Incident solar diffuse radiation on the floor

edit

Oh, but you said "incident", these are what's absorbed. So, you could just divide these by the floor surface solar absorptance. The problem with the diffuse is that there's the initial diffuse from the windows, but then there's diffuse reflections from all surfaces in the zone. The algorithm tracks what's absorbed but never really adds up the incident diffuse.

Yes E+ never adds up the incident diffuse... also I cant divide above output by floor surface solar absorptance because of the fact that E+ calculate it in different and a bit complex way. May I suppose all transmitted diffuse solar radiation will fall on the floor? Is consequent error big? My zone is a greenhouse with 4080 m2 floor area and 6m height. all surface are glass.

No, you cannot assume that all transmitted diffuse solar will fall on the floor. The entering diffuse from any given window is distributed by area to all other surfaces in the zone (including other windows) except for surfaces facing in the same direction. For example,the diffuse entering through a south-facing window will not cast any diffuse on other south-facing surfaces.

However, it is true that Surface Inside Face Initial Transmitted Diffuse Absorbed Solar Radiation Rate divided by the surface solar absorptance is equal to the initial incident diffuse before allowing for reflections.

That's completely true. I set solar absorptance to 0.9999 then I sum beam and absorbed diffuse and compare it with above way, the answer was same. Thank you