Half way there

HiminsHimins Solar Expert Posts: 54 ✭✭✭✭
Well, 3 of my six panels are mounted, and connected in series to the outback fm80 CC. This is day 2, and with heavy clouds including snow. My input voltage has hovered around 90v. These panels are fastened to a vertacle south facing exterior wall in n.w. minnesota. I understand that these panels wired in series would tripple the voltage but the amp should remain constant. I am concerned about the string of 3 when I see 90 v input with heavy clouds,even though these panels are rated @230v each. As I understand it, 90v is approaching 1000w, and if the sun ever shines up here, I presume it will well exceed that output. How is this possible? And will this damage my CC?
My other string of 3 will be mounted sometime next week, into a combiner box with the first 3. I will then have 2 strings of 3, my question should be ...will this double my amp to somewhere near 16?
Finally, after downloading every voltage drop spreadsheet, I choose #6 awg wire to connect both the panels and the CC. The distance between the CC and battery bank required a pair of 18 feet, or a total of 36 feet with both wires. The string of 3 panels wired in series required one pair of wired 6' long, also using the same #6awg wire.

I will paste the panel's specs below.
Short Circuit Current (Isc): 8.80 Amps
Open Circuit Voltage (Voc): 36.75 Volts
Maximum Power Current (Ipm): 8.14 Amps
Maximum Power Voltage (Vpm): 29.00 Volts
Temperature Coefficient (Voc): -0.351%/°C
Temperature Coefficient (lsc): 0.053%/°C

Electrical Characteristics
Maximum Power (Pmax)*: 230 Watt
Tolerance of Pmax: +10%/-5%
Type of Cell: Polycrystalline silicon
Cell Configuration: 60 in series
Module Efficiency (%): 14.7%
Maximum System (DC) Voltage: 600 Volts
Series Fuse Rating: 15 Amps
NOCT: 47.5°C
Temperature Coefficient (Pmax): -0.485%/°C
*Measured at (STC) Standard Test Conditions: 25°C, 1 kW/m2 insolation, AM 1.5

Mechanical Characteristics
Dimensions: 64.6" x 39.10" x 1.8" (994 x 1640 x 46 mm)
Cable Length: 43.3" (1100 mm)
Weight: 44.1 lbs (20.0 kg)
Max Load: 50 psf (2400 Pascals)
Operating Temperature (cell): -40 to 194°F (-40 to 90°C)


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Half way there

    Don't panic. Panels are current sources, not Voltage sources; as the sunlight increases on them the Voltage will not change much at all but the available current will go up.
    For instance Voc on one of those panels is 36.75; for three it is 110.25. What you are seeing is panels lightly loaded bringing the Voltage down a bit, but still just above 3X Vmp (within the accuracy of the controller's metering). The only thing that will really push Voltage up is cold temps, and then you might see 144 Voc which will shut the controller down until the panels warm up and the Voc drops (I think the FM's auto-restart; the older MX didn't).

    With greater intensity of sunlight the current potential will increase, up to a theoretical maximum power of Vmp * Imp. Note that the charge controller may choose some other combination at any given time to get what it thinks is best for charging the batteries. And if the current isn't needed by the batteries it won't be produced.

    Cold will also increase the Vmp, but it won't likely be any more than 1.3X Vmp or 117 Volts (* 8.14 Imp or 952 Watts for one string). Even if you put more than 80 Amps * system Voltage panels on the input the controller will just 'clip' the excess power and its output current will not exceed 80 Amps.

    No worries. :D
  • HiminsHimins Solar Expert Posts: 54 ✭✭✭✭
    Re: Half way there

    THANK YOU, That makes sense now and a great deal of relief knowing how much work it took. The OB CC is a real treat to work with. Now to work out the wire size...
    Again, thank you very much. Cariboocoot
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Half way there

    welcome to the forum
    as was said, do not worry about the voltage you are seeing as the vmp x 3 is 87v. that happens when the pvs are fully loaded and if the load is not drawing all the pvs can provide the input voltage will start rising until it reaches the voc point for all 3 in series with 0 current.

    i notice you mention you have a wall mount. i would be interested in hearing how you did this and with what as wall mounts are few and far between. pics would be nice too if you can provide us with some.
  • HiminsHimins Solar Expert Posts: 54 ✭✭✭✭
    Re: Half way there

    Good morning Niel,
    These wall mounts are strictly amature and home made. My budget allowed only enough money for panels without frames. I fell off my roof in Aug. and have been suffering a broken heel bone and when these panels arrived, one was broken. The company has shipped a replacement at no cost. Back to the frames, my neighbor volinteered to help me mount these and came by with some wood frame materials commonly used for insulated glass. Simply a clear pine board with a one inch dato on the inside quite suitable for these panels. I merely mounted one for the top and bottom of the exterior wall, set the panels inplace, dropped some insulation behind them, and stood them inside the dato. Then installed some small blocks commonly called window stop. Mine are extreemly crude but look ok on this old farm house(I'm easy to please). I have to have these panels installed before the ice and snow hits so I didn't get very fussy. I can see how the appearance would be quite attractive with just a little more time and effort. I need to go on a photo run for another company, another friend of mine built a passive solar floor heating system for a comercial building that is getting some attention. I may post these photos here on a different forum.
    I'll be mounting three more panels this week, with this broken foot, I have my hands full but will try to shoot a photo off...Thanks again for your help Niel.
    One more question...will the #6 awg wire run of 18 feet from the CC to the batt suffice for two strings of three on these panels? I am not sure if you have to measure the distance...or the wire lenght which is thirty six feet for both wires...I expect to generate about sixteen amps under full sun.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Half way there
    Himins wrote: »
    I merely mounted one for the top and bottom of the exterior wall, set the panels in place, dropped some insulation behind them, and stood them inside the dado.

    What do you mean 'insulation'? It is usually not a good idea to insulate the back of a panel... they work better when air-cooled.
    One more question...will the #6 awg wire run of 18 feet from the CC to the batt suffice for two strings of three on these panels? I am not sure if you have to measure the distance...or the wire lenght which is thirty six feet for both wires

    #6 is more than adequate. You're looking at a 0.54% voltage drop.

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • HiminsHimins Solar Expert Posts: 54 ✭✭✭✭
    Re: Half way there

    I wondered bout the insulation, but these panels will be subjected to -30, even-40 degree tempratures this winter...the insulation is 3/4 inch cheap styrofoam. My thoughts were to bring the temps up just a bit to prevent other(unknown) problems and some type of backing as they could be subjected to minor impacts...wind blown objects etc. I'm now certian this insulation will be removed early this spring unless you advise me to remove it now. Thanks for the insight vtMapps
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Half way there

    Himins; you are more likely to have problems with the PVs having insulation behind them than without. Higher panel temps decrease Voltage, lower panel temps increase it. All your insulation will likely do is reduce power in Summer.

    My panels sit out all Winter in temperature down to -40. No problem at all.
    As a rule, electrical devices do not mind cold but they sure don't like heat.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Half way there

    too which needs to be low as well.

    in the case of multiple strings being paralleled only one string should be considered at a time and usually starts will the farthest out with the longest run. do not add all of the v drop %s for each string to the final total. maybe an example here can help you and others understand.

    let's go with 3 strings of identical arrays with the only difference being how far out from being combined at the combiner. say array 1 which is closest has .45% v drop loss, array 2 has 1.0% loss, and array 3 has 1.5% loss up to the combiner. from the combiner to the cc you find it to have a much heavier wire due to the combined currents of all 3 strings and the long length of the run. let's say it has 1.0% loss from the combiner to the cc. from the cc to the batteries let's say .54% as vtmaps said he figured.

    for array 1 it goes .45% + 1.0% + .54% = 1.99% total. this is good for under a 2% requirement, but just barely. if future expansions are to be considered then one may wish to invest in the larger wires now as you would most likely need them later and would cost you more in the future.

    for array 2 it goes 1.0% + !.0% + .54% = 2.54% total. this is not acceptable under a 2% max goal, but is acceptable under a 3% max goal and depends on what you will deem to be acceptable losses. now higher than 3% i don't recommend, but sometimes if it is close it may be viable to the system owner depending on circumstances. for reference the nec states that from source to load the losses should not exceed 5%, but we in solar like lower losses because our power is more costly to lose.

    and that brings us to array 3 with it going 1.5% + 1.0% + .54% = 3.04% total. again this may be ok as it may be circumstances such as an extra long run to the array due to extra distances to avoid shading or for whatever reasons. now if this is avoidable by going with thicker wire then you should do so and it would be quite a bit thicker to achieve under 2% and may be unrealistic to get that goal.

    that all assumes the strings will stay the same arrangement for you may opt to add another identical pv to each string upping the voltage while keeping the same current. this would effectively lower the v drop %. raising the battery voltage also has this same effect from the cc to the batteries.

    v drop %s are a direct every day loss from your power production. over the course of say 20 years this can amount to a staggering loss of wattage just by allowing the extra percentage point in going from 2% to 3%. example-1kw system at a modest 3hrs per day production. the extra 1% represents .01 x 1000w = 10wh. x3 for the average day is 30wh per day. at 365 days per year this is 10.950wh or 10.95kwh. over 20 years is 10,950wh x 20 = 219,000wh or 219kwh. wire costs often play a factor here in whether recouping the v drop % loss is viable, but sometimes it is simple to do as i stated in an example already. raising the voltage raises the reference that the v drop loss is pitted against and can result in a lower loss. sometimes systems can see a big difference. here is an example,
    2 identical pvs with these made up specs going through 75ft of #8 and assuming the use of an mppt cc just to account for variables in voltage,
    30v vmp
    8a imp

    for parallel you have 30v vmp we'll use as the reference voltage at 2 x 8a imp = 16a imp total. i found the result to be 3.2% v drop loss.

    now put those 2 pvs in series though that same wire to get 60v vmp total at 8a imp. this resulted in a v drop % of .8%!!!!!!! why you ask? cutting the current in half cuts the v drop % in half. doubling the voltage also cuts the v drop % in half. for the same identical pvs with the same total wattage capability the v drop losses where able to be cut by 4x just by this rearranging. are there other things that can do this you might ask? yes. cutting the distance the wire runs in half cuts the resistance in half and therefore cuts the v drop % in half. along these same lines is you go to a larger wire size by 3 numbers for the awg scale this is also cutting the resistance in half and will also cut the v drop % in half. that means that in that same example that if you further took these steps that you could go from .8% to .2%. that's like going from #10 to #7 or #6 to #3 and that's quite a change. (note that the odd gauge numbers are not always available.) i should also point out that with those same pvs in parallel and you cut the distance the wire travels and go 3 spaces on the gauges that this too would lower the v drop from the original 3.2% to .8%.

    my example makes it sound like it is an easy task to lower the v drop losses, but this is not always the case. i took that scenario to dramatize how this could change the losses vastly for some minor changes. some of these changes may not always be possible in some system designs. you might encounter more subtle changes that can be additive to achieve one's goal and could be just a few feet of wire off of the pv run of wire or even for the cc to batteries wire run. upping pv voltages one has to watch as controller efficiencies also could drop with higher voltages, but if in conjunction with upping the battery voltage this could be a twofold gain as v drop losses are then minimized in the cc to battery run and the ratio of the voltage conversion is also cut in half improving the cc efficiency.

    now don't be offended that i included all of this info as i don't want to infer that you are dumb as you weren't asking for all of this, but i just wanted to elaborate on this for others to have an understanding of how this can work as it isn't always obvious to the overview of all of this. i get in the mood to write a minibook once in awhile.:roll:
  • HiminsHimins Solar Expert Posts: 54 ✭✭✭✭
    Re: Half way there

    Well this is what i get from all that Niel
    39.65 feet of 8 amp wire 0.128 +
    18 feet of 16 amp wire 0.1152
    brings me to a total of 0.2432
    or 2.43% voltage drop
    All wire is #6awg
    There are 2 strings of 3 panels ea, One string needs 6' to reach the combiner box, string 2 needed 12' to reach the CB, each panel has 44" of wire attached.
    If I did this correctly Niel, well a tip of the hat to you my friend...thank you for sharing. if i did it wrong...well something went wrong when I added the total lenght of wire instead of starting at the farthest string. That part didn't stick.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Half way there

    i have to apologize as i tried posting that earlier and it was much longer that what you see and it somehow got cut. i thought i deleted it as i had to leave the forum and didn't want it to be posted incomplete, but there it is. too much got lost somehow, but i do remember my saying you need to eliminate the insulation as that is for thermal panels and not photovoltaics. i'll trust you did the correct calculations for yourself as i was talking in generalities not just to you, but to many who may be reading.

    i guess that my post was cut in 2 is somewhat ironic and appropriate for a thread titled half way there.:roll::p
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