Conventionally, we need complete and accurate property diagrams or equations of state of the working fluid to calculate the thermal efficiency of ORCs (organic Rankine cycles). In this paper, we develop by entropy-generation analysis several analytical expressions for the thermal efficiency of ORCs. These analytical formulas are functions of several dimensionless variables, enabling fast and accurate calculations without recourse to thermodynamic diagrams or equations of state. The entropy-generation analysis clarifies the thermodynamic mechanism behind the effect of superheat in the evaporator on the thermal efficiency. Among the derived dimensionless variables, cT(3)/r (T3 is the evaporating temperature, r is the heat of evaporation at the evaporating temperature, and c is the average heat capacity of the saturated liquid) reflects the influence of the critical temperature on the thermal efficiency and can serve as an indicator of evaluating working fluids in the same group: thermal efficiency generally decreases with cT3/r for the working fluids in the same category. (C) 2016 Elsevier Ltd. All rights reserved.