How is evaluated the "Low-light" efficiency ?

[André Mermoud]

The efficiency behaviour involved in the results of PVsyst is a "pure" application of the "One-diode" standard model, corrected for exponential Rshunt as function of the irradiance.

In the simulation process, the "Irradiance loss" is the deficit of efficiency as function of the irradiance during the simulation, by respect to the efficiency at nominal conditions (STC, 1000 W/m²).

You can see the graph "Efficiency as function of Irradiance level" in the "graphs" tool of the PV module definition.

The "Low-light efficiency" is mainly dependent on the Rserie parameter, and the exponential behaviour of the Rshunt (as well as di²MuTau for amorphous technology).

- The Rserie resistance: the loss goes with the square of the current (R * I²), therefore increasing quadratically with power. Take now a module under 200 W/m2 irradiance: if the Rserie is high (bad), the losses are higher at STC, and the STC will be lower. Therefore inversely, for two modules having a same Pnom at STC, the module having a high Rserie will have a higher relative efficiency at low levels.

- The Rshunt exponential behaviour : when the irradiance diminishes, the Rshunt increases exponentially (and therefore de corresponding loss diminishes). Now if you have a good Rshunt at STC (low losses), you have nothing to gain when coming the low irradiances levels. As a contrary, with a bad Rshunt at STC (as in amorphous modules), the Rshunt loss will strongly diminish toward low levels, therefore increasing the efficiency.

See detailed explanations.

Therefore modules with identical Pnom at STC , but a bad (high) Rserie, or a high Rshunt dynamics will behave better under low irradiances, i.e. under real operating conditions during the year !

Here is a typical diagram of the efficiency as a function of Irradiance.

During the simulation, the corresponding "Low-light" loss is the difference between the STC efficiency (1000 W/m2 @25°C) and the effective efficiency under the present irradiance and 25°C.