Environmental sustainability and thermodynamic evaluation of a solar dryer with zigzag airflow during henna leaf drying

Abstract

This study developed and evaluated a solar dryer with a zigzag airflow pathway for drying henna leaves under Egyptian climatic conditions. The system was designed to improve air‐product contact in tray drying by directing heated air alternately above and below the drying trays. Drying experiments were conducted at two airflow rates, 0.09 and 0.14 m ³ /s, three‐layer thicknesses, 2, 4, and 6 cm, and four tray positions. An Arduino‐based local data‐logging system was used to record temperature and relative humidity during drying. Henna leaf moisture content decreased from 62.7% to below 5% wet basis. The shortest drying time was 10 h at 0.14 m ³ /s and 2 cm layer thickness, whereas the longest drying time was 26 h under the lowest airflow and highest loading condition. The corrected peak drying rate ranged from approximately 0.28 to 0.35 g water g ⁻¹ dry matter h ⁻¹ . The maximum solar‐collector energy efficiency reached 67.8%, while the drying‐room exergy efficiency ranged from 25.27% to 86.66%, depending on airflow rate, layer thickness, and tray position. Higher airflow enhanced useful energy recovery, but its effect was interpreted as a balance between increased mass flow rate, shorter air residence time, and improved vapor removal. A simplified environmental assessment showed that the energy payback time decreased from 9.83 years at 2 cm layer thickness to 3.27 years at 6 cm layer thickness, while lifetime CO ₂ mitigation reached up to 2399.2 kg CO ₂ .