Thermodynamics

584 | Thermodynamicsin Fig. 10–24. In this cycle, energy is recovered from the exhaust gases bytransferring it to the steam in a heat exchanger that serves as the boiler. Ingeneral, more than one gas turbine is needed to supply sufficient heat to thesteam. Also, the steam cycle may involve regeneration as well as reheating.Energy for the reheating process can be supplied by burning some additionalfuel in the oxygen-rich exhaust gases.Recent developments in gas-turbine technology have made the combinedgas–steam cycle economically very attractive. The combined cycle increasesthe efficiency without increasing the initial cost greatly. Consequently, manynew power plants operate on combined cycles, and many more existingsteam- or gas-turbine plants are being converted to combined-cycle powerplants. Thermal efficiencies well over 40 percent are reported as a result ofconversion.A 1090-MW Tohoku combined plant that was put in commercial operationin 1985 in Niigata, Japan, is reported to operate at a thermal efficiency of44 percent. This plant has two 191-MW steam turbines and six 118-MW gasturbines. Hot combustion gases enter the gas turbines at 1154°C, and steamenters the steam turbines at 500°C. Steam is cooled in the condenser by coolingwater at an average temperature of 15°C. The compressors have a pressureratio of 14, and the mass flow rate of air through the compressors is 443 kg/s.Q inCompressorCombustionchamber6 7GAS CYCLEGasturbineT7Airin5Heat exchanger98ExhaustgasesQ inGASCYCLE832PumpCondenserSTEAMCYCLE63Steamturbine 2519STEAMCYCLE41Q out4Q outsFIGURE 10–24Combined gas–steam power plant.