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The "Mismatch loss" is mainly due to the fact that in a string of modules (or cells), the lowest current drives the current of the whole string.

Now when installing real modules in the field, the characteristics of each module are never rigorously identical. The parameters (Isc, Vco, Pmpp) usually present statistical distributions, which may be rather Gaussian,  or with a square shape for Pmpp,  if they result from a sorting at the output of the manufacturing process.

See the previous page "Array Mismatch Losses"  for the possible causes and the default values.

Tool for the evaluation of the array mismatch from I/V char4acteristics dispersion

PVsyst proposes a  tool for understanding, and statistically estimating the corresponding power loss (button "Detailed calculation"). This tool first creates a statistical sample of modules, setting Vco and Isc values according to a gaussian or square distribution.  Then it adds the I/V characteristics of each module in each string (add voltages) and then gathers the strings in the array (add currents). Finally it draws the resulting I/V curve of the array, and identifies the MPP value which may be compared to the MPP value of an array with identical modules.

You can observe on these results that while the power loss is usually rather low (due to displacing on the curve), the loss may be much more important when the system runs at fixed voltage.

This loss result is of course very different with each statistical sample (each run of the tool). Therefore a button "Histogram" uses this tool a great number of times and performs an histogram of the Power Loss values with different samples.  

This allows for an estimation of the Mismatch Loss parameter, which has to be specified by the user as an input parameter, and remains a fixed loss during the detailed simulation. This loss will appear on the final Loss diagram. The default value proposed by PVsyst has been chosen at 1% for Pmpp, and the double for fixed voltage uses.

In older versions (V<6.xx) the default value was 2%, but in version 6 we reduced this due to the narrower tolerance range of most modern PV modules. In the new version 7, we passed again to a default value of 2%.

This factor is usually set to 2% in most PV simulation software. In our calculations, this may be justified by the flash-tests uncertainty. But there is no "absolute value" of course.

Now you can sometimes obtain a good knowing of your module sample, by using the Isc, Voc or Pmp distributions for each module, often given by the manufacturers. In the present time it is not possible to import directly these data in PVsyst for the estimation of the resulting mismatch (this will be done in the future).

But you can establish the statistical estimator RMSE of your real distribution  (for example using the Excel function "StDev" in english, or "EcartType" in French), and use it as input parameter for this tool.

 

Mismatch and Ageing

The new tool Degradation tries to take the discrepancies in long-term degradation characteristics between modules into account.

PVsyst proposes a mechanism (Monte-Carlo random process) for this evaluation, but we have never found any study concerning this evolving dispersion and therefore we cannot give any assessment for the proposed RMS values.