When inspecting operating crystalline silicon PV modules by infrared thermography (IRT), the resultant thermal images usually present specific thermal image patterns, according to each type of fault (e.g. cracks, PID, failed bypass diode, etc). It is also known that when a PV module is short-circuited (SC) under illumination, its thermal image temporarily shows a so-called "patcwhork" pattern, with some cells apparently warmer than others (in some cases with a ΔΤ up to 5-10oC or even more). In this case, the several warmer cells are not actually related with faults or hot spots. Besides, this pattern is observed even in brand new modules. Moreover, such a pattern is always the same for the same module; which means that random cells but always the same cells are warmer or colder. If the same module is then inspected again with IRT under normal, operating conditions (i.e. at MPP), assuming there is no other fault occurring, it will normally present a uniform temperature distribution and pattern, as for a "healthy" module.
In my knowledge and experience, it is erroneous to say that under SC conditions such a "patchwork" pattern is related with occurring failures, cracks, mismatches, etc, as it is mentioned in some articles. On the other hand, trying to explain this "effect" in a physics/material/electrical point of view, I cannot conclude to a certain explanation. I have a suspicion that it may has to do with the inevitable difference in the quality (and, thus, Isc) of each cell, in the cell matrix of the module, but it's a bit strange to me that this causes such high temperature differences, and only under SC conditions.
Could anyone have an explanation on this issue or a point to discuss it further?
Thanks in advance.
I don't know the answer, but it is probably a combination of factors. Part of the answer depends on how the cells in the panel are connected. Another hot spot is around the junction box. Poor soldering is also a problem. Isc is the highest current and short circuit voltage is near zero. This would affect the weighting of the terms in the diode equation. The diode equation is a simplification, so some temperature terms may become more important if the voltage term drops to zero.
As far as I know, when one sets the PV panel is short circuit mode, the electrons simply pass from the top (negative) side of the illuminated PV module to the back (positive) and shaded part of the same module. It then migrates through the silicon material and the PN junction. The energy (and indeed electrical flow) gained by the electron from the sun therefore flows through the silicon material and is eventually changed into heat within the cell. Given that not all electrons recombine with exactly the same way and also many other intrinsic factors in the solar cell, it is impossible to have a homogeneous temperature throughout. The temperature increase is high depending on the amount of solar radiation incident on the panel at the time of testing. In other words, at lower radiation you will get lower temperatures. Having said that, many times the quality of the cell material itself (especially polycrystalline) varies along the ingot. In many instances, the PV modules are made of a mixture of these cells (i.e. they are not grouped separately) and hence this gives rise to more defects on the module level.
I had ask simillar question. The details in link below.
I had run such experiments with real monocrystalline Si PV-batteries since June 2014. I had measure the photo-current, and back-side temperature of two identical PV-batteries with different electrical loads. The experimental results are out of my understanding.
My own hypothesis that temperature differences are result of heat transport from front-side to back-side of PV-panel. Something like Thermoelectric (Peltier) effect.
I guess that this effect sign (positive or negative) depends on soldering technology .
https://www.researchgate.net/post/Part_2_In_which_condition_does_the_maximum_heat_dissipation_in_a_solar_cell_occur_SC_or_OC_or_MPP_Maximum_electrical_Power_Point
The Anton Driesse answer is clear.
Next step is to explain the opposite tempereture sign relative to OC-mode for different cells.
Let us to continue discussion with may question "What are the factors which limite overheating of single cell in PV-battery?"
https://www.researchgate.net/post/What_are_the_factors_which_limite_overheating_of_single_cell_in_PV-battery
Article PV-battery Power and Heat Balance
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