The idf format of OpenModelica is not related to the idf of EnergyPlus. However, there are a few ways to couple EnergyPlus with Modelica.
- You could export EnergyPlus (in your case I believe the building model) as an FMU for co-simulation and import in a Modelica tool. See http://simulationresearch.lbl.gov/fmu/EnergyPlus/export/index.html
- You could export the Modelica model as an FMU for co-simulation, and import it into EnergyPlus. See http://apps1.eere.energy.gov/buildings/energyplus/pdfs/externalinterfaces_application_guide.pdf
- You could link Modelica and EnergyPlus to the Building Controls Virtual Test Bed. For EnergyPlus, you could use an FMU (see 1) or its ExternalInterface with the Simulator actor of the BCVTB, see http://simulationresearch.lbl.gov/bcvtb/releases/latest/doc/manual/tit-EnePluCon.xhtml. For Modelica, you could again use an FMU or the Modelica BCVTB interface, see http://simulationresearch.lbl.gov/modelica/releases/latest/help/Buildings_Utilities_IO_BCVTB.html
Note that the FMU support of OpenModelica still has a few gaps, as does its coverage of the Modelica language. Hence, the above tool chain has been tested with Dymola but not extensively with OpenModelica yet.
