Modelica.Media: BaseProperties versus setState_XXX

Question

The Modelica Standard Library comes with the Modelica.Media library which makes available thermodynamic properties of fluids. Quoting from the Modelica.Media documentation:

Media models in Modelica.Media are provided by packages, inheriting from the partial package Modelica.Media.Interfaces.PartialMedium. Every package defines:

• [...]
• A BaseProperties model, to compute the basic thermodynamic properties of the fluid;
• setState_XXX functions to compute the thermodynamic state record from different input arguments (such as density, temperature, and composition which would be setState_dTX);
• [...]

There are - as stated above - two different basic ways of using the Media library which will be described in more details in the following section.
One way is to use the model BaseProperties.
[...]
The second way is to use the setState_XXX functions to compute the thermodynamic state record from which all other thermodynamic state variables can be computed [...]

My colleague prefers BaseProperties (he spends most time modeling components),
I prefer the setState_XXX functions (I spend most time writing a property library).
Now we want to develop a simple&small component library together and probably we should agree to use one of the two approaches.

Can you recommend a publication that explains the advantages/disadvantages of the two approaches? Publications that promote the use of the setState_XXX function are preferred of course... ;-)

Are there some simple rules to decide which one of the two approaches to use when modeling a component (e.g. a very simple turbine)? The components in Modelica.Fluid seem to use both.

Answer 1

The 2 types of patterns for computing properties can both be used for all types of components, but BaseProperties have been designed to make life for the Modeller easy for components with dynamic states, i.e. usually for the storage of mass and energy in volumes. You need to write just the conservation equations, instantiate BaseProperties, equate the relevant variables and you are done. That is often overkill (more equations than minimally needed) for components with a stationary mass and energy balance, like simple valves, pumps and turbines. For that type of components (no dynamic states), the setState_xxx method provide a way to work with the minimally necessary number of equations. I think that is also what you will see in Modelica.Fluid: BaseProperties are used together with dynamic equations for mass- and energy storage, and setState elswhere.

The minimum number of equations is not the whole story w.r.t. computational efficiency, but in geeneral models shoudl not ocmpute more than what is actually needed.