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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.

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