Influence of Evacuated Tube Count and Stone Dust Particle Size on the Thermal Behavior of a Passive Solar Dryer

ABSTRACT This study presents a foundational experimental investigation into optimizing the heat‐source–thermal‐storage pairing of an evacuated tube collector (ETC) – assisted passive solar dryer under no‐load conditions. The effects of ETC count (one, two, and three collectors) and stone dust particle size (≤ 300 μm, ≈600 μm, and ≈1.18 mm) on thermal behavior were systematically evaluated. Performance was quantified using peak tray temperature ( T ), midday mean temperature (Td), and evening temperature drop (Δ T ) between sunset and 21:00 h. Results show that increasing ETC count raised T from 52°C–59°C (single ETC) to 70°C–80°C (two ETCs), while three ETCs achieved maxima of 92.4°C (fine stone dust), 84.6°C (medium), and 81.2°C (coarse). Stone dust texture strongly influenced thermal retention: fine dust cooled fastest (Δ T ≈37°C), coarse dust retained heat longest (Δ T ≈31.8°C), and medium dust provided the most balanced response (Δ T ≈33.6°C). Although three ETCs delivered the highest peak temperatures, the marginal gain beyond two ETCs was limited, indicating diminishing returns in collector augmentation. These findings provide a low‐cost and practical pathway for improving passive solar drying performance in rural applications.