Solar Shading Edge Cases have Strange Results

Hi,

I've been working with the SAM UI's shade modeling using the "3D shade calculator" and directly modifying "beam irradiance shading losses by time step" (PVWatts performance model) and discovered 2 things:

1) Relative to an unshaded solar module, SAM finds that a x% shaded module may undergo <x% of power output loss. This was surprising since I expected each 1% of shading to lead to >1% power loss. I could be wrong on this but....

2) Hardcoding 100% shading on the "beam irradiance shading losses by time step" still leads to significant solar power output rather than the expected 0kW solar output.

I've been experimenting extensively with the UI to make sure I hadn't missed something, but I'm at a loss. Would really appreciate clarity on what's happening here.

Thanks.

Hi Hugh,

The beam shading factors only reduce the beam (direct normal) component of the solar irradiance incident on the array (in the plane of area, or POA). The small amount of power generated by the system when all beam irradiance is blocked is from the diffuse component of the irradiance. To completely block all sunlight from the array in a given time step, you would need to set the beam irradiance shading for that time step to 100%, and also enable sky diffuse shading loss and set the diffuse shading to 100%.

Best regards,
Paul.