Second-law-based screening of working fluids for medium-low temperature organic Rankine cycles (ORCs): Effects of physical and chemical properties

Abstract

Abstract The diversity of both organic fluids and performance criteria makes the selection and evaluation of working fluids for organic Rankine cycles (ORCs) full of challenge. To overcome this challenge, this paper first reports on second-law-based analytical expressions for thermal efficiency and a turbine size parameter, which reveal key dimensionless parameters relating to operating temperatures and thermodynamic properties of organic fluids. This paper systematically screens more than 70 working fluids involving a wide range of physical and chemical properties. This study shows that specific turbine size generally increases with critical temperature, reduced ideal gas heat capacity, and the atom number of working fluids. Different-family fluids may lead to ten-fold relative differences in specific turbine size even though they have the same atom numbers. The thermal efficiency of ORCs appears to be independent of critical temperature, or at most, a weak function of critical temperature for low boiling point wet fluids and other halocarbons. This is a result of Jacob number being independent of or dependent weakly on the critical temperature of organic fluids such as alkanes, alkenes, and benzenes.