Are two partially shaded total 260W panels worth it

[Shaneesprit]

I have a single solar panel 138 watts charging 2 batteries, the panel faces slightly away from the sun - i never even considered that the boom may also cast a shadow on it.
However even in the UK winters i have had no problems - only the bilge pump connected while on its mooring.
While sailing had no problems either, i do have engine to charge the batteries but i never use that unless using the engine for drive.
The last owner used to put the panel out at the best location for the time and keep moving it! one of my first jobs was bolting it to the roof and fitting LED lighting.

[nilespf]

dinghysailor, Regarding:
I have been told that if you only have 2 or 3 panels then to minimise the effect of shading have equal nominal capacity panels and connect them in series (not parallel) to a MPPT charge controller that is specified to accept the maximum voltage of the panels connected in series.



It depends upon how each panel has been configured by the manufacturer, i.e what ratio (bypassed to total) of the shaded cell array. A 36 cell array with two bypass diodes will effectively lose 1/2 power if you put your thumb on one cell. The same array with 3 diodes could yield 1/3 power if you shade 2 cells (one in each array controlled by a bypass diode) Two panels wired in series will have the same current in each panel but have the benefit of additive voltage from each panel. Two panels in parallel will have the same voltage across them but have the ability to sum currents.


PN

[nilespf]

dinghysailor, regarding: Three in number 60W semi flexible panels that have 23.9V open circuit voltage and a short circuit current of 3.15amps connected in series to a Victron MPPT 75/10 charge controller (max 10amps at 75v input)


Using the OCV and the short circuit current number you have supplied and assuming you have panels with 2 diodes then shading in a manner that causes one of the diodes to become active results in the shaded panel to yield 14V @1.6A. If you put this panel in series with the other 2 non-shaded panels you get approximately 1.6A @38V which is OK for the controller
Put that same shaded panel in parallel with the other two non-shaded panels and you end up with 14V @ 7.9A
It is apparent which configuration supplies more power but will the Victron boost circuit be able to take advantage of the 100 plus versus 59 W configurations. Not all controllers come as close to the theoretical maximums as others and for this example you can for comparison sake ignore them. If you only care about floating the batteries then either configuration will work. If you want to take the batteries from bulk charge to float as fast as possible then you have to go with the parallel configuration. If your budget can handle 3 controllers, putting each panel on a controller will maximize output power and keep a shaded panel from effecting the other two.


pn

[Pizzazz]

Some good real world examples here. I have a 100W panel on the dodger, sometimes it makes 4.5A, sometimes as low as 1A. I get about 20AHrs a day from it if I am generous with my calculations. It is not worth it except as an emergency backup, for the radio and the autopilot.

There is no good place for solar on a small boat. The costs add up (panels, controllers, support structures) and it looks ugly to me. I believe it is much better to invest in a better alternator and a portable generator as backup. Run the engine for two hours, make 120-140 AHrs, which is typical daily consumption, use excess current to make some water, vacuum, you also get hot water. Not so bad.

I have thought about adding more solar if I decide to do some longer distance cruising but I do not really mind the engine noise underway. So, solar is really only useful at anchor when you want absolute piece and you can set up the panels clear of shading.

I am thinking of putting one of these small $200 gas generators in the anchor locker (it fits well), routing the exhaust out and into the water. They produce around 50A, leave it on while on the way to the island and five hours later you have 300AHrs. To me this makes more sense.

[paul2884]

Originally Posted by Tenedos

That would require to power an LTE modem, a wifi camera and an access point. I certainly don't want to drain the battery when I am away.

Thoughts, suggestions?

could you reduce your power usage by using somthing like the "Alfred" app on an old phone, then you would just need a 12v USB charger. any solar panels should cope with that.

[dinghysailor]

Nilespf - thank you for the useful information. From what you say i should definitely connect the three panels in paralel not series. My budget is not unlimited (naturally) but the space available on the boat to fit solar panels is very much limited so if I go for solar power I need to make the most use of that space and it seems that means one MPPT controller per panel. Too bad that adds about 50% to the cost of the installation compared with one MPPT controller for three panels.


I am a bit puzzled though, I need to do more reading to satisfy myself that I am understanding all this. Being new to this I thought that if I have three panels in series and within each panel there are 36 cells in series with two bypass diodes, each diode in parallel with a string of 18 cells, then altogether that is 6 strings in series, each string protected by a bypass diode. If one string becomes inactive due to shading then the current from the five active strings will pass through the bypass diode that is in parralel with the string that has a shaded cell(s). In that case I would have thought that the power available from the three panels is 5/6 times the power that would be available without shading, less the relatively small power that is being dissipated in the bypass diode that is in paralel with the inactive string. Is that wrong?



Anyway I think I need to start again with planing a new electrical system for this boat. Although I am a dinghy sailor, as my name here, the boat in question is a performance sailing trimaran with only a tiny petrol outboard motor for auxiliary propulsion. At present the only battery charging is from that outboard, there is not even a battery charger to connect to a shore supply. The existing battery is at the end of its life. There is space on the main hull for three in number 60W solar panels but there would pretty well always be some shading on these. If panels were fitted on the tops of the floats they would be reasonably free from shading but this is a Farrier folding design which would make the panels a bit vulnerable when the floats are folded. If shading is going to drastically reduce the power I can get from solar then I am very tempted to get a petrol generator instead, despite reading many dire warnings about the hazards that these present if used on a boat. Some people say that boat insurance policies will be invalidated by just having a petrol generator on board but our insurance policy does not say so.


The two safety issues people raise with petrol generators are fire and CO poisoning. For fire, I already have a petrol outboard so the petrol fire hazard is already present, as is the case with a large proportion of all the small craft afloat today. At least, on this boat, the petrol tank is in a purpose made compartment in the cockpit with ventilation directly overboard, the same compartment could be used for storing a small portable generator and its fuel. As for CO poisoning, I realise that you must not run the generator inside the boat (pretty obvious) nor must it be run anywhere near an open cabin hatch or window. There are various locations on the trimaran that would suit these requirments, for example there is a platform right at the stern, behind the aft cabin and reasonably high above the water. As a further precaution I would be happy to have CO detectors in each of the three cabins.



I think it has to be either solar or generator, not both since weight is critical which is probably why there is only a very rudimentary electrical system on the boat at present. If I fit solar then find that it doesnt do the job I have to do remedial work to the boat to remove the panels and wiring whereas if I get a generator then find that I really should have gone for solar, or something else (an Efoy fuel cell maybe, but I dont like that the fuel for those would not always be easy to come by) then I just leave the generator on shore, no damage to the boat. We dont race the boat at present but if we ever did want to do serious racing then we could also just leave the generator and fuel on shore. The generator I have in mind is Yamaha EF1000is, I think this is about the smallest and quietest available, the alternative from Honda is a few mm too large to fit the likely storage space. Looking at the specs this generator should put up to around 40 to 50amps from a marine battery charger into a nominal 12v LiFePo battery and that would be available even if there is a prolonged cloudy period, unlike with solar. For our purposes we would probably only need to run the generator for an hour or so once every couple of days and only when there is no shore power available. A tempting option.

[john61ct]

Yes 1:1 MPPT per panel will be most efficient, and gets rid of any "need" for diodes between panels

The smaller cheap Victrons are suitable.

Choose a high voltage panel, 24V nominal (40+Voc rating) or higher

and especially, maximum watts/amps output per square area.

The issue of intra-panel wired diodes is not clear, they can introduce inefficiencies too, I would avoid getting into the weeds on that unless getting advice from a known solid well-respected professional.

These guys have a good rep

Northern Arizona Wind and Sun https://www.solar-electric.com/marine-rv.html

[KTP]

I like the idea of one controller per panel except that if you have a larger battery bank, like 48V the problem is almost no controller does boost. Very few solar panels also have a open circuit of 70+ volts either.

[foggysail]

There are both advantages and disadvantages to individual controllers especially as one’s panel count increases. I prefer a single controller operating with a 64Vmp panel arrangement ( two parallel pairs series connected) Note that the higher voltage arrangement minimizes power loss between the panel to controller wiring.

Consider the disadvantages caused by the separate wiring scheme required for each panel/controller. I use a single pair of wires between my series connected panels and my controller where as with individual controllers I would require 4 pairs......8 individual wires. And don’t overlook the real estate and weather protection needed for the controllers. Each of the controllers should have a separate wiring that monitors the voltage at their battery bank. More wiring to do.

[CarlF]

The trade off of shade and panel cost is just a fact of life on a sailboat. You do the best you can -

Yes - even 5% shading can make a panel "drop out" depending on where the shade falls. Type of panel makes some difference - but not a lot.

Shade on series connected panels has a bigger effect than parallel connected panels. But it's bad for either.

A regulator per panel (or perhaps a pair of panels right next to each other with the same shading angles) is best. I use three Victrons for six panels. The Victrons are designed except that the temperature sensor is in the unit not on a cable to the battery. Mount them near the battery so they see about the same temp and you're fine.

I've found that the best compromise is to make sure that each panel is mounted so it has no shade for part of the day and doesn't share a regulator with panels with other shade times. (and moving the boom is often the best way to increase the hours of full sun)

Dodgers are often shaded enough that it can be hard to justify the panel cost for the watts gained. Worse, when you are standing at the helm steering, you are looking right over the shiny panels on the dodger that give off lots of glare.

[FlyingScot]

I posted this on another thread here: http://www.cruisersforum.com/forums/...ng-210456.html

Mono Pro: Highest total output per unit area over the course of a day.
Mono Con: Price and they hate shadows.

Poly Pro: Cheaper and more shadow tolerant than mono.
Poly Con: Not as good under lower light conditions, so less total output per day.

Thin Film Pro: Resist shading, more heat tolerant and more flexible.
Thin Film Con: Less efficient and not really cheap.

[Lance]

I think the inclusion of a Smart Charger and preferentially one that has a Multi Power Point Tracker (MPPT) will reduce losses from shading .

[foggysail]

My thoughts about partially shaded panels. Modern/better solar panels use internal active diodes across cell strings. These so called bypass diodes allow panels to provide power at a reduced Vmp even when partially shaded.

Solar panel are current sources, not voltage sources. So the output current should remain about the same, partially shaded or unshaded, Vmp though will be lowered. Luminance is alway present during normal daylight hours although at reduced intensity under shaded conditions. So reduced power should be possible even from shaded cells.

There are good reasons to use higher Vmp panels or connecting lower Vmp panels in series. Wiring losses between the panels and the controller is one the other that I can think of has to do with a panel’s Vmp output when partially shaded conditions inhibit cell strings causing a panel’s output voltage to fall below the active voltage input range of the controller. I personally avoided using paralleled 18Vmp panels for that reason. My solar consists of four 32Vmp panels, two pairs in parallel connected in series. My Victron controller sees 64Vmp from the panels. I am pleased with my configuration, it works well for our purposes. JMHO.

EDIT: Presumed MPPT controllers above of course

[nilespf]

dingysailor, regarding: "I am a bit puzzled though, I need to do more reading to satisfy myself that I am understanding all this. Being new to this I thought that if I have three panels in series and within each panel there are 36 cells in series with two bypass diodes, each diode in parallel with a string of 18 cells, then altogether that is 6 strings in series, each string protected by a bypass diode. If one string becomes inactive due to shading then the current from the five active strings will pass through the bypass diode that is in parralel with the string that has a shaded cell(s). In that case I would have thought that the power available from the three panels is 5/6 times the power that would be available without shading, less the relatively small power that is being dissipated in the bypass diode that is in paralel with the inactive string. Is that wrong? "



You are correct. I forgot to add the third panel in my series computations. Sorry about that. Your explanation is spot on. The bypass diode is in parallel with "I'ts cells" but in series with the current flow of the panel. With 3 parallel wired panels the voltage output of the "system" will be the worse case panel. We are neglecting the voltage drop across any active bypass diode. If the corner case limit will be only a 50% panel in parallel with two more 100% panels, I would be OK with a parallel arrangement. Any decent controller has buck/boost AC to DC converters but not all of them track down to the same lower input voltage range. If one panel gets close to dark, a shared controller will be taxed to get nominal rated output power (if ever) from sub 9v input even if the P input is 80W.



The only way the designer can get a low input DC voltage into 15V plus is to chop faster & put in a larger transformer. All the manufactures have that lower limit but not all advertise it. They all are in the same race for conversion efficiencies.



You have it figured out, now it's trying to get good data from the manufacturer's and to walk the plank on shading. I have 4 panels and when the circumstances are worse case i.e. low input voltage to a controller, I lose greater than 35%.


PN

[dinghysailor]

OK, to summarise, three identical panels could be connected in parrallel to one MPPT controller, or in series to one MPPT controller (provided that controller is specified for the total voltage), or each panel could be connected individually to one of three MPPT controllers. Assuming that each panel has 2 bypass diodes, and presumably one blocking diode, then if one of the two cell strings in one of the three panels becomes inactive due to shading:


Option 1: One panel becomes reverse biassed so produces no power, total power is 2/3 times power without shading.
Option 2: One string out of six is bypassed by the current flowing through the other five strings so total power is 5/6 times power without shading, less a small loss in the bypass diode that is conducting current.
Option 3: The panel with the shaded and hence bypassed string produces half power less a small power loss in the conducting diode, so total power is a bit less than 5/6 times power without shading. However, if the voltage produced by the panel that has shading falls below the minimum voltage at which the MPPT controller will work properly then little or no power will be contributed by that panel so total power will then be similar to Option 1.



So under this particular shading condition, Option 2, with all the panels in series to one controller is at least as good as any other option for power output, it also has the lowest parts cost, permits the smallest gauge wiring and has the least length of wiring, so would appear to win overal.



This is what I was told by someone outside this forum, although he was not someone with specialist expertise in this area. If there is anyone here who really understands this subject fully, I think it would be useful to readers if they could either confirm my conclusion above, or refute it. Altough I am considering only three panels, similar considerations could be relevant to those who have more than three panels.



I would say that this is making me think twice about whether to have solar power on our boat. The boat has a big carbon fibre reinforced mainsail that does not stow well, it becomes a bulky package when flaked down on the racks that extend each side of the boom, so shading could often be worse than just a part of one panel. Maybe the power output that we would actually get even with bright sun would be only a fraction of what one would expect from the rated output of the panels. In that case, as I said before, I am tempted to just get a small portable generator.