Investigation of energy, exergy and techno-environmental analysis of a hybrid PVT-based active direct solar dryer while drying henna leaves

Current literature lacks comparative studies on the effects of solar and open sun drying (OSD) methods on the drying characteristics and color properties of henna leaves. Furthermore, energy and exergy analyses, techno-environmental assessments, and sustainability evaluations remain uninvestigated for hybrid photovoltaic-thermal (PVT)-based active direct solar dryer (ADSD) in this context. Filling these research gaps through systematic study is critical to advancing sustainable drying technologies for agricultural products. The current study used ADSD and OSD to dry henna leaves, with three-layer thicknesses of 2, 4, and 6 cm. The obtained results showed that the drying time was between (14 and 16 h) and (15 and 19 h) for the ADSD and OSD, respectively. The lower color change (ΔE) value was observed with samples dried on OSD at a layer thickness of 2 cm, followed by the samples dried on ADSD at a layer thickness of 6 cm. The energy efficiency of the ADSD was ranged between 9.11 and 41.66%, and the highest drying efficiencies were 14.97%, 31.98%, and 34.57% at layer thicknesses of 2, 4, and 6 cm, respectively. The exergy outflow, exergy loss, and exergy efficiency of ADSD were in a range of (3.13-260.16 W), (218.64-1396 W), and (1.42-16.36%), respectively. Also, the improvement potential (IP) was in the range of 2.16-11.92 W, while the sustainability index (SI) and waste exergy ratio (WER) varied from (1.01 to 1.20) and (0.79 to 0.94), respectively. In addition, energy payback time (EPBT) was 6.89, 2.71, and 1.77 years at layer thicknesses of 2, 4, and 6 cm of henna leaves, respectively. Additionally, average annual CO_₂ emissions in lifetime values for ADSD were 133.46 kg/year. In contrast, net CO_₂ mitigations in lifetime values for ADSD were found to be 13.45, 40.42, and 63.90 tons, at layer thicknesses of 2, 4, and 6 cm of henna leaves, respectively. The study demonstrates that the ADSD significantly reduces drying time compared to OSD for henna leaves, particularly at 6 cm thickness, which also improves drying efficiency and sustainability. ADSD exhibits lower energy payback time (1.77 years) and higher CO₂ mitigation (63.9 tons) at 6 cm. Despite some color retention advantages in OSD, ADSD proves more energy-efficient and environmentally sustainable, making it a viable solution for agricultural drying.