[EnergyPlus_Support] Re: calculating emissivity of materials

["Phil" <philcook76@xxxxxxxxxxx>]

Wait a minute. This is not correct. The emissivity and the absorptivity of any material is wavelength dependant. This is why E+ gives us the option of specifying the absorptivity at three different wavelength ranges. In building science, people often simply refer to "absorptivity" when they mean the "absorptivity at the solar frequencies" and the "emissivity" when they mean "emissivity at the thermal frequencies". This is why these values are not related - incorrect use of terminology. 

However, Kirchhoff's law of thermal radiation states that for an opaque body in thermal equilibrium, the emissivity = the absorptivity at a particular wavelength. It is the last part that is important in this discussion. E+ assumes that within the range of wavelengths specified, the absorptivity = the emissivity which is a reasonable assumption. But the absorptivity in the thermal band is not related to the absorptivity in the solar band which is where the low-e paints come in. The thermal absorptivity/emissivity of most materials (including a painted surface) is about 0.9 no matter what the solar and visible absorptivity/emissivity is.

The question I have is why does E+ have a specific field for entering the solar absorptivity and one for the visible absorptivity given that the visible spectrum is contained within the solar spectrum. How does it use these different values?

By the way, room temperature is about 300K not 40K.

Talk to you later,
Phil.

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "D. Charlie Curcija" <curcija@...> wrote:
>
> Emissivity is not the same as absorptance/absorptivity.  Not even close.
> Emissivity is measured in Far IR range (thermal radiation), while
> asbortptivity is measured in Solar (UV, VIS, Near IR) range.  Big
> difference, since solar radiation is at approximately 5600K, while thermal
> radiation is at approximately room temperature (plus/minus 40K).  
> 
>  
> 
> BTW, this is a common misconception and very often these quantities are
> confused, leading some to believe that white painted surface has low
> emissivity, which is not true.  Unless the paint is specifically designed to
> be low-emissivity (not easy), all paints have emissivity of 0.9 regardless
> of color or appearance.  However, shiny white paint will have very low
> absorptivity, while dull black paint will have very high absorptivity.  This
> behavior is utilized in the design of cool roofs, where low solar
> absorptivity is sought.
> 
>  
> 
> Since all surfaces in buildings are painted, it is safe to assume that
> emissivity is always 0.9, which is probably the reason that there is no
> field to enter it, however this should change as we see more and more
> specially designed low-e paints, even in window frames.
> 
>  
> 
> D. Charlie Curcija 
> DesignBuilder Software 
> 16 Bridge St. 
> Millers Falls, MA 01349
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> Fax: (413) 256-4823 
> cell: (413) 575-3487 
> email:  <mailto:curcija@...>
> curcija@... 
> web:  <http://www.designbuildersoftware.com/>
> http://www.designbuildersoftware.com 
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> From: EnergyPlus_Support@xxxxxxxxxxxxxxx
> [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Linda Lawrie
> Sent: Wednesday, June 02, 2010 10:22 AM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Subject: Re: [EnergyPlus_Support] calculating emissivity of materials
> 
>  
> 
>   
> 
> At 03:39 AM 6/2/2010, ifigeneiat wrote:
> 
> I would like to know how E+ is calculating the emissivity of materials (not
> the glazing ones). I am guessing it is an indirect proceedure, as there is
> no specific input field for this subject. 
> 
> 
> I believe the input absorptance/absorptivity for materials is the same as
> emissiv



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