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What happens when the simulation runs?

Schematics (Advanced level)

The standard level interface covers the most common simulation tasks, but sometimes it is useful to examine other variables than those available here or to replace a component model. To accomplish this one turns to the advanced level interface, where the system is described in a mathematical sense with components containing equations, variables and parameters.

Access to the advanced level is provided for both the Standard and Expert editions of IDA ICE. However, Expert edition users can manually edit (reconnect) component diagrams at the advanced level, while Standard edition users can only inspect, log variables and change parameters. Some of the examples below require the Expert edition, but this will then be mentioned in the introduction.

Work at the advanced level is best presented in terms of demonstration, so the written account here is quite brief. Look also at the User’s web page for more information on work at the advanced level, demonstration movies etc.

In some cases the system structures at the standard and advanced levels match each other quite well. The air handling unit, for instance, has different components: fans, coils etc. The same description can then be used for both the standard and advanced levels. The same is true for the primary system. However, for the actual building description entirely different structures are used for the standard and advanced levels.

Most components at the advanced level are described with equations. Components are interconnected by creating equalities between variables that appear on interfaces. An example is the interconnection between the cooling and heating coils in the air handling unit. Both the outflow interface of the heating coil and the inflow interface (terminal) of the cooling coil contain variables for: pressure, temperature, massflow, moisture and carbon dioxide concentrations.

To work on the advanced level, select Build model in the Simulation tab. This creates the schematic view of the current building. If the model has been built previously, Schematic is already available as a view of the system. The following figure shows the appearance of the air handling unit in the schematic view (which is also the standard view for the air handling system).

The air handling unit

The behavior of a component is described by equations, variables and parameters. The difference between parameters and variables is that the former will never change their value during a simulation, whereas a variable might. A window area is an example of a ”natural” parameter and the room air temperature is always a variable.

The description of most components is done using a special language called NMF (Neutral Model Format). Click on the Code tab of any of the AHU components, to see the NMF code. It contains the following main sections:

From version 4, some models in the ICE library are instead described by the Modelica language (www.modelica.org).

We will now go through a few examples of useful operations at the advanced level of ICE.

Example 1: Presenting more data in an existing diagram

Suppose we are interested in the air temperature after the heat exchanger in the air handling unit and would like to see the graph together with the other temperatures of the AHU. Open the AHU window and double click on the connection between the heat exhanger and the heating coil. A small window is opened showing the connection between the two interfaces; SUPOUT (SUPply OUT) of the heat exchanger (hx) is connected to the AIRFLOWIN interface of the heating coil (hc). To see the actual variables of the connection, double click on the box hx.SUPOUT. TSUPOUT is the variable we are looking for. Double click on it to get the form at the lower.

All relevant information about the variable is displayed in this window. At the bottom of the window, there is a combo box for logging the variable to a diagram. Select “AHU temperatures” in the combo box and give a meaningful name to this variable in the corresponding field. Make a simulation, and inspect the added graph in the diagram.

Example 2: Shade control by zone temperature (Expert edition required)

Normally, window integrated shading is controlled by the amount of radiation that penetrates the glazing. However, in some applications it can be useful to let the zone air temperature determine whether shades should be drawn instead. It is possible to define customized controls for shading (and other devices) at the standard level.

We look at a simple case consisting of a single zone with a window in one wall and intend to have an external blind shading the window depending on the zone air temperature and governed by a thermostat. First insert an external blind. Double click on the window to open the window form. Search in database under “Integrated Window Shading” and select “External blind” as Device as shown below.

Next we define the control system to use. Right below the previous choice in the window form select “New custom control” as Control and give an appropriate name. An empty macro form appears. Drag a thermostat from the Control palette and connect the AirTemp link on the “Zone” box to the measure link on the thermostat as shown below. Click and hold to draw the connection. Similarly, connect the out signal link of the Thermostat to the macro output “Shading signal”.

To achieve the right performance of the thermostat we have to give a dead band with sign; in this case a negative sign to get out signal 1 for high measure signals and 0 for low. Finally we insert a Constant field from the Utility palette and connect it to the setpoint link of the thermostat. With a given setpoint value (22°C in the figure below) the control system is defined. After the simulation the effect of the shade control can be checked in the diagram “Heat balance”.

For the AHU systems, the presentations for the standard and advanced user interface levels are the same (for the distinction between standard and advanced levels). However, in the case of the zone models, the presentations are completely different. This section explains the principal features of the zone models, and also provides a brief overview of the advanced level in this context. Actually, most users will never have to deal with the advanced level, but it is useful to know it exists, and the physics are more easily explained from the point of view of the advanced level.

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