Multi-purpose design optimization of oil production and drying of St. John's Wort plant using a new type of thermal energy storage solar drying system

In this study, a new generation solar drying system integrated with thermal energy storage and powered solely by its own renewable energy production was developed. The system aims to reduce dependence on conventional energy sources while improving the quality of dried herbal products. Hypericum perforatum (St. John's Wort), known for its medicinal properties, was selected as the model plant for the drying and oil extraction processes. To ensure drying continuity during periods of insufficient solar radiation, RT28HC was used as the phase change material (PCM). Within the scope of the study, experimental and numerical analyses were carried out under three different air velocities (1, 2, and 3 m/s), as well as in natural sun drying and shade drying conditions. The initial moisture content of the plant, measured at 82%, was reduced to 10.2% in 7 hours at 1 m/s, 9.1% in 6 hours at 2 m/s, and 8.7% in 5 hours at 3 m/s. Color analysis of the samples dried at 3 m/s revealed L* = 32.56, a* = 8.12, and b* = 17.03, indicating that the visual quality of the plant was well preserved. In the oil samples obtained at the same velocity, the color values were recorded as L* = 26.45, a* = 6.37, and b* = 14.89. Energy efficiency ranged from 48.7% to 62.9%, while exergy efficiency varied between 4.13% and 6.45%. The results demonstrate that the system ensures energy-efficient and uninterrupted drying, maintains product quality, and offers a sustainable solution for agricultural drying applications.