Heat pump-centered low-carbon drying technologies for agricultural products: A review on energy efficiency and sustainability

Agricultural products—including fruits, vegetables, grains, edible fungi, and aquatic goods—exhibit high water activity, rendering them highly susceptible to microbial contamination, nutrient loss, and quality deterioration during post-harvest handling. Drying is a critical preservation technology that reduces moisture content, inhibits biochemical reactions, and extends shelf life to enable long-term storage and distribution. However, conventional methods such as open sun drying and hot air drying (HAD) suffer from low energy efficiency, lengthy processing cycles, thermal degradation of product quality, and high greenhouse gas (GHG) emissions—outcomes that conflict sharply with global carbon peaking and neutrality goals. Positioning heat pump drying (HPD) as the core technological route, this review systematically examines multi-energy complementarity, hybrid drying configurations, non-thermal pretreatments, and intelligent digital control within the context of agricultural low-carbon drying. Recent advances—particularly in HPD, solar/renewable energy integration, hybrid systems, non-thermal assisted drying, and smart controls—have established these technologies as central to achieving efficient, low-carbon, and high-quality agricultural drying. Focusing on literature from 2021 to 2026, this paper provides a comprehensive synthesis of these developments. It elaborates on the thermodynamic mechanisms, energy-saving principles, performance enhancements, quality retention characteristics, and carbon reduction potentials of various technologies, with specific attention to high-temperature heat pumps and desiccant wheel-coupled systems. Furthermore, the review constructs a standardized, multidimensional sustainability evaluation framework by systematically synthesizing exergy analysis, energy efficiency metrics, life cycle assessment (LCA), techno-economic analysis (TEA), and carbon footprint quantification. It critically analyzes the conceptual foundations, optimal combination rules, and selection criteria for hybrid drying systems, while identifying key bottlenecks and research gaps impeding industrial scaling. Finally, future trajectories are outlined, emphasizing multi-energy integration, advanced thermal storage, green refrigerants, non-thermal synergy, digital intelligence, and circular economy models. This work aims to provide theoretical grounding, technical reference, and practical guidance for researchers, engineers, and policymakers involved in the innovation and deployment of low-carbon agricultural drying systems.