Research Papers: Gas Turbines: Electric Power

Combined Cycle Off-Design Performance Estimation: A Second-Law Perspective

[+] Author and Article Information
S. Can Gülen

J Joseph

 GE Energy, 1 River Road, Building 40-412, Schenectady, NY 12345joseph1.j@ge.com

J. Eng. Gas Turbines Power 134(1), 011801 (Nov 07, 2011) (11 pages) doi:10.1115/1.4004179 History: Received April 25, 2011; Accepted April 27, 2011; Published November 07, 2011; Online November 07, 2011

A combined cycle power plant (or any power plant, for that matter) does very rarely—if ever—run at the exact design point ambient and loading conditions. Depending on the demand for electricity, market conditions, and other considerations of interest to the owner of the plant and the existing ambient conditions, a combined cycle plant will run under boundary conditions that are significantly different from those for which individual components are designed. Accurate calculation of the “off-design” performance of the overall combined cycle system and its key subsystems requires highly detailed and complicated computer models. Such models are crucial to high-fidelity simulation of myriad off-design performance scenarios for control system development to ensure safe and reliable operability in the field. A viable option in lieu of sophisticated system simulation is making use of the normalized curves that are generated from rigorous model runs and applying the factors read from such curves to a known design performance to calculate the off-design performance. This is the common method adopted in the fulfillment of commercial transactions. These curves; however, are highly system-specific and their broad applicability to a wide variety of configurations is limited. Utilizing the key principles of the second law of thermodynamics, this paper describes a simple, physics-based calculation method to estimate the off-design performance of a combined cycle power plant. The method is shown to be quite robust within a wide range of operating regimes for a generic combined cycle system. As such, a second-law-based approach to off-design performance estimation is a highly viable tool for plant engineers and operators in cases where calculation speed with a small sacrifice in fidelity is of prime importance.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 1

Part-load curve for 2 × 1 209E CC power plant (vertical axis is heat rate as percent of base rating)

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Figure 2

Gas turbine Brayton cycle. Variations in the cycle for three part-load controls described in text are also shown (qualitatively): Constant airflow (A), constant TET (B), and constant TIT (C).

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Figure 3

Relative GT-CC part-load performance for different control philosophies described in the text. The numbers indicate the δ between TET values of constant TIT control and the base TET.

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Figure 4

Relative GT-CC part-load performance for different control philosophies. (The dashed lines correspond to the ideal model calculations in Fig. 3.)

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Figure 5

Ambient and part-load variation of RBC exergetic efficiency. For a 1 × 1 CC system with 50- and 60-Hz frame GTs and OT-OL [once-through, open-loop (water-cooled condenser)] heat rejection system. Design point is ISO base load.

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Figure 6

Individual lost work contributors and RBC net output as a fraction of total GT exhaust exergy; dependence on ambient temperature

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Figure 7

Individual lost work contributors and RBC net output as a fraction of total GT exhaust exergy; dependence on CC load

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Figure 8

Exergy analysis of data from two different performance tests of a 2 × 1 CC power plant. Listed GT exhaust and HRSG stack gas temperatures are for the mixed flow of two units. HRSG lost work is inclusive of transition duct and casing heat losses.

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Figure 9

Gas exergy formula multiplicative correction factors [to be applied to Eqs. 10,11] for ambient (i.e., reference) temperature




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