Why use Solar Panel Strings - is more better

[SolarHead]

What are the reasons for using multiple strings in a solar panel array? I guess the obvious would be;

1. Shading. More strings means, if shading occurs then hopefully its contained to a string and the rest of the array can perform.
2. If a solar panel failed or was in the process of failing then hopefully the power drop would be contained within the one string, and not affect the entire array.
more?

I guess too what I am asking, is the idea to have as many strings as possible, but staying within voltage minimums and maximums required by the inverter? I have 4 strings of 9. For example, would I have been better off to have done 6 strings of 6 ? I don't plan to re-wire my array, just wondering what the thought process and/or design thoughts are when considering number of strings in an array.

 

[sunshine_eggo]

It's less a question of better or worse and more of a question of MPPT limitations and wiring losses.

MPPT with very high series voltage often have a notably lower PV input current limit. You essentially are forced to series as many as you can and are restricted to the number of strings you have in parallel by the PV input current.

MPPT with the typical 150-250Voc limitations usually have PV input current limits similar to their charge current, so you are again restricted by the MPPT... only 2-4 panels in series strings and possibly several strings in parallel.

Long PV wire runs benefit from higher Vmp, so that's another consideration.

1 & 2) Vmp is the kicker in both cases:

String 1: 6S, 1 panel fully shaded
String 2: 6S, no shading

1) In string 1, the shaded panel can't contribute to either voltage or current, so the 6S array will perform optimally with 5S numbers. Unfortunately, the parallel string with no shading performs optimally at 6S voltage. What ends up happening is the array will perform at some voltage between 5S and 6S (closer to 5S) and the whole array will be compromised by more than just the one shaded panel.

2) If the fully shaded panel failed and went short circuit, it would behave as shaded. If it failed open circuit, that string would essentially no longer be present, and the second string would operate as a solo 6S string without any influence from the inop string.

 

[DerpsyDoodler]

It's less a question of better or worse and more of a question of MPPT limitations and wiring losses.

MPPT with very high series voltage often have a notably lower PV input current limit. You essentially are forced to series as many as you can and are restricted to the number of strings you have in parallel by the PV input current.

This is kind of moot, as if you understand how to build the system you can take this into account during your design phase and choose accordingly.

The bottom line is multiple strings with multiple controllers give redundancy and increase resilliency. Be it because of partial shading, panel failure, controller failure, or whatever the failure may be, and for aiming your strings to capture peak power at different times of the day, offering more consistent production throughout the day.

Anything else is overthinking the response to this question.

 

[rogher]

I think that series connection will perform better under poor conditions, that is when the sun is low and not due South, because you will have a higher voltage for the controller to work with. I believe the inverter will just shut down at low voltages, so higher voltages keep them harvesting for longer. The controller will, of course, need to be able to cope with the increased voltage, though.
Rather than put strings in parallel, I'd put them through separate controllers for greater resilience and redundancy.
Life can get complex if the nature of the strings in parallel is different, but separate (cheaper) controllers can 'iron-out' differences in panel output due to technology, shading, or orientation.

 

[SolarHead]

I think what I'm absorbing from all of these contributions is, its not super duper detailed, you just more-or-less balance the needs of the charger(s) with their required voltage (min/max), and keep a reasonable amount of strings for redundancy and shading. No need to have 20 strings with 20 panels that would be wasteful and wouldn't work because not enough voltage on each string. Most inverters seem to require about 5 or 6, maybe 7 panels in series just to get up to the required minimum voltage to operate. So, most common you see is 4 strings, 6 strings, 8 strings. and then if you play with mixing series and parallel is where the flexibility comes into play with what you can do with the requirement of voltage and amps of the inverter. The combination of each can usually land you right about where you need to be.

I have 4 strings of 9 (total 36). The strings are all in series of 9 panels. I have 4 strings total, with 2 each going to a combiner box (I have 2 combiner boxes). I was just wondering if there was something else I had not thought about, or had the knowledge of if it made better sense to have say 6 strings of 6 (total 36) but doesn't seem to be much benefit over the 4 series of 9. It would give me maybe more redundancy but its going to reduce voltage since its more strings (in series of course). I'm right about where I want to be in regards to amps and volts of my array and again I just wondered if there was any benefit I was leaving on the table so to speak by having them wired the way I have them now.

When I got started with solar and was a complete newbie, I was thinking okay I need to wire these to I get the most power, but from what I can tell, series or parallel or any combination there of is going to result in the same wattage production. The wiring configuration really just helps you get the redundancy (points of failure are spread out, and shading) and volt/amp levels to fall within the chargers requirements on min/max. I guess those are really then the only reasons that dictate the number of strings one would use.

 

[DerpsyDoodler]

Most inverters seem to require about 5 or 6, maybe 7 panels in series just to get up to the required minimum voltage to operate.

Avoid making generalizations like this. First step in the process is to determine your power needs with a power audit. I have a link to this forum that assists with that in my signature.

Based on those needs and the amount of time you would like to have power in a "grid down" (or "low solar production") situation, you will determine the size of battery bank you need (this assumes you're not building a grid-tie battery-less system).

Once you know what your consumption is, you'll know what you need to produce to both carry loads and charge batteries.

At that point, based on how much power you need and the solar irradiation of your location, you will determine how many watts of panels you need. With this information you can determine how to arrange them for best production. Based on that, shop for controllers and/or inverters and/or AIOs that meet your prescribed needs.