Measured RPOA Equivalent

[David Bowersox]

Pertaining to capacity or PR testing, the following equation is frequently referenced for total irradiance calculations:

E_Total = E_POA + E_Rear * 𝜑

...where 𝜑 is equal to the module bifaciality factor.

My question concerns the E_Rear term: if a PV system has a pyranometer installed in the rear plane of array in such a way that it is unobstructed by structures such as the torque tube, and the E_Rear used for the physical PV system is the measured RPOA from this pyranometer, what do you recommend using for the E_Rear term to calculate an equivalent model-side E_Total? Would the E_Rear term be the output variable GlobBak, a combination of factors such as GlobBak + BackShd, or something else? 

 

Reference: Waters, Martin, Chris Deline, Johan Kemnitz, and Jeffrey Webber. 2019. Suggested Modifications for Bifacial Capacity Testing: Preprint. Golden, CO: National Renewable Energy Laboratory. NREL/ CP-5K00-73982. https://www.nrel.gov/docs/fy20osti/73982.pdf

[Auriane Canesse]

From a rear side measurement of the POA, you still have to apply a shading factor corresponding to the torque tube, cables, etc ("structure shading factor") in order to compute how much light effectively reaches your panel. This effective light corresponds to GlobBak.

All relevant irradiance for bifacial calculation are defined here
https://www.pvsyst.com/help/project-design/bifacial-systems/bifacial-systems-results.html

 

[David Bowersox]

Thank you Auriane - just to ensure that I have this correct, it's best to apply the rear-side structural shading factor to the measured RPOA because it is most important is to account for the light that is realistically reaching the rear side of the modules? This equation for E_rear would look something like the following using the measured RPOA...

E_rear = RPOA * (1 - "Structural shading factor")

 

So, please correct me if I'm wrong, but it sounds like it's accurate to say that GlobBak is not by itself analogous to measured RPOA because it has been reduced by a structural shading factor, where an unobstructed RPOA pyranometer would not be. I understand that what's most accurate may be to calculate the incident irradiance on the rear side of the module, but in my experience many engineers desire to modify measured sensor values as little as possible. In such a case, would it be more analogous to combine GlobBak + BackShd so that in the performance analysis both E_rear terms reflect the rear side irradiance before being reduced by the shading factor?

[Rafael Santos]

That's a very interesting topic!

I am also carrying out a performance analysis, with RPOA measurement... Since it is not possible to use it on the .MET file (like GPOA), I am trying to fit the bifacial parameters (structure shading + transparent fraction) running multiple simulations and comparing the results (GlobBak) to the measured GPOA, to see which one fits best.

Apart from the absolute values, all simulations showed the shape curve similar to GHI, while the measured curve showed a shape similar do GPOA:

 

Is this have something to do with the assumption that the re-emitted irradiance is isotropic? This text below was taken from PVsyst help:

 

Or am I doing something incorrect? Maybe "Globak" is not the right variable to compare with RPOA? Even so, would the shape of the curves would be that different?

 

Best regards,

Rafael

[Rafael Santos]

Continuing the question: I am thinking about
 

1 - ignoring this shape difference and count it as uncertainty, or

2 - Consider the module as monofacial, and "boost" GPOA values with the mantioned formula:

E_Total = E_POA + E_Rear * 𝜑 or

E_rear = RPOA * (1 - "Structural shading factor")

 

The second option makes sense? If so, which formula would be more accurate?

 

Thanks!

Rafael