Overview and outlook of research and innovation in energy systems with carbon dioxide as the working fluid

Carbon dioxide (CO2, R744) is a natural working fluid with interesting thermophysical properties that have stimulated strong attention by the academic and industrial communities for a broad range of energy applications. The technology readiness level of CO2-based energy systems is very diverse due to the increasing consideration that the fluid has been receiving since the 1990s. Hence, the state of the art in CO2 energy research spans from fundamental thermofluid and chemistry science to commercial system innovations. After a brief compendium on ongoing activities, this paper proposes a roadmap for CO2 energy research with reference to the cooling, heating and power sectors. The key knowledge gaps and the main challenges at system and component levels are critically discussed. Pathways to advance the understanding and the technological maturity of CO2 energy systems are also outlined.