Special Features of the Material Model Steam
Some conventions should be observed when modeling energy systems with steam suppliers or steam power processes. The aim of this article is to teach the basics of handling and to deal with special features or any difficulties that may arise in advance.
The construction of an energy system with steam components takes place – as with the other energy suppliers – by simply connecting the generators and demands and by using the material model Steam. Generators and demands can be connected in series or in parallel.
Concept State Determination
The material model Steam maps the medium water in the aggregate states “liquid” and “vaporous” as well as in the two-phase wet steam area. In TOP-Energy, a basic concept for modeling with the material model Steam is the clear user specification of the thermodynamic state at every point in the system. This is done by means of the State Determination component, which must be inserted in every section of the steam network.
In this component, the thermodynamic state of the fluid is unambiguously determined by the specification of pressure and temperature or steam mass fraction (see component State Determination).
If the State Determination is missing, error message 8315 appears in the Simulator window (see the following figure) during the simulation:
A model specific error has occured in component . . .:
The thermal state of the medium is undefined. Please check if you forgot to use an appropriate component ‘State Specification’.
Mixing of Material Flows
Due to the above-mentioned specification of the states, no discharge or mixing states are calculated in the TOP-Energy model. However, if material flows with different states are to be combined in the model, the component Mixer can be used. The mixed state must be specified by a State Determination component. The user should enter a plausible value (see component Mixer).
Mass and Energy Flows
When parameterizing the components, the output of the generators and the demand units can be specified either in a mass flow unit (e.g., tons of steam per hour) or in an energetic unit (e.g., kilowatt). Particularly in the case of energy-converting components (such as steam turbines or steam generators), data sheets are often available in mass-related units.
For a plausible solution of the system, there must be a sink at each source – and vice versa – if there is no return flow. For this reason, in TOP-Energy, steam supply systems can be equipped with sources and sinks, e.g., with a Condensate Sink, a Source of Steam or a Feedwater Source (see the second figure below).
This must always be taken into account when creating a model. Otherwise no physical solution can be found, as, for example, in the schema on the first figure below. Due to a missing source, an error message 8359 Plausibility check would be issued for components with the following wording:
The sinks of the mass flows in the energy system cannot be matched with corresponding sources.
If the sink is missing, the error message is:
The sources of the mass flows in the energy system cannot be matched with corresponding sinks.
In these cases, the missing sources or sinks must be added.
For more information, see the articles about Modeling Feedwater Preheating (Steam) and Heat Recovery Steam Generator. Please also note the hint on the special presentation of steam in the energy balance time series in ETA.
