Working Fluid Selection And Active Design For Organic Rankine Cycle Based On Reduced Temperature

Efficient,economical and the large-scale utilization of medium and low-grade energy can not only reduce human demand for fossil fuels,but also indeed reduce greenhouse gas emissions.Medium and low grade thermal power conversion technology based on organic Rankine cycle is one of the most effective and economical technologies for the generation and utilization of medium and low grade energy.Aiming at the organic Rankine cycle power generation system,from the point of view of matching the characteristics of heat source and taking the exergy efficiency as the objective function,working fluid selection and mixture parametric design are carried out under the reduced temperature,and theoretical analysis is validated by experiments with R245 fa as the working fluid.The thermodynamic model of ORC is established.Thermodynamic calculation of pure working fluid and mixture are both carried out respectively.The effects of heat source temperature,evaporation temperature and physical properties of working fluid,temperature slip and mixing ratio on thermal efficiency,exergy efficiency and net power output are analyzed.The key physical parameters of working fluid are critical temperature,constant pressure specific heat capacity and latent heat of vaporization.The working fluids are selected using exergy efficiency as the index to analysed the changes of the cycle's performance at the optimum evaporation temperature.Compared the thermodynamic performance of mixture and pure working fluid,it can be concluded that the performance of mixture is better than that of pure fluids.The simplified model for calculating thermal efficiency,net output power and exergy efficiency are proposed,which can be used for pure working fluids and mixtures.Based on the validation of model accuracy,the correlation of heat source transition temperature is explored.When the heat source temperature is lower than the heat source transition temperature,with the increasing of evaporation temperature,the exergy efficiency increases first and then decreases.There is an optimal evaporation temperature.Under the constant reduced evaporation temperature(0.85),the working fluids are selected with the exergy efficiency as the index.It can be concluded that the working fluids will achieve the maximum exergy efficiency when the critical temperature approached the heat source temperature.The optimal working fluids are R114,R245 fa,R245ca and R365 mfc when the heat source temperature are 130?,150?,170? and 190? respectively.It can be summarized the exergy efficiency of different working fluids are roughly the same under total reduced temperature.This rule has a guidance for performance prediction and reverse design of working fluids.The theoretical model of exergy efficiency has been modified and applied to the mixture calculation,which is calculated by critical temperature,specific heat capacity at constant pressure and latent heat of evaporization.Using the Key mixing rule,the proportion of mixture can be determined according to the virtual critical temperature and Jacob Number(Ja).The optimum pure working fluid R245 fa corresponding to the heat source at 150? can be replaced by the mixture R245ca/R152a(0.675/0.325).The experimental study of organic Rankine cycle power generation system using R245 fa as working fluid are done in this paper.The effects of working fluid flow rate,evaporation temperature and superheat degree on thermal efficiency,exergy efficiency and net output power are tested.Under the given experimental conditions,the maximum thermal efficiency of the cycle is 3.57%,the maximum net output power is 0.53 kW.Fixed the heat source condition,with the increasing of working fluid flow rate,the system thermal efficiency decreases,the net output power increases first and then decrease,and the exergy efficiency decreases.