Drying kinetics, moisture diffusivity, and quality preservation of mint leaves using an IoT-assisted hybrid solar dryer with air recirculation

Abstract

Drying of aromatic herbs such as mint (Mentha spicata L.) is often constrained by unstable environmental conditions, leading to extended drying durations, inefficient energy utilization, and deterioration of product quality. In this study, an advanced IoT-enabled hybrid solar dryer integrating a flat-plate solar collector, LPG-assisted auxiliary heating, and a controlled air recirculation system was developed and experimentally evaluated to provide controlled thermal conditions and enhanced energy efficiency. Drying experiments were conducted at air temperatures of 50, 55, and 60 °C combined with recirculation ratios of 70%, 80%, and 90% under both winter and summer climatic conditions. The system utilized real-time sensing and automated control to dynamically regulate airflow distribution, maintain temperature within ±2 °C, and stabilize relative humidity inside the drying chamber. The drying performance was quantitatively evaluated using moisture ratio, drying rate, effective moisture diffusivity (D_eff), activation energy, and thermodynamic parameters including enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG). The results demonstrated that increasing both drying temperature and recirculation ratio significantly enhanced moisture removal and reduced drying time due to improved heat and mass transfer conditions. Under optimal conditions (60 °C and 80% recirculation), the drying time was reduced to approximately 105–120 min, compared with 180–210 min at lower operating conditions, representing a reduction of nearly 40–50%. The final moisture content decreased from an initial range of 77–87% (wet basis) to 7.5–7.6%, meeting safe storage requirements. The effective moisture diffusivity ranged from 2.7 × 10⁻⁹ to 6.08 × 10⁻⁹ m²/s, increasing proportionally with temperature and airflow recirculation, indicating enhanced internal moisture diffusion. Seasonal analysis revealed faster drying under summer conditions; however, the hybrid configuration maintained stable performance across varying ambient conditions.