Installing serial solar panels

[ramblinrod]

Originally Posted by TeddyDiver

Aaaargh!
Isn't that just what he said... So you're wrong saying he's wrong when you are both right. The point is in parallel you loose the other panel completely but in series you loose only 1/3 of that same panel.

BR Teddy

What are you talking about?

If one has two 100W panels connected in series, you can't have 2.5 A going through the shaded panel and 5 A going through the non-shaded. They are in series. If one is 2.5 A, both are 2.5 A.

So in theory, if the panels were equipped with bypass diodes in the various series string of cells within each panel, one would expect there to be no significant difference between series or parallel connected panels when it comes to shading.

But please view the video in post 30.

This is a perfect example of my experience between series and parallel connected panels.

In practice, shading absolutely kills panels connected in series, regardless of how many diodes are indicated in the spec sheet.

[ErikElmgren]

Originally Posted by travellerw

P.S. back to the original question about MC4 connectors.. We USED to have MC4 connectors on our panels. I cut all of them off after we had multiple (4) failures. We no longer use them on our boat and I wouldn't rely on them as a "cutoff" for the panels. Not the best solution, but we throw towels over the panels.

Thanks for the real live example of series connection.

If you disconnect MC4 connectors under load you will get arching and this will at least deteriorate the connectors. If you were doing that I'm not surprised that they started failing. If they failed without being used to break the load then it's more worrying as I thought these were high quality parts.

[travellerw]

Originally Posted by ErikElmgren

Thanks for the real live example of series connection.

If you disconnect MC4 connectors under load you will get arching and this will at least deteriorate the connectors. If you were doing that I'm not surprised that they started failing. If they failed without being used to break the load then it's more worrying as I thought these were high quality parts.

We never disconnected them under load. They failed due to moisture intrusion and pin corrosion. To be fair, all our connectors were just the cheap ones on Amazon. Maybe brand name ones would fair better, but we just eliminated them to remove that failure point.

[john61ct]

Yes, included MC4s are rarely good quality.

Re switches breakers etc are all those really needed in the first place?

If you want to save money, fuses and say Anderson plugs at the controller should suffice whatever your topology choices right?

[DotDun]

Originally Posted by ramblinrod

First of all, calling others "ignorant" is not cool. (I believe I've called you on this before in another thread.)

What is the rule about voltage sources connected in parallel?

Oh yeah, THEY MUST BE EQUAL.

So you are completely wrong, or is that "ignorant" to you?

If you have two voltage sources in parallel, they must be the same voltage. It's the law.

When a boater has two solar panels in parallel, the shaded panel producing zero current, produces zero current, it does not affect the unshaded parallel panel whatsoever.

So before you go calling others names, get your "alternative facts" straight.

Is that a Canadian law????

Hmm, don't tell the net metering folks! The whole industry would fall apart if their inverters had to be the same voltage as commercial power!

[DotDun]

Originally Posted by ramblinrod

OK, lets look at a very common scenario, comparing apples to apples, and using Blue Sea Marine products as the source (which I highly recommend BTW).

Common Scenario:

3 x 100W panels rated 15 Vdc at 5 A ea.

Connect them in series and an SPST switch, rated 45 Vdc or better costs $48.

https://www.bluesea.com/products/900...Battery_Switch

Connect them in parallel, and a Blue Sea Contura III, 8282 costs $12.

https://www.bluesea.com/products/828...Black_-_OFF-ON

How many switches?

One per panel for 3 panels?

Series = 3 x $48 = $144 + Tax

Parallel = 3 x $12 = $36 + Tax

Now lets talk breakers

Series = 3 x 7080 = 3 x $54 = $162

Parallel = 3 x 7402 = 3 x $27 = $81

Or, put 3 switches in a nice little panel with a switch and resettable breaker for each for $111

https://www.bluesea.com/products/827...ite_3_Position

Big, clunky, costly components for series connected panels, or small unobtrusive, affordable components for parallel connected panels.

The choice is yours.

Why are you using battery switches? Why not use 2-pole solar disconnect switches? Why a switch on each panel?

[Paul Elliott]

Originally Posted by DotDun

Is that a Canadian law????

Hmm, don't tell the net metering folks! The whole industry would fall apart if their inverters had to be the same voltage as commercial power!

Yes, this is a law, derived from "Kirchhoff's voltage law". The confusion here is that solar cells are not constant-voltage devices, but instead are largely constant-current.

Under some conditions, a partially-shaded panel in parallel with an unshaded panel *may* deliver some current, but this depends on the nature of the shading, and any bypass diode configuration.

With more extreme shading, even with bypass diodes the shaded panel will contribute essentially zero current because the total voltage output from this panel is too low and the panel will become reverse-biased by the voltage from the unshaded parallel panel.

[ramblinrod]

Originally Posted by DotDun

Why are you using battery switches? Why not use 2-pole solar disconnect switches? Why a switch on each panel?

This was in response to those who wish to have switches or breakers in place to disconnect / isolate panels.

These were just comparisons through a single supplier (that I commonly use) for an apples to apples comparison.

For smaller systems (up to 400W) I don't recommend any switches or breakers. To me, they are just another unnecessary potential failure node, and added cost for little value.

For the rare occasion where a panel may require isolation, a towel over and disconnecting the MC4s works just fine. In my experience, if one doesn't bend or twist the MC4 while making or breaking they are good for, well, I've never had one fail in about 1000 connects / disconnects performing installations and trouble shooting installs by others.

My point is, and perhaps I was playing devil's advocate a little too strongly, there is no "best way" between series and parallel solar panel installation. Every case is different and requires evaluation of user needs, expectations, boat design / layout, etc.

In some cases, serial connection may work best, in other parallel, and yet in others series / parallel.

As with boat designs, there are pros and cons to every design compromise. During the eval, if one only considers the pros of one option and the cons of others, this skews the conclusion in an unrealistic way.

Series:

Pros:

1. Reduces current and voltage drop per resistance / meter so smaller gauge cables can be used. (This is really just one thing.)

2. Provides higher voltage in poor sunlight conditions so MPPT controllers can work better. (This is mostly theoretical, 10% more of near nuthin is still pretty much nuthin.)

Cons:

1. Increases voltage to potentially dangerous levels. Regardless what some may say, it is wise to use lower voltage when possible from a safety perspective. A person is more likely to be killed by 220 Vac when standing in water than with 110 Vac. Similarly, a person is more likely to get a nasty shock from 60 Vdc than 15.

2. Increases cost of wiring components like switches and breakers (if desired), because it more likely to cause contact arcing and deterioration.

3. To limit the devastating impact of shading, a number of bypass diodes have to be included. Lets say, 3 per panel. For a standard silicon diode this is a .7 Vdc voltage drop (6%) per diode, when it is biased to pass current.

Parallel

Pros:

1. Reduces impact of shading. Anyone can argue till the cows come home, but through all the theory about what bypass diodes may do out the window, and do the tests on real systems and you'll see what I mean. The impedance of a forward biased p/n junction (diode) has a significant impact on the current bypassed. It is not a dead short, like most are pretending.

2. Reduces voltage so more common, less costly components like switches and breakers can be used.

3. No bypass diodes required to reduce the devastating impact of shading. Using smaller panels (100W or less) bypass diodes not required (shading of one cell and loss of that panel will be less than shading one cell in a large panel with bypass diodes).

Cons:

1. Voltage in low light conditions may be insufficient for charging. What is the actual impact? Maybe a loss of 1 A/hr per day in a 400W system?

2. Higher current requires larger gauge cable to reduce voltage drop.

In a 15 V x 400 Watt system with a 20 ft round trip solar panel cable run to the controller, max current = 27 A, requires minimum 90C rated conductor size of 14 AWG (per ABYC E11).

For 10% Voltage drop, around 12 AWG. Retail = C$0.70/ft x 20 ft = $14.

For 3%, around 6 AWG. Retail C$3/ft x 20 feet = $60.

The huge difference of $44!!!!! Negligible in the grand scheme, and more than covered by one switch or breaker if connecting in series.

[DotDun]

Originally Posted by Paul Elliott

Yes, this is a law, derived from "Kirchhoff's voltage law". The confusion here is that solar cells are not constant-voltage devices, but instead are largely constant-current.

Net metering works because the consumer side inverter is ~3v higher than commercial power. If the voltage was the same, no current would flow back to the grid. Apparently Kirchhoff didn't understand this.

In PV cells the current is dictated by the amount of photons causing the electrons to jump the gap. They are certainly not constant-currents devices, especially in shading situations.

Originally Posted by Paul Elliott

Under some conditions, a partially-shaded panel in parallel with an unshaded panel *may* deliver some current, but this depends on the nature of the shading, and any bypass diode configuration.

With more extreme shading, even with bypass diodes the shaded panel will contribute essentially zero current because the total voltage output from this panel is too low and the panel will become reverse-biased by the voltage from the unshaded parallel panel.

Each bypass diode creates essentially a smaller panel, i.e. a 60 cell panel with 3 bypass diodes becomes (3) 20 cell panels.

[Paul Elliott]

Originally Posted by DotDun

Net metering works because the consumer side inverter is ~3v higher than commercial power. If the voltage was the same, no current would flow back to the grid. Apparently Kirchhoff didn't understand this.

When connected to the mains, the consumer-side inverter output is *not* 3V higher than the mains voltage. It is exactly equal. Kirchhoff was pretty smart!
The inverter's open-circuit or unloaded voltage will be higher, but this is neither here nor there. An inverter has a completely different output circuit (and V/I characteristics) than a solar panel.

Yes, with the right kind of shading a 36-cell panel with two bypass-diodes becomes effectively a single 18-cell panel with an additional diode-drop.

[DotDun]

Originally Posted by Paul Elliott

When connected to the mains, the consumer-side inverter output is *not* 3V higher than the mains voltage. It is exactly equal. Kirchhoff was pretty smart!
The inverter's open-circuit or unloaded voltage will be higher, but this is neither here nor there. An inverter has a completely different output circuit (and V/I characteristics) than a solar panel.

Yes, with the right kind of shading a 36-cell panel with two bypass-diodes becomes effectively a single 18-cell panel with an additional diode-drop.

The grid tie inverter (GTI) must match the phase of the grid and maintain the output voltage slightly higher than the grid voltage at any instant.

https://en.wikipedia.org/wiki/Grid-tie_inverter

The ones I am familiar with run 3v higher than the grid.

[Paul Elliott]

Originally Posted by DotDun

https://en.wikipedia.org/wiki/Grid-tie_inverter

The ones I am familiar with run 3v higher than the grid.

By now, we are arguing semantics and probably using different points of measurement. Since this no longer relates to solar panel connections, I will abandon the discussion.

[ramblinrod]

Originally Posted by DotDun

Is that a Canadian law????

Hmm, don't tell the net metering folks! The whole industry would fall apart if their inverters had to be the same voltage as commercial power!

I said 2 voltage sources in parallel (referring to a DC circuit).

An inverter, and a whole pile of voltage sources, with a whole pile o %$#% in between, is not 2 DC voltage sources in parallel.



The positive and negative connection nodes between the two sources have to be one voltage. Has to be. How can a single node be two different voltages at the same time?

That Kirchoff, he was a fart smucker. He developed his "First law" that defines this, 172 years ago. Catch up dude.

[ramblinrod]

Originally Posted by TeddyDiver

If that's the way and components you want to use
Series = 1x$48+1x$54=$102 one string, one breaker
Parallel = 3x$12+3x$27=$117 three strings...

BR Teddy

I don't know what you are talking about. One can put 732 breakers and switches on panels in series if they wish to artificially inflate the cost, but I don't really get the point.

There is no reason to put 3 in on the parallel array and only one on the series array.

If one want to be able to isolate the array, for either series or parallel it takes one, if one wants to isolate every panel, for either series or parallel it takes 3.

[DotDun]

Originally Posted by ramblinrod

I said 2 voltage sources in parallel (referring to a DC circuit).

An inverter, and a whole pile of voltage sources, with a whole pile o %$#% in between, is not 2 DC voltage sources in parallel.



The positive and negative connection nodes between the two sources have to be one voltage. Has to be. How can a single node be two different voltages at the same time?

That Kirchoff, he was a fart smucker. He developed his "First law" that defines this, 172 years ago. Catch up dude.

Your right! I need to catch up!