Comparative Thermodynamic Environmental and Economic Analyses of Combined Cycles Using Air and Supercritical CO₂ in the Bottoming Cycles for Power Generation by Gas Turbine Waste Heat Recovery

Abstract

This study aims to improve existing fossil gas turbine power plants by waste heat recovery. These power plants function with an air simple cycle (ASC) and are implemented where water resources are limited. Modeling and simulation of ASC and two advanced energy conversion systems are performed. They are the gas turbine–air bottoming cycle (GT-ABC) and gas turbine–supercritical carbon dioxide bottoming cycle (GT-sc-CO2BC). The main intent is to assess the benefits of employing sc-CO2 as a working fluid in a closed Brayton bottoming cycle compared to air, based on the energetic and exergetic performance and economic and environmental impact. Analyses of ASC, GT-ABC, and GT-sc-CO2BC are performed for various topping gas turbine powers: large (plant 1); medium (plant 2); and low (plant 3). The results of the energetic and exergetic analyses indicate that there is a significant improvement in the output power (ranging from 22% to 25%); and energy and exergy efficiencies of GT-ABC and GT-sc-CO2BC (up to 8% and 11%, respectively) compared to that of ASC. To provide better insight into the behavior of these technologies and achieve their better integration, we investigate the influence of varying the bottoming compressor pressure ratio, the ambient temperature, and the gas flow rate in the bottoming cycle. The results of the environmental and economic analyses show that the amount of CO2 emissions in GT-sc-CO2BC is reduced by 10% more than in GT ABC. The results also show that GT-ABC improves the NPV between 17.69% and 30% but GT-sc-CO2BC improves it even more, between 25.79% and 33.30%.

Keywords

• exhaust heat recovery
• gas turbine improvement
• combined cycle
• air bottoming cycle
• supercritical carbon dioxide bottoming cycle