SAM's calculation of efficiency for bifacial PV arrays

Dear SAM staff, 

How does SAM calculate the "Subarray 1 Module efficiency | (%)" shown in "Data Tables", for the case of bifacial arrays?

When simulating a scenario with the "module is bifacial" option checked, and then running the simulation again with that option unchecked, SAM provided very similar efficiency levels (difference < 0.02% on average). Therefore, it seems that there is a calculation to apply the proper power balance, considering "total power" and "total irradiance".

I tried using the equation below (efficiency and effective irradiance), but I did not get the same results as those reported by SAM.

η = Pmp / (A.GE) , with GE = Gfront + φ.Grear

I would appreciate if you could explain the calculation, or point out a reference paper / method.  

Thank you. 

Hello,

SAM reports the module efficiency to give you an indication of how well the module performs. SAM calculates this output by dividing the module power by the solar irradiance incident on the module surface. It does not use the module efficiency to calculate the module power.

For monofacial modules, you can replicate the calculation of module efficiency using simulation results:

Subarray 1 Module Efficiency (%) = Subarray DC Power Gross (kW) * 1000 W/kW / [ Module Area (m2) * Number of Modules * Subarray 1 POA Front Total Irradiance after Shading and Soiling (W/m2) ] * 100%

For bifacial models, SAM does not report the rear-side incident irradiance value required to replicate the calculation.

We will address this issue in the Fall 2023 version of SAM. See github.com/NREL/SAM/issues/1418 to track our progress.

Thank you for reporting this problem and for your patience while we researched its cause.

Best regards,
Paul.

Dear Paul, 

Thank you very much for adding this topic to the Forum - and for the reply.

Regards,
Caio