Jean,
Thanks for the compliment. Funny that this topic has now come up, because last week I was just reviewing a nice paper presented at SimBuild 2010 by a colleague, Peter Lyons in Canberra, that explored exactly this topic, "A comparison of window modeling methods in EnergyPlus 4.0". Quoting from page 2 of Peter's paper,
"The impact of using the FSM further depends on the shape of its intrinsic solar spectral irradiance function. This can exert an additional and sometimes dramatic influence on calculated solar and visible properties of a glazing system. Optical calculations in EnergyPlus, for window layers input using full spectral data, use a spectral weighting data set that is derived from the Optics 5 data file, ISO-9845GlobalNorm.std. ISO 9845-1:1992 is based on two spectra, ASTM E891 Direct (the standard used by the U.S. National Fenestration Rating Council - NFRC) and ASTM E892 Global (the standard used in the European ISO/CEN system). An extensive discussion may be found in Gueymard and W.C. duPont (2009). In contrast to EnergyPlus, the modeling tool WINDOW 5 (the reference simulation software for NFRC) uses a different default spectral weighting data set, W5_NFRC_2003.std which is based on ASTM E891 Direct, only. The main issue with E891 and E892 are that they are now obsolete and have been withdrawn by ASTM. Gueymard and duPont strongly recommended that all existing spectra should be replaced by ASTM G173 (ASTM, 2003):
"...This [weighting set] difference accounts for most of the variation in SHGC values reported by EnergyPlus and WINDOW 5 for full spectral data window layer input. This variation is more pronounced for window constructions of three glass layers or more. Users intending to select a window construction based on SHGC value for energy code compliance should base their selection on the value reported by WINDOW 5 since this is the officially recognized value. "
In summary, EnergyPlus uses a weighting function to disaggregate the solar radiation from the weather file to its spectral components, which are then combined with the WindowGlassSpectralData to derive the net radiation transmitted, reflected, etc. However, the spectral weighting data used in EP Version 4.0 were obsolete, so that Chris Gueymard, a recognized expert on solar radiation models, recommended that they be replaced.
The solar radiation data on the weather files are strictly broadband single values, although Chris has developed a detailed Clear Sky Model called REST2 that also provides detailed spectral values, which can and will vary depending on the atmospheric conditions. All that, though, is an order of magnitude more precise than warranted for the (calculated) solar radiation on the weather files.
Joe
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 108D Moraga CA 94556 yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx www.whiteboxtechnologies.com (o) (925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"
On 5/21/2013 2:24 AM, Jean marais wrote:
I would guess the physical window does not change, therefore the ability of the window to absorb, transmit or reflect energy at a specific wave length remains constant and the infalling radiation at said wavelength is what is changing.
I have no idea how and if the energy at different wavelengths is varied or calculated during run time. I believe solar data is hour based, meaning at least the results are only varying on an hourly bases.
Anyone else? The weather guru here would be Dr Joe Huang at whitebox technologies. He wrote some of the weather processing models.
Jean
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "FrancescoP" <direzionecontraria@...> wrote:
>
> I'm modeling a sunspace and comparing the results with monitored data. The inside temperatures are influenced a lot by the model that is used for the windows, therefore I'm reading about the object MaterialProperty:GlazingSpectralData
> I have a question: how the is spectral composition of solar radiation considered in the simulation? I mean, is it considered constant or does it change during the simulation (in my layman's opinion, the distribution of energy among the different wavelengths, in percentage terms, depends on the cloudiness)?
> Regards
>
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