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Approximating DOAS to AHUs in EnergyPlus

asked 2016-10-11 09:07:41 -0500

updated 2018-09-18 17:44:35 -0500

It's not currently possible to model this configuration explicitly in EnergyPlus, although it is in design. What is a reasonable approximation/simplification for this system?

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answered 2016-10-11 17:02:34 -0500

mleach gravatar image

updated 2016-10-11 17:03:05 -0500

It depends on the system configuration but I have done this a few different ways, none of which are perfect. These are the steps I generally take:

  1. Represent DOAS heating and cooling using components on the outdoor air path of each AHU. Say that the DOAS provides air at 70 F. You can put a heating coil and a cooling coil on the AHU outdoor air path (before the OA mixer) and apply scheduled setpoint managers to the outlet node of each coil, each set to 70 F. In this same way, you could apply ERV, evap cooling, etc.
  2. The tricky part is correctly capturing the fan energy. I have tried a few ways to do it and can think of at least one other, but none of them are perfect. I have added the DOAS fan power as a process load but then you don't capture its fan heat at all, and this approach really only works well for a CAV DOAS. For a VAV DOAS I have represented the DOAS fan by applying a relief fan to each AHU (such that the flow through the fan is always equal to the OA intake, as long as you don't have exhaust fans affecting the flow balance, even if you have DCV applied). Again, however, fan heat is not captured. I haven't tried this, but you could also increase the W/cfm of the AHU supply fans to represent the fan heat added by the DOAS, but the exact amount of heat getting added will vary with a variable speed supply fan, and the heat is really getting added to the wrong place (after the OA mixer, as opposed to before). So, the bottom line is you probably can't capture the DOAS fan heat correctly no matter what you do, unless there's some other way I haven't thought of.
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Thanks @mleach. In exploring this option I found that OS limits the number of fans on an air loop to two. https://github.com/NREL/OpenStudio/is...

MatthewSteen gravatar imageMatthewSteen ( 2016-10-14 09:34:11 -0500 )edit
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I ended up using IES VE for this particular configuration. However, the OpenStudio limit to the number of fans on an air loop was removed by this merged pull request.

MatthewSteen gravatar imageMatthewSteen ( 2017-04-30 20:39:54 -0500 )edit

@MatthewSteen, Thanks for guiding me here. I am also trying to make this configuration in IES. However, I have a lot of problem with room load and system load sizing in IES 2016. every time that I bring a system from template library and I run the system load sizing, some of the parameters are still zero. and I end up with 4000+ unmet load hours. Any suggestion about this will help a lot. maybe I am missing something in IES.

NasimMirian gravatar imageNasimMirian ( 2017-05-01 08:48:16 -0500 )edit

@mleach for the 2nd step you mentioned you used the fan power as a process load. How is that done in OpenStudio else EnergyPlus? How is that done for VAV DOAS? Can you share screenshots or the IDF file for others?

rkbest gravatar imagerkbest ( 2018-03-19 16:08:35 -0500 )edit

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Asked: 2016-10-11 09:07:41 -0500

Seen: 1,337 times

Last updated: Sep 18 '18