Potential improvements of supercritical recompression CO2 Brayton cycle by mixing other gases for power conversion system of a SFR

Highlights: → S-CO2 cycle could be enhanced by shifting the critical point of working fluids using gas mixture. → In-house cycle code was developed to analyze supercritical Brayton cycles with gas mixture. → Gas mixture candidates were selected through a screening process: CO2 mixing with N2, O2, He, and Ar. → CO2-He binary mixture shows the highest cycle efficiency increase. → Lowering the critical temperature and critical pressure of the coolant has a positive effect on the total cycle efficiency. - Abstract: A sodium-cooled fast reactor (SFR) is one of the strongest candidates for the next generation nuclear reactor. However, the conventional design of a SFR concept with an indirect Rankine cycle is subjected to a possible sodium-water reaction. To prevent any hazards from sodium-water reaction, a SFR with the Brayton cycle using Supercritical Carbon dioxide (S-CO2) as the working fluid can be an alternative approach to improve the current SFR design. However, the S-CO2 Brayton cycle is more sensitive to the critical point of working fluids than other Brayton cycles. This is because compressor work is significantly decreased slightly above the critical point due to high density of CO2 near the boundary between the supercritical state and the subcritical state. For this reason, the minimum temperature and pressure of cycle are just above the CO2 critical point. In other words, the critical point acts as a limitation of the lowest operating condition of the cycle. In general, lowering the rejection temperature of a thermodynamic cycle can increase the efficiency. Therefore, changing the critical point of CO2 can result in an improvement of the total cycle efficiency with the same cycle layout. A small amount of other gases can be added in order to change the critical point of CO2. The direction and range of the critical point variation of CO2 depends on the mixed component and its amount. Several gases that show chemical stability with sodium within the interested range of cycle operating condition were chosen as candidates for the mixture; CO2 was mixed with N2, O2, He, and Ar. To evaluate the effect of shifting the critical point and changes in the properties of the S-CO2 Brayton cycle, a supercritical Brayton cycle analysis code with a properties program, which has the most accurate mixture models, was developed. The CO2-He binary mixture shows the highest cycle efficiency increase. Unlike the CO2-He binary mixture, the cycle efficiencies of CO2-Ar, CO2-N2, and CO2-O2 binary mixtures decreased compared to the pure S-CO2 cycle. It was found that the increment of critical pressure led to a decrease in cycle operating pressure ratio which resulted in a negative effect on total cycle efficiency. In addition, the effects from changed minimum operating condition and property variations of multi-component working fluid changed the recuperated heat in the cycle which was closely related to the cycle performances.