Efficiency of irreversible Brayton cycles at minimum entropy generation

Abstract

System level optimization of power plants is mainly based on the thermodynamic laws. Various objective functions are proposed that are based on the 1st and 2nd law of thermodynamics. Given the fast technological advancement of power plants, it is important to understand which optimization objective should be used in practical applications. The objective is to find out whether an entropy-based optimization may be useful when designing gas turbine engines operating on Brayton cycle. The study considers three configurations of open Brayton cycle: a regenerative cycle, a reheat regenerative cycle, and an intercooled regenerative cycle. The operational regimes at maximum thermal efficiency, maximum work output and minimum entropy production of these power cycles are compared. The results reveal that a reduction in cycle entropy production is neither equivalent to an increase in its thermal efficiency; nor to an increase in its work output. Under special circumstances, minimum entropy production design may be identical to maximum thermal efficiency design and/or maximum work output design. The minimum entropy production, the maximum thermal efficiency, and the maximum work output criteria may be equivalent at the condition of fixed heat input. The two first criteria may also lead to an identical design at the condition of fixed work output. It is demonstrated that for practical applications, thermodynamic optimization of gas turbine power plants should continue to be based upon maximum thermal efficiency or maximum work output criteria. An entropy-based design should be avoided.