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



Hello everyone. I am really grateful for all the helpful feedback and comments.
Hopefully the discussions were helpful to others beside me too.

My 2 cents: 
A material can have different absorptance over different wavelengths, especially near and far infrared. Documentation suggests that Thermal absorptance is about long wave/far infrared regions. So, for a cool roof coating, it should be ideal to have thermal absorptance high and low solar and visible absorptance. In region of a particular wavelength though, absoptance and emissivity have to be equal or nearly equal.
I think that is agreed upon by all?
Now, the seemingly low energy savings are most likely because of regions that have been discussed - high insulation of roof. 
If the fraction of heat load that is entering due to solar flux on roof is small to start with, even a 100% reduction in that load should not matter much to comfort hours/energy savings. 
In my case, since the majority of load is internal (high occupant density), I was sort of convinced by low reduction in discomfort hours. Still, I wanted to check what others had found.
However, if there are some doubts regarding the modelling inside EPlus, hopefully future editions will take care of it.

Regards,
asit

 



On Sat, Jul 12, 2014 at 8:30 PM, Julien Marrec julien.marrec@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

Asit,

I'm catching up with the thread. The reason why I asked for your roof construction is similar to Dru's comment: with high insulation, the effect of cool roof is minimized.

Looking at the data you sent, unless I made a mistake the R-value is R-SI 5.95 and R-IP of 34 (excluding interior and exterior air film). That's a pretty high level of insulation, so I'm not surprised you're not getting crazy savings from a cool roof.

That being said, it doesn't mean that E+ does the calc right, I'll leave that to the experts...

Best,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
LinkedIn (en) : www.linkedin.com/in/julienmarrec


2014-07-10 6:11 GMT+02:00 Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:

 

The entire roof's composition is as follows: Construction,
    C1 Roof 90.1-2007_nonRes CZ2-8,  !- Name
    A1_Built-up Roofing - .375" / 9mm,  !- Outside Layer
    Insul_Roof_90.1-2007 nonRes_CZ1,  !- Layer 2
    Insul_Roof_90.1-2007 nonRes_CZ2-8,  !- Layer 3
    INSUL_R2.8-RoofInsulation_1"(25mm);  !- Layer 4

  Material,
    A1_Built-up Roofing - .375" / 9mm,  !- Name
    VerySmooth,              !- Roughness
    0.0254,                  !- Thickness {m}
    0.155736,                !- Conductivity {W/m-K}
    1121.2917282,            !- Density {kg/m3}
    1380.72,                 !- Specific Heat {J/kg-K}
    0.9,                     !- Thermal Absorptance
    0.2,                     !- Solar Absorptance
    0.2;                     !- Visible Absorptance

  Material,
    Insul_Roof_90.1-2007 nonRes_CZ1,  !- Name
    MediumRough,             !- Roughness
    0.1321816,               !- Thickness {m}
    0.051912,                !- Conductivity {W/m-K}
    256.29525216,            !- Density {kg/m3}
    836.8,                   !- Specific Heat {J/kg-K}
    0.9,                     !- Thermal Absorptance
    0.7,                     !- Solar Absorptance
    0.7;                     !- Visible Absorptance

  Material,
    Insul_Roof_90.1-2007 nonRes_CZ2-8,  !- Name
    MediumRough,             !- Roughness
    0.1427988,               !- Thickness {m}
    0.051912,                !- Conductivity {W/m-K}
    256.29525216,            !- Density {kg/m3}
    836.8,                   !- Specific Heat {J/kg-K}
    0.9,                     !- Thermal Absorptance
    0.7,                     !- Solar Absorptance
    0.7;                     !- Visible Absorptance

  Material,
    INSUL_R2.8-RoofInsulation_1"(25mm),  !- Name
    MediumRough,             !- Roughness
    0.0254,                  !- Thickness {m}
    0.051912,                !- Conductivity {W/m-K}
    256.29525216,            !- Density {kg/m3}
    836.8,                   !- Specific Heat {J/kg-K}
    0.9,                     !- Thermal Absorptance
    0.7,                     !- Solar Absorptance
    0.7;                     !- Visible Absorptance

I have modified the solar and visible Absorptance to get effect of reflective coating. 
By the way, I have not been able to figure out how to get surface temperatures. I would appreciate any help in the matter.

Regards,

asit


On Tue, Jul 8, 2014 at 6:04 PM, Asit Mishra <asitkm76@xxxxxxxxx> wrote:
Hi Jeremiah and Julien,
                                   I am out of station for a couple of days, I will get back to you soon on the data.

Regards,

asit


On Tue, Jul 8, 2014 at 12:29 PM, Julien Marrec julien.marrec@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

Asit,

I'm curious, could you tell me what is your roof composition or at least U-value/R-value?



--
Julien Marrec, EBCP, BPI MFBA
Energy&Sutainability Engineer
T: +33 6 95 14 42 13
www.julienmarrec.com

DoYouBuzz : www.doyoubuzz.com/julien-marrec_1
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr
LinkedIn (en) : www.linkedin.com/in/julienmarrec


2014-07-08 5:11 GMT+02:00 Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:

 

Thanks again for the quick response. 
Of course now I am perplexed for a different reason. It seems I had done everything right (specifying appropriate values of thermal, solar and visible Absorptance) for simulating a cool roof paint/coating. But somehow this did not produce any dramatic change in the indoor temperature regime.
I had expected more hours with comfortable temperature since a major source of heat gain (solar gain through roof) was significantly reduced. But the
benefit was only about a couple of dozen hours.

Regards,

asit



On Mon, Jul 7, 2014 at 9:08 PM, Jeremiah Crossett jcrossett@xxxxxxxxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

Sorry-  missed a word... 

Here is a quick illustration 
0.9,    !- Thermal Absorptance = 0.9 Thermal Emissivity
0.92,  !- Solar Absorptance = 0.08 Solar Reflectivity  




â??â??
Jeremiah D. Crossett  | Senior Analyst  | LEED Green Associate 
â??â??
120 E. Pritchard St.  | Asheboro, NC 27203 
â??â??
 | Mobile 503-688-8951
  





On Mon, Jul 7, 2014 at 8:34 AM, Jeremiah Crossett jcrossett@xxxxxxxxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

Here is a quick illustration 
0.9,           !- Thermal Absorptance = 0.9 Emissivity
    0.92,               !- Solar Absorptance = 0.02
    0.92;               !- Visible Absorptance  = 0.02



â??â??
Jeremiah D. Crossett  | Senior Analyst  | LEED Green Associate 
â??â??
120 E. Pritchard St.  | Asheboro, NC 27203 
â??â??
 | Mobile 503-688-8951
  





On Mon, Jul 7, 2014 at 5:46 AM, Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

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 Crossett jcrossett@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 
â??â??
120 E. Pritchard St.  | Asheboro, NC 27203 
â??â??
 | Mobile 503-688-8951
  





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: Asit Mishra <asitkm76@xxxxxxxxx>


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