Simulation and Analysis of the Supercritical ORC Heat Exchanger

The traditional organic Rankine cycle (ORC) is operated below critical point. However, the specific heat of the working fluid undergoes tremendous change near the critical point. This can improve the thermal performance of the system due to the enhancement of heat transfer coefficient within the heat exchanger. However, the strong temperature dependence of thermo-physical properties of the working fluid especially at near the critical point requires much more efforts in designing a heat exchanger. Hence, more elaborate calculation involving stepwise 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 input parameters for the second segment, and the process is carried out further on. The fluid properties are calculated with the actual bulk temperature of each segment. With increasing number of segments, better resolution of temperature distribution of both heat source and working fluid within the heat exchanger is achieved. In the present study, a plate heat exchanger was numerically examined by using R-245fa as a working fluid at a supercritical condition. The effects of the working pressure and mass flow rate were examined in detail. For all cases in this study, the maximum of the total heat transfer rate was achieved by a working pressure of 3700 kPa, especially close to critical pressure. It is found that at a working pressure of 4000 kPa and mass flow rate ranging from 1 kg/s to 1.75 kg/s, the total heat transfer rate was independent of the mass flow rate.