Partial & Unpredictable shading best practices
- Thread starter Dzl
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Bob B
Emperor Of Solar
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It would be pretty easy to solder a wire and terminal on the ones without it .... and you could make the wire whatever length works best.
Amazon is getting a little strung out on shipping times, so Ebay is getting more of my business than usual.
svetz
Works in theory! Practice? That's something else
All diodes are like check-valves in that they allow one way flow current. The difference between a blocking diode and a bypass diode is their location (purpose) and electrical characteristics. There are a wide number of specialty diodes (even a a PV cell is a bunch of diodes), but blocking/bypass are just terms used in the pv solar panel industry AFAIK.
blocking diode - prevents the battery from draining into the panels when its voltage is higher then the panels (e.g., night time). Usually built into the SCC.
bypass diode - prevents a panel with some shade from dragging down the string by having power from other unshaded panels "bypass" it, otherwise power from other panels would back-feed into it and shorten it's life. These can be external to the panel, but are commonly found in the Jbox to prevent an "internal-to-the-panel-string" from dragging the panel down in the event of partial shade. Other than specific obstructions, you typically won't find these external to the panel as that would mean the whole panel is shaded and that typically means the string is shaded as well. The Maxim cell optimizer can replace this diode, it's similar to how a microinverter works and allows maximum power from the panel, but works within the panel, probably the same price as diodes when buying in small quantities .
Microinverters and optimizers work at a panel level, there are no "string" of voltage sources that can adversely impact one another, so they are ideal for situations where some partial shading might occur.
See also: https://diysolarforum.com/threads/shade-on-your-solar-panel-can-void-your-warranty.241/
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Also prevents power generated from a lit string to flow into a shaded string.
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KBWaldron
Solar Enthusiast
These days, with few panels directly connected to batteries (I.e. No SCCj, that is the more common purpose.
@Dzl wiki page start written out with your text and some images. Feel free to edit.
KBWaldron
Solar Enthusiast
Thanks for that link. I found one's spec'd the way I need them from the same vendor. By the way the form factor is referred to as a stud.
Looks good, I've been working on rewriting some of what I have in the OP and adding some links with the intention of adding it to the wiki. Should be ready shortly
Maybe Svetz or someone else can clarify, but from what I've learned so far I am pretty certain you are correct. using optimizers or microinverters is like putting a tiny MPPT controller on the back of each panel. So there really can't be any series or parallel connection between panels. I believe the optimizers are then connected in series and routed to the inverter.
Here is a very short explanation from Aurora Solar:
svetz
Works in theory! Practice? That's something else
More like on "the inside" of each panel. They don't replace microinverters or Solar Edge's DC optimizers - those optimize the panel rather than the cells.
Short answer: That's what Maxim claims.
Long answer
Great question. I don't know. Let's see.... When panels are in series the voltages sum but the current stays the same. But all of the current must pass through every panel:
I don't know if this is accurate, but if you think of shade as adding resistance to a cell, it's easy to see how one cell being shaded will affect the output of an entire string. With bypass diodes, any shaded part should be cut-out and shouldn't impact the overall string, but if multiple sections are shaded there are no clear paths so with added resistance energy is lost as heat.
My guess would be that the cell optimizers are therefore passing the energy from previous panels directly upstream and not running them through each cell. Instead, the cell collects the energy from the panel as if it was all by itself, then outputs it at the appropriate voltage to merge with the incoming power. Keep in mind that's just a guess.
@upnorthandpersonal @svetz below is what I plan to add to the Wiki entry on shading, wanted to make sure we are on the same page before updating, if you have any input or see anything in need of correction let me know, I would prefer not adding anything more as its already a bit longer than I had hoped, but I'm open to alterations or subtractions. Let me know what you think. @svetz I would appreciate your input specifically on the section on panel level optimization and the maxim chip.
Finally I have included a section on blocking diodes, but I am having reservations about reccomending them, I understand the basics of how they work and their purpose but I'm having trouble finding solar industry or solar educational sources that reccomend using them in modern systems or examples of systems that use them. Why is that? Maybe just ignorance on my part. What are your thoughts?
Strategies to minimize losses due to partial shading
Even a little bit of shade has the potential to drastically reduce the output of your PV system if strategies aren't taken to minimize losses. The most effective solution by far is to avoid shade if at all possible. For many people, especially those with boat and vehicle based systems, avoiding shade entirely is not a possibility. In these cases it is important to take steps to mitigate the consequences of shading. The following is an overview of strategies for improving performance in partial shad conditions.
Avoid Shade
There is no substitute for this, this is far and away the best option. Every other strategy is just a way to mitigate the consequences.
Connect your panels in parallel, or a sensible combination of series and parallel. If you wire in series, the current of the entire string of panels will be reduced to that of the shaded panel. If you wire in parallel, the unshaded panels won't be pulled down by the single shaded panel. (more info)
Choose panels that make smart use of bypass diodes. Bypass diodes allow current flow around or 'bypass' a shaded part of a solar panel. This is important for efficiency and safety. Similar to how wiring in parallel limits losses due to shade to the shaded panel, bypass diodes limit losses to the shaded portion of the panel. They do this by creating several strings of cells within a panel, if one string is shaded current from the other strings can flow around the shaded string through the bypass diode, this limits losses and prevents heat buildup in the panel. Almost all modern panels have bypass diodes, but the effectiveness will be based on the number of diodes, and how the strings are laid out.
Below is an example of a conventional panel (left) with 3 strings of cells laid out lengthwise, and an example of a panel that makes smart use of diodes and half cut cells (right) to create 6 strings of cells in a 2x3 configuration.
Choose panels that are optimized for partial shade. It is beyond the scope of this article to discuss specific technologies or product lines. But you should be aware that some panels are better optimized for partial shade than others.
Use multiple MPPT controllers. This is commonly done in the marine industry where partial shade due to sails, rigging, etc. The ideal configuration is one controller per panel, but one panel per zone (for instance port side and starboard side) would also improve efficiency. There are two advantages to multiple MPPT controllers, first, it has the same advantages as parallel wiring, second, it improves efficiency of shaded and unshaded panels by calculating their 'maximum power points' individually for the conditions of each panel rather than for the average conditions of the array as a whole.
Panel Level Optimization. Panel level optimization is an umbrella term for techniques of optimizing the output of each panel individually as opposed to optimizing the output of your array as a whole. The latter is the traditional method and works great so long as conditions are mostly uniform across the entire array. Panel level optimization is beneficial when conditions (such as partial shading, orientation or angle) differ across your array. The 'maximum power point' (most efficient point) is tailored to the specific conditions of each individual panel rather than the average conditions across the whole array. There are several types of panel level optimization:
Finally I have included a section on blocking diodes, but I am having reservations about reccomending them, I understand the basics of how they work and their purpose but I'm having trouble finding solar industry or solar educational sources that reccomend using them in modern systems or examples of systems that use them. Why is that? Maybe just ignorance on my part. What are your thoughts?
Strategies to minimize losses due to partial shading
Even a little bit of shade has the potential to drastically reduce the output of your PV system if strategies aren't taken to minimize losses. The most effective solution by far is to avoid shade if at all possible. For many people, especially those with boat and vehicle based systems, avoiding shade entirely is not a possibility. In these cases it is important to take steps to mitigate the consequences of shading. The following is an overview of strategies for improving performance in partial shad conditions.
Avoid Shade
There is no substitute for this, this is far and away the best option. Every other strategy is just a way to mitigate the consequences.
Connect your panels in parallel, or a sensible combination of series and parallel. If you wire in series, the current of the entire string of panels will be reduced to that of the shaded panel. If you wire in parallel, the unshaded panels won't be pulled down by the single shaded panel. (more info)
Choose panels that make smart use of bypass diodes. Bypass diodes allow current flow around or 'bypass' a shaded part of a solar panel. This is important for efficiency and safety. Similar to how wiring in parallel limits losses due to shade to the shaded panel, bypass diodes limit losses to the shaded portion of the panel. They do this by creating several strings of cells within a panel, if one string is shaded current from the other strings can flow around the shaded string through the bypass diode, this limits losses and prevents heat buildup in the panel. Almost all modern panels have bypass diodes, but the effectiveness will be based on the number of diodes, and how the strings are laid out.
Below is an example of a conventional panel (left) with 3 strings of cells laid out lengthwise, and an example of a panel that makes smart use of diodes and half cut cells (right) to create 6 strings of cells in a 2x3 configuration.
Choose panels that are optimized for partial shade. It is beyond the scope of this article to discuss specific technologies or product lines. But you should be aware that some panels are better optimized for partial shade than others.
Use multiple MPPT controllers. This is commonly done in the marine industry where partial shade due to sails, rigging, etc. The ideal configuration is one controller per panel, but one panel per zone (for instance port side and starboard side) would also improve efficiency. There are two advantages to multiple MPPT controllers, first, it has the same advantages as parallel wiring, second, it improves efficiency of shaded and unshaded panels by calculating their 'maximum power points' individually for the conditions of each panel rather than for the average conditions of the array as a whole.
Panel Level Optimization. Panel level optimization is an umbrella term for techniques of optimizing the output of each panel individually as opposed to optimizing the output of your array as a whole. The latter is the traditional method and works great so long as conditions are mostly uniform across the entire array. Panel level optimization is beneficial when conditions (such as partial shading, orientation or angle) differ across your array. The 'maximum power point' (most efficient point) is tailored to the specific conditions of each individual panel rather than the average conditions across the whole array. There are several types of panel level optimization:
- Per panel MPPT controllers (as explained above)
- Solar Optimizers - Calculate the maximum power point for each panel individually and are connected together in series with one high voltage DC transmission line to a central inverter.
- Microinverters - similar to Solar Optimizers, but additionally transform the DC output of the panel to AC and have no central inverter
- Maxim Chips - emerging technology that promises to extend the benefits of 'panel level optimization' to the sub panel (string) level and do so at a lower cost. There have been some issues bringing this technology to market.
Rookie answer to your rookie question (so take it with a grain of salt):
There is probably a straightforward way to charge batteries with a system that uses microinverters, but I think that they are mainly intended for grid-tied solar systems. DC optimizers do basically the same thing as microinverters without inverting to AC, it seems these would make more sense for battery storage.
Here is an article on battery backup with microinverters
Are micro inverters & battery backup compatible?
Can you use a micro inverter off grid? Or even for grid connect solar with batteries? Yes you can - here's how it can be done.
www.solarquotes.com.au
svetz
Works in theory! Practice? That's something else
An AC powered battery charger. The Enphase batteries operate like the Tesla powerwall, they input and output AC, but have DC batteries, inverters, and charging mechanisms all hidden away inside.
svetz
Works in theory! Practice? That's something else
They've been around for years and Jinko sells them installed in some of their panels, so not really "emerging". The only issues I'm aware of is that it's known to cause some TV interference in some cases. They are readily available, you can buy them off places like digikey.
That said, like most of the stuff that interests me, they are fringe. Despite NRELs glowing report, hardly anyone knows about them or uses them. It's probably just poor marketing, that most owners don't want to retrofit diodes with chips, the TV issue and you really don't want shade on your panels anyway.
Perhaps reword to:
4. Maxim Cell Optimizers - When a panel's internal diodes bypass a shaded section of a panel, that power is forever lost. Maxim has a $5 replacement chip for diodes that allows that power to be recaptured and operates the cells at their maximum power point. Essentially for a shaded substring of cells it acts like a boost converter raising the voltage of the stranded energy so that it may join the output of the rest of the panel. It has a 120 mv voltage drop which is lower than the typical bypass diode. Despite NREL issuing an overall positive report on their findings few solar panel manufacturers have incorporated them and it remains a fringe component; for example the Jinko MX panels that have the chip integrated are not available in Australia. The chips are compatible with and will help panels in strings, parallel, or with microinverters. These chips have been known to cause TV reception issues when placed in close proximity to TV antennas and wires.
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svetz
Works in theory! Practice? That's something else
Original reports on the issue are from 2016... hard to imagine Maxim hasn't fixed it by now.
I feel your pain. This pretty much describes me as well.
I suppose I meant emerging in a broader (and more conservative) sense. Market awareness, maturity of the technology, long term track record, integration by panel manufacturers and/or solar installers.
My thinking is that they sound great on paper, really great! Better and cheaper than microinverters or dc optimizers. When I first read your posts about them I got excited, but the fact that they sound so promising, yet haven't really taken the industry by storm, combined with not being sold in australia due to TV reception issues (which seems like it might indicate much broader RF noise issues), has made me a little wary. Most of the buzz about them seems to be around 2016 (which I believe was when they became publicly available) and since some reviewers and installers that previously wrote positively about the tech have moved away from reccomending them.
"emerging" may not be the most precise term to choose (and I'm open to rewording) But it was my attempt at conveying that the tech is promising but that 'the jury is still out' on whether the bugs can be worked out and whether they can be competitive in the consumer/residential market.
This is a great explanation. It is much longer than what I was aiming for (and I already failed to be as concise as I had hoped) in this entry. The way I I was thinking of this entry was a list of strategies/technologies for dealing with partial shade with just enough meat on the bones to convey the concept, the advantages, and the disadvantages in one or two sentences. A sort of 101 level high level overview. If you had to condense it down to 1 or 2 lines how would you explain the maxim chip?
But I really like your explanation (and have learned a lot from your resources on the maxim chip). I wonder if it deserves its own longer entry that could be linked to from this entry--either its own entry, or an entry like the article you linked to earlier that compares explains the different types of panel/sub-panel level optimization.
Possibly (hopefully), but its also possible that this is part of why Maxim hasn't gained more market share or been integrated into more panels.
This is part of what i intended to convey with calling it 'emerging' there just isn't a ton of clarity/info out there yet on real world usage.
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