Synergizing Heat Pump and Solar photovoltaic-thermal Systems for Efficient Drying of Onion Slices: Kinetics, Energy, and Quality Analysis

Onions are among the most widely cultivated and oldest vegetables globally, ranking second in overall production. Despite their popularity and high consumption, significant post-harvest losses occur due to their limited shelf life and high production levels, which pose challenges to economic stability and food security. Therefore, it is crucial to efficiently preserve this essential crop while maintaining its quality for related industries. The drying process, a fundamental post-harvest procedure, can be optimized under various conditions using a photovoltaic-thermal (PV-T) hybrid solar dryer equipped with a heat pump, providing an eco-friendly solution. This study examines the dehydration behavior of white onion slices at three temperatures (50, 60, and 70°C) and varying airflow rates (0.008, 0.016, and 0.024 kg/s). The results are categorized into three primary areas: drying characteristic curves, energy aspects, and quality parameters of the dehydrated onions. The research findings indicate that drying duration ranged from approximately 173 to 676 minutes. Moisture ratios from the experiments were modeled using nine mathematical drying kinetics models, with the Midilli et al. model demonstrating the best fit for the hybrid solar drying of white onion slices. Effective moisture diffusivity values were found to range from 1.9809 × 10⁻⁹ to 5.1977 × 10⁻⁹ m²/s, and activation energy was measured at 17.09, 22.07, and 26.89 kJ/mol across different conditions. Specific moisture extraction rates and dryer efficiency varied from 0.15 to 0.32 kg/kWh and 35.84% to 76.25%, respectively. Both independent variables significantly influenced quality parameters, including total phenolic content, vitamin C retention, color, and rehydration ratio.