Importance of Auxiliary Power Consumption for Combined Cycle Performance

Abstract

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.