An Assessment of Subcritical and Trans-critical Organic Rankine Cycles for Waste-heat Recovery

Abstract

Abstract Organic Rankine cycle (ORC) systems are increasingly being deployed for waste-heat recovery and conversion in industrial settings. Using a case study of an exhaust flue-gas stream at a temperature of 380 °C as the heat source, an ORC system power output in excess of 10 MW is predicted at exergy efficiencies ranging between 20% and 35%. By comparison with available experimental data, the thermodynamic properties (including those in the supercritical region) of working fluids are shown to be reliably predicted by the SAFT-VR Mie equation of state; this verification is quite important as this is the first time that the SAFT-VR Mie equation of state is used for thermodynamic property prediction of working fluids in their supercritical state in trans-critical ORC systems. Various cycle configurations and the use of working-fluid mixtures are also investigated. ORC systems operating on trans-critical cycles and those incorporating an internal heat exchanger (IHE) are seen to be beneficial from a thermodynamic perspective, they are, however, more expensive than the simple ORC system considered (subcritical cycle with no IHE). Furthermore, ORC systems using pure working fluids are associated with slightly lower costs than those with fluid mixtures. It is concluded that a basic ORC system utilizing pure working fluids shows the lowest specific investment cost (SIC) in the case study considered.