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Re: [EnergyPlus_Support] simulating a coat of reflective paints (cool roofs)



"Solar and Visible absorptance are low while Thermal absorptance is high" is not correct...they will be similar, but probably a little less than the emittance of IR spectrum.


2014-07-11 10:44 GMT+02:00 Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:
 

So to summarize, the so called cool paints/cool coats have a high emissivity in long wave/far infrared regions while a high reflectance in visible and near infrared wavelengths. 
Since the coatings can be assumed to be opaque, absorptance = emissivity = 1 - reflectance.
Therefore, for such a coating, Solar and Visible absorptance are low while Thermal absorptance is high.
Please do let me know if I have confused anything.

Following from this though (and assuming my understanding of all the terms is correct), simulation does not show any significant benefit being obtained from use of these coats. I did not find any significant reduction in number of discomfort hours and Jeremiah (as he had mentioned in a previous email in this thread) did not find significant changes in cooling energy needs. This seems contrary to the fervor with which these coatings are being advertised by manufacturers and also contrary to my personal experience with using these coats in residences. 
I did my simulation for occupancy levels used for offices.
Is that the region why there was not much benefit (internal loads significantly outweigh solar loads) or is there some other reason or am I doing something wrong?

Regards,

asit


On Thu, Jul 10, 2014 at 10:58 PM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 


Actually, you where right: absorbtance = 1 - reflectance, but the column heading in your datasheet uses misleading language with the word "refectance" instead of "emittance".

Mit freundlichen Grü�en- Sent from my iPhone (excuse the brevity)

i. A.
Jean Marais
b.i.g. bechtold

On 10.07.2014, at 13:32, "Asit Mishraasitkm76@xxxxxxxxx[EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

In other words, all the values are being kept high (> 0.7) ?
I was incorrectly interpreting then that Absroptance = 1 - reflectance and hence I was giving low values to these properties.
Thanks for correcting my erroneous assumption.

Regards,
asit



On Thu, Jul 10, 2014 at 4:48 PM, Jean Marais jeannieboef@xxxxxxxxx[EnergyPlus_Support]<EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

Eg.
Cool paint AK-103
Solar Absorbtance = 0.7324 (Solar Direct Reflectance is incorrectly named)
Thermal Absorbtance = 0.939
Visual = ? ca. 0.75 to 0.95 (very little energy in this bandwidth so errors can be tolerated as they have comparitively small effect)


2014-07-07 14:46 GMT+02:00 Asit Mishra asitkm76@xxxxxxxxx[EnergyPlus_Support]<EnergyPlus_Support@xxxxxxxxxxxxxxx>:
 

Dear Jeremiah,
                          I am very thankful for the two spreadsheets. They will be very useful in my work. 
I was in particular referring to the paints that have been called cool roof paints.
These are a few values for solar direct reflectance as quoted from a database provided by US-India Joint Center for building Energy Research and Development. (I am attaching the original document as well)

Thermatek Heat Reflective Paint:  0.8973
Cool paint AK-103, Aroma paints: 0.7324
Sun cool - LHP coating: 0.9097

Actually, when I started out, I did what Jean had mentioned - change the reflectivity/solar absorptance etc. of the outermost layer for a wall or roof. The introduction of a thin film 
was just for convenience so that I could make it an outermost layer in any kind of wall without having to change properties of standard layers. 
Either way, results still did not show much change in the year round indoor temperatures, i.e. before and after use of the reflective coats. 
Regarding emmisivity, I wanted to know exactly which property to control to change emissivity.
The typical cool roof paints have a high reflectivity and a high emissivity as well. 
Looking at a typical material specification, I am unable to see how I change this for a surface/the outermost layer.
example Material,
    A1 - 1 IN STUCCO,        !- Name
    Smooth,                  !- Roughness
    2.5389841E-02,           !- Thickness {m}
    0.6918309,               !- Conductivity {W/m-K}
    1858.142,                !- Density {kg/m3}
    836.8000,                !- Specific Heat {J/kg-K}
    0.9000000,               !- Thermal Absorptance
    0.9200000,               !- Solar Absorptance
    0.9200000;               !- Visible Absorptance

I am sorry if my question is not making full sense. And thank you both for your immediate response and help.

Regards,

asit


On Mon, Jul 7, 2014 at 9:46 AM, Jeremiah Crossettjcrossett@xxxxxxxxxxxxxxx[EnergyPlus_Support]<EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

I  my experience unfinished metal is the only thing that warrants using anything far off from default, and only modify absorbence for parametric studies.  Attached is a spreadsheet you can find online that can be used to fit product data into E+, just remember that solar [and visible] absorbence is the inverse of the reflectance info you can get from manufactures.  Also attached is a spreadsheet where  absorbence / reflectance = albedo..

Hope this helps 





â??â??
Jeremiah D. Crossett  | Senior Analyst  | LEED Green Associate 
  





On Sun, Jul 6, 2014 at 6:59 PM, Asit Mishra asitkm76@xxxxxxxxx[EnergyPlus_Support]<EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

Hello,
         I would like to have some idea on how you all might have approached simulating the effect of a reflective coat of paint. The way I went about it is defined a material with low absorptivity and added a thin layer (0.5 mm) to the roof/wall construct.
This did not seem to produce desired results
Secondly, normally what I have seen in material specifications, while values for absorptivity can be specified, there was no obvious way to specify a high emissivity (apart from indirectly doing so using surface roughness).
Please advise

Regards,

asit









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Posted by: Jean Marais <jeannieboef@xxxxxxxxx>


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