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How to adjust the Voc temperature coefficient ?


André Mermoud
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The temperature Coefficient of the open circuit voltage muVoc is not an input parameter, it is a result of the one-diode model.

It is closely related to the choice of the Rserie value.

You could try to choose the RSerie value giving your specified coefficient, but it is not always possible (the Rserie values may be out of its allowed range "0 to Rsmax"), and it is not a good idea, as other criteria are much more significant for the choice of Rserie (namely the low-light efficiency).

NB: Rsmax is the maximun possible value allowing the curve to pass through the 3 reference points (i.e. solving the equations).

It is not possible to adjust at the same time a specidfied muPmpp value (temperature coefficient on Pmpp) and muVoc. This is a property of the one-diode model.

And the muPmpp value is very important, as it is involved in the operation during the simulation.

While the muVoc is not implied in the simulation process. It essentially acts on the sizing limit for the determination of the maximum possible voltage (which should not overcome the VmaxAbs of the inverter, nor the Vmax of the modules, for the lower design temperature conditions).

You have the choice of using two different values during the system sizing:

- by default, the muVco as result of the model (which depends on the temperature)

- if you define the muVoc as specified by the manufacturer in the module's parameters, you can ask for using this value during the sizing process ("Project's settings" button in the project's dialog)

NB: you will find the "manufacturer" specification for muVoc value in the PV module definition dialog, page "Additional data > Secondary parameter"

NB: In some special conditions (namely with modules specified with a very low gamma value, i.e. Rseries very close to its maximum), for calculation reasons the the lower value of the diode saturation current is limited to 0.1 pA. This limit is sometimes attained with very low temperatures (< 0°C), which leads to positive muVco values. Don't worry: the model still works correctly, only the temperature behavior below this temperature is slightly affected !

We are here at the boundaries of the one-diode model. If you want to decrease this low-temperature limit behaviour, you should increase a little bit the Voc (STC) value, and readjust the Rserie for getting the equivalent Low-light behaviour. This will result is a more robust model.

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