Apparent Intensity Dependence of Shunts in PV Modules Revision of the Shunt Parameterization in the De Soto Model and PVsyst

Abstract

It is common practice in PV system simulation to use the De Soto model, which describes how to use the 1-diode equivalent circuit model for modules. De Soto's model scales the shunt with irradiance, making it disappear toward zero W/m². Also, the commercial software PVsyst uses a parameterization that reduces the shunt effect when the irradiance goes down. However, the solar cells that make up a module typically do not have an illumination-dependent shunt. We therefore investigate the origin of the intensity-dependent apparent shunt in modules. We show that this apparent shunt (derived from the slope of the quasi-linear region from I_SC onwards) is a misinterpretation for module I-V curves and has little to do with a shunt conductance, although this slope method serves well for determining the shunt conductance of individual cells. Instead, the module I-V curve slope of the quasi-linear region from I_SC onwards is strongly influenced by even small I_SC mismatches between the cells. Such mismatch can occur from small illumination inhomogeneity even for A+ solar simulators in the laboratory, or from cell production variation. Abandoning the practice of using the I-V curve slope to determine the shunt value for equivalent circuit models of modules (and the corresponding shunt scaling in the De Soto model or PVsyst) contributes to physically more meaningful I-V curve parameterizations and bears the opportunity for further improved accuracy of PV system energy yield prediction.