Advanced power plant design methodology using process integration and multi-objective thermo-economic optimisation

Thermo-economic modelling techniques are well known techniques to optimise power plant designs. These methods are usually based on the definition of a superstructure that includes the major options of the design. If this approach has proved to be appropriate for the optimisation of several conventional NGCC (Natural Gas Combined Cycles), it reveals some weaknesses when dealing with particularly complex systems where heat integration leads to a lot of possible heat exchange configuration. This is for example the case in advanced cycles in zero emission plants where numerous heat exchanges between the gas turbines, the steam network and the CO2 capture units can be considered. In this situation, the superstructure approach is not anymore practical and new modelling techniques are needed. In this paper, we present a new modelling technique developed to be used in the context of a multi-objective optimisation framework. This method uses a thermodynamic model of the energy flows of the energy conversion units. The results of the model allow the calculation of the hot and cold streams to be considered in the heat exchanger network. A heat cascade model using the Delta T-min concept is used to compute the optimal integration of the heat exchange in order to maximise the energy conversion. In this model, a special steam cycle model has been developed to represent all the possible heat exchange interactions and to compute the optimal flowrates in the system with a minimum of structural information. The third part of the model is the thermo-economic estimation of the system cost to deduce the performances of the system. An optimisation method, based on a multi-objective evolutionary algorithm is then used to identify the most important system configurations. The method is illustrated on an AZEP (Advanced Zero Emission Plant) combined cycle design.