Application of drying kinetics in solar indirect dryer for medicinal herbs

Solar drying is a well-known, sustainable, and cost-efficient technique for preserving medicinal herbs. Indirect solar drying also enhances temperature control and hygiene for maintaining bioactive compounds. A study of drying kinetics helps optimize product quality and process efficiency. However, a limited number of studies are available for comparing drying models across herbs under specific conditions. This study represents 7 models for four herbs: Menthe , Coriandrum sativum , Trigonella foenum-graecum , and Azadirachta indica, using a solar dryer. The Logarithmic model is best for Menthe (RMSE: 0.0001, r: 0.9998, MBE:0.0131). For Coriandrum sativum Two-Term model is best (RMSE: 0.0011, r: 0.9986, MBE:0.0338). The Page model was suitable for Trigonella foenum-graecum (RMSE: 0.0085, r: 0.9945, MBE:0.0921). Azadirachta indica, the Page model performed the best fit (RMSE: 0.0016, r: 0.9982, MBE:0.0040). This drying occurred under 850 W/m 2 solar radiation, with an inlet temperature of 25–27 ℃ and a peak outlet temperature of 58–59 ℃, reducing the moisture content below 15% in 4 h. The system demonstrated effective energy conversion, achieving a maximum collector efficiency of 28% and a peak drying efficiency of 75% in the active moisture removal stage. An economic assessment indicated the system’s feasibility, with a capital expenditure (CAPEX) of $722.18, an annual operating cost (OPEX) of $155.18, and a drying cost of $0.088 per kg for all four medicinal herbs. The results highlight that drying model selection depends on both herb-specific properties and Gujarat’s climatic conditions, providing useful guidance for the design, optimization, and economic assessment of solar dryers for medicinal herb processing.