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.

1.
2007,
2007-08 Gas Turbine World Handbook
,
Pequot
,
Fairfield, CT
.
2.
1996, “
Performance Test Code on Overall Plant Performance
,” ASME PTC Paper No. 46-1996, Appendix A, p.
69
.
3.
Gülen
,
S. C.
, and
Smith
,
R. W.
, 2010, “
Second Law Efficiency of the Rankine Bottoming Cycle of a Combined Cycle Power Plant
,”
ASMEJ. Eng. Gas Turbines Power
,
132
, p.
011801
.
4.
Kelly
,
G. M.
, 1975, “
Cooling Tower Design and Evaluation Parameters
,” ASME Paper No. 75-IPWR-9.
5.
Hensley
,
J. C.
, ed.,
Cooling Tower Fundamentals
,
2nd ed.
,
SPX Cooling Technologies
,
Overland, KS
.
6.
Heat Exchange Institute
, 1995,
Standards for Steam Surface Condensers
,
9th ed.
,
Heat Exchange Institute
,
Cleveland, OH
.
7.
Hensley
,
J.
, 1992, “
Maximize Tower Power
,”
Chemical Engineering
,
Access Intelligence
,
New York
, pp.
74
82
.
8.
Thermoflow, Inc.
, GT PRO Version 18.0.2, 29 Hudson Road, Sudbury, MA 01776, website: www.thermoflow.comwww.thermoflow.com
9.
Tawney
,
R.
,
Khan
,
Z.
, and
Zachary
,
J.
, 2003,”
Economic and Performance Evaluation of Heat Sink Options in Combined Cycle Applications
,” ASME Paper No. GT2003-38834.
10.
Cotton
,
K. C.
, 1998,
Evaluating and Improving Steam Turbine Performance
,
2nd ed.
,
Cotton Fact Inc.
,
Rexford, NY
.
11.
Wright
,
J. S.
, 1994, “
Steam Turbine Cycle Optimization, Evaluation, and Performance Testing Considerations
,” Paper No. GER-3642E, www.gepower.comwww.gepower.com
You do not currently have access to this content.