mixing and matching solar panels

[jack11]

This may have been brought up here before, but I can't find any into, so here it goes.

The problem I am having is the following:
I build a system for certain required solar wattage, and buy the panels to match the wattage requirements and desired string configuration (array V and I).
After some operating time, this system needs to be upgraded to more solar input watts, but the panels I have used are no longer available or difficult/costly to get.
However, I have some other solar panels, but with different Voc/Vmp and Isc/Imp specs.

One specific example is as follows:
I have a MPPT system consisting of 4  320W panels (4*320=1280W).
For a number of reasons, the panels are connected in 2 strings, each string has 2 panels in it connected in series, and the 2 strings are connected in parallel.
For each panel: Voc=64.8V, Isc=6.24A.
For each string: Voc=129.6V, Isc=6.24A.
For both strings: Voc=129.6V, Isc=12.48A.
The MPPT controller sweeps between Vbatt and Voc for the whole array (24V to 129.6V).

At STC:
For each panel: Vmp=54.7V, Imp=5.86A.
For each string: Vmp=109.4V, Imp=5.86A.
For both strings: Vmp=109.4V, Imp=11.72A.

I need to add up to about 640W of solar input to take advantage of the additional controller's capacity.

I cannot get these exact 320W panels I have previously used. But, I have some other panels with different specs.

I am looking for feedback from someone who has mixed and matched solar panels before, and the results they got.
Some questions I have regarding the string I plan to add in parallel to this 1280W system are (I can't expand in series because of the max array V limit of the controller):
What happens if it has Voc much different from 129.6V?
What happens if it has a much different I-V curve shape from the panels I currently use (the optimal max-power point is much different from 109.4V)?
What sort of panel specs mismatch the system can tolerate and still produce more or less optimally, and what sort of mismatch it cannot tolerate?
Etc.
…
Would anyone care to elaborate on this, maybe using the numbers from the example I gave above?
I am looking for general guidelines for mixing and matching solar panels, what makes sense to do and what does not.

[DamonHD]

Hi

I suspect that as long as you build parallel strings whose constituent panels are well matched and of which the total Voc(max) and Vmp are tolerably close to the existing strings, you should be OK.

I have a weird mixture of 12V nominal panels on a 12V nominal MPPT and have been able to put on excess by being able to buy opportunistically which may compensate for theoretical performance loss.

That MP V/I curve looks to have a very sharp knee I know, but I suspect reality often gets in the way anyway with dirt and shade and so on...

Rgds

Damon

[XeonPony]

As stated #1 goal is to match the voc and Imp as close as possible, even if the pannels are a lower wattage since it is in perallel it add watts as volts remain constent!

you can not series a lower wattage to a higher wattage pannel how ever, but since this is not the goal! Things will tend to average out in the middle so again keeping the Vmp as closely matched to your array is the most critical!

[jack11]

thank you all,

after thinking some more, I've also come to believe that matching the Vmp of the string I'd add in parallel is key to success.

For example, to show the results of a gross mismatch in Vmp, from the spare panels I have I could make up a string whose Vmp and Voc are quite a bit less than 109.4V and 129.6V of the current system.

I believe the controller could not even "see" this added string, because it would be masked by the Voc = 129.6 of the strings I already have (I am not sure what would happen if I connected in parallel stings with Voc of 129.6V and 74.2V. Since there is no current flow (blocking diodes) the 129.6V strings should probably "win")

So, the controller sees the two existing strings only in its MPPT sweep, and the added string contributes nothing because the  Vmp = 109.4V of my current system is way beyond the Voc = 74.2V of the added string (its power drops to zero after 74.2V).

Can anyone give a specific MPPT system example where they more or less matched the strings' Vmp, made up of random panels, and the system did gain additional power, and how much gain?

[DamonHD]

Hi,

1) Make sure that the new string's Voc cannot exceed the inverter's Vmax even in the very coldest conditions.

2) A *good* MPPT inverter will do a big sweep across the voltage range and discover all the extra power available from a new string even if a *bit* (maybe 10%--20% at most I suggest) below the existing ones, and note that a lower Vmp string will still give (slightly sub-optimal) power even if at a slightly higher voltage: it's not on or off.

Rgds

Damon

[Flux]

I agree with Damon and Xeon, use the nearest you can get and if anything err on the higher voltage side as long as it NEVER exceeds the inverter maximum input volts.

The case is simpler to understand with direct battery charging, if voc is below battery volts it will contribute nothing. If the voltage is higher than battery it will add something. When Vmp is equal to battery volts you get maximum. for higher voltages you get something approaching Isc.

With mppt things are not so simple as it will depend on the algorythm used to track the mppt and not all converters will behave the same so try to keep the error as small as possible. As long as Voc of the added panels are above the point where the mppt tracks you will add something. If you are marginally low it will force the mppt to track at slightly lower voltage but if too low it will either get ignored or pull the main bank way off track.|

Flux

[bob golding]

on the same subject but not directly related. i am using 3 200 watt 36 volt panels connected directly to the battery bank in parallel( 8 430 amp hour 6volt surrettes.) at the moment i have the batteries connected for 24 volts. i intend to up the panels to 6 of the same type in parallel with the 3 i already have. ok simple so far. i need another inverter and notice 12volt ones area lot cheaper than 24v. the original reason for having 24 volt was for the wind turbine, sadly still in pieces waiting for me to repair it and make some new blades. for now i need more power for a project i am working on that needs more  than the 300 watt inverter i have, which is fine for most things. if i reconfigure the battery bank for 12 volts how will this affect the charging? at the moment everything is fine . the panels  and the battery self regulate quite nicely will changing to 12volts alter this greatly?

[Flux]

Basically using the panels direct on 12v will keep a similar charging current, ( virtually Isc) hence you effectively halve the panel output. There will be a bit of an improvement in that Isc on 12v is higher than I mp on a near charged 24v battery on a hot day.

Going from 3 to 9 will give a 3 fold improvement at 24v but only half that charging 12v. No real way round it except a mppt controller .

Flux

[jack11]

So, this problem of adding a mismatched string may make the power curve's profile pretty non-linear (P = fct(V)).
For example, the linear region from 0 to almost Vmp may now have a plateau in it, or a second local maximum, coming from the power contributed by the mismatched string.
How is this going to affect the controller's MPPT function in finding the global maximum?
It sure would be nice to have a MPPT controller reporting to the PC the shape of the power curve after each sweep, and which max power point it found. Mine does not, does anyone know of any controller that does?

Also, a good explanation of what happens when you connect mismatched PV cells in parallel can be found at:
http://www.pveducation.org/pvcdrom/modules/mismatch-for-cells-connected-in-parallel

And the same for the mismatched strings, also considering the effects of the bypass diodes, is here:
http://www.pveducation.org/pvcdrom/modules/mismatch-effects-in-arrays

They give some answers to what happens to Voc and Vmp when there are mismatches, and show other problems that may develop.

[jack11]

On bob's topic, I have a similar simple system to his that we take camping with us, 2x230W panels with Vmp of 30V connected in parallel, and directly connected to 2 deep cycle batteries wired for 24V.
This results in a pretty good match where the batteries charge up to about 27.5V.

Based on what I've seen, I'd be worried about connecting Vmp = 36V panels to the 24V batteries though, because of possible overcharge.
I'd be even more worried about connecting these 36V panels to 12V batteries, even greater possibility of overcharge, the batteries may be in perpetual equalization now and all the electrolite may boil off.

[bob golding]

i am not worried about overcharging as i have a morningstar ts45 charge controller. for the cost of a mppt controller i can get another 6 panels which is why i prefer to go down that route.

[Flux]

Yes for a small set up it is more cost effective to add panels rather than get a mppt controller as long as the match is good. Changing to 12v in this case will not work well.

Unless you are looking to get a professional inverter to replace a cheapie one I haven't found much difference in price between 12 & 24v, it just narrows the choice somewhat.

Flux

[bob golding]

Yes for a small set up it is more cost effective to add panels rather than get a mppt controller as long as the match is good. Changing to 12v in this case will not work well.

Unless you are looking to get a professional inverter to replace a cheapie one I haven't found much difference in price between 12 & 24v, it just narrows the choice somewhat.

Flux

after a bit more digging i have found a couple of 24volt ones at reasonable price. given the potential problems with going to 12 volt i will leave it as it is. also i intend to repair the turbine for the winter so changing everything back to 24v again makes it more trouble than it is worth.

[eco007]

Yes for a small set up it is more cost effective to add panels rather than get a mppt controller as long as the match is good. Changing to 12v in this case will not work well.

Unless you are looking to get a professional inverter to replace a cheapie one I haven't found much difference in price between 12 & 24v, it just narrows the choice somewhat.

Flux

after a bit more digging i have found a couple of 24volt ones at reasonable price. given the potential problems with going to 12 volt i will leave it as it is. also i intend to repair the turbine for the winter so changing everything back to 24v again makes it more trouble than it is worth.

I think you need to check the voltages of each panel individually with a dvm, a good meter that will able to test the short circuit current of each pv up to about 10 amps.