[Bldg-sim] Internal mass modeling guidance in E+?

Doug Hittle hittle.doug at gmail.com
Mon Jan 14 18:58:03 PST 2013


I believe that E+ and predecessors take/took what ever was described
as "internal mass" and parsed it into an equivalent "partition" of
some "equivalent"  area. This can lead to some counter intuitive
results. Most might expect that "internal" or "thermal" mass is going
to moderate energy transfer ("store"). But suppose I describe a
banquet hall full of folding mdf or plywood tables (light weight).
Compared to an empty room, the area available to absorb radiation and
convect it to the room is increased by easily 1/3. All of a sudden the
room is "fast" and energy gains are translated into cooling loads much
more quickly.

So let's agree with Joe that your question can be answered by "walking
around." In looking around my office I find that one partition is
covered with book cases and that the other side of the partition is
also covered with books. If I thought this was an important feature
that needed to be modeled (which I don't and which isn't) I would not
describe any internal equivalent mass, I would simply describe a
partition made up of books-air space-gyp board wall-air space-books.

But why do I think this is irrelevant additional work? I can weigh
some books to determine their density. We think books are heavy but
imagine putting an equivalent volume of concrete in your backpack.
Then they are not so "thermally massey". Also the books never receive
direct sunlight and the temperature in my office varies little because
it is in my heavily insulated, air tight home. Since the guzinta
equals the guzouta over time and since the guzinta and guzouta, even
for this massive double book lined partition, are near zero because
the materials are at constant temperature, more or less, the mass of
the books is not very important to the room energy balance.

I don't know what thermal mass is anyway. The term must have evolved
partly from passive solar designs, where energy storing materials
exposed to the sun are important,  and from the brick trade
association who left a confusing legacy of claims that a masonry house
had a higher R-value because of the "thermal mass." Mind you I think
bricks are great, I have some myself. However, there is nothing in the
definition of R-value that is labeled "brick."

My apologies Dr. Wang. I expect that what I have just said is more or
less useless as an answer to you questions. However, a productive
guide to a first approach might be found in:

Consider a Spherical Cow: A Course in Environmental Problem Solving, John Harte

It is easier to bound a problem than to defend anybody's guess at the
mean. The correct answer always seems to be "It Depends."

Doug Hittle




On Mon, Jan 14, 2013 at 1:16 PM, Qinpeng Wang <qpwang at gatech.edu> wrote:
>
> Dear colleagues,
>
> I'm looking into furniture/internal mass modeling in energy models particularly within the environment of EnergyPlus but also not limited to that.
>
> I have two questions I guess:
>
> 1, Are there any modeling guide or reference building report that may give recommendations/instructions on how to represent furniture/internal mass inside a building if there is no much detailed information? I guess all I'm asking is are there any published data-set, or default values in terms of material property and area to represent furniture like what could be easily found about schedules and occupancy density?
>
> COMNET modeling guide has only one paragraph about it and it mentioned "The interior thermal mass and modeling assumptions in the baseline building shall be the same as the proposed design."
> Currently what I have is from Building America House Simulation Protocols: "The internal mass of furniture and contents shall be equal to 8 lb/ft2 of conditioned floor space. For solar distribution purposes, lightweight furniture covering 40% of the floor area shall be assumed."
>
> 2, DOE reference commercial buildings have internal mass objects in the idf. file, where does the information come from?
>
> Appreciate your input!
>
> Best,
>
> Qinpeng
>
> --
>
> Qinpeng Wang, PhD Student
>
> College of Architecture
>
> Georgia Institute of Technology
>
> Atlanta, GA 30332-0155
>
>
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