Applications and development of CO₂-based reverse Rankine/reverse Brayton cycles in novel energy system architectures

Amid the accelerating global pursuit of carbon neutrality and the phased elimination of high-GWP refrigerants, natural working fluid CO2 has re-emerged as a leading candidate for next-generation heating and cooling technologies. Owing to its distinctive thermophysical characteristics—most notably the pronounced temperature glide during supercritical heat rejection—CO2 heat pump systems exhibit relatively high energy efficiency across an exceptionally wide span of supply temperatures. This capability enables thermodynamic performance and application flexibility that surpass almost all of conventional refrigerants, placing CO2 at the forefront of commercial, industrial, and emerging high-temperature applications. This paper provides a comprehensive and structured review of transcritical CO2 cycles, covering both reverse Rankine and reverse Brayton operating modes, and mapping their technological evolution across three representative temperature tiers: (i) high-temperature heat supply up to 120°C, (ii) ultra-high-temperature steam generation in the 200–300°C range, and (iii) ultra-super-high-temperature applications approaching 500–600°C. For supply temperatures below 120°C, the transcritical reverse Rankine cycle has achieved significant maturity in fields such as building HVAC, commercial refrigeration, industrial drying, and vehicle thermal management. As for supply temperature around 300°C, recent research highlights its growing penetration into industrial process heating, where CO2 systems offer high-grade steam generation, improved exergetic performance, and opportunities for integrated heating–cooling services. Advancements in component materials, ejector-based cycle enhancement, and system-level heat recovery have further expanded the feasible temperature envelope and improved part-load operation. When the supply temperature target exceeds approximately 500°C, the working conditions require a shift toward the reverse Brayton cycle. In this ultra-super-high-temperature domain, CO2 heat pump technology is increasingly recognized as a strategic enabling component of Carnot Battery–type thermal energy storage systems. These systems utilize a high-temperature heat pump to convert electricity into storable heat within molten-salt or solid-particle reservoirs, followed by power-cycle discharge via supercritical CO2 turbines. Such configurations offer several advantages: high round-trip efficiency, compact storage, rapid response, and strong compatibility with large-scale renewable integration and fossil-plant repowering. The resulting power-to-heat-to-power pathway provides long-duration, dispatchable energy storage and enhances grid flexibility while leveraging the inherent safety, low cost, and non-flammability of CO2. Overall, CO2-based reverse Rankine and reverse Brayton technologies demonstrate exceptional operating versatility, with effective temperature coverage extending from –40°C to nearly 600°C. Their ongoing development positions CO2 heat pumps and CO2-driven thermal energy storage as pivotal solutions for deep decarbonization of the built environment, industrial processes, and the power sector. The insights consolidated in this review aim to support future research, technology road-mapping, and deployment strategies for building a more resilient, efficient, and low-carbon global energy system.