Prediction accuracy of thermodynamic properties using PC-SAFT for high-temperature organic Rankine cycle with siloxanes

Abstract

High-temperature organic Rankine cycle (ORC) with a heat source temperature over 250 °C can be used as the bottom cycle to form a combined cycle with a heat engine such as gas turbine or internal combustion engine. The accurate calculation of thermo-physical properties of the working fluid is of great significance for the performance prediction of an ORC system. PC-SAFT Equation of State (EoS) based on statistical theory can be used to estimate the thermo-physical properties of different working fluids and even to develop new working fluids. However, its accuracy needs to be investigated comprehensively. In this study, the prediction accuracy for the thermal properties based on PC-SAFT for linear and cyclic siloxanes is evaluated and the influence of the uncertainties on the performance of a high-temperature ORC is estimated. First, a program based on the PC-SAFT is developed to compute the properties of 8 siloxanes (MM, MDM, MD2M, MD3M, MD4M, D4, D5, D6). Then, the parameters of the EoS are fitted. Next, the accuracies of PC-SAFT along with the saturation lines and in the superheated regions are determined and compared with the results of Peng-Robinson (PR) EoS. Finally, the ORC performance is estimated using PC-SAFT. The results indicate that the accuracy of PC-SAFT is better than PR EoS. Only the deviation for the compressibility factor in the superheated region is slightly higher than PR. When PC-SAFT is used for the performance prediction of ORC system, the prediction accuracy increases apparently with the increase of molecular chain length. The deviations of the thermal efficiency by PC-SAFT relative to that of Refprop are basically less than 8% for the siloxanes except MM.