Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page

Shading calculations with very large walls

(note that I can't add images due to points)

We've been performing some sensitivity analyses on various parameters to a very simple model for the purposes of shading impacts. The results we see typically align with our expectations, however strange behaviour manifests when the height and width of the wall increases.

We vary the height and width X of a wall from between 3 m to 1280 m.

The vertical wall is X m wide * X m high pointing north (southern hemisphere / Melbourne Australia), and a 1m x 1m window is in the wall.

The size of the window remains the same for this study, and its position remains the same in each run - 1 m up and 1 m left of the bottom-left corner of the wall. We model two scenarios - one with no shading fins, and another with shading fins on both sides. Then we collect the incident solar radiation on the window and calculate a ratio between shaded / unshaded results.

While the shading ratio remains 'stable' at small wall dimensions ( 3 m through to 40 m ), this changes as the dimensions increase. See below for an excerpt of the results. Remember - the window and its shading remain the same

W 3 m / H 80 m => 0.720

W 10 m / H 80 m => 0.720

W 20 m / H 80 m => 0.720

W 40 m / H 80 m => 0.721

W 80 m / H 80 m => 0.722

W 160 m / H 80 m => 0.722

W 320 m / H 80 m => 0.746

W 640 m / H 80 m => 0.796

W 1280 m / H 80 m => 0.894

My expectation is that no matter the size of the wall, the shading ratio would always remain the same.

It is difficult to determine what is going on here, though I'm assuming it's a product of the shading algorithm.

This model is being run with the 'FullExterior' Solar Distribution

edit retag close merge delete

Sort by » oldest newest most voted

I wonder if the additional wall area is increasing the diffuse solar reflected from the ground which then enters the window. I would think a ray striking the wall, scattering backwards, and some of that hitting the ground, would never enter the window (ray tracing), but it's worth a look at which components are affecting the window's incident solar. If you haven't already, I would look closer at all these reports.

Zone,Average,Surface Window Transmitted Solar Radiation Rate [W]
Zone,Average,Surface Window Transmitted Beam Solar Radiation Rate [W]
Zone,Average,Surface Window Transmitted Beam To Beam Solar Radiation Rate [W]
Zone,Average,Surface Window Transmitted Beam To Diffuse Solar Radiation Rate [W]
Zone,Average,Surface Window Transmitted Diffuse Solar Radiation Rate [W]

more

Thanks Richard, we've run a few tests based on your suggestion, the notes below based on switching from smaller -> bigger walls

Solar Distribution set to FullExteriorWithReflections -

shaded - ground diffuse decreases, sky diffuse increases (largest change), sky diffuse surface reflected decreases (small change)


Solar Distribution set to FullExterior -

shaded - sky diffuse increases (same amount as with reflections-shaded changes)


Interesting results... thoughts?

( 2019-11-28 21:42:01 -0500 )edit

This is out of my area. I would try to think of a way to verify the total solar striking the surface/window, on a per m2 basis.

( 2019-12-10 09:13:02 -0500 )edit