Spectral mismatch

Fabian Plag, Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116, Braunschweig, Germany

Spectral mismatch calculations in accordance with the standard requirements

The measured spectral responsivity (SR) of a DUT PV module is required to consider and compensate the spectral mismatch effect in the performance measurement of the module when using solar simulators or natural sunlight. The reference device rates the apparent irradiance during a measurement differently than the DUT PV module does. For a world-wide comparability, the measurement needs to be corrected by referring to the reference spectrum E_λ,STC(λ) defined in the standard [1].

A spectral mismatch of the modules short-circuit current that is measured under a spectral irradiance E_λ(λ) that differs from E_λ,STC(λ), occurs if the SR s(λ) of the module differs from this of the reference device s_ref(λ).

The spectral mismatch factor SMM is defined as follows:

(1): [math]\displaystyle{ SMM=\frac{\int_{\lambda}{E_{\lambda\mathrm{,STC}}(\lambda)\ s_{\mathrm{ref}}(\lambda)\ d\lambda}}{\int_{\lambda}{E_\lambda(\lambda)\ s_{\mathrm{ref}}(\lambda)\ d\lambda}}\frac{\int_{\lambda}{E_\lambda(\lambda)\ s(\lambda)\ d\lambda}}{\int_{\lambda}{E_{\lambda,\mathrm{STC}}(\lambda)\ s(\lambda)\ d\lambda}} }[/math]

Note that also the relative SRs can be used in equation (1) because the absolute values cancel out.

By dividing the measured short-circuit current I_SC,meas trough SMM, the (corrected) I_STC can be obtained.

(2) [math]\displaystyle{ I_{STC}=\frac{I_{\mathrm{SC,meas}}}{SMM}. }[/math]

Another application of spectral corrections is described in energy rating procedures in accordance with the standard IEC61853-3 [7] to compensate the impact of varying spectral irradiances on the calculated standardized energy yield.

References

[1] IEC 60904-3:2008, Photovoltaic devices - Part 3: Measurement principles for terrestrial photovoltaic (PV) solar devices with reference spectral irradiance data.

[2] IEC 60904-7:2008, Photovoltaic devices - Part 7: Computation of the spectral mismatch correction for measurements of photovoltaic devices.

[7] IEC 61853-3:2018, Photovoltaic (PV) module performance testing and energy rating - Part 3: Energy rating of PV modules.