For some series-connected PV cells, if one or some cells are partially shaded, they produce less voltage. Why hot spot occurs at this condition? Can you analyze the circuit both with/without bypass diode?
Yes Tony answer is very satisfactory. However i want to add to this answer some circuit analysis to see how the shadowed cell becomes reverse biased. To be specific assume that the string is shortcircuited and the shadowed cell has a partial shadowing as you desire. Then since the cells are connected in series only the smallest cell current will continue to pass in the string according to the continuity principle. Consequently the surplus of the photcurrent above that of the shadowed cell will be forced to pass through the cell internal diode causing it to be forward biased. So, every cell will develop a forward voltage say Vsur, the surplus voltage. Assuming all cells except the shadowed one producing the same voltage, the according to Kirchhoff voltage law, all these voltages will appear on the terminals of the shadowed cells reverse baisining it with N Vsur where N is the number of the unshadowed cells.
Solar cells have relatively high substrate doping and their breakdown voltage is relatively low. The solar cells when breaks down they break in small area which means that a high current at high voltage is dissipated in small volume leading to heating these weak points than the surrounding regions. The remedy is by using backward diodes as explained by Tonny.
I hope that i added some thing useful to Tonny answer.
PV cells are dc current source and the shaded cell produce less current. and hot spots do not occur unless the module is shorted where a reveres bias applied
The PV cells are p-n junction diodes. During shaded condition, they act as diode in reverse biased and offers maximum resistance to the flow of current. When one or some cells are partially shaded, the part which are shaded acts as reverse biased p-n junction and offers resistance to the flow of current produced in unshaded part. The current thus dissipate in heat (I2R loss) and causes hot spot.
When a diode is reverse biassed it does not conduct much current, so you would not get any hot spot because there is no power dissipated. What happens is that the reverse biassed diode goes into avalanche mode where the depletion layer breaks down to allow charge to flow. This can occur at high voltage - so you have high voltage with high current and that means high power. The shaded cell is destroyed quite quickly! To prevent this from happening it is usual to fit reverse diodes across a number of cells in series which limits the reverse bias to a value that the cell pn junctions can safely tolerate. This eliminates hot spots. Unfortunately it also eliminates any power generated in that series of cells, when only one is shaded; the others could produce useful output, but it is lost.
Dear Tony
Thank you a bunch. Your answers are always comprehensive and quite helpful.
Yes Tony answer is very satisfactory. However i want to add to this answer some circuit analysis to see how the shadowed cell becomes reverse biased. To be specific assume that the string is shortcircuited and the shadowed cell has a partial shadowing as you desire. Then since the cells are connected in series only the smallest cell current will continue to pass in the string according to the continuity principle. Consequently the surplus of the photcurrent above that of the shadowed cell will be forced to pass through the cell internal diode causing it to be forward biased. So, every cell will develop a forward voltage say Vsur, the surplus voltage. Assuming all cells except the shadowed one producing the same voltage, the according to Kirchhoff voltage law, all these voltages will appear on the terminals of the shadowed cells reverse baisining it with N Vsur where N is the number of the unshadowed cells.
Solar cells have relatively high substrate doping and their breakdown voltage is relatively low. The solar cells when breaks down they break in small area which means that a high current at high voltage is dissipated in small volume leading to heating these weak points than the surrounding regions. The remedy is by using backward diodes as explained by Tonny.
I hope that i added some thing useful to Tonny answer.
Dear Tony and Dear Prof Abdelhalim
One question! In the case that no bypass diode has been used, why when a PV cell is shaded, the PV array does not give out any current? (the load draws no current) I want a circuit analysis of this situation.
Thank you again
because the shaded cell is reverse biased by the entire power generated by non-shaded cells, due to insufficient dissipation of large amount of heat generated locally leads to hot spots in the module
It is necessary to use 1 bypass diode for every 10 to 15 cells.
Generally 2 being used for 36 series connected cells.
because the shaded cell becomes load for the remaining, leading to very less or negligible current, you can simulate in matlab latest edition or in labview
you can find very good papers on the shaded cell analysis on IEEE and Elsevier
because the shaded cell is reverse biased by the entire power generated by non-shaded cells, due to insufficient dissipation of large amount of heat generated locally leads to hot spots in the module
It is necessary to use 1 bypass diode for every 10 to 15 cells.
Generally 2 being used for 36 series connected cells.
because the shaded cell becomes load for the remaining, leading to very less or negligible current, you can simulate in matlab latest edition or in labview
you can find very good papers on the shaded cell analysis on IEEE and Elsevier
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