PR versus. PR weather corrected

[daniel.luzzi]

Greetings,

 

When I read the information provided in Project design > Results > Performance Ratio  PR, it is clear that the PR is calculated considering the STC values (25°C, 1000 W/m²) of the system, as cited below:

 

"The Performance Ratio is the ratio of the energy effectively produced (used), with respect to the energy which would be produced if the system was continuously working at its nominal STC efficiency. The PR is defined in the norm IEC EN 61724."

 

So, if I'm calculating the PR from a solar plant and comparing with the simulation, during a year long basis, should I consider the Module Reference Temperature equals to 25°C?

 

For me it is not clear if PVsyst uses the monthly average array temperature for calculating the PR as the reference or the 25ºC. One of the main reasons to question it is because of the following text: "Mathematically, if the  TArrayAver is calculated with the same data, the  yearly PR(corr) value should be equal to the yearly PR.  " In my opinion, I can't understand how is it possible if we are comparing different TArray.

[André Mermoud]

Please remember the basic "function" of the PR: this  represents all the losses in your system, from a  reference input energy to the final yield E_Grid.

The  reference input  energy ("nominal theoretical")  is the energy you would have with the incident irradiation in the collector plane  (Globinc [kWh/m2] * Array area [m2]) multiplied by the power which would be produced by your PV module when working at STC (W per kW), which is defined as the nominal (nameplate) power of your PV array. For this definition the temperature is obviously the STC temperature (25°C).

Now the operating losses include the loss due to the array temperature. This is another thing which acts on the PR value. Normally you will have a higher PR in winter than in summer.  The PR "corrected for temperature" is a tentative proposed by the NREL for the evaluation of the PR that you should find when you are measuring it on a little period in the year.

This correction cannot be absolute,  as it is based on an average TArray during a given period, which is not exactly the hourly TArray. However when calculating the PR corrected  over the whole year, it should be equal to (close to) the global PR, which involves the instantaneous array temperature.