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Technical Brief

Demonstration of an Inverse Relationship Between Thermal Efficiency and Specific Entropy Generation for Combustion Power Systems

[+] Author and Article Information
Y. Haseli

School of Engineering and Technology,
Central Michigan University,
Mount Pleasant, MI 48859
e-mail: hasel1y@cmich.edu

K. Hornbostel

Mechanical Engineering and Materials Science,
University of Pittsburgh,
Pittsburgh, PA 19104
e-mail: hornbostel@pitt.edu

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received May 7, 2018; final manuscript received July 2, 2018; published online August 9, 2018. Editor: Hameed Metghalchi.

J. Energy Resour. Technol 141(1), 014501 (Aug 09, 2018) (6 pages) Paper No: JERT-18-1317; doi: 10.1115/1.4040793 History: Received May 07, 2018; Revised July 02, 2018

Maximum thermal efficiency is commonly assumed to correspond to minimum entropy generation. However, previous work has disproven this assumption for various power generation systems. In order to reconcile these two optimization approaches, second law analysis is performed here in terms of specific entropy generation (SEG), defined as the total entropy generation per mole of fuel. An inverse relationship between thermal efficiency and SEG is derived here, and it is shown that maximum thermal efficiency always corresponds to minimum SEG for lean fuel/air mixtures. Furthermore, the maximum efficiency limit of conventional power plants is shown to differ from the Carnot efficiency. Finally, a modified second law efficiency is introduced, and it is shown that the exhaust combustion products are bounded by a theoretical minimum temperature.

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References

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Figures

Grahic Jump Location
Fig. 1

Schematic of (a) a gas turbine cycle, and (b) a steam power cycle. The dashed rectangles show the system boundaries. Thin arrows represent mass flows, and thick arrows represent energy flows.

Grahic Jump Location
Fig. 2

Mole fraction of the reaction products included in the equilibrium calculations for methane combustion: (a) ϕ ≥ 1 and (b) ϕ < 1. All reactants are at 298.15 K and 1 bar.

Grahic Jump Location
Fig. 3

Mole fraction of the reaction products included in the equilibrium calculations for methanol combustion: (a) ϕ ≥ 1 and (b) ϕ < 1. All reactants are at 298.15 K and 1 bar.

Grahic Jump Location
Fig. 4

Mass and energy inputs/outputs of (a) an actual combustion-driven power plant, and (b) a Carnot engine

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