Mathematical modeling, drying kinetics, and economic analysis of a hybrid photovoltaic thermal solar dryer for henna leaves.

Solar dryers offer a sustainable and efficient method for drying many agricultural products, preserving their quality, color, and medicinal properties while minimizing energy consumption and environmental impact. Current research on henna processing reveals a significant gap in drying engineering studies, creating a critical barrier to process optimization and quality enhancement. While numerous studies have investigated henna's physical and chemical characteristics, the engineering aspects of drying-including heat and mass transfer mechanisms, equipment design, and process parameter optimization-remain substantially understudied. This knowledge gap hinders the development of efficient, standardized drying methods that could improve product quality, reduce energy consumption, and increase production yields in commercial henna processing. So, during the present study, a direct solar dryer integrated with a photovoltaic system was used for drying Henna leaves at Aswan University, Egypt, during January 2025. Where a comparison study was conducted between the drying of Henna leaves by the developed direct solar dryer (DDSD) and open-air drying (OAD) at three-layer thicknesses of 2 cm, 4 cm, and 6 cm. The comparison study between both drying systems was established in terms of mathematical modeling, drying parameters, EMD, and economic analysis. The obtained results showed that the equilibrium moisture contents of Henna leaves samples dried in OAD and (DDSD) reach ranged between 2.52 and 3.23% (2.17 and 2.69%) on a dry base. Applying the DDSD to dry Henna leaves resulted in a reduction in drying time by approximately 7.14%, 13.33%, and 18.75% for layer thicknesses of approximately 2, 4, and 6 cm, respectively. Additionally, the EMD of the Henna leaves dried using the DDSD ranged from 2.84 × 10 to 22.96 × 10 m/s. Furthermore, Lewis (Newton), Weibullian, and Page were the most appropriate mathematical drying models for Henna leaves at layer thicknesses of approximately 2, 4, and 6 cm, respectively, for dried samples by DDSD. On the other hand, the economic analysis revealed that the DDSD has the potential to generate substantial cost savings, amounting to 3,348 USD per year. Additionally, the payback period was calculated to be 0.077 years (less than one month), demonstrating the system's rapid return on investment and economic viability.