Thermodynamic analysis of three ejector based organic flash cycles for low grade waste heat recovery

Systems based on Organic Flash Cycle (OFC) are promising technologies for waste heat recovery application due to a better match of the hot and cold heat transfer curves. However, the large amount of exergy lost during the throttling process, results in a low thermal efficiency for OFC. In this work, ejectors are introduced in basic OFC to replace the low pressure throttle valve, high pressure throttle valve and both two throttle valves, which namely Single Ejector and Single Flash evaporator based OFC (SESF-OFC), Single Ejector and Double Flash evaporators based OFC (SEDF-OFC) and Double Ejectors and Double Flash evaporators based OFC (DEDF-OFC), respectively. The thermodynamic analysis including energy analysis, exergy analysis, the parametric analysis and comparison analysis are evaluated by using steady-state mathematical model and thermodynamic laws. The ejector allows a high pressure fluid to entrain and compress a low pressure fluid to a medium pressure. Hence, the mass flow rate or expansion ratio of turbine can be enhanced, results in all of three ejector based OFCs are more efficient than the basic OFC. When the high temperature fluid with temperature of 453.15 K is set as heat source, the maximum global second law efficiency (ηII,G) of 38.95% belongs to DEDF-OFC followed SEDF-OFC (37.92%), SESF-OFC (33.68%) and basic OFC (31.43%). The SEDF-OFC is more efficient than SESF-OFC, which indicates that the high pressure throttle valve replaced by ejector is a more effective method than the low pressure throttle valve done. Moreover, the four OFCs have been compared with conventional Organic Rankine Cycle (ORC), and the comparison results show that the ηII,G of basic OFC and SESF-OFC are lower than that of ORC, while SEDF-OFC and DEDF-OFC are more efficient than ORC, and the average increment of SEDF-OFC and DEDF-OFC are 10.37% and 15.01%, respectively. Hence, the ejector based OFCs are alternative methods for waste heat recovery application with a high efficiency.