Potential improvements of supercritical CO₂ Brayton cycle by mixing other gases

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 sodium-water reaction. To prevent any hazards from sodium-water reaction, a SFR with the Brayton cycle using Supercritical Carbon dioxide (S-CO₂) as working fluid can be an alternative approach to improve the current SFR design. There are some key advantages of a S-CO₂ cycle. The size of turbo-machineries in a S-CO₂ cycle is much smaller than those in other cycles. Furthermore, high efficiency can be achieved with lower turbine inlet temperature and simpler cycle configuration. However, the S-CO₂ Brayton cycle is highly 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 near boundary between supercritical state and subcritical state. For this reason, heat rejection temperature and the minimum pressure of cycle are just above the CO₂ critical point. In other words, critical point acts as a limitation of the lowest operating condition of the cycle. In general, lowering rejection temperature of thermodynamic cycles can increase the efficiency. Therefore, it can be predicted that changing the critical point of CO₂ can result in an improvement of the total cycle efficiency with the same layout.In order to change the critical point of CO₂, a small amount of other gases can be added. The direction and range of critical point variation of CO₂ depends on the mixed component and its amount. It should be noted that not only the critical point changes but also all other thermo-physical properties vary with concentration of species in a gas mixture. To evaluate the effect of shifting the critical point and changes of properties on the S-CO₂ Brayton cycle, a simple Brayton cycle analysis tool with property data of multi-component gas was developed.This work will investigate which material and composition of gas mixture is the best coolant for Balance Of Plant (BOP) of a SFR. Several gases that show chemical stability within interested range of cycle operating condition have chosen as candidates for the mixture. By using the developed cycle tool, optimized cycle of gas mixture will be compared with the reference case of pure S-CO₂ cycle, and characteristics of the proposed supercritical gas mixture cycle will be discussed in thermodynamical point of view.