Page 131 - Entrophy Analysis in Thermal Engineering Systems

124 Entropy Analysis in Thermal Engineering Systems

one must focus on enhancing the design of the topping cycle. For this, a
designer should look for ways to utilize the thermal energy efficiently.
The design of the combined cycle may be altered by including a heat
exchanger between the gas turbine and the HRSG to preheat the air before
entering the combustor; see Fig. 8.11. The thermal energy of the flue gases
would then be recovered within both HRSG and heat exchanger.

8.6.3 Modified design

The modified combined cycle shown in Fig. 8.11 is indeed an integrated
regenerative gas turbine and a steam cycle. To examine the performance
of the proposed layout, the cycle efficiency is calculated using the same data
given in Table 8.4 and plotted against the compressor pressure ratio in
Fig. 8.12 (denoted by HRSG &H.Ex) for TIT¼1273K and E¼0.85.
The thermal efficiency of the combined cycle of Fig. 8.7 (denoted by HRSG
Only) is also provided in Fig. 8.12 for comparison.
Two main observations can be made in Fig. 8.12. First, the maximum
thermal efficiency of the cycle with “HRSG & H.Ex” is 0.564, whereas that
of the combined cycle with “HRSG Only” is 0.527. Second, the maximum

Fig. 8.11 The modified layout of the combined gas/steam turbine power cycle.