Page 122 - Entrophy Analysis in Thermal Engineering Systems

Irreversible engines—Open cycles 115

Fig. 8.4 Distribution of the specific entropy generation throughout the gas turbine
cycle at the condition of minimum SEG for three values of TIT.

is the combustion process followed by the exhaust gas cooling process. The
irreversibility of the fuel compressor is negligible compared to the other
components, so it is excluded in Fig. 8.4. Except the turbine, the contribu-
tion of all the irreversible processes to the minimum SEG slightly decreases as
TIT increases. However, this trend is opposite for the expansion process
within the turbine. As seen in Fig. 8.3 that the thermal efficiency improves
by an increase in TIT. In practice, the upper limit of TIT is constrained by
the durability of the turbine blades.
A further observation in Fig. 8.4 is that a major source of inefficiency in
the gas turbine cycle is the significant amount of thermal energy that is
wasted with no further use in the downstream. In modern designs of gas tur-
bine engines, a recuperator is placed after the turbine to recover part of the
thermal energy of the hot flue gases. Depending on the application, the
recovered heat is used to either preheat the pressurized air before the com-
bustor or produce superheated steam to run a downstream Rankine cycle.
The former design is referred to as the regenerative gas turbine cycle, and the
latter is known as the combined cycle.
Recycling the heat recovered from the hot exhaust gases back to the pro-
cess boosts the overall power generation efficiency. Both regenerative gas
turbine and combined cycles will be discussed in the upcoming sections.
Further improvement in the efficiency of the gas turbine cycle would be
possible by enhancing the efficiencies of the compressor and turbine.