How to interpret the Low-light measurements ?

[André Mermoud]

The IEC 61853-1 standard now recommends the measurement of the PV module characteristics under a matrix of different irradiances and temperatures (namely 1100, 1000, 800, 600, 400, 200 and 100 W/m2, for temperatures from 15°C up to 75°C). The low-light efficiency is the basis for the determination of the uncertain parameters of the one-diode model in PVsyst, mainly the Rserie value.

These measurements should be performed with high quality flash-test instruments (AAA class). The irradiance attenuation is ensured by calibrated filters.

The results are usually provided as ( Imp, Vmp, Isc, Voc ) data sets, for each (Irradiance, Temperature) condition. Let's define:

- PmpMeas = Imp * Vmp measured at GMeas = nominal irradiance after filter

- PmpSTC = Imp * Vmp measured at GSTC (= 1000 W/m²).

The relative efficiency is then (PmpMeas / GMeas) / (PmppSTC / GSTC) - 1

Filter uncertainties

One problem is that the filters are not perfect. They may have inaccuracies either in irradiance and in spectral response.

Then, an error of 1 % on the irradiance means an error of around 1% on the relative efficiency.

For avoiding this problem we can follow the hypothesis that the short circuit current is proportionnal to the irradiance (including spectral mismatch).

From experimental basis, this hypothesis is proposed by the Sandia National Laboratories (USA), as a direct measurement of the incident irradiance.

In the "theory", it is the basic hypothesis of the one-diode model.

During the analysis of the low-light data, a correct estimation of the relative efficiency requires to modify the GMeas nominal response of the filter by using a corrected irradiance GmeasCorr = IscMeas * GSTC / IscSTC

Electrical measurement setup

Another key point of the measurement of low-light efficiencies is the electrical measurement of the voltage: this should be measured as close as possible of the PV module terminals (so-called "four-wire" measurement).

If the voltage is measured at the measurement instrumentation level, the cable resistance will add with the Rserie of the module.

As an example, if you have 2 x 5 m of 4mm2 cable (AWG 11), the parasitic resistance will be of 45 mOhm.

For a usual 250 Wp modules of 60 cells, this has to be compared to the Rs = 310 mOhm.

The addition of this parasitic resistance will induce an error in the modelled low-light efficiency of

+0.33% at 800 W/m²,

+0.69% at 600 W/m²,

+1.11% at 400 W/m²,

+1.58% at 200 W/m².

These errors are very important, when we usually see (wait) over-efficiencies between +0.3 and max. +1% in the 600 - 800 W/m² range.

NB: The current measurement is not affected by voltage drops due to parasitic resistances.

 

Four-wire voltage measurement