Molecular property methods for assessing efficiency of organic Rankine cycles

This paper presents a robust method for assessing the efficiency of organic Rankine cycle (ORC) plants based on the molecular structures of the working fluids employed. The developed methodology uses molecular group contribution methods and does not require equations of state or extensive experimental data. The maximum utilization efficiency ɳu* of an ORC plant was correlated with two thermodynamic properties of the working fluid, namely, its critical temperature Tc and reduced ideal gas heat capacity Cp0/R. The developed correlations predict ɳu* with an average error of 0.9-1.5 percentage points. The optimum ORC heat source temperature Ths* can be predicted with an average error of 3.5 °C to 6.6 °C. The developed methodology was validated using a numerical model of an optimized ORC. It was then used to estimate ɳu* and Ths* of 92 working fluids with low global warming potentials (GWP100 < 150) and low flammability values (LFL > 0.1 kg/m3). Lastly, best candidate next-generation, low-GWP working fluids were selected for a more detailed examination.