Performance analysis of gas turbine cogeneration systems.

Abstract

Cogeneration is a highly efficient approach to generating electricity and process heat from the same fuel source. It is an approach of utilizing waste energy products for a useful purpose that significantly improves the optimal provision of the different grades of energy from high-quality, high-grade electricity or power to low quality and low-grade heat. As a result, combined heat and power can be applied to various situations using various technologies. In the present work, performance of two gas turbine cogeneration systems is analyzed using the first and second laws of thermodynamics. A basic gas turbine cogeneration system and a steam injection gas turbine cogeneration system are investigated. In the analysis the system components, compressor, combustion chamber, turbine and heat recovery steam generator are modeled. The CO2 emissions of both systems are also evaluated. In this investigation the decrease in CO2 emissions was found in the steam injected gas turbine cogeneration system as compared to the basic gas turbine cogeneration system. In the parametric study the influence of pressure ratio and turbine inlet temperature on the performance characteristics of the systems such as specific work, energy and exergy efficiencies are investigated. In the efficiency calculations all the three forms of outputs: power, heat and cogeneration are considered. Thus power generation, heat generation and cogeneration efficiencies are estimated. The carbon dioxide emissions are estimated for both systems. In addition, the effect of pressure ratio and turbine inlet temperature on the CO2 emissions is studied. The results show that pressure ratio and turbine inlet temperature are the key operating variables to optimize the performance of the cogeneration systems. The second law analysis revealed that the maximum exergy destruction occurs in the combustion chamber. In this analysis, the chemical and physical component of the exergy is considered. The role of specific heat as the function of temperature in estimating the performance of cogeneration is also investigated and the performance results are compared with the results assuming constant specific heat. The results with specific heat as the function of temperature are more realistic and accurate compared to the actual performance of the cogeneration systems. The analysis and the results are useful for optimizing the operating parameters of the gas turbine cogeneration systems and thereby enhancing the system performance. The study is also useful for choosing appropriate cogeneration system for a specific application.