Introduction: Drying is one of the most important post-harvest processes for medicinal plants, particularly for heat-sensitive species like peppermint (Mentha piperita). Currently, the common methods of drying medicinal plants in Iran are divided into two main categories: traditional methods (using direct sunlight) and industrial methods (using thermal dryers). Traditional methods face several limitations, including long drying time (4 to 7 days), exposure to environmental pollutants, insect and bird attacks, non-uniform humidity changes, and complete dependence on atmospheric conditions. On the other hand, industrial methods, although they have a higher drying speed, have significant energy consumption (2-3 kWh per kilogram of product) and are associated with greenhouse gas emissions and high operating costs. Traditional drying methods face challenges including high energy consumption, long processing times, and quality degradation. This research comprehensively investigates an innovative hybrid solar-assisted heat pump dryer integrated with flat side reflectors to address these limitations while promoting sustainable energy use in agricultural processing.

Materials and Methods: This research was conducted to evaluate the performance of a combined solar-heat pump dryer equipped with concentrating mirrors for drying mint leaves. The experimental system comprised four main components: a closed-cycle drying unit with heat pump recovery, a three-pass solar air collector, flat side reflectors, and an advanced monitoring and control system. The solar collector featured a black-painted aluminum absorber plate with triple-pass airflow configuration and eight aluminum fins at 48° angle. Two flat glass mirrors (0.96 m² each) were mounted at 60° angle for solar concentration. Comprehensive experiments were conducted using a factorial design with three primary factors: drying temperature (40, 50, 60°C), heat pump application (with/without), and flat side reflectors (with/without). Fresh peppermint leaves (100 g samples) were dried until reaching 15% moisture content (wet basis), with continuous monitoring of thermal parameters, energy consumption, and mass transfer kinetics. Various parameters including drying time, collector inlet and outlet air temperatures, drying chamber temperature, relative air humidity, solar radiation intensity and electrical energy consumption were measured. Performance indices including collector thermal efficiency, drying efficiency, specific moisture absorption rate and heat pump performance coefficient were calculated. The obtained data were analyzed using SPSS statistical software and means were compared using Duncan's multiple range test at a significance level of 5%.

Results and Discussion: Analysis of variance of different experimental factors on the thermal efficiency of the solar collector showed that the effect of temperature, heat pump, flat side reflectors, and the interaction effect of temperature and heat pump, temperature and concentrating mirrors, and temperature and heat pump and mirror on the drying time was statistically significant at the 1% level. The results of the study showed that the use of concentrating mirrors and heat pumps had a significant impact on improving the thermal efficiency of the solar collector. The average efficiency of the solar collector without the use of flat side reflectors was measured at 29.7%, while with the use of concentrating mirrors this value increased to 36%. The use of a heat pump alone also increased the thermal efficiency of the collector by 10%. The highest thermal efficiency was related to the treatment at 40 ° C in the case of using flat side reflectors, which was measured at 49.1 and 48.5% in the two conditions of using and not using a heat pump, respectively. The integrated system demonstrated remarkable performance improvements. Simultaneous application of heat pump and flat side reflectors achieved an 84.5% reduction in drying time (from 240 to 37 minutes) and 65.3% enhancement in collector thermal efficiency. The flat side reflectors individually contributed to 21.2% improvement in thermal efficiency, while the heat pump alone provided 10.1% efficiency gain. Maximum drying efficiency of 6.08% and superior SMER of 0.087 kg/kWh were obtained at 60°C with both subsystems operational. Energy consumption analysis revealed 61.2% reduction under optimal conditions. The enhanced performance is attributed to improved heat transfer through flat side reflectors and efficient energy recovery via the heat pump system, which reduced relative humidity and increased drying potential.

Conclusion: The hybrid solar-heat pump dryer with flat side reflectors presents a technologically advanced and environmentally sustainable solution for medicinal plant processing. The system achieves substantial reductions in drying time and energy consumption while maintaining product quality through controlled temperature conditions. This research contributes to sustainable agriculture by demonstrating the practical integration of renewable energy technologies for food and pharmaceutical applications, with potential for significant economic and environmental benefits in medicinal plant processing industries.























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