[TRNSYS-users] External shading factor

[Damien Gondre <dgondre.etamine@gmail.com>]

Hello,

I have a question about “external shading factor” defined in TRNBuild. 
Does this factor apply to beam radiation or total radiation?
In others words if I define an external shading factor “Eshad”, a view factor to sky “VFS”, and a window g-value “gw” and if my window receives beam radiation “Ib” and diffuse radiation “Id”, does the total radiation “It” that is transmitted through the window is:

1)    It = (Ib*(1-Eshad) + VFS*Id) * gw (?)
Or
2)    It = (Ib * (1-Eshad) + Id*(1-Eshad)*VFS) * gw (?)
Or
3)    It = (Ib+Id) * (1-Eshad) * gw (?)

Equations 1) and 2) make sense to me but I don’t know which one is correct (if one is actually correct). 3) would be more surprising (because the VFS doesn’t apply in this case). 

I looked at the user guide and the Type 56 mathematical description but I couldn’t find any answer. I also looked at the Trnsys-users archives and even if external shading factors have been discussed several times I didn’t find the answer to my question.

Thanks,

Damien Gondre.