Shade is sometimes unavoidable, especially for mobile and marine systems. I'm hoping that this thread can serve as both a resource and a place for conversation on ways to minimize losses due to shading.
I am far from the most informed on the topic, but I've learned a thing or two:
Avoid Shade where possible. This is obvious but can't be understated, every other point is just mitigation, avoiding shade is always the best solution where possible.
Use parallel wiring or a sensible combination of series and parallel wiring so losses stay localized to only the shaded panel.
Use panels that make good use of Bypass Diodes to minimize losses and reduce heat buildup
Use multiple smaller MPPT controllers so that each panel's (or each group of panels') maximum power point is calculated independently
And a few things I haven't had the chance to learn about yet, but are supposedly beneficial in partial shade:
Low 'reverse bias voltage' panels
Solar Optimizers Similar in concept to using multiple MPPT controllers, solar optimizer allow per panel maximum power point optimization
Microinverters I believe these have the same benefits as Solar Optimizers (per panel maximum power point optimization), as well as converting the DC output of the panel into AC.
Regarding Low Reverse Bias Voltages, this is what I've read:
If you don’t need to know why -skip to the next point and just believe me that: SunPower panels perform better in partial shade. But if you are remotely interested, let me do my best to briefly explain why:
A standard silicone cell has the positive on one side of the cell and the negative on the other. Because of the large electrical separation between the positive and negative, when the cell is shaded, it needs to produce a comparatively higher reverse bias voltage to overcome the resistance. A typical conventional cell has a ‘reverse bias voltage’ of 15V to 20V. This high voltage on a standard panel causes detrimental hot spots and reduces the panel’s efficiency. When the reverse bias voltage gets high enough, it will activate the protective bypass diode. While bypass diodes are installed in each third of a panel to protect it from hotspots, as each bypass diode activates, the panel reduces its production by one-third. To make things worse, bypass diodes have a limited lifetime. The more often they are activated, the sooner they will fail.
Sunpower has designed the Maxeon cell to have positive and negative on the back of the cell – just a fraction of a millimetre apart. When shade occurs, instead of a huge 15 to 20 volts, the “reverse bias voltage” is 2.5 volts (X series) or 5.5 volts (E series). In summary, the advantage of the Maxeon back-contact in shaded conditions are:
The reverse voltage is lower, therefore
the heat created is less, therefore
more current and power flows, and
bypass diodes operate less frequently giving more powerand increasing their lifetime.
What I'm asking is how much difference in output, reliability and longevity between different manufacturer solar panels. Are there any brands that are head and shoulders above the rest?
Sunpower has a very good reputation on all three fronts, they have some features that make them stand out, but not "head and shoulders above the rest"
I know that most people on here say if you need more solar .... just add some more panels .... but in my case, I need to maximize what I am getting per square foot. I would also like to do the best I can to minimize the effects of shading
I am in the same boat as you, watts / sq ft, and performance in variable less ideal conditions (partial shade) matter to me. We are the exceptions to the "a watts a watt, don't waste money on high efficiency panels" wisdom (which holds true for most situations).
do diodes really do any good .... are there any Mfg who do a good job of this and do they test to really work in partial shading?
Yes. I am not an expert, but I've been doing my best to learn about this (its hard to find info since partial shading is a secondary or non-concern for most stationary installations).
So far, the brands that stand out to me in terms of partial shading performance are Sunpower and Rec Solar. Of those two, Sunpower seems to have the advantage in overall efficiency of their panels in unshaded conditions. They have different methods of addressing partial shade.
Rec Solar's Twin Peak line uses half cut cells which splits the panels into 6 sections (as opposed to 3 sections for traditional panels) meaning if only one section is shaded that's a theoretical 17% loss of output as opposed to a 33% loss for a standard panel, Or if shade affects the 1 entire short side of the panel the half cut cells would lose 50% whereas the traditional panel would lose 100%. I believe their may be other companies that use half cut cells, but Rec Solar is the brand I'm aware of and I believe the originator of the concept. Here is a (marketing) video that explains the advantages of half cut cells
Sunpower's technologies that make them well suited for partial shade are a bit less straightforward, and I can't adequately explain them here. If you read through the threads I link to at the bottom of this post you can learn about some of them. They also make use of bypass dioes (the P-series uses 6 like the Rec panels (but all in a lengthwise orientation), The solid copper backing (IBC), low reverse bias voltage, smart use of bypass diodes, and high efficiency make them suitable for mobile use in terms of shading performance, durability, and space efficiency. This is a marketing video but it highlights some of the technologies and shares some apparently independent partial shade test data.
Yes. @Carly posted a picture that does a good job showing both the advantage of bypass diodes and of the half cut cells.
I am thinking I would like to use higher voltage panels and MPPT controller to reduce current from the panels .... but parallel would be better for partial shading .... maybe parallel higher voltage panels?
Yes, this is what I plan(ned) to do (now my plan involves multiple small MPPT controllers but the same principle of high voltage panels in parallel still mostly applies)
Sunpower's X series panels are 96 cell panels with a Voc in the high 60's, and some panels (can't remember which brand) are pushing 100V per panel.
Another thing worth noting is that the focus on voltage drop and line efficiency from panels to SCC is somewhat more relevant to stationary builds than mobile builds (where wire runs are usually quite short so (1) losses are less, and (2) oversizing the wire to compensate for losses is cheaper).
I have asked similar questions and started relevant posts to yours at various points in my research there is some good info in some of them, check them out:
I think that's a great idea, I'm not sure I feel knowledgeable enough yet to go into too much depth detail. But I could give a good high level beginner friendly explanation and we could build from there, or just keep it simple.
I think that's a great idea, I'm not sure I feel knowledgeable enough yet to go into too much depth detail. But I could give a good high level beginner friendly explanation and we could build from there, or just keep it simple.
You haven't mentioned blocking diodes. If you are going to expect shade then you'll want them. I'm mobile so won't always be able to avoid it. They are in the combiner box, at the end of each series circuit, where the parallel circuits join. Unlike bypass diodes which normally aren't conducting the blocking diodes are normally conducting. When one string is shaded, it's voltage drops and the other strings then start forcing power back through those panels. The bypass diodes in the shaded string do not protect from this.
That is the job of the blocking diode, which becomes reverse biased in this situation and stops conducting thus protecting the shaded string.
These diodes need to be higher power and high quality, as under normal circumstances they are dissipating power.
For my parallel string which are pushing up to 8 amps through each diode with a .4 volt drop that comes out to 3.2 Watts. I'm going with big can 10 watt diodes from a reputable manufacturer, on a heat sink. With 4 parallel strings that's a total of 13 watts being dissipated. And yes, I'm not happy about the loss but I want to protect my panels.
If you are planning on using blocking diodes it is another reason to want to keep the series string amps low.
@Bob B I would also benefit from a diagram, I'm a visual learner.
Here is a quick intro video to bypass and blocking diodes I will note that the video indicates that per panel or per series string MPPT controllers is a better (but more expensive) solution compared with blocking diodes.
A Blocking Diode is: like a one-way electrical 'check valve' that prevents power from a string in full sun being pushed into a shady string of panels.
A Blocking Diode should be located: between parallel connected PV modules or strings. Blocking diodes are unnecessary if you have do not have parallel connected panels or strings.
The problem a blocking diode solves is: (this is where I still need clarification and/or correction) (1) It improves efficiency in uneven shading by preventing current from flowing from the sunny string to the shaded string instead of the battery (2) does it also off protection to the panels somehow??
The tradeoff is: voltage drop of roughly 0.5V and some amount of power loss.
So .... does anyone have a diode part number .... and link where it can be purchased if possible ....that would be hefty enough for most any solar panel combination.
Same for bypass diodes. I know many solar panels have bypass diodes for strings of cells within the panel, but do they also have one in their combiner to bypass the entire panel?
All modern panels have bypass diodes. They’re in the junction box on the back of the panel. There will be one less than the number of cell strings on the panel.
Often they’re undersized on the cheaper panels and will eventually burn out. I inspected and tested them all on the used panels I bought. None were burned out but that was expected as they were from a solar farm and so it was unlikely the panels experienced shading, and there was no evidence of lightning strike. The ones I’ll be using in my combiner box are similar to this. The physical design allows easy connection to line breaker and connection to a common busbar heatsink.
All modern panels have bypass diodes. They’re in the junction box on the back of the panel. There will be one less than the number of cell strings on the panel.
Often they’re undersized on the cheaper panels and will eventually burn out. I inspected and tested them all on the used panels I bought. None were burned out but that was expected as they were from a solar farm and so it was unlikely the panels experienced shading, and there was no evidence of lightning strike. The ones I’ll be using in my combiner box are similar to this. The physical design allows easy connection to line breaker and connection to a common busbar heatsink. View attachment 11496
