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

Abstract

• A thermo-economic analysis of Brayton cycle with CO 2 -based mixtures is performed. • The Pareto fronts of 3 working fluids are obtained by multi-objective optimization. • SCO 2 -Xe cycle shows the best thermodynamic performance compared to others. • CO 2 -Kr shows the potential for development through thermo-economic analysis. The Supercritical carbon dioxide recompression Brayton cycle (SCO 2 RBC) is regarded as one of the most promising thermal power conversion systems and mixing other gases with CO 2 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 SCO 2 RBC is investigated via thermodynamic analysis. The effects of the key parameters on the thermo-economic performance of recompression Brayton cycles using CO 2 , CO 2 /xenon (mass fraction 0.55/0.45) and CO 2 /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 optimize 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 CO 2 /krypton cycle has the huge development potential. Specifically, the optimum exergy efficiencies of Brayton cycles using CO 2 , CO 2 /xenon and CO 2 /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 SCO 2 RBC while the levelized cost of energy also increased by 10.04%. The cycle efficiency of the supercritical CO 2 /krypton cycle is 9.44% higher while the cost is 2.49% higher compared to the SCO 2 RBC. There are suitable decision variables to make the cost of the cycle using CO 2 / krypton lower than that using CO 2 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 CO 2 -based binary mixtures can reduce the total exergy destruction effectively.