Installing serial solar panels

[OceanSeaSpray]

Originally Posted by Paul Elliott

You have to completely and fully shade a cell or string or panel (as I and others have done in some of our testing) before the contribution from a shaded parallel panel drops to zero.

Taking the situation to the limit like this is certainly an educational experiment, but before making decisions it is important to remember that reality isn't that black and white... literally!

[HankOnthewater]

Noelex, thank you for you post #165. Yes, we are all really interested in the original question: series or parallel?
Jedy, thank you for your 3 links to previous threads! I read them all, as I have been following this thread.
Paul, thank for your tests.

Indeed there are many scenarios, different panels, different controllers, many different diode configurations.
Just on diodes. I noticed that in this thread 3 different types have been mentioned: bypass, blocking and series diodes. Am I correct that a series diode and blocking diodes are the same kind?
Here are some links to diode use in panels (just read one link, as all of them are similar):
http://www.electronics-tutorials.ws/...ss-diodes.html
Blocking and By-Pass Diodes Used in Solar Panels
https://www.altestore.com/blog/2016/...-solar-panels/
https://www.solar-electric.com/learn...ical-tips.html
Blocking Diode and Bypass Diode for solar panels - SINOVOLTAICS | Solar Technology & Asia

Each 12 Volt solarpanel has (most of the time) 36 'cells' that produce about 0.5 Volt each. So a string of those 36 cells (or 36 cells in series) produces 17-20 Volts. A single solarpanel may have more than one string of 36 cells. These strings are parallel within each panel.
Here-in may be a second source of confusion: the words parallel and series, even before we connect more than one solarpanel.

I know a little about electricity, but I do not call myself a recent expert. I have installed quite a few a solar-setups on boats, last one was about 10 years ago. I always used panels in parallel, as there were then no or few MPPT (maximum power point tracking) controllers used, just the traditional PWM (pulse width modulation) type of controller.

Can I can add another question to this thread?
Still considering a 12 volt system, when using a PWM type of controller, should panels always be parallel, not in series?
An answer should help many with older setups, that this discussion is only relevant when upgrading to new controller or new installations.

[Paul Elliott]

Originally Posted by HankOnthewater

Can I can add another question to this thread?
Still considering a 12 volt system, when using a PWM type of controller, should panels always be parallel, not in series?
An answer should help many with older setups, that this discussion is only relevant when upgrading to new controller or new installations.

Yes, always in parallel. With a PWM or other non-MPPT controller you want the optimum panel voltage to roughly match the battery charging voltage. Any extra voltage will just be wasted. If you were to connect two 36-cell panels in series and run this through a PWM charger into a 12V battery you would be throwing away half the available charging power.

[Paul Elliott]

Originally Posted by noelex 77

Series connection. Result 67.7w going into the batteries.

We have a single MPPT controller. The optimum Vmp is 16 + 15.5 = 31.5 v. The current running through the two panels must be identical, so this is 2.5A. Total output is 31.5 x 2.5 = 78.8w (the other possible Vmp is close to 16v; the output here will be similar). The loss in the single controller will be slightly higher because of the greater voltage conversion. Say 9w. The output is now 69.8w. Let's say 3% loss for wiring and resistance over circuit breakers and we have 67.7w going into the batteries.

noelex, thanks for this post. I've been wanting to explore these marginal cases, since, as you say, the "optimum" panel / controller configuration can be different based on small changes in shading.

In the case you illustrate above, yes, the current has to be equal in the series-connected panels. But at a point somewhere between partial and full shading the bypass diode will start conducting and the series current will rise to that of the unshaded panel (assuming there is enough voltage available).

It's not intuitively obvious to me where this transition occurs, so I built my own Spice solar panel model and will be playing with that in addition to the testing with real panels. (Spice is an electrical / electronic circuit simulator.)

noelex, are you using a simulator, or working by hand off the published V/I curves for your examples?

[noelex 77]

Originally Posted by Paul Elliott

It's not intuitively obvious to me where this transition occurs, so I built my own Spice solar panel model and will be playing with that in addition to the testing with real panels. (Spice is an electrical / electronic circuit simulator.)

That sounds great. The theoretical modelling of solar production is much more complicated than the simple examples (including mine) suggest. Spice might enable you to produce a realistic model.

Originally Posted by Paul Elliott

In the case you illustrate above, yes, the current has to be equal in the series-connected panels. But at a point somewhere between partial and full shading the bypass diode will start conducting and the series current will rise to that of the unshaded panel (assuming there is enough voltage available).

A simple way of looking at the situation of diffuse shade is to imagine each solar cell.
In the case I illustrated, each solar cell would be producing 0.43v and generating 3A.

Solar cells exchange voltage and current in a rather counter intuative way. This can be seen with the I/V curve. Basically the solar cell in the above conditions cannot produce significantly more current than 3A even if the voltage is dropped. So the second solar panel or any of the cells within the panel, cannot get close to 5A that the unshaded cell is capabable of producing.

If the bypass diodes were activated then the full 5A produced by the first solar panel could flow though the series conection, but in this case there is no power contribution from the shaded panel at all. In fact, the voltage drop in the bypass diodes means even the output of the first panel is reduced.

So if all the bypass diodes in the second panel were activated, series connection would produce a voltage of 16v less the drop in the bypass diodes so say a net 15.3v @ 5A = 76.5 w. With the power loss in the controller and the voltage drop in the wires the net output for both panels would be 66.4W. The result is very similar and once again way less than the 114W produced by the parallel connection. I am not convinced the bypass diodes would be conducting in this example, but if they are the output (slightly) drops.

I stress this is just one example. We cannot conclude from this one case that parallel is better than series, just as we cannot conclude from the other theoretical examples in this thread that series is better than parallel.

For extra bonus marks try to calculate what would happen if the cells of the shaded panel were only producing 0.5A. Once again, the important principal is that if the panels are connected in series, the current traveling through both panels must be equal. . Does the current in the unshaded panel drop from 5A to 0.5A?. Or do the bypass diodes become "activated" so the shaded panel produces no power, but the unshaded panel can continue to produce its 5A? If so what activates the bypass diodes? The cells of the shaded panel are still producing voltage. Where is the reverse voltage bias?

Originally Posted by Paul Elliott

noelex, are you using a simulator, or working by hand off the published V/I curves for your examples?

No, I am just working it out by hand. It is very important to understand the I/V curves when calculating theoretical solar output. Solar cells behave electrically in an unusual way, but they all (mono and poly) have reasonably similar characteristics. I did not use a particular panel's output curve, but I think the numbers are representative.

[ramblinrod]

Originally Posted by Paul Elliott

I'm not following you here. Any two-node passive device has just one impedance, measured at the two nodes. There are not different "across" and "through" impedances.

An ideal voltage source has zero impedance.

An ideal current source has infinite impedance.

Real-world devices are not ideal. We add series resistance to an ideal voltage source for a first approximation of a real-world voltage source. We add parallel resistance to simulate a real-world current source.

Solar cells are even more complicated, but the usual model consists of a current source, a diode, and two resistors:

Absolutely 100% correct.

The point I was attempting to make, was that the source impedance or equivalent circuit has negligble bearing on the discussion regarding the voltage being equal across panels connected in parallel.

[DotDun]

Ramblin,

Here is another example of how you lose credibility, IMO. You speak as an authority on sizing solar systems but yet propose the most ridiculous design. Also, you are attempting to use your ideal solar system to dictate the energy usage on a boat. That's not the way it works, you match the solar system to usage, not the other way around.

Originally Posted by ramblinrod

Interesting that as I scanned the link thread, by chance I'm sure l, the 4 I picked were all small panels. It really doesn't have to do with the size of boat. It is all to about energy consumption and replenishment.
If a skipper on a 100 ft yacht is very energy conservative, and consumes only 100 A-hrs per day, 400 W of solar should be plenty. If he is not so energy conscious and consumes 200 A-hrs per day, and brings the bank up to 80% by other means every morning, and wants solar just to top up the bank on nice days to keep the batteries functioning nicely, he still only needs 400 W.

1) This demonstrates to me that you never been close to a 100 ft yacht, let alone work on an energy audit and design a solar system for one.

2) 400W of solar will not replenish 100ah on a 100 ft yacht. Why? Because there isn't a 100 ft yacht floating on this planet with a 12v house bank!

3) One ice maker on a 100 ft yacht will consume everything a 400W solar system delivers. There are most likely 5-6 more refrigerators/freezer on such a vessel. Now get to the pumps, electronics, etc. I'll bet a 100 ft yacht would consume everything a 3kw solar system would put out and have to be very energy conservative to make that work.

4) I would love to witness you propose a 400W solar system to a skipper of such a vessel and then tell him that he is not very energy conscious if he goes over this budget. (I suggest you not stand next to the railing when doing this)

Let me finish your next sentence!

Originally Posted by ramblinrod

4x100W panels, in an unshaded location is a nice system for just about any size boat under 30 ft. Putting on several more hundred Watts in locations prone to shading, doesn't really give anyone any more.

I have 825W and no shading!

Originally Posted by ramblinrod

I've seen lots of boats with large panels on the lifelines or
side rails. Doesn't make it smart. At least one is in the shade most of the day. I laughed watching a Youtube where the guy was touting his rail mounted panels he could angle toward the sun with an elevation adjustment only. Of course this only worked for the unshaded panel and when the sun was a abeam the boat. Several episodes later he had only one panel; tacked the boat and forgot to raise the panel. In a later episode yet, the panels were moved up
onto an arch aft of the boom. Much better for solar production, but
now the boat heeled more due too all the weight being higher.

Several examples in the post you referenced where 1kw or more caused no ill effects to the boat. Your experience must be from boats ~30 ft and smaller, yet you propose a system for a 100 ft yacht and claim if it doesn't work the skipper is not so energy conscious! Yeah, that is ridiculous!

Originally Posted by ramblinrod

Yup, there are pros and cons to every desgn decision. Action before considering them properly, usually means sub optimal performance and dissatisfaction, when one learns how good it could have been.

[DotDun]

Originally Posted by noelex 77

I think this is a very important topic. We should know which option is best, serial or parallel. If one method of connection gives, on average, the best results for a shaded panel then this method is likely to be the best system for nearly all sailboats. So the conclusions are important for everyone installing a solar system.

I think the thread lost its way a little, but my reading of the posts is that overall the consensus is that serial wins. This mostly stems from the theoretical calculations showing the output is better for serial connected panels if one string is shaded. These calculations are mostly correct, although some of the more minor effects such as the voltage drop over the blocking diodes have been ignored. However, I don't think this one isolated theoretical case is representative.

To mathematically model solar panel output with shade is incredibly complex and picking one isolated case and drawing conclusions from that is wrong. There are cases where the theoretical output of parallel connected panels beats series connected panels. There are even scenarios where wiring the panels in series with a shaded panel can drop the output of an unshaded panel, occasionally significantly.

If we want to theoretically model solar panel output we need to consider all these options. Diffuse shadows are particularly complex and the results are very different depending on the type of shadow chosen.

Let's take a simple example where parallel connection wins convincingly. One panel is in full sun and one has a diffuse shadow. The two panels can have their own controller or be connected in parallel or series.

The maximum output of the panel in full sun is 16v @ 5A = 80 w. The maximium output of the panel in diffuse shade is 15.5v @ 3 A = 46.5 w

Seperate controller for each panel. Result 111.1w going into the batteries:

Here we have the full 126.5w less the self consumption and inefficiencies in the controllers. As we have two controllers, this may amount to around 12w so the output is now 114.5w. Let's say 3% loss for wiring and resistance over circuit breakers and we have 111.1w going into the batteries.

Parallel connection. Result 114.1w going into the batteries.

Here we have a single MPPT controller. This controller must pick the optimum Vmp for the array as a whole. This would fall very close to the lower voltage so for simplicity let's assume it is 15.5v (the actual optimum Vmp will be slightly higher). The current output of the first panel will rise slightly at this lower voltage so panel one produces 15.5v @ 5.1A = 79.1w. Panel two produces 15.5v @ 3 A = 46.5 w. We only have a single controller so the self consumption and inefficiencies in the controller will be less, let's say 8w. The output is now 117.6w. Let's say 3% loss for wiring and resistance over circuit breakers and we have 114.1w going into the batteries.

Series connection. Result 67.7w going into the batteries.

We have a single MPPT controller. The optimum Vmp is 16 + 15.5 = 31.5 v. The current running through the two panels must be identical, so this is 2.5A. Total output is 31.5 x 2.5 = 78.8w (the other possible Vmp is close to 16v; the output here will be similar). The loss in the single controller will be slightly higher because of the greater voltage conversion. Say 9w. The output is now 69.8w. Let's say 3% loss for wiring and resistance over circuit breakers and we have 67.7w going into the batteries.


The above calculations are slightly simplified, but I have taken into account most of the factors for this one scenario. You can see parallel connection wins. It even beats separate controllers for each panel, but importantly series connection is by far the worst option in this example. In this series example the second panel contributes nothing. However, it would be silly to extrapolate this one example to all cases of shade. It is just one example. For other types of shade two controllers will be best and for others series connection will be best. So the above calculations while accurate are a waste of time unless we model all the possibilities and integrate them into a realistic model taking into account the likelihood of these options. This is incredibly complex.

Basically, I think we cannot use theoretical calculations to determine if series or parallel connection is best. The argument used in this thread by others is that a single example where series connection beats parallel connection means we should draw the conclusion that series connection is best, is a false conclusion. Just like the above example should not be used to conclude parallel connection is best.

So we need experimental data not theoretical calculations. So keep the data coming Paul . This is the only way we will answer the question. However, even here we need a sophisticated experimental protocol. Producing shadows by laying sheets of card on the panels is not the same as shadow from the boom where a reasonable amount of light is still falling on the shaded area.

My suspicion is that parallel wins, but not because of the above calculations. I have no vested interest in one method of connection. I would just like to know the right answer.

Keep in mind, most MPPT controllers require vbat +5 to start, hence MPPT may not work well on 36 cell panels, depending on battery soc.

[noelex 77]

Originally Posted by DotDun

Keep in mind, most MPPT controllers require vbat +5 to start, hence MPPT may not work well on 36 cell panels, depending on battery soc.

The new Victron models do, but as you say if you want to run 36 cells in parallel there are perhaps better MPPT controllers. The high start up voltage is quite rare and is more to do with Victron trying to decide a sensible wake up routine rather than any inherent need for this voltage difference.

Solar cells generate reasonable voltage even under poor light where they cannot produce any useful current. If the controller wakes up at an early stage the self consumption from the MPPT circuitry can consume more power than the array is producing, loosing significant power.

The Victron requirement is not as bad as it seems, as it is looking at Voc not Vmp. Voc is typically 3-4v over Vmp for a 36 cell panel. Once the Victron controller has woken up it only needs 1v over battery voltage so the problem only effects the initial startup. On wake up the panel will be cool and close to its specified Voc, which is typically around 21V. There will be no voltage drop in the wiring and the battery voltage is likely to be low so I think it will only rarely be a problem, even with the Victron controller.

Most other MPPT controllers are fine with parallel connection of 36 cell panels. In fact I think this is probably the most efficient system, although I am always interested in alternative theories.

[ramblinrod]

Originally Posted by DotDun

Ramblin,

Here is another example of how you lose credibility, IMO. You speak as an authority on sizing solar systems but yet propose the most ridiculous design. Also, you are attempting to use your ideal solar system to dictate the energy usage on a boat. That's not the way it works, you match the solar system to usage, not the other way around.



1) This demonstrates to me that you never been close to a 100 ft yacht, let alone work on an energy audit and design a solar system for one.

2) 400W of solar will not replenish 100ah on a 100 ft yacht. Why? Because there isn't a 100 ft yacht floating on this planet with a 12v house bank!

3) One ice maker on a 100 ft yacht will consume everything a 400W solar system delivers. There are most likely 5-6 more refrigerators/freezer on such a vessel. Now get to the pumps, electronics, etc. I'll bet a 100 ft yacht would consume everything a 3kw solar system would put out and have to be very energy conservative to make that work.

4) I would love to witness you propose a 400W solar system to a skipper of such a vessel and then tell him that he is not very energy conscious if he goes over this budget. (I suggest you not stand next to the railing when doing this)

Let me finish your next sentence!


I have 825W and no shading!



Several examples in the post you referenced where 1kw or more caused no ill effects to the boat. Your experience must be from boats ~30 ft and smaller, yet you propose a system for a 100 ft yacht and claim if it doesn't work the skipper is not so energy conscious! Yeah, that is ridiculous!

DOT,

This forum is for boaters with vessels of all sizes.

There are lots of boats less than 100 ft and far fewer more than 100 ft.

I suspect the average boat of people following this thread is around 30-40 ft

I have stated many times in this thread and others, that when configuring a solar charging system for any vessel, one reviews the vessel design, expected energy consumption, and user needs.

My point in the prior post, is that the solar charging system is not sized based on LOA, and especially not alone.

As I stated, a 400W solar charging system may be absolutely suitable for a 100 ft yacht or larger, if all the boater needs, based on this review, is a 400W system, as I clearly stated.

There are lots of large boats with solar charging systems of 400W and less.

Now rather than your attempt to change the subject...

Remembering Kirchoff's First Law, if that system had 4 x 100W panels in parallel, and one panel had 15.0 Vdc across it, what voltage would the other panels have across them?

[hellosailor]

Paul-
"I don't understand this above. Can you re-phrase for me?"
Yes, although I think others have made it clear by now. "Diodes" are either bypass or blocking and that's a functional description of where they are, but the terms are often confused. If a "12v" panel has no internal diodes, that's acceptable when the panel uses only one string of 36 cells. So there "should" not be any kind of bad diode to be found in a single panel with only 36 cells. Unless there were, literally, 36 bypass diodes, one on each cell in the string, to allow the panel to put out more power when any one panel was in shade. I don't think anyone is making panels that way, it is more cost effective to just deal with shade if it happens. And since "shade" will mean a passing bird over most fixed panel installations...not the issue it is on boats.

Dotdun-
I haven't read new manuals lately, but recall that even ten years ago the Blue Sky MPPT controllers didn't mention needing bat+5v before they would start. Maybe I missed it, but IIRC they started working shortly after sunrise, when the panels might have a no-load voltage like that, but voltage under load could be very different, like 6V. I'd suspect the controller requirement would be based on panel output with just the load of the controller as the factor there.

And just as with inverters, MPPT efficiency rises as you get closer to their maximum ratings. Again, IIRC, they might be 96% for a 12v panel but 98% for a 36V (3*12) panel array. And again, 96% at half their rated power but 98% when near their full capability. So, not a big deal but potentially a 4-5% combined gain or loss in efficiency when matched properly.

[s57ra]

Originally Posted by noelex 77

I think the thread lost its way a little, but my reading of the posts is that overall the consensus is that serial wins. This mostly stems from the theoretical calculations showing the output is better for serial connected panels if one string is shaded. ...

I have just installed 4x165W mono cells. First i planned to have setup with 2 parallel Victron BlueSolar MPPT 100/30, each connected to 2 panels in parallel to have some redundancy in the system.

Panels are rated Imp=8,9, Vmp=19,5 V, Voc=23

After reading all those threads, i was determined to go to serial setup and maybe use only one controller and have the other one stored for spare.
Unfortunately i have been struck by the facts of reality...

If i connected 2 panels in parallel, i got about 220W at 4pm, very close to the 2xsinglePanel output. Partial shading (similar to one from the mast) killed performance of one panel, but i still got about 125W (one full panel plus few watts from other).
I then switched to serial configuration and under same conditions, the total power dropped to only 30W or so.
I checked the junction box, there were 3 shotky (bypass) diodes 20SQ045 (picture)
and voltages across them were strange, with no shading 13V, 5V and 2V. Unfortunately i cannot reverse-engineer the panel's schematics.

From the information i have up to now, the only option remains parallel connection and 2 controllers (all panels can produce more than 30A, exceeding rating of MPPT). But if someone of you serial freaks can resolve this, i'd be grateful (maybe additional logic to bridge shaded panel or something).

[noelex 77]

Originally Posted by hellosailor

If a "12v" panel has no internal diodes, that's acceptable when the panel uses only one string of 36 cells. So there "should" not be any kind of bad diode to be found in a single panel with only 36 cells. Unless there were, literally, 36 bypass diodes, one on each cell in the string, to allow the panel to put out more power when any one panel was in shade.I don't think anyone is making panels that way, it is more cost effective to just deal with shade if it happens. And since "shade" will mean a passing bird over most fixed panel installations...not the issue it is on boats.

Nearly all 36 cell panels have bypass diodes fitted. Open the panel up, or look on the specification sheet. The bypass diodes are installed fundamentally to protect the cells from overheating, rather than shade tolerance. Without bypass diodes a 36 cell panel is likely to have a short life. I am not sure why you think there has to be 0 or 36 bypass diodes, 2 or 3 diodes is the norm.

Originally Posted by hellosailor

Again, IIRC, they might be 96% for a 12v panel but 98% for a 36V (3*12) panel array. And again, 96% at half their rated power but 98% when near their full capability. So, not a big deal but potentially a 4-5% combined gain or loss in efficiency when matched properly.

This is the wrong way around. Controllers become less efficient when the panels are wired so the voltage is long way above battery voltage. Look at this graph from an Outback regulator. This performance is typical. You can see the effeciency falls as the panel voltage becomes much greater than the battery voltage:

[noelex 77]

Originally Posted by s57ra

From the information i have up to now, the only option remains parallel connection and 2 controllers (all panels can produce more than 30A, exceeding rating of MPPT). But if someone of you serial freaks can resolve this, i'd be grateful (maybe additional logic to bridge shaded panel or something).

Thanks for the report. I like to understand things from first principals, but I think the theoretical modelling of solar panel performance is way beyond my ability. So practical experimentation is the only way of settling the question.

Most user reports from people that have trialled both series and parallel connection are similar to yours, with parallel connection winning convincingly in shade conditions.

BTW a single 30A controller is not enough for your system. Even with series connection of the panels, the output current to the batteries will be greater than 30A.

[Paul Elliott]

Originally Posted by hellosailor

Yes, although I think others have made it clear by now. "Diodes" are either bypass or blocking and that's a functional description of where they are, but the terms are often confused. If a "12v" panel has no internal diodes, that's acceptable when the panel uses only one string of 36 cells. So there "should" not be any kind of bad diode to be found in a single panel with only 36 cells. Unless there were, literally, 36 bypass diodes, one on each cell in the string, to allow the panel to put out more power when any one panel was in shade. I don't think anyone is making panels that way, it is more cost effective to just deal with shade if it happens. And since "shade" will mean a passing bird over most fixed panel installations...not the issue it is on boats.

I still don't know what you mean by "should" (in quotes).

I've got 36-cell panels from two different manufactures, one set about 15 years old, the other is one year old. They all have two 18-cell strings, with bypass diodes across each string. These diodes can fail. As I mentioned, one of my new panels had an open connection in the bypass circuit (the diodes themselves were OK), but I suppose any of these diodes can go bad. It sounds like they generally go open-circuit, but diodes do short out as well (usually in an overload situation).

I'm quite familiar with solar panel diode configurations, I am just confused by your statement. Probably my fault. Anyway, I'm looking forward to panels with embedded per-cell micro-MPPT controllers. I know, it's not going to happen! Rooftop and utility are the solar panel applications that count, and they don't need that stuff (your "bird" example shows why). Boats get the high-volume table scraps or high-cost custom designs.