Another view on the optimization of the Brayton cycle with recuperator

Marija Živić ,Zdravko Virag ,Antun Galović

doi:10.17559/tv-20160520115040

Abstract

Nondimensional expressions for the work output, thermal efficiency, heat input, and entropy generation number in the Brayton cycle with a recuperator are given as functions of seven input parameters: pressure ratio, temperature ratio, recuperator effectiveness, turbine and compressor isentropic efficiencies, nondimensional pressure drop in the combustor and recuperator and the ratio of specific heat capacities. The expressions for pressure ratios which maximize or minimize the considered quantities are derived. Since it is not possible to maximize all the quantities at a unique value of pressure ratio, three optimization criteria: (i) maximum work output, (ii) maximum thermal efficiency, and (iii) maximum of their weighted sum are defined. For each criterion two diagrams from which one can select optimal pressure ratio and read out all basic quantities defining the Brayton cycle are provided. In the case of criterion of maximum thermal efficiency, when the recuperator effectiveness is lower than the certain limit, the Brayton cycle without a recuperator is better than that with it. In the case of other two criteria, the Brayton cycle with a recuperator is always better than that without it, regardless of the value of the recuperator effectiveness.

Highlights

Global warming caused by increased emissions of greenhouse gases is a major environmental problem nowadays to which power generation contributes greatly
Gas turbines operate using the Brayton cycle, which may be a simple one when it is used for limited peak power generation, or there may be various variants of advanced Brayton cycle increasingly used for the base load applications
The criterion of maximum thermal efficiency is favourable in stationary power plants because the ratio of the fuel consumption to the gained power output will be the most favourable

Summary

Introduction

Global warming caused by increased emissions of greenhouse gases is a major environmental problem nowadays to which power generation contributes greatly. Cheng and Chen [8] investigated the effect of regeneration on the power output and thermal efficiency in an endoreversible, regenerative Brayton cycle They calculated the maximum power output and the corresponding thermal and the second law efficiencies using the optimal values of cycle temperatures. Wang et al [11] Another view on the optimization of the Brayton cycle with recuperator derived analytical expressions of the dimensionless power and the thermal efficiency of an irreversible, closed, intercooled regenerative Brayton cycle coupled to variable temperature heat reservoirs. SanchezOrgaz et al [13] developed a theoretical model of a multistep, regenerative, closed Brayton cycle with an arbitrary number of intercooled compression steps and reheated expansion steps They used both the maximum power output and the maximum thermal efficiency optimization objective. The goals of this study are: to summarize the optimization with respect to three different thermodynamic criteria by using two concise diagrams for each criteria, and to point out that, depending on recuperator effectiveness, the regenerative Brayton cycle could be thermally less efficient than the one without recuperator

Energy analysis of a regenerative Brayton cycle

Work output

Heat input and heat output per cycle and the cycle thermal efficiency

Criterion wmax

Conclusions