Working Fluid Selection Of Low-temperate Waste Heat Recovery System Based On Organic Rankine Cycle

Organic Rankine Cycle(ORC) is the Rankine cycle where organic fluid is used instead of water as the working fluid. ORCs have been applied for electricity generation by recovering low grade heat source and promoting turbine to work. Its development and utilization is important for improving the energy efficiency and reducing environmental pollution.In this paper, the model of ORC system with pure fluid is established based on the laws of thermodynamics. According to the selecting principles, there are 14 kinds of organic working fluids including R600, R601, isohexane, hexane, R601 a, cyclohexane, heptane, pentane, R123, R365 mfc, R141 b, R245 ca, R245 fa and benzene are preliminary screened to do the study considering thermodynamics and economic aspecsts.The results show that most alkanes have higher thermal efficiency and pressure ratio and lower mass flow rate than other kinds of working fluids. And the cyclohexane which belongs to alkanes is considered as an ideal working fluid with higher thermal efficiency, lower mass flow rate and appropriate pressure ratio for the low-temperature waste heat recovery system.At the same time, considering the shortcomings of pure working fluid in heat exchange process, we have analyzied the ORC system with the mixture working fluid. In this paper,R245 fa which has flame retardant propertyes and cyclohexane which is better in Rankine cycle thermal performance were chosen to make up of the zeotropic mixture. We also establish the model of ORC system with chosen mixture as working fluid. Calculations are compared based on the given conditions of the system resulting from trading-offing the linearity, thermodynamic properties and economical efficiency between pure working fluid and the chosen zeotropic mixture with ratio of the exergy loss, system thermal efficiency, pressure ratio,irreversible loss and UA as performance index.The results show that the exergy loss ratio and the quadratic difference of slope of mixture are less than that of pure working fluids, and its linearity between enthalpy and temperature are better than pure working fluids, its heat transfer process are closer to the ideal heat transfer process. Under most of the mass fractions conditions,the mixture has better thermodynamic performance with higher thermal efficiency and system net work. But its higher UA and mass flow rate lead to worse economic performance compared to the pure working fluids.Meanwhlie,under R245fa+cyclohexane(0.8, 0.2) mass fraction condition, the mixture is considered as an ideal working fluid with the lowest exergy loss ratio and quadratic difference of slope, the highest thermal efficiency and net work, and lower mass flow rate in the low-temperature waste heat recovery system.