Shading and Serial vs Parallel with modern solar panels with MPPT controllers.

[FilterGuy]

It is generally accepted that if you have shading on some of the solar panels, it is better to put them in parallel so the shaded panels don't impact the unshaded panels.
However, with modern panels that have two or 3 bypass diodes and MPPT controllers, is parallel really better? I am not sure it is.

Note: This turned into a long post, but stick with it to the end to see my non-conclusion to this question

Let's take a hypothetical panel that has no bypass diodes and creates 30V and 10A (300W) with no shade:


Now let's assume one of the cells gets shaded.

The shaded cell will block most of the current and have a high voltage drop across it so the panel output voltage and current will be very low. The resulting power output is negligible.

Now let's add in the bypass diodes


Now the current bypasses the portion of the panel that is blocking. For this example, that means the output current will be ~10A and the output voltage will drop by ~1/3 to ~20V for a production level of 200W. (Much better than negligible, but still a lot less than full power).

Now let's put two of these in series with one cell shaded.



The current will remain the same and the voltages will add. The result is 50V and 10A for 500W total production.

Finally, let's put two of the panels in parallel with one cell shaded. (This is where it gets interesting/confusing)



What will the output be? The unshaded panel is going to be producing 30V and the shaded panel is going to be producing 20V. If the panels were just driving a resistive load, the 30V would overwhelm the 20V panel and the total output would come from the unshaded panel; 300W (or less if the current is back feeding into the shaded panel). However, an MPPT controller will try to find the best operating voltage and current for maximum production of the total array. This means the voltage and current will be the best power point for the array, but sub-optimal for either panel. Since the I-V curves of panels are not linear, I don't think we can assume it will average to 250W per panel.... but I do not know what it will be. My gut tells me that the best case is that the MPPT does find a point where both produce 250W for a total of 500W. (The same as in serial panels). However, if they are creating the same power, it would still be better to go with serial because the lower current and higher voltage allow for smaller conductors and lower percent voltage drops.

Has anyone seen any data or studies on this?

 

[wattmatters]

Now for fun add modern half cut cell panels into your diagrams.

It will also depend on how good the global MPPT algorithm is. Modern high quality string inverters are very good at doing sufficiently regular global sweeps of the P-V curve and will find the optimal point without getting stuck at a local maxima.

But then there is the issue of whether there are sufficient loads as well. Often the PV array is not required to generate at its MPP, so it will be operating (by design) at a sub-optimal power output. In which case it's a bit moot.

Of course cabling a single string is easier/cheaper than multiple strings in parallel.

End of the day, do what you can to avoid persistent hard shading as it's not healthy for the panel. Quality panels will have decently robust bypass diodes. Cheaper panels may see these diodes die if exposed to such conditions for long enough.

 

[bgflyguy]

I haven't seen a video of this being demonstrated, but I have seen a lot of videos of partial shading all but killing output in series strings.

What about using multiple mppts like micro inverters?

 

[FilterGuy]

I haven't seen a video of this being demonstrated, but I have seen a lot of videos of partial shading all but killing output in series strings.

Yes. It is particularly true that shading can kill the whole string if there are no bypass diodes. However, bypass diodes dramatically change the equation....that is why you now see them in most quality panels.

What about using multiple mppts like micro inverters?

Yes, MPPT controller per panel will get the most possible out of the array, regardless of shading.

I am quite impressed that companies like enPhase have been able to drive the cost of an MPPT controller and a small inverter down to where the solution is competitive. Of course, the modern NEC code requirements for PV arc-fault protection, pv ground fault protection and rapid shutdown have made it far easier for micro-inverters to compete. These requirements all add complexity that a micro-inverter hides. Once you factor in labor, micro-inverters will typically beat a string inverter in the price for grid-tie, no-battery installs. The last few bids from solar install companies that I have seen were all enPhase micro inverters and panels.

The cost and functionality equations get less clear when you add a battery into the mix.....but even that seems to be swinging toward micro-inverters for professional home installs by professional installers. (They are installing power-wall type batteries..... not putting systems together like we do on this forum)

 

[wattmatters]

Once you factor in labor, micro-inverters will typically beat a string inverter in the price for grid-tie, no-battery installs.

Not in Australia. You can expect an Enphase system to be 20-40% more expensive when compared with equivalent wattage quality string inverter systems.

 

[FilterGuy]

Modern high quality string inverters are very good at doing sufficiently regular global sweeps of the P-V curve and will find the optimal point without getting stuck at a local maxima.

Yes, a good MPPT will always track the best I-V point. However, the question is what *is* the optimal I-V point in the situations described? Will the best point for a serial configuration be better, the same, or worse than a parallel configuration? I don't know, but my gut tells me serial will be the same or better...... at which point serial wins for the other benefits it offers. (This assumes the MPPT can take the serial voltage)

End of the day, do what you can to avoid persistent hard shading as it's not healthy for the panel.

Yup.... but sometimes it is unavoidable.

Quality panels will have decently robust bypass diodes. Cheaper panels may see these diodes die if exposed to such conditions for long enough.

Yup.... you typically get what you pay for.

 

[FilterGuy]

Not in Australia. You can expect an Enphase system to be 20-40% more expensive when compared with equivalent wattage quality string inverter systems.

Interesting. Is that installed cost with labor?

Does Australia have requirements for rapid-disconnect? This requirement in the US code can be costly and complicated to implement but micro-inverters take care of it without any extra equipment.

 

[FilterGuy]

Now for fun add modern half cut cell panels into your diagrams.

Yes.... trying to logic out what half-cut does in series and parallel configs makes my head hurt.

 

[ArtDeco]

Slightly off topic, but are very cloudy skies (moving clouds) likely to have the same effect on production as hard shade ?

 

[FilterGuy]

Slightly off topic, but are very cloudy skies (moving clouds) likely to have the same effect on production as hard shade ?

There are so many variables that the best answer is probably 'it depends'...... sorry I can't be more definitive.

 

[wattmatters]

Interesting. Is that installed cost with labor?

Yes, fully installed by professional installers. Grid tied solar PV here is way cheaper than in the US.

Does Australia have requirements for rapid-disconnect?

No. That's mostly a US thing. Most of the rest of the world doesn't specify such a requirement. Our standards have a detailed range of requirements which must be met for grid-tied solar PV installations.

Slightly off topic, but are very cloudy skies (moving clouds) likely to have the same effect on production as hard shade ?

No. Primarily because the shade from clouds affects an entire array rather than individual panels or parts of a panel.

 

[meetyg]

My gut feeling (and some unscientific testing) tells me that you are correct. Modern panels in series with bypass diode combined with a good MPPT controller is probably better than parallel.

When you wire panels in parallel, from what I understand, you should use blocking diodes in between to minimize current going from the unshaded panel to the shaded one (a panel can sink current, not just source it). Adding blocking diodes will drop the voltage a bit, which would negatively affect output as well (at least in the long run).

 

[efficientPV]

Blocking diodes will only help if there is a significant failure of a panel or two in a string. Saying a shaded string will draw power from other strings is a myth. I've experimented with back feeding panels to melt snow off. It takes more voltage than an open circuit string can produce.

 

[timselectric]

Blocking diodes will only help if there is a significant failure of a panel or two in a string. Saying a shaded string will draw power from other strings is a myth. I've experimented with back feeding panels to melt snow off. It takes more voltage than an open circuit string can produce.

Were you successful in melting snow?
Was it cost effective?
I was thinking about doing some experiments this winter, myself.

 

[ArtDeco]

Yes, fully installed by professional installers. Grid tied solar PV here is way cheaper than in the US.


No. That's mostly a US thing. Most of the rest of the world doesn't specify such a requirement. Our standards have a detailed range of requirements which must be met for grid-tied solar PV installations.


No. Primarily because the shade from clouds affects an entire array rather than individual panels or parts of a panel.

That makes sense and agrees with my experience, but then so does "It depends ...".

 

[efficientPV]

Were you successful in melting snow?
Was it cost effective?
I was thinking about doing some experiments this winter, myself.

Not really. I've seen it done. It was a morning after a very light snow cover. It was about 32F and figured it would make a dramatic video. It wasn't a condition you would even care about. It took a lot of current, you must stay below the panels rated current. And it was taking forever and stopped the test. A solar panel has a large surface area to dissipate that power. I'd say it isn't worth it unless the panels are near vertical and you have the force of gravity to help out.

 

[timselectric]

Not really. I've seen it done. It was a morning after a very light snow cover. It was about 32F and figured it would make a dramatic video. It wasn't a condition you would even care about. It took a lot of current, you must stay below the panels rated current. And it was taking forever and stopped the test. A solar panel has a large surface area to dissipate that power. I'd say it isn't worth it unless the panels are near vertical and you have the force of gravity to help out.

That's what I figured, but was willing to give it a shot.
Thanks, for saving me the time.
Next test is heat trace on the back of panels.
Then, water source heating and cooling.
Cool the panels in summer, while providing pre heated domestic water. And warm the panels as needed for snow melting.
I'll find something that works, and doesn't break the wallet.
Hopefully

 

[efficientPV]

It would be the same heat to melt snow. Panel current would likely be more efficient since it would be in direct contact. Just saying it takes a lot of power and would you gain that back with the power created by the panel vs just waiting for natural melting.

 

[timselectric]

That's the ultimate question.
It will be powered by excess production, that would normally be wasted. So, time will tell.

 

[ArtDeco]

That's what I figured, but was willing to give it a shot.
Thanks, for saving me the time.
Next test is heat trace on the back of panels.
Then, water source heating and cooling.
Cool the panels in summer, while providing pre heated domestic water. And warm the panels as needed for snow melting.
I'll find something that works, and doesn't break the wallet.
Hopefully

How would you have excess production if the panels were covered in snow? When my panels were snow covered (all of February 2 years ago) my production was 0. A really big battery maybe ?