Multi-criteria optimization of a regenerative gas turbine power cycle

It is shown in this chapter that to optimize a regenerative gas turbine power plantoperating on the basis of an open Brayton cycle by maximization of work output, firstlaw and second law efficiencies, and minimization of total entropy generation rateassociated with the power cycle, as fundamental thermodynamic optimization objectives,means to find an optimal for overall pressure ratio of the cycle. The study accounts forcomponents efficiencies and pressure drop throughout the cycle. It is found that atregenerator effectiveness of 50 percent, maximum work output, maximum 1^st lawefficiency and minimum entropy generation are coincident; though this value of theeffectiveness is irrelevant from practical perspective. However, in general, optimizationof any of these four objectives results in different design regimes. It is shown that entropygeneration is a basic requirement to drive a Brayton - type heat engine, and it is incorrectto consider the Carnot efficiency as the upper limit of the 1^st law efficiency of the plant.The results indicate that a real engine must operate at a region imposed by maximumwork output and maximum 1^st law efficiency. In other words, the pressure ratio of thecycle must lie between pressure ratios obtained by maximization of the work output andmaximization of the 1^st law efficiency. Furthermore, a criterion is established forutilization of a regenerator, which leads to introduce Critical Pressure Ratio beyondwhich employing a regenerator would be no longer useful. For the regeneratoreffectiveness greater than 0.8, the 2^nd law efficiency may be considered as a trade-offbetween the maximum work and maximum 1^st law efficiency designs, given that for theregenerator effectiveness around 0.8, a design based on the 2^nd law efficiencymaximization would be almost equivalent to the maximum work output design.