circulation, improve thermal distribution uniformity, minimize stagnant airflow regions, and provide improved structural stability under wind loading conditions. Four representative months (March, June, September, and December) were selected to evaluate seasonal performance and simulate year-round operating conditions. Both leafy products (Molokhia and mint) and non-leafy products (tomato and onion) were experimentally and analytically examined under different climatic conditions. The results showed that the proposed system effectively utilized available solar energy, achieving an average thermal efficiency of approximately 50%. The required solar energy to reach the target moisture content was approximately 7.8, 8.1, 4.4, and 3.35 kWh/m² for tomato, onion, Molokhia, and mint, respectively. Leafy products exhibited lower energy demand and were successfully dried within one day, while tomato required approximately 1.5-2 days and onion required 2-3 days to reach safe moisture levels .Thermal analysis indicated a progressive increase in air temperature from ambient conditions to absorber plate temperatures, with the highest performance observed in June and the lowest in December. Furthermore, experimental validation conducted on 15 March showed strong agreement between predicted and measured results, confirming the reliability of the proposed analytical model. Overall, the proposed octagonal greenhouse solar chimney dryer exhibits improved thermo-aerodynamic performance compared with conventional greenhouse dryer geometries and provides a reliable low-energy solution for agricultural drying applications.