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 Post subject: How to establish the "PAN files"?PostPosted: Wed Jul 16, 2014 12:22 pm

Joined: Mon Apr 16, 2012 7:29 pm
Posts: 1662
The "PAN files" hold all the definitions specific for a PV module in the database of PVsyst. These definitions may be either a file (*.PAN), or a CSV string in the EXCEL document representing the PV modules database.

NB: this denomination comes from the very beginning of PVsyst in 1992, when the program was written in French (the file extension ".PAN" stands for "Panneau PV" which is the French word for PV module).

The main parameters governing the performance of a PV module are:
- The PNom value which is the basic definition of the module (nameplate value),
- The STC specifications Isc, Voc, Imp, Vmp, normally taken directly from the datasheets,
- The temperature coefficients musc (alpha), muVco (beta) and muPmpp (gamma), from the datasheets.
- The optical performance, i.e. Incidence Angle Modifier (IAM) which induces a loss for not-normal incidence angles, especially important for the diffuse part.

The model of PVsyst requires a set of additional parameters which affect the operating performances:
Rshunt Shunt resistance, the inverse of the slope of the I/V curve around V = 0 at STC.
RshExp Characteristics of the exponential-like behavior of Rshunt as function of the irradiance,
Rsh(0) Intercept of this exponential at Irradiance = 0
Rserie An internal parameter of the one-diode model. Should not be confused with the slope of the I/V curve at I = 0, which is named RserieApp (for apparent) in the software.
The Rserie is the main parameter governing the low-light performance.
Gamma (diode ideality factor) is closely related to the Rserie value during the calculation of the model parameters, and is not considered as an additional unknown parameter.
muGamma is a linear deviation of the Gamma value (diode ideality factor) as function of the temperature. It can be viewed as a correction for getting a specified muPmpp (in PVsyst, mu stands for "temperature coefficient).
d2MuTau Specific parameter for the recombination term, only relevant for amorphous and CdTe modules.
IAM Incidence angle modifier as a function of the incidence angle.

These additional parameters are usually not mentioned on the datasheets, and the difficulty is to find their values according to further information for a given module.

Rserie is the most important parameter and has a great impact on the Low-light efficiency.
Its determination is crucial for the model.
See How should the Rserie value be specified? for details.

Rshunt, RshExp, Rsh(0), (and Rserie)
Sometimes measurements are performed for a set of different irradiances and temperatures according to IEC-61853-1. Then this data is used to perform a general fit, using all parameters Rserie, Rshunt, Rsh(0), RshExp and muGamma as variables.
In such kind of fit, the Rshunt, Rsh(0), RshExp are usually "adjusting" parameters, which may take values very different from their physical observed value, in order to satisfy the fit.
Of course the result of a fit (residues) is better when using more parameters. However if the parameters are not well implied in the equations, erratic values may lead to unstable solutions: slightly different measured values may give completely different parameters, leading to erroneous results when applying the parameters to modules with slightly different STC input data.

This is the case mainly when we extend the results of the fit to other modules of different powers (i.e. different qualities of manufacturing).
This is the reason why we prefer keeping these parameters concerning Rshunt (i.e. Rshunt, RshExp and Rsh(0)) as close as possible to the default values, modifying them only if it is really necessary to reflect direct measurements. And we adjust the Rserie parameter in order to get the measured low-light relative efficiencies at 25°C. We do that identically for all modules of a power series, but we don't have any proof of the validity of this hypothesis as we never got systematic measurements for different power classes of a same module type.
As another example, the Temperature behavior of the Pmpp issued from the global fit may be significantly different from the direct measurement at 1000 W/m2 specified on the datasheets.
In a general way, we sometimes cannot accept the results of these multi-parameters fits for the database, as we don't have a mean for extending it to other modules of different powers in the same series. The methodology is too different from the standard parameter's choice used by all other manufacturers, and this may lead to irrelevant discrepancies between manufacturers in the simulation results.

Incidence Angle Modifier (IAM) function
The IAM function represent the additional reflection losses when the irradiance is not perpendicular to the PV module area, with respect to a normal incidence.
The IAM profile is specified within the PAN file. The standard behavior in PVsyst is the ASHRAE parametrization with bo = 0.05.
If you want to specify a custom profile in the database of PVsyst, we will require a full report of measurements performed indoor by a third party laboratory.
Since january 2017, we don't accept outdoor measurements anymore - even if they are performed according to IEC 61853-2 - because the proposed methodology seems not sufficiency reliable. See the post
How to deternime the IAM profile ?.

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