To MPPT or not to MPPT?

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Lefty Wright
Lefty Wright Solar Expert Posts: 111 ✭✭
My PV array is mounted on a tower 40 feet from the house.

Since I live in the woods occasional shading of part of my array is unavoidable. For this reason my system is 12V with all my panels wired in parallel.

I am adding more panels and will need a larger charge controller.

Since my system is made up of panels all wired in parallel is there any advantage in spending the extra money on a MPPT controller over a PWM type?

How important is the equalization offered by MPPT?

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: To MPPT or not to MPPT?

    The equalize function is not really part of the MPPT/PWM/PWC question.

    Newer, more complex (and typically larger), charge controllers will offer an Equalize Option as well as a remote battery temperature sensor. Both functions are very important for longer battery life and better charging of the battery bank to 100% capacity.

    MPPT type controllers also happen to be newerr designs and larger controllers too.

    Regarding your question... MPPT controllers can operate at the same panel voltage of PWC controllers, and for (typically) cold weather, can increase the total power into the battery by 0-30%.

    However, the big advantage for you is that you already have existing copper wire from your tower to your battery shack. If you up the panel voltage from ~17 volts to ~100 volts Instead of just placing more 17 volt panels in panels in parallel), you can send over 5x as much power over the same wire.

    Also, you can make use of the larger (>100 watt) solar panels that have non-standard (for a battery) Vmp. These panels typically have higher Vmp, are cheaper, and easier to wire and mount (fewer panels and connections) than getting twice as many of the smaller panels.

    The problem you may have with replacing your current PWC controller is mixing the existing with new solar panels and matching the Vmp and currents so that you can use them all in one string.

    You may be left with the choice of leaving your existing panels and controllers in place, and adding a second array+wiring+controller. Or, taking out the old panels and controller (giving them away, placing them on Ebay) and installing a new set of matched panels and MPPT controller.

    Lastly, if you are making a major overall and increase in power--it might be time to look at your current 12 volt system and decide if changing it to a 24 or 48 volt battery bank will be helpful or not (note, with MPPT controllers, many of them are switchable between 12/24/48 volts without need for any panel rewiring, assuming you when for > 60 volts Vmp series wiring--so you can make the battery bank voltage change when it is more convinient/cost effective).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Lefty Wright
    Lefty Wright Solar Expert Posts: 111 ✭✭
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    Re: To MPPT or not to MPPT?

    I see the advantages you mentioned.

    But my problem with series strings is shading. My panels are mounted 20 feet above the ground in a horizontal line and as the sun moves one panel after another will be partially shaded.

    I figure series strings would suffer more than parallel panels. That's why I think I'm stuck on 12V. I can afford to lose one panel at a time. If I wired them in series I'd lose the whole string.

    I now have two panels on a 2000E MPPT controller and others on a PWM controler, all wired 12V, and I think the MPPT controler uses more power than it saves.

    But I have no real hard data. I guess I'd have to replace the battery with a constant load to get good output measurements.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: To MPPT or not to MPPT?

    Well, shading is a problem no matter what...

    The "best" shade tolerant system would be to use a 12 volt battery bank and a single string of solar panels in series up to the safe maximum controller input voltage...

    As each panel gets shaded, it will drop out and the diodes will bypass it...:

    Example, 6x17 Vmp panels in series (mythical round numbers "12 volt" 5 amp 75 watt solar panels)

    6 x 17 volts = 102 volts DC (*5 amps = 510 watts)
    5 x 17 volts = 75 volts (* 5 amps = 375 watts)
    etc.

    This should work pretty well (down to when the last panel or two get shaded). The advantage is you are dealing with 10 amps vs 60 amps of current.

    If you were to wire these at 2 series strings of 3 (still six panels), then when the first parallel string got one panel shaded, the whole series string current output will drop pretty much to zero (because the other string is still outputting 100% voltage)--so a series parallel array out put would drop to 50%.

    You could save money (if you are getting a 60-80 amp Outback FM xx type controller) by putting two series/parallel strings of 6 (12 panels total) if the panels are stacked and both of the strings get equal shading (voltage drops on both at roughly the same rate, so don't get the 50% drop).

    In the end, you might look at how much time your array is spending in "increasing shading"... If it takes 20 minutes for the array to be shaded in late after noon, the total amount of energy loss may not be worth the price of just adding more panels vs more copper+controllers.

    Obviously, if you could move your panels farther away into "full sun", then the higher voltage MPPT controller would be really nice.

    Regarding the 2000E controller--it is MPPT, but cannot handle anything more than 30 Voc... The others can manage Voc>135 vdc and are better suited to high voltage series strings.

    You can find smaller MPPT controllers (like the Blue Sky) that don't use as much power as the larger Outback/Xantrex 60+ amp units.

    There is the Morning Star 15 amp MPPT controller which is 92%+ efficient down to 25 watts. It also goes to 75 volts Vmp.

    Note that most MPPT controller can charge both 12 and 24 (and the Outback/Xantrex 48+ volt) battery banks... There output current is limited, but their total output power is based on P=I*V -- so they can handle more solar panels at higher bank voltages.

    Anyway, only you can tell how much shading costs you vs these other options.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Lefty Wright
    Lefty Wright Solar Expert Posts: 111 ✭✭
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    Re: To MPPT or not to MPPT?

    Thanks, Bill.

    But I'm now confused.
    The "best" shade tolerant system would be to use a 12 volt battery bank and a single string of solar panels in series up to the safe maximum controller input voltage...

    As each panel gets shaded, it will drop out and the diodes will bypass it...:

    Are we talking about series or parallel?

    I thought that when a series wired group of panels has one panel shaded the whole group is shut down. (no current flow)

    My KC 120's have built in bypass diodes. How would I install bypass diodes in a series string?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: To MPPT or not to MPPT?

    LW,

    My favorite answer... "It depends".

    Think of the strings as a set of "D" cell 1.5 volt batteries, each with 1 amp output.

    And you have a 1.0 volt rechargeable NiCad battery.

    You can put 6x1.5 volt batteries in parallel, and if one battery fails "open" (like a switch opened up) the others will still supply current to the 1 volt battery (with less available current).

    Now, put the 6 batteries in series and a "magic box" that can down converter the voltage (efficiently) to 1.0 volts and charge the battery. 9V*1amp=9watts

    If one battery has "failed open" the other batteries are still able to provide their current to the NiCad battery (if the one failed battery has a diode in series, so that when the battery fails, the voltage will "reverse" and the diode will conduct around the "open" battery). 7.5V*1amp=7.5watts

    The voltage is less, but our "magic down converter" is taking the lower voltage (which is still higher than needed) and converting the remaining output to power the the 1 volt rechargeable battery.

    Now, take your 6 string of series batteries, and put a second string in parallel... Both strings are 6x1.5v=9volts. 9V*2amp=18watts

    But, when one battery fails, that second string only has 5x1.5v=7.5volts.

    That second string will not output any current because the other string's voltage is still so high (still at 9 volts).

    To be honest, it is possible that a MPPT controller may figure out it can get more current at 7.5 volts at 2 amps (15 watts), than one 9 volt string at 1 amp (9 watts)--but, it is unknown and, probably unpredictable, whether a MPPT controller will figure out there are two maxima instead of one, and that the second one at, at the lower voltage (the "sweep" range would have to be rather large).

    NAWS sells replacement diodes--for example. For panels that don't have bypass diodes, you place the doide in parallel, "backwards", with the panel--so it does not conduct when the panel is "on", and when the panel is shaded, its voltage tries to "reverse" (because of the other series connected panels still with sun) and diode will bypass the "off" panel (that is in high resistance because of no sun).

    Sorry, it is confusing to explain just by typing... I probably need to wave my hands to help. ;)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Lefty Wright
    Lefty Wright Solar Expert Posts: 111 ✭✭
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    Re: To MPPT or not to MPPT?

    Bill, I hate to sound thick, but I don't get it.

    If a panel in a series string is shaded is it not an open? So there is no current flow in that string.

    I don't see what a reverse biased diode wired in parallel with a shaded panel would do. Where is the current flow comming from? Unless the diode is shorted or the other panels shaded and the battery voltage reversed there will be no current flow through the diode.

    I have never seen a schematic showing how bypass diodes are wired into the circuit so maybe there is more to this than I think.

    Then there is the problem that I'm an electrician. When people discuss electricity in non-electrical terms I often can't follow. Like the term "combiner box". I know about J boxes, junction boxes, pull cans, etc., but never heard the term "combiner box" until landing on this site. It's not an electrical term.

    At least not in the non-solar part of the trade.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: To MPPT or not to MPPT?

    I was trying explain one (or a couple) shaded panels in a series string.

    A solar panel, with no sun, first drops the amount of current it can deliver (to near zero amps) then the voltage drops to near zero volts. The panels don't go "shorted" when shaded, the go "hi resistance".

    So the reason that no current can flow in the string, first off, is because there is no low resistance path (i.e., copper wire) for current flow.

    So when a panel is shaded, the panels cannot just "push" the current through the high resistance panel, a reversed biased diode is needed to provide an alternate current path around the panel that has no sun.

    Once the current has an alternate way to flow (past the one shaded panel via the reversed biased diode in parallel with each solar panel), then the rest of the string of 5x15 volts @ 1 amp can continue to flow--looking like just 5 series panels connected together to the MPPT controller.

    The other issue with multiple parallel strings... If we have one string with 5x17volt panels in parallel with another with 6x17volt panels, the "Low Voltage" string will not pass any current while the other panels are at 1 panel higher voltage. Just like 6x D battery string in parallel with a 5x D battery string.

    Here is a better explanation with a drawing... Please note that the drawing shows the Diodes on their own connections to the solar panel... Electrically, the two "left panel connections" are the same "-" negative contact, and the "to right connections" are the "+" postive polarity.

    The "Combiner Box" (as you have guessed) is really just distribution box (typically with breakers or fuses), but wired up "backwards"--to take current from the "branch circuits" and combined them into the main output (aka: feed). I never heard the term before either.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Lefty Wright
    Lefty Wright Solar Expert Posts: 111 ✭✭
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    Re: To MPPT or not to MPPT?

    Hark! A drawing!

    Thanx for your patience, Bill. I now see how the bypass diodes work. If the diode connections in the drawing are the same electrical point as the "power" connections on the panels I see how the diodes would conduct when the impedance of the individual PV panel increases.

    I would call the bypass diodes forward biased when a panel is shaded. Not that it makes any difference.

    I'm now thinking that the extra money spent on a MX60 (or equivalent) may be worth it.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: To MPPT or not to MPPT?

    LW,

    You are correct, the diodes are "forward biased" when the panel is shaded and the other panels are not.

    However, you install them "backwards" or "reversed biased" wrt to the panel voltage markings so as they don't short the panels when they are in normal operation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset