Simulation and analysis of the supercritical orc heat exchanger

The traditional organic Rankine cycle (ORC) is operatedbelow critical point. However, the specific heat of the workingfluid undergoes tremendous change near the critical point. Thiscan improve the thermal performance of the system due to theenhancement of heat transfer coefficient within the heatexchanger. However, the strong temperature dependence ofthermo-physical properties of the working fluid especially atnear the critical point requires much more efforts in designing aheat exchanger. Hence, more elaborate calculation involvingstepwise integration is needed as far as accuracy is concerned.Therefore the heat exchanger is divided into several segments.The outlet temperatures of the first segment serve as the inputparameters for the second segment, and the process is carriedout further on. The fluid properties are calculated with theactual bulk temperature of each segment. With increasingnumber of segments, better resolution of temperaturedistribution of both heat source and working fluid within theheat exchanger is achieved. In the present study, a plate heatexchanger was numerically examined by using R-245fa as aworking fluid at a supercritical condition. The effects of theworking pressure and mass flow rate were examined in detail.For all cases in this study, the maximum of the total heattransfer rate was achieved by a working pressure of 3700 kPa,especially close to critical pressure. It is found that at a workingpressure of 4000 kPa and mass flow rate ranging from 1 kg/s to1.75 kg/s, the total heat transfer rate was independent of themass flow rate.