Enhancing s-CO2 Brayton Power Cycle Efficiency in Cold Ambient Conditions Through Working Fluid Blends.

Supercritical carbon dioxide (s-CO2) Brayton cycles have emerged as a promising technology for high-efficiency power generation, owing to their compact architecture and favorable thermophysical properties. However, their performance degrades significantly under cold-climate conditions-such as those encountered in Greenland, Russia, Canada, Scandinavia, and Alaska-due to the proximity to the fluid's critical point. This study investigates the behavior of the recompression Brayton cycle (RBC) under subzero ambient temperatures through the incorporation of low-critical-temperature additives to create CO2-based binary mixtures. The working fluids examined include methane (CH4), tetrafluoromethane (CF4), nitrogen trifluoride (NF3), and krypton (Kr). Simulation results show that CH4- and CF4-rich mixtures can achieve thermal efficiency improvements of up to 10 percentage points over pure CO2. NF3-containing blends yield solid performance in moderately cold environments, while Kr-based mixtures provide modest but consistent efficiency gains. At low compressor inlet temperatures, the high-temperature recuperator (HTR) becomes the dominant performance-limiting component. Optimal distribution of recuperator conductance (UA) favors increased HTR sizing when mixtures are employed, ensuring effective heat recovery across larger temperature differentials. The study concludes with a comparative exergy analysis between pure CO2 and mixture-based cycles in RBC architecture. The findings highlight the potential of custom-tailored working fluids to enhance thermodynamic performance and operational stability of s-CO2 power systems under cold-climate conditions.