[Radiance-general] illuminance on external facade

[Ery Djunaedy]

All,

Thanks for the comments.

A bit more background information. We are doing a coupled 
EnergyPlus-Radiance simulation on blinds control. The blinds control is 
in EnergyPlus, and we use Radiance to calculate the illuminance in the 
space.

One of the blinds control algorithm that we tested uses the vertical 
illuminance at the exterior facade. As you said, there is no easy way to 
calculate this in EnergyPlus. So we need to have another Radiance run 
just to calculate the vertical illuminance at the external facade. I am 
not happy with having to call Radiance twice, because it adds a bit of 
complexity and a bit of time to the whole scheme. So that is why I was 
wondering about simple calculation of vertical illuminance if we know 
the total solar irradiance on the surface.

So I dig further. The file below contains some graphs with simple 
regression on irradiance and illuminance on external facade:
https://www.dropbox.com/s/mqs4vycp1vc01w7/vertical_illum_solar.zip


The irradiance is the total solar incidence (direct, diffuse plus ground 
reflection) calculated by EnergyPlus and the vertical illuminance is 
calculated by Radiance (gendaylit and rtrace). They both came from the 
same weather file (Boise TMY3 EPW).

Apart from anomalies on the some points on West and North facade, the 
slope is about 100-110 lm/W. I will dig deeper to look at some outliers 
in the data.

Any thoughts?

Ery



On 03/20/2013 05:54 PM, Andrew McNeil wrote:
> Ery,
>
> German is correct.  179 lumens/watt is the efficacy for uniform 
> distribution of visible light.  And only useful after you've converted 
> to visible energy.
>
> The efficacy for the solar spectrum varies based on properties of the 
> atmosphere (which is constantly changing).  The efficacy for direct 
> sun is generally 80-120 lumens/watt.  The efficacy for sky is 
> typically 100-140 lm/watt.  The efficacy of light on a facade is 
> further complicated since it is a constantly changing mix of direct 
> and sky.
>
> Sorry I can't give you an easy number!
>
> Andy
>
>
> On Wed, Mar 20, 2013 at 3:36 PM, Germán Molina Larrain <gmolina1 at uc.cl 
> <mailto:gmolina1 at uc.cl>> wrote:
>
>     Dear Ery,
>
>     the 179 is the magic number just for Radiance... is the Luminous
>     Efficacy. My understanding is that the Luminous Efficacy of the
>     clear sky is around 100; but it vary.
>
>     My answer would be NO. But wait for others, there might be some
>     hope yet.
>
>     Regards,
>
>     German
>
>
>
>     2013/3/20 Ery Djunaedy <ery.mailinglist at gmail.com
>     <mailto:ery.mailinglist at gmail.com>>
>
>         Folks,
>
>         This may not be a relevant question for Radiance list, but I
>         ask it anyway.
>
>         If I have the solar incident (irradiance, direct and diffuse)
>         on the external facade, is there a simple way to
>         calculate/estimate the illuminance on the facade? The magic
>         number 179?
>
>         Thanks,
>
>         Ery
>



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