multiple panel efficiency

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Dapdan
Dapdan Solar Expert Posts: 330 ✭✭
in Advanced Solar Electric Technical Forum #1
Hey all,

My current set up has 2x2(series x parallel) into a C40 and 2x5into a C60 as my signature indicates they are all identical panel running on a 24V system. Here is what is going on maybe some one can shed some light on it.

My 2x2 string together outputs say 80Ah and the 2x5 string outputs 223Ah. These are actual number taken today from my CC. They are all operating under the same environmental conditions. What i want to know is if it is a fact that a single panel within a grouping of panels will out perform a single panel operating on it own as my number suggest.

i.e in the 2x2 single series string giving 40Ah

as opposed to in the 2x5 a single series string giving 44.6Ah

Is a normal characteristic of all solar panel operating in a series/parallel string.

Just wondering....:confused:


Cheers...
Damani
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
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    Re: multiple panel efficiency

    their overall exposure to the sun is different and would be the main reason for this. it is also possible the pvs in the bigger array have more output capability too, but i lean it's the amount of exposure the larger array receives.
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  • Dapdan
    Dapdan Solar Expert Posts: 330 ✭✭
    #3
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    Re: multiple panel efficiency

    Hey neil,

    I don't see how they could have different exposure. They are all facing the same direction and are at the same angle in the same place. The only slight difference is that 10 of the panel are the KC130TM and the other 4 are the KC130GT the have the exact same name plate specs. It is a major difference of nearly 5Ah (12.5%)over a solar day, not that I am complaining. Just wondering if as an array vs single panel if the unit performs better while in an array. Really in an array I think panel would have a high temp compared to a single panel on it own so performance should decrease slightly?

    Cheers...
    Damani
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #4
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    Re: multiple panel efficiency

    You base this all on the C40/60 amp gauge and I know for a fact that is far from accurate ... could be off 5%+ easy.

    You have different wiring and panels are never 100% matched, typical variations are in the 5% range as well
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  • Dapdan
    Dapdan Solar Expert Posts: 330 ✭✭
    #5
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    Re: multiple panel efficiency

    Guppy,

    I accept that even identical panels have a variation. The wire runs are all less than 30' and are all 10g, so i don't think i have a lot of line loss variation since i am running a 24 v set up. If i have losses it should be in the bigger array since the current goes up to a max of 56amp and the other array 20amps.

    Is the error in the meter face plate for the C series CC really that high. Are they any more accurate meters out there? Additionally, even if the meter have manufacturer accuracy issues what i think should be considered is the relative accuracy. The difference in accuracy between the two CC and meter shouldn't be as high as 5%.

    Therefore I think there maybe other factors that are at work. I will switch inputs on the meters and see if I get the same type of readings. Inorder to rule out the influence of the CC + meter.

    Cheers
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #6
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    Re: multiple panel efficiency

    The meters are just LCD displays, they have no current measurement in them, that's on the C40/60 board, which is a piece of 10 gauge wire as the shunt. I know the controllers inside and out and yes, 5% would be doing good for what that controller has internally for current measurement.

    Since the "shunt" is a piece of copper wire, it will always report high at higher currents, as the wire heats up as the current increases, increasing the resistance. Current reported is just the voltage drop across the shunt/wire so your 100% guaranteed that it will report high at large current vs small currents and explains what your seeing
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  • Dapdan
    Dapdan Solar Expert Posts: 330 ✭✭
    #7
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    Re: multiple panel efficiency

    Ok SG,

    you are the expert. so the most accurate way of see if this is taking place would be using a meter using a more accurate shunt. So if one had say a linklite and used it on both array set up it would give a more accurate assessment right?

    Cheers...
    Damani
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #8
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    Re: multiple panel efficiency

    You have to read up on what the specs are on your measuring unit and shunt. Trying to do better than 5-10% in accuracy starts getting into worrying about all sorts of things instrumentation wise, generally speaking, its gonna cost 10X more to improve your reading from 10 to 1%, same thing again from 1% to 0.1%. I know this first hand as I can measure 0.1%, including from my test arrays.

    The real question is what is the benefit of this capital out lay going to do for you?

    Personally, if your willing to fork over 3-400 bucks, get more solar, that will give you more harvest ... spending it so you have higher precision digits on some display won't keep the lights on longer, of course IMHO
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #9
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    Re: multiple panel efficiency

    I agree with SG (of course--I would be wrong to disagree with SG at anytime ;) )...

    Your panels, if they are within 5% or so of each other--are doing fine. And without a pyrometer and expensive test gear--trying to get more accurate is just not worth the money.

    If you are debugging a system--disconnecting your panels and measuring Voc and Isc (voltage open circuit and current short circuit) should indicate if the panels are pretty much OK--and all panels should match pretty closely--the one that does not may have a problem...

    Connecting each panel to a load (such as a car battery or your battery bank) and measuring the current with a DVM is the gold standard--but usually not worth the effort for debugging.

    However--getting a Battery Monitor for your system would be a wonderful addition any way... Both to learn how it works--and to have a simple "fuel" gauge that you or anyone else (spouse, guests, weekend rentals) can check at a glance (i.e., if 50% or less battery capacity--stop the loads and/or start the genset).

    And if you have AGM/Sealed batteries--it is the only accurate method to check state of charge in an operating system (need to let the bank rest for 3 or more hours before measuring with an accurate DVM to properly asses state of charge).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Dapdan
    Dapdan Solar Expert Posts: 330 ✭✭
    #10
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    Re: multiple panel efficiency

    SG,

    I don't plan on get to involved on the absolute accuracy of any type of meter. I asked specifically about the linklite because it is my intent to get one in the near future and was just wondering on it accuracy in relation to the C series type metering interms of the shunt design for each. surely the shunt design on the linklite is better than that on the C series hence a more consitent/reading of current as it relates to valve of input current. I have no need for a 1% or 0.1% accuracy just a knowledge that the accuracy measuring say 20A is the same/similar (small deviation than C series) for that at say 60A. Further to this there must be some shunts that are more consistent or better than others in terms of maintaining the same "relative resistance" for low and high current.

    In your professional opinion which meters out are the best in terms of cost versus accuracy (T2020, linkpro, linklite etc) or the best shunts available to give good consitent performance at a reasonble price. Does not have to be very accurate but very consitent over it range of its current readings.

    Cheers...
    Damani
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #11
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    Re: multiple panel efficiency

    Well, by your numbers your at 4.4% now ... so you would indicate you are looking for closers numbers.

    I have never owned a battery monitor, as I said, read the specification, they will have the tolerances listed.

    The Linklite spec http://www.xantrex.com/web/id/273/p/2609/pt/7/product.asp

    showes +/- 0.4% , you then have to add the tolerance of the external shunt, typically +/- 1-2% shunts are always external on these devices

    No harm would be done using one, except the hole in the wallet, I still think more PV is the better investment
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  • Dapdan
    Dapdan Solar Expert Posts: 330 ✭✭
    #12
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    Re: multiple panel efficiency

    SG,

    Right now i have 14 kyocera and they are more than enough to run my house. I agree it would be better to get more panels with the money but i think a meter will allow me to be manage my panels. I intend to set up some panels at my office so i want to divide my current array and install some at the office. Therefore the battery meter(ver an observation period) will help me decide how many i can remove and be able to meet my energy needs at home.

    I am looking at the getting some of the suns hopefully (and accessing a discont being a member on your forum) froim the miami guys. however it cheaper for now to say move four of my panels to run lights at my office since i can import the meter without a huge freight and tax bill to Barbados as compared to say the panels. as an example, when i imported my first set of 10panels that cost me around $6000 I paid an additional $1000 in Vat and taxes here in Barbados and then another $300 for ship freight from Miami (which = 2panels)

    Cheers...
    Damani
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #13
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    Re: multiple panel efficiency

    dapdan,
    my point was even though the 2 arrays may seem the same they are not. all of the variables mentioned combined will create discrepancies and the meters i had honestly forgotten about their role in this as sg is quite right. put the c60 in the c40's place and see if it's different. the shunts themselves could also be off a bit too.
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  • 2manytoyz
    2manytoyz Solar Expert Posts: 373 ✭✭✭
    #14
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    Re: multiple panel efficiency

    I ended up installing a LinkPRO on my small setup. Different reason than many here. My original intent was emergency backup power in my hurricane prone home in FL. I started with a battery bank, grid based charger, and an inverter. The solar panels were added later, and are now becoming a prime interest as I'm able to move household items over to solar power exclusively.

    That said, a big issue for those of us using battery power in an emergency is trying to avoid going below the sacred 50% mark. Knowing that battery voltage means NOTHING with the batteries under load. Measure instantaneous current and guess state of charge? Fine if the loads stay constant, but that doesn't happen either.

    So in my case, the LinkPRO with a shunt was the right choice. It tracks current flow in/out of the bank. I told it the size of the battery bank, it does the rest. I can have it set off bells & whistles, or even remotely start a generator (if mine had that feature) when I reached the 50% mark.

    I can't speak for the actual accuracy, but the numbers do track with Prostar 30M, and my Extech clamp-on DC meter. While my Extech isn't calibrated, the two we have at work are, and know these to be a decent brand. Someday I'll compare mine against the calibrated models just to verify the numbers.

    I do have to say that my (solar) current readings are changing either constantly, or only staying put for a few seconds (this is my opportunity to compare figures). We usually have some wispy clouds, rather than pure blue skies, so the energy the panels receive does vary.

    http://2manytoyz.com/linkpro.html

    I paid about $250 for this meter, and it was easy to install. Details above.
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