Page 49 - Advanced Gas Turbine Cycles

26 Advanced gas turbine cycles

0.9, and the combustion pressure loss is 3% of the inlet pressure to the chamber. The
method of calculation is given in Chapters 4 and 5, but it is sufficient to say here that it
involves the assumption of real semi-perfect gases with methane as fuel for combustion
and no allowance for any turbine cooling. The work terms associated with the abstraction
and delivery to the atmosphere are ignored in the valuation of the fuel exergy, which is
thus taken as [-AGO].
The thermal efficiency, the work output as a fraction of the fuel exergy (the maximum
reversible work), is shown as no. 1 in the figure and is 0.368. The internal irreversibility
terms, xFR/[-AGo], are shown as nos. 2, 3, and 4 in the diagram, for the combustion
chamber, compressor and turbine, respectively. It is assumed that there is no hear rejection
to the atmosphere from the engine, i.e. IQ = 0 (no. 5), but there is an exergy loss in the
discharge of the exhaust gas to the atmosphere, (BP4 - Gm)/[-AGo], the last term of
Eq. (2.49), which is shown as no. 6 in the diagram.
The dominant irreversibilities are in combustion and in the exhaust discharge.

2.7. A final comment on the use of exergy

We shall later give more detailed calculations for real gas turbine plants together with
diagrams similar to Fig. 2.9. Exergy is a very useful tool in determining the magnitude of
local losses in gas turbine plants, and in his search for high efficiency the gas turbine
designer seeks to reduce these irreversibilities in components (e.g. compressor, turbine,
the combustion process, etc.).
However, it is wise to emphasise the interactions between such components. An
improvement in one (say an increase in the effectiveness of the heat exchanger in a
[CBTX], recuperative plant) will lead to a local reduction in the irreversibility or exergy
loss within it. But this will also have implications elsewhere in the plant. For the [CBTXII
plant, an increase in the recuperator effectiveness will lead to a higher temperature
entering the combustion chamber and a lower temperature of the gas leaving the hot side
of the exchanger. The irreversibility in combustion is decreased and the exergy loss in the
final exhaust gas discharged to atmosphere is also reduced [6].
Therefore, plots of exergy loss or irreversibility like Fig. 2.9, for a particular plant
operating condition, do not always provide the complete picture of gas turbine
performance.

References

[I] Haywood, R.W. (1980). Equilihrium Thermodynamics, Wiley, New York.
[2] Gyftopoulos, E.P. and Beretta, G.P. (1991), Thermodynamic Foundations and Applications, MacMillan,
New York.
131 Kotas, T.J. (1985). The Exergy Method of Thermal Power Analysis, Butterworth, London.
[4] Horlock, J.H., Manfrida, G. and Young, J.B. (ZOOO), Exergy analysis of modem fossil-fuel power plants,
ASME J. Engng Gas Turbines Power 122, 1-17.
[5] Horlock, J.H. (2002). Combined Power Plants, 2nd edn, Krieger, Melbourne USA.
[6] Horlock, J.H. (1998), The relationship between effectiveness and exergy loss in counterflow heat exchangers,
ASME Paper 1998-GT-32.