Analysis of Brayton Cycle Characteristics of CO2-SF6 Mixed Working Fluid

The supercritical carbon dioxide (S-CO2) Brayton cycle is an efficient energy conversion system. It has the advantages of simple and compact, low noise and high efficiency. Therefore, it has attracted much attention from academia and industry in recent years. However, the current S-CO2 Brayton cycle study usually considers the application scenarios with ambient temperature 20 ℃, and the minimum cyclic temperature is 32 ℃. The adaptive analysis and optimization of low temperature cold source scenarios such as deep sea and polar regions are not optimized. CO2-SF6 mixture has lower critical parameters than pure CO2, so that it is expected to further reduce the compression power, improve the turbine pressure ratio, and thus improve the cycle efficiency with low ambient temperature. In this paper, the PR equation and van der Waals mixing rules were used to calculate the CO2-SF6 mixture thermal properties with the Nishiumi interaction coefficient model. The thermal properties calculated are in good agreement with the experiment data in literature. Based on the calculated thermal properties, a simple recuperation cycle was analyzed with a maximum pressure of 12 MPa, a minimum cyclic temperature of 15-55 ℃, and a minimum pressure slightly higher than the critical pressure. The results show that replacing the pure CO2 with a CO2-SF6 mixture can effectively improve the turbine pressure ratio, thereby improving the cycle efficiency, and reducing the critical temperature and the optimal compressor inlet temperature. And the use of CO2-SF6 mixture can effectively decrease cycle efficiency degradation when the compressor inlet temperature is lower than the quasi-critical temperature, thereby improving the stability of the cycle under the low temperature cold source. The preliminary analysis results show that the use of CO2-SF6 mixed working fluid instead of pure CO2 working fluid can effectively improve the performance of the cycle in the low temperature environment, which also indicates that the mixed working fluid scheme is one of the effective solutions to expand the applicable temperature range of the S-CO2 Brayton cycle. © 2023 Atomic Energy Press. All rights reserved.