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In a thermal power plant, high-temperature heat as input to the power plant is converted to electricity as one of the outputs and low-temperature heat as another output. As a rule, in order to achieve high efficiency, the temperature of the input heat should be as high as possible and the temperature of the output heat as low as possible.
For gas turbine generators, the input temperature to the gas turbine is relatively high (some 900°C to 1,200°C) but the output temperature of the flue gas is also relatively high (some 450°C to 650°C). For steam turbine power plants, the output temperature of the cooling water is significantly lower (some 20°C to 40°C), but the input temperature to the steam turbine is also significantly lower (some 420°C to 580°C). Therefore, by combining both processes, high input temperatures and low output temperatures can be achieved and the power plant efficiency can be increased.
The output heat of the gas turbine flue gas is utilized to generate steam in a heat recovery steam generator (HRSG) and therefore is used as input heat to the steam turbine power plant.
The HRSG can be designed without or with supplementary firing. Without supplementary firing, the efficiency of the combined cycle power plant is higher, and the plant is more flexible to in response to fluctuations of electrical load. Whereas the gas turbines usually are fired by relatively expensive fuels such as natural gas, gas from coal gasification or light fuel oil, the supplementary fuel may be a less expensive type such as heavy fuel oil or coal.
In the case of generating only electricity, power plant efficiencies of up to 58% can be achieved. In the case of combined heat and power generation, the efficiency increases to about 85%.
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