Photovoltaic module analysis techniques
[1] During their lifetime, PV modules experience severe changes in weather and working conditions, leading to large temperature variations (day - night, summer - winter, irradiance) and mechanical stress (wind, snow, hail).
This article gives an overview about common analysis techniques used for operation and maintenance (O&M) of PV modules in the field.
[3] Possible defects, which can be identified by visual inspection, are glass breakage, electro-chemical corrosion, burn marks (in front or back sheet), delamination of front glass or back sheet, browning (evoked by atmospheric oxygen or heating), snail tracks, soiling and others.
Thus, to compare the resulting IV curve to ones taken in different conditions, it needs to be adapted to STC by correction factors for measured irradiance and cell temperature.
[4] Modern inverter or maximum power point tracker (MPPT) are able to measure the IV curve of the connected string (series circuit of multiple PV modules).
A decrease in shunt resistance results in an increased slope of the IV curve close to ISC.
[2] Electrical mismatches between the cells of a module result in a step wise behaviour of the IV curve.
An infrared (IR) camera allows to take an image of the module temperature with high spatial resolution, this is called thermography.
Simultaneously any shadow on the module must be prevented (e.g. cast by clouds, buildings, operator or camera).
[8] While UVF imaging with a camera gives statements about the luminescence intensity and thus the fluorophore density, UVF spectroscopy analyses the kind of present fluorophores by measuring the emitted spectrum at a precise spot of the module.
[10] For electroluminescence (EL) imaging, excitation of the Silicon is triggered by an external forward current, applied to the module connectors by a power supply.
The emerging forward bias forces majority carrier to cross the p–n junction, resulting in increased recombination.
[10] Usually, EL is done in laboratory conditions, where a dark environment ensures separation of emitted and surrounding radiation.
To get rid of any noise (in laboratory noise due to electronics and statistical fluctuations of the emitted photons, outdoor additionally the ambient radiation) in the collected signal, background subtraction of the EL image is done.
[10] Photoluminescence (PL) imaging is done by use of an external light source to excite carrier inside the Silicon of the solar cells.
If no circuit is applied or the load on the module is high enough, the excited carrier have no way to leave the solar cell and will recombine, resulting in luminescence radiation.
Therefore, PL can be performed during the whole production cycle of solar cells (EL only if connectors are mounted) and PV modules in operation can stay connected to the grid, whereas for EL they have to be disconnected and the external power supply attached.
additionally to the DMs detectable with EL, PL can measure the minority carrier lifetime in the cell material, the diffusion length and the diode voltage.
