Question 5.1: Estimate the effect of changes in the thermal efficiencies o......
Estimate the effect of changes in the thermal efficiencies of the topping gas turbine plant, \eta_{GT}, and of the bottoming steam turbine plant, \eta_{ST}, on the overall efficiency \eta_{CC} of the combined-cycle power plant shown in Figure 5.2.
Consider the following three cases:
• Case I with \eta_{GT} = 0.35 and \eta_{ST} = 0.3
• Case II with \eta_{GT} = 0.4 and \eta_{ST} = 0.32
• Case III with \eta_{GT} = 0.415 and \eta_{ST} = 0.335
Determine the increase in combined-cycle power plant efficiency \eta_{CC} in cases II and I II as compared with case I.
1. Case I with \eta_{GT} = 0.35 and \eta_{ST} = 0.3
\eta_{C C}=\eta_{GT}+\eta_{S\Gamma}\,(1-\eta_{G\Gamma})=0.35+0.3(1-0.35)=0.5452. Case II with \eta_{GT} = 0.4 and \eta_{ST} = 0.32
\eta_{C C}=\eta_{GT}+\eta_{ST}\left({1-\eta_{\mathrm{ GT}}}\right)=0.4+0.32(1-0.4)=0.5923. Case III with \eta_{GT} = 0.415 and \eta_{ST} = 0.335
\eta_{C C}=\eta_{GT}+\eta_{ST}\,(1-\eta_{GT})=0.415+0.335(1-0.415)=0.611Hence, the combined-cycle power plant efficiency \eta_{CC} increases in case II by 100 × (0.592/ 0.545–1) = 8.6% and in case III by 100 × (0.611/0.545–1) = 12.1%.