I have to jump in here, because I've been working for the past four years with Oak Ridge National Laboratory to develop the Roof Savings Calculator (www.roofcalc.com) that's meant to be an online calculator to replace the previous ones developed by LBNL for DOE and EPA.
We are currently in the midst of an one-year effort to first do interprogram comparisons between the RSC, previous LBNL results, and EnergyPlus, which were inconclusive, and then validate the RSC against detailed monitored data provided by LBNL. As a one-sentence introduction, the simulation engine behind the RSC is DOE-2.1E coupled with AtticSim, the only attic simulation program I know of that handles radiant heat transfer between the attic surfaces as well as with the duct system, as well as natural or forced ventilation in the attic. The role of DOE-2.1E in doe2attic is to take the heat flows and duct losses computed by AtticSim to calculate the net impact on the heating and cooling energy use of the building.
I wouldn't put so much faith in the numbers from the CRRC, since they were all based on DOE-2.1E simulations done by LBNL that were never calibrated nor backed up by any measured data. Although modeling a "cool roof" may seem simple as pie, it is actually quite difficult to determine the energy impact because of the intervening attic or plenum, let alone any compounding effects on the duct system.
Furthermore, since a "cool roof" affects only the radiative heat gain of the roof, any credible simulation must keep track of the radiative exchange between the attic interior surfaces. Since DOE-2.1E does not do that (it tracks only air temperatures), I regard all the numbers in the CRRC as suspect. Specifically, I find the statement in the CRRC that the heating penalties from "cool roofs" are negligible to be particularly troubling, since it leads the CRRC to conclude that "cool roofs" are still beneficial in Chicago (look at the map) or virtually anywhere in the US.
From the simulations that I've done with doe2attic and EnergyPlus, I found the cooling savings to be 20-50% greater, but the heating penalties to be anywhere from 3 to 10 times larger than reported by the LBNL studies that form the basis of the CRRC numbers. Consequently, my sense is that the "cool roof" savings in cooling-dominant locations will remain unchanged, but that moving north they will be progressively degraded by the heating penalties, so that they're negative in heavily heating-dominant locations.
Other comments:
1) My experience using EnergyPlus to model attics was not in failing to get cooling savings, but the opposite of getting attic air temperatures peaking 10-20 C above measured data.
2) It seems you're modeling the roof as a monolithic single layer with an extremely low conductivity, and without a plenum or attic space. That's much too simplified to draw any conclusions about the EnergyPlus' algorithms vis-a-vis "cool roofs".
3) Looking at previous posts, there seems to be some confusion about the difference between reflectivity and emissivity. A "cool roof" pigment should have be high in both, i.e., it should reflect incoming radiation, as well as emit long-wave radiation when it's hot. A surface that has high reflectivity but low emissivity would be like a bare metal sheet, it wouldget very hot in the sun, which is how the term "cat on a hot tin roof" came about.
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 108D Moraga CA 94556 yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx http://weather.whiteboxtechnologies.com for simulation-ready weather data (o) (925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"
On 7/11/2014 12:55 PM, Jeremiah Crossett jcrossett@xxxxxxxxxxxxxxx [EnergyPlus_Support] 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.
Jeremiah D. Crossett | Senior Analyst | LEED Green Associate
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 isEmmitance 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 timehaving a solar emmitance which is low, simply because the IR band carries so much of the total solar band's energy. The IR and visualbands fall within the solar spectrum. And a large part of the rest of the solar spectrum passes straight through most materials and sodon'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 Grüßen- Sent from my iPhone (excuse the brevity)
i. A.Jean Maraisb.i.g. bechtoldTel. +49 30 6706662-23
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 Grüßen- Sent from my iPhone (excuse the brevity)
i. A.Jean Maraisb.i.g. bechtoldTel. +49 30 6706662-23
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:
Visual = ? ca. 0.75 to 0.95 (very little energy in this bandwidth so errors can be tolerated as they have comparitively small effect)Eg.
Cool paint AK-103
Solar Absorbtance = 0.7324 (Solar Direct Reflectance is incorrectly named)Thermal Absorbtance = 0.939
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: 0.8973Cool paint AK-103, Aroma paints: 0.7324Sun 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 filmwas 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, !- NameSmooth, !- Roughness2.5389841E-02, !- Thickness {m}0.6918309, !- Conductivity {W/m-K}1858.142, !- Density {kg/m3}836.8000, !- Specific Heat {J/kg-K}0.9000000, !- Thermal Absorptance0.9200000, !- Solar Absorptance0.9200000; !- 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
On Sun, Jul 6, 2014 at 6:59 PM, Asit Mishra asitkm76@...[EnergyPlus_Support]<EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
asitRegards,Please adviseSecondly, 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).This did not seem to produce desired resultsHello,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.
Posted by: Asit Mishra <asitkm76@...>
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