How should the Rserie value be specified?

[André Mermoud]

The determination of the PAN file parameters, and especially Rserie, have evolved over time according to our progressive understanding of the one-diode model parameters significance and implications.

When computing the one-diode model, the Rserie and Gamma value (diode ideality factor) are strongly interrelated variables. At the beginning, we chose to fix the Gamma parameter as default value in the model's calculation, as it was independent of the module size.

Rseries and Gamma (diode ideality factor) interrelation in the one-diode model

 

In PVsyst Version 5, according to our long-term outdoor measurements on several modules, we first fixed the default value Gamma to 1.3 for mono-, and to 1.35 for poly-crystalline modules.

The practice revealed that this choice was very conservative.

NB: This default is specified in the hidden parameters, you can change it, if desired.

With the development of PVsyst Version 6, we had the opportunity to analyze a set of measured data, recorded outdoor by the Sandia National Laboratory for about 100 modules (Sandia model). This lead us to change the value of the default Gamma to 1.1 which has become the default since version 6, and up to V 6.25.

Gamma value recomputed for the modules of the Sandia database.

 

However, we observe a big discrepancy between outdoor measurements and the usual indoor measurements performed by flash-tests. When low-light measurements are available, we adjust the Rserie accordingly.

In most of the recently reported measurements, the relative efficiency lies between 0.5 and a maximum of 1% in the 600-800 W/m2 range, and is around -3% for 200 W/m2. When data overcome these values we are extremely careful when checking the reported measured data. It seems that the most recent modules, and some special technologies, may show very good low-light performances that overcome these averages.

Low-light performance for different hypothesis (Rseries choices)

 

Now we got many independently measured values at different irradiances, according to the new IEC-61853-1 norm for recent modules (indoor measurement). We observe on these data that the the low-light performance are rather comparable for all modules, with a relative efficiency drop of about 3% or less (up to 1%) at 200 W/m2.

Therefore since the version 6.26, the default Rserie value is fixed in order to get a -3% relative efficiency at 200 W/m2.

Otherwise in the database, when the manufacturer provides low-light efficiencies, we use to set the Rserie in order to get the low-light performances approaching the measurements at best (although we don't know the evolution of the low-light performance according to the power of then module within the series, i.e. its quality).

Effect on the yiels

We can mention that an increase of 1% on the 600-800 W/m2 region represents an increase of about 1% for the annual yield. This is not very dependent on the climate.

Effect of Rseries or Ganna choice on the yield

 

NB:The indoor performance reported by manufacturers usually corresponds to very low Gamma values, often below 1 and down to 0.9.

Remember that from the physics point of view, the one-diode model is a simplification of the two-diodes model, with Gamma=1 for the diode describing the diffusion loss, and Gamma=2 for the diode describing the recombination. The Gamma value in the single diode model should represent a mix of these 2 diodes, so that its value should lie between one and two.

The discrepancy between outdoor and indoor measurements is not fully understood. Probably the unavoidable diffuse component in the outdoor measurements is higher at low irradiances, and is subject to higher IAM losses. Furthermore, the module sample measured at Sandia is rather old, and the low-light performance may have increased in recent technologies. Recent measurements performed by research teams seem indicate a better match between indoor and outdoor measurement for a same module.

As a side note we mention, that when a manufacturer specifies the Rserie values (and sometimes also modifies the Rshunt, Rsh(0) and RshExp), we always require a measurement of the low-light performance at 200, 400, 600 and 800 W/m2, performed by an independent institute. Even in this cases, it happens that we need to perform a small correction on the submitted values, to ensure a coherent behavior of the modules in the simulation.

Representativity of the measured modules

The measurements are usually performed using a very limited sample of modules (1 – 3 modules) chosen from the middle or upper part of the power range of a given manufacturing batch. The results are then applied to all modules of the range.

Several questions now arise :

- How representative is the measured module for the entire batch?

- How is accounted the LID degradation? The official measurements are usually performed after LID, when the individual performances measured for each module at the output of the manufacturing line are obviously before.

- Will it stay representative for the future productions?

- How was the module chosen (most of the times by the manufacturer)?

- What is the evolution of the low-light performance within the manufacturing batch? When establishing the parameters in the database, we assume that it stays constant at all powers. But this is really a doubtful hypothesis. To confirm it, one would need a systematic measurement of several modules from the same manufacturing batch, but with different powers (i.e. manufacturing quality). We never saw any publication nor got data of this kind.