Thermodynamic analysis and mathematical optimisation of power plants

Abstract

A new methodology is presented for a new design of power plants, which combines the benefits of thermodynamics, economics and mathematical optimisation. The design process is divided into two mains stage, namely analysis stage and design stage. The analysis stage comprises thermodynamic analysis, sensitivity analysis and economic analysis, and the design stage includes mathematical optimisation. Thermodynamic analysis is based on the Combined Pinch and Exergy Representation (CPER) (Zheng 1996) and σ - E p diagram. CPER gives an overview of the exergy losses in the heat exchange system and provides information on exergy input for shaftwork production. σ - E p diagram on the other hand provides more detailed information on the exergetic efficiencies of subsystems, including not only heat exchangers but also turbines, pumps, etc. This exergetic efficiency is based on the concept of avoidable exergy loss (Feng and Zhu, 1997), thus providing realistic targets on exergy loss reduction in the processes. These two tools together enable the designer to quickly locate the inefficiencies in the process and evaluate the effects of process changes on shaftwork. Once the promising modifications from the thermodynamic analysis are found, sensitivity and economic analyses are performed to evaluate the effect of individual changes on the performance of the plant and its economics. The changes with good economic potential together with the base case form a superstructure leading to a MINLP formulation. Because only the economically viable changes are included, the superstructures will be potentially much simpler and hence the number of integer variables is reduced. Another benefit of the analysis stage is that the results give upper and lower bounds and initial points for the optimisation. With a reduced superstructure, good initial starting point and feasible bounds industrial problems can be tackled effectively.