Panels on 2 Roof Planes on a single string?

vidhanvidhan Registered Users Posts: 14
I live in Hawaii, latitude 20 degrees and am doing a grid-tied system. I have 12 Evergreen ES-190 panels which I will roof mount. The roof pitch is 14 degrees. I can fit 10 panels on the south facing hip of the roof. I am thinking to mount the remaining 2 panels on the east facing hip and connect them all in a single string.
Is this a bad idea?

At this latitude there is not an appreciable difference in insolation from one roof to the other on the average, but I am concerned about whether MPP tracking will be thrown off by doing this. Does anyone know how much performance will be degraded if I hook up the system this way?

My alternative is to not use the extra two panels. Would I generate more power from the 12 than the ten?

Aloha,
V

Comments

  • FL SUNFL SUN Solar Expert Posts: 94 ✭✭✭✭
    Re: Panels on 2 Roof Planes on a single string?
    vidhan wrote: »
    I live in Hawaii, latitude 20 degrees and am doing a grid-tied system. I have 12 Evergreen ES-190 panels which I will roof mount. The roof pitch is 14 degrees. I can fit 10 panels on the south facing hip of the roof. I am thinking to mount the remaining 2 panels on the east facing hip and connect them all in a single string.
    Is this a bad idea?

    At this latitude there is not an appreciable difference in insolation from one roof to the other on the average, but I am concerned about whether MPP tracking will be thrown off by doing this. Does anyone know how much performance will be degraded if I hook up the system this way?

    My alternative is to not use the extra two panels. Would I generate more power from the 12 than the ten?

    Aloha,
    V

    I am assuming your panels are ES-190-SL (or RL) and not ES-A-190 or ES-B-190's as the last two would need more than 12 in a series to generate the voltage needed for grid tie.

    Of course it's always best to mount an entire string on the same plane. Since your pitch is so shallow (3/12), and you are at a lower lattitude, you may be able to make the two on the other hip work out somewhat. If that is your only alternative for mounting, I would make sure they are mounted in a way to not get shading on the 2 rogues. I would still expect a penalty in voltage, though.

    I would not string any less than 12 of these 190's together as hot roof temps may bring your voltage to near cut-out threshold for your inverter. 10 panels would not work very well in my opinion.

    Which inverter are you using?
  • icarusicarus Solar Expert Posts: 5,090 ✭✭✭✭
    Re: Panels on 2 Roof Planes on a single string?

    Since this is posted in the off grid section, I would consider the panel configuration and operating voltage.

    You could also consider using PV watts to do a series of calcs with different configurations.

    More info please?

    Tony
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Panels on 2 Roof Planes on a single string?

    i will assume this being in offgrid to be a mistake on the op's part and as such i am moving it. as to pvs in a series string, keep them all aimed the same way somehow.
  • FL SUNFL SUN Solar Expert Posts: 94 ✭✭✭✭
    Re: Panels on 2 Roof Planes on a single string?
    niel wrote: »
    as to pvs in a series string, keep them all aimed the same way somehow.

    Since the panels in a system are such a major investment in cost, i too recommend mounting them by the book to maximize your return. Same plane mounting with no partial shading is important in maintaining high dc voltage. 12 of these panels is minimum at best for grid tie. 122 deg F ambient (roof very easily) 12 panels in series is 237 volts.

    Vidhan, have you explored all possibilities? Do you need a solar awning over a window, or can you do some sort of pole or ground mount for the other 2? If you have a 3 kw inverter, you could plan to add more 190's in the future up to a total of 15 or 16 in the string depending on your inverter.
  • vidhanvidhan Registered Users Posts: 14
    Re: Panels on 2 Roof Planes on a single string?

    Sorry, I meant to post this with grid-tied.

    My panels are Es-190SL and I was considering using a Fronius IG 2000 which has a low input voltage requirement.

    Ultimately, I would like to enlarge and reconfigure this whole system.

    Say, I was to use 20 ES-190SL Panels, putting a string of 10 on the south facing roof and a string of 10 on the east facing roof. Would I need a separate Fronius IG 2000 for each string or could I get by with a single Fronius IG 4000?
  • vidhanvidhan Registered Users Posts: 14
    Re: Panels on 2 Roof Planes on a single string?

    By the way, Fronius claims that the IG2000 operates within a DC Voltage Range of 150-450 volts. So I thought 10 panels would be no problem.

    Aloha,
    Vidhan
  • FL SUNFL SUN Solar Expert Posts: 94 ✭✭✭✭
    Re: Panels on 2 Roof Planes on a single string?
    vidhan wrote: »
    By the way, Fronius claims that the IG2000 operates within a DC Voltage Range of 150-450 volts. So I thought 10 panels would be no problem.

    Aloha,
    Vidhan

    Yes, 10 in a string is no problem with that Fronius model according to their sizing tool.
    http://www.fronius.com/cps/rde/xchg/SID-C6E1357A-2CCC2583/fronius_usa/hs.xsl/2714_8236.htm

    The 4000 inverter would handle 2 strings of 10 just fine. The other string could be mounted on the east roof if you have a good eastern exposure. Whether you use one 4000 or two 2000 inverters is up to your cost and space allowance.
  • vidhanvidhan Registered Users Posts: 14
    Re: Panels on 2 Roof Planes on a single string?

    Using two strings of ten panels each on two different roof pitches, would there or wouldn't there be a problem tracking the MPP?

    Also, from what I could gather from the pv watts calculator, the overall output from the east facing roof would only be 5% less than the output of the south facing roof, but that doesn't mean that at any instant the outputs would be within 5%, that's just a yearly average.

    I'm just trying to figure out if the differing string outputs would mean we should run two separate inverters.

    Aloha,
    Vidhan
  • FL SUNFL SUN Solar Expert Posts: 94 ✭✭✭✭
    Re: Panels on 2 Roof Planes on a single string?

    That is an interesting point you have. As long as your input dc voltage is within MPPT range, your penalty should be negligible. With each string maintained on its own plane, coupledwith your shallow roof pitch, and low latitude, you should be fine.

    Anyone else who can add some input on this subject please chime in.
  • vidhanvidhan Registered Users Posts: 14
    Re: Panels on 2 Roof Planes on a single string?

    Hey Florida Solar,

    Thanks for the help. As I looked into this I found on the Fronius website a white paper entitled FRONIUS IG – Reaction to Non-Optimal Conditions. It was a pretty technical look at the issues of shading, similar panels in different orientations, and dissimilar panels in an array. They actually quantified the losses that would be due to their inverter-and it comes to about 1%-negligible, as you said.

    So, we'll go with the IG4000 on this system, it looks like.

    Aloha,
    vidhan
  • mradtkemradtke Solar Expert Posts: 53 ✭✭✭✭
    Re: Panels on 2 Roof Planes on a single string?

    Vidhan,

    Please read my posts in this forum about Fronius MPP hunting.

    I chose the Fronius inverter because I too had low DC voltages and few inverters would work. Since the problem that plagued me was related to both high mains voltage as well as low panel voltage, you may not fall into the MPPT error, but if you choose a Fronius, be sure to watch for the problem.

    The Power One inverter that I replaced my Fronius with has 2 independent MPP tracking inputs that suite my dissimilarly oriented arrays just fine and might be a good choice for you (as well as dual inverters). As others on this forum might tell you, please take the Fronius white paper with a grain of salt. There are errors in it and as far as I know the tests haven't been replicated.

    Aloha,
    Mike
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