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Research Papers: Gas Turbines: Electric Power

# Second Law Efficiency of the Rankine Bottoming Cycle of a Combined Cycle Power Plant

[+] Author and Article Information
S. Can Gülen

Raub W. Smith

The sign is $+$ for heat transfer into the CV and − for heat transfer out of the CV.

The properties herein are calculated using JANAF package (29) with the zero enthalpy reference set to $To$. As such the enthalpy and entropy of the exhaust gas at $To(=Tamb)$ are exactly zero.

This is essentially a futile task due to the paucity of information regarding cycle details, auxiliary loads, and the impossibility of weeding out the margins.

The error introduced by this assumption is $<1°C$ for all calculations of interest herein.

100% methane gaseous fuel is assumed.

J. Eng. Gas Turbines Power 132(1), 011801 (Sep 30, 2009) (10 pages) doi:10.1115/1.3124787 History: Received July 09, 2008; Revised July 15, 2008; Published September 30, 2009

## Abstract

A significant portion of the new electrical generating capacity installed in the past decade has employed heavy-duty gas turbines operating in a combined cycle configuration with a steam turbine bottoming cycle. In these power plants approximately one-third of the power is generated by the bottoming cycle. To ensure that the highest possible combined cycle efficiency is realized it is important to develop the combined cycle power plant as a system. Doing so requires a solid understanding of the efficiency entitlement of both, topping and bottoming, cycles separately and as a whole. This paper describes a simple but accurate method to estimate the Rankine bottoming cycle power output directly from the gas turbine exhaust exergy, utilizing the second law of thermodynamics. The classical first law approach, i.e., the heat and mass balance method, requires lengthy calculations and complex computer-based modeling tools to evaluate Rankine bottoming cycle performance. In this paper, a rigorous application of the fundamental thermodynamic principles embodied by the second law to the major cycle components clearly demonstrates that the Rankine cycle performance can be accurately represented by several key parameters. The power of the second law approach lies in its ability to highlight the theoretical entitlement and state-of-the-art design performances simultaneously via simple fundamental relationships. By considering economically and technologically feasible upper limits for the key parameters, the maximum achievable bottoming cycle power output is readily calculable for any given gas turbine from its exhaust exergy.

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## Figures

Figure 1

Simplified CC Rankine bottoming cycle diagram

Figure 2

Heat release diagram for a single-pressure HRSG

Figure 3

Comparison of OEM-reported CC ST outputs with the predictions from Eq. 12 using the GT exhaust data

Figure 4

RBC exergetic efficiencies from the OEM data in Ref. 18

Figure 5

Distribution of GT exhaust gas exergy in CC Rankine bottoming cycle

Figure 6

Rankine bottoming cycle technology curves. (Note that 593°C=1100°F, each 100°C increment is 212°F.)

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