Research progress of CO2 air source heat pump system for heating

The heat pump cycle based on the natural working fluid CO2 has become a cutting-edge research direction in the field of sustainable thermal energy technologies due to its excellent thermophysical properties (high specific volume, significant temperature-glide effect in the supercritical region) and environmental friendliness (ODP = 0, GWP = 1). This paper systematically reviews the research progress of transcritical CO2 heat pump systems in terms of basic cycle optimization and efficiency enhancement technologies. Existing studies indicate that innovative methods such as cascade, multistage compression, mechanical subcooling, and ejectors can effectively improve the system performance. At the level of application research, this paper elaborates on the technical breakthroughs of transcritical CO2 heat pumps in typical scenarios such as energy saving in the drying process, heat pump heating and hot water supply systems, and air-conditioning systems for new energy vehicles. Regarding system regulation and control, this paper summarizes the regulation and control methods for transcritical CO2 heat pump systems, and analyzes the applicable scenarios, advantages, and disadvantages of each method. In terms of techno-economic analysis, it has been identified that CO2 heat pumps commonly confront the challenge of high initial investment costs, which hinders their extensive promotion and application. Finally, this paper indicates that current systems still face technical bottlenecks that urgently need to be addressed, including the reliability of high-pressure operation (with a design pressure >10 MPa), dynamic operating condition control strategies, and cost-benefit optimization. It also anticipates that through system optimization, technological improvement, and the large-scale implementation of control strategies, the cost of CO2 heat pumps can be reduced. By integrating waste heat recovery and solar-thermal storage integration technologies, it will be possible to facilitate the large-scale application of CO2 heat pump technology in fields such as civil buildings, commercial uses, and industrial applications.