Advances, challenges and outlooks in frost-free air-source heat pumps: A comprehensive review from materials, components to systems

Air-source heat pumps (ASHPs) have dominated the heat pump market, accounting for over 90% of the market share, due to their potential to reduce emissions, low operational costs, and ease of maintenance. However, ASHPs are often inefficient in the summer and prone to frosting in the winter. Frost-free air-source heat pumps (FFASHPs) have recently emerged as a promising solution for both heating and cooling, achieving high efficiency and eliminating frost. This review provides a comprehensive summary of the latest advancements in FFASHPs, focusing on materials, components, and systems. Specifically, we begin by discussing the frost-free mechanism of FFASHPs, followed by a systematic presentation of the solutions used in FFASHPs in terms of their materials and properties. Additionally, we summarize breakthroughs in heat and mass exchangers, including the heat and mass transfer characteristics, models, and coefficients. We also discuss regeneration methods of solutions in terms of driving force, regeneration rate, and efficiency. Furthermore, we demonstrate the principle and potential of energy savings, regional suitability, and design and operational optimization of the system. Finally, we provide insights and perspectives on FFASHPs to further improve their efficiency and promote their application, including the development of a thermophysical property and corrosion database for solutions, intelligent screening of solutions, uniform correlation expressions of heat and mass transfer coefficients, comprehensive analysis and comparison of different regeneration approaches, obtaining dynamic properties of FFASHPs for predictive control and dynamic programming, and developing design standards for FFASHPs.