Nick,
I am curious - and you might need to consider - whether absorbtance is
really the same thing as absorbtivity (and check my spelling using b instead
of p - not sure there, either). Similarly, are emission and emissivity
truly synonymous? I'm pretty sure that Mr. K's law refers to behavior in a
situation, whereas eQuest is asking for material properties.
Is anyone else clearer on this than I am?
Lars Fetzek, EI
Phoenix Engineering Group
Tampa, Florida
_____
From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Monday, September 28, 2009 6:49 PM
To: Ed Garcia
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Cool Roofs
Hi Ed,
I would account for emissivity by specifying the absorptance property of the
surface in the detailed edit mode (spreadsheet view is fastest way to edit
multiple roof constructions). For the purposes of defining roof
constructions to figure out their thermal behavior*, Emissivity =
Absorptance.
The solar reflective index (SRI) is another way of defining the same thing,
based on a different scale between some grade of white and black paint
rather than absolutes (zero-to-1.0). There are probably charts out there
that could translate SRI into absorptance/emissivity if you have only the
SRI to go off of.
Right-click in eQuest one of the cells for entering your construction
absorptance, and you'll be led to a DOE help file that has a handy reference
chart of absorptances of various exterior finishes.
-----
* Since I know we're a group of learners, I figured you and others might
be interested to hear (correct?) my logic behind "emissivity = absorptance"
for modeling roof constructions in eQuest and other energy modeling
ventures...
First, let me relate my understanding of emissivity through glass: Light
energy hitting glass does one of three things - (A) reflect off the surface,
(B) transmit through the glass, (C) absorb into the glass as heat energy.
Absorbed light energy (option C) is simultaneously radiated (aka "emitted"
ala "emissivity) from both sides of the glass (into and outside of your
building). "Low-e" glass is glass that has a low emissivity, and thus
doesn't get really hot and heat up your building via heat radiation as much
in the summer when hit by daylight.
Where emissivity is a fraction of radiation emitted from a surface,
absorptance is a measure of the fraction of radiation absorbed. One is
energy out, the other is energy in.
To ground my explanation a bit, I googled up something from my
thermodynamics courses that had gone a bit fuzzy:
Kirchoff
<http://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation> 's
law: "At thermal equilibrium, the emissivity
<http://en.wikipedia.org/wiki/Emissivity> of a body (or surface) equals its
absorptivity <http://en.wikipedia.org/wiki/Absorptivity> ."
If we assume, for the purposes of thermal energy modeling, that within each
modeled hour the roof construction is in thermal equilibrium (not in the
process of heating up or cooling down), we can say "radiation in = radiation
out," and apply Mr. K's law above, concluding Emissivity = Absorptance.
The potential inaccuracy of this assumption lies in the reality of how a
"cold" roof, for example, in the morning hours will absorb more heat into
its construction than it emits, until it reaches a thermal equilibrium point
later in the day. In the evening hours, the reverse will be true and the
roof will radiate heat at a higher fraction than its absorptivity.
That said, I am semi-confident that eQuest and other programs using a
"construction by layers" methodology will compensate for such an inaccuracy
by approximating the actual absorption/radiation effects of thermal massing
over time. If I'm wrong on that point, I will have to lick my wounds and
hope someone on this list can set me straight before I drive this eQuest
train into a wall!
In other words, I'd love to be corrected if it's in order =). Otherwise,
hope this helps!
~Nick
cid:489575314 at 22072009-0ABB
NICK CATON, E.I.T.
PROJECT ENGINEER
25501 west valley parkway
olathe ks 66061
direct 913 344.0036
fax 913 345.0617
Check out our new web-site @ www.smithboucher.com
From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Ed Garcia
Sent: Monday, September 21, 2009 9:44 AM
To: Subject: [Equest-users] Cool Roofs
I a new user to eQuest and need to model cool roofs. I see the user
specified roof properties, but I do not see how to add the SRI or emissivity
of the roof. Would appreciate some guidance.
Regards,
Ed
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