Optimum exergetic performance parameters and thermo-sustainability indicators of low-temperature modified organic Rankine cycles (ORCs)

The optimum exergetic performance parameters and thermo-sustainability indicators (TSIs) of low-temperature Organic Rankine cycles (ORCs) are presented. The study objectives are (i) to determine the exergetic performance, and TSIs of modified ORCs based on different thermodynamic inputs and (ii) compare the optimum exergetic performance parameters and the TSIs of the ORCs. The TSIs considered include, waste exergy ratio (WER), exergy efficiency (EE), exergetic sustainability index (ESI) and environmental effect factor (EEF). Additionally, a component by component exergy analysis was first performed, followed by the application of the TSI models established by modifying the existing TSI model for open cycle thermal plants. The results indicated that the increase in EE across the ORCs was not greater than 0.3%. The exergy destruction gap across the ORCs was between 1.2 and 23%. The ESI was low with the basic ORC while the modified systems had improved ESIs. However, best exergetic performance and TSIs were obtained at optimum conditions of 0.127 <= p(1) <= 2.144 MPa, 0.17 <= p(2) <= 2.5 MPa and 416 <= t(3) <= 426. The study inferred that system configuration and type of working fluids are paramount in determining the ORC performance. Moreover, the results obtained can further be used to evaluate the functional sustainability limits of working fluids.