Performance of supercritical Brayton cycle using CO₂-based binary mixture at varying critical points for SFR applications

The supercritical carbon dioxide Brayton cycle (S-CO₂ cycle) has attracted much attention as an alternative to the Rankine cycle for sodium-cooled fast reactors (SFRs). The higher cycle efficiency of the S-CO ₂ cycle results from the considerably decreased compressor work because the compressor behaves as a pump in the proximity of the CO₂ vapor-liquid critical point. In order to fully utilize this feature, the main compressor inlet condition should be controlled to be close to the critical point of CO₂. This indicates that the critical point of CO ₂ is a constraint on the minimum cycle condition for S-CO₂ cycles. Modifying the CO₂ critical point by mixing additive gases could be considered as a method of enhancing the performance and broadening the applicability of the S-CO₂ cycle. Due to the drastic fluctuations of the thermo-physical properties of fluids near the critical point, an in-house cycle analysis code using the NIST REFPROP database was implemented. Several gases were selected as potential additives considering their thermal stability and chemical interaction with sodium in the temperature range of interest and the availability of the mixture property database: xenon, krypton, hydrogen sulfide, and cyclohexane. The performances of the optimized CO ₂-containing binary mixture cycles with simple recuperated and recompression layouts were compared with the reference S-CO₂, CO ₂-Ar, CO₂-N₂, and CO₂-O₂ cycles. For the decreased critical temperatures, the CO₂-Xe and CO₂-Kr mixtures had an increase in the total cycle efficiency. At the increased critical temperatures, the performances of CO₂-H ₂S and CO₂-cyclohexane with the recompression layout were superior to the S-CO₂ cycle when the compressor inlet temperature was above the critical temperature of CO₂.