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Re: [EnergyPlus_Support] simulating a coat of reflective paints (cool roofs)
I guess this needs quantification. What I mean is
Emmitance is low = 0.3 - 0.5 (wave length or bandwidth dependant)
Emmitance is high = 0.9 - 0.98 (wave length or bandwidth dependant)
You simply don't get materials (at least not mass produced and cheep) that will have an IR emmitance = high whilst at the same time
having a solar emmitance which is low, simply because the IR band carries so much of the total solar band's energy. The IR and visual
bands fall within the solar spectrum. And a large part of the rest of the solar spectrum passes straight through most materials and so
don't impact as useful energy. The visual spectrum holds relatively little energy, so if a material had a low visual emmitance, but a "normal"
high IR emmitance, the net effect on the emmitance of the entire solar band is small and (although less than that of the IR emmitance)
should be farely close, allbeit a little less, than the IR emmitance.
Visual emmitance is easy to judge because we can see it. White reflects quite well, but is not as good as a mirror, and even a mirror isn't perfect.
Physical theory may differ slightly, but practically speaking this is good enough for our error margins in building simulation.
Mit freundlichen Grü�en- Sent from my iPhone (excuse the brevity)
i. A.
Jean Marais
b.i.g. bechtold
Tel. +49 30 6706662-23
On 11.07.2014, at 17:58, "<mailto:ecoeficiente@xxxxxxxxx>ecoeficiente@xxxxxxxxx [EnergyPlus_Support]" <<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
>
Jean, I agree with the "Solar and Visible absorptance are low while Thermal absorptance is high". I don't know why you say it is wrong...
>
>
>
> ---In <mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>EnergyPlus_Support@xxxxxxxxxxxxxxx, <jeannieboef@...> wrote :
>
> "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 <mailto:asitkm76@...>asitkm76@... [EnergyPlus_Support] <<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>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, '<mailto:jeannieboef@...>jeannieboef@...' <mailto:jeannieboef@...>jeannieboef@... [EnergyPlus_Support] <<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>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
Tel. +49 30 6706662-23
>>>>> On 10.07.2014, at 13:32, "Asit Mishra<mailto:asitkm76@...><mailto:asitkm76@...>asitkm76@...[EnergyPlus_Support]" <<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>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 <mailto:jeannieboef@...><mailto:jeannieboef@...><mailto:jeannieboef@...>jeannieboef@...[EnergyPlus_Support]<<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>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 <mailto:asitkm76@...><mailto:asitkm76@...><mailto:asitkm76@...>asitkm76@...[EnergyPlus_Support]<<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>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 Crossett<mailto:jcrossett@...><mailto:jcrossett@...><mailto:jcrossett@...>jcrossett@...[EnergyPlus_Support]<<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>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
<http://www.phasechange.com/><http://www.phasechange.com/><http://www.phasechange.com><http://www.phasechange.com>www.phasechange.com
<http://i.imgur.com/Sldxf.jpg>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Sun, Jul 6, 2014 at 6:59 PM, Asit Mishra <mailto:asitkm76@...><mailto:asitkm76@...><mailto:asitkm76@...>asitkm76@...[EnergyPlus_Support]<<mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx><mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>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
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>> Posted by: Asit Mishra <<mailto:asitkm76@...><mailto:asitkm76@...>asitkm76@...>
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