ABSTRACT
Decarbonization for a net-zero future has gained momentum in recent years due to the impact of greenhouse gases on our environment. Various governments and industries have announced their strategies to achieve low-carbon or net-zero carbon strategies, which will require the use of renewable energy resources and alternative fuels. Achieving a net-zero carbon future may take anywhere between 20–50 years. In the interim, it is apparent that there is a need to make the current energy-producing devices such as internal combustion engines and gas turbines more efficient, helping to bridge the gap. One such concept is waste heat recovery (WHR) from hot exhaust gases.
In this paper, Aspen Plus software is used to model and analyze WHR systems, based on different variations of the Joule Cycle: Inverted Joule Cycle, Closed Joule Cycle (CJC), CJC with heat recuperation, two-stage Ericsson Cycle (approximated as a CJC with two stages of compression with intercooling between the compression stages, two stages of expansion with reheating between the expansion stages, and heat recuperation), three-stage Ericsson Cycle (approximated as a CJC with three stages of compression with intercooling between the compression stages, three stages of expansion with reheating between the expansion stages, and heat recuperation), CJC with two stages of expansion, reheating and heat recuperation, and CJC with two stages of compression, intercooling and heat recuperation. In multistage compression, the temperature of the inlet stream into all the compressors was kept constant using intercoolers, and equally, in multistage expansion, the temperature of the inlet stream into all the turbines was also kept constant through reheaters. Air, argon, and carbon dioxide are separately investigated as working fluids. Sensitivity studies of the exhaust gas temperature and the system pressure ratio and their impact on the cycle thermodynamic efficiency and specific net work output were carried out for the considered working fluids and a comparative analysis of the different system models was also performed. The results show that for an exhaust gas temperature of 600°C, theoretical cycle thermodynamic efficiencies up to about 40%, and specific net work output values up to about 160 kW/kg of exhaust gas can be obtained, with the two-stage Ericsson cycle and the CJC with heat recuperation and intercooling striking a pragmatic balance between 232system efficiency and system complexity. Also, the results show that argon is the most effective working fluid at low-pressure ratios while carbon dioxide is most suitable for high-pressure ratio applications.
