figuring panel strings and mppt

Gttdnbman
Gttdnbman Registered Users Posts: 7 ✭✭
I wanted to double check what I was told by a salesman. I was told I need 12- Suntech 305 watt panels, IMP 8.43A, Vmp 36.2V. My system is a 24volts, I was told that my current Outback 60 MPPT charger would work. Can anyone give me a string layout and formula showing me that I would stay under the max of the MPPT?

Thanks,
Brent

Comments

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    3660 watts of panels is too much for your controller in a 24 volt system.  Ideally, with those panels you would put two to a string and have a string Vmp = 72.4 volts. 

    What do you have for a battery?  Does it need 3600 watts of panels?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Gttdnbman
    Gttdnbman Registered Users Posts: 7 ✭✭
    I was told I would need 10-210AH Gel, loads are 100 watts of lighting 600 watts of fans for night and a electric fridge (468KWH a year). Thoughts? I already have the FM60 and Magnum4124 inverter.
  • Gttdnbman
    Gttdnbman Registered Users Posts: 7 ✭✭
    Off Grid in Africa
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    you need to flesh out the times that the appliances will be running, other than the fridge, to be able to work out an answer to your questions....
     
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Gttdnbman said:
    I was told I would need 10-210AH Gel, loads are 100 watts of lighting 600 watts of fans for night and a electric fridge (468KWH a year). Thoughts? I already have the FM60 and Magnum4124 inverter.


    Hi Brent,

    Those 10 batteries are most likely 12 V.   This would equate to five parallel strings ...   not  a very efficient way to get battery Capacity.

    Some Gel batteries have a maximum charge current rating that is relatively low (often about 5% oc 20 Hr  Capacity),  which can mean that those Gel batteries are not well-suited for off-grid,  or perhaps not well suited for some Back up power situations.

    Some Gel batteries made in Europe  appear to not have such a low Max charge current rating.

    Know that finding batteries that are suited for cyclic use,  in Africa can be a bit of a challenge.

    If you could find Flooded batteries that are made for cycling,  near your location,  those may well be best.  It does appear that you have been using Solar charging on an existing system,  so you might be using (these or some) Gels,  currently.

    vtmaps is correct about your best choice for string configuration on a 24 V system,  AND about the inability of your FM-60,  or MX-60 Charge Controller to use all of those PVs.

    FWIW,   Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Gttdnbman
    Gttdnbman Registered Users Posts: 7 ✭✭
    So what makes an efficient string? For batteries and panels. 

    Just a little back ground I serve at a NGO school in the bush in Zambia. We have school buildings and staff houses, each school class has a Zambian teacher and a North American teacher. For the most part each structure has its own solar system. We have two Plug and play Outback systems, two-Opti Solar (African) with Outback MPPT and two Magnum with Outback MPPT's. Most of this has all been inherited to me for the last facilities guy so I am learning little by little as problems arise and as we move forward. Most of our batteries are AGM, one has Bosch Silver Calcium and one has Gel. Probably the most frustrating thing here in Zambia is that nothing is consistent, once something is out of stock it seems like it gets replaced by something different also there is a lot of LOW quality items for sale locally. So we have shipped in what we can from back home. Anything I glean from here helps!
  • scheek
    scheek Registered Users Posts: 26 ✭✭
    vtmaps said:
    3660 watts of panels is too much for your controller in a 24 volt system.  Ideally, with those panels you would put two to a string and have a string Vmp = 72.4 volts. 

    What do you have for a battery?  Does it need 3600 watts of panels?

    --vtMaps
    So for clarity and to help me understand, I'm new to this, he is to use 24V panels in series (pushing 36.3 v ea) to acquire a 72.4 volt panel array (trying to stay under a 100 v?) So to say he has 48V system is not technically correct? He now has a 72.4 Volt system/8.43A using MPPT?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    For clarity--We tend to talk about 12/24/48 VDC battery banks (lead acid "nominal" battery voltage). Note that even these nominal voltages range from ~10.5 to 16.4 volts (for a 12 volt bank, from "near dead" to equalization voltage).

    With solar panels, folks talk about 12 volt or 24 volt panels--We try to talk about exact Vmp (voltage @ maximum power) and Imp (current @ maximum power). For example a "12 volt panel" (minimum panel Vmp voltage required to optimumly recharge a 12 volt battery) is Vmp~17.5 to 18.6 volts (a "36 cell panel" typically).

    There are also "24 volt panels"--But things get confusing... A 24 volt panel may have Vmp~36 volts (72 cell panel) which will work find for charging a 24 volt battery bank. However, it can also be a Vmp~30 volt panel (60 cell panel)--That voltage is too low for recharging a 24 volt battery bank (hot panel can have Vmp-hot down towards 24 volt volts, and you need >~29 volts to quickly/fully recharge a battery bank).

    An PWM type charge controller is (at its most basic) an On/Off switch between the panel and the battery bank. On, the battery charges, off the battery is not being charged (the "switch" will turn on/off 10's to 100's of times a second, "more on time" per cycle lets more current through, more off time per cycle, less average current flow--hence PWM, pulse width modulation).

    An MPPT (Maximum Power Point Tracking) charge controller a a "buck mode switching DC power supply" between the solar array and the battery bank. More or less think of it as a DC version of a "variable AC transformer"--It can "match" the high voltage/low current from the solar array and efficiently "down convert it" to low voltage/high current needed by the battery bank. (another way to think of it is like a single speed bicycle PWM vs a 10 speed bike MPPT--Does not "make more energy" but better matches the source of power to the load--Battery bank).

    With a "higher end" MPPT type charge controller, you can run a wider array of solar voltage into your fixed battery bank. For example a PWM controller uses Vmp~36 volts (standard test conditions), but the MPPT controller can used Vmp~36 volts to 100 volts STC for the array input (and there are controllers that can take Vmp-array~400 volts too).

    MPPT controllers are not perfect--They are (for example) most efficient around Vmp-array 2x battery bank voltage (i.e., 24 volt battery bank ~48 volt Vmp-array is "ideal").

    As the differences increases, the controller efficiency can drop... From ~98% peak to ~95% peak. Here is an Outback FM controller running on a 24 volt battery bank (from the user manual):
    Note that that dropping from 98% to 95% efficiency is not a big deal with respect to your system design (you won't miss 3% power loss during charging).

    However, from the controller's point of view, it is a big deal. One is 2% power loss "heating" the charge controller--The other is 5% heating--or 2.5x more heat generated in the controller--Needing better ventilation and higher fan speeds (noise). So, while you can run an "inefficient" high voltage array--You need to make sure that the charge controller is well cooled (not kept in a "hot electrical closet").

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    I suggest that a DC-DC buck converter is most efficient and cool when the input voltage is as low as possible - ie, just a few volts above the maximum voltage that the batteries need.    Ie, in the above graph, there is no line for 48V, but I suspect it would be slightly lower than the 34V line.

    I am available for custom hardware/firmware development

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Just remember hot panels run Vmp at 80% or so of STC.

    You need a couple volts drop across the mppt charge controller. And some for wiring voltage drop. Lastly, you need 14.75 volts minimum for quick/full battery charging.

    I think Ryan/Halfcrazy from midnite suggested 130% of battery charging voltage minimum STC Vmp for the array voltage.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Skippy
    Skippy Solar Expert Posts: 310 ✭✭
    Hey bill.  Just skimming the forums here, and read your last few posts . . .

    I just picked up 2 panels - 255 watt with a VMP of 30.2

    Then picked up the morningstar CC  (TS-MPPT-45) to run the panels . . .

    right now, I have the panels wired together for 60.4 Volts @ 510 watts going into the CC

    Did I read right that it would be better to change that to each individual panel separate for a Voltage of 30.2 @ 255 watt going into the CC ?

    I Agree, either way is well within the specifications of the charge controller, I just never thought about alternative wiring . . .

    The tristar 45 can handle up too 150 V ---  1,200 watts @ 24 Volts
    2 - 255W + 4 - 285W PV - Tristar 60 amp MPPT CC / 3 - 110W PV -wired for 36V- 24V Sunsaver MPPT CC / midnite bat. monitor.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Skippy said:
    Hey bill.  Just skimming the forums here, and read your last few posts . . .

    I just picked up 2 panels - 255 watt with a VMP of 30.2

    Then picked up the morningstar CC  (TS-MPPT-45) to run the panels . . .

    right now, I have the panels wired together for 60.4 Volts @ 510 watts going into the CC

    Did I read right that it would be better to change that to each individual panel separate for a Voltage of 30.2 @ 255 watt going into the CC ?

    I Agree, either way is well within the specifications of the charge controller, I just never thought about alternative wiring . . .

    The tristar 45 can handle up too 150 V ---  1,200 watts @ 24 Volts


    Hi Skippy,

    I am NOT Bill,  but am certain that he is NOT recommending that you switch to single PVs in parallel  --  30.2 Vmp.   This is simply not high enough voltage for any MPPT CC to work well,  especially with Flooded batteries ...   cannot find the batteries that you are using.   Even dreaded Gel batteries would probably not be too happy with an MPPT CC,  at with cool batteries on hot days,  outside.


    And,  YES,  Midnite Solar is recommending 30 % STC Vmp headroom,  above the highest voltage that the battery will even need,  at least for Flooded batteries.

    FWIW,   Vic


    Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.