Influence of physical shape and salting on tomato drying performance using mixed mode solar and open-air methods in semi-cloudy weather

SD Solar drying is increasingly recognized as a sustainable and energy-efficient solution for preserving agricultural products, offering a practical alternative to fossil fuel-dependent methods and traditional open sun drying (OSD). However, its overall performance is highly influenced by environmental variability and system design. This study provides a detailed evaluation of a newly developed direct solar dryer (DDSD) for tomato dehydration, conducted under real and fluctuating climatic conditions in Aswan, Egypt, from February 22 to 27, 2025. During the trial period, solar irradiance ranged widely from 88 to 826 W/m² due to intermittent cloud cover, while ambient temperatures fluctuated between 22 and 34 °C-conditions representative of actual field environments. Tomato samples were prepared in three physical forms-halves, quarters, and 6 mm slices-and subjected to two pretreatment methods (salted and unsalted) to assess their effects on drying kinetics. The DDSD demonstrated significantly better performance than OSD, reducing drying durations by 25-39.6%. The most efficient results were achieved for salted 6 mm slices, which dried in just 9 h-substantially faster than the 29 h for unsalted halves in DDSD and 48 h in OSD. These samples also exhibited the highest effective moisture diffusivity (D_eff) (5.92 × 10⁻⁹ m²/s), reflecting enhanced internal moisture transport. Among 12 drying models evaluated, the Logistic model most accurately described the drying behavior in the DDSD, with an excellent statistical fit (R² = 0.999524, χ² = 6.74 × 10⁻⁵, RMSE = 0.006868). Economically, the DDSD, integrated with a photovoltaic (PV) system, required a modest initial investment of $520 and achieved a payback period of just 1.82 years for salted slices due to faster processing and increased throughput. From an environmental perspective, the system is projected to offset approximately 105.68 metric tons of CO₂ emissions over a 20-year lifespan, with an energy payback time of only 1.10 years and potential revenue of $1321.04 from carbon credits. These findings underscore the DDSD's potential as a cost-effective, environmentally sustainable, and technically efficient solution for agricultural drying in solar-rich regions.