Author Topic: solar panels in series vs parallel  (Read 28276 times)

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defed

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solar panels in series vs parallel
« on: July 22, 2010, 06:22:41 PM »
let's say you have 3 panels, rated at 100w (20v @ 5a for simplicity sake).

when wired in series, you get 300w (60v @ 5a).  in parallel you have 300w (20v @ 15a).

now let's say the actual output of the 3 panels is 100w, 90w, and 100w.  in series, is it correct that the total output is 'held back' by the lowest panel?  270w output (90x3).

in parallel, would the total output still be the sums?  290w output.

in other words, does 1 'weak' panel only bring down the output of all others when in series, and just lowers the overall output by it's own deficiency when in parallel?

hopefully this make sense.

thanks
Dave

dnix71

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Re: solar panels in series vs parallel
« Reply #1 on: July 22, 2010, 09:27:08 PM »
In theory. In real life many thin film panels are not made to string like that. They can't pass the current without a voltage drop from internal heating. There isn't enough meat to carry the current. Poly and Mono panels on the other hand usually can be strung with little loss. If the panels already have bypass diodes, then you are all set to string in series. If not, schottky bypass diodes can installed but it's hard to find high voltage and high amperage schottky diodes. There are other bypass diodes available, but those have greater forward voltage drops, so you lose out collecting some power in overcast weather and at dusk and dawn.

If there is any shading [clouds, frames, etc.] stringing panels usually results in serious power losses even with the bypass diodes.

The power output in watts should be the same in either case. If you have long wire runs from the panels to the controller, it pays to boost the voltage since that allows smaller wire to carry the same current. Anything over 60v is considered high voltage under the NEC and all wiring like that is expected to meet code. That might be consideration, depending on where you are installing the setup.

If this is home brew, don't make permanent connections until you have had the chance to experiment. Try different ways and see what works best in your case. If you are paying for this commercially, get a couple of opinions, but the final say will depend on the code and the brand of panels you buy because they have to be UL/NEC/UN/CSA whatever rated for that particular application and setup.

defed

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Re: solar panels in series vs parallel
« Reply #2 on: July 23, 2010, 05:50:49 AM »
thanks,

i am looking at getting some mono/poly panels.  the battery bank will be 24v (possibly 48v in the future).  i want to get 24v panels to avoid having to have them in series (unless i went 48v later).  my concern is that if i buy a few panels now, w/ a certain rating, and then can't find the exact same ones later when i add on, i wasn't sure how they would interact if the volts/amps varied.

so if i have (3) 24v panels, they would be combined at the charge controller.  if all panels were 100w but varied in voltage/amperage, 20v/5a, 21v/4.75a, 19v/5.25a....would this provide a simple 20v at 15 amps, or something less due to one panel having low amps and one having low volts?

i guess it's also a matter of panel deterioration over time, not just a initial matching issue.  if one panel starts losing power faster, in series, it would drag all the others down with it, but does it affect the other panels in parallel?


Airstream

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Re: solar panels in series vs parallel
« Reply #3 on: July 23, 2010, 08:53:40 AM »
Remember full charging & equalizing a 24V bank can require 31+VDC (14.7 x 2 plus diode & 3-5% line losses)

ghurd

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Re: solar panels in series vs parallel
« Reply #4 on: July 23, 2010, 09:13:14 AM »
The current will be the same in all the series panels.

The voltage is added together in series..

Changing the current changes the voltage of each panel.
The lower amp rated panel will pass a bit more amps than rated and operate at a lower voltage.
The higher amp rated panels will pass a bit less amps than rated and operate at a higher voltage.

Bypass diodes in a 24V system will not do any good (unless wired for higher voltage to an MPPT).
For example, 12V crystalline panels with 20Vo, with 2 bypass diodes per panel, one for each '6V' string.
If a bypass diode is 'bypassing', that leaves that section of panel as a 0.7V drop.
Leaving 3 sections of 10Vo, minus 0.7V diode drop, minus semiconductor losses in the controller.  Say 28.5Vo?
Pull a few dozen ma, and the panel's voltage drops like a rock.
Leaving no charging.   Unless the batteries are dead flat.

In parallel, all the panels will operate at the same voltage.  (google KVL)
They should be operating at the battery voltage plus hopefully less than 1 or 1.5V.  But they will be operating at a voltage below the max power voltage.
The amps add together.

Dnix, I don't follow the concept that matched thin film can not carry the amps in series.
5A is 5A.  Series or parallel.

If you are not 100% certain about the final system, and thinking about 24V panels,
I would tip the scales toward 24V panels with 4 bypass diodes, that have the option to rewire to 12V output.  They will generally have a higher resale value if the situation arises.
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dnix71

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Re: solar panels in series vs parallel
« Reply #5 on: July 23, 2010, 01:19:01 PM »
GHurd, I double my HF thin film panels because my mppt works better in that voltage range, but they lose something when you do that. They weren't made to double up.
Some thin film are. Harbor Freight panels may be a special case, but they are common.  Tripling the HF panels triples the voltage, but the output suffers a lot mpre if I do it. Double seems to be the best compromise.

I'm typing this in tropical storm Bonnie. It just made landfall in Miami about an hour ago. My output today has been all over the place. I briefly saw over 5 amps in heavy overcast, but right now am getting less than 1 amp boost. Without the doubled voltage, the HF panels wouldn't be putting out enough to charge a 12v system. I added a 12k uF cap on the panel side to smooth out the ups and downs of voltage. That makes my mppt more stable along the edges (dusk, dawn and heavy overcast). Otherwise it click-click-click-clicks searching trying to find stability.

defed

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Re: solar panels in series vs parallel
« Reply #6 on: July 23, 2010, 04:53:23 PM »
Remember full charging & equalizing a 24V bank can require 31+VDC (14.7 x 2 plus diode & 3-5% line losses)

right, i was just using simple #'s in my examples.

defed

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Re: solar panels in series vs parallel
« Reply #7 on: July 23, 2010, 08:24:19 PM »
this is more complicated than i thought....here i figured solar was simple...plug some panels into a controller and batteries and away you go!  wind turbines are a cake walk compared to this!

ghurd

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Re: solar panels in series vs parallel
« Reply #8 on: July 24, 2010, 08:09:47 AM »
It is that simple.
Parallel panels to the controller terminals marked "Panels".
Battery to the terminals marked "Battery".

Until MPPT or something complicated is introduced.
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defed

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Re: solar panels in series vs parallel
« Reply #9 on: July 24, 2010, 08:56:35 AM »
It is that simple.
Parallel panels to the controller terminals marked "Panels".
Battery to the terminals marked "Battery".

Until MPPT or something complicated is introduced.
G-

ok, but i'm still trying to wrap my head around how panels w/ lower output (either due to losses from age or mfg tolerances) affect the output of the total array in different configurations.

i was looking at going MPPT from the start so i didn't have to buy more stuff later to upgrade.

defed

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Re: solar panels in series vs parallel
« Reply #10 on: July 24, 2010, 09:05:14 AM »
i presume a panel w/ these specs is for 24v...

Power (W)                           195 Watts
Open Circuit Voltage (V)          33.90 Voc
Short Circuit Current (A)            8.15 Isc
Maximum Power Voltage (V)    27.10 Vmp
Maximum Power Current (A)      7.20 Imp
Weight (lb)    41 lb
Connector    MC-3

does the open circuit voltage have any real relevance?  if not, why list it?  i take it that the 27.1v is the max power under load conditions?  similar to a turbine after cut-in.
« Last Edit: July 24, 2010, 09:07:13 AM by defed »

defed

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Re: solar panels in series vs parallel
« Reply #11 on: July 25, 2010, 06:12:49 AM »
it seems that when using MPPT, you want to feed the most voltage into it as possible.  for some reason, i was under the impression that you wanted input voltage to equal output voltage (like a regular controller) and the MPPT just adjusted the volts/amps...but it seems it likes to have the most voltage possible and least amps possible for its initial input.  ie...(3) 100 watt panels (20v @ 5a) would want to be input as something like 150v at 2 amp.  not 100% sure how the panels are hooked together (presume series?), still doing alot of reading.
« Last Edit: July 25, 2010, 06:35:26 AM by defed »

ghurd

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Re: solar panels in series vs parallel
« Reply #12 on: July 25, 2010, 10:26:49 AM »
That panel is something unusual.  Too much for efficient 12V, not at all enough for 24V.
It may be intended for grid-tied systems.  It would be OK for a MPPT system.
Typical panels have a peak power operating voltage of about 17V per 12V of system. (34V for a 24V panel).
Typical panels have an open circuit voltage of about 21V per 12V of system (42V for a 24V panel).

Maybe read some of the data on the Blue Sky (MPPT controllers) website.  Might have to read the 2512iX pdfs.  Not sure where it is, but there are a couple pretty good entry level descriptions of MPPT there somewhere.

Mis-matched parallel panels into a MPPT will get something unusual going on.
Since the mismatched panels will have different peak efficiency operating points, the controller will likely have problems figuring it out.
The controller is expecting the output chart to look like ' ^ ' but it will look more like ' m '.
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defed

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Re: solar panels in series vs parallel
« Reply #13 on: July 25, 2010, 12:04:28 PM »
thanks Ghurd.  those panels i mentioned seem to be evergreen seconds, at least by looking at the pics of them...but no one says that they are the CORRECT pictures.  also, i don't recall seeing any evergreens w/ specs like those.

the reason i was interested in them, they are less than $2/watt.

i will have to read more on MPPT and see how it reacts to combining different panel arrays.  i saw some mention of hooking a series pair of, let's say, 50w panels and a pair of 100w panels, and then this was paralleled to the controller.  if i was just buying all the panels i would ever need now, i wouldn't be having such a hard time w/ this...but i want to buy 2 or 4 panels now, and then be able to use them w/ other panels, that may not be exactly the same, in the future w/o buying all new components.


ghurd

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Re: solar panels in series vs parallel
« Reply #14 on: July 25, 2010, 12:26:02 PM »
I believe if you stay with the same type of panels, and a standard type of panels, the MPPT should be OK.
Like it would be OK to parallel a series string of 2 x 50W crystalline 17Vmp panels, with a series string of 2 x 100W crystalline 17Vmp panels.

But I would not want to add a series pair of thin film with them, or a series pair of 15Vmp 30 cell self-regulating panels with them.

Stick with something standard, and expansion will be easier.
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Rover

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Re: solar panels in series vs parallel
« Reply #15 on: July 25, 2010, 12:41:50 PM »
Concur with Ghurd, I have a blue sky mppt , and I initialy had all my mismatached panels (some hf amorphous, 50 watt Mono, and 3 100w poly's running through it. I've since moved the mono and the hf to a pwm that I had laying around, and left my  polys going into the blue sky. My total has gone up 10-20% since doinhg that

It also helps that my 3x100s are located in the same spot , and the others about 15 ft away, and there is a large variation in sun light between the two spots. I did not have diodes in between so I was probably suffering some loss for having some panels shaded and the same time others had full sun. (which probably confused he MPPT even more)




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DamonHD

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Re: solar panels in series vs parallel
« Reply #16 on: July 25, 2010, 12:59:12 PM »
I have a very mixed bag running parallel into my MPPT controller and it all seems to work:

1 x 20W silicon monocrystaline (nom 12V)
1 x ES62 Unisolar triple-junction (nom 12V)
many random TopRay amorphous nominal 12V at all sorts of orientations and pitch to catch different bits of sun!

I trust Morningstar to have a robust algorithm to seek the highest peak!

Rgds

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Rover

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Re: solar panels in series vs parallel
« Reply #17 on: July 25, 2010, 01:04:40 PM »
It may work for you but I'd have to assume there is some inneficency in the combination, unless all of your panels have the same characteristics and MPP. Otherwise some of your panels may be at MPP  while others aren't and the controller is doing its best. But it still won't provide the best MPPT.
 
Rover
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dnix71

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Re: solar panels in series vs parallel
« Reply #18 on: July 25, 2010, 01:56:12 PM »
I'm with GHurd on this one. Those specs sound like direct grid tie panels. I answered a Craigslist ad a couple of years ago and a guy had a shed full like that he was selling.

I didn't buy any. He wanted too much per watt and they were used, plus he had a $2k Xantrex grid-tie inverter to go with it. He also had a garage full of sour 6v traction batteries. He bought the setup thinking he could go off-grid, but didn't do the math. You can't run a/c from solar down here in south Florida. Even with a roof full (5.5kw) if a cloud passes by, poof, there goes your power.

I also passed on the panels because of the odd voltage. Too high for my 12v setup without an mppt, and back then there weren't any inexpensive mppt's that I trusted to actually work. Big panels, too. 2 would cover my Ford van from end to end.

DamonHD

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Re: solar panels in series vs parallel
« Reply #19 on: July 25, 2010, 03:05:37 PM »
Rover: agreed it won't be optimal, but it's better than a plain PWM controller with those same panels, and I don't really want the complication (and expense and extra wiring) of multiple small controllers.

It's one of these things like trackers, I suspect, that it's almost always easier and cheaper to buy more panels then increase complexity to get the same improvement in output.

Rgds

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defed

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Re: solar panels in series vs parallel
« Reply #20 on: July 25, 2010, 06:29:21 PM »
i found some BP's that are for 24v (43v open, 36v loaded) that aren't that much more.  BP is a big name, but w/ all of their troubles lately, and mixed news on their financial well being, what are the bets that they will be around for warranty or getting more similar panels in 2 yrs?



ghurd

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Re: solar panels in series vs parallel
« Reply #21 on: July 26, 2010, 11:58:14 AM »
Buy a well made, good quality panel, and hope for the best.

If you decide by how large and experienced the company is, you would have bought maybe Shell, Siemens, Photowatt, or Chronar?
(all sold out, at least in the US)

I figure there is very little profit in solar panel manufacturing, or they wouldn't get out of the business.
Nobody says "Wow.  We are making too much money with this.  Lets sell it.'

I figure BP is in solar for good press, which they need now more than ever.
Imagine the bloggers if BP got out of solar after the gulf incident.
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DanG

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Re: solar panels in series vs parallel
« Reply #22 on: July 26, 2010, 01:05:35 PM »
If BP sells off their PV line the receiver would be obligated to honor warranties.

If BP goes bankrupt (they didn't let the scummy finance people sink) then your vendors may or may not honor warranty issues - but my money is on BP being around for the next 100 years at least...

Tritium

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Re: solar panels in series vs parallel
« Reply #23 on: July 26, 2010, 02:49:56 PM »
thanks Ghurd.  those panels i mentioned seem to be evergreen seconds, at least by looking at the pics of them...but no one says that they are the CORRECT pictures.  also, i don't recall seeing any evergreens w/ specs like those.

the reason i was interested in them, they are less than $2/watt.

i will have to read more on MPPT and see how it reacts to combining different panel arrays.  i saw some mention of hooking a series pair of, let's say, 50w panels and a pair of 100w panels, and then this was paralleled to the controller.  if i was just buying all the panels i would ever need now, i wouldn't be having such a hard time w/ this...but i want to buy 2 or 4 panels now, and then be able to use them w/ other panels, that may not be exactly the same, in the future w/o buying all new components.



I have 4 of those panels.  I run them as 2 in a series with 2 parallel series groups into an Outback MX60 MPPT to a 12V battery bank (6 sams club golf cart batteries, 6V 220aH, series for 12V, paralleled for 660aH).  Panels are usually held right around 53v input by the controller.

Thurmond