MPPT with mismatched panels?

[daz]

This is the first time that I will be using a MPPT cc and I just have a few questions.
I read awhile ago on a post that it is best to use matching solar panels with your MPPT, and that mismatched ones would cause some inefficiency (cant find that post now ).

I have a 230W panel, and would like to add more solar panels. The problem comes in that I cannot get that exact 230W panel. I have looked around and I can get a 215W or a 265W solar panel (those were the closest I can find at the time). The system will be run as a 48V (already have 2x230W panels, both in series for nominal 48V).
So...my questions...

1) I am geting a 20A MPPT (150voc) cc, and if I get the 2 new panels (either the 215W or 265W @34.4V), can I connect it to the existing panels (in parallel?)?
2) Will the cc battle to find the optimum power point? Or will it be fine, just not as efficient as having all the panels exactly the same?
3) If there will be inefficiency, will it be alot, or really negligible?

If I remember correctly, the post said that you could use other solar panels, so long as the current/voltage specs were within 5%. Is that correct?


Thanks!

[Sunking]

1) I am geting a 20A MPPT (150voc) cc, and if I get the 2 new panels (either the 215W or 265W @34.4V), can I connect it to the existing panels (in parallel?)?
2) Will the cc battle to find the optimum power point? Or will it be fine, just not as efficient as having all the panels exactly the same?
3) If there will be inefficiency, will it be alot, or really negligible?

I think all your question are a moot point because a 20 amp MPPT controller has the following input power limitations vs battery voltage:

250 watts @ 12 volt
500 watts @ 24 volt
1000 watts @ 48 volt

So unless you plan on using 48 volt battery, your idea is shot to pieces.

[daz]

I think all your question are a moot point because a 20 amp MPPT controller has the following input power limitations vs battery voltage:

250 watts @ 12 volt
500 watts @ 24 volt
1000 watts @ 48 volt

So unless you plan on using 48 volt battery, your idea is shot to pieces.

Thank you for the reply Sunking!
Yes, this is a 48V system. It is only using the 2 x 230W panels (in series) at the moment, and I would like to expand it. That's why I am asking about the mismatch panels relative to the MPPT cc. First time using MPPT like this....so not sure what the inefficiency/output will be!

[inetdog]

Thank you for the reply Sunking!
Yes, this is a 48V system. It is only using the 2 x 230W panels (in series) at the moment, and I would like to expand it. That's why I am asking about the mismatch panels relative to the MPPT cc. First time using MPPT like this....so not sure what the inefficiency/output will be!

If you are adding a mismatched panel to a series string, the voltage does not matter (as long as Voc total is within the MPPT input limits) but the current, Imp, should be either higher than the Imp of the existing panels or within 10% for two panels in series, 5% for 3-5 panels in series, even closer for more than that.

For mismatched panels in parallel the Imp does not matter, but the Vmp of the added panel should be within 10% of the Vmp of the others AND the Voc of the lower voltage panel should be greater than the Vmp of the higher voltage panel.

[daz]

If you are adding a mismatched panel to a series string, the voltage does not matter (as long as Voc total is within the MPPT input limits) but the current, Imp, should be either higher than the Imp of the existing panels or within 10% for two panels in series, 5% for 3-5 panels in series, even closer for more than that.

For mismatched panels in parallel the Imp does not matter, but the Vmp of the added panel should be within 10% of the Vmp of the others AND the Voc of the lower voltage panel should be greater than the Vmp of the higher voltage panel.

Thank you Inetdog!! That really helps!

I do have another question for you, if you do not mind.
There are already 2x230W panels in the existing system (in series for nominal 48V system). Instead of using my two new panels in series, and then wiring it in parallel to the existing panels, can I wire the new panels as you said in the first part of your answer? That is, I place each new panel in series with one existing panel, and then place both of them in parallel? (that should mean both strings will be identical, with one old panel and one new panel, which would be better for the cc?)

[Naptown]

Post the specs on both panels both existing and new or proposed new

[inetdog]

(that should mean both strings will be identical, with one old panel and one new panel, which would be better for the cc?)

It does not matter which is "better for the CC". What counts is which will give you the most power from the total of four panels.
You can do the approximate calculation for both configurations, changing the current for the high current panel to equal to the current of the lower current panel and see what you get.
Do the same for paralleling two mismatched series strings and see what you get.

[daz]

Post the specs on both panels both existing and new or proposed new

Thanks for the reply Rich!
I don't have those numbers with me, for the existing panels. I will check the specs on the existing panels when I go past the site tomorrow and post them here.
Basically I am just looking for a "rule of thumb" when trying to match panels togethor. That way for future projects I know exactly how it should be done!

Specs for the proposed new panels are;
Wmp : 200W
Vmp : 27.2V
Imp : 7.4A
Voc : 33.4V
Isc : 7.8A
NOCT=45

The only reason I am going for the 200W (I know it is lower than the existing 230W), is because that is the closest the manufacturer has. The next size up that they have is a 265W, and the price goes up quite a bit! Pricewise, although I will lose some power (because the new panels are 30W less) it is 'cheaper' overall, per watt of usable power, than going for the 265W.

[daz]

It does not matter which is "better for the CC". What counts is which will give you the most power from the total of four panels.

Absolutely! I will go crunch some numbers and see what I can come up with. Thanks!

Your previous post was extremely helpful as well (post number 4). Exactly what I was looking for!
I am going to the site tomorrow, so I will get the specs for the existing panels then (will post them here as well). Then onto some number crunching

[daz]

Post the specs on both panels both existing and new or proposed new

Here are the specs:
Existing Panels;
Wmp : 230
Vmp : 29.7
Voc : 36.3
Imp : 7.74
Isc : 8.75

New (proposed) panels;
Wmp : 200
Vmp : 27.2
Voc : 33.4
Imp : 7.4
Isc : 7.8


I installed the 20A MPPT and it is charging nicely! Now I just have to decide on the new panels.

From what Inetdog said, I went and crunched some numbers. From what I calculate, placing the two new panels (wired in series) in parallel with the existing panels (also wired in series), I will generate 823.62W per hour.
If placing all solar panels (new and existing) in series, I generate 842.12W per hour.
So if my numbers are correct, I should place all the panels in series to get the maximum amount of power out of the system!

Thank you for that advice Inetdog!!! I was just going to wire the panels as in my first example, but since I have sat down and done the actual calculations, I now know which way will yield the best results! Thanks!

[Naptown]

Slow down there cowboy.
With 4 in series you run the risk of over voltage to the charge controller.
Back up and recalculate what you are doing or what charge controller are you using?

[solarside]

The MPPT will adjust and get you the most power for what you have no matter and of course you want to match your panels but they always change over time anyway getting old, bird poop and things like that so some small mismatching is expected. When you go series with the panels the least amps wins! Whatever the lowest amps is on one panel that will be your max amps for the whole string. Also be careful with OC voltages going into the 100V plus area, dangerous!

[daz]

Slow down there cowboy.

No problem....I will stable the horse for now! haha.

With 4 in series you run the risk of over voltage to the charge controller.
Back up and recalculate what you are doing or what charge controller are you using?

With all 4 panels in series, I added the total open circuit voltage, and it comes to 139.4V. The charge controller that I have can have a maximum input of 150Voc, so I am assuming it can handle the voltage? The 139.4V is the maximum, as it is the open circuit voltage. The panel voltage decreases slightly with heat (-115.30mV/C'), so the maximum the CC should ever see is the 139.4V? Or is there something that I missed somewhere?

[Naptown]

Panel voltage also goes up when it gets cold and you are close to the limit. and Cold means anything below 77F

[daz]

Panel voltage also goes up when it gets cold and you are close to the limit. and Cold means anything below 77F

Sorry I had to Google. Apparently 77'F = 25'C.

Good point! Durban has a sub-tropical climate, so it very rarely gets lower than 25'C!! Average here is usually 25-30'C. The only time the temps drop, is if there is a cold front, which does happen every now and again. At the moment we are having a cold front that has come over from the Atlantic side, so the temperature is about 21/22'C. But I do not think that should be a problem for me? Because when the cold weather is here, it is usually overcast/raining, which normally drops my power output to less than 10% (usually about 2-5%). So ya, that's about the only time we got cold weather (colder than 25'C), when it's overcast/raining.

According to the panel spec sheet, the power output is affected 115mV per 'C. So 1V = 8.6'C? Which means, taking an extreme low of 15'C, it would boost output voltage by 1.15V (assumung I have calculated that right??).

With that info, what do you think? Should it be alright?
Or should I not take the chance, play it safe, and just install the new panels in parallel with the existing panels??


Thanks for all your help so far Rich. It is really appreciated!