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Re: [EnergyPlus_Support] Window5 data file
Rob.
A couple of questions about how to get E+ to model spectrally selective glazing correctly:
Can I assume that the "SpectralDataFile" named in the W5 EnergyPlus report data file (last column of each Layer# line) that can be referred to in the E+ ConstructionFromWindow5DataFile will cause the simulation refer to a
WindowGlassSpectralData object with the same name? Can I also assume that if there is no object with that name that the tool defaults to SpectralAverage data?
Or...to get spectral data into E+, do I have to create a glass material and a construction to refer a glass material layer to a WindowGlassSpectralData object?
Thanks again,
r3
----- Original Message -----
From: Rob Hitchcock
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Tuesday, May 06, 2008 10:43 AM
Subject: Re: [EnergyPlus_Support] Window5 data file
David,
See my comments interspersed below.
Rob Hitchcock
=======
Results from experiments with window5 files, reversed layering for
interior windows and low-E surface location:
I was concerned with the idea of creating a reverse-layer low-E window
subsurface for an adjacent window in an interior zone, since each glass
material 'layer' actually has its own layers with different spectral
qualities (back and front in the material definition). If the same
materials are used for the reverse/rotated construction, front becomes
back and back, front (outside to inside), and the lo-E surface ends up
on the wrong side. Which zone should have the 'real' window definition?
Am I thinking about this correctly?
My guess is that a flipped-face, reverse-layer construction
(double-negative, so actually a correct order of 1,2,3,4 surfaces) for
the interior window of the outboard zone would yield the most 'correct'
results, as this is the zone where the exterior conditions (surface
temperature, diffuse solar) for the interior zone are determined (??).
ROB - Yes, this is the correct approach if I understand your
description. You want to reverse the order of the layers, and you want
the coating to be on the correct face of its layer (i.e., it has to be
flipped between the two window constructions).
If this is correct, the hypothesis would be that because the lo-E
coating is on #2 surface for the outboard zone, the Surface Inside
Temperature of the interior window of the outboard zone should be warmer
than if the layers were the 'wrong' way round. Also, there should be a
higher deltaT between the inside and outside surface temperatures in a
cooling situation.
I built two pairs of window constructions, one normal (loE on #2
surface) and one reversed (loE on #1 surface). The flipped material puts
the loE coating on the correct face for the outboard zone, but not for
the inboard zone, while the 'normal' material definition puts it on the
correct face for the inboard zone (and is ALOT more intuitive), but not
the outboard reversed-layer zone.
ROB - You have to manually input the opposing surfaces rather than use
the auto-generate capability of the UnenteredOtherZoneSurface input method.
The hypothesis above proved out for this test. I'm still a bit
uncomfortable with the results, which show about a 9% difference is
resultant cooling loads (favoring the non-intuitive flipped-reversed
construction) for a 70% glazed space inboard of a 100% glazed
double-skin space. There is about 18% difference in Zone Diffuse Solar
from Interior Windows (lower for the flipped-reversed material), which
makes some sense, as the flipped version in W5 has an SHGC about 5% higher.
ROB - Beware the Diffuse Solar report variables for Interior Windows.
As of E+ v2.1 we changed the treatment of diffuse solar to address
exactly the situation you are attempting to model. Try comparing values
for "Initial Transmitted Diffuse Solar Transmitted out through Inside of
Window Surface", described in the I/O Ref.
One question is, what is used to determine inside surface temperature of
the glass in the interior zone, and therefore convection and resultant
air heat gains? Is it the exterior temperature of the outboard zone
surface? Effective Uvalue for the flipped material construction is
significantly higher than for the normal construction, but I believe
this is not a useful value to understand the e+ calculations, as the
engine is only concerned about material temperatures and conductivities
of the layers, right?
ROB - A full window heat balance calculation is performed each time step
accounting for absorption, reflectance, and transmittance of all layers
in the window construction. See "Window Heat Balance Calculation" in
the E+ Engineering Reference.
Hope someone can clear this up.
By the way, there is a slight difference between doing the same
operation using Window5 data files instead of explicit e+ materials and
constructions. Is this simply due to the more specific optical data
provided in the W5 datafile compared to using the "Spectral Average"
optical data type?
ROB - We are working on correcting some issues related to differences
between W5 import and window constructions input through the IDF. In
general, you will always see differences between using full spectral
data and spectral average data, whether input comes from W5 or directly
through your IDF.
And lastly, what happens if a different set of materials are used for
the layers of an adjacent surface? ie, can I use a flipped-face low-E
glass for one construction, and a non-flipped glass material for the
reversed construction? No time for more experiments.
ROB - I thought this is exactly what you had done.
r3
[Non-text portions of this message have been removed]
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