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Research Papers: Gas Turbines: Electric Power

Importance of Auxiliary Power Consumption for Combined Cycle Performance

[+] Author and Article Information
S. Can Gülen

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

J. Eng. Gas Turbines Power 133(4), 041801 (Nov 18, 2010) (10 pages) doi:10.1115/1.4002254 History: Received May 12, 2010; Revised May 26, 2010; Published November 18, 2010; Online November 18, 2010

The key product of a combined cycle power plant is electric power generated for industrial, commercial, and residential customers. In that sense, the key performance metric that establishes the pecking order among thousands of existing, new, old, and planned power plants is the thermal efficiency. This is a ratio of net electric power generated by the plant to its rate of fuel consumption in the gas turbine combustors and, if applicable, heat recovery boiler duct burners. The term in the numerator of that simple ratio is subject to myriad ambiguities and/or misunderstandings resulting primarily from the lack of a standardized definition agreed upon by all major players. More precisely, it is the lack of a standardized definition of the plant auxiliary power consumption (or load) that must be subtracted from the generator output of all turbines in the plant, which then determines the net contribution of that power plant to the electric grid. For a combined cycle power plant, the key contributor to the plant’s auxiliary power load is the heat rejection system. In particular, any statement of combined cycle power plant thermal efficiency that does not specify the steam turbine exhaust pressure and the exhaust steam cooling system to achieve that pressure at the site ambient and loading conditions is subject to conjecture. Furthermore, for an assessment of the realism associated with the two in terms of economic and mechanical design feasibility, it is necessary to know the steam turbine exhaust end size and configuration. Using fundamental design principles, this paper provides a precise definition of the plant auxiliary load and quantifies its ramification on the plant’s net thermal efficiency. In addition, four standard auxiliary load levels are quantitatively defined based on a rigorous study of heat rejection system design considerations with a second-law perspective.

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References

Figures

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Figure 1

OEM data from a recent trade publication (1)

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Figure 2

Heat rejection system exergy loss (i.e., lost work) and auxiliary load for a closed-loop system with a CT (design parameters are shown in Fig. 3)

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Figure 3

Key temperatures and temperature deltas for a closed-loop system with a CT

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Figure 4

Heat rejection system exergy loss (i.e., lost work) and auxiliary load for an OT-OL system with Tcw,in=63°F(17.2°C)

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Figure 5

Heat rejection system exergy loss (i.e., lost work) and auxiliary load for an OT-OL system with Tcw,in=53°F(11.7°C)

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Figure 6

Comparison of estimated and actual (bid) power consumption of major parasitic loss contributors

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Figure 7

Impact of ST exhaust end annulus area on total CCPP performance penalty as condenser pressure changes

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Figure 8

Comparison of CCPP aux load estimates from Table 1 with actual bid data

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