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Old 03-08-2010, 15:57   #1
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Solar Panels Wired in Series / Parallel

I decided to upgrade from a PWM to a MPPT controller and change my eight panels in parallel to series/parallel.

I have two banks of 4 panels rated for 12v (approx. 17.x volts open circuit voltage) and all are presently wired in parallel. The wiring runs are long (20+ feet) and I can reduce wiring losses without up-sizing wire by going with higher voltage. One bank is on each side of the boat.



MPPT controllers have the ability to manage different voltage levels on each side, whereas PWM controllers are intelligent switches that limit current. The MPPT controller I'm getting can down-convert as high as 72 volts in solar output to a 12v battery system. This opens up some options.

So which is more efficient?...

A) 4 parallel pairs of series panels at nominal 24v (2x17v=34v actual), each parallel pair having output diodes to make the overall array more shadow-resistant? (Two banks each side of the boat).

B) 2 parallel banks of series panels at nominal 48v (4x17v=68v actual), each made up of 4 panels in series. (One bank each side of the boat). Blocking diodes used only on the output of each series, in event one side of the boat is in the shadow of sails/rigging.

The blocking diodes I'm using now have a measured insertion loss voltage of about 0.4 volts. I would guess this is less of an issue if the overall PV output is at a higher voltage, because it is a lower percentage loss. Can anyone confirm that assumption?

I'm leaning toward option B, although option A would be more tolerant of partial shadows and more fault-tolerant in the "real world". Thoughts?
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Old 03-08-2010, 17:07   #2
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Hi SFT,

Splitting the array port/starboard makes sense. Given the layout in your photo, and assuming port/starboard symmetry, I would use option B (2 banks of 4 panels, each bank with its own MPPT). Splitting into 2-panel banks is unnecessary.

Some friends and I spent many hours agonizing over such matters back when I worked on solar cars (the curvature of their upper bodies, combined with the shadow from the canopy, caused all sorts of trouble for the MPPTs). We eventually concluded that there's no point splitting up a bank that's exposed to identical conditions, unless the MPPTs are too small to handle the whole bank. (It's worth noting that, one one car, we had a bypass diode on each individual cell- on another, we had them on each cluster of 6-10 cells, set up to bypass just that cell, dropping the voltage slightly but keeping the string active, if a cell failed).

Of course, you won't see open-circuit voltage with the MPPT in place; it'll try to track the point of maximum V*I which, depending on your panels, might be anywhere from 65% to 95% of V_oc. So the actual voltage ratio across the buck-boost MPPT is not as severe as it might seem.

Oh, and keep the MPPTs dry and protected, and don't touch the little buggers when they're energized... don't ask me how I found that out.
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Old 03-08-2010, 17:30   #3
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If you're concerned about the wiring length then the series option is a good idea. Voltages would add but the current would remain the same. The power provided would be unchanged (all in parallel), though.

The only problem I can see with your series/parallel arrangement is the loss of a panel or a broken connection in the series wiring. Then the power would be reduced to zero and it's possible to damage the solar panels.

Assuming the blocking diodes are rated for the maximum voltage and power the panels can produce, you should be fine.
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Old 03-08-2010, 18:38   #4
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Thinking about this further and doing a bit more reading, now option A is looking better. Option B will need bypass diodes in addition to blocking diodes, and any partial shading is more likely to lead to cell damage.
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Old 03-08-2010, 19:23   #5
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Sailfast I have been looking at the solar thing & found not a big difference with shading as most panels have diodes built in. The bigger factor is the installation on boats is having to mount the panels & flat. Where I am it is winter & the performance is well down because of the sun angle. In practical terms the flat factor is .5 of the tilted figures & the winter factor a further 70% less again. i.e. 400w results in 130w for about 4.5hrs of the day winter figures. (Went to the trouble of collecting data with a data logger at 1min periods) So just check you have a good excess over your actual needs.

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Old 03-08-2010, 21:35   #6
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The diodes that are built in might not handle the loads of option B. Any bypass diodes need to be able to handle the load of the entire series array, and I'm learning from reading that it becomes much more extreme when the voltage goes up to higher levels. If I put 4 panels in series the voltage would exceed 68 volts, and a nice big seagull dropping could conceivably become a very expensive hot-spot failure.

Anyone have info to disagree with that?
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Old 04-08-2010, 07:51   #7
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SFT, if you're concerned about the capacity of the bypass diodes, you'll probably be more comfortable with option A.
I've dealt with panels wired in series to upwards of 120 V, with no problems, but these were modules we assembled ourselves using carefully selected diodes rated for this kind of load.
If the cost of two extra MPPTs isn't an issue, there is nothing wrong with splitting into four banks- it's just that there isn't much benefit to doing so if all the components are appropriately rated.
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