open-circuit voltage

[Dfarid]

Hello ,

I'm encountering an issue with calculating the theoretical open-circuit voltage of my PV system. The formula I'm using results in discrepancies of over 5% specifically for low irradiance conditions (below 200 W/m²), where the calculated voltage is consistently lower than the measured one. Interestingly, for higher irradiance levels, the results are accurate.

The formula used :
Vtheo = m * Vostc * [1+Beta * (Tcell-25)] + Ncell * m * 8,62 * 10^-5 * (Tcell + 273) * ln(G/Gstc)
m : NO. of Modules

I've already checked the units and ensured that all parameters are correctly inputted.

Do you have any suggestions on how to address this problem? Are there additional considerations or factors I should take into account?

Thank you in advance for your assistance!

 

[DougfromdaUP]

Welcome to the Forum.

Five percent sounds pretty close to me. I’d compare your numbers to the results from one of the online calculators, too, before I finalized any plans.

 

[gotbeans]

There's some more info here & a calculator

Maximum open circuit voltage calculator

I'm posting this in a thread here since it might be useful for people. Please keep in mind that the below calculator contains bugs and doesn't do error checking - it's literally been 5 minutes to put together. Ok, so what does it do? It calculates the maximum open circuit voltage you would see...

diysolarforum.com

 

[profesor79]

Hello ,

I'm encountering an issue with calculating the theoretical open-circuit voltage of my PV system. The formula I'm using results in discrepancies of over 5% specifically for low irradiance conditions (below 200 W/m²), where the calculated voltage is consistently lower than the measured one. Interestingly, for higher irradiance levels, the results are accurate.

The formula used :
Vtheo = m * Vostc * [1+Beta * (Tcell-25)] + Ncell * m * 8,62 * 10^-5 * (Tcell + 273) * ln(G/Gstc)
m : NO. of Modules

I've already checked the units and ensured that all parameters are correctly inputted.

Do you have any suggestions on how to address this problem? Are there additional considerations or factors I should take into account?

Thank you in advance for your assistance!

The measurment depends on device class and the range that is used.
in most cases if the measurment is in lower band of the range it can produce more %diff (especially with cheap multimeters) than it's defined in the class.

 

[Dfarid]

Thanks for the quick responses!!

According to the datasheet, the cell arrangement is specified as '132 (2*(11*6)).' Does this mean that there are 132 cells in total, or could it indicate two parallel strings of 66 cells each? I've noticed that inputting N=132 gives issues, but when I use N=66, the values are perfect

 

[RCinFLA]

PV cells shunt leakage resistance consumes a higher percentage of illumination generated current at low illumination levels. On a per silicon cell basis, shunt resistance is typically between 100 ohms for poor quality cells to over 1000 ohms for high quality cells.

When illumination generated current reaches the panel shunt leakage current the panel voltage quickly rises to Voc. Below that illumination level the panel open circuit voltage collapses.

Even same model panels have some variations in shunt leakage resistance between panels. Cells used for same panel are usually selected for matching leakage current.

Wafer defects spots are usually the dominate cause of shunt leakage. Just about all PV cells have some defect spots of varying quantity and severity.

'Low illumination' panels have cells selected for low leakage current. Cheap panels generally have greater shunt leakage cells and poorer low light performance.

Shunt leakage effects becomes more significant below about 200 w/m^2 (20%) full sun illumination.

The PV cells' shunt leakage resistance also prevents you from in-circuit bypass diodes testing with DVM.