Approximating the Temperature–Entropy Saturation Curve of ORC Working Fluids From the Ideal Gas Isobaric Heat Capacity

Abstract

Recently, we proposed an approximate expression for the liquid−vapor saturation curves of pure fluids in a temperature−entropy diagram that requires the use of parameters related to the molar heat capacity along the vapor branch of the saturation curve. In the present work, we establish a connection between these parameters and the ideal-gas isobaric molar heat capacity. The resulting new approximation yields good results for most working fluids in Organic Rankine Cycles, improving the previous approximation for very dry fluids. The ideal-gas isobaric molar heat capacity can be obtained from most Thermophysical Properties databases for a very large number of substances for which the present approximation scheme can be applied.

Keywords

• ORC working fluids
• temperature–entropy saturation curve
• saturation properties
• wet and dry fluids
• ideal-gas heat capacity