Theoretical study of organic Rankine cycle for working fluid selection

This paper establishes a mathematical relationship between fluid properties and cycle performance to establish a criterion for working fluid selection. The theoretical model for predicting cycle performance incorporates thermodynamic principles and several thermophysical correlations based on fluid properties. To achieve maximum net power output, a theoretical expression for the optimal evaporation temperature is derived as a function of heat source temperature and fluid properties. The relationship between fluid properties and cycle performance is analyzed by introducing a new feature parameter χ, which consists of operating temperatures, latent heat of vaporization, the slope of the saturated liquid line, and the slope of the isobaric line of expanded vapor. The effectiveness of the feature parameter χ as a selection criterion is validated by comparison with results from previous studies. Results show that the theoretical model achieves high accuracy, with maximum relative deviations below 3 % compared to numerical simulations. Regardless of whether the outlet temperature constraint is present, the standard deviation of the optimal evaporation temperature remains below 3  K, with a coefficient of variation of less than 0.8 %, indicating that the fluid properties have no significant impact on the optimal evaporation temperature. By accounting for the non-isothermal condensation process, the net power output shows a consistent linear relationship with the feature parameter χ, with the Pearson correlation coefficient remaining 1 across four cases. Case studies of working fluid selection for ORCs in maritime applications indicate that the feature parameter χ serves as an effective selection criterion for dry and isentropic working fluids.