Modeling and Cost Optimization of Combined Cycle Heat Recovery Generator Systems

Abstract

The combined cycle power plant is made up of three major systems, the gas turbine engine, the heat recovery steam generator and the steam turbine. Of the major systems the gas turbine engine is a fixed design offered by a manufacturer, and the steam turbine is also a fairly standard design available from a manufacturer, but it may be somewhat customized for the project. In contrast, the heat recovery steam generator (HRSG) offers many different design options, and its design is highly customized and integrated with the steam turbine. The objective of this project is to parametrically investigate the design and cost of the HRSG system, and to demonstrate the impact on the overall cost of electricity (COE) of a combined cycle power plant. There are numerous design parameters that can affect the size and complexity of the HRSG, and it is the plan for the project to identify all the important parameters and to evaluate each. For this study, the design parameter chosen for evaluation is the exhaust gas pressure drop across the HRSG. This parameter affects the performance of both the gas turbine and steam turbine and the size of the heat recovery unit. Single-pressure, two-pressure and three-pressure HRSGs are all investigated, with the tradeoffs between design point size, performance and cost evaluated for each system. A genetic algorithm is used in the design optimization process to minimize the investment cost of the HSRG. Several system level metrics are employed to evaluate a design. They are gas turbine net power, steam turbine net power, fuel consumption of the power plant, net cycle efficiency of the power plant, HRSG investment cost, total investment cost of the power plant and the operating cost measured by the cost of electricity (COE). The impacts of HRSG exhaust gas pressure drop and system complexity on these system level metrics are investigated.