Which are differences in yield between version 6 and version 5 ?

[André Mermoud]

In the version 6, the models have in principle not been significantly changed.

However several modifications of default values may explain significant differences in the final results:

Defaults values

Transposition model

In the previous versions (up to Version 5), PVsyst proposed the Hay transposition model as default as it was judged more "robust" than the Perez model. In a recent study, Pierre Ineichen found that the Perez model is giving slightly better results (in terms of RMSD of hourly values) in any case (see "Global irradiance on tilted and oriented planes: model validations", P. Ineichen, 2011). Therefore the Perez model is proposed as default in the version 6. The Perez model gives yearly values significantly higher than the Hay model, of the order of 0% to +2% depending on the climate and the plane tilt.

PV module: Rserie parameter

Up to Version 5, the default Rserie value was chosen in order to obtain a gamma value (Diode ideality factor) of 1.30 for mono- and 1.35 for poly-crystalline modules, according to our first measurements on some modules. This leads to underestimated low-light performances. But:

- By comparison with the Sandia model (obtained by outdoor measurements with dozens of modules), we observed that Gamma should rather be of the order of 1.1 to 1.15.

- The low-light data, measured indoor (flash-test) by different independent laboratories, are compatible with still lower Gamma values of the order of 0.9 to 1.

We still do not understand quite well this discrepancy between indoor and outdoor measurements.

However in the version 6, we fixed the default Gamma value to 1.1, which significantly decreases the irradiance losses of previous simulation (by 2-3% depending on modules and climate).

This will affect all modules for which the Rseries was not specified in the database by the manufacturers. When manufacturers propose enhanced Rseries resistances, we require that they provide independent low-light measurements for assessing the proposed values. See What explains the difference of yield between different modules?

The default gamma value is specified in the Hidden parameters, topic "PV modules". You can change it even in the version 5 if desired.

Module quality and Mismatch losses

In the previous versions the default "Module quality loss" was chosen as the medium value between the lower tolerance and 0. In the version 6, the database also mentions the higher tolerance limit for modules. The default "Module quality loss" is now defined as the quarter between the lower and the higher tolerance. This doesn't change anything for symmetrically defined tolerances, but will provide a negative loss factor (gain) for positive sorted modules (for example -0.75 for a -0/+3% module).

The mismatch loss parameter was previously proposed by default as 2%, corresponding to PV module samples with an Isc dispersion of the order of 5%. Nowadays the PV modules are specified with narrower tolerance limits, and the delivered samples for a given project are often with 2-3% dispersion. Therefore we diminished this mismatch default loss to 1%.

Simulation differences

Losses with derate factors (Module quality, Mismatch, Soiling)

In the version 5, some loss parameters (derate factors) were specified by respect to the STC power, when the result was evaluated as a percentage of the "previous" energy. This gave a discrepancy in the final results, which were higher (by about 10%] than their parameter. In the version 6 the derate loss factors are specified by respect to the "actual" energy and the results are identical to the parameters.

Array Energy calculation

In the simulations of the version 5, the electrical behaviour calculation was done globally for the whole sub-array. Therefore if you had, for example, 9 strings on 2 MPPT inputs, the calculation was equivalent to 2 inputs of 4 1/2 strings.

In the version 6 the calculation is performed for each inverter separately (one with 4 and one with 5 strings). This may induce difference in case of overpower conditions.