Mini review on a technological map of R744 heat pumps

The recent global challenges in current socioeconomic conditions have raised efforts for alternative ways of searching for gas- and oil-fuelled boilers that supply hot water to buildings for heating and/or domestic use. Heat pumps, which are in the heating capacity range of 20 kW-200 kW, are strong candidates for replacing fossil-fuelled boilers. Among other heat pump solutions, carbon dioxide-based solutions are more popular thanks to their environment-friendly and energy-efficient operating conditions; thus, they have been under technology radar over the last years, especially for residential and industrial purposes. This paper presents a mini-review of the carbon dioxide heat pump technologies from the component- to system-level, as appears in literature and industry, by generating a technological map of the solution throughout time. The review starts with an overview; then focuses on the R744 heat pump components; which are followed by discussions related to the system-level studies