Evaluate this solar setup - Recommendations and Criticisms

EurovanCamper

Many thanks to Bill @BB. and @Cariboocoot for their assistance 10 years ago with my first solar installation. It has been working wonderfully and I couldn’t have done it without your help.

 

Please criticize this new project I’m working on. 

(I bet you couldn’t make a better solar system within these constraints.) /s

 

I am adding a second solar installation to a sailing catamaran and would like your input.

The existing solar array was installed by the previous owner. It is helpful but we only get about 3-4 days on our 3 x12v 200Ah = 600 Ah lithium LiFePO4 battery bank (Renogy) before we hit 20% SOC and must run the generator to charge back up. This costs diesel and limits what we can do.

I have tracked our energy use between generator charging and during the month of February we had an average draw of 5.1 amps at 13.2 V for about 1.6kWh of use per day. I don’t have data for earlier in the winter so I'm not sure how large the deficit is then. Sizing for a slightly larger deficit and future loads I’d like to generate about 2.5kWh per day.

I calculated my solar array would need to be:

2.5kWh / 0.77 charging inefficiency / 2.84 hrs of insolation (November in Key West, FL) = 1,143 W of solar

 

My mounting location will be aft of the hardtop, horizontally (0 degree inclination), in a new rack not larger than 144” x 78”. This will be ~7 ft above the white fiberglass aft deck providing great reflection for a bifacial panel.

I researched and found availability of 3 x 400 W Q.Cells Q.Peak Duo BLK ML-G10+ Bifacial panels for a 1200W array

Voc= 45.3V      Isc= 11.14A     Vmp=37.13V     Impp= 10.77A     V Temp Coeff = -0.27 %/C     I temp= 0.04 %/C     3 bypass diodes 6x 22 half cut cells     74” x 41.1”    so 3 side by side in portrait makes 74” x 124”.

These would be connected to a Victron SmartSolar 150Vmax / 100A max MPPT charge controller. The documentation states this controller can accept up to 1450W of solar. My calculations show this may be a little over-paneled for the controller if Q.Cells estimate of up to 30% more production for the bifacial is accurate. I would route each panel to its own charge controller, but the pathway to run the cables from the solar panel is not large enough to accommodate 6 wires necessary for this, and the space I have to install the charge controller is not large enough for 3 controllers.

Voc max of the array if connected in series is 146.9 @ -5C. This shouldn’t be an issue unless the bifacial panels can produce higher voltage than Voc + Vtemp Coefficient with sun reflected on the bottom. Anyone know how the Voc is impacted on a bifacial panel? I am also not frequently in cold weather and don’t plan to be anywhere below 40F, but it could happen.

Shading due to the sailboat mast and boom is unavoidable which is why I think hooking the panels up in a series string makes more sense than connecting them in parallel, (based on reading a number of your posts in this forum and elsewhere). If the panels were installed in parallel and one cell is shaded or one bypass diode is activated, then the whole panel no longer produces, or brings the whole Vmp of the array down to that 2/3 producing panel? One diode activated would bring the max output to 1066 W (series with one section of a panel not producing)  instead of 800 W (Parallel with a whole panel not producing). I think this is where I get the most confused. Series or Parallel?

Balance of system:

10AWG wire from panels to a 20A 150 V DC breaker (MNEPV20-150-1PNP). Might be 30ft each (+&-)

10 AWG wire from breaker to Charge Controller

4 AWG wire from charge controller to 125A 80VDC breaker (MNEPV125-80-1PNP) then 4AWG to the battery bus bars which have a shunt monitor and then 2/0 wire going to each of the batteries.

Breakers in a Midnight Solar Baby Box and mounted next to the charge controller.

 I have one additional 12V 200Ah Lithium Renogy battery I may add to the bank to increase from 2.5 days of reserve to 3.4 days of reserve power.

Let me know what you think.

littleharbor2

Those large panels have plenty high enough voltage to not have to series wire for MPPT . Series strings are most affected by shadowing, not parallel. For 3 or more parallel panels you need to fuse each panel.

EurovanCamper

So current drops faster than voltage in a shading situation? 
I thought the bypass diode would be activated and the voltage of the whole series array would drop but the current would be unaffected, so you would have 2 panels + 2/3 of one panel producing at full power. If they are in parallel, the lower voltage of the panel with the activated bypass diode brings the voltage of the whole array down to the 2/3 producing panel, and the total output is lower.
Or does the voltage of a partially shaded panel remain largely the same and the current is impacted?

littleharbor2

EurovanCamper said:

So current drops faster than voltage in a shading situation? 
I thought the bypass diode would be activated and the voltage of the whole series array would drop but the current would be unaffected, so you would have 2 panels + 2/3 of one panel producing at full power. If they are in parallel, the lower voltage of the panel with the activated bypass diode brings the voltage of the whole array down to the 2/3 producing panel, and the total output is lower.
Or does the voltage of a partially shaded panel remain largely the same and the current is impacted?

That may be true if just the 1/3rd of the panel is shaded that each diode controls. What if that shadow crosses the entire panel? The whole string is affected.
 The diodes still work in parallel. If your panels were 12 volt panels then I would recommend series wiring. For your 12 volt system, the higher voltage panels you have will do just fine with your MPPT controller.

Dave Angelini

Good advice here so far! Parallels!   Typical to over panel 30% or more if if it is rare like your sail app.   A fast mppt will help with shade tolerance. Not sure about victron but probably good. Solar MP4 connectors offshore are a maintenance issue and many just but splice crimp and heat shrink. Voc is not a big issue on bifacial. Low temp is and you are not anywhere near it. Don't go to Alaska !   Cheers!