Dear all
We are trying to get a reasonable estimate for SHGC in the field.
Our project is a retrofit of a relatively recent build that has overheating
problems. No records exist of the original glazing spec.
The glazing has a slight green tint; i.e. the "look and feel" of a fairly
modern spectrally selective product with g-value (sorry SHGC) of about 0.4.
However, a team member has now produced some records indicating this is
significantly higher (more like 0.6).
Does anybody have any experience or know of any research relating to this?
*I was thinking of something like this, taking measurements on the
horizontal and vertical plane inside and out:
https://www.uv-light.co.uk/meters/irradiance-and-exposure/
Our saint gobain rep tells me their spectrographic equipment is 50,000 GBP
- a bit out of our price range! (They've offered to test a sample for us!).
Cheers
Chris
Hi Chris ?
That instrument won?t do it, because it is not sensitive to infrared. More than half of irradiance is in the near IR (aka solar infrared). You need a pyranometer (LiCor is our go-to, or an Eppley
Best,
Nathan
Nathan Brown, BEMP, LEED AP ~ Associate
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At LBNL, in collaboration with University Federal Santa Catarina (UFSC),
Brazil, we have designed partable window energy meter that measures SHGC in
the filed. The instrument is not yet commercialized, but two companies are
exploring commercialization (D&S and EPDT). In a mean time, UFSC can
manufacture unit for you. I can supply you with contact information if you
are interested. Also, attached link to US DOE newsletter about the
instrument:
https://energy.gov/eere/articles/us-brazil-collaboration-leads-innovative-device-reduces-energy-use-buildings
.
Charlie
Hi Nathan
Does the li-cor measure short enough wavelengths? I thought a lot of the uv
was < 400nm. I guess that most "selective coatings" have quite a big
crossover to the visible range in their reflectivity - hence the slight
green or blue hues. So, I can imagine that missing some of the uv-b isn't
too big a deal.
Both responses are super interesting. Thank you very much!
Chris
Correction > 400 nm misses uv-a as well.
Hi Chris ?
The LiCor manual shows that the pyranometer is sensitive to wavelengths between 390 nm to 1100 nm. That means the LiCor misses most of the UV (and longer near IR). So the LiCOr is not perfect by any means, but it?s pretty good, given the coverage of the visible and shorter end of the near IR, which represent most of the energy in solar heat gain. So it?s a matter of how precise you need to be - LiCor gets you most of the way there, but you may need something more robust depending on the application. My instinct says that it?s good enough for telling the difference between 0.4 and 0.6 SHGC.
Best,
Nathan
(from https://heatisland.lbl.gov/sites/all/files/Solar%20Energy%20Spectrum.jpg)
Nathan Brown, BEMP, LEED AP ~ Associate
LOISOS + UBBELOHDE
- - - - - - - - - - - - - - - - - - - - - - - - - - -
1917 Clement Avenue, Building 10A
Alameda,CA 94501 USA
- - - - - - - - - - - - - - - - - - - - - - - - - - -
510 521 3800 VOICE
510 521 3820 FAX
- - - - - - - - - - - - - - - - - - - - - - - - - - -
www.coolshadow.com
Hi Chris ?
A bit more reading shows that the LiCor Pyranometer is not designed for SHGC: the manual says the following:
?The spectral response of the Li-200 does not include the entire solar spectrum, so it must be used in the same lighting conditions as those under which it was calibrated. Therefore, the LI-200 should only be used to measure unobstructed daylight. It should not be used under vegetation, artificial lights, in a greenhouse, or for reflected solar radiation?
The solar radiation getting through the glass is obstructed, so the LiCor should not be used for measuring SHGC and I?d probably look elsewhere if the measurement is important enough to verify in the field.
Best,
Nathan
Nathan Brown, BEMP, LEED AP ~ Associate
LOISOS + UBBELOHDE
- - - - - - - - - - - - - - - - - - - - - - - - - - -
1917 Clement Avenue, Building 10A
Alameda,CA 94501 USA
- - - - - - - - - - - - - - - - - - - - - - - - - - -
510 521 3800 VOICE
510 521 3820 FAX
- - - - - - - - - - - - - - - - - - - - - - - - - - -
www.coolshadow.com
Please note that SHGC is not equal to solar transmittance. Even if you
would be able to make perfect Tsol measurement in the filed, that could be
way off in predicting SHGC. This is because, SHGC consists of two parts,
one is Tsol (directly transmitted solar radiation and the other is inward
flowing fraction of absorbed solar energy in the window. For the most part
inward flowing fraction is NOT small and it depends on the glazing and
framing configuration. In its extreme, single glazed tinted glass would
have SHGC more than twice than Tsol and often triple Tsol.
Charlie
Hi Charlie ?
Thank you for correcting my oversight about Tsol vs SHGC! They are not the same, and I was forgetting about the inward flowing fraction of absorbed energy component of SHGC.
Best,
Nathan
Nathan Brown, BEMP, LEED AP ~ Associate
LOISOS + UBBELOHDE
- - - - - - - - - - - - - - - - - - - - - - - - - - -
1917 Clement Avenue, Building 10A
Alameda,CA 94501 USA
- - - - - - - - - - - - - - - - - - - - - - - - - - -
510 521 3800 VOICE
510 521 3820 FAX
- - - - - - - - - - - - - - - - - - - - - - - - - - -
www.coolshadow.com
We found this issue with interstitial venetian blinds - super high internal
surface temperatures.
Thanks Charlie. I'd actually misread Nathan's explanation of ir as
retransmitted whereas what I think Nathan were describing is the ir
component of the transmitted. Under en 410 this Tsol is an integration of
wavelengths from 380 to 1100nm to the best of my memory.