Abstract
Combined-cycle power plants seem an almost ideal solution for coping with demands for electrical power and heat: they give unique flexibility over wide ranges of loading. The gas turbine, because of its ability to burn a variety of fuels, may become the work-horse of all expanding electricity-supply systems. In this paper a system of intercooled low-pressure steam-injected gas turbine (turbo-STIG) with cogeneration is compared with an intercooled cogenerated one. Design point thermo-economic evaluation shows that the first out-performs the second by about 21% in power output and 16% in overall efficiency, with a payback period of 1.5 years. Off-design performance and useful energy in the bled steam were evaluated over wide ranges of the operating variables, namely compressor pressure ratio, turbine inlet temperature and injected steam:air ratio. A comparison between the two systems demonstrates the superiority of the first system. Sensitivity analysis shows the remarkable effect of turbine inlet temperature on the energy in the bled steam.