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Re: [EnergyPlus_Support] Thermal Emissivity
Quentin,
Mike Witte covered this about a year ago in the list...
EnergyPlus opaque materials require three inputs related to radiant
exchange:
Absorptance:Thermal
Absorptance:Solar
Absorptance:Visible
Thermal applies to the infrared wavelengths for radiant heat transfer.
Visible is applied to daylighting and the visible light fraction from
LIGHTS. Solar refers to the full solar spectrum including the visible and
thermal wavelengths. The solar absorptance is applied to incident solar
radiation.
Values for these properties are available from various handbooks. Some
sources with data for various colored roof materials are cited below. Some
handbooks will state emittance or reflectance data rather than absorptance.
To convert to absorptance, use the following rules. Emittance=Absorptance
for a given wavelength region (solar, visible, or thermal).
Absorptance+Reflectance+Transmittance=1.0. For opaque materials,
transmittance is zero, so Absorptance = 1.0 - Reflectance.
Data sources for roof material absorptance properties:
LBL site with roofing data - http://eetd.lbl.gov/CoolRoofs/
The Florida Solar Energy Center (FSEC) has published several reports
regarding roof properties and their effect on cooling loads. A list of
publications may be found at:
http://www.fsec.ucf.edu/bldg/resources/publications/index.htm
Two especially pertinent reports are:
http://www.fsec.ucf.edu/bldg/pubs/cr670/index.htm - Lots of measured
property data
http://www.fsec.ucf.edu/bldg/pubs/cr1336/cr1336.htm - Table 1 lists solar
reflectances for six new and weathered materials
"Quentin Jackson"
<quentin@buildingworksh To: EnergyPlus_Support@xxxxxxxxxxxxxxx
op.co.nz> cc:
Sent by: Subject: [EnergyPlus_Support] Thermal
EnergyPlus_Support@yaho Emissivity
ogroups.com
07/26/2005 08:25 PM
Please respond to
EnergyPlus_Support
Hi there,
I want to be able to simply change the thermal emissivity of a building
material - do I do this by using the thermal absorbtance field under
regular materials. If so how does absorbtance relate to emissivity?
Cheers,
Quentin
--
Quentin Jackson - Director
BUILDING WORKSHOP
PO Box 9585 | Level 1, 1 Marion Street | Wellington
www.buildingworkshop.co.nz <http://www.buildingworkshop.co.nz>
Phone 04 3846583
Fax 04 9165755
Mobile 0274 205650
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