4 strings of 3 or 6 strings of 2

jcheil
jcheil Solar Expert Posts: 722 ✭✭✭
in Advanced Solar Electric Technical Forum #1
I have 12 of these configured into 4 strings of 3.
Those 4 strings go into a combiner/breaker box that feeds 125 feet of (2) #2 AL direct burial wires to my FM80.

After reading many designs on here would I be better going to 6 strings of 2?
I got the #2 AL wire for free and I know it is overkill. However, how much efficiency am I losing/gaining by changing to 3 panels vs 2 (in the strings)

Feel free to be VERY technical please :)

Attachment not found.
Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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Comments

  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #2
    Re: 4 strings of 3 or 6 strings of 2

    Assuming your modules are reasonably well matched, you are better off running as high a voltage as you can. The limiting factor is the module's max Voc and the max input voltage of the Charge controller which is 150V. Usually can't string more than 3 of typical modules, but yours have a pretty low Voc of 32V or so, so should be able to string 4 in series. You need to do the temp. adjust calcs though as Voc can be significantly higher at low temps. Probably not too bad in Florida though.
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  • SolInvictus
    SolInvictus Solar Expert Posts: 138 ✭
    #3
    Re: 4 strings of 3 or 6 strings of 2

    6 series strings means a lower voltage, higher current, greater power loss in the wiring and more fuses than 4 series strings.

    For the SV-T-205, 6 strings give 7.36 A * 6 = 44.2 A at maximum power point and for 4 strings, 7.36 A * 4 = 29.4 A.

    6 strings increases the power loss in the wiring by 44.22 / 29.42 = 2.26 times.

    The Vmpp of the series strings must be higher than the maximum voltage of your battery array. With two PV panels in series, the Vmpp = 27.9 V * 2 = 55.8 V which is okay for your 24 V battery array. The FM-80 is rated at 2,500 W for 24 VDC systems, but the NEC Recommended Solar Maximum Array STC Nameplate is:
    12 VDC systems 1000 Watts
    24 VDC Systems 2000 Watts
    48 VDC systems 4000 Watts
    60 VDC Systems 5000 Watts

    which means your battery array should be at least 36 VDC because your twelve 205 W PV panels are rated at 2,460 W. You are pushing your FM-80 to the maximum which may be causing you to lose some power on hot, sunny days. I wonder if the FM-80 is more efficient with a higher voltage and lower input current. It might be best if you reconfigure your battery array to 48 VDC but that voltage probably will not work with your Cotek SK 3000 PSW Inverter.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: 4 strings of 3 or 6 strings of 2

    i'd advise going with 3 in series and put in some heavier wire as your v drop losses are apt to be too large. run it through a v drop calculator that includes aluminum wires and use the vmp as the voltage x 3 for the string. 3% v drop may be livable with under 2% best. you might get away with 4 in series, but that depends on the max low temp for your area. if the temps go low enough on the pvs it could present a high enough voltage to damage the cc and is why i said 3 in series.

    if too much power is present for the cc the cc will freeze the output current at its max.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #5
    Re: 4 strings of 3 or 6 strings of 2
    jcheil wrote: »
    I have 12 of these configured into 4 strings of 3.
    Those 4 strings go into a combiner/breaker box that feeds 125 feet of (2) #2 AL direct burial wires to my FM80.

    After reading many designs on here would I be better going to 6 strings of 2?

    In order to answer your question you need the FM80 efficiency curves for a 24 volt system. I posted them here:
    http://forum.solar-electric.com/showthread.php?15907

    In that thread the issue was the same as yours... 2x3 or 3x2? I found for my cable and array that two in series was better than three in series. The efficiency gain in the controller made up for the higher power loss in the cable. That may or may not be the case for you. Is your FM80 running hot with the fan on a lot? If so, that might be a deciding factor in whether to change array configurations.

    To use those efficiency charts you will have to do some interpolation... if you need help, just ask. If you can figure it out, let us know how the analysis turned out.
    SolInvictus wrote: »
    You are pushing your FM-80 to the maximum which may be causing you to lose some power on hot, sunny days. I wonder if the FM-80 is more efficient with a higher voltage and lower input current. It might be best if you reconfigure your battery array to 48 VDC but that voltage probably will not work with your Cotek SK 3000 PSW Inverter.

    The FM80 is LESS efficient at higher input voltage (for a given battery voltage). SolInvictus is correct, you are at or near the limit of what your FM80 can do. The suggestion to go 48 volts is also good... you could reconfigure your batteries into two parallel strings instead of four (much better arrangement) and leave your panels configured as is. The FM80 would be much happier handling that power at higher (48 volt) output voltage. Unfortunately it will cost you a new inverter and new battery chargers.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #6
    Re: 4 strings of 3 or 6 strings of 2
    vtmaps wrote: »
    In order to answer your question you need the FM80 efficiency curves for a 24 volt system. I posted them here:
    http://forum.solar-electric.com/showthread.php?15907

    In that thread the issue was the same as yours... 2x3 or 3x2? I found for my cable and array that two in series was better than three in series. The efficiency gain in the controller made up for the higher power loss in the cable. That may or may not be the case for you. Is your FM80 running hot with the fan on a lot? If so, that might be a deciding factor in whether to change array configurations.

    To use those efficiency charts you will have to do some interpolation... if you need help, just ask. If you can figure it out, let us know how the analysis turned out.



    The FM80 is LESS efficient at higher input voltage (for a given battery voltage). SolInvictus is correct, you are at or near the limit of what your FM80 can do. The suggestion to go 48 volts is also good... you could reconfigure your batteries into two parallel strings instead of four (much better arrangement) and leave your panels configured as is. The FM80 would be much happier handling that power at higher (48 volt) output voltage. Unfortunately it will cost you a new inverter and new battery chargers.

    --vtMaps

    I would LOVE to go to 48v, but that would mean a new inverter and charger (anyone interested in buying a year old 24v inverter and charger?).

    I am not having any issues with it right now. Anf some summer days it does max out at 84amps. Generally it is in the mid to upper 70's all the time. I just read a lot on here that people have mentioned that it is better to be closer to the battery voltage. Right now it is set up as 3 in series. If I went to 2, that would put me closer to the battery voltage, but i guess I am wondering if it would really make any kind of a difference? And it doesn't seem to run too hot. The fan comes on frequently but not any more than I would expect.

    With the #2 AL at 125' I wouldn't think I would have any concern with v-drop even if they were in 2-series??
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #7
    Re: 4 strings of 3 or 6 strings of 2
    jcheil wrote: »
    With the #2 AL at 125' I wouldn't think I would have any concern with v-drop even if they were in 2-series??

    Your current configuration has a 2.25% voltage drop on the 125 ft #2 aluminum cable. With your panels 2 in series you will have a 5% voltage drop over that same cable.

    Your current configuration loses 55 watts in the cable at full (seldom achieved) power. Putting the panels 2 in series will result in a cable power loss of 125 watts in the cable at full power. NOTE: the power loss goes as the square of the power produced, so at half power your cable losses are 1/4 of the full power numbers.

    Using the Outback efficiency chart it looks like the 2-in-series configuration is about 1% more efficient than your current setup. At 2000 watts power handling, that is about 20 watts extra heat in the controller with your current setup.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #8
    Re: 4 strings of 3 or 6 strings of 2
    vtmaps wrote: »
    Using the Outback efficiency chart it looks like the 2-in-series configuration is about 1% more efficient than your current setup. At 2000 watts power handling, that is about 20 watts extra heat in the controller with your current setup.

    --vtMaps

    So, if I understand correctly, it sounds like it is not worth changing the current setup even at just the cost of 4 more MC4 cables and another 2 breakers?
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #9
    Re: 4 strings of 3 or 6 strings of 2
    jcheil wrote: »
    So, if I understand correctly, it sounds like it is not worth changing the current setup even at just the cost of 4 more MC4 cables and another 2 breakers?

    I wouldn't bother. Does your system (over all) meet your needs? If not, I doubt that reconfiguring your existing array will be a major part of the solution.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #10
    Re: 4 strings of 3 or 6 strings of 2
    vtmaps wrote: »
    I wouldn't bother. Does your system (over all) meet your needs? If not, I doubt that reconfiguring your existing array will be a major part of the solution.

    --vtMaps

    It meets my needs perfectly and have no issues. I just wanted to make sure that it was configured the "most efficient" way.
    Thanks for all the help.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #11
    Re: 4 strings of 3 or 6 strings of 2
    jcheil wrote: »
    It meets my needs perfectly and have no issues. I just wanted to make sure that it was configured the "most efficient" way.
    Thanks for all the help.

    It's not the most efficient transmission of array power, but it is the most efficient GIVEN that you don't want to raise your system voltage, don't want to buy a new, higher voltage controller, and don't want to put in a thicker cable.

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