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How to model PAU connected to multiple AHUs

asked 2022-01-26 04:05:09 -0500

Keigo's avatar

I want to model HVAC system: PAU (primary air handling unit) + AHU as shown below. One PAU is connected to multiple AHUs. The PAU supplies outdoor air to AHUs.

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Can it be modelled in EnergyPlus in the first place? If yes, how can it be modelled? If no, is there any other software that can model it?

I tried some tricky modelling as follows, but in vein.

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For Failed model 1, I tried to connect AirLoopHVAC:ZoneSplitter of PAU Airloop to OutdoorAir:Node of AHU1 AirLoop and AHU2 AirLoop, but I got severe error saying "Node OUTSIDE AIR INLET connects to no component" and "An outlet node in AirLoopHVAC=PAU is not connected to any zone".

For Failed model 2, I added zone 21 and zone 22 and used AirLoopHVAC:SupplyPlenum and AirLoopHVAC:ReturnPlenum instead of AirLoopHVAC:ZoneSplitter and AirLoopHVAC:ZoneMixer, but I got the same severe errors.

OutdoorAir:Mixer is essensital for AirLoopHVAC and cannot be deleted.

I don't know how to connect PAU supply air duct to AHU1 and AHU2.

Currently, I have to compromise and change the Air-side Schematic as shown below.

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For Compromise model 1, one large PAU is divided into two small PAUs.

For Compromise model 2, There is only one PAU, but it supplies outdoor air to each zone directly.

In terms of energy consumption, these HVAC systems might consume almost the same energy as the original system (PAU connected to AHUs), but I want to model the original system and compare with Compromise model 1 and Compromise model 2 in order to confirm it.

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answered 2022-01-26 10:10:48 -0500

This looks like a good use for the AirLoopHVAC:DedicatedOutdoorAirSystem object, which represents a central dedicated outdoor air system (DOAS) and delivers outdoor air to multiple AirLoopHVAC (central air) systems. You can specify the number and names of these AirLoopHVAC systems and use an AirLoopHVAC:Mixer object and AirLoopHVAC:Splitter object to define how air moves between the central DOAS and AirLoopHVAC systems. You need to reference these mixer/splitter objects in the central DOAS object, which isn't shown as input field entries in the Input Output Reference but it is visible in the example object (copied below, input fields 4-5).

AirLoopHVAC:DedicatedOutdoorAirSystem,
  AirLoopHVAC DOAS,        !- Name
  AirLoopDOAS OA system,   !- AirLoopHVAC:OutdoorAirSystem Name
  ALWAYS_ON,               !- Availability Schedule Name
  AirLoopDOASMixer,        !- AirLoopHVAC:Mixer Name
  AirLoopDOASSplitter,     !- AirLoopHVAC:Splitter Name
  4.5,                     !- Preheat Design Temperature {C}
  0.004,                   !- Preheat Design Humidity Ratio {kgWater/kgDryAir}
  17.5,                    !- Precool Design Temperature {C}
  0.012,                   !- Precool Design Humidity Ratio {kgWater/kgDryAir}
  5,                       !- Number of AirLoopHVAC
  PSZ-AC:1,                !- AirLoopHVAC 1 Name
  PSZ-AC:2,                !- AirLoopHVAC 2 Name
  PSZ-AC:3,                !- AirLoopHVAC 3 Name
  PSZ-AC:4,                !- AirLoopHVAC 4 Name
  PSZ-AC:5;                !- AirLoopHVAC 5 Name
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Thank you for your answer! Embarrassing... I have used E+ v8.9.0 due to compatibility issues with our company's modelling software and I haven't noticed AirLoopHVAC:DedicatedOutdoorAirSystem. I'll use it.

Keigo's avatar Keigo  ( 2022-01-26 11:25:35 -0500 )edit

No problem! It looks like this object was added to EnergyPlus in v9.2, in case you didn't want to update all the way to v9.6.

Aaron Boranian's avatar Aaron Boranian  ( 2022-01-26 15:25:16 -0500 )edit

We can still select only DirectIntoZone for Air Outlet Type in HVACTemplate:System:DedicatedOutdoorAir. So, for now, HVACTemplate has no feature to use AirLoopHVAC:DedicatedOutdoorAirSystem, right?

Keigo's avatar Keigo  ( 2022-01-26 19:52:53 -0500 )edit

One more question. I'm looking into example file SmallOffice_CentralDOAS.idf. This example file has SetpointMnager:Scheduled objects on Cooling Coil Air Outet Node and Heating Coil Air Outled Node in order to set DOAS supply air temperature. I think SetpointMnager:Scheduled objects are necessary to control supply air temperature. But then, for what purpose are Preheat Design Temperature, Preheat Design Humidity Ratio, Precool Design Temperature and Precool Design Humidity Ratio in AirLoopHVAC:DedicatedOutdoorAirSystem used? For autosizing?

Keigo's avatar Keigo  ( 2022-01-27 02:01:39 -0500 )edit

I used EnergyPlus v9.2.0 and successfully modelled AirLoopHVAC:DedicatedOutdoorAirSystem although it took me hours to notice this bug. The biggest lesson for me this time is that I should always use the latest version.

Keigo's avatar Keigo  ( 2022-01-27 07:03:23 -0500 )edit

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Asked: 2022-01-26 04:05:09 -0500

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Last updated: Mar 15