Thermodynamic and multi-response optimization for passive indirect solar drying of moringa leaves under natural convection

Agricultural drying processes remain highly energy-intensive and are commonly associated with low thermal efficiency, uncontrolled moisture removal, and considerable environmental impact. Although passive solar dryers have been widely proposed as sustainable alternatives for decentralized agricultural processing, existing studies primarily focus on isolated drying kinetics, thermal behavior, or optimization techniques without integrating thermodynamic irreversibility assessment and multi-response optimization within a unified framework. Experimentally validated energy-exergy-Specific Moisture Extraction Rate (SMER)-based optimization methodologies for passive natural-convection solar dryers remain insufficiently explored in the open literature. Therefore, this study develops and experimentally investigates a passive indirect solar dryer for moringa leaves under natural convection while establishing an integrated thermodynamic and statistical optimization framework. The proposed system consisted of a flat-plate solar collector, a multi-tray drying chamber, and a roof-mounted turbine ventilator operating entirely through buoyancy-driven airflow without external electrical power input. Energy analysis, exergy assessment, SMER, tray-wise drying evaluation, and Response Surface Methodology (RSM)-based multi-response optimization were simultaneously applied to evaluate system performance. Experimental results showed that the moisture content of moringa leaves decreased from 72 to 78% to 8–10% (wet basis) within 8–9 h under solar radiation levels of 320–850 W/m 2 . The collector generated a temperature rise of 20–30 °C, producing thermal efficiencies of 42–44% with a maximum value of 52%, while exergy efficiency varied between 18% and 28%. The maximum SMER reached 0.55 kg/kWh, and the RSM model predicted experimental responses with deviations below 4%. The proposed framework provides a generalized thermodynamic-optimization methodology for sustainable passive solar drying applications under decentralized and off-grid conditions.