Confusion Regarding Irradiance Calculation

I 'm trying to understand the irradiance calculations but there seems to be some errors between the NREL SAM software results and the formulas shown in  www.nrel.gov/docs/fy18osti/67399.pdf . 

• I'm running a simple fixed axis PV simulation No financial model.
• For my location/resource I am using 1 location phoenix_az, and my Albedo-sky diffuse model uses Perez and i've selected 'DNI and DHI' from the 'Weather File Irradiance Data' section.
• I've selected SunPower SPR-X21-335 from 'CEC Performance Model with Module Database'. I've defined a custom inverter using 'Inverter Datasheet' option. My parameters are: 400Wac, 100% efficiency, 240Vac nom Vac, 60Vdc max, 18Adc max, 10Vdc min mppt, 35Vdc nom, 60Vdc max mppt, 1 mppt input, 0W of consumption at night and i'm using the default temp derate curve. 
• My system design have 1 inverter, 1 module per string, 1 strings in parallel in subarray, fixed axis, 20deg tilt, 180deg azimuth, 0.3 GCR
• No changes to 'Shading and Layout' tab
• Set all losses to 0 
• No changes to 'Grid Limits'

There are two main areas of confusion from the irradiance results when I compare them to the Technical Reference Document ( www.nrel.gov/docs/fy18osti/67399.pdf ). 

1. From the doc the eq'n for I the nominal POA irradiance is I=Ib + Id + Ir. From the Simulate results from SAM i'm assuming Ib is POA front beam irr after shading and soiling, Id is POA front diffuse irr after shading and soiling and Ir is the front total irr after reflection. However from NREL SAM software my values at 8am are Ib=225.612 W/m2, Id=55.2022W/m2, Ir=248.651W/m2 and I=280.814W/m2. Shouldn't I=529.4652 using the formula from the doc ?
2. G is defined as the POA total irr after shad and soiling in the doc (poa_eff) which from my SAM results are 280.814W/m2 which is also the same as I above. The doc also has eqn's for G=Gb + Gd + Gr where Gb=GbshadO, Gd=GdshadO, Gr=GrshadO, Gbshad=IbSbnsS1xssSbss, Gdshad=IdSdnsSdss, Grshad=IrSrss. however in my SAM results Sbns, Srss, Sdss, Sgss and Sbss are all 0 so therefore G, Gb, Gd, Gr, Gbshad, Gdshad, and Grshad should also be 0 not 280.814W/m2 from my results. Why G not 0 ? For the factors i'm taking my results from SAM and assuming Sdss is self-shading non-linear sky diffuse irr factor, Sgss is self-shad non-linear ground diff irr factor, Sbss is self-shading linear beam irr factor, O is soiling beam irradiance factor, Sbns beam irr shading factor, Srss ground diff factor/self-shad reflected derate.

If you could explain questions 1 and 2 to me that would be great because I don't understand based on the SAM results. 

Hi David,

For #1, you should be able to verify the following for the irradiance incident on the front surface of the modules in the array:

Subarray 1 POA front beam irradiance after shading and soiling (W/m2) +
Subarray 1 POA front diffuse irradiance after shading and soiling (W/m2) =
Subarray 1 POA front total irradiance after shading and soiling (W/m2)

The "after reflection" variable shows the irradiance incident on the cell after it passes through the module front glass cover.

As explained in the "Nomenclature" section of the reference manual, the variable names "I" and "G" are to distinguish between irradiance at on the module cover surface before and after soiling and shading. SAM's "POA after shading and soiling" output variables are equivalent to the "G" variables in the manual, as shown in Table 7. The table also shows that the "POA front total irradiance nominal" output variable is equivalent to I. SAM does not report Ib, Id, or Ir as output variables.

The shading and soiling losses inputs in SAM are given as percentages that are converted to fractions in the code as shown in Equation 7.4. If the soiling loss inputs are set to 0%, they are treated as a fraction of 1.

Best regards,
Paul.

Is there a reason Ib, Id and Ir are not provided in SAM ? Are they provided in the python module Pvwattsv1 or would the only way to obtain these values would be to use the formulas in the document ? 

Which irradiance is the one which is actually used in the single diode model params function and which one is used when finding the PV cell temperature ? Using NOCT cell temp model it has G which is the effective irradiance transmitted to cell so would this be the POA front total irradiance after shading and soiling or the after reflection or are different irradiances used for different things  ? In my case I only care about a fixed axis setup as described in my question.


I have a separate question as well. I'm currently using Pvwattsv1 for the SAM simulations but I'm wanting to speed up the simulation. Is there a way to speed up the python based simulations if I am running multiple simulations for different panels and locations ? 

Hi David,

Those variables are not reported as SAM outputs because they are intermediate variables, and we thought it would be confusing to have even more irradiance variables in the outputs than there already are.

The "effective" irradiance values shows as "G" in the manual are the inputs to the module model. That is indicated in Table 9.

That irradiance is adjusted by the module model as shown in Equations 10.20 and 10.25.

I think you meant to type pvsamv1 instead of pvwattsv1. I don't think there is a way to speed up the SSC module runtimes.

Best regards,
Paul.

Ok thanks. Last thing, is the final G value in eqn (10.25) which is used in (10.26), reported/saved in either SAM or pvwattsv1? 

So is that final G value (irradiance transmitted to cell) which is used in 10.26 and 10.37 not reported by SAM ? Is the only way to actually know this value would be to use the formulas in the doc and do it by hand ? If it isn't reported is there any plan to add it as an output ? I think it would be quite useful to actually know that final value.