Exergy Analysis for the Dual-pressure Evaporation Organic Rankine Cycle System

Dual-pressure evaporation cycle contains two evaporation processes with different pressures and an isobaric condensation process. It is an emerging study focus in the organic Rankine cycle (ORC) field because it can significantly increase the heat-power conversion efficiency and improve the adaptability to heat sources with various characteristics. An exergy analysis was carried out in this study for the dual-pressure evaporation ORC system using pure fluids. The thermodynamic perfections of dual-pressure evaporation cycle and its heat absorption process were quantitatively studied. Characteristics of the exergy loss distribution in the system were revealed. Results show that the variations of exergy loss in the heat absorption process and its thermodynamic perfections are closely related to the critical temperature of working fluid. The dual-pressure evaporation cycle can remarkably reduce the exergy loss in the heat absorption process, and thereby increase the thermodynamic perfection of ORC. While, the exergy loss in the heat absorption process remains the largest when the heat source temperature is low, compared to other thermodynamic processes. The vital thermodynamic process which limits the heat-power conversion efficiency to further increase will change as the heat source temperature increases. To reduce the exergy loss in the desuperheated process becomes crucial when the superheat degree of working fluid in the high-pressure stage is large.