Transmission Fraction Definition

Hello,

I am wondering if I can get some clarification around the definition of "Transmission Fraction" as defined on the Module tab of a bifacial PV design.

This is part of the definition from the SAM software help file:

"The ratio of array area that allows light to reach the surface behind the array to the area that blocks light, expressed as a fraction between 0 and 1. The array area is the total area occupied by the modules, including space between the modules.... NOTES The transmission fraction is by definition related to the ground coverage ratio (GCR), so you should verify that the GCR input is consistent with the transmission fraction. SAM does not automatically verify that the two inputs are consistent."

This is the definition from SAM help slide deck from NREL (link:  sam.nrel.gov/images/webinar_files/sam-webinars-2018-pv-bifacial.pdf )

"A fraction between 0 and 1 specifying the percentage of array row area (including through the modules) that allows light to transmit through
from front to rear."

So my question is, does transmission fraction include the east-west space between tracker rows (i.e. the space defined by GCR) or is it only the ratios of total area and opaque area within one tracker row?

I am trying to determine a proper assumption for this value, and if it does include the east west space the value might be something like 25-35%, whereas if it's only the area within one tracker row it would be much lower (2% or less). 

Thank you,
Kyle Dean, P.E.

Hi Kyle,

Sorry these descriptions are not clearer. I will revise the description in Help for the next version of SAM (Fall 2021).

The transmission fraction is meant to account for light that may pass through or around the modules in each row, so it should not be calculated based on the row spacing. The bifacial model calculates the space between rows based on the GCR and the geometry you specify on the Shading and Layout input page.

You can explore the source code for the calc_rear_side() function here: github.com/NREL/ssc/blob/573f3316d2b871328ec23cfcff8b7abe7fa8513f/shared/lib_irradproc.cpp#L2456 .

Best regards,
Paul.