12v vs 24v Solar

[mitiempo]

Originally Posted by kevins

Is there anything wrong with connecting 2 - 12 volt panels in series to create a 24 volt "panel" and then feeding that to the MPPT controller. (of course if the mppt controller can handle the setup)

why did/would I buy a 12 volt panel when the 24v would/would have given me a possible better option.

thanks
Kevin

Series panels are only a good idea if there is zero shading. Shading has less effect on parallel systems.

[CampDavid]

You need to focus on wattage more than voltage if you have MPPT control
Shading will kill a series system while it will reduce a parallel system

24 PLUS volts smaller wires

With 405 watts 12 volt nominal panels I use 6 awg wire to the control
.5 volt line loss is 2 plus amps in out put

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[lindabarzini]

So that was a completely wrong hypothesis. I have now learned it is cheaper and more efficient to buy 24v or higher solar panels. The key is the MPPT controller for the system.

[MaxWebXperienZ]

Originally Posted by lindabarzini

So that was a completely wrong hypothesis. I have now learned it is cheaper and more efficient to buy 24v or higher solar panels. The key is the MPPT controller for the system.

I'm an electronics engineer [or at least my school said I was]. I feel sorry for people looking for answers about solar that aren't because it's even taken me awhile to sort things out. The controller should be able to compensate and maintain a constant voltage out of it with a wide variation of input voltage from the panels. If the controller is capable then shading will have the same effect no matter the panel voltage. In practice I really don't know much about the controllers that are available, maybe they don't compensate as well as I might expect.

Higher [series] panel voltages are better because the energy can be transmitted with lower losses due to heating of wires.

An app like this can help you home in on appropriate wire size: Wire Resistance and Voltage Drop Calculator

Maybe you could post some links to controllers and I can read the specs and tell you what I think will happen with various installations?

[sailor1924]

I chose to pay a little more per watt and a lot less on shipping and bought 8 @ 50 watt panels and wired them in parallel.
When I set up my system I decided that being ketch rigged the biggest problem I would have is shading.
All of previous comments are correct about higher voltage being a more efficient in terms of transmission loss and that bigger panels are cheaper per watt, but if a series
wired big panel is shaded even a small amount you can lose as much as 70% of output. I also chose to not spend money on a MPPT controller which is only really useful for higher voltages then 12v panels. I paid more in wire costs with a parallel wired system but so far I've not yet failed to meet my daily needs for refrigeration, computers and miscellaneous uses even though shading knocks my output down to 30% of max swinging on a mooring. I'll be adding 2 @ 100 watt 12 volt panels this year to hopefully cover my auto pilot consumption. The other benefit I've seen is that the Tristar controller is able to top up my batteries to a fully charged voltage on a daily basis, some thing neither my wind generator or diesel generator running a Heart inverter\charger will do.

[MaxWebXperienZ]

I would really like for somebody to produce a line of DC operated appliances. Voltage is not standardized at this point, as evidenced by this forum so maybe each item would have to come in different voltages.

The whole thing of converting to Solar and then accommodating stuff that runs on AC bothers me. I'm a purist, don't want the unnecessary multiple conversions and don't like being bathed in the AC field. I'm looking around my office here and not seeing a single item that doesn't run on DC internally, not anything at all. So I have to have AC house wiring to a room that could be well served by DC. There is already plenty of stuff for RV's and boats that runs off DC, I loved that about my travel trailer, could park it, turn on the propane and have everything from lighting to cooking complete with refrigeration and the nice peaceful feeling I get when I'm away from the AC power. I'm working out the strategy for a new home and it comes down to DC for everything but the cooking area because refrigeration takes a lot of power if you want to be able to entertain a lot of guests. Cooling with air conditioning is ridiculous. Even in the Mojave Desert houses are built without shade and with thin walls that don't insulate, then cooled with huge amounts of electricity, via central air, no less, swamp coolers use about 15% the power and do a great job with the fresh air. All this s%^t needs some fundamental upgrading basically...

One thing about a multihull is that solar panels can be mounted all over the place [seemingly, I don't really know boats] so the wiring runs from panel to end use point are short. Several panels and controllers is better than one big one if it's planned out well.

[noelex 77]

Originally Posted by lindabarzini

So that was a completely wrong hypothesis. I have now learned it is cheaper and more efficient to buy 24v or higher solar panels. The key is the MPPT controller for the system.

There is lot of debate about series versus parallel connections of solar panels/cells. The general consensus is that for a sailboat where shading is an issue, parallel connection is better.

Thus all things being equal, nominal 12v (36 cell) panels are best. 24v panels (which are internal wired as two 12v panels in series) are less efficient with shading. There is some advantage with the higher output voltage reducing the voltage drop in the wiring, but providing suitable wiring physically fits, the main implication of this is cost. This effect is small compared to the effects of shading.

However, large high effeciency panels are only generally available with a higher voltage output. So in practice fitting large, high voltage panels results in the best output per surface area as well as the most cost effective solution.

[mitiempo]

What it comes down to is the kind of boat, where will the panels be located, and what size will work in the desired location. Sloop has less shading issues for example. Is there an arch or are you willing to build/buy one? Then you work out what will be the best for your situation.

MPPT will gain over PWM, even with 12 volt nominal panels which have a max voltage of between 18 and 20 typically. In real world testing over 20% gain with MPPT.

[poppyduc]

This is the output from our setup on a Seawind 1160 cat 12 volt batteries (3 x 200 amp AGM)
Replaced the 2 x 125watt 12 volt panels with 2 x 250watt 24 volt and new Victron 100/50 regulator with Bluetooth.
As can be seen output good output on a light cloudy day.
Very pleased so far seems to make current fm first light till night fall now.


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[dave777]

Originally Posted by noelex 77

Thus all things being equal, nominal 12v (36 cell) panels are best. 24v panels (which are internal wired as two 12v panels in series) are less efficient with shading

Within each solar panel there are many series solar cells. It's not clear to me that a 24v nominal panel would suffer from shading differently than a 12v panel.

If you have more than one panel, seems to me that going 24v is better, and with shading
a concern they could be in parallel.

Of course, the only way of knowing is testing. There are $3 paralleling MC4 connectors available, so you could easily switch between series and parallel on your own boat and know for sure. Could vary by time of year. If you're going to be moored at the same spot for 2 weeks, you could change them as shading would vary with prevailing winds vs. sun & location.

[poppyduc]

all very true.
as the Viltron 100/50 can take up to 100 volts will try next trip out in series to see the difference at 48 volts I have wired the plugs close so testing is a simple process to try out different setups and the bluetooth makes monitoring so simple with its 30 day memory.

[Arthur Garfield]

@poppyduc, I look forward to your results ! Tks.

[Texlan]

Originally Posted by noelex 77

There is lot of debate about series versus parallel connections of solar panels/cells. The general consensus is that for a sailboat where shading is an issue, parallel connection is better.

Thus all things being equal, nominal 12v (36 cell) panels are best. 24v panels (which are internal wired as two 12v panels in series) are less efficient with shading.

This is usually not actually the case (though I initially believed the same thing.) Turns out most modern 24 volt panels are wired internally as three 8 volt nominal sections in series with bypass diodes, allowing unshaded sections to develop power bypassing the current limited sections. Check the datasheet on your panel(s) for junction box bypass diode(s). 24v panels wired in series benefit from the same bypass diodes -- as sections and or whole panels are shaded the voltage will drop but current will continue to flow unimpeded. your MPPT charger will adapt accordingly.

With such panels it makes more sense to wire in series (higher system voltage = lower wire loss, less total wire run, and you save money and complexity in combiner box breakers)

My 2 cents

Sean
WK7R

[noelex 77]

Thanks for your thoughts Sean. The issue of series or parallel connection is far from cut and dried, but my reading of the experimental and theoretical evidence favours parallel connection.

The bypass diodes you mention are fitted to all solar panels both 12v and high voltage panels. Anywhere from 2 to 8 bypass diodes are common. Their main purpose is to protect the solar cells from localised heating and damage, but they do have a role to play increasing the output with shade.

In fact, if there were no bypass diodes and multiple panels were wired in series the output of the total array would be zero if there was deep shade anywhere on any panel in the system. On a boat there is very often some shade on at least one panel so fortunately for those that wire their panels in series this is not the case due to the bypass diodes. However, the bypass diodes don't solve all the shading problems associated with series connection. The diodes alow the current to be passed around the high resistance of the shaded cells, but they do so with some voltage loss as the current has to pass through multiple diodes these voltage losses add up.

Let's take a simple example of two (nominally) 12v 100w panels each with 4 bypass diodes. We can connect these in parallel or series.

If we take the case of one panel substantially shaded by say the boom:

- If we connect the panels in parallel the unshaded panel ignores the shaded panel and produces its full output The shaded panel produces zero. The total output is potentially 100w.

- If we connect the panels in series the current of the panel in sun must pass through the shaded panel. Shaded solar cells have a very high resistance and will pass very little current. However, the bypass diodes can pass current. In this case the current must pass through 4 bypass diodes in the shaded solar cell. These are generally conventional diodes each with a voltage loss of about 0.5v. 4x0.5=2v are lost from the (nominal 12v) panel that is full sun. At best, the panel has lost a significant amount of power. At worst, its voltage (or at least its Vmp) has dropped below the charging battery voltage and the output will be very low. Thus the shaded panel has significantly reduced the output of the panel in full sun.

Mathematical modelling of shaded solar cell arrays is very complex and the above is only one very simple scenario but it does illustrate how with shade the output from solar panels connected in series can be much worse than parallel even with bypass diodes.

If you do want to connect panels in series it is worth considering installing an additional bypass diode that bypasses the whole panel. That would reduce the loss in the above case to only 0.5v. Still less than with a parallel connection, but it is a substantial improvement.

Hopefully there will be more people willing to alternate series and parallel connection and report the real practical results (thanks Poppyduc). I think this is the best way of answering the question.There have been some reports and these seem to favour parallel connection, but the results show a lot of variation and not many people have installed an additional whole panel bypass diode before doing the test so I think we need to keep an open mind until a definite consensus is reached.

[Texlan]

LOL.. I just lost a post I worked on and off for two hours but apparently timed out writing..

Please pardon my brevity as I rewrite the salient points...

Solar panels use Schottky bypass diodes with a forward voltage drop of .15 to .45 volts.. I would conjecture the losses aren't too significant in many installations -- In this example panel inexpensive grid tie manufactured panel (of which i use two 240watt versions) Vikram Solar Pvt. Ltd. | 60P Prime Series | Solar Panel Datasheet | ENF Panel Directory two panels in series, 1 completely shaded, would result in a voltage drop of (absolute max) 1.35 volts through its 3 bypass diodes. At a NOCT(normal operating cell temperature) maximum production of 180.24 watts, the 1.35 drop from the bypassed panel would net 171.48 watts, a loss of only 5%.

In my installation I switched from parallel to series because at elevated cell temperatures the maximum power point voltage would fall below my LiFeP04's set float voltage of 27.2v. I would still charge, but lower and lower current the warmer the cells were.

I wound up netting more energy per day in my situation, primarily due to several factors --
1, due to the higher operating voltages the charge controller would wake up and produce usable power much earlier in the day and would go to sleep much later. A couple hours worth of extra production at less than max insolation..
2, the higher voltage produced more usable power in cloudy days, where the paralleled panels' max power point would quickly drop below my charge profile voltages.
3, my panels are oriented so the mast could only shade a maximum of 2 out of the 3 sections on one panel. So in my particular case, in a partial shade situation I am producing at least 30% more power in series, even accounting for bypass diode voltage drop, than in a parallel array, where partial shading would immediately drop the entire partially shaded panel below my charge profile minimum and render it net zero production.

Added benefit: The fact my MPPT controller is switching lower currrent along the long run (read: antenna) to the solar panels creates MUCH less RF noise in and around my boat. My VHF doesn't break squelch randomly like it used to. I am an extra class amateur radio operator and care much about these things..

Devil's advocate may state that the higher your array voltage from your battery voltage, the less efficiently your mppt charger will operate -- but -- we're talking percentage points here.

And of course, everything is percentage points here...

I am not saying series is the answer or parallel is the answer -- truely, I think that a "general consensus" is a bad idea here. We all have different operating temperatures, shading considerations, battery system charge profiles, panel specifications, installation considerations, etc. I just want to offer up additional considerations to the subject that many people are probably not aware of -- it is important to read the data sheets on your solar panels and carefully consider both temperature effects and shading effects on arrays and net energy production before we commit one way or another. YMMV.

And anyways, doing some empirical testing on both methods isn't that hard as long as you take that into consideration during the installation.

Respectfully,
Sean
WK7R