Efficient drying through hybrid microwave and heat pump: optimization and simulation

Escalating global energy demand, rising greenhouse gas emissions, and increasing food security concerns have intensified the need for energy-efficient and sustainable food preservation technologies. Although drying is widely used, conventional convective and heat-pump drying systems often suffer from long drying times, high energy consumption, and quality degradation, particularly for high-moisture, heat-sensitive foods. This study investigates intermittent microwave-convective drying integrated with a heat-pump dehumidification system (IMCD-HP) using a combined experimental and modelling-based optimisation framework. A physics-based 3D numerical model coupling heat transfer, moisture diffusion, electromagnetic microwave heating, and computational fluid dynamics (CFD) was developed to simulate multi-sample drying under uniform inlet air velocity and controlled conditions. The model was experimentally validated using apple slices dried at 40–60 °C, 220 W microwave power, and an air velocity of 1 m/s. RSM was employed to optimise drying air temperature, microwave ON time (MOT), and pulse ratio (PR) to minimise drying time, energy consumption, and colour change. Model predictions closely matched experimental results, accurately capturing temperature evolution (25–60 °C), moisture reduction (6.4 to ∼ 1.2 kg/kg d.b.), and drying kinetics. Pulse ratio was identified as the most influential parameter, with optimal conditions obtained at 50 °C, 20 s MOT, and PR = 3 (composite desirability = 0.6889). Under these conditions, IMCD-HP achieved rapid moisture removal, nearly two-fold enhancement in effective moisture diffusivity, substantial energy savings (0.25 kWh in 12 min), and improved colour retention. The results demonstrate the strong potential of IMCD-HP for scalable, energy-efficient, and high-quality food drying applications.