Performance of Supercritical CO_2 Brayton Cycle with Additive Gases at Varying Critical Points for SFR Application

摘要

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_2 cycle when the compressor inlet temperature is above the critical temperature of CO_2.