[Radiance-general] illuminance on external facade
Guglielmetti, Robert
Robert.Guglielmetti at nrel.gov
Wed Mar 27 08:51:32 PDT 2013
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<mailto: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<mailto: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<http://climate-based-daylighting.com/>
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