Performance of supercritical CO ₂ Brayton cycle with additive gases at varying critical points for SFR application

The supercritical carbon dioxide Brayton cycle (S-CO ₂ cycle) has received attention as alternative to the energy conversion system for a Sodium-cooled Fast Reactor (SFR). The high cycle efficiency of S-CO ₂ cycle is attributed to significantly reduced compressor work. This is because the compressor operates like a pump in the vicinity of CO ₂ critical point. To make use of this feature, the minimum cycle operating range of S-CO ₂ cycle, which is the main compressor inlet condition, should be located close to the critical point of CO ₂. This translated into that the critical point of CO ₂ is the limitation of the lowest cycle condition of S-CO ₂ cycles. To increase the flexibility and broaden the applicability of the cycle, changing the critical point of CO ₂ by mixing additive gases could be adopted. An increase in the efficiency of the S-CO ₂ cycle could be achieved by decreasing critical point of CO ₂. In addition, increasing critical point of CO ₂ could be utilized to obtain improved cycle performances at ascending heat sink temperature of hot arid areas. Due to the rapid fluctuations of thermo-physical properties of gas mixtures near the critical point, an in-house cycle analysis code coupled to NIST property database was developed. Several gases were selected as potential additives through the screening process for thermal stability and chemical interaction with sodium. By using the developed cycle code, optimized cycles of each gas mixture were compared with the reference case of S-CO ₂ cycle. For decreased critical temperatures, CO ₂-Xe and CO ₂-Kr showed an increase in the total cycle efficiency. At increasing critical temperatures, the performance of CO ₂-H ₂S and CO ₂-cyclohexane is superior to S-CO ₂cycle when the compressor inlet temperature is above the critical temperature of CO ₂.