LBNL spreadsheet should use the word emissivity, not
emittance. Technically emittance is related to the energy emission from the
whole body (volumetric), while emissivity is related to surface emission of
energy. In this case, we are clearly talking about surface emission.
Otherwise, there is no other difference between the two. They are both in
far IR (FIR) portion of the spectrum.
It is also important to remember that formula 1 = absorptance +
reflectance + transmittance applies to single wavelength only, but it is
commonly applied to average of the portion of the spectrum, so in the solar
portion of the spectrum (UV+VIS+NIR) this is considered as acceptable, and for
opaque surfaces absorptance = 1 – reflectance. In FIR, same formula
stands, however we are not interested in absorptance in FIR, but emissivity, however
emissivity is equal to absorptance (or better yet absorptivity). Emissivity
is commonly measured using portable emissometers, which gives emissivity
directly as a result of measurement or FIR spectrophotometers, where
transmittance and reflectance are measured and emissivity is calculated from
emissivity = 1 – reflectance.
Charlie
-------------------------------------------------------------------
D. Charlie Curcija
DesignBuilder Software
16 Bridge St.
Millers Falls, MA 01349
Tel: (413) 256-4647
Fax: (413) 256-4823
cell: (413) 575-3487
email: curcija@xxxxxxxxxxxxxxxxxxxxxxxxx
web: http://www.designbuildersoftware.com
STATEMENT OF CONFIDENTIALITY
The information contained in this electronic message and any attachments to
this message are intended for the exclusive use of the addressee(s) and may
contain confidential or privileged information. If you are not the
intended recipient, please notify DesignBuilder
Software immediately at either +1 (413) 256-4647 or at home@xxxxxxxxxxxxxxxxxxxxxxxxx, and
destroy all copies of this message and any attachments.
From:
EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On
Behalf Of csimmons290
Sent: Wednesday, February 17, 2010 11:11 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Re: Surface
properties:reflectance/absorptance
It looks like you have everything you need. infrared = thermal and i believe in
this case you can assume emittance = emissivity (although technically the terms
aren't equivalent)
LBNL has an SRI calculator that you could try using by plugging in the
"solar reflectance" and "infrared emittance" from the
supplier.
coolcolors.lbl.gov/assets/docs/SRI%20Calculator/SRI-calc10.xls
Also, the equation you should use is:
1 = absorptance + reflectance + transmittance
For an opaque material tranmittance = 0
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx,
R- Theru <rtheru@...> wrote:
>
> Hi everyone,
>
> I am trying to simulate a layer of concrete tiles coated with cool barrier
technology. From a supplier's data sheet on the material's properties, I have
data for solar reflectance, NIR reflectance, visible reflectance, UV
reflectance, infrared emittance and Solar Reflectance Index (SRI). However, I
need to use thermal absorptance (emissivity), solar absorptance and visible
absorptance.
> Firstly, am I right in converting solar and visible reflectance to the
respective absorptance values by using the formula A=log(1/R)?
> Secondly, how can I derive emissivity from emittance or the other
available data?
> Thanks in advance
>
__._,_.___
The primary EnergyPlus web site is found at:
http://www.energyplus.gov
The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/
Attachments are currently allowed but be mindful that not everyone has a high speed connection. Limit attachments to small files.
EnergyPlus Documentation is searchable. Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.
Change settings via the Web (Yahoo! ID required)
Change settings via email: Switch delivery to Daily Digest | Switch to Fully Featured
Visit Your Group | Yahoo! Groups Terms of Use | Unsubscribe
__,_._,___