[EnergyPlus_Support] Re: spectral composition of solar radiation

["FrancescoP" <fpasserini.tn@xxxxxxxxx>]

Thank you for the very detailed answers!
I took some time to read the suggested articles.
Two  issues as user of EnergyPlus.

1. Joe Huang wrote: "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."
Were they replaced in the last versions?

2. From my point of view, the conclusions of P. Lyons et al. (2010) were unexpected:
"In many cases there is much better agreement between the FSM [note: Full Spectral Model] and SWM [note: Simple Window Model] than between the FSM and SAM [note: Spectral Average Model]"
www.ibpsa.us/pub/simbuild2010/papers/SB10-DOC-TS04A-04-Lyons.pdf
It seems that if I don't know the spectral characteristics of the windows that I'm modeling, using the object WindowMaterial:SimpleGlazingSystem is better than using the object WindowMaterial:Glazing with SpectralAverage as "Optical Data Type", doesn't it? Isn't it strange?

With Regards
Francesco 


--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "cgueymard" <chris@...> wrote:
>
> I'd like to offer some supporting info, as well as corrections, to what Joe said. The 2009 paper Peter mentions is this one:
> 
> http://www.sciencedirect.com/science/article/pii/S0038092X08003460
> 
> That research highlighted two things:
> - The WINDOW software is using an antiquated spectral weighting function; ASTM G197 would now be the logical choice since it was specifically developed for fenestration applications (vertical windows or roof skylights). In contrast, ASTM G173 was developed for other applications, such as PV.
> - A single spectral distribution (as currently implemented) is fine for rating purposes, but cannot be considered realistic for hourly energy simulations, since the spectral distribution changes dynamically.
> 
> These variations are mainly driven by the sun geometry (air mass) and variable atmospheric constituents (mainly clouds, aerosols and water vapor), which are time and location dependent. Spectral calculations under cloudless sky conditions can be done with the SMARTS code, which was used to generate the ASTM G173 and G197 standard spectra, among others. SMARTS would be the model to use here. Contrarily to what Joe said, REST2, which is a simpler model derived from SMARTS, would not be appropriate.
> 
> The idea of developing hourly predictions of the spectral distribution at any location for energy simulation purposes has been floating around for many years. See ASHRAE RP-1143 in particular for preliminary developments and "Advanced Solar Irradiance Model and Procedure for Spectral Solar Heat Gain Calculation," ASHRAE Trans. 2007. The task at hand involves increased computer calculations and considerable atmospheric data to access dynamically in one way or another. Although solar heat gain calculations would definitely be more accurate if based on hourly spectral information, the increased complication is an important aspect of the question. The impetus for a drastic change (and necessary financial support) would have to come from ASHRAE, NFRC and other institutions, but is still too weak.
> 
> Chris
> 
> -- 
> Chris A. Gueymard, PhD
> Solar Consulting Services
> P.O. Box 392
> Colebrook, NH 03576, USA
> Florida's Office:
> Tel. (386) 402-8949
> www.SolarConsultingServices.com
> Publication list: http://scholar.google.com/citations?user=s_wkStcAAAAJ
> 
> 
> 
> 
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Joe Huang <YJHuang@> wrote:
> >
> > 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
> >
>




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