Blocking diodes or not?

[Redpacket]

So a discussion on blocking diodeswas underway in another thread but was off topic for that thread (no problems with mods keeping threads close to title, helps the site be logical, etc).
That thread is here: https://secondlifestorage.com/t-Calculating-Solar-System-DC-Wire-Size-and-System-Installation

I'm still interested in should we be using blocking diodes (different to bypass diodes) in systems where there are parallel strings on panels & one or more stringmay get shaded differently.

Is it a problem to have an example (but likely)situation where most of you array is putting out Vmp x number ofpanels in series. Then there's say one string that's shaded.
Is it detrimental for the shaded panels to have the full system voltage applied when they're not generating themselves (ie assuming no blocking diodes)?

On one hand, pretty much any light in the sky seems to bring panel voltage up, so shaded strings voltage might only be a bit less than the system voltage - so probably not an issue.

On the other hand, Australian Standards AS 5033 for solar panel installs, etc does mention including them in parallel string situations,as do links provided by Sean (from the other post)
https://www.electronics-tutorials.ws/diode/bypass-diodes.html

The problem of night time current bleed back through panels connected to batteries seems to be solved by have pretty much any charge controller between the panels & batteries.

The problem of forcing voltage onto a shaded panel is not so clear.

There's also the(less likely) situation of when there's a fault in a panel like a shorted bypass diode or shorted cells. Then the applied voltage from the rest of the system starts becoming more serious I think.

Thoughts?

 

[TheBatteries]

I have my panels set up such that as the sun comes up in the morning, it progresses across the panels so there's a part that's shaded at first. I debated using blocking diodes for a while but ended up deciding against it. The diodes create heat, decrease efficiency, and are costly. I did quite a bit of testing by putting my DC clamp meter on individual leads when some strings were partially shaded and did not experience any current flowing "backwards" as some say happens. The current being generated on the partially shaded string was just lower than the other strings.

That being said, I'm not an expert and maybe someone else knows better than I...

 

[Sean]

Rather than repeat the content of the previous thread, perhaps the OP could link to it ?

And perhaps this thread could be confined to evidence based (and supported by calculation or reference material) fact, not opinion, or credential flagging.

The only addition comment I will add to the previous thread is folks need to determine if the panels they are intending to use, have bypass diodes within the rear junction box (most modern panels do) - and to consider if they understand that blocking diodes will aid performance, or as per the change of direction towards the end of the previous thread, they perform a protective function in the event of wire damage/other unusual issue.

 

[wim]

Always avoid panels in parallel on different surfaces... i am not a fan of blocking diodes, thats why there isusual more than one MPPT input on a inverter.
Use different mppt inputs for each serial string of panels, no diodes;thats how its done these days.

 

[completelycharged]

"Therefore, the user must evaluate the efficiency of using blocking diodes for performance improvement for his particular case. If, forexample, the final aggregation consists of paralleling (k = 1), and m is small (e.g., 4 strings), then, the use of blocking diodes (in this case, only 4) is certainly warranted." Source : NASA 1979 : BDM Design Guidelines for large photovoltaic arrays (also blocking diode reason for use is transient lightning protection surge damage)

Even NASA back in 1979 understood the need in certain situations for blocking diodes on a pure energy and protection basis...

 

[Sean]

....... also blocking diode reason for use is transient lightning protection surge damage.

SPDs could well be a separate topic, albeit related to array protection - to introduce it here will distract from the OPs focus, thats an opinion.

PV panel cell design and layout have progressed a little since 1979, that's a fact.

 

[completelycharged]

Can you provide supporting evidence for the "that's a fact" ?

 

[Sean]

Can you provide supporting evidence for the "that's a fact" ?

Yes, clearly - but in doing so this thread would be dragged off at a tangent.

If you want to personally abuse me, feel free to PM me (obviously I won't reply), if you want to discuss lightning protection for PV arrays perhaps start another thread, if you'd like to continue debating the merits of blocking diodes, and their correct usage you are obviously free to add more on topic content here.

Whatever you do, please don't derail another thread, something we were both guilty of previously.

Mods, feel free to remove this post.

 

[completelycharged]

I thought that since 1979 with the evolution of different solar junction types and the doubling of efficiency that solar had actually changed quite a lot, with concentrated junctions, thin film, flexible, etc. All I was reflecting was your request to back up opinions with facts.

Source :




The NASA document includes the use of blocking diodes for voltage surges due to lightning flashes from damaging dark cells - how is this distracting from reasons for using blocking diodes ? That is why I included the source as a relevance to blocking diodes.

You then go on to try and direct the responses to your definition of a post that ended with the one word "Thoughts?"

 

[Sean]

I was going to post a few images and some words about the detailed balance limit, but I'm keen to avoid a repeat of a previous thread - the fact that improvements have been made isn't contentious. Im not a fan of debating using images but you seem to prefer that, and I suppose it helps quickly illustrate a point.

But the image you've posted does omit mention of the pioneering work done by Shockley at BL in the early 60s, which is of interest, if only in as much that his work on balance limits remains true today (well I find it of interest)

Again, mods free feel to remove this off topic content.

 

[Korishan]

determine if the panels they are intending to use, have bypass diodes within the rear junction box (most modern panels do) - and to consider if they understand that blocking diodes will aid performance, or as per the change of direction towards the end of the previous thread, they perform a protective function in the event of wire damage/other unusual issue.

This is something I've thought was happening on a lot of panels. The diodes are built within the panel.
With that, I will have to agree that adding diodes doesn't really add any extra benefits. If anything it drops the voltage of the output again, as the inbuilt ones are already doing.

If panels already have these built in, then there's no reason to add more. The only thing would be to know they are there, wire everything properly, and know where to look if a panel all of a sudden stops working; possibly the diode(s) failed somehow.

Keeping with the OP's original question:
Are the blocking diodes really of any benefit?
- If this is to include the ones already builtin, then yes. They cover probably the majority of the concerns that were pointed out in this and the previous thread. Backfeeding to the panel from other panels, and for helping to protect panels from lightning
- If this is meaning to add secondary diodes external to the panels, then probably not

Now, the biggest question, and this is regional. Are they required for proper installation?
- This is a question you'd have to ask your local solar installers, which I would ask different ones to get a consensus of the regulations. It's possible that they are required by certain zoning regulations (ie. commercial buildings yes, residential no; vice versa) or territory (in the city yes, in the country no; vice versa)
- Probably another good source would be where you get construction permits

 

[Jon]

Im no expert, just relating what Im doing and why, Im interested in others opinions and feedback.
We have a microinverter based 8kW PV system as our roof is multi faced and has a lot of close trees which throw a lot of shade, I dont think we have all our panels unshaded at any one time.
We currently have a pool pump which is running on a timer during the day, in an effort to reduce the power we use from our main system/the grid were installing a DC pool pump.
Im putting in a 4P 3S array of 2nd hand 250W panels, on the roof of our pool area shade which faces SE (in Southern Hemisphere) at about 10* and gets shading from close trees.
The pump manufacturer recomends 3S 300w panels, because of orientation, shading and the price of good 2nd hand panels Im using 4 of 250W panels instead of a single 300W panel.

From what Ive read Ive decided the best configuration is to make 3 of 4 panel parallel arrays and connect those 3 arrays into a series string rather than 4 of 3S strings and parallel them together.
Im using blocking diodes inside the parallel arrays.
Based on what Ive read I believe that the diodes in the panel junction box do a good job of combatting shading losses across the panel, my logic is that by making a 4P sub-array Im effectively making a single 750W panel so I should carry the diode logic up one level inside that sub-array.

Please be kind, Im learning this as I go along, feel free to use my system to explain what I could do better.

Mods; if youd prefer Ill start another thread asking for advice.

Thanks
jon

 

[completelycharged]

Bypass diodes and blocking diiodes are different and easily mixed up.

AS/NZS 5033 Clause 4.3.10 - blocking diode requirement for direct battery connection situations, diode rating specification, diode positioning wiring diagram for parallel strings, and explicit statement they are not to replace the function of a fuse.

UK : "Blocking diodes are not commonly used in a grid connect system as their function is better served by the installation of a string fuse. However, for multi-string arrays with some types of PV module, particularly thin-film types, it may not be possible to provide adequate overcurrent / reverse currentprotection with string fuses or MCBs alone.This is due to the fact it may not be possible to specify a fuse/MCB which is greater than Isc x 1.25 but less than the reverse current rating of the module. In this situation blocking diodes should be used in addition to string fuses."
Source : 2012 Microgeneration Certification Scheme (MCS) Guide to the installation of Photovoltaic systems


This is an outline of part of my wiring setup (I have several strings across 3 separate controllers) :







New and old panels, two differnet makes with different (close) Imp Vmp and Voc. High shading on some panels at differing times of the day.


The diodes drop around 0.7V with a string Vmp of 120V, so around 0.5% losses. Cost per MC4 diode was around $3 each. The NASA document also details the economic case outline regarding the power losses from the diode and back feeding. Yes, it's from 1979 but the basic math is the same today.

It's not explicit in AU/NZ code so if you don't want to use them there is no problem as far as I know (dont have access to a copy of the full wiring code)

UK depends on the exact wording in the current amended 17th Edition and new 18th Edition wiring regulations (come into effect Jan 2019), which I don't have access to at the moment. MCS is the accreditation scheme for the UK, so they are not talking BS in the guide.
USA.. pass.


If you have multiple parallel strings fused and connected into a single controller do some research on fault current handling to see if your panels are adequately protected.

This is all asside from reverse current flow degredation, which differs in extent depending on panel type.

 

[Redpacket]

Rather than repeat the content of the previous thread, perhaps the OP could link to it ?

Link added

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This is something I've thought was happening on a lot of panels. The diodes are built within the panel.

I don't think most panels include blocking diodes, they do typically include bypass diodes.
To be clear, bypass diodes in normal (non shaded) operation do not lower the voltage of the panel & are in parallel with sections of the panels (usually 3 sections).
I don't think manufacturers would not be keen on adding blocking diodes as they would drop the panels performance fractionally due to the small 0.7V drop across them and you know marketing likes a positive spin.

Re lightning protection, I strongly doubt diodes would cope with currents or voltages involved, they might help a bit with induced pulses from nearby strike, but direct hits, all bets are off, they'd be toast! Semiconductor devices are recognized as not being much with lightning.

Local regulations is a good point.

It seems the need for blocking diodes with shading is likely marginal (bypass diodes definitely needed here).
Like said before, thin film panels likely do need them (due to higher reverse current leakage), mono & noncrystalline panels not.

Panel/wiring fault condition protection & regulations seem to me to be the two stand-out reasons in favour of blocking diodes.

Agree strings where panels facing different directions should:
a) feed into separate MPPT inputs &
b) probably have blocking diodes if a) is not done

 

[Sean]

Just for clarity, could you confirm if you have added an additional diode across the output of each panel, or is that a rather stylised representation of the inbuilt bypass diodes ?

 

[completelycharged]

Panels will normally never be supplied with blocking diodes fitted as they are only needed if at all at the ends of the strings. Inline MC4 style are more normal - no screwdriver to install, which also allows live working on part of the installations when the PV panels are exposed. Cheaper build costs..

Lightning protection detailed in the NASA document is related to the light intensity from the flash. They quote a figure of 1200 times normal sunlight at a distance of 10m, which will decrease by the square of the distance, so only significant pulses for the close calls (sub half second flash bang). They actually added in zenner protection suggestion, but a typical charge controller with input capacitors may provide enough protection to cap the voltage at the controller end. Still leaves the panels under an interesting voltage pulse condition if they are not all facing the same direction / shading.

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I am showing where the factory fitted bypass dioded and my MC4 inline blocking diodes are located.

 

[Sean]

I am showing where the factory fitted bypass dioded and my MC4 inline blocking diodes are located.

Is that electrically how you understand the factory fitted diode? functions, as depicted ?

 

[Redpacket]

My setup is similar to completelycharged's diagram.
I used heavy duty flat pack schottky type diodes mounted on heatsinks inside (each string is separately cabled in my system).
There's a clear plastic safety cover over them.
Here's a pic:

 

[completelycharged]

It is how my panels are wired at the terminal box, also proven by the diode I had to take out of a panel when 150 miles away from the nearest shop that was likely to actually sell anything that contained a diode.

Internally other panels can be wired differently depending on the cell configuration and number of parallel/series arrangements internally and may also have more than one bypass diode fitted.

It's not how I understand them to be fitted it is how they are fitted to my panels.

 

[Redpacket]

I am showing where the factory fitted bypass dioded and my MC4 inline blocking diodes are located.

Is that electrically how you understand the factory fitted diode? functions, as depicted ?

That is the normal setup for bypass & blocking diodes....
The bypass diodes across each panel may actually be more than one diode & they're usually mounted in the junction box on the back of the panel.