Avoidable thermodynamic inefficiencies and costs in an externally fired combined cycle power plant

The real thermodynamic inefficiencies in a thermal system are related to exergy destruction and exergy loss. An exergy analysis identifies the system components with the highest exergy destruction and the processes that cause them. However, only a part of the exergy destruction in a component can be avoided. A minimum exergy destruction rate for each system component is imposed by physical, technological, and economic constraints. The difference between the total and the unavoidable exergy destruction rate represents the avoidable exergy destruction rate, which provides a realistic measure of the potential for improving the thermodynamic efficiency of a component. The calculation of avoidable cost rates associated with both exergy destruction and capital investment is described in the paper and is applied to the exergoeconomic evaluation of an externally fired combined cycle power plant. For each plant component, avoidable and unavoidable exergy destructions and investment costs are calculated. The assumptions required for these calculations are discussed. Modified exergoeconomic variables assist in identifying the real potential of improving single plant components. In addition, some aspects of the design and improvement of externally fired combined cycles are discussed. The results of this study show that the concepts of avoidable exergy destruction and avoidable investment cost are very useful in designing cost-effective energy conversion systems. (C) 2005 Elsevier Ltd. All rights reserved.