Total POA Irradiance not equal to front + back?

I'm attempting use recorded weather data to model the irradiance onto the front and back of a vertical system and compare to the vertical reference cells we have set up.

I'm looking at,
1. "Subarray 1 POA front total irradiance  after reflection (IAM) (W/m2)"
2. "Subarray 1 POA rear total irradiance  after reflection (IAM) (W/m2)"
3. "Subarray 1 POA total irradiance  after reflection (IAM) (W/m2)"

And noticing that 3 seems to be less than the sum of 1 and 2. Is there some additional loss calculation that comes in before the total is calculated?

Hello,

From my initial investigation, it looks like 3 should be 1 + ( 2 * bifaciality factor ):

github.com/NREL/ssc/blob/274560a9eba141154f7dd52a8d3e0fe5690b3b84/ssc/cmod_pvsamv1.cpp#L2393

When I try that equation, I get a small discrepancy between the values I calculate and those in SAM that I think is due to rounding. I'll investigate further and let you know what I find.

Best regards,
Paul.

It seems an odd choice that bifaciality factor would be applied when calculating total POA irradiance rather than afterward when calculating resulting DC output, but good to know. I'll keep that in mind.

It looks like the reason 3 is less than the sum of 1 and 2 is that the bifacial model applies an electrical mismatch loss that is not included in 2.

The rear total irradiance after reflection value is recorded here in Line 1967: github.com/NREL/ssc/blob/274560a9eba141154f7dd52a8d3e0fe5690b3b84/ssc/cmod_pvsamv1.cpp#L1967

And the mismatch loss is applied later, in Line 2064: github.com/NREL/ssc/blob/274560a9eba141154f7dd52a8d3e0fe5690b3b84/ssc/cmod_pvsamv1.cpp#L2064

I think the reasoning for this is that although the model applies the electrical mismatch loss to the rear-side irradiance, because it is an electrical loss, it does not make sense to show its effect on the rear-side irradiance.

Best regards,
Paul.