Analysis, economical and technical enhancement of an organic Rankine cycle recovering waste heat from an exhaust gas stream

Abstract

Any effort with the aim of increasing the total electrical power generation (EPG) due to the existed constraints in vessels is desired in power plants. Using Organic Rankine Cycle (ORC) for recovering waste heat of an exhausting gas is considered as an auxiliary way for improving EPG value. In this study, four ORC arrangements were first modeled for six refrigerants by EES software and the first and second laws of thermodynamics efficiency (η I and η I I, respectively) and mass flow rate were studied for these cycles. Based on modeling results, the best arrangement was selected for each refrigerant such as; F-type (Reference cycle+ Intermediate recuperator+ Final recuperator) for R123, H-type (Reference cycle+ Intermediate recuperator) for R114 and n-butane and R-type (Reference cycle + Final recuperator) for n-pentane, n-heptane, and toluene. The η I value as a technical objective function was then optimized for the above cycles by Aspen HYSYS. n-heptane and toluene cycles were chosen due to higher first-law efficiency value and then were studied under different source temperature condition and n-heptane cycle showed better adaptability. Afterward, exergoeconomic was applied on n-heptane cycle and maximum cycle pressure was chosen as a design variable for economically optimizing net final income (NFI). Finally, NFI value is increased from 423.1$/kW·h to 609.9$/kW·h about 44.2%, while the second-law efficiency value is just decreased from 25.6% to 20.6% about 5%.