Page 129 - Entrophy Analysis in Thermal Engineering Systems

122 Entropy Analysis in Thermal Engineering Systems

where S 13 is the entropy of the flue gases at the ambient pressure and
temperature.
Finally, the specific entropy generation of the cycle is determined by
adding the entropy generation within the gas cycle (GC), steam cycle
(SC), and due to rejecting waste heat to the environment.
X X

SEG ¼ Φ i + Φ j + Φ g,L + Φ c,L (8.59)
GC SC
where i and j refer to the components of the topping and bottoming cycles,
respectively.

8.6.2 Illustrative example
The thermal efficiency and specific entropy generation of the combined
cycle are plotted against the compressor pressure ratio in Fig. 8.9 for three
values of TIT. These results are obtained for the operating parameters given
in Table 8.4. The steam temperature is set to be 15K below the temperature
of the combustion gases entering the HRSG [3]. The thermal efficiency
peaks at a pressure ratio at which the specific entropy generation approaches
a minimum value. Fig. 8.9 shows that a design of the combined cycle power
plant based on the maximum thermal efficiency is the same as that of the
minimum specific entropy generation.

Fig. 8.9 Variation of the thermal efficiency and specific entropy generation of the
combined cycle with the pressure ratio at three different values of TIT.