Series vs Parallel - 2 x 50 Watt Panels on RV, Shading...?

jdzljdzl Registered Users Posts: 11
So, I know enough to be dangerous, but trying to wrap my head around shading and the effects on series vs parallel with two modules is really throwing me for a loop. On a bigger system I wouldn't worry about it so much - cut a tree, add another panel, whatever...

On this particular install, there was only enough room on the roof for 2 x 50 Watt Renogy panels, one is mounted above the windshield at about 35 degrees or so, the other is mounted about 3 feet behind at 0 degrees (parallel to the earth). Due to this layout, it's quite likely that one of the panels winds up partially or completely shaded.

Initially I was planning to wire in series and use an MPPT controller (The KID) to try and make the most of the [little] power I have. After doing some reading, it sounds like the current would be limited to the lowest panel in the series? So then I was thinking of going parallel with a more traditional PWM controller (SunSaver 10)? Holding that thought, I just did some tests up on the roof of the RV...

0 degree panel, sun at a sharp angle in relation to it, not shaded. 20v open circuit, 0.7a short circuit
35 degree panel, totally shaded. 19v open circuit, 0.2a short circuit

Now based on what I'd read, I was expecting to connect them in series and have them put out 0.2a at 40v or so, but I ended up getting 0.7a at 40v?

So based on that, it seems like:

Series = 0.7a @ 40v = 28w
Parallel = 0.9a @ 20v = 18w

Is this because I'm dealing with short circuit current with no load? Very confused...

Edit: Think I'm starting to get it... At short circuit, voltage is 0, so above calculations are totally incorrect... I'd need a resistor to get "real world" values. That being said, does anyone have feedback on parallel vs series in a situation like this?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Series vs Parallel - 2 x 50 Watt Panels on RV, Shading...?

    The only Voltage you can read on a PV without a load is open circuit, and it doesn't mean much. With a load on it will drop to Vmp or even less. Thus your Voltages of 19 & 20 or 40 combined aren't what you will get with maximum current output.

    The short circuit current (Isc) is what you want to go by. The real problem is that with panels in two different locations/shading conditions the insolation will not be the same on both at any given time. As a result you will never see 2XVmp * Imp with a series connection.

    With the current you report you're clearly not getting anything close to full, direct sun as a 50 Watt panel should have specs around 17.5 Vmp and 3 Imp or so.

    What happens with shading is not current limiting, but current drop as the current from the well-lit panel passes partially or completely through the bypass diodes of the other panel. Depending on the particular panel and the conditions of the moment this can end up being Imp @ Vmp -Vdrop for the bypass diode (usually very low).

    So we have the standard condition of using the panels in parallel, where the Voltage of either will equalize and they will both put out whatever current they can from the sun available.

    If you do not have long wire runs to overcome the higher array Voltage affords no real advantage. Nor would the MPPT controller (about 10% at best under normal conditions) providing the panels aren't unusually high Vmp.
  • jdzljdzl Registered Users Posts: 11
    Re: Series vs Parallel - 2 x 50 Watt Panels on RV, Shading...?

    Makes sense, thank you very much!
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