A limiting efficiency of subcritical Organic Rankine cycle under the constraint of working fluids

Highlights: • A limiting efficiency is derived for subcritical Organic Rankine cycle. • A new expression of cycle perfectness is proposed from the limiting efficiency. • The limiting efficiency is lower than the Carnot efficiency for any working fluid. As a theoretical upper bound of cycle efficiency, Carnot efficiency doesn't contain detailed information on the properties of working fluids. A nature idea emerges how to derive the efficiency limit under the constraint of working fluids and how to quantify it by considering the properties of working fluids. Therefore, in this contribution, a limiting efficiency is proposed for subcritical Organic Rankine cycle (ORC). For the calculation of limiting efficiency, a limiting factor is defined on the basis of the saturated slope of liquid at the reduced temperature 0.9. Furthermore, in order to represent the extent to which the practical efficiency approaches to the limiting efficiency, a new expression is proposed for thermodynamic perfectness. 13 pure fluids and 3 mixtures are employed to demonstrate the effects of working fluids on the limiting efficiency and thermodynamic perfectness. For pure working fluids, the fluid with a higher critical temperature possesses higher limiting efficiency and cycle perfectness. For mixtures, the limiting efficiency generally locates between those of pure fluids, while the thermodynamic perfectness varies greatly with the composition. Although the proposed limiting efficiency can't be achieved by practical cycles, it can provide guidance for the selection of working fluids and the construction of ORC.