Recent Advances in Heat Pump Drying System for the Food Industry: Structure, System Optimization, Applications, and Prospects

Heat pump drying (HPD) has garnered significant attention in the food industry due to its unique energy efficiency, environmental friendliness, and exceptional product quality. However, HPD still faces several challenges in practical applications, including quality loss during food drying, a narrow drying temperature range, slow heat and mass transfer during drying, and limited applicability in cold regions. These issues impact the performance, drying efficiency, and product quality of HPD. This review aims to comprehensively analyze the structure and optimization schemes of HPD systems and their applications in the food industry, with a focus on single and hybrid HPD systems. The environmental impact on the drying system can be minimized by modifying the air circuit mode of HPD systems. The optimization of single HPD systems involves the use of appropriate air circulation media, refrigerants, multi-stage, and cascade systems, which can enhance system performance and expand the drying range of HPD. Optimization of hybrid HPD systems includes advanced technologies (infrared, ultrasonic, microwave, etc.) and clean energy sources (solar, biomass, geothermal, etc.) to assist the HPD, which helps increase drying rates during the later stages of HPD drying and conserve energy. In the improvement of HPD systems, balancing the drying characteristics of different types of food, the performance and simplicity of the system, and economic efficiency remains a huge challenge. This review fills the gap in previous reviews on optimization strategies for HPD in the food drying industry and provides guidance on expanding applications of HPD in food drying through advanced technological updates.