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

[Justin Spencer]

There's also an issue here about whether or not there's any modeling of
internal partitions or fixtures included in your model. My personal
experience in modeling residential models is that they always lag more than
you expect, so I've regularly responded by jacking up the internal mass. I
certainly can fit metered HVAC data a lot better when I add mass beyond
what the BABM suggests. I've guessed that's because my partitions don't
quite match the amount of mass we're interacting with.

I get what Doug is saying that there isn't much temperature change
occurring in his bookshelf, but when you have literally several tons of
exposed mass, even a degree or two of temperature change is quite a bit of
heat. Those granite counters currently in fashion do something for the
building mass.

What's funny about this is that the conventional rule of thumb is that
residential construction is "light" and has little impact from mass, while
commercial buildings are "heavy" and have lots of mass impacts. My personal
experience in studying this is that mass is extremely important in
residential construction because they "float" a lot of the time and also
have higher solar gains than commercial spaces.

One possible area of fruitful research I've thought about is to try and get
some utility direct load control data and look at the "response time" of
residences in terms of how long it took to have the temperature rise a
certain amount when the HVAC system was constrained. You'd at least get
some good data about how responsive the houses are supposed to be and alter
what you can in your models to get them to better match real life (whether
or not you actually figured out the root cause of the problem in the
model).

On Mon, Jan 14, 2013 at 7:58 PM, Doug Hittle <hittle.doug at gmail.com> wrote:

> 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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