The influence of composition shift on organic Rankine cycle (ORC) with zeotropic mixtures

In the medium-temperature and low-temperature energy utilization, organic Rankine cycle (ORC) is one of the most technically feasible methods. In order to improve temperature mismatch in the process between constant temperature phase change and varying temperature heat source, organic Rankine cycle with zeotropic mixtures is put forward. The characteristics of temperature glide for zeotropic mixtures during evaporation and condensing processes lead to better temperature match with varying temperature heat source and heat sink, and therefore result in a lower system's irreversibility and higher system performance. This paper mainly discusses the influence of composition shift on organic Rankine cycle with zeotropic mixtures. Evaporator, condenser, expander and feed pump models are respectively developed, and then the circulating composition is calculated, based on which, the system parameters and performance of organic Rankine cycle with zeotropic mixtures are obtained in the case of composition shift that means that the circulating composition is different from the charge composition. The results show that composition shift significantly influence the performance of organic Rankine cycle with zeotropic mixtures, which will result in a lower output work of expander, a higher power consumption of pump, a lower net output work and lower thermal efficiency. Meanwhile, it also discusses the local composition shift characteristic in the process of phase change and effects of charge and charge composition on composition shift.