Specific Entropy Generation in a Gas Turbine Power Cycle

Abstract

Numerous studies have shown that the minimization of entropy generation does not always lead to an optimum performance in energy conversion systems. The equivalence between minimum entropy generation and maximum power output or maximum thermal efficiency in an irreversible power cycle occurs subject to certain design constraints. This article introduces specific entropy generation defined as the rate of total entropy generated due to the operation of a power cycle per unit flowrate of fuel. Through a detailed thermodynamic modeling of a gas turbine cycle, it is shown that the specific entropy generation correlates unconditionally with the thermal efficiency of the cycle. A design at maximum thermal efficiency is found to be identical to that at minimum specific entropy generation. The results are presented for five different fuels including methane, hydrogen, propane, methanol, and ethanol. Under identical operating conditions, the thermal efficiency is approximately the same for all five fuels. However, a power cycle that burns a fuel with a higher heating value produces a higher specific entropy generation. An emphasis is placed to distinguish between the specific entropy generation (with the unit of J/K mol fuel) and the entropy generation rate (W/K). A reduction in entropy generation rate does not necessarily lead to an increase in thermal efficiency.