Author Topic: diffeerent size panels  (Read 5692 times)

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thirteen

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diffeerent size panels
« on: September 11, 2014, 11:45:32 AM »
I have 3 24v - 240 w panels now. I am trying to put my panels upon my hill side. I can get a good price on 24v  255 w panels. Because of the fixed position how much of a problem will i encounter trying to incorporate my 3 24v 240w panels with my 8 255w panels. They will be hooked in serris of 4 ea except for my 3 240w's. I plan on putting them facing straight West in a fixed position. Or in a straight East position. They will feed into the same power lines. Some of the time they may be overpowered some  by the 255w panels but very little.  Will the 15 w make that big of difference.  I am trying to use my panels together in sets of 4 for higher voltage because I have about 550 ft of power line to get to my controler and bank and I will have voltage loss. I plan on using my Flex max 80 (Outback) as my controler. I will expand to 4 more panels late next year. For a total of 12 24v 255w panels I will have to get a bigger controler when adding the next set of 4 255w panels.. I could get 1 more 240 w panel to make it 4 for more balanced panels system. Just not sure of my ideas working properly.
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lifer

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Re: diffeerent size panels
« Reply #1 on: September 11, 2014, 01:32:47 PM »
Think of PV panel as a big (series) diodes array. The 3 X 240W branch will output 3 x 24V = 72V while each of the 4 x 255W branches will output 4 x 24V = 96V (at maximum power point, for example).

To acomodate this situation (those branches being parallel connected) the output voltage of each branch should be identical. That means the 4 x 24V branches should lower their output and the 3 x 24V branch should have a bigger output voltage. The maximum voltage of a panel is the open circuit voltage and the minimum one are the short circuit voltage. In each case, the panels performances are very degraded and chances are the 4 x short circuit voltage will still be greater than 3 x open circuit voltage so the 3 x 240W panels are reverse polarised and won't generate any energy (it's like leaving those panels open).

You better try to trade off those 240W panels and get only 255W panels (multiple of 4) or get another panel (240W or 255W) to complete that 3 panels branch.   

Another solution it's to use a separate charge controller for the 3 x 240W panels.

Mary B

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Re: diffeerent size panels
« Reply #2 on: September 11, 2014, 06:42:07 PM »
Separate controller or order 1 more panel so they balance.

lifer

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Re: diffeerent size panels
« Reply #3 on: September 12, 2014, 06:41:37 AM »
The short answer it's yours! I had the impression that he wanted to use a 3 panels branch in parallel with 4 panels branches - but I was wrong. Anyway, it's better to have the same panels type for further reconfiguration (series of 6 - 8 panels).

madlabs

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Re: diffeerent size panels
« Reply #4 on: September 12, 2014, 10:46:48 AM »
The Flexmax is an MPPT controller, so unless the mVp of the panels are ther same or close, you will be losing some juice that way. I agree with ditching the other panels.

Another thing to take a look at is the Flexmax power curve and see where the sweet spot is. For example, the Midnite Classic that I own can process the most power with an input voltage of around 100VDC. If you run at less or more voltage the controller can't handle as much power. Important if you want to be able to connect as many panels as possible in the future.

Jonathan

thirteen

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Re: diffeerent size panels
« Reply #5 on: September 13, 2014, 12:51:57 AM »
Am I thinking this wrong?
a panel 24v 255w open circiut voltage is 37.4 v dc
a panel 24v 240w open circiut voltage is 37 v dc

4 x 37.4 = 149.6 v
4 x 37    = 148    v
The max voltage for a Flexmax 80 is 150v so it would be at the top of the safty margin. I have a Flexmax 60 now.
I would need a mppt controler that would be able to handle the high voltage.  175/200 v max.
This is based on the open circiut high voltage on the upper end of the panels.
Their operating voltage is around 30v. the operating curent is 8.03 for the 240w and 8.3 for the 255w
Where can I find out a voltage loss chart for 525 ft of power line from panels to control room? I might be within working range of my panels I wish to install with the power loss. I'll have to study the power curve for the best usage. 
I would only have to find a single 24v 240w panel.
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madlabs

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Re: diffeerent size panels
« Reply #6 on: September 13, 2014, 01:08:06 AM »
I use the calculator on this page for wire loss.

What is the mVp of your panels? That is the voltage the controller will try to run the system at. I did a quick look for the data sheet for your controller, but I didn't see a power curve. Find the curve and see how much power your controller can process at the mVp voltage.

Jonathan

lifer

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Re: diffeerent size panels
« Reply #7 on: September 13, 2014, 02:47:26 AM »
Look at these two charge controllers (80A - 600V DC maximum input voltage):

Xantrex-MPPT-80-600

Conext-MPPT-80-600

Actually, it's the same Schneider Electric product with different brand names. Good luck!

Flux

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Re: diffeerent size panels
« Reply #8 on: September 13, 2014, 03:41:09 AM »
I can see where the confusion comes from. I initially thought you were trying to combine a string of 3 240W with strings of the bigger ones. As long as you buy the extra 240W one to make all strings 4 there is no problem. As long as each string has the 4 same panels all is well.

If you string 3 x 240W with one 255W it will still work but the big one will be reduced to the same as if it was 240W. The minor difference in o/c volts doesn't matter but the major difference between a string of 3 or 4 is not viable for parallel connection.

As for whether adding panels facing directly east or west is worthwhile depends on your location but early morning and late evening has little sun energy, here in the UK the gain would be small.

Flux

Rob Beckers

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Re: diffeerent size panels
« Reply #9 on: September 13, 2014, 09:28:05 AM »
Am I thinking this wrong?
a panel 24v 255w open circiut voltage is 37.4 v dc
a panel 24v 240w open circiut voltage is 37 v dc

4 x 37.4 = 149.6 v
4 x 37    = 148    v
The max voltage for a Flexmax 80 is 150v so it would be at the top of the safty margin. I have a Flexmax 60 now.
I would need a mppt controler that would be able to handle the high voltage.  175/200 v max.
This is based on the open circiut high voltage on the upper end of the panels.
Their operating voltage is around 30v. the operating curent is 8.03 for the 240w and 8.3 for the 255w
Where can I find out a voltage loss chart for 525 ft of power line from panels to control room? I might be within working range of my panels I wish to install with the power loss. I'll have to study the power curve for the best usage. 
I would only have to find a single 24v 240w panel.
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Thirteen, yes you are actually thinking this wrong...  8)

Unless you live in Hawaii there's a good chance you will kill an otherwise fine FlexMax80 by hooking up four regular 60-cell panels in series.

As you noted, your panels have an open circuit Voltage of around 37 Volt. Very typical for 60-cell modules intended for grid-tie use (these days all in the 250 - 285 Watt range). That Voltage is measured when the cells are at 25 Centigrade (about 78F)!! When silicon solar cells get colder that Voltage goes up, when they get warmer it goes down. By quite a bit.

For example:

Here in the Ottawa area we can see winter temperatures as low as -35C (about the same in F). That is the temperature commonly used over here to calculate the maximum panel Voltage (to get more of a safety margin it would not be a bad idea to add another 5C though). The temperature coefficient of (nearly) all regular silicon solar cells is -0.34% per degree Centigrade.

So we have a temperature difference of 35 + 25 = 60 Centigrade vs. that standard 25C temperature.

That means a difference in Voltage of 60 * 0.34 = 20.4%

That means the open Voltage of that panel at -35C will be Voc = 1.204 * 37 = 44.5 Volt (!!)

That means a string of 4 panels, as you proposed, will generate 4 * 44.5 = 178 Volt! Well above the 150 Volt absolute limit for a FlexMax80!

Maybe your lowest temperature (n)ever measured is not quite as low as -35C for where you live. Just plug it in and follow the same calculation as above. For almost all locations in the US you will still go well above the 150 Volt limit of the charge controller. This is something to keep in mind for any MPPT charge controller one plans on using.

-RoB-

thirteen

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Re: diffeerent size panels
« Reply #10 on: September 13, 2014, 11:55:55 PM »
Flux
The reason for the E=W panel is I have a mountian that gets in the way around 5-6 right now for an hour and in the winter (Dec to 1 Feb) there is only about 1 1/2hr  of sunlight but close to ESE. and in the afternoon I could get maybe 1 hr but that is facing WSW. I live in a steep sided canyon. I've thought of a tracker but people seem to have problems up here during the winter plus the snow gets up around 5 -8 ft deep where my panels will be. You cannot walk up to them you have to crawl and during the winter months if you slip it is about 450 ft to the road below. Just plain steep.
Rob
I do get a temp of around -20 but usually only for 3 weeks at most. I does dropp down to that but only for 6-10 days unless it changes but that has been the pattern for 8 years. Thanks for giving that information on the voltage. I had not thought of that. I either need to go with a bigger MPPT or drop to panels of 3 instead of 4.
Madlabs
What chart are you using for the voltage loss it did not come thru. These are Canadian Panels. I got a chart on line but the information I  could not read.
Lifer
I'll look those up I do believe I will need somthing that can handle the voltage that I could have that Rob erxplained. 
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XeonPony

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Re: diffeerent size panels
« Reply #11 on: September 16, 2014, 09:38:29 PM »
here is a nice one I use for Vdrop
Ignorance is not bliss, You may not know there is a semie behind you but you'll still be a hood ornimant!

Nothing fails like prayer, Two hands clasped in work will achieve more in a minute then a billion will in a melenia in prayer. In other words go out and do some real good by helping!

thirteen

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Re: diffeerent size panels
« Reply #12 on: September 16, 2014, 10:50:48 PM »
I used it but not sure what cma of conductor  105.600 represents? Could you explain it to me? 13
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Mary B

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Re: diffeerent size panels
« Reply #13 on: September 17, 2014, 06:10:30 PM »
Good voltage drop calculator http://www.nooutage.com/vdrop.htm

XeonPony

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Re: diffeerent size panels
« Reply #14 on: September 21, 2014, 09:03:09 PM »
circular mills area. surface area of wire. not importent for you really

total paqnel wattage devided by string voltage at minimum

for my system 540w @ 32V mppt so 540w/32 = 16.875A so well round it up to 17A

So 32V
17A
50 foot run
Copper conducter
Target V drop lower then 3%

I could use 6 guage wire

By entering the data and using an educated gues for my starting guage I just scroll down the wires till I meet my target.
Ignorance is not bliss, You may not know there is a semie behind you but you'll still be a hood ornimant!

Nothing fails like prayer, Two hands clasped in work will achieve more in a minute then a billion will in a melenia in prayer. In other words go out and do some real good by helping!