Max distance between panels on 1 string ?

Zivva

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Hi,

Haven't found the information anywhere, then I thought I would ask ...

Got some remaining space on a roof for 3 panels. Those 3 panels would have a total VOC < MPPT startup voltage, then I can not connect them directly to my inverter.

1. I could build a series string with them with other exact same panels but there is a distance of 10ft / 3 meters between the 2 sets of panels. Is that distance reasonable enough or would create too much of a voltage drop between panels of a same string ?

2. All panels are fitted with 12 AWG / 4mm2 cable from their junction box, could fit a thicker cable between the 2 sets of panels to limit the too much voltage drop if any ?

Thanks for your advices
 

brum

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Voltage drop depends on the cable. Calculate the drop and divide it on the MPP voltage your panels have. This will give you the power loss. With it you can decide on your own if this is acceptable. 3 meters should not be an issue.

On 4mm2 wire and 10A current you'll get less than 0.1V drop in one direction, so if the total PV array voltage is 100V this is 0.2% losses. Not something I would bother about.

Having the same panels and having them facing in the same direction is a lot more important. Consider if there are objects that can throw partial shade on some of your panels. If yes - you may need power optimizers to get the maximum output.
 

Zivva

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Voltage drop depends on the cable. Calculate the drop and divide it on the MPP voltage your panels have. This will give you the power loss. With it you can decide on your own if this is acceptable. 3 meters should not be an issue.

On 4mm2 wire and 10A current you'll get less than 0.1V drop in one direction, so if the total PV array voltage is 100V this is 0.2% losses. Not something I would bother about.

Having the same panels and having them facing in the same direction is a lot more important. Consider if there are objects that can throw partial shade on some of your panels. If yes - you may need power optimizers to get the maximum output.
Thank you so much.

Each panel has 45v of VOC. Looks like having a string of 6 (3 next to each others on the roof + 3 others 3 meters away) with no shade for all of them shouldn't be an issue then.
 

brum

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What is the MPPT range and the max PV voltage of the inverter?
 

Hedges

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Extra voltage drop in longer PV wire between panels in one string vs. another parallel string is negligible compared to power/voltage curve of PV panels. With two parallel strings at same voltage, the difference in voltage across PV panels of one string vs. the other will let both be so close to maximum power it makes no difference.

But your situation is just a single string? Extra length of wire, whether between panels or for the home run, is just power loss of IR drop. Unless it is so excessively long that Vmp gets dropped below minimum of MPPT.

All panels of a given string should be identical orientation, or at least very close to same.
Two parallel strings can be of different orientations. A field study showed 2% reduction in power vs. having them on separate MPPT.
Even when sun fall such that one string has sun and the other is in the shade, no noticeable loss.

With a single string, shade on a panel or part of a panel should only cause loss of the power from the shaded diode-bypassed sections.
With two strings in parallel, shading a small percentage of panels in a string (I tested 9s2p with one panel shaded) only causes a loss in power approximately equal to the shaded panel(s). It does not cause loss of power of that entire string, nor does it reduce power from unshaded string by a similar number of panels. The power/voltage curve just changes a bit, and the two strings are close enough to their peak.

With lots of shading (e.g. 25% to 50% of one string), the power/voltage curve has two local maxima. If the lower voltage local maxima is higher power, only some MPPT will find it; others sit at the lower maxima. Either way, significant power is lost compared to separate MPPT.
Note that "half cut" panels are internally wired something like 2p3s, and partial shade on just one panel can cause loss of 50% of the power of a single PV string, unless MPPT is smart enough to find the much higher local maxima at lower voltage.

You can even mix-n-match panels. Generally on separate parallel strings. Should have similar Vmp for the string. I've used 24x "12V", 12x "24V", 8x "36V" all in parallel. If you needed to add a panel to an existing string, best to get one of equal to higher current so it doesn't limit production of the rest. And equal to higher voltage if another string in parallel.
 

brum

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Startup : 150v
MPPT range : 125 - 425v
PV input voltage : 370v (100v - 500v)

Go for single string and everything will be OK. As said if they are facing the same direction then you are good to go with single string.
 

Hedges

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Each panel has 45v of VOC.

Definitely single string of 6 panels.
3 in series could drop below minimum MPPT voltage on a hot day. Vmp under STC is probably around 35V ~ 40V.
 

Zivva

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Definitely single string of 6 panels.
3 in series could drop below minimum MPPT voltage on a hot day. Vmp under STC is probably around 35V ~ 40V.
Yah it's kind of hot everyday here (Indonesia ...)
 

Hedges

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So every day is an "85/85 test"?
Hope your electronics has sealed enclosure or at least conformal coat.

Batteries don't like to be hot. Possibly shade and earth-sheltered to moderate temperature. Some people keep everything inside an air conditioned room (which should also serve to dehumidify.)


All panels are fitted with 12 AWG / 4mm2 cable from their junction box, could fit a thicker cable between the 2 sets of panels to limit the too much voltage drop if any ?

Rarely does wire gauge matter for PV. If you had 6 panels in parallel would need to be large enough for ampacity, and if long then larger for resistance.
I use 12 awg for each string, about 150' away. The strings are about 380Vmp, 6A.

The usual guidelines for <3% voltage drop aren't particularly relevant for PV, because MPPT can accept a wide range of voltage and harvest power. It matters for drop from battery to inverter, from SCC to battery, and for AC.
Even if a PV string lost 25% or 40% of power at peak current, that is just power loss, doesn't otherwise negatively impact performance so long as voltage remains above minimum for MPPT. Giving the price of copper vs. silicon, I would use skinny wire and add more panels to make up for the loss, within voltage limits.
 

Zivva

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Closer to 90° than 85° and humidity ranges from 60 to 80% ...

Batteries & inverters are in an enclosed room with 24/7 AC. Humidity, salty air & dust are more a concern than just temperature.
 
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