Micro-cracks: Solar Module

Recently discussed a lot about micro cracks in solar modules with lot of peoples, now a days new consultants without practical knowledge of the subject scaring investors and trying to make more money by giving them wrong inputs about the subject and so as to activation of bypass diode .

Here i am just sharing my talks with expertise in this matter so that everybody will get some idea about the phenomena associated with micro-cracks and activation of bypass diode:

Microcracks are well studied and they impact power in two ways –

• 1. They isolate an area of the cell big enough to drive it into reverse bias.

The first effect is the dire, but the reality is the probability of this is very low with modern 5 or 6 (or more) busbar modules.

The reverse bias voltage needs to exceed the sum of all the other forward bias voltages from the normal behaving cells (about 14V or so) and then the diode turns on. And if it is a crack it stays on and the diode is not really designed for this. The cracked cell will also heat, your module is not that great at that point.

This is the great fear of cracks, but it is actually really unlikely to happen for 5 or 6 or more busbar cells

And if it happens on a half cell module it will not even activate the bypass diode, the power point tracker will just shunt off the poor substring, the cell wont even heat in this case. Split cell modules are a very nice design.

• 2. They break fingers and increase the series resistance of the cell.

The second effect is surprisingly minor on power. We should try to eliminate them as much as possible and the good manufacturers have. There are still issues with installation and wind loading during operation. Glass-glass modules are fantastic for this. Chosing your racking / installation and deployment method is key too. But a crack here and thère won't kill you. We need to have some perspective. And we need to stop using EL images of 3 busbar modules to scare people when the issue is completely different on a 5 or 6 busbar module.

You can see this really clearly in EL and PL testing. If the crack breaks through the grid fingers, then it means the generated electrons don't have an optimal path to travel to the nearest busbar, they will usually travel down a longer path than normal. The finger are not always broken straight away by the cracks, but over time with thermal loading they will be.

The effect is very minor and it can be modelled with a few modelling tools. If every cells is cracked you might be able to detect it on a module. Otherwise it is hard to get a tester that is accurate enough.

And remember, higher series resistance has a bigger impact on one sun power than it does on total yield, because the impact of series resistance loss reduces when the sun is less bright and there is less current flowing.

Why some people are claiming the effect is more at low irradiance?

The edge of a crack can be a source of shutting and recombination and this is worse at low irradiance. This is usually only significant when there are a lot of fine spider-web like cracks right across the cell. One bright note is that when the cell is shunted it wont cause reverse bias problems. Just one or two cracks on a cell does not usually have a big shunting effect.

On a half cell module it will not even activate the bypass diode, the power point tracker will just shunt off the poor substring, Why diode will not activated and how MPPT will shut off the poor substring?

Lets call a normal module 72 cells all in series. Each group of 24 is in parallel to a bypass diode, so there are three diodes. When a cell is shaded by enough, it goes into reverse bias to a value equal to the turn on voltage of the bypass diode plus the 23 other cells in forward bias. This group of 24 cells contributes little to no power and the bypass diode helps to bypass the current of the other two groups of 24 so you at least get 2/3 of the power.

Now the split cell module. Now you have 144 half cells. Imagine the module sitting on its short edge. There are 6 columns just like a normal module, top half and bottom half. The first two columns in the top half are 24 split cells in series. The first two columns in the bottom half are also 24 split cells in series. The top half 24 cells and the bottom half 24 cells are in parallel with each other and the bypass diode. So the net performance (and voltage and current) of those first two column are almost the same as for a normal module only is comes from a series - parallel arrangement of half cells instead of a normal series arrangement. The split cell performance is actually better due to packing arrangements and lower series resistance.

The other groups are arranged the same so you have top half columns 3 and 4 in parallel with bottom half columns 3 and 4 and so on. The these three groups are all in series.

So now the shading arrangements required to turn on the bypass diode are really complicated so the IV curve actually will have a lot of lumps and bumps. Lets say you shade one full half cell somewhere on the bottom half of the module. The module IV curve will have a local max power point at a high voltage where it is making about half its power due to this section of cell contributing no current to the parallel arrangement with it top cell counterparts (and all other groups constrained to this current too). It will also have a local max power point at a high current where the module is making about 2/3 of its power and the bypass diode is turned on.

For a more common shading arrangement such as all the half cells along the bottom of the module being shaded at once (inter row shading), then the module is just making half the power from the top cells just due to the parallel arrangement, without the bypass diodes turning on.

Unable to understand the above statement how bypass diode will turned on because in one of the statement you said during interrow shading module will produce half of its power but if this will keep happening all over the time than all the bypass diode should activated?

It is tricky to explain on paper. The module has an IV curve, and then using this you can calculate the power as a function of voltage and current. There can be many local maxima in this power curve, sometimes the bypass diode is turned on and sometimes off.

For the interrow shading, lets say 6 half cells are completely shaded all along the bottom edge. This means that each of the three series connected strings in the bottom half of the module wont be contributing any power. No significant amounts of current will be flowing through any of them. But the counterpart series string they are in parallel to in the top half will still be make close to their full power. So even with that full interrow shading, the module is at half power with no diode stress. Compared to a normal module. In that situation the module would make no power.

In that case as lower cell is not producing and continuously shaded means hot spot may applicable so as which leads to power dissipation resulting in the increase in surface temperature and so as to fire may happen? Am I thinking in right direction?

So a hot spot from shading happens when a cell is driven into reverse bias by the shading. A reverse bias cell is a sink for power rather than a source. If the power sunk into the reverse bias cell is high enough or there is some manufacturing fault in the cell there can be "hot spots". The beauty of the bottom row shading in a split module is that the bypass diode does not turn on. You just simple have the lower strings not contributing power to the parallel arrangement with the upper string. So no diode turning on and no hot spot risk.

its takes a while to learn how all this hot spot and reverse bias and diode stuff works from first principles. Most people just repeat what they think they have learnt and a lot of time the subtitles are missed.

Note:

As long as the shaded cell is reverse biased but not past the reverse break down voltage it passes very little current, even if it is only partially shaded. Once the bypass diode is forward biased those cells no longer contribute to the module output. The current either goes through the forward biased diode or the cells, it won't get split.

Keep in mind that this changes when modules are connected in parallel. If the voltage applied to a module from the outside stays above the voltage that allows the diode on the shaded cell to bypass then the shaded cell will be forced into reverse breakdown. For instance if we have a 60 cell module in a string of modules in parallel with other strings and an inverter controlling the string voltage with MPPT this is what might happen; a cell is shaded and to activate the bypass diode the module voltage has to fall below 40*Vmp(cell)-0.7. If the inverter holds the voltage above this value using the MPPT circuit and the other parallel string voltages then the diode will remain open, the shaded cell will go into reverse breakdown, and the string current will be significantly reduced. The string voltage will drop slightly because of the reverse biased cell but the MPPT will still be on a local maximum, fat dumb and happy. Now if the inverter is smart it will lower the string voltage far enough to forward bias the diode and restore the full string current. This will be another MPPT local maximum at a higher power than the one it was on at the higher voltage. This is why we have shade smart inverters now, they search for other MPPT maximums and find the maximum of the local maximums.