Half-cut panels, shading, and MPPT range.

[fafrd]

I’m planning a new DC-coupled array based on half-cut panels which will suffer from partial shading.

I’m hoping to use 3S strings and as I’m understanding the interaction of shading, bypass diodes, and half-cut cell panels, I thought I share what I’m coming to understand as far as the requirements that places on Vmppt range of an MPPT charge controller.

Half-cut panels change the equation because if a panel is only shaded on one-half (upper or lower in portrait), they can put out Imp/2 @ Vmp.

So if you have a string of 3 panels where 2 are ever half-shaded, you want your MPPT to run at full 3x Vmp and half-current (Imp/2) for ~1/2 max power instead of cutting out both shaded panels using bypass diodes which would result in only ~1/3rd max power.

So what happens when 1 panel wants to pass twice the current of the other panels?

It will increase panel voltage beyond Vmp and begin approaching Voc to the point that current is reduced by half.

If we make a simple linear model of Imp @ Vmp and 0A @ Voc, voltage will increase to at least Vmp + (Voc - Vmp)/2. Since the actual power curve extends out past this linear approximation before dropping more steeply, the actual voltage needed to drop current to Imp/2 will be slightly higher than this estimate.

So the power being output from the unshaded panel will be reduced from Imp x Vmp to > Imp/2 x [Vmp + (Voc-Vmp)/2] which will be somewhere beyond half max power.

What this means as far as an MPPT controller is that it must have the range to extend to 3 x Vmp + (Voc-Vmp)/2 in order to fully-extract this available power.

My 450W panels are 41V Vmp and 49.6V Voc, so I need the Vmppt range to extend to at least 123V + 4.3V = 127.3V.

But on a cold day, Vmp can be as much as 43.6V and Voc can be as much 52.7V so I’d ideally like an SCC that can control Vmppt all the way to 130.8V + 4.55V = 135.35V.

Epever’s 200V MPPT charge controllers control Vmppt to 144V and WZRELB’s 170V MPPT controller control Vmppt all the way to 150V, so looks like either will fit the bill as far as controlling 3S strings of these 450W half-cut panels with partial-shading.

But this is another constraint to consider when planning an array of half-cut panels which will be impacted by shade...

 

[fafrd]

By the way, one thing you can do to mitigate this possible energy loss is to use a Tigo optimizer on the partially-shaded panel.

Tigo’s optimizers will buck to a maximum of 33%/25%, meaning that current of Imp/2 can be increased to 1.33 x Imp/2 = 2/3 Imp at a reduced voltage of 3/4Vmp (total power out of the partially-shaded panel remains 1/2 of Vmp x Imp).

The unshaded panel now needs to reduce current to only 2/3 Imp rather than 1/2 Imp, which it can do at a voltage slightly higher than Vmp + (Voc - Vmp)/3 resulting in power output of over 2/3 Imp x [Vmp + (Voc - Vmp)/3] or more than 2/3 Imp x Vmp or 67% of max.

In the case of my 450W panels, (Voc - Vmp) = 22% Vmp so a third of that equals 7.33% and total power out of the unshaded panel in that current-constrained condition will increase from over 50% x (100% + 11%) = >55.5% without a Tigo optimizer to over 67% x (100% + 7.33%) = >71.6% (about a 30% increase).

The two panel string will be putting out 53% of max power with no optimizer, versus 60.8% of max power versus a theoretical limit of 75% of max power if the two panels were connected in parallel.

So you will be losing 19% of available power with the Tigo Optimizer versus 29.3.% without an optimizer.

It’s a pity Tigo optimizers won’t buck closer to 100%/50% (doubling current and halving voltage) - that would make them a much better fit for partially-shaded half-cut panels...

 

[curto]

By the way, one thing you can do to mitigate this possible energy loss is to use a Tigo optimizer on the partially-shaded panel.

Tigo’s optimizers will buck to a maximum of 33%/25%, meaning that current of Imp/2 can be increased to 1.33 x Imp/2 = 2/3 Imp at a reduced voltage of 3/4Vmp (total power out of the partially-shaded panel remains 1/2 of Vmp x Imp).

The unshaded panel now needs to reduce current to only 2/3 Imp rather than 1/2 Imp, which it can do at a voltage slightly higher than Vmp + (Voc - Vmp)/3 resulting in power output of over 2/3 Imp x [Vmp + (Voc - Vmp)/3] or more than 2/3 Imp x Vmp or 67% of max.

In the case of my 450W panels, (Voc - Vmp) = 22% Vmp so a third of that equals 7.33% and total power out of the unshaded panel in that current-constrained condition will increase from over 50% x (100% + 11%) = >55.5% without a Tigo optimizer to over 67% x (100% + 7.33%) = >71.6% (about a 30% increase).

The two panel string will be putting out 53% of max power with no optimizer, versus 60.8% of max power versus a theoretical limit of 75% of max power if the two panels were connected in parallel.

So you will be losing 19% of available power with the Tigo Optimizer versus 29.3.% without an optimizer.

It’s a pity Tigo optimizers won’t buck closer to 100%/50% (doubling current and halving voltage) - that would make them a much better fit for partially-shaded half-cut panels...

Hey - you seem to know your stuff in relation to these 1/2 cut panels - can i ask a question ?

I am on grid and have a string of 13 1/2 cut CS 370w panels.

In the morning 7 of the panels are shaded until about 12:30 at this time of the year - my initial thoughts were to put TIGO optimizers on these 7 panels - they are in landscape mode rather than portrait - but this has not worked out as the string output is held down to around the 1 amp mark until they all get into full sun.

Initially i thought that with them in landscape mode and the shadow extending across both the top and bottom 1/2 of the panel that the TIGOs would kick in a bypass the panels entirely and let the 6 unshaded panels work - this has not proven to be the case - so i am thinking now of splitting the string into 7 and 6 panels and let the voltage boost of the TIGOs take care of the voltage mismatch for the shorter string - do you think this would work ? Or could you suggest a different approach ?

Craig

 

[fafrd]

Hey - you seem to know your stuff in relation to these 1/2 cut panels - can i ask a question ?

By all means - pretty certain that’s one of the primary reasons for this Forum .

I am on grid and have a string of 13 1/2 cut CS 370w panels.

In the morning 7 of the panels are shaded until about 12:30 at this time of the year - my initial thoughts were to put TIGO optimizers on these 7 panels - they are in landscape mode rather than portrait - but this has not worked out as the string output is held down to around the 1 amp mark until they all get into full sun.

Initially i thought that with them in landscape mode and the shadow extending across both the top and bottom 1/2 of the panel that the TIGOs would kick in a bypass the panels entirely and let the 6 unshaded panels work - this has not proven to be the case - so i am thinking now of splitting the string into 7 and 6 panels and let the voltage boost of the TIGOs take care of the voltage mismatch for the shorter string - do you think this would work ? Or could you suggest a different approach ?

Craig

We can get lost in the language so it’s important to be sure you communicate the shading pattern clearly (or attach a picture or photo).

My shading is horizontal. The sun starts reaching the Apex of the roof and slowly shifts downwards until the panels are unshaded.

For my shading situation, half-cut panels are much more productive in portrait orientation rathe than landscape.

I’m portrait, as soon as the upper-halves of all panels are unshaded, the whole array is operating at 50% power.

In general, optimizers are not going to do much of anything for you if you are dealing with a relatively widespread / uniform shading issue (like mine),

Optimizers may help to some degree if you have one or a small number of partly-shaded panels in an otherwise unshaded array, though the bypass diodes alone will achieve much of the same without needing optimizers.

Optimizers are most helpful when using a single MPPT controller fed by two differently-sized strings (either different number of panels or different panel specs).

So you’re going to have to describe more about your system including MPPT voltage range of your SCC and string architecture you are using before I can help you understand whether there is any role your optimizers may play in improving output.

I have a 3P1S string of three PV half-cut panels served by a single MPPT so that as soon as any half-panel is shade free, it starts contributing 50% of max possible output to the charge controller.

I see you’ve got 13 panels and it sounds like they are organized into 2 parallel strings of 6S and 7S, is that right?

Served by a single MPPT or 2?

And then you’ve got to make clear what shade pattern you are hoping to squeeze more output from (as well as minimum Vmppt of your SCC as well as Vmp of your panels…

 

[curto]

Hey - thanks for the quick response.

Ok we have a Solis 25Kw inverter - it has 15Kw of panels attached to it.

The longest string is 14 panels (all CS 370) which is the string that gets the first and earliest sun in the day - this produces more than 500v - more than enough to awaken the inverter whose startup voltage is 350v.

The string that i am having problems with is a newer one (installed in April last year) it is an L shaped string of 13 panels (all CS 370)

This string is on a single MPPT to the Solis inverter - the MPPT range on this inverter is 200-600v

All of the panels in this string are in landscape orientation due to the shape of the roof.

We are in the Southern Hemisphere (so our Summer now)

If you imagine the short section of the L faces due North and has 7 panels

The longer section of the string faces NW and has 6 panels - these panels suffer significant morning shade in Winter so to resolve this (we thought) we placed TIGO TS4 optimizers on each of these panels.

What we are finding is that the Eaves of the 2nd storey of the house run along the top 3 panels in this string (and as they are in Landscape) they are suffering shade essentially across the whole length of the panel - over about 1/2 of the total area i.e. both 1/2 cut sections are effected.

What we were hoping would happen was that the TIGOs would isolate all of these panels out of the string and leave us with the 7 other panels - producing from around 8AM when they are in full sun - but this is not the case (and we have proven it by isolating the string down to just the 7 panels) and it then roars into life - around 8AM or so - depending on the time of the year.

So what we are thinking of doing is to split the string into two - one of 7 panels and the other 6 (all with TIGO optimizers) - we would then parallel them into the same single input on the inverter. The thought being that this string of 6 panels would be voltage boosted by the TIGOs to match up with the other panels and when they were shaded and producing very low amps - it would be added to the other string rather than dragging the amps right down as it is doing now.

Does this sound like it would work - or can you see a smarter way to do it ?

Craig

 

[fafrd]

Hey - thanks for the quick response.

Ok we have a Solis 25Kw inverter - it has 15Kw of panels attached to it.

The longest string is 14 panels (all CS 370) which is the string that gets the first and earliest sun in the day - this produces more than 500v - more than enough to awaken the inverter whose startup voltage is 350v.

You need to specify Vmp of your panels, but I’m guessing ~40V which would give ~560V for your 14S string and mean minimum string length is 9S to exceed the startup voltage of 350V.

The string that i am having problems with is a newer one (installed in April last year) it is an L shaped string of 13 panels (all CS 370)

This string is on a single MPPT to the Solis inverter - the MPPT range on this inverter is 200-600v

Not understanding if it is a separate MPPT or shared with the other string but will assume the new 13-panel string has its own independent MPPT (since you can’t do much of anything if it doesn’t).

Also, not understanding what it means to have a ‘startup voltage’ of 350V but a minimum Vmppt voltage of 200V. You need 9 unshaded panels to deliver a string voltage of over 350V but only 6 to exceed 200V. Can your inverter function with a 6S string or not?

All of the panels in this string are in landscape orientation due to the shape of the roof.

And you’ve got no possibility to orient them in portrait? If the shadows are horizontal (side to side, not top to bottom), landscape orientation nullifies any advantage from half-cut panels.

We are in the Southern Hemisphere (so our Summer now)

If you imagine the short section of the L faces due North and has 7 panels

So these 7 panels have no shading issue, correct?

The longer section of the string faces NW and has 6 panels - these panels suffer significant morning shade in Winter so to resolve this (we thought) we placed TIGO TS4 optimizers on each of these panels.

So these 6 panels are in landscape orientation and essentially blocked by horizontal shade until midday, right?

What we are finding is that the Eaves of the 2nd storey of the house run along the top 3 panels in this string (and as they are in Landscape) they are suffering shade essentially across the whole length of the panel - over about 1/2 of the total area i.e. both 1/2 cut sections are effected.

No output from any of those 3 horizontally-shaded panels until the sun clears…. What about the other 3 panels in that sub-string?

What we were hoping would happen was that the TIGOs would isolate all of these panels out of the string and leave us with the 7 other panels - producing from around 8AM when they are in full sun - but this is not the case (and we have proven it by isolating the string down to just the 7 panels) and it then roars into life - around 8AM or so - depending on the time of the year.

So a 7S panel alone will startup properly and deliver power from te MPPT, correct. 7S should get you right around 280V at Vmp and probably closer to 350V at Voc, so you are just barely making it past the startup voltage of 350V and the the MPPT can increase output by dropping under 300V…

Whenever bypass diodes of a panel are activated, that drops string voltage by ~0.8V, or let’s say 2.4V per each fully-bypassed panel.

So when the 6 shaded panels are being bypassed, that’ll drop Voc string voltage by close to 15V, which is probably enough to keep the total string voltage under 350V.

Optimizers will increase current but at the cost of even lower voltage, so they won’t help at all…

So what we are thinking of doing is to split the string into two - one of 7 panels and the other 6 (all with TIGO optimizers) - we would then parallel them into the same single input on the inverter.

Yes, going from 13S to 7S in parallel with 6S will work much better. The 7S string will start to produce and the 6S string will be choked off until it is completely unshaded.

Once the 6S string is unshaded, it will deliver sub-optimal output because the voltage it set for 7S which is higher than it should be for 6S.

But that is where your Tigo Optimuzers finally pay off.

If every panel in the unshaded 7S string has an optimizer, they will increase current by ~17% and drop voltage by ~17% to deliver 100% output at 6Vmp rather than 7Vmo (which will allow the 6S string to deliver optimal output once it is finally unshaded.

So now at 6S, your MPPT voltage should drop to ~240V, still above your minimum.

2 6S strings of those panels would never start up with your inverter, but a 7S string with optimizer will and will then be able to drop down to the 6S range needed to support the shorter 6S string once it is unshaded.

I’m not a big fan of optimuzers in general, but yours is one of the rare situations where they truly pay off…

The thought being that this string of 6 panels would be voltage boosted by the TIGOs to match up with the other panels and when they were shaded and producing very low amps - it would be added to the other string rather than dragging the amps right down as it is doing now.

Does this sound like it would work - or can you see a smarter way to do it ?

Craig

Optimizers don’t boost voltage, they boost current (at the expense of voltage).

But because you’ve confirmed that a 7S string will startup your inverter (and that the Vmp minimum is 200V which should be less than 6xVmp), a 7S string in parallel with a 6S string should work well for you.

Each panel in the 7S string needs an optimizer. Optimizers won’t hurt anything on the 6S string, but they aren’t needed…

Good luck and let us know how the reality matches up against the theory .

 

[curto]

You need to specify Vmp of your panels, but I’m guessing ~40V which would give ~560V for your 14S string and mean minimum string length is 9S to exceed the startup voltage of 350V.

My understanding with this ongrid inverter - Data sheet here - is that there are two voltages that matter

1) Startup Voltage (of any string) is what is used to wake the inverter up in the morning and hence it is "dead" until this voltage is reached
2) MPPT voltage range - the range within which the MPPT algorithm for the inverter will be able to actively seek the sweet spot;

Not understanding if it is a separate MPPT or shared with the other string but will assume the new 13-panel string has its own independent MPPT (since you can’t do much of anything if it doesn’t).

The string in question is a completely seperate string of 13 panels on its own MPPT input (the inverter has 4 MPPTs)

Also, not understanding what it means to have a ‘startup voltage’ of 350V but a minimum Vmppt voltage of 200V. You need 9 unshaded panels to deliver a string voltage of over 350V but only 6 to exceed 200V. Can your inverter function with a 6S string or not?

Yes once the inverter has started up (by any string) - then the MPPT range kicks in.

And you’ve got no possibility to orient them in portrait? If the shadows are horizontal (side to side, not top to bottom), landscape orientation nullifies any advantage from half-cut panels.

No unfortunately not - that section of the roof is not able to fit two panels in portrait. Yes we sot of figured out that the 1/2 cut was not going to help - but was hoping that the panel diodes/Tigos would have taken them out of the string completely until they were unshaded.

So these 7 panels have no shading issue, correct?

Yes - correct - once the sun clears the house across the street then these 7 panels have full sun from approx 9AM

So these 6 panels are in landscape orientation and essentially blocked by horizontal shade until midday, right?

Actually the top 3 panels are the only ones with horizontal shade after 9am - the bottom 3 panels are also clear.

No output from any of those 3 horizontally-shaded panels until the sun clears…. What about the other 3 panels in that sub-string?

yes well technically those 3 bottom panels should be able to produce - but was prepared to sacrifice their output if splitting the 13 string into two shorter ones would achieve more overall output.

So a 7S panel alone will startup properly and deliver power from te MPPT, correct. 7S should get you right around 280V at Vmp and probably closer to 350V at Voc, so you are just barely making it past the startup voltage of 350V and the the MPPT can increase output by dropping under 300V…

Yes thats my thoughts - but the inverter is already started by my larger 14s string so it is (i believe) the MPPT Voltages that then come into play

Whenever bypass diodes of a panel are activated, that drops string voltage by ~0.8V, or let’s say 2.4V per each fully-bypassed panel.

So when the 6 shaded panels are being bypassed, that’ll drop Voc string voltage by close to 15V, which is probably enough to keep the total string voltage under 350V.

Optimizers will increase current but at the cost of even lower voltage, so they won’t help at all…

Yes, going from 13S to 7S in parallel with 6S will work much better. The 7S string will start to produce and the 6S string will be choked off until it is completely unshaded.

Once the 6S string is unshaded, it will deliver sub-optimal output because the voltage it set for 7S which is higher than it should be for 6S.

But that is where your Tigo Optimuzers finally pay off.

If every panel in the unshaded 7S string has an optimizer, they will increase current by ~17% and drop voltage by ~17% to deliver 100% output at 6Vmp rather than 7Vmo (which will allow the 6S string to deliver optimal output once it is finally unshaded.

Wow this is the part that feels backwards to me - i thought that the TIgos were Boost units and boosted the voltage not the current. !!

So now at 6S, your MPPT voltage should drop to ~240V, still above your minimum.

2 6S strings of those panels would never start up with your inverter, but a 7S string with optimizer will and will then be able to drop down to the 6S range needed to support the shorter 6S string once it is unshaded.

I’m not a big fan of optimuzers in general, but yours is one of the rare situations where they truly pay off…

Optimizers don’t boost voltage, they boost current (at the expense of voltage).

But because you’ve confirmed that a 7S string will startup your inverter (and that the Vmp minimum is 200V which should be less than 6xVmp), a 7S string in parallel with a 6S string should work well for you.

Each panel in the 7S string needs an optimizer. Optimizers won’t hurt anything on the 6S string, but they aren’t needed…

Good luck and let us know how the reality matches up against the theory .

OK we have terrible weather and high winds at the moment so not going to get onto this until the middle of next week - but will do so and then report back.

I will build it up slowly and see where i get to and keep you updated.

Thanks for all your help and time on this

Craig

 

[curto]

OK just a quick updated - managed to get onto the roof today (was not until all the panels were in full sun) (Well overcast day - but no extra shadow on any of the panels)

To simplify things to start with i have split the panels into two equal length strings of 6 panels each (left one panel disconnected for the time being)

I have paralled these two 6s strings into a single MPPT input and they are producing the amount of power i would expect in the current overcast day.

Will report back tomorrow where will hopefully see the string come to life much earlier than previously witnissed

Craig

 

[fafrd]

OK just a quick updated - managed to get onto the roof today (was not until all the panels were in full sun) (Well overcast day - but no extra shadow on any of the panels)

To simplify things to start with i have split the panels into two equal length strings of 6 panels each (left one panel disconnected for the time being)

I have paralled these two 6s strings into a single MPPT input and they are producing the amount of power i would expect in the current overcast day.

Will report back tomorrow where will hopefully see the string come to life much earlier than previously witnissed

Craig

Why would you prefer a 622P string when your optimizers will support a 6S1P string in parallel with a 7S1P string (and that will also get your string to startup voltage more quickly)?

 

[curto]

I don't prefer that but wanted to simplify things to start with so made sure the string were the same lengths etc.

Once i have a sunny day (forecast for tomorrow) i want to see how the MPPT in the inverter reacts to the strings when one is in shade etc, once i have a baseline i can then look at adding the extra panel in and see what effect that has.

The startup voltage does not appear to be an issue as the longest string on a seperate MPPT always has plenty of volts to get the inverter going.

WIll keep up you updated as we have two suuny days coming up

Craig