Analysis For The Influence Of Pinch Point Temperature Difference In Heat Exchanger On Organic Rankine Cycle System

In industrial production processes, a large number of low temperature waste heathas being discharged into atmosphere. Therefore, the industrial energy utilization ratiois no more than33%in China. It not only results in the enormous loss of energy, butalso makes the environment situation increasingly grim. As one of the most promisingtechnologies in recovering low grade waste heat, the organic Rankin cycle (ORC)system with many advantages, for example, simple structure, flexibility and highefficiency etc, has been increasingly paid attention. Because the problem of the lackingof energy becomes outstanding, the ORC system has a broad application prospect.In view of the characteristics of low temperature flue gas released by industrialboiler, the thermodynamic model and economic model of the ORC system underconstraint condition have been established based on theoretical analysis and simulationsoftware of EES. The pinch point temperature difference (PPTD) in heat exchanger,which is correlated with heat flux，the irreversibility of heat transfer process and heattransfer area, has important impact on the system efficiency of waste heat recovery andinvestment cost. Despite there have been many studies associated with the effect of theparameters on performance of the ORC system, a detailed analysis about the effect ofthe PPTD has rarely been found. Therefore, the influence of PPTD and evaporatingtemperature on the system exergy efficiency and net power output per heat transfer areahave been analyzed to improve the performance of the ORC system. Meanwhile, theelectricity production cost (EPC) has been used to evaluate the economic performanceof the ORC system based on direct search method. We also discuss the relationshipbetween the critical temperature of working fluid and the system economic performance.Results indicate that:(1) the irreversibility losses of heat exchangers account for80%ofthe total exergy destruction in the ORC system under the optimal PPTD condition. TheEPC decreases first and then increases with the increase of the PPTD in evaporator andcondenser. And then the minimum EPC and the optimal system parameters can beobtained based on the optimal evaporating temperature.(2) For different working fluids,the minimum EPC decreases first and then increase generally with increasing of thecritical temperature of working fluids. The ORC system with R123, n-pentane, R11andR141b, which critical temperature between180℃and205℃, show excellent economicperformance for waste heat source temperature between160℃and200℃.(3) A new power/cooling combined system by coupling ORC and vapor compression refrigerationcycle (VCRC) together has been presented. The detailed analysis of working principleand structural characteristic has been researched. There exists the optimal turbine inletpressure for the maximum refrigerating capacity and system exergy efficiency. And theperformance of the power/cooling combined system can be improved obviously byadding a regenerator. R134a, which is a good eco-friendly organic working fluid, wouldlike to be recommended for the combined system.