Thermodynamic analysis and optimization of organic Rankine cycles based on radial-inflow turbine design

Abstract

In recent years, high-temperature organic Rankine cycle (ORC) with a heat source temperature over 300 °C has attracted interests, which has wide range of applications from industrial point of view. However, few researchers have focused on the coupled design and optimization of the high-temperature ORC system and radial-inflow turbine (RIT). In this work, a coupled design of the ORC system and RIT with the heat source inlet temperatures of 300, 350 and 400 °C is developed using non-dominated sorting genetic algorithm. The ORC thermal efficiency and turbine performance factor (PF) are introduced as optimization objectives. Two mixtures of siloxanes, seven pure siloxanes and nine pure hydrocarbons are considered as working fluid candidates. In addition, the thermodynamic comparison between mixtures of siloxanes and pure working fluids is presented. Moreover, the influences of the turbine inlet total temperature, total-to-static expansion ratio, and specific speed on the ORC thermal efficiency, turbine performance, and turbine size are investigated. The results show that the turbine power output and cycle thermal efficiency are more sensitive to the expansion ratio while turbine size is more sensitive to the turbine inlet total temperature and specific speed. In addition, PF increases with the rise of the turbine inlet total temperature, expansion ratio and specific speed. Moreover, compared with pure working fluids, mixtures of siloxanes have higher efficiencies, but larger turbine and heat exchanger size. Cyclohexane, toluene, m-xylene and hexane with higher PF and smaller size of turbine and heat exchanger are promising working fluids in applications where the compact turbine and heat exchanger is preferred, such as vehicle waste heat recovery.