MPPT Wiring Tricks

[witzgall]

Hi all;

We currently have one 120w 12v Kyocera panel on Alchemy. it is mounted 1.5 feet above the rear of the bimini, allowing it to be aimed at the sun.
I just upgraded the solar controller to a Morningstar 15 MPPT. I would like to add another panel.

Here is what I am thinking. Mount the second panel on the bimini itself. Since it will be lower then the first panel, there will be times when it is blocked by that panel. Wire them together in series. This allows me to keep the existing wiring. The new controller is rated at 200w @12v, or 400w @24 v. The two panels together will be around 250w.

So when one panel is "working", only 12v makes it to the controller, so I am not surpassing the ratings. When sun hits both panels, we are at 24v nominal to the controller, and still within ratings for wiring and controller. The MPPT controller automagically adjusts for the voltage between nominal 12-24v.

Will this work? Am I missing something? Will current flow if they are wired in series, and only one panel is getting sun? Does it mater which panel is "First" in the series wiring?

Chris

[Captain Bill]

When Wired in series when one panel is shaded you get no current, not half the volts. The panel being shaded does not conduct significantly. Assuming it is not in total darkness you'll get a small amount of current, but not anywhere near the nominal output. With no load on the panels you might in fact still see 24 volts, but as soon as you try to draw any power from it you would not get any significant output.

[Fuss]

Originally Posted by Captain Bill

When Wired in series when one panel is shaded you get no current, not half the volts. The panel being shaded does not conduct significantly. Assuming it is not in total darkness you'll get a small amount of current, but not anywhere near the nominal output. With no load on the panels you might in fact still see 24 volts, but as soon as you try to draw any power from it you would not get any significant output.

Tell me if this true....for 3x KD135 watt panels at 17v each for a 24v system into a tristar mppt 60.
If the panels are good quality like Kyocera then they have blocking diodes so that when one panel is not producing amps it does not take anything away from the remaining panels that are producing amps.
Otherwise it would never make sense to wire panels in series if there is even the remotest chance of one of them being shaded.
When one of my Kyoceras is in the shade then the others put out their amps as if the shaded one is not there.
Im sure thats true for my serial wired setup.
(can someone verify what I just wrote)

[Paul L]

I think you are misreading the ratings on the Morningstar controller. The 200w is for 12v output to the battery. The 400w is for 24v output to the battery. Since yours is a 12v system, it would be 200w max. It is basically a 15amp controller.

[Paul Elliott]

Ignoring the question of the power rating for the controller, here's the deal on the panel wiring:

With the two panels in series, as you suspect the shaded panel will not deliver any appreciable power. Because it becomes a high impedance when shaded, it will also prevent the un-shaded panel from delivering power.

The normal blocking diodes will prevent reverse current through a panel, but will not let current flow around the shaded panel. For that you need a "Bypass Diode", which is wired from (+) to (-) across a panel. In fact, you may not need the blocking diodes.

So, with two panels in series, and bypass diodes across each panel, you will see both panels when they are in the sun, and one panel (minus any diode drops) when the other is shaded.

[capnorv]

Usually an MPPT controller also has a maximum input voltage. Typically 50,100,150, I don't know what it is on your Morningstar. Suffice to say that if it is at least 35 volts, you will be fine putting your panels in series, that is what MPPT's benefit is, ringing as much amperage as possible from marginal conditions, that and being able to transport current at higher voltages. Your blocking diodes do take a very small nominal amount of power in this situation, but you are on the right track. Have you thought of triing to use your existing mount as one side of each panel mounting to get more sun exposure, just a thought, don't know your situation.

[Fuss]

Originally Posted by Paul Elliott

So, with two panels in series, and bypass diodes across each panel, you will see both panels when they are in the sun, and one panel (minus any diode drops) when the other is shaded.

Hi Paul,
Thanks for this information....
I wonder why this information is not made clear when wiring in series....
Is it possible to give a link to a bypass diode that can be easily installed on the panels as i would like to buy one for each panel.
John

[goboatingnow]

Your blocking diodes do take a very small nominal amount of power in this situation,

Diodes, While they cause voltage drop, which is irrelevant with an MPPT controller, they do not consume any power, only that which they loose in heat, which is miniscule. In practice once you compensate for voltage drop, diodes have no impact on power.


Only a small point, carry on..
Dave

[jaredko]

Paul is correct about the need for bypass diodes. Here is the manual for your panels, diodes are covered in section 11 and it states that they are installed by the factory, so you should be good to go.

http://www.kyocerasolar.com/pdf/spec...120Install.pdf

[capnorv]

Originally Posted by goboatingnow

Diodes, While they cause voltage drop, which is irrelevant with an MPPT controller, they do not consume any power, only that which they loose in heat, which is miniscule. In practice once you compensate for voltage drop, diodes have no impact on power.


Only a small point, carry on..
Dave

miniscule, nominal, phonetics, mean the same thing. But then I've only been installing this stuff since 1979

[jaredko]

Just wanted to add that nearly all modern panels have bypass diodes built in, most have a separate set of diodes on each of the two cells, so panel with the top half shaded will put out half the power. Generally speaking, series wiring is the way to go if you have an MPPT controller as you need lower gauge wiring to the controller and it handles shading better.

[goboatingnow]

Originally Posted by capnorv

miniscule, nominal, phonetics, mean the same thing. But then I've only been installing this stuff since 1979

sorry wasnt trying to be smart, merely that peole get all het up over diode drops , when in fact designed correctly no power is lost.

Dave

[Fuss]

Ok... so ... do we need bypass diodes or not as im confused at this point after reading all the posts.

(if I read the kyocera link a few posts back it looks like they are needed???)

[jaredko]

They are already installed, you are ok to wire them in series as is.

[Captain Bill]

Originally Posted by Fuss

Tell me if this true....for 3x KD135 watt panels at 17v each for a 24v system into a tristar mppt 60.

Originally Posted by Fuss

If the panels are good quality like Kyocera then they have blocking diodes so that when one panel is not producing amps it does not take anything away from the remaining panels that are producing amps.
Otherwise it would never make sense to wire panels in series if there is even the remotest chance of one of them being shaded.
When one of my Kyoceras is in the shade then the others put out their amps as if the shaded one is not there.
Im sure that’ true for my serial wired setup.
(can someone verify what I just wrote)

Fuss, I think you are talking about bypass diodes, Blocking diodes are typically used to prevent battery power from draining backward through solar panels at night. Good controllers typically take care of that problem for you and do not require blocking diodes

Bypass diodes work inside the panel so the panel can still produce power if some of the cell groups are shaded. Within a panel cells are arranged in series so they produce the desired output voltage. In small panels may mean that they only have one series group. In larger panel these series groups are arranged in parallel to produce a higher amperage capability. The bypass diodes are connected between these series groups so that if one is shaded the others are not affected and the panel still produces some portion of its power. If the shadow falls across all of the series group the panel effectively produces no power. In theory bypass diodes can be placed between each cell in a series group, but no one seems to do that as that would effectively drop the panel voltage.

In a series circuit amperage is constant and voltage is additive. In a parallel circuit voltage is constant and amperage is additive. So let’s just assume for a minute that there are two 120 watt panels putting out 12 volts each at 10 amps and that each of these panels has 4 series groups wired in parallel putting out 12 volts at 2.5 amps each. I used these round numbers to keep the math simple, I realize that a nominal 12 volt panel's MPP is nearer to 17 volts.

Keeping in mind the fact that in series circuit amperage is constant, if the panels are mounted in series and both are in full sunlight the array can produce 10 amps (max output of each panel) at 24 volts. Now let’s assume that a shadow fall across one of the series groups in the first panel. That panel now is still producing 12 volts from the other three series group but is now capable of producing only 7.5 amps. Since Voltage is additive in the series circuit, the total output of the array is still 24 volts, but since amperage is constant, the amperage becomes limited by the partially shaded panel and is now limited to 7.5 amps. If the shadow falls across another series group the output falls to 5 amps, and if the shadow falls across all of the series groups in the panel the output is now zero amps. Since the amperage is constant in a series circuit the output from the whole array is now zero. Since daylight is somewhat diffuse and shadows seldom produce absolute darkness the output never actually drops to zero, but can come pretty close. Bypass diodes work between parallel groups within a panel or between parallel wired panels. Bypass diodes are important because a panel that still has a voltage applied by the other series groups tend to get very hot. These bypass diodes protect the panel.

Now all this being said, what would the role of a panel bypass diode be in a series connected array? I would suspect that until there was shade on all of the series groups within the panel, the panel would not bypass. One would get 24 volts and 2.5 amps from the array in the example above. If the panel was completely shaded then the panel would bypass and one would get 12 volts at 10 amps at the controller. Also keep in mind that the panel that is unpointable becomes the limiting factor in the OP’s example. While the pointable panel might well be able to produce near it's maximum output through many hours of the day the flat mounted panel would only do that near noon and with the sun directly overhead. So while the pointable panel might be capable producing 8+ amps, the flat mounted panel might be only capable of producing 4 at the same time. The total output of the array would be limited to 4 amps or 96 watts (24x4) whereas a parallel array would be producing 144 watts (12x12).

Unfortunately I'm not familiar enough with the tristar MPPT 60 to know if its voltage regulator is capable of both Boost and Buck conversion. Boost conversion is when a lower voltage is boosted at the expense of amps to the desired voltage. Buck is when amps are boosted at the expense of volts. Assuming you have bypass diodes on all of your panels you would be able to produce power is any one of the panels was completely shaded even if your controller was only capable of buck conversion. If it is capable of boost then you can produce usable power with two panels in shade. It gets a bit complicated if the panels are in partial shade.