Solar Panels - Series or Parallel ?

[noelex 77]

There has been lots of posts on the optimum way to connect solar panels on a boat. In series or parallel.
For a long time I have been suggesting people try both metods and report the results.
So far there does not seem to be any practical results.

My regulator has recently developed a problem, so the purchase of a new regulator which will accept a higher voltage. Will allow me to make the change.
As the wiring is a bit inaccessible I thought I would first do some theoretical calculations to determine the difference.

To make it simple I assumed

2X12v panels each panel is dived into 2 sub panels of 6v each with bypass diodes. Each panel can produce in practical conditions 1A

The sub panels are connected in series, hidden within the 12v panel, but we can chose to wire the 2 12v panels in series or in parallel.

In series we have 4 6v sub panels connected in series.
In parallel we have 2 6v sub panels connected in series (because this is how panels are constructed), but the 12v panels are connected in parallel

.

No shade:
Series and parallel perform essentially the same. There is a slight reduction in wiring loss for series connection, and the panels may produce power slightly earlier and later, but the regulator will be less efficient with the greater voltage difference. Overall maybe a couple of percent improvement with series connection.
Series 2.0+ A
parallel 2.0 A

Full shade on all panels:
No advantage with series or parallel connection
Series 0.0 A
Parallel 0.0 A


Full Shade on one Sub Panel out of 4:
If there is shade on one sub panel the result is the same no matter which sub panel is effected
Series 1.4 A (will still have 3 out of 4 sub panels working, but there is some loss in the bypass diodes)
Parallel 1.0 A



Full Shade on two Sub Panel out of 4:
Here the theoretical model gets more complex.
If we call the 2 6V sub panel that make up the first 12v panel sub panel 1 and 2. The 6v panels that make up the second panel are 3 and 4.

There are 6 possibilities

Shade on sub panels 1&2
Series 0.9 A ( Not 1.0 A because there will be some loss in the bypass diodes)
Parallel 1.0 A

Shade on sub panels 3&4
Series 0.9 A
Parallel 1.0 A

Shade on sub panels 1&3
Series 0.9 A
Parallel 0.0 A (the voltage is under the nomimal 12v so no power is produced)

Shade on sub panels 1&4
Series 0.9 A
Parallel 0 .0 A

Shade on sub panels 2&3
Series 0.9 A
Parallel 0.0 A

Shade on sub panels 2&4
Series 0.9 A
Parallel 0.0 A



Overall my theoretical calculations suggest series connection of 24V in a 12v system is better
I will try some practical experiments over the next few days to see if I can get a definitive answer, but my theoretical model supports series connection
Comments welcome.


Note many solar panel have move sub panels separated by bypass diodes than the 2 I have assumed in this theoretical model, but the calculations and explanation gets more complex without any change in the recommendation.
I have also assumed a reasonably high loss in the bypass diodes.
The calculations are based on a series input of 24V on a 12v system. The calculation for increasing the voltage beyond this I will leave to others, as it is not possible in my system.
I have assumed very deep full shade.

[steamgoat]

Series or parallel panels is amost as contentious as a 'anchor' thread !!
I purchased two Kyocera panels from Arizona Wind & Sun, I asked which was best, they said, if there was never any chance of shading go in series, if there was a possibility of occasional shading, go parallel. I havent constructed the arch yet but I think I will go parallel.
Bruce.

[noelex 77]

Yes there have been a million threads on the subject on various forums with many different opinions. It seems to me there should be a simple correct answer.
My theoretical calculations strongly support connecting 2 X 12v panels in series, with an improvement in shaded performance despite the advice you have been given (and contrary to my own expectations), but perhaps there is a mistake in my assumptions that someone can point out.

[44'cruisingcat]

I have 6 panels connected to an Outback power systems MPPT charge controller.

Used to have all 6 in series, but read in the Outback manual that for a 12 volt system, having 24 volts from the solar panels was the most efficient.

So I've gone to a series/parallel setup, but actually THINK(1) I'm getting less power from the system on cloudy days. Probably try all in series again, and see how it looks.

1: (It's difficult to know for sure - on good days the system goes into absorb/float modes, so you don't get the full input, and comparing cloudy days is guesswork - how cloudy is cloudy?)

Based on your testing it would seem all in series would be a little better.

[noelex 77]

Originally Posted by 44'cruisingcat

I have 6 panels connected to an Outback power systems MPPT charge controller.

Used to have all 6 in series, but read in the Outback manual that for a 12 volt system, having 24 volts from the solar panels was the most efficient.

So I've gone to a series/parallel setup, but actually THINK(1) I'm getting less power from the system on cloudy days. Probably try all in series again, and see how it looks.

1: (It's difficult to know for sure - on good days the system goes into absorb/float modes, so you don't get the full input, and comparing cloudy days is guesswork - how cloudy is cloudy?)

Based on your testing it would seem all in series would be a little better.

Thanks the more practical results the better. Considering how many boats have multiple solar panels with MPPT controllers we should have such basic information related to how we wire them up for optimum output.

The greater the difference between the panel voltage and the battery voltage the greater the losses in the charge controller, but the losses in the charge controller are only small and in general these loses will be overwhelmed by the difference in panel output.
I am calculating the differences (and hopefully measuring the differences in the near future) between parallel connection of 2X12v panels and series connection. If we can sort out what is best for a typical boat installation the next stage is to look at the effects of higher voltages.

[goboatingnow]

The greater the difference between the panel voltage and the battery voltage the greater the losses in the charge controller, but the losses in the charge controller are only small and in general these loses will be overwhelmed by the difference in panel output.

Yes put higher voltages generally allows the panel to work closer to it's MPPT. Equally switched convertors suffer little additional heat losses even if the Vin/Vout ratio is quite high

The comment is very true for conventional series pass or shunt regulators

Dave

[PeteAndJudith]

Like so: Solar Panel Installation - A Spirit Solar Case Study - Spirit Solar

[twistedtree]

Originally Posted by noelex 77

Yes there have been a million threads on the subject on various forums with many different opinions. It seems to me there should be a simple correct answer.

For every complex problem there is a simple answer - and it's usually wrong.

I think the reality here is that it's complex, and the best answer depends on a number of factors. Things like:

- Does the charge controller accept high input voltages, and if so, what's the max allowed?

- How many panels can be wired in series without exceeding the max input voltage. This needs to be a worst case calculation using high light and low temp. There's a limit to how many panels can be in series, and if you have more then some degree of parallelism is required.

- If you are forced to parallel, the strings need to be the same. For example, you might have 4 panels, but your charger can only handle 3 in series without exceeding it's max input voltage. This will force you to wire 2 strings of 2 panels each.

- Is shading more probable in one area over another? If you have a series/parallel combination, you can pick which panels are in series and which are in parallel. This can let you optimize for the most likely shading scenarios.

No matter how you slice it, some degree of engineering is needed to get an optimal configuration, or to at least confirm that you intuition (or buddy down the dock) is right.

[stevensuf]

run in series and if one is shaded the output for the lot falls, if in parallel, you only loose the output of the one in the shade, drawback for parallel is thicker cable as more amps and more amps for the controller to deal with.

im doing mine in parallel as im bimini mounting, so might get some boom/sail shade, if youre mounting on an arch with no shade then series may be better or part series part parallel to get the voltage right.

[prroots]

Originally Posted by noelex 77

Overall my theoretical calculations suggest series connection of 24V in a 12v system is better
I will try some practical experiments over the next few days to see if I can get a definitive answer, but my theoretical model supports series connection
Comments welcome.

Thanks. Your theoretical calculation really interests me since I've just finished mounting my two Photowatt PW2050-21 panels and have started testing them with my Morningstar TriStar MPPT 45 with an input voltage rating of 150. The panels are 18 Volt nominal and rated at 210 watts each. The specs say that each panel has 4 bypass diodes.

http://www.photowatt.com/documents/p.../PW2050_EN.pdf

Given the controversy surrounding series vs parallel I have arranged the wiring to make it very easy to switch back and forth. Since this thread is getting old I'm wondering if you have any empirical evidence to support either parallel or series.
Pete

[s/v Jedi]

Anybody actually wiring in series will find that Noelex is absolutely correct: series is better when your panels have bypass diodes. I have never seen a panel on a boat that didn't have them; my 8 year old panels have 6 each.

There's always naysayers (they typically have a parallel setup that can't be changed to series because they have an old controller that can't take the higher input voltage and they don't want to buy a new controller )



ciao!
Nick.

[Dreaming Yachtsman]

This is not necessarily a recommendation one way or the other, but I have four Kyocera 130W panels wired in two banks, each bank having two panels in series. The two banks are then paralleled together at the input of an Outback MX60 MPPT controller. The panels are on an arch at the stern so they are out of the shade a fair amount of time (unless the wind/current comes from the south).

I have not tried other wiring configurations but for the first year, the panels were mounted along the rails on clamps that enabled them to be rotated towards the sun for optimum efficiency. The clamps wore out and they kept getting hit with waves so I moved them to the stern.

[prroots]

Originally Posted by s/v Jedi

Anybody actually wiring in series will find that Noelex is absolutely correct: series is better when your panels have bypass diodes. I have never seen a panel on a boat that didn't have them; my 8 year old panels have 6 each.

There's always naysayers (they typically have a parallel setup that can't be changed to series because they have an old controller that can't take the higher input voltage and they don't want to buy a new controller )



ciao!
Nick.

As mentioned, my panels each have 4 bypass diodes, but I don't know which cells they cover. It's kind of odd since my 54 cells/panel are not divisible by 4? Maybe there are two strings of 13 and two of 14?

I hope to prove whether serial or parallel is better as soon as the weather cooperates. I need fairly constant irradiance for long enough to switch back and forth. My Morningside TriStar MPPT 45 can accept up to 150 VDC so I can easily go either way. I have provided some early test results here under a clear unshaded sky with panels in the horizontal position:

http://www.cruisersforum.com/forums/...tml#post884485

Pete

[prroots]

Originally Posted by noelex 77

There has been lots of posts on the optimum way to connect solar panels on a boat. In series or parallel.
For a long time I have been suggesting people try both metods and report the results.

It would be interesting to repeat your theoretical analysis with my two nominal 18 Volt panels and 4 bypass diodes/panel. The shading of one string in one panel in parallel with other panel would still provide enough Voltage for batteries. Would it be ~26 - 6.5 - 0.5 = 19 Volts, where 26 is typical Voltage, 6.5 is Voltage of each string and 0.5 is the diode drop? Wonder what happens to Voltage and IV curve when this panel is placed in parallel with the other one producing 26 Volts? In the case with the shading of two strings in the same panel in parallel configuration, there would be insufficient Voltage to charge batteries. BTW, I tried sending you a private message, but it says your private message quota has been exceeded
Pete

[prroots]

I'll make a first attempt at repeating your theoretical analysis under the the case of two 18 Volt panels with 4 bypass diodes/panel (ie, my case).

Here are my assumptions:

1. each panel generates 4 watts unshaded or 8 watts total
2. each subpanel generates 1 watt (ie, 1/4 of panel output)
3. each diode drop equals loss of 0.1 watt
4. panel puts out 0 watts if panel voltage < 15
5. each panel outputs 26 Volts
6. each subpanel outputs 6.5 Volts
7. each diode drop is 0.5 Volts

no shade:

series: 8w
parallel: 8w

shade on 1 subpanel:

series: 6.9w
parallel: 6.9w

shade on 2 subpanels in same panel

series: 5.8w
parallel: 4w

shade on 2 subpanels; 1 in each panel

series: 5.8w
parallel: 5.8w

shade on 3 subpanels in same panel

series: 4.7w
parallel: 4w

shade on 3 subpanels with 2 in one panel and 1 in the other

series: 4.7w
parallel: 2.9w

shade on 4 subpanels in same panel

series: 3.6w
parallel: 4w

shade on 4 subpanels with 3 in one panel and 1 in the other

series: 3.6w
parallel: 2.9w

shade on 4 subpanels with 2 in each panel

series: 3.6w
parallel: 0w

shade on 5 subpanels with 4 in one and 1 in the other

series: 2.5w
parallel: 2.9w

shade on 5 subpanels with 3 in one and 2 in the other

series: 2.5w
parallel: 0w

Overall, it still seems to favor the series configuration. If one assumes that each of the above cases were equally probable, the average for series is 4.7 watts and for parallel is 3.8 watts. Comments welcome.
Pete