Review on development of air source transcritical CO₂ heat pump systems using direct-heated type and recirculating-heated type

Transcritical CO₂ heat pumps have found worldwide applied because of the environmental friendliness and other thermodynamic advantages, however, the inefficiency of the CO₂ cycle has been the most issue blocking its popularization. This review has reported the recent state-of-the-art for improving performances in the Transcritical CO₂ heat pumps using both direct-heated type and recirculating-heated type. The features and issues of IHX, multi-compression with flash tank or economizer, ejector, parallel compression, cascade and combined cycles are cited and analyzed, respectively. Besides, variable of optimization methods and control strategies, especially the extremum seeking control, are referred. After been discussed and compared, the application range of majority modifications and the historical map of the researches are summarized, which aims at providing an overview of the variable requirements in different regions and the most appropriate modifications regarding to variable requirements for scholars, developers, manufacturers and policy makers.