A review of transcritical carbon dioxide heat pump and refrigeration cycles

• A full-scale overview of transcritical carbon dioxide heat pump and refrigeration systems.
• The latest researches on transcritical carbon dioxide heat pump and refrigeration systems.
• Our latest researches on transcritical carbon dioxide expander.

As a natural refrigerant, carbon dioxide is safe, economic and environmentally sustainable which can be used in heat pump and refrigeration systems especially in transcritical cycles. From the early 1990s, in which the carbon dioxide transcritical cycle began, theoretical and experimental researches, as well as commercial system development, has improved to make the transcritical system performance to a level similar to that of conventional heat pump systems. This paper presents an overview of transcritical carbon dioxide heat pump and refrigeration systems. The paper introduces a summary of the history and main application of carbon dioxide's use as a refrigerant firstly. Secondly, the properties of supercritical pure carbon dioxide and that containing polyalkylene glycol (PAG) lubricants are analyzed and reviewed. In Section 3 the paper began with an analysis of some special characteristics of the basic carbon dioxide transcritical cycle such as the optimum system high pressure and so on, and then followed by a performance analysis and comparison of several novel transcritical cycles. The study finally presents a review of research on transcritical carbon dioxide heat pump systems, which covers the main components and research hotspots, such as heat transfer and expander.