In order to accurately model large-scale power systems such as boilers and steam turbines, accurate correlations for steam properties are required. The International Association for the Properties of Water and Steam has industry-standard correlations for water and steam properties at pressures up to 10MPa and temperatures up to 1073 K. Higher temperatures are possible too, but not at the full range of pressures. These correlations are in many cases stipulated requirements when consulting engineers are contracted to perform analysis - in order that competing proposals are competing on a fair basis. In large-scale design, even quite small variations in steam properties can result in large differences in overall cost estimates.
The current models/thermodynamics.a4l library in ASCEND does not allow for compressibility of liquids and is therefore too simplistic for use at the elevated pressures seen in large-scale power systems.
Connection to freesteam
Freesteam provides a free open-source implementation of the standard IAPWS IF-97 steam properties correlations for use with ASCEND via the external relation mechanism. Example models using these property correlations include those in models/johnpye/rankine.a4c, which demonstrate a range of steam power cycle calculations.
The IAPWS-95 (pdf) steam properties correlations have been implemented and can currently be used to obtain properties of single-phase steam. We are working on getting a stable two-phase model that is able to correctly accomodate the saturated region using a separate IAPWS correlation for the saturation curves (pdf).
This work in ASCEND is to facilitate modelling of solar thermal and conventional power station systems.
For further information about IAPWS correlations and a list of some other IAPWS-savvy software, see Freesteam.
To see the current state of the steam tables work, go over to John Pye's ASCEND models at models/johnpye.
There is some interesting information to be gleaned from the Modelica ThermoFluid library, . This set of Modelica/Dymola models includes IAPWS-IF97 as well as some interesting although very complex approaches to the problems of modelling thermodynamic processes involving water and steam.