Series panels

mountainman · May 2018

Renogy sells a 12 volt 500 watt kit. 150 volt cc with 5 100 watt panels in series. Can some one explain why this is a bad idea?

mcgivor · May 2018

The kit uses an MPPT controller which converts the series voltage to a value needed to charge the batteries, somewhere in the neighborhood of 14.4V, the current supplied to the batteries is correspondingly increased over what the panels supply at ~150V. Which may be a little inefficient usually around double nominal is preferable but it will work.
Advantages include better optimization of the available production especially in cold climates, as opposed to a PWM controller with parrallel panels. Wiring from array to controller can be of a smaller gauge, only a single overcurrent device is needed, no combiner box required. The only disadvantage would be if there any shade issues, even on one panel or even part of one, the whole strings output would be compromised.
So it's not a bad idea in and of itself as long as it's understood that any shade is a bad thing. Here is some very useful information supplied buy the host of the forum, well worth reading

https://www.solar-electric.com/learning-center/batteries-and-charging/mppt-solar-charge-controllers.html
https://www.solar-electric.com/learning-center/

Photowhit · May 2018

Who said it was a bad idea? I guess in truth I would prefer to see the voltage coming in at about double the system voltage. but it's possible to build a system like this. I find most of them over priced for what you get.

I think the bigger trick is not being able to find a 'kit' that fits your needs and often people buy kits rather than starting with the loads they would like to run.

Saying "oh, look electricity lets see what we can do with it..." is like buying a quart of gasoline and saying " let's see how far this will take us..." It might be a fun trip for a few miles, but if you are heading to a friends house and run out of gas half way, it'll not seem like a productive idea.

mountainman · May 2018

As far as efficiency comparing double the volts to 5 x the voltage roughly what are losses.

Estragon · May 2018

With cold batteries and warm panels it could be a problem. Assuming the panels are ~17-18 Vmp and a cold bank may need >16Vabs, mppt may have trouble.

mountainman · May 2018

Also would with such a down conversation be more stress heat etc on the controller?

mcgivor · May 2018

mountainman said:

As far as efficiency comparing double the volts to 5 x the voltage roughly what are losses.

Can you post a link to the exact controller, there may be a few percentage points lost, 87% as opposed to 92%, just random numbers, the manual should provide the information.

mountainman · May 2018

Panels are 18.9 vmp. average low 20f . Occasional 0. I think I have the link attached

mountainman · May 2018

How it works from renogy

BB. · May 2018

OK, so the difference between a "lower" voltage and a higher PV Array voltage is around 98%vs 95% efficiency (the greater the difference between Vbatt and Varray, the less efficient the charge controller).

At the system level, a 2-3% drop in efficiency is pretty much lost in the weeds (not easy to measure without a laboratory setup) and will not affect your day to day harvest (that you can see).

However, from the MPPT charge controller point of view, that is a 2% of energy being turned into "waste heat" vs 5% being turned into waste heat... Or >2x more heat that the MPPT controller needs to dissipate through its heat sink and air flow/cooling system.

So, if you run a high(er) Vmp-array voltage--You need to make sure that the MPPT controller has very good air circulation (don't mount in a closet/under a shelf/etc.) and double check that it is not running too warm.

-Bill

mountainman · May 2018

So not the most efficient way to do it. But no chance of over voltage or severe damage to the cc as long as ithe has sufficient room to cool.

mike95490 · May 2018

mountainman said:

Renogy sells a 12 volt 500 watt kit. 150 volt cc with 5 100 watt panels in series. Can some one explain why this is a bad idea?

The 5, 100w panels in series is lame for a 12V battery. Way too much input/output differential and will keep the controller warmer than needed 5 panels is a odd number, and could have too high of voltage in cold weather.

mike_s · May 2018

mcgivor said:
as opposed to a PWM controller with parrallel panels... The only disadvantage would be if there any shade issues, even on one panel or even part of one, the whole strings output would be compromised.

Renogy panels include bypass rectifiers, so shading loss is less than with parallel connected panels. When parallel connected, shading on any panel will kill the whole output from that panel, because the output voltage will drop below the voltage of the other panels, so it no longer contributes current. When series connected, you don't lose any current, just the voltage of the cell strings which are shaded. Shading one cell in a parallel connected array kills the whole panel's contribution. In a series array, only 1/3 of the panel power is lost (for the most common panel configuration).

mcgivor · May 2018

mike_s said:

mcgivor said:
as opposed to a PWM controller with parrallel panels... The only disadvantage would be if there any shade issues, even on one panel or even part of one, the whole strings output would be compromised.
Renogy panels include bypass rectifiers, so shading loss is less than with parallel connected panels. When parallel connected, shading on any panel will kill the whole output from that panel, because the output voltage will drop below the voltage of the other panels, so it no longer contributes current. When series connected, you don't lose any current, just the voltage of the cell strings which are shaded. Shading one cell in a parallel connected array kills the whole panel's contribution. In a series array, only 1/3 of the panel power is lost (for the most common panel configuration).

Most modern panels have bypass diodes which generally divide the panel into 3 parallel strings of individual cells, usually lengthwise, a shadow across a vertical string or individual cell, would disable that string but the remaining 2 strings will produce full voltage at reduced current. Conversely if thr shadow is cast across a single horizontal cell row, the entire panel loses output so in a parallel panel arrangement, that particular panel is lost but the remaining panels will still contribute. Series wise in the same panel layout, the shadow across one horizontal row of cells renders that panel void, but will also cripple the entire string, so no output. Naturally this is merely hypothetical for explanation purposes, shadows move, so different portions would be shaded as the angle of the sun changes. If shadows are unavoidable, designing a layout and configuration which minimizes the effect, such as rotation of the panel with horizontal shadows will improve output. The purpose of the comment regarding parallel versus series was with respect to the OP's particular arrangement of 3 panels in series, every individual setup is different.

mike_s · June 2018

mcgivor said:

Series wise in the same panel layout, the shadow across one horizontal row of cells renders that panel void, but will also cripple the entire string, so no output.

Nope. You'll lose the output for that panel only. The rectifiers will simply bypass it, and allow current from the other panels to flow.

Estragon · June 2018

I'm confused (not ususual). Where is (are) the rectifier(s)? Are you saying the panels invert the DC current to AC internally, then rectify to DC at some point?

My understanding is most panels have diodes to prevent reverse current, but maybe these are different?

mike95490 · June 2018

Panels only produce DC. They have diodes, sometimes embedded in the glass sandwich, sometimes in the J-box on the back. The diodes allow power to bypass a shaded portion of the panel to allow the array to produce some power, instead of no power. But if you have a string of panels, and some shade hits parts of some of them, your array voltage may drop enough the system shuts down.

mcgivor · June 2018

mike_s said:

mcgivor said:

Series wise in the same panel layout, the shadow across one horizontal row of cells renders that panel void, but will also cripple the entire string, so no output.

Nope. You'll lose the output for that panel only. The rectifiers will simply bypass it, and allow current from the other panels to flow.

When the term rectifier is used in your text, are you referring to a panel bypass diode, independent of the panel as manufactured, with built in bypass diodes?

Estragon · June 2018

> @mike95490 said:
> Panels only produce DC. They have diodes, sometimes embedded in the glass sandwich, sometimes in the J-box on the back. The diodes allow power to bypass a shaded portion of the panel to allow the array to produce some power, instead of no power. But if you have a string of panels, and some shade hits parts of some of them, your array voltage may drop enough the system shuts down.

That's what confuses me. My understanding is a rectifier circuit would use something like a diode and capacitor to turn AC into DC.

mcgivor · June 2018

It's the context the term is used in, rectifier implies rectification ie. AC to DC, where a diode or group of diodes cut half of the AC waveform to produce DC. In strictly DC circuits, it is a one way valve of sorts, allowing current to flow in one direction, not actually performing rectification as such.

Estragon · June 2018

That's pretty much my understanding... I didn't know a rectifier was another name for a diode in a DC circuit though, hence my confusion.

mike_s · June 2018

Sorry, I figured people on an forum about electrical stuff would be familiar with electronics terms. "Rectifier" is a very common term for a diode used in power applications (where high current is important and high speed isn't), to distinguish them from small signal diodes (which have the opposite qualities). Never quite understood why they're still even called diodes, though, no one calls a transistor a triode.

But, It's now very clear that your confusion is not only in terminology, but in how they work and what they do.

mike_s · June 2018

Estragon said:

My understanding is most panels have diodes to prevent reverse current, but maybe these are different?

That would be a blocking power diode (does that make it less confusing for you?), not a bypass one. They keep power from backflowing from a battery into the panel when it's dark. A controller should serve that function, but some of those cheap battery maintainer panels which plug into a car's 12V socket have them. This might aid your understanding: Link.

mike95490 said:
But if you have a string of panels, and some shade hits parts of some of them, your array voltage may drop enough the system shuts down.

The discussion was comparing series vs. parallel panels. There is no case in which a parallel connected system would continue to provide power with the same shading which would drop a series connected system below the system voltage. So, what's your point?

Any shade on a parallel connected panel will kill all of its contribution. You would have to shade at least one cell in all three rows to kill the contribution from a series connected panel, and the output from the rest of the string would continue to flow. That's for a common system charging 12V batteries, as mentioned in the OP. It's a little different when the system voltage is higher than the panel voltage, in that there can be both series and parallel wiring involved, but series/MPPT still provides more protection from shading, as it will continue to produce power under a wider range of shading conditions.

Estragon · June 2018

> @mike_s said:
...
>
> But, It's now very clear that your confusion is not only in terminology, but in how they work and what they do.

Quite likely. That's why I ask questions to clarify things.

mcgivor · June 2018

Back to the series parallel question, there is more to the science than making a blanket statement that one layout is better than another, if one chooses to read through the attached Pdf to gain knowledge, it's pretty intense, be forewarned. Here is the conclusion of what the best configuration is, not being argumentative in any way, shape or form.

Conclusion

Series connection of solar cells in an array is essential to get practically utilisable voltage. A number of such strings are

connected in parallel to get the requisite power. As there is a substantial power loss due to non uniform illumination of a

series string care should be taken to see that all the cells connected in series receive the same illumination under

different patterns of shading. Such a care will give a better protection to the array and at the same time the total energy

output will also be higher. In this paper, the series connected and parallel connected SPVA is compared under different

shaded conditions. It is found that parallel connected SPVA is dominant under shaded condition. So, the parallel

connection is the best possible configuration. The problem of high current output in parallel connected system requires

the definition of new configuration.

mike_s · June 2018

That article simply doesn't apply. They're using panels with no bypass rectifiers.

mcgivor · June 2018

mike_s said:

That article simply doesn't apply. They're using panels with no bypass rectifiers.

Can you answer the question previously asked, the reference to bypass rectifier/diode, is that either a individual cell bypass, a string bypass or an entire panel bypass?

mike_s · June 2018

Sure, I can Google it for you this time. We're discussing Renogy panels. They have bypasses wired thusly:

Image: http://theouterlimits.ws/projects/airstream/solar/photos/thumbnails/RNG-100D-PanelSchematic.jpg

Estragon · June 2018

I haven't read the pdf yet, but would I be correct in saying that in a typical array, shading of parallel segment(s) will reduce current produced (voltage remaining about the same), whereas shading of a series segment reduces voltage (current remains about the same)?

A bypass diode / rectifier in parallel with cell(s) prevents high string voltage from going through higher resistance shaded cells by providing a lower resistance path around it/them. All else equal, voltage will drop by roughly 0.5v x number of cells bypassed?

mcgivor · June 2018

mike_s said:

Sure, I can Google it for you this time. We're discussing Renogy panels. They have bypasses wired thusly:

As do most panels, nothing special there, if there were bypass rectifiers/diodes on each cell, that would be something different.

mike_s · June 2018

That's right. So, take the most basic comparison case of a two sets of two panels, one set wired in series and the other in parallel.

In the series wired set, any single shaded cell will kill the output from that panel. So, with one remaining panel, output drops to 1/2 capacity. For series panels bypassed like the Renogy shown above, a single shaded cell will kill 1/2 the voltage from that panel, but current stays the same and is bypassed around the section that cell is in, so the output drops to 3/4 capacity. Larger panels often have 3 bypassed columns, where the output would drop to 5/6. But in all cases, the performance is equal or better under identical shading conditions when using series connected panels.

That's a bit simplified. In the series case, the group of cells the shaded cell is in becomes high impedance, and stops producing current. The remaining group of cells then isn't producing a voltage higher than the battery, so the bypass can't kick in, so no current flows for the entire panel. If you measured Isc, you'd see full current but 1/2 voltage because the bad section got bypassed. But when driving a controller to charge a battery, the load is up around 14 V, not a short circuit.

Series panels

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