[Radiance-general] illuminance on external facade
Stephen Selkowitz
seselkowitz at lbl.gov
Thu Mar 28 00:13:04 PDT 2013
Rob and others
The very high numbers (>300) are not physically plausible (at least on this
planet with its atmosphere and sun). You can get numbers in the 200l/w
range if you filter sunlight through spectrally selective glass. If I had
to guess, without doing any homework, its an artifact of the derivation as
follows: I assume diffuse radiation is calculated by processing measured
global irradiance and measured or assumed beam, and that diffuse
illuminance is calculated by processing measured global illuminance and
measured or assumed beam, so you are comparing two "small" numbers, each
the result of subtraction of two much larger numbers, so that relatively
small errors in the original calcs are magnified by that derivation. If
someone has time to dig into the source data you could verify or expose
another source. Of course it could be direct measurement and just a batch
of poorly measured/calibrated/reported data.
Steve
**********************************************************************
Stephen Selkowitz
Building Technology and Urban Systems Department (510) 486-5064
Bldg.90-3111
fax (510) 486-4089
Lawrence Berkeley National Laboratory
SESelkowitz at lbl.gov
Berkeley, CA 94720
http://buildings.lbl.gov/ <http://BTECH.lbl.gov/>
**********************************************************************
On Wed, Mar 27, 2013 at 8:51 AM, Guglielmetti, Robert <
Robert.Guglielmetti at nrel.gov> wrote:
> **
> Hi John,
>
> Thanks for looking at this. This is indeed interesting. When I get back in
> the office I can at least ask around if the measured versus derived
> illuminance question is answered somewhere in the weather file. As to the
> reason(s) why the efficacy for diffuse horizontal flaps around, I'm
> stumped, and unclear if its cause for alarm. Anyone?
>
> Rob
>
>
>
> Rob Guglielmetti
> NREL Commercial Buildings Research Group
> Golden, CO 80401
> robert.guglielmetti at nrel.gov
>
>
> -----Original Message-----
> *From: *John Mardaljevic [j.mardaljevic at lboro.ac.uk]
> *Sent: *Monday, March 25, 2013 03:44 PM Mountain Standard Time
> *To: *radiance-general at radiance-online.org
> *Subject: *Re: [Radiance-general] illuminance on external facade
>
> Hi Ery,
>
> Tinkering with the Boise TMY3 data I was a little surprised to see how
> some of the efficacy values turned out. For those unfamiliar, the Boise
> TMY3 climate file (much like many of the others from the Energy+ website)
> contains both illuminance and irradiance data for these quantities:
>
> - Global horizontal (Gh)
> - Diffuse horizontal (Dh)
> - Direct normal (Dn)
>
> Sometimes illuminance data is derived from irradiance measurements, though
> nowadays it is more likely to be measured separately. Which it is could be
> identified in the climate file (I haven't checked).
>
> The following table shows how the mean luminous efficacy (within 5klux
> bands of global horizontal illuminance) varies for: the global horizontal;
> the diffuse horizontal; and, the direct normal quantities:
>
> gh_lo gh_hi Eff_gh Eff_dh Eff_dn
>
> 5000.00 10000.0 106.145 135.606 60.1583
> 10000.0 15000.0 106.610 147.202 73.0784
> 15000.0 20000.0 107.339 258.167 81.6775
> 20000.0 25000.0 108.312 140.848 88.2379
> 25000.0 30000.0 107.261 156.417 92.4818
> 30000.0 35000.0 106.713 184.455 94.1426
> 35000.0 40000.0 106.456 197.996 96.1488
> 40000.0 45000.0 106.142 213.014 97.9623
> 45000.0 50000.0 105.894 177.705 99.2376
> 50000.0 55000.0 105.992 173.584 100.170
> 55000.0 60000.0 105.812 179.285 100.867
> 60000.0 65000.0 106.057 300.989 101.701
> 65000.0 70000.0 105.593 129.234 101.205
> 70000.0 75000.0 105.347 145.367 101.420
> 75000.0 80000.0 105.333 193.799 101.418
> 80000.0 85000.0 105.044 135.980 101.445
> 85000.0 90000.0 104.697 134.219 100.965
> 90000.0 95000.0 103.924 123.413 100.588
>
> The global efficacy value (Eff_gh), as you noted, is between 100 and 100
> lm/W -- I'd expect global vertical to be very similar to global horizontal,
> at least for sun illuminated orientations. Direct normal efficacy across
> the ranges is also pretty much how I expected it to vary. However, diffuse
> horizontal efficacy seems to flap-around quite a bit -- four maxima, two of
> them quite conspicuous. I wouldn't have been too surprised to see general
> trends, but this was unexpected. Can't say that I've noticed or given much
> thought to it before, but I am a little intrigued by those high values,
> i.e. > 200 lm/W.
>
> Best
> John
>
> John Mardaljevic
> Professor of Building Daylight Modelling
> School of Civil & Building Engineering
> Loughborough University
> Loughborough
> Leicestershire
> LE11 3TU, UK
>
> Tel: +44 1509 222630 (Direct)
> Tel: +44 1509 228529 (Pam Allen, secretary)
>
> j.mardaljevic at lboro.ac.uk
> http://www.lboro.ac.uk/departments/cv/staff/profile/367.html<http://www.lboro.ac.uk/departments/cv/>
>
> Personal daylighting website:
> http://climate-based-daylighting.com
>
>
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>
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