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Re: [EnergyPlus_Support] simulating a coat of reflective paints (cool roofs) [2 Attachments]



Possibly cool roofs save 10-30% when packaged roof top units are pulling outside air from the local hot micro climate? 

The table I had before was off due to some quick copy paste formula-- here it is corrected for the model attached earlier. 

E+DefaultSRI78SRI85IMPOSSIBLY HIGH
Energy Per Conditioned Building Area [kWh/m2]226.3225.04 224.81223.64
[kWh/m2] Savings1.261.492.66
% Savings 0.56% 0.66%1.18%

The hysteresis PCM model I have been working on seems to be more stable by switching from the melt to freeze curve of vise verse when temperatures change directions rather then going up and down a slope. Its most useful feature is to provides the user with the physical state of the PCM- so it is possible to determine if the PCM is transitioning as expected. 

I will write a paper on it as you suggest, but there is also the one from last years ASHRAE conference, attached as well as some slides I made on the concepts. 



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 Fri, Jul 11, 2014 at 1:18 PM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

That's right, but e+ is not alone in this respect. Commercial code is not doing much better. Greenroof is another example... I guess it should be about experimental validation. Greenroof has been known to be pretty ineffective in e+ unless irrigated or rained on (many weather files have zeros in the rain data column which average users won't expect). I believe, that this aspect has shown itself to be real though. So we have learnt from it. Your work on the hysteresis effect of PCMs shows how significant that effect is (still hope to read your paper).

It would be nice if the working limits of some of the models were more clear. E+ is perhaps too forgiving...Trnsys, for example, terminates if a chiller goes outside of its performance matrix data...e+ uses curves and warnings.

On 11.07.2014, at 21:55, "Jeremiah Crossett jcrossett@xxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

 

I guess- except that Energy Plus shows next to nothing in terms of Energy Savings, while very credible groups such as the cool roof rating counsel claiming 10-30% cooling energy savings, and a billion $ of annual savings for the USA.  

Honestly I think that this is a fallacy/bug/issue with Energy Plus.  Coming from the perspective of a manufacture, who has had issues with incorrect models vastly under predicting our product performance, and having worked on development of a new model that is more accurate for our product and having found quite a few bugs in Energy Plus, or have found even more times that the issues I tend to with the actual model-- I can not imagine that cool roofs do not save cooling  (and raise heating?) energy!  

Has anyone on the list ever seen anything like 10-30% savings for a cool roof in an E+ model before? If not I think that the software may need some improvement for its cool roof methodology. 

http://coolroofs.org/documents/CoolRoofings-General.pdf

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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 Fri, Jul 11, 2014 at 12:40 PM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

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

On 11.07.2014, at 17:58, "ecoeficiente@xxxxxxxxx [EnergyPlus_Support]" <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 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 asitkm76@... [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@...' jeannieboef@... [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@...[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@...[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@...[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@...[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@...[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





Posted by: Asit Mishra <asitkm76@...>







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