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Got panels...now what?

pda1

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Feb 16, 2021
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I just got 10- 260w pv panels and need some further guidance in setting everything up. They're 24v panels (34v before any load) Amps are about 6 per panel.

The panels were tested today, beautiful cloudless day, and total output of all panels summed up= 2.3 kW (again, this is the sum of all the outputs I measured of individual panels.

Main interest is in running some small appliances for a short term (such as desktop computers and maybe a small freezer/refrigerator) and a 12v submersible water pump in the well (Nemo pump). In a off grid situation, which these panels are for, the assumption is there will not be much power consumption...maybe 100 watts per hour at most.

So here are a few of my questions;

1- What roof mounting system do you recommend? Our roof's pitch is 12/12 (45°) facing South. Also have a portion of the roof facing Southwest with the same pitch. The shingles are fiberglass/asphalt- roof is new.

2- Should I split the system and put 5 panels on the South facing roof and 5 on the SouthWest facing roof?

3- Is mounting the panels on the ground preferable? (yes, I understand they have to be tilted.....and "it all depends".)

Now for the things I don't exactly know about....

4- On any array, should the panel outputs be fed into a "combiner box"...and then one feed to the MPPT?

5- Batteries. LifePo batteries are expensive but might well make up the cost in having to replace flooded lead acid. Any ideas?

These are just some initial questions which will help me consider the various possibilities for panel arrangement and power usage.

Do you have any suggestions or ideas?

Thanks,

Peter
 
2- Should I split the system and put 5 panels on the South facing roof and 5 on the SouthWest facing roof?

I'd split them and use 2 separate charge controllers for each group to get a better efficiency.

3- Is mounting the panels on the ground preferable? (yes, I understand they have to be tilted.....and "it all depends".)

If you have a space - definitely yes. You can set them to best angle/orientation. And will be much easy to maintain/clean them. Even a small amount of dust can reduce efficiency significantly.

5- Batteries. LifePo batteries are expensive but might well make up the cost in having to replace flooded lead acid. Any ideas?

First you need to multiply acid battery Ah by 2 in comparison with LiFePo4. Second LiFePO4 battery will last much longer than lead acid. Plus flooded battery requires a constant maintenance. I'd use cheap but proven LiFepo4 batteries like SOK, AmpereTime, Chins etc.
 
Is mounting the panels on the ground preferable? (yes, I understand they have to be tilted.....and "it all depends".)
If you live in snow country it is much easier clearing snow off from the ground then a roof.. but your 12/12 roof will make it easier for it to slide off. It also means you aren't messing with your roof and putting holes, cabling, etc on it. There is also extra precautions/code that should be followed when mounting on a roof that don't apply if mounted on ground. I don't have specifics since mine are all ground mounted but remember seeing references to this.

So if you have the room and access to sun on ground I would prefer on the ground.. it does mean however you need to pay for some form of mounting structure to replace what simply would be your roof.
 
All good ideas.

I'm learning slowly about pv solar and do have quite a bit of experience in residential electrical work and electronics.

Ok, a ground mount would be better and easier.

While understanding that there are so many variables and the beginning intention was really just to have a solar well pump....since the panels have arrived the size of the project has increased.

Here are some questions;

1- The panels I bought from SanTanSolar have a Voc (I think that's what it's called) of about 34vdc . For the moment I'd like to use 12v batteries for storage and it seems I'll get LiFePo. Is 34v sufficient to charge a LiFePo 12v?

2- I'm concerned about the wire size and cost for running wires from the Combiner box to the MPPT controller which will probably be 100 feet away. I'm certain it all depends upon how the strings are wired. Do you have any recommendations for wiring the panels together so I don't have to spend plenty of money on heavy gauge wire?

3- MPPT controllers- use more than 1 if the panel strings vary in wiring- series. parallel or series/parallel.

4- Pure sine wave inverters- why do even the big ones only have 1 or 2 120vac receptacles?

Lastly, I must sign-off as a storm is coming....

Thank you,

Peter
 
I'd split the array across two angles and connect them in parallel on one MPPT charge controller. Some loss in efficiency, but better utilization of the charge controller (more hours production.)

Certainly 34V is more than enough for 12V LiFePO4. But using MPPT, you would connect multiple panels in series so it wouldn't be 34V.
5 x 34Voc = 170V under nominal conditions.
170V x 1.16 = 197Voc in very cold weather (1.16x is a conservative adjustment; you can use the panel's temperature coefficient and your location's record cold temperature for a better number.)
That requires a 200V or higher controller, for this 5s2p array.

Alternatively, 2s5p. Put 6 on one roof face and 4 on the other. 2 x 34V x 1.16 = 79Voc, so a 100V controller is sufficient.

A hybrid inverter would let you program desired battery current, which could be less than maximum PV output. The extra available power would be used only when inverter was also drawing current.

12V is not the way to go except for very small systems. Consider 24V or 48V.

Inverters that have receptacles aren't big. 12V inverters aren't big. Big inverters are 48V and hardwired to breaker panels.
 
1- The panels I bought from SanTanSolar have a Voc (I think that's what it's called) of about 34vdc . For the moment I'd like to use 12v batteries for storage and it seems I'll get LiFePo. Is 34v sufficient to charge a LiFePo 12v?
You'll need a charge controller. You feed the solar panel into the charge controller and the charge controller outputs to the battery.

Do you have any recommendations for wiring the panels together so I don't have to spend plenty of money on heavy gauge wire?
Wire gauge is determined by the current you expect it to carry. So to keep wiring costs down you need to keep the current down.

PV panels in series keeps the current down while increasing voltage. Panels in parallel keeps the voltage down while increasing the current.

IOW to keep wiring costs down you'll want your panels in series. But to do that requires a charge controller which can handle a higher voltage input.

The higher the voltage the charge controller MPPT input can accept, the more panels in series you can connect.
 
Concerning panels in series and MPPT controllers;

If 2 panels are put in series, let's say 72v and 6a (432w) output once connected in series, and then connected to the MPPT controller which is then used to charge a 12v battery, will the MPPT convert the voltage down to a usable charging level and increase the current?

In other words the MPPT will still provide 432w available to charge the battery or fed into an inverter but the voltage will drop and the current increase? (system losses are excluded here).

Thanks,

Peter
 
MPPT:
Yes, that's exactly what they do, deliver same watts at a lower voltage like a transformer does for AC. They also dynamically adjust the voltage ratio to maximize output.

Some have documentation showing conversion efficiency vs. voltage, which usually drops a bit for higher voltage input, but at higher voltage lower current the losses in wire decrease.

If one panel out of several in series is shaded, or a diode-bypassed portion of a panel is shaded, the MPPT controller will adjust to whatever voltage/current delivers the most power. So long as voltage remains high enough, typically Vbattery + 5V.
 
Ah....most informative! I was hoping that would be the situation.

Thank you,

Peter
 
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