Research Papers: Energy Systems Analysis

Specific Entropy Generation in a Gas Turbine Power Cycle

[+] Author and Article Information
Y. Haseli

School of Engineering and Technology,
Central Michigan University,
Mount Pleasant, MI 48859

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received July 12, 2017; final manuscript received August 28, 2017; published online September 28, 2017. Editor: Hameed Metghalchi.

J. Energy Resour. Technol 140(3), 032002 (Sep 28, 2017) (8 pages) Paper No: JERT-17-1356; doi: 10.1115/1.4037902 History: Received July 12, 2017; Revised August 28, 2017

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.

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Fig. 1

Comparison of the thermal efficiency of a regenerative gas turbine cycle versus the effectiveness of the regenerator at the regime of maximum thermal efficiency, maximum work output, and minimum entropy generation [5]

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Fig. 2

Schematic of a gas turbine cycle

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Fig. 3

Comparison of the present model and a simplified model [3,29]: (a) thermal efficiency and (b) work output (Joule per mole of air)

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Fig. 4

Thermal efficiency (solid lines) and specific entropy generation (dashed lines, unit: J/mol K) of the gas turbine cycle versus the pressure ratio at three different TITs

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Fig. 5

Illustration of entropy generated (J/mol K) due to each irreversible process at the condition of minimum specific entropy generation for three TITs

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Fig. 6

Thermal efficiency (solid lines) and specific entropy generation (dashed lines, unit: J/mol K) of the gas turbine cycle versus the TIT at three different values of pressure ratio (p2/p1)

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Fig. 7

The effect of fuel type on the thermal efficiency (solid lines) and the specific entropy generation (dashed lines, unit: J/mol K) of the cycle. The three different colors correspond to different TITs: 1200 K (blue lines), 1100 K (brown lines), 1000 K (green lines).

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Fig. 8

Comparison of the minimum specific entropy generation (J/mol K) of the cycle operating with different fuels at three TITs. The numbers within the graph are the heating values in kJ/mol.

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Fig. 9

Comparison of the minimum specific entropy generation (J/g K) of the cycle operating with different fuels at three TITs. The numbers within the graph are the heating values in kJ/g.

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Fig. 10

Thermal efficiency (solid lines) and entropy generation rate (dashed lines, unit: kW/K) of the gas turbine cycle versus the pressure ratio at three different TITs; Fuel: methane, air mass flowrate: 2 kg/s

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Fig. 11

Comparison of the trends of the specific entropy generation (J/K mol fuel) and entropy generation rate per unit molar flowrate of the air (J/K mol air) versus the compressor pressure ratio (TIT = 1200 K)




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