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-CO2 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 CO2 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 CO2-containing binary mixture cycles with simple recuperated and recompression layouts were compared with the reference S-CO2, CO2-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-H2S 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.