Radiance and solar gain factor
pmr
I am actually using Diva for Rhino&Grasshopper to study the impact of various shading system geometries on my facade. Since glass materials are only defined by 3 transmission values (R,G,B), I would like to know how I could use solar control glass (defined by 2 values, light transmission and solar gain) during my analysis. In fact, I am finding it quite hard to understand what radiation maps really represents: is the part of energy contained in the invisible spectrum displayed?
Thank you very much,
Max D
yes, as far as I know Radiance will consider the total spectrum,
however solar control glazing would be ignored when the -i switch is
used; that is the case when running irradiance studies; in fact, all
glass materials are ignored, as are trans materials and others I
cannot recall just now. That's just the logic of the engine, as the
daylighting components of DIVA deal with the visual spectrum.
Decreasing visual transmittance would attenuate light intensities, but
it would not necessarily reflect the way a given window behaves from a
thermal point of view, due to, e.g., the order of coatings within an
assembly. You'd need to use EnergyPlus simulations to really get a
grip on that!
Best wishes,
Max
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J. Alstan Jakubiec
I had to think about this one for a little while. For what it's worth, the
documentation language in rpict is extremely difficult to understand:
> −i
> Boolean switch to compute irradiance rather than radiance values.This
> only affects the final result, substituting a Lambertian surfaceand
> multiplying the radiance by pi. Glass and other transparentsurfaces are
> ignored during this stage. Light sources still appearwith their original
> radiance values, though the −dv option (above)may be used to override
> this.
So what does this mean? Anything seen /through the window directly/ will
not account for the widow, but the distribution of irradiation inside the
space will account for the transmittance properties of the window. You can
clearly see this effect by running two radiation map images inside of a
space, looking out the window: one with glass and one without glass. What
is seen through the "window" is identical in both cases, but the
distribution of light in the room is completely different.
That said, we can return to Pierre's original question. Can you use this
to find the spatial distribution of cumulative heat gains inside a space?
Well, yes and no. You would need to completely redefine your material
properties so that R, G and B are defined in relationship to the
short-wave radiation incident from the sun/sky rather than the visible
spectrum. This is particularly important for glass which may have a 60%
visible transmittance and a 20% solar heat gain coefficient.
One major limitation here is that there are only three color channels,
rather than a continous spectrum like you might appreciate in such an
analysis.
Best Regards,
Alstan
PS. Pierre, as we have closed the DIVA-Google-Group, if you'd like to
continue this thread, please do so on www.diva4rhino.com.