Performance Optimization of an Irreversible Brayton Cycle, and Proposing New Definitions for Second Law Efficiency and Exergy

In this study, the optimal performance of an irreversible regenerative Brayton cycle is sought through power maximization using the finite-time thermodynamic concept in finite-size components. Optimization is performed on the maximum power as the objective function using a genetic algorithm. In order to take into account the time and the size constraints in the current problem, the dimensionless mass-flow parameter is used. The behavior of the system parameters, such as maximum output power, exergy, exergy destruction, first and second law efficiencies, and effectiveness of the heat exchangers are investigated using the dimensionless mass-flow rate parameter. The influence of the unavoidable exergy destruction due to finite-time constraint is taken into account by developing the definition of thermal exergy. According to the results, the external exergy destruction increases and consequently the second law efficiency and heat exchangers effectiveness decrease with an increment of the dimensionless mass-flow rate parameter. However, as the dimensionless mass-flow rate parameter tends to zero, the efficiency and the power of the system approaches Carnot efficiency and zero value, respectively. Finally, the improved definitions are proposed for the heat exergy and the second law efficiency which will be compared with the conventional definitions and then their cumulative effects on cycle’s performance will be discussed.