[Radiance-general] illuminance on external facade

[John Mardaljevic]

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

Personal daylighting website:
http://climate-based-daylighting.com

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