Learn the EnergyPlus Data Format (Part 3)
Last time, we mentioned that there are many other Object-level comments, and Field-level comments and the list can be found on the top of the IDD comments sections (those lines starts with a !). Most of these comments are self-explanatory for instance the "\minimum" for a numeric number. Here I take out some not-so-intuitive but important comments and try to make them clearly by examples:
Whenever you see this comment appears in the Object-Level comments, you know this EnergyPlus object is dynamically extensible. The explanations for this comment in IDD is for software developers, what it means for energy modelers is:
"Objects that is dynamically extensible does not have limitations on the number of fields it has in the IDD file." That is typically seen in objects like the "BranchList", which by default has 500 fields. In a situation such as a large building, all the reheat coils connect to a hot water loop through the "BranchList" object; this may cause the number of object inputs exceed the 500 field limitation. Having this comment means it is safe to put more than 500 fields (but must be correct!) to the "BranchList" object.
Additionally, you should pay attention to the <#>. The "#" refers the number of fields that should be extended as a group. If this is 2, then it means two following fields have to be extended as a group, and their order must be preserved. For example, in "Branch" object, the size of the extensible field group is 2, and they must be in the order of "Object Type" then "Component Name."
Most of the field-level comments are very easy to understand. So I will focus on the comments that may not be so intuitive to most of the energy modelers.
\object-list and \reference
The reason I put these two comments together is that they are so connected with each other. The relationship can be described in the image below:
A field that has the \object-list comment means its input is linked to the name of an object elsewhere in the same model. The linkage is established by some anonymous keys (e.g. zoneNames, materialNames) identified in the IDD file. For example, the "Zone or ZoneList Name" field under the Peopleobject has an \object-list comment point to a ZoneAndZoneListNames key. This ZoneAndZoneListNames key link to object Zone and object ZoneList. This means the field "Zone or ZoneList Name" can only have a list of object names from the identified Zone and ZoneList objects.
The \external-list comment often appears in the output objects (e.g. Output:Variable), which point to a specific output file in the simulation folder. The \external-list comment explains that the field should reference to the RDD or MDD output file through keys:
autoRDDvariable: point to the .rdd file.
autoRDDmeter: point to the .mdd file.
autoRDDvariableMeter: point to the .rdd file and .mdd file.
Objects such as the Output:Variable needs these output files been generated, otherwise, they won't be able to pull out a full list of available output variables or meters.
So the easiest way is to browse through available variables for outputting is to run a simulation. Alternatively, if you are very familiar with the output variable names, you can also enter these variable names in the Output:Variable object. Or find a .rdd or a .mdd file from the previous simulation, and paste these files into the same folder.