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How can I model a VFD in EnergyPlus

Assume I have a working E+ building model with a constant-speed fan or pump. What's the best way to model the addition of a VFD to this building model? Do I just need to change the fan or pump object to variable speed? Or is there more to it than that? I'm interested in seeing the energy savings from adding a VFD.

What controllers might need to be changed? How is the process different for fans and pumps?

Thanks in advance!

RyanStochastic's avatar
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RyanStochastic
asked 2014-10-12 16:58:04 -0500
__AmirRoth__'s avatar
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__AmirRoth__
updated 2017-05-17 12:43:22 -0500
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@RyanStochastic - I would suggest appropriately tagging your question (fans, pumps, vfds, energyplus, etc) and clarifying what you're looking for. Is this an input question, i.e. how to change your model from constant speed to variable, or is it an output question where you're interested in the effects of the change?

MatthewSteen's avatar MatthewSteen (2014-10-12 17:46:40 -0500) edit
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@MattSteen, thanks for the suggestions. I've edited the question accordingly. It's both an input and output question, i'm interested in HOW to simulate the system with and without a VFD, and the results of the simulations for each case.

RyanStochastic's avatar RyanStochastic (2014-10-12 22:02:23 -0500) edit
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For pumps, it is usually enough to just replace the pump with a variable speed. You might check if you are getting flow in bypass branches and if there is flow it may be desirable to remove the bypasses.

For air systems, you have more to do than just replace the fan. The air system generally needs to be changed from a constant-volume-variable-temperature control approach to a constant-temperature-variable-flow approach. The zone's air terminal unit will need to be changed to one of the VAV types. The supply air deck temperature needs to be controlled with a setpoint manager.

Archmage's avatar
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Archmage
answered 2014-10-13 09:23:28 -0500
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It can be as simple as adding the correct objects, but it is also important to select appropriate coefficients for the performance curve that these objects use. Ideally, user will have actual curves provided by field measurements, but manufacturer info or just "typical" curves could be ok as well, depending on the application. There is a paper titled "How to Avoid Overestimating Variable Speed Drive Savings", by J. B. Maxwell that can be found with a bit of Googling, which describes some typical curves.

Matt Doiron's avatar Matt Doiron (2014-10-13 17:06:31 -0500) edit
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