2 strings with different panels

[Engebretsen]

Hi all

I'm new to the forum and also new to the use of Classic 150.

I have had a setup running for a year now and so far its running pritty good. But I still need to tune a bit and I have a few questions I cant figure out.

One thing I'm wondering about, is how to add another string of solar panels. The issue is that they are not the same as the first ones.

Currently I have installed 3 panels in series of Sharp NQR256A. And I have an additional 3 panels of Sharp NURD285 that i want to install in parallel. I have seen some posts on the forum discussing some of the same, but it's not really clear to me if they are using a MPPT CC or what the actual result will be. So to be sure I hope you guys can help me with my setup.

1. I see that the voltage at maximum point differs a bit. 27,53V on NQR256A and 31.3V on NURD285. If I mount the new panels just under (in same angle as) the old ones, how will the Classic react to this? Will it be possible?

2. For maximum wattage of the system, the efficiency is not a critical point, but is it possible to estimate a maximum effect of this? The current setup has been reaching 1000W a few times. This is mostly for cable, fuse and diode dimensioning.

3. More critical is charging in bad sun conditions. The current system is able to charge at almost 50W even if the sun not hitting directly. Will adding the 3 new ones be able to double this? That means float will be a lot more significant.

4. Do I need to install 2 diodes, one per serie string, to prevent currents going the wrong way if I add the 3 new ones?

5. I have the possibility to add the 3 new ones at a different angle (azimuth?). Current setup is pointing south-west. And by adding new ones to a second roof pointing south-east I can get 3-4 more hours in the morning of sun. Is this even an option if I'm limited to the one classic controller? If so, what will be the best option? Identical angles (both azimuth and altitude) or different azimuth. Swapped pritty much 90 degrees.

I know its a lot of questions, its my first time working with a setup of this size. Any help or tips will be very much appriciated!

General info on setup:
SMA 6.0H Inverter
Leaf pack battery bank in 48V - 7s6p (2x cells in each pack)
REC BMS
Classic 150 with CANbus communication adapter.
System is installed in an area where theres no sun a few months per winter, so I consider the low power efficiency is important.
Datasheets on panels attached.

Please ask if theres any additional info you need.

[ClassicCrazy]

It is recommended for string voltages to be within 10% of each other. Your two series strings would be a bit more than 10% . When it is too far off then the MPPT doesn't work correctly.
Maybe some others willl have another suggestion that would make your two strings work on the same controller.

For two strings you don't need diodes - you would want to get a combiner box and have a breaker for each string - usually 15 amps each is used . That makes life easier to work on and diagnose any problems in the future between stings since you can easily cut each one off or both.  You then still have a main PV input breaker at the controller box that is rated for the wire you are using from combiner to controller.  Midnite has a small combiner box that would work.

Adding more PV won't effect Float  - when it gets past Absorb and goes to Float voltage then it takes whatever current it needs to maintain the Float voltage.

Not sure why you would want to mess around with different angles - if the controller you have will handle the current from the PV then you can put everything at the same angle. Your best bet would be to try and find 3 more panels that match either of your other 3  if possible.

Larry

[Engebretsen]

Hi, thanks for your reply!

Then I will have to check a bit if there are any of the original panels somewhere, that would be the best. And hope that someone are willing to trade panels . I'm a bit unsure now why the supplier agreed on selling me these three additional ones for adding to the first. Ill check with them too.

What is a combiner box? I have a separate fuse box with 2x 16A fuses to separate the two strings and put the negatives and the positives toghether before the classic. If there are no diodes needed, then I guess I have whats needed?

Yes you are correct about that regarding float. My intention is to raise the lower part of the charging so it can sustain whats needed to stay in float charge for a longer time to overcome the internal power used by the classic, Inverter and BMS. I am not sure how adding a new string will affect this.

And thats also what I'm thinking about putting the panels on a different angle, to try and keep a charge of lets say 50-100W for as long as possible.

I have actually two months every year without sun on the panels due to mountains, so I guess the question comes down to; how many panels do I need to gather enough reflected energy from the atmosphere to keep the battery voltage stable instead of decreasing per day? . Without the knowledge for proper measurement of this, my best bet is to add the 3 extra panels that I have (or similar ones) and test it.

But just to make it clear, if I have panels within the 10% difference in Vmpp, then adding a string in parallel at a different angle shouldn't be a problem at least? The result will be increased time of exposure to sun, not so much raising the max effect?

EDIT: I have information from the supplier that there should be no issue adding the three extra in parallel, but the result will be that the new panels wont be producing more then the old ones. So in reality it will be like having 2p3s of the same 256W panel. Is this correct? If so, then I am ok with it.

[Vic]

Hi Engebretsen,

Larry is correct with his comments.

The difference between the Vmp of those two PVs is almost 14%.   Ideally,   for parallel strings of PVs,   Vmps should match within 5%.   And,  have a maximum Vmp difference of 10% (IMO).

Your 256 Watt PVs are very,  very unusual  --  they use 48 cells to make up the panel.   Have never,   ever seen any other 48 cell PVs.  You will continue to have trouble finding matching 48 cell PVs,   should you want to expand the system further.   Perhaps your supplier could allow you to return the 256 PVs for some credit,  and you could use the 285 watt PVs  which are 60 cells.   60 cell PVs are probably the most common layout,   and in production,   still.

Clouds and shading reduce power production  by large amounts.   Often down to  5 or 10% of what would be produced at that time of day,  if the sun was not blocked.

There can be some benefit in having PV arrays pointing at different Azimuths  (compass headings),   to spread power production,   more evenly throughout the day.

All opinions,   Vic

[Engebretsen]

Hi again, thanks for the info.

I will keep to using the same roof so I dont need to worry about the different azimuth agles.

Can you explain what you mean by the percentages? You say ideally within 5% and max 10%.

What is setting these limits? Why do you not want to go outside these values?

Can the panels be damaged by connecting them together?

If the only result of this mix of panels, will be that the 285W panels won't produce all 285W but lets say 256W as the others, then I'm very much OK with the production.

Will this in any way harm the controller, because of the difference?

[Vic]

Hi Engebretsen,

All that I was trying to say,  was,  "The difference between the Vmp of those two PVs is almost 14%.   Ideally,   for parallel strings of PVs,   Vmps should match within 5%.   And,  have a maximum Vmp difference of 10% (IMO)".

The above is the "standard wisdom".   With a greater difference than about 10% Vmps for PVs in parallel,   the MPPT Charge Controller (CC) may have a problem finding the best Power Point.   In certain Modes,   the CC might "hunt",   etc.   In varying illumination conditions the behavior of the Vmp Tracking may be different.

This "wisdom" is to help guide the purchase of PVs that are reasonably compatible with the existing ones.    As stated before,   IMO,  those 48-cell existing PVs  are quite unusual.  And generally it is quite possible that you will be unable to find reasonably compatible PVs with the existing ones,   perhaps,  EVER.
 
Have little to add.   Just  my opinions.   Perhaps speak with the supplier of the original 256 W PVs,   to see if you could return them for some credit ...   Or perhaps that supplier might be able to find some additional 48 cell units.  In the USA,   48-cell PVs  are not at all common.

Wish you the best of luck.   Vic

[Vic]

Engebretsen,

Another thing,  or two;   The second part of that standard "wisdom",  is that when adding PVs in series,   the Imp differences should ideally be within 5% of each other,   with a maximum recommended difference of 10%,  or less  ...

Do not know your budget,  or the amount of space you have for PVs.  But,  you could run strings of one 256 W,   and two 285 W PVs in series,  for a Vmp of about 90.1V for the string.

Later,  if you needed more PV power,  you could add strings of three 60 cell PVs to that first configuration of two 60s and one 48 cell PVx  --  the strings of three 60 cell 285s would be within about a 4.2% difference in string Vmps compared to the string/s with one 256 W and two 285 W PVs (about 93.9 V for three 285s).

Just a thought ...    Vic

[Engebretsen]

Hey Vic, thats an awesome idea! Thank you for that! I do have space to mount the three new ones close to the others so changing up the series should be no problem.

The voltages will then indeed be within the limits.

Do I have to consider anything on the amps when mixing panels in series?

The 256W have Ampp 9.33A and 285W are at 9.1A. Does this mean that maximum current through the string will be 9.1A? It would be no problem, but good to know how it works in this situation.

As I see it now, the setup will give:
9.1A at 86.36V for string one
9.1A at 90.13V for string two

Then the Classic will find a midpoint between the two?

[Vic]

Let me look at the Data sheet for the 256s,   and calculated Imp from the STC power,   and the Vmp   ...   will check back later.   It is dinner time here.

Vic

[Vic]

    ...
Do I have to consider anything on the amps when mixing panels in series?

The 256W have Ampp 9.33A and 285W are at 9.1A. Does this mean that maximum current through the string will be 9.1A? It would be no problem, but good to know how it works in this situation.

As I see it now, the setup will give:
9.1A at 86.36V for string one
9.1A at 90.13V for string two

Then the Classic will find a midpoint between the two?

Hi Englebretsen,

The primary thing to consider in mixing PVs in series is the Imp of each.  Having all Monos,   or all multicrystals in each string may help,   may not.   Certainly,  mounting strings of PVs with different mounting dimensions could be a bit of a hassle.

Yes,   in series strings of different PVs,  the STC Imp of the string will be about that of the PV with the lowest Imp.   As you know,   we seldom see STC outputs,  without reflections,   or perhaps with very cold weather.  Often  at Solar Max,  we see about 75%  of STC power ratings.

As I see it,  string one will be two two 285 W PVs,   and one 256. This would give a calculated STC string Vmp of about  31.3V X 2 = 62.6V plus 27.5V for the 256,  which equals about 90.1 V for that string.

Three 285s would give STC string V of about 93.9V.

Am simply summing the STC Vmps of each PV in the string to get the string V.   When using strings of PVs with different STC Vmps,   one would expect that the Pv with higher Isc might well have a bit higher Vmp at STC conditions,  due to it being loaded a bit less,  causing its Vmp to rise a bit (IMO).

Since there are variations between PVs with identical specs,   and the output of the PVs vary considerably due to variations in Irradiance and temperature,   clouds,   haze,  and humidity (etc),   have not spent too much time trying to calculate what will be the power output,   assuming certain conditions,   but that is just me ...

The above calcs may have some flaws,   but this is the way that we look at it.

The Classic should find a MPP that is between the Vmps of each of the two candidate PVs,  for the Irradiance and cell temperatures at that moment.

Hope that the above makes some sense.   Vic

[Engebretsen]

Hi Vic

Your info and explainations are very much appreciated!

I will indeed aim to mount the three new panels and mix up the series. Thanks to you, this situation turned out a whole lot easier hehe.

Regarding the efficiency compared to STC, maybe my system is in a extraordinary position, since the classic have already logged peaks at almost 1000W on the 3x 256W panels! And 700-800W is normal when the theres full sun.  Will be exciting to see when the three next are up

Unfortunately it looks like I wont manage to mount them before the winter this autumn. Aiming for some time in the spring instead.

Thanks again for your help