How is the steam cycle heat rate of a combined cycle unit calculated?

A combined cycle plant is a highly fuel-efficient process that involves generating electricity in combustion turbines and utilizing the procedure's hot exhaust (waste heat) to create water vapor, which is then used to generate additional electricity in a steam turbine. The heat rate is defined as a measure of energy efficiency that defines how much fuel it takes to generate a kilowatt-hour of electricity.

The combined cycle plant involves the sequential use of the fuel energy in both the gas turbine-generator and the steam turbine-generator. The steam turbine operates in conjunction with the gas turbine, providing extra generation at an effective heat rate of zero to the station, ignoring any duct burners.

Therefore, heat rates are defined for the gas turbines and for the combined cycle, but not the steam cycle alone.

Using the GADS fuel and generation data and the performance reports from GADS Open Source, you can determine the approximate overall heat rate of the combined cycle plant and an approximate heat rate for each of the gas turbines. Based on these heat rates, it is possible to infer the approximate heat rates for the steam turbine; however, the computation is somewhat involved.

Normally, the overall heat rate (Btu/kWh) for the combined cycle is calculated by summing the fuel burned in each of the gas turbines and the fuel burned in the duct burners (expressed in Btu) and dividing the summed Btu value by the total generation(kWh) from the gas turbine generator and the steam turbine generator.

So, the answer is that the steam cycle heat rate is not normally calculated by itself.  It can be accurately calculated by testing and performing a detailed heat balance for the steam cycle.

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