PixelCounting also doesn't seem to handle reflections

[sullivj1]

Issue overview

Running a model with shading surfaces around the windows and the solar distribution set to FullExteriorWithReflections I found significant differences in results between the PixelCounting and PolygonClipping shadow calculation algorithms.

Testing found that the reason for these differences was that with PixelCounting there appears to be no reflected solar radiation. When using PolygonClipping I observe expected non-zero outputs for Surface Outside Face Incident Sky Diffuse Surface Reflected Solar Radiation Rate per Area and Surface Outside Face Incident Beam To Diffuse Surface Reflected Solar Radiation Rate per Area, and, as we would expect, if the shading reflectance is increased from 10% to 90% the reflected solar on the window also increases.

Using PixelCounting all the values for these outputs are 0, and the resulting zone temperatures are noticeably colder and are unaffected by ShadingProperty:Reflectance inputs.

This suggests that further warnings need to be added around the use of PixelCounting following the previously identified issues with it and transparency (#9653). Though now this is more an unconditional "if you have significant shading reflections then PixelCounting may significantly underestimate your solar gains and this point I'm not sure anyone should even consider using it".

Details

Some additional details for this issue (if relevant):

• Windows 10
• EnergyPlus v22.1.0
• Unmethours link or helpdesk ticket number

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

• [ ] Defect file added (list location of defect file here)
• [ ] Ticket added to Pivotal for defect (development team task)
• [ ] Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)

[nealkruis]

Thanks for reporting this, @sullivj1. I'll look into this when I can get around to it.

[sullivj1]

Alright, I think I might know what the problem here is. It's not actually that PixelCounting doesn't handle reflections, it's that it's not handling them specifically with regards to mirrored shading surfaces.

My original model had shading around the windows like this, with the front surfaces facing outwards. This works fine with PolygonClipping, but has reflects no light into the windows with PixelCounting.

Reversing all the shading, so that they face towards the window, and PixelCounting now reflects light into the windows, while PolygonClipping works as normal.

[nealkruis]

@sullivj1 Thanks for the investigative follow up! Can you share both of these files with me? That would help me understand what is changing in the source code.

[sullivj1]

@nealkruis Here they are Pixelcounting_shading.zip