Dynamic behavior of supercritical organic Rankine cycle using zeotropic mixture working fluids

Abstract

As a prominent technology for recovering low-grade waste heat, supercritical organic Rankine cycle (ORC) exhibits a better performance due to the higher endothermic temperature and better thermal match with the heat source. The irreversibility in system can be reduced greatly when the zeotropic mixtures are used as working fluid to match the heat source and sink profiles. Affected by the fluctuations in waste heat sources, there is a challenge for ORC to recovery waste heat. An improved dynamic model of supercritical ORC using zeotropic mixture R134a/R32 as working fluid is developed and dynamic behaviors of supercritical ORC are analyzed. It is found that an abnormal fluctuation may occur in some parameters due to the effects around pseudo-critical point and thermal inertia of the heater. A fitting correlation to predict the response time based on different heat transfer coefficients of heater is found. Besides, three dynamic regimes are defined to investigate the effects of heat source frequencies and system thermal inertia on the dynamic response. As the heat source frequency increases or the heat exchange in the heater is enhanced, the fluctuation amplitude of the pressure decreases in the heater.