Mismatch Loss Parameter; Irradiance and Temperature

[Dan Nicksy]

Good afternoon all,

As a starting point, I have read and understand the post "How to define the "mismatch loss" parameter" in the FAQ section of the Forum (http://forum.pvsyst.com/viewtopic.php?f=29&t=47&p=47&hilit=mismatch#p47).

In doing some independent research, I have come across the idea that mismatch in a PV array is due not only to differences in current and voltage properties of individual modules in a string, but also caused by variations in irradiance and temperature in different parts of the array. I have two questions:

1) Please confirm that this irradiance and temperature mismatch is not accounted for in the PVsyst "Detailed Calculation" of mismatch button.

2) If I decide to include some amount of mismatch for irradiance and temperature, is it appropriate to include it in the Mismatch Loss parameter, or is there a more suitable way to include it in the simulation?

Thanks,

Dan

[André Mermoud]

In PVsyst, the mismatch parameter is a constant parameter that you can set at any desired value.

Now in the explanations I have not mentioned this mismatch in irradiance on a big array. You can include it if you want.

However please observe:

- The mismatch in important when acting on the current, i.e. in a given string. But between strings in parallel, the mismatch loss is negligible. Therefore this contribution should only concern short-distance irradiance discrepancies.

- The cloud's effects are transcient phenomena. Their duration may be considered as negligible when calculating averages over hourly values.

- Such variability may be accounted in the meteo data uncertainties, especially in case of unstable conditions.

[anjana961]

Hi all,

I would like to know how PVsyst calculates the efficiency of module at STC and non STC conditions from corrected Rshunt values.This is for the purpose of calculating irradiance loss

[André Mermoud]

PVsyst uses the one-diode model with slight modifications (especially an exponential behavior of the Rshunt as function of the irradiance).

See the Help "Physical models used > PV module, model description".

[anjana961]

Thanks Andre,

I would like to know how exactly do i calculate the values of EArrayRef and EArrayNom because when i manually calculated these values using formula given in PVsyst help i didnt get my answer matching exactly with values generated by PVsyst i.e with a difference of abt 5000 when the value was in 10 million.

To the above mentioned two values what formula can i apply to calculate irradiance and thermal loss?

In one calculation, I calculated the values Iph(photocurrent) and V ( output voltage) from panel using following calculations

Iph(G, T) = (G/Gref)· [iph,ref + μIs c· (T − Tref )]

V = (Vmppref + tempcoeffof VmppSTC.(Tc-Tref))

where Tc- calculated cell temperature

Tstc - cell temperature at reference conditions

Vmmp - max power point voltage at STC

μIs c - temperature coefficient of Isc

Then i calculated Output power from array by using above calculated values (Iph*V) and tried calculating thermal loss by applying temp coeef at Pmpp but later found that this output power that i have calculated already incorporated deductions in irradiance and thermal loss.Now that i have effective input irradiance to array and this output power, how do i obtain split up of irradiance and thermal loss that occurs in between my input and this output power that i have calculated.

[André Mermoud]

PVsyst uses the one-diode model.

It is very difficult and hazardous to get satisfying results with such simplified models.

[anjana961]

Hi Andre,

I would like to know whether E_Array_Ref value is the array output before the occurence of irradiance and thermal loss.Which of the two values (E_Array_Ref or E_Array_Nom) is used as base to calculate the losses that come after panel level in a grid connected system.

[André Mermoud]

The EArrRef is the STC energy according to the "nameplate" reference power of the modules. This value is used as reference for the PR calculation.

The EArrNom is the result of the one-diode model at STC. It should be the same, but may have a slightly higher value than the EArrRef. It is used as starting point in the Array Loss diagram.

See our FAQ Why is the Pmpp of my module different than the specified value ?