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Double skin facade providing outside air to AHUs

asked 2016-04-26 10:53:41 -0500

updated 2017-05-05 09:23:22 -0500

I've got a building where there's a double skin facade (actually two, one west, one east), spanning 2 stories. The double skin is open to the outside, but not the inside.

In the winter the outside air is drawn in the cavity by dampers located at the bottom. In the summer the dampers are closed and the air is drawn through direct evaporative coolers.

Each zone is served by a water source heat pump which takes its outside air from the double skin cavities.

How would you go about modeling this in EnergyPlus? I cannot seem to figure out a way to have the outside air of the AHU being drawn from the cavity.

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answered 2016-04-26 11:24:51 -0500

Archmage's avatar

I don't think EnergyPlus has a direct way to do this.

If you have a side calculation to determine the facade-heated-outdoor air's drybulb, you could assign it to the OA inlet node using EMS actuator for Outoor Air System Node Conditions.

It might be considered thermodynamically similiar to use the return air feature in WindowProperty:AirflowControl. The heat gets added to the return air instead of the outdoor air, but only the fraction of heat gained that goes out the relief air is lost from the unit.

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Not a bad idea for the WindowProperty:AirflowControl, but I see two problems in my case:

  • This is a lab. All the OA air is supplied through the AHUs, but a bunch of it gets exhausted directly from the fume hoods and don't go through the AHU and its built-in ERV. I'm afraid I'd be over estimated the gain
  • My cavity is 2.5 m wide between the two skins, I kinda feel like WindowProperty:AirflowContro was meant for much smaller gaps.
Julien Marrec's avatar Julien Marrec  ( 2016-04-26 12:02:04 -0500 )edit

I guess I could simulate the cavity itself with unconditioned air being introduced and taken from it, get the resulting zone temperature, and feed that back in EMS. Whether that'll be any accurate is another question :)

Julien Marrec's avatar Julien Marrec  ( 2016-04-26 12:12:57 -0500 )edit

Yeah, I agree that wide of DF cavity should probably be it's own thermal zone. I think your suggestion would work well enough. It is pretty easy EMS to map the conditions at one node to an OA inlet.

Archmage's avatar Archmage  ( 2016-04-26 12:22:32 -0500 )edit

I didn't even think about actually coupling the zone node and the outside air node, thanks (was gonna do two sims and create an 8760 schedule)! I guess to take it further I could try to aggregate the outside air requirement (mass flow rate I guess) from the AHUs and use that as airflow in the cavity, maybe feeding it into a ZoneInfiltration:DesignFlowRate so that I don't double count fan power. Think that's possible @Archmage?

Julien Marrec's avatar Julien Marrec  ( 2016-04-26 12:55:47 -0500 )edit
Archmage's avatar Archmage  ( 2016-04-26 12:57:38 -0500 )edit

answered 2016-04-27 07:52:57 -0500

BL's avatar

updated 2016-04-27 08:25:07 -0500

I agreed with @lutinfr answer. In IES, you created a separate zone to represent the air cavity. In the interior wall, you can set the interior wall as glazing material or whatever the actual material is. The air flow rate though the cavity can be specified as OA ventilation etc. Or you can create zero energy use fan or something like a dummy mechanical component to control the air cavity zone. Please see attached image.

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answered 2016-04-27 03:16:59 -0500

lutinfr's avatar

I have not experienced this case in E+, I'm IESVE user. But I think the concept for getting this done is the same. As usual with double skin facade, I model cavity zones as thermal zones, separated them as individual thermal zone at each floor, cavities are connected vertically. In case I realize this for study, I separate them to 2 or even 4 at each floor to see roughly how the temperature gradient inside the cavity. Then using air net work on IESVE to connect them all and connect to AHU. Hope this can help.

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Asked: 2016-04-26 10:53:41 -0500

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Last updated: Apr 27 '16