Combined cycle plant efficiency increase based on the optimization of the heat recovery steam generator operating parameters

Abstract

At the present time combined cycle (CC) power plants allow to meet the growing energy demand with the least fuel consumption. Thus it is of great interest to define a strategy for the optimization of these systems, in order to get greater performances and efficiency from them. The purpose of the most part of the world manufactures involved in this sector is to reach overall thermal efficiency of 60% in the short term period, above all by improving the gas turbine inlet temperature. In the present work, it is shown how it could be possible to reach the same performances by best fitting the existing technology. In particular we point our attention to the optimization of the heat recovery steam generator (HRSG), as a first step in the analysis of the whole plant, according to a hierarchical strategy. We handle this problem adopting both a thermodynamic and a thermoeconomic objective function instead that with the usual pinch point method. Thermodynamic optimization has the purpose to diminish energy losses, expressed on exergy basis, while the aim of the thermoeconomic optimization is the minimization of a cost function, sum of the cost of exergy inefficiencies and the cost of the HRSG. Proposed methods have been applied to some HRSG configurations, including some present commercial plants. The results of the application of the thermoeconomic optimization leads to a meaningful increase of the thermal efficiency of the plant that approaches the 60%, obtained with and increases of the heat surface and a decrease of the pinch-points. The economic evaluations are referred to US dollars ($) even if the costs are referred to European scenario, assuming 1 US $ = 1.1 euro.