Molecular property methods for assessing efficiency of organic Rankine cycles

Highlights: • ORC efficiency was predicted based on molecular structures of working fluids. • ORC efficiency is strongly correlated with working fluid properties: C_p⁰/R and T_c. • C_p⁰/R and T_c were evaluated using molecular group contribution methods. • ORC efficiency was assessed for 92 working fluids with low global warming potential. 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 ɳ_u* of an ORC plant was correlated with two thermodynamic properties of the working fluid, namely, its critical temperature T_c and reduced ideal gas heat capacity C_p⁰/R. The developed correlations predict ɳ_u* with an average error of 0.9–1.5 percentage points. The optimum ORC heat source temperature T_hs* 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 ɳ_u* and T_hs* of 92 working fluids with low global warming potentials (GWP₁₀₀ 0.1 kg/m³). Lastly, best candidate next-generation, low-GWP working fluids were selected for a more detailed examination.