Thermodynamics

A 1350-MW combined-cycle power plant built in Ambarli, Turkey, in1988 by Siemens of Germany is the first commercially operating thermalplant in the world to attain an efficiency level as high as 52.5 percent atdesign operating conditions. This plant has six 150-MW gas turbines andthree 173-MW steam turbines. Some recent combined-cycle power plantshave achieved efficiencies above 60 percent.Chapter 10 | 585EXAMPLE 10–9A Combined Gas–Steam Power CycleConsider the combined gas–steam power cycle shown in Fig. 10–25. The toppingcycle is a gas-turbine cycle that has a pressure ratio of 8. Air enters thecompressor at 300 K and the turbine at 1300 K. The isentropic efficiency ofthe compressor is 80 percent, and that of the gas turbine is 85 percent. Thebottoming cycle is a simple ideal Rankine cycle operating between the pressurelimits of 7 MPa and 5 kPa. Steam is heated in a heat exchanger by the exhaustgases to a temperature of 500°C. The exhaust gases leave the heat exchangerat 450 K. Determine (a) the ratio of the mass flow rates of the steam and thecombustion gases and (b) the thermal efficiency of the combined cycle.Solution A combined gas–steam cycle is considered. The ratio of the massflow rates of the steam and the combustion gases and the thermal efficiencyare to be determined.Analysis The T-s diagrams of both cycles are given in Fig. 10–25. The gasturbinecycle alone was analyzed in Example 9–6, and the steam cycle inExample 10–8b, with the following results:Gas cycle:h 4 ¿ 880.36 kJ>kg1T 4 ¿ 853 K2q in 790.58 kJ>kgw net 210.41 kJ>kgh th 26.6%h 5 ¿ h @ 450 K 451.80 kJ>kgT, K13003'4'2'3500°C7 MPa4505'7 MPa3001'215 kPa4sFIGURE 10–25T-s diagram of the gas–steamcombined cycle described inExample 10–9.