Thermo-economic analysis and multi-objective optimization of supercritical Brayton cycles with CO2-based mixtures

The Supercritical carbon dioxide recompression Brayton cycle (SCO2RBC) is regarded as one of the most promising thermal power conversion systems and mixing other gases with CO2 to change the critical point represents an effective strategy to improve the performance of the Brayton cycle. In this work, the feasibility of using xenon and krypton as additives to the SCO2RBC is investigated via thermodynamic analysis. The effects of the key parameters on the thermo-economic performance of recompression Brayton cycles using CO2, CO2/xenon (mass fraction 0.55/0.45) and CO2/krypton (mass fraction 0.755/0.245) as working fluids are discussed by means of parametric analysis. A non-dominated sorting genetic algorithm is employed to simultaneously opti-mize the cycle exergy efficiency and the levelized cost of energy. The total thermal conductance, the main compressor outlet pressure, pressure ratio and split ratio are selected as the decision variables. The optimum solutions with their corresponding decision variables are selected from the Pareto frontiers. The results show that the supercritical CO2/krypton cycle has the huge development potential. Specifically, the optimum exergy ef-ficiencies of Brayton cycles using CO2, CO2/xenon and CO2/krypton as working fluids are 0.585, 0.646 and 0.664. The addition of xenon can improve cycle efficiency up to 12.08 % higher than SCO2RBC while the lev-elized cost of energy also increased by 10.04%. The cycle efficiency of the supercritical CO2/krypton cycle is 9.44 % higher while the cost is 2.49 % higher compared to the SCO2RBC. There are suitable decision variables to make the cost of the cycle using CO2/ krypton lower than that using CO2 while achieving the same exergy efficiency when the required exergy efficiency is in the range of 0.60-0.62. The exergy destruction distribution of Brayton cycle illustrates that high temperature recuperator is the highest exergy destruction component and using CO2- based binary mixtures can reduce the total exergy destruction effectively.

Automated summary

This study investigates the feasibility and performance optimization of using xenon and krypton as additives in the supercritical carbon dioxide recompression Brayton cycle (SCO2RBC). The results show that the supercritical CO2/krypton cycle outperforms other cycles and can achieve cost reduction through suitable decision variables.