In this paper, a simple methodology of applying the Peng Robinson (P R) EoS (Equation of State) to easily, quickly, and inexpensively evaluate and screen the performance potentials of many thousands of working fluids in ORC (organic Rankine cycles) is presented. The P R EoS can be applied both to well-described (considerable experimental data is available) and not-so-well-described (little or no experimental data is available) working fluids. For not-so-well-described working fluids the P R EoS can be constructed from thermodynamic parameter estimates using simple group contribution methods. In particular, they can be used to estimate the critical state properties, ideal gas specific heat at constant pressure, and acentric factor from knowing only a working fluid's molecular structure and its NBP (normal boiling point) temperature. The simulations presented in this paper represent a large variety and number of working fluids for several different ORC applications and operating conditions. The simulations using the P R EoS based on knowing only a working fluid's molecular structure and its NBP temperature showed comparable accuracies to simulations based on using much more complex EoS based on large amounts of experimental data. (C) 2014 Elsevier Ltd. All rights reserved.