A system for the space/shading constraints of a boat - Feedback requested!

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KodiakMike
KodiakMike Registered Users Posts: 1
in Solar Beginners Corner #1
Hello - I live with my family on a 45' sailboat, currently in New Zealand, and am adding PV to our energy mix. I've gotten to the point where I could use some feedback from people who know something about PV - many thanks in advance!

Current system: 4 x 6V 335Ah AGMs in series/parallel, charged by 2 wind generators (one good, one not so good), a Westerbeke diesel genset, and 100 A alternator on the main engine.

I have two smaller spaces (each ~36"x22") and one larger space (~60"x28") available for panels. I'm limited to panels easily available in New Zealand, and had come up with two CNPV 50W panels (Vmp = 19.0) for the small spaces and one Suntech 140W panel (Vmp = 18.6) for the large space, wired in parallel through a Rogue MPPT 2024. Talked to the Rogue guy, and he said that keeping Vmp of different panels "within a volt or 2" would be adequate for the MPPT.

BUT, shade will be a constant problem, and I expect that all 3 panels will be shade-free none of the time, practically speaking. So, 2, questions for dealing with shade:

1) Would I be better replacing the 140W with 2 65W Suntechs? My thinking here is that in situations where the 140W panel would be shaded, one of the smaller panels might be shade-free, so that 2 small panels would be better than one large. Big downside, though, is price - $260NZD for the 140W, $485NZD for 2 x 65W, plus additional cost for fabricating brackets for the 2 panels.

2) Would I be better using separate Genasun MPPT controllers for the 2 x 50W string and the 140W panel? Or even separate MPPTs for each panel? Downside here is more expense, more complexity (always an evil to be avoided on a boat!). So would want a concrete advantage if I were going to go with multiple controllers.

So, again, thanks for any help. I've read through a few threads here and it seems like there's a lot of experience in this group!
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  • SolInvictus
    SolInvictus Solar Expert Posts: 138 ✭
    #2
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    Re: A system for the space/shading constraints of a boat - Feedback requested!

    You have a 12 V battery array and PV panels whose voltage is appropriate for 12 V systems. Thus an MPPT charge controller will not help much without connecting the PV panels in series which is a bad idea because you have a shading issue. Your most cost effective choice would be a 20 A PWM charge controller. The only advantage I see in having two separate charge controllers is redundancy in case one breaks while you are out to sea.

    With two 50 W and one 140 W PV panels in parallel, you probably need fuses on the positive leads of the two 50 W panels. If you connect two 50 W and two 65 W PV panels in parallel, then you probably need fuses on all 4 of them.

    It is hard to say whether you should use a single 140 W panel or two 65 W ones because it depends on the shade. A less expensive PWM charge controller might make the higher price of two 65 W panels more palatable.

    The CNPV 50 W panel is mono-crystalline, but the specifications I found for the Suntech STP140-12/Tb are contradictory. The specification indicates mono-crystalline, but the title and photo indicate poly-crystalline. If one is mono and the other is poly, then you should check how the voltages change as a function of temperature and age because they will not be identical. For the Suntech:

    Open Circuit Voltage (Voc): 22.4V
    Optimum Operating Voltage (Vmp): 17.6V
    Temperature Coefficiency of Pmax: -(0.48 ± 0.05)%/K
    Temperature Coefficiency of Voc: -0.075 %/K

    but I can not find the data for the CNPV 50 W.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: A system for the space/shading constraints of a boat - Feedback requested!
    KodiakMike wrote: »
    and am adding PV to our energy mix.

    Welcome to the forum,

    fortunately you have other sources to charge your batteries so the solar is not essential and anything you get from it is a bonus.

    One thing you might consider (if you have an appropriate space) is putting panels vertical. Some of us folks in snow country put our panels vertical to shed snow and find that we get fantastic power because the sun reflects off the snow on the ground.

    I don't know how well that work for you on the water... it may depend on the sun angle. Might be worth testing with a small panel.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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