Reference Conditions are not equal to STC -> Temperature Correction on Gamma does not work.

[marianne.hartung]

I am trying to create a PAN File for a PV module where I don't know the IV curve at STC. My Gref and Tref in the "Definition of a PV module > Basic data" sheet are 800 W/m² and 45°C. In this case it seems that the "temperature correction on Gamma" tool does not work correctly. I will give you an example so that you will be able to reproduce my problem. I select "Tools>PV modules" and in "Choosing a PV module" I choose "50 Wp 25 V, Si-poly, CS 50, Centennial Solar, Photon Mag. 2009". In a first step I retreive the performance of this module at 800 W/m² and 45° from the "Definition of a PV module > Basic data > internal model result tool". I obtain Impp = 2.2 A, Isc = 2.42 A, Vmpp = 16.3 V, Voc = 20.2 V, Tc = -0.49 %/°C. Now I try to create a new PAN File for the CS 50 module based on this information. In "Definition of a PV module > Basic data > Manufacturer specifications or other Measurements" I enter Gref = 800 W/m², Tref = 45°C, muIsc = 1.2 mA/K, and the values for Impp, Isc, Vmpp, Voc. Then I go to the "Definition of a PV module > Model parameters > Temper. coeff" sheet and check that the temperature coefficient at STC is set to -0.43%/°C. The corresponding "Gamma temp. coeff" is -0.04 %/°C. If I go back to "Definition of a PV module > Basic data > Internal model result tool", I get -0.44 %/°C at 1000 W/m² and 25°C. First I suspected that this special set of one diode model parameters somehow resulted in a minimum temperature coefficient of 0.44 and smaller values could not be achieved. Therefore I tried putting -0.44%/°C in the "Definition of a PV module > Model parameters > Temper. coeff" sheet. I expected that the gamma temperature coefficient should not change and I would again get the -0.44 %/°C in the "Definition of a PV module > Basic data > Internal model result tool". However, this was not the case. For -0.44%/°C in the "Definition of a PV module > Model parameters > Temper. coeff" I get a "Gamma temp. coeff" of -0.07%/°C and a temperature coefficient of -0.45 %/°C at STC from the "Definition of a PV module > Basic data > Internal model result tool". Can you help? Thanks a lot in advance!

[André Mermoud]

First, I observe that your values are quite close.

However I can make several comments:

- A model is never the reality. Especially the one-diode model is highly non-linear.

You establish it at STC (1000 W/m2 and 25°C) and at other conditions (derived from the model), with probably rounding errors in the Isc, Voc, Imp, Vmp values. Therefore this is not exactly the same model's parameters.

- The true Pmpp value is not perfectly linear with the temperature: that is, muPmpp it is not constant with the temperature.

This is a result of the model, and also the reality (please have a look on official measurements reports of TÜV or others if you avail).

- The muGamma correction is a little "ad hoc" correction just for matching the muPmpp of the model to the specified value. In you case a discrepance of 0.04% to -0.07% is really very low.

Personally, I am really surprised that the one-diode model behaves with such an accuracy in this case !!!

[marianne.hartung]

Thanks for your answer. Unfortunately, I think that the observed effect is not a rounding error but a problem with the "gamma temperature correction" tool. It works fine, if reference conditions are at STC. Otherwise problems occur. The example below should explain this. I am using the same PAN File as I have explained in my previous post. It is based on the "50 Wp 15 V, Si-poly, CS 50, Centennial Solar, Photon Mag. 2009" PAN File, but the reference conditions are 800 W/m² and 45°C. The inputs for Imp, Isc, Vmp, Voc and muIsc have been adjusted accordingly (cf. to my previous post.)

I try different values for the temperature coefficient at STC in the "Definition of a PV module > Model parameters > Temper. coeff" sheet. This is what I get:

specified Tc ------> resulting gamma ------> resulting Tc (Internal model result)

-------------------------------------------------------------------------------------------------

-0.43 %/°C -------> -0.04%/°C -------> -0.44%/°C

-0.44 %/°C -------> -0.07%/°C -------> -0.45%/°C

-0.45 %/°C -------> -0.09%/°C -------> -0.46%/°C

-0.46 %/°C -------> -0.11%/°C -------> -0.47%/°C

-0.47 %/°C -------> -0.14%/°C -------> -0.48%/°C

-0.48 %/°C -------> -0.16%/°C -------> -0.50%/°C

The first column of the table is the temperature coefficient at STC that I specify in the "Definition of a PV module > Model parameters > Temper. coeff" sheet. The second column of the table is the resulting "Gamma temp. coeff" from the same sheet. The third column is the temperature coefficient from "Definition of a PV module > Basic data > Internal model result" tool at 1000 W/m² and 25°C.

I understand that the temperature coefficient is not constant with temperature. However, in the example above both temperature coefficients are at STC. In the "Definition of a PV module > Model parameters > Temper. coeff" sheet one has to specify the temperature coefficient at STC. In the "Definition of a PV module > Basic data > Internal model result" tool I selected 1000 W/m² and 25°C.