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interior air walls for splitting nonconvex zones

asked 2020-01-29 07:36:56 -0500

updated 2020-03-10 08:58:58 -0500

In real world there are plenty of cases where zones are nonconvex polygons.

It is a well known issue that energyplus cannot handle this, and it's up to the engineers designing simulation to do an alternative.

One of the best alternatives to circumvent this shortcoming is for example to split the zone into two or more convex zones, using as a separator a thin wall of air that lets all/almost all heat exchanges to occur between zones.

Following of course Honestly, it's knowledgeable that EnergyPlus is a complex simulation engine and documenting such into a couple of PDFs is no easy task, although there isn't easy information to be found regarding the issue of air walls, and have used this topic to check them out so, thanks.

Only question i have is about these materials you are using in Open Studio:

  Air Wall Material, !- Name
  MediumSmooth, !- Roughness
  0.01, !- Thickness {m}
  0.6, !- Conductivity {W/m-K}
  800, !- Density {kg/m3}
  1000, !- Specific Heat {J/kg-K}
  0.95, !- Thermal Absorptance
  0.7, !- Solar Absorptance
  0.7; !- Visible Absorptance

So the basic idea here is using this Air Interior Wall surfaces to split non-convex L buildings/zones, or polygons with holes (aka interior rings) that create nonconvex shapes.

How? Adding an extra interior wall to L corners (this is done using computational geometry) and or ring corners, and then assigning them air wall material for the zone interface surfaces (one for each contiguous zone).

The issue with this material which would be supposed to mimick somehow the properties of airflow that seem not matching and if you can explain would be welcome are the following properties:

Roughness: shouldn't it be smooth (it's the most possible resembling to air layer or surface)
Thickness: shouldn't it be minimal (in documentation they do not recommend lower than 0,003 so 0,001 for example) Solar and Visible absorptance shouldnt it be lower to match the material properties accordingly.

EDIT (10/03/2020):
In a response to a question in comments, due to its potential interest in more people checking this topic here is added further information.
Convex zone zone having a floor polygon projection (footprint) where each vertex connects to all others through a line that is always inside the polygon (i use this definition because i work with computational geometry and actually this is one way to compute if it is convex with algorithms, there are other definitions, like for example all external angles must be above 180º)

Non-Convex Zone opposite of convex (an L-Shape is one of the most common building non convex zones).
Why does this matter?
EnergyPlus engine part that treats beam solar radiation and reflectances from exterior surfaces that strike the building and, ultimately, enter the zone is not prepared to handle non-convex geometries because in these cases it would require further interactions that where not considered, specifically due to Interior. This engine part is defined in ... (more)

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@EngFranc What exactly is energyplus unable to handle in nonconvex zones? I know there are some problems with simulating lighting from fenestration, but I've never gotten a clear answer as to what all the problems are. Could you please add that to the post?

mldichter's avatar mldichter  ( 2020-02-28 12:07:01 -0500 )edit

@mldichter ok i will edit it and since you asked i will also add a solution i am adopting to deal with these in a good fashion.

EngFranc's avatar EngFranc  ( 2020-03-10 08:33:49 -0500 )edit

2 Answers

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answered 2020-01-29 09:41:15 -0500

If you are using the latest EnergyPlus version 9.2, there is a new Construction:AirBoundary object that was added for this case of an "air wall" separating two zones from each other. Inputs here can specify how EnergyPlus calculates solar gains, inter-zone mixing airflow, and radiant heat exchange between the two zones across the surface using this air boundary construction.

This is easier and faster than the approach you've done in OpenStudio. The "Air Wall Material" you've added is an opaque Material object -- so no light will pass through unless you draw a window into that surface. You will also need to add Zone Cross Mixing object(s) to account for the inter-zone mixing airflow.

You may be interested in also following this post related to multiple surfaces using this new air boundary construction separating the same zones from each other.

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Hello Aaron, thanks for very nice feedback. Can you post for users of energyPlys v9.1 what are the best suited surface, construction and material objects (if possible in plain IDF formats), according to your experience to achieve what i explained accordingly. Like if one toggles all absorptances to 0 we do get a neutral surface where the only issue is it blocks mass but it can sort the nonconvexities, is it what you are suggesting if you know and feedback its thanks in advance.

EngFranc's avatar EngFranc  ( 2020-01-30 08:51:11 -0500 )edit

If you're using OpenStudio, you can use the Air Wall Zone Mixing measure to automatically apply mixing airflow across the air wall surface in both directions. The measure will also alter the surface to become adiabatic. In addition to this post and this post, there are many others on Unmet Hours you can find on this topic by searching for "air wall".

Aaron Boranian's avatar Aaron Boranian  ( 2020-01-30 09:24:57 -0500 )edit

If you download example.osm, open it in open studio, then export the idf. You will see that it is exactly the same, still something is wrong and my assumption to hack the values remains (check my reply) unless someone adds new information.

EngFranc's avatar EngFranc  ( 2020-01-30 11:43:40 -0500 )edit

answered 2020-01-30 11:41:29 -0500

updated 2020-01-30 11:42:13 -0500

Ok i have re-engineered the example.osm of Air Wall Zone Mixing Building Component Library, by opening it in OS and export the IDF. Furthermore opening and checking IDF code with the needed code, for more purist energy modelers who rely on IDFs scripts.
Again, the same issue, if you export that model.osm to idf you will see the specified interzone surface is the same as above.
So the takeaway is that something is wrong with that solution, because it will generate an opaque surface. If it not can you explain only with IDF code what classes are missing, if no reply i'm sure my assumption is ok and will hack those values which seem opposite to what a neutral interzone surface should be.
The interzone surface:

  Surface 2,                              !- Name
  Wall,                                   !- Surface Type
  Air Wall,                               !- Construction Name
  Thermal Zone: Space 101,                !- Zone Name
  Surface,                                !- Outside Boundary Condition
  Surface 21,                             !- Outside Boundary Condition Object
  NoSun,                                  !- Sun Exposure
  NoWind,                                 !- Wind Exposure
  ,                                       !- View Factor to Ground
  ,                                       !- Number of Vertices
  24.384, 0, 3.048,                       !- X,Y,Z Vertex 1 {m}
  24.384, 0, 0,                           !- X,Y,Z Vertex 2 {m}
  0, 0, 0,                                !- X,Y,Z Vertex 3 {m}
  0, 0, 3.048;                            !- X,Y,Z Vertex 4 {m}

the construction:

  Air Wall,                               !- Name
  Air Wall Material;                      !- Layer 1

the material:

  Air Wall Material,                      !- Name
  MediumSmooth,                           !- Roughness
  0.01,                                   !- Thickness {m}
  0.6,                                    !- Conductivity {W/m-K}
  800,                                    !- Density {kg/m3}
  1000,                                   !- Specific Heat {J/kg-K}
  0.95,                                   !- Thermal Absorptance
  0.7,                                    !- Solar Absorptance
  0.7;                                    !- Visible Absorptance
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Asked: 2020-01-29 07:36:56 -0500

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Last updated: Mar 10 '20