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PostPosted: Mon Mar 25, 2013 6:41 pm 
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Joined: Mon Apr 16, 2012 7:29 pm
Posts: 1658
PVsyst is not able to give definitive answers when comparing the yield of 2 different modules of same technology.
A detailed analysis of the parameters has to be performed for such an assessment.

Let's limit this analysis to Crystalline modules (mono or poly).

The yield (specific energy production, or Performance Ratio) depends on the STC values, but also on 2 additional parameters Rserie and Rshunt (and also the behaviour of the Rshunt value according to irradiance, i.e. Rsh(0) and Rsh(exp)). These parameters are not mentioned in the Standards and in the usual specifications, but they have a great influence on the Low-light performance, and therefore on the annual yield.
In absence of further information, these values are set at default values in PVsyst.

If Rshunt can be measured (which is very difficult and unreliable for crystalline technology, when using flash-test I/V curve data), it may be specified. But manipulating the defaults for Rshunt irradiance evolution is not advised, as these values seem to reflect a rather "stable" behaviour.

The main effect on the low-light behaviour of the model is the Rserie value, which is not directly measurable (see I can't specify my measured Rserie).
When not available, PVsyst chooses a default value which corresponds to a pre-defined Gamma value (diode ideality factor) when solving the one-diode model equations.
According to my early outdoor measurements on some modules, this value is presently set to Gamma=1.35 for poly and 1.30 for mono, which leads to poor low-light efficiency performances.
After comparisons of the one-diode model with the US Sandia model (based on outdoor measurements) on several modules, I discovered recently that this value should be reduced to about Gamma = 1.10 to 1.15. This gives relative low-light efficiencies (by respect to STC) between -0.5% to 0% under 800 and 600 W/m2, and therefore an increased annual yield (about 2% to 3% higher than the previous case, depending on climate).

Now when analysing Low-light efficiency data provided by the manufacturers (measured with flash-tests by 3rd-party institutes ), it appears that these efficiencies are still higher, usually around +0.1 to +0.4% for 800 and 600 W/m2 by respect to STC. This discrepancy between models established on outdoor and indoor measurements are not well understood. They may be partially attributed to the fact that the flash-test uses full beam component, when the outdoor have a part of diffuse component, depending on the irradiance and suffering of IAM effect.

Therefore in the PVsyst database:
- either the Rserie/Rshunts are not specified, and established to Default values (i.e. checkbox "default" checked). This gives under-estimated performances in the present version 5, but will be improved in the future version 6, by requiring Gamma values = 1.10 and 1.15.
NB: These modified values may already be defined in the version 5 in the "Hidden parameters".
- or these parameters are specified by the manufacturers. In this case we don't accept Gamma values below 1.10, unless the manufacturer can provide an assessment using Low-light efficiencies, measured by an independent institute.

Other parameters may give deviations between modules:

- The "Module quality loss" parameter takes half the lower tolerance as default value.
This will induce a difference of 1.5% between modules with +/-3% tolerance, and positive-sorted (-0/+5%) modules.

- Some modules are defined with STC values Imp*Vmp higher than the nameplate specification (for taking positive-sorting into account). The simulation will use the Imp*Vmp for calculation, and the PNom for reference, resulting in overestimated indicators like Specific production or Performance Ratio.

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