Thermodynamic evaluation of combined cycle plants

Abstract

Abstract The application of the exergy concept for the thermodynamic evaluation of energy conversion systems and chemical plants is steadily growing. However the general application of this concept is complicated by the large variety of parameters that is used to present the results of such evaluations. Easily understandable diagrams that offer a quick overview of the main results of such an evaluation will be very helpful. Large power plants, as for example combined cycle plants, consist of a large number of apparatuses. The thermodynamic modeling of these plants requires the computation of the thermodynamic properties at inlets and outlets of all apparatuses. These results allow for the calculation of the exergy values at all considered points after defining an appropriate environment. Using these exergy values exergy losses and efficiencies of all considered apparatuses can be determined. However, additional parameters and methods for presenting losses are necessary to understand the origin of exergy losses and the options for further improvements. Exergy efficiencies of power cycles show the actual losses but do in general not clearly indicate the potential for improvement. The use of the so-called internal exergy efficiency of a power cycle will be helpful to understand this potential. Also value diagrams and exergy flow diagrams are very useful to understand the thermodynamic performance of complicated systems. In this paper the application of these tools is demonstrated for the evaluation of alternative designs of combined cycle plants. Three system designs are established for this purpose and modeled using the computer program Cycle-Tempo. The considered combined cycles use the same gas turbine but have different steam bottoming cycles. Differences do originate from the number of pressure levels at which steam is generated in the HRSG (Heat Recovery Steam Generator). The evaluation includes respectively a single pressure, double pressure and triple pressure HRSG. The steam pressures are optimized with regard to overall plant efficiency using a multi-parameter optimization procedure. The evaluation shows that the application of the internal exergy efficiency of a power cycle is in particular useful if the temperature of heat transfer from the cycle will be affected by the cycle performance, i.e. in the case of gas turbine cycles. The value diagrams show how the increasing number of pressure levels of steam generation will reduce the losses due to heat transfer in the HRSG but also the exergy loss due to the exhaust of flue gas to the stack. The exergy flow diagrams show that the main exergy losses of combined cycle plants occur in the combustion process. Possibilities to reduce these losses are limited. Serious improvement of the efficiencies of future combined cycle plants is conceivable by applying high temperature fuel cells.