Calculating Voc - How is irradiation taken into account

[Andrew]

To calculate Voc, we've trypically taken the cell temperature into account (which is related to irradiation and ambient temperature). In a current project the inverter manufacturer is recommending that in addition we should be considering the impact of irradiation on Voc following the relationship V0c = kT/q*ln(I_L/I_0+1).

Does PVsyst consider this impact in calculating the Voc under minimum ambient site conditions, and if not is it valid to consider for furture versions?

In my current project it makes the difference between 29 modules per string and 30.

Many thanks,

Andrew

[André Mermoud]

In PVsyst the Voc(temp) is calculated by the one-diode model.

The expression given here is a direct result of the "pure" one-diode model: you can easily get it by simply writing the one-diode model equation for I = 0

PVsyst adds some corrections to this "pure" model, so that the result may be slightly different.

[Andrew]

Thanks Andre,

To confirm, is the Voc for this check calculated at 1000W/m2? And can this be set for other irradiation levels in PVsyst?

If not, is it valid to use the irradiation correction below to test the Voc at different irradiation levels? Considering that at the sites minimum temperatures, the irradiation is likely to be low.

[Pablo]

Hello all,

I would like to confirm if PVsyst considers an irradiation of 1000 W/m2 when V0c is calculated? Normally the lowest site temperatures are done at low levels of irradiation.

Is there any way to set other irradiation value? This parameter has a lot of influece on the number of modules per string, type of tracker structures and inverter selection, so it would be very interesting that PVsyst takes it into account, specially in Utility-Scale projects.

Thank you so much.

[André Mermoud]

This is a worst case choice.

Please carefully read the FAQ post How to adjust the design temperatures ?

[Pablo]

Hello André,

According to the post you sent me, it sais that the Voc(Tmin) requirement corresponds to:

- Choosing 1000 W/m2 means that in worst case, in winter the sun may suddenly appear on the array (cloud), and in this case the cell's temperature may be the ambient temperature.

These choices are a universal common practice, adopted by everyone (not only PVsyst users).

However I don't know whether they are mentioned in an IEC norm about system sizing.

As I understand, PVsyst takes into account the coldest day of winter and considers that the minumum temperature will be during the day while the sky is cloudy. When there are clouds the irradance is not zero, so the cell temperature will not the same as the ambient temperature according the theorical formula (according to IEC 60891). The cell temperature will be the same just when there is no sunlight (or very low irradiance).

On other hand, to suppose that the irrandiance will be 1000W/m2 when the minimum ambient temperature is done, is a consideration very conservative. It will depend on each placement and it would be interesting that the irradance could be changed for the maximum Voc scenario (like the reference low temperature can be modificaded).

The IEC 60891 tells the procedure for temperature and irradiance corrections, so I consider that PVsyst should allow to change the irradance for the maximum Voc calcule. With a historical meteorological data is easy to know the irradance at the lowest temperatures.

Thank you in advance.

[André Mermoud]

The Voc(Tmin) requirement corresponds to a safety requirement.

The procedure used in PVsyst for the evaluation of the max. voltage is a universal common practice, adopted by everyone in the PV community (not only PVsyst).

We will certainly not modify this procedure.

See the FAQ How to adjust the design temperatures ?.

[Pablo]

Hello André,

I understand that PVsyst estimates the maximum voltage taking into acccount a safety scenario, even I could be agree with you in calcule this value as default as Voc (Tmin=-10ºC,Gmin=1000W/m2). However it is an unreal scenario which never will happen and you cannot justify these calcules saying that is a common practice in the PV community. If you take any meteorological database of the last 50 years and you calcule teh voltage hour by hour(doesn't matter the location), you never will see a scenario so pessimistic whose lowest voltage is reached at 1000W/m2...

To do the design for the worst scenario contemplated in the meteorlogical database is a very good safety requirement.

I don't expect that PVsyst changes their criteria to calcule the maximum Voc, it is logical that you consider conservative values by default (-10ºC, 1000W/m2). I am just asking you to include the option to change the default Irradiation used for the max. voltage calcule. In the same way you allow to modify the reference low temperature in the Project's parameters, I think that could be interesting alow it because the IEC norm (IEC 60891) explains the procedure for temperature and irradiance corrections.

Thank you very much in advance,

Pablo.