Thermal and drying performance of a solar dryer with evacuated tubes and sand storage

Solar drying is increasingly being adopted as a sustainable alternative to conventional fuel-based preservation systems. This study investigates the effect of evacuated tube collector (ETC) count and sand particle size on the performance of a solar dryer integrated with sand-based thermal storage. Experiments were carried out using fine-grained sand (FGS), medium-grained sand (MGS), and coarse-grained sand (CGS) across one-, two-, and three-ETC configurations. Performance evaluation was based on drying rate (g/min) and drying efficiency (%). The results show that drying rate improved consistently with an increase in ETC count, peaking at 0.77 g/min with FGS under three ETCs, followed by MGS (0.63 g/min) and CGS (0.58 g/min) in the same configuration. Fine sand exhibited superior thermal conduction, leading to faster moisture removal, while medium and coarse sands demonstrated steadier drying patterns during late hours. Drying efficiency displayed an opposite trend: the FGS-1 ETC setup achieved the highest efficiency of 70.7%, whereas efficiency declined to 33.4% for two ETCs and 15.8% for three ETCs, regardless of sand size. This analysis highlights the trade-off between maximising drying rate and maintaining energy efficiency.