upgrade to 24 volt system from 12 volt - inverter help needed

hrdrokhrdrok Registered Users Posts: 3
i will keep this simple - looking at changing out a really old Xantrex 1512 inverter to less old Xantrex 4024 inverter 

Details, Live in the sub arctic so half the year is generator driven, charge and use, looking to reduce my fossil "fool" consumption

my current system

Solar - 3000 w (6x250 24v) into  2 x Tristar 60 amp mppt
Batteries - 12 x Trojan 6V LR16
Generator - Northern lights 6kW - 2 x 30 amp @ 120V
Inverter - Trace 1512 - 1500 w 12v - 60 amp input - 30 amp bypass and 30 amp charge

My change option

Inverter - trace 4024 - 4000 wattt output (not really required) - 24 volt system

I can change my batteries to 24 volt system no problem with a jumper or 2...
I can add solar panels with 24 volt with current mppt's to 6000 w

My question - 

The trace 4024 also accepts 60 amp - 30 amp bypass and 30 amp charge split

So, will a 30 amp 120 volt into this inverter charge at twice the rate in a 24 volt system vs a 12 volt system? (2 times a fast?)

I would love to reduce my diesel usage

TIA


Comments

  • vtmapsvtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    edited April 2016 #2
    hrdrok said:
    So, will a 30 amp 120 volt into this inverter charge at twice the rate in a 24 volt system vs a 12 volt system? (2 times a fast?)
    Welcome to the forum,

    Your current system has 6 strings of batteries in parallel.  That is terrible design and will certainly be improved by reconfiguring them into 3 strings at 24 volts.  Of course, even 3 strings is sub optimal, but much better than 6 strings.

    To answer your question... No, it won't charge faster.    Your AC input to the charger is 30 amps at 120 volts = 3600 watts.  
    The DC output to the batteries will go from about 14.8 volts to about 29.6 volts.  Due to losses in the charger, the 3600 watts would be reduced to about 3200 watts.  That is 216.6 amps at 14.8 volts or 108.3 amps at 29.6 volts.  Either way, during bulk charging you are putting 3.2 kilowatthours of energy into the batteries every hour. 

    I'm not sure exactly which batteries you have... if they are 370 amphours then your energy storage is 12 batteries X 6 volts X 370 amphours = 26.64 kilowatthours.   Your batteries hold the same amount of energy (kilowatthours) whether you configure them at 12 or 24 volts. 

    Your charge rate is about 11%.  That's pretty reasonable... we generally don't recommend much higher than 13% charge rate, so you don't have much room to increase your charge rate.

    The way to save money on diesel is to use a smaller generator... a 4 kilowatt generator makes 3600 watts more efficiently than a 6 kilowatt generator.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • VicVic Solar Expert Posts: 3,162 ✭✭✭✭
    edited April 2016 #3

    Hi hrdrok,

    Perhaps you left out a bit of detail,  or there is a Typo in the Solar power in your post  "Solar - 3000 w (6x250 24v)"
    6X250 W = 1500 W  of PV.   Do you have two arrays,  each of which is 1500 W?

    And on your generator,  does this genset have the ability to produce all of its output at 120 VAC?,  Or perhaps you plan to use an external transfer switch to switch your AC from one winding of  the genset to your AC loads,  while the other 120 VAC winding on the genset connects to the AC input of the inverter just for charging (?).  Otherwise,  since the venerable SW 4024 inverter produces and accepts 120 VAC,  so it would be unable to use the full capability of the genset,  UNLESS the genset has a switch that allows paralleling both windings of the AC Alternator,  as far as I can see it ...

    Believe that the Maximum Continuous Charge Current (at 24 VDC) from the  SW 4024 is 110 A DC,  FWIW.

    Just a drive-by post.   Vic


    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 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.
  • hrdrokhrdrok Registered Users Posts: 3
    thanks for the reply, i inherited the 12v with the OTG house, my view of the number one benefit of going to a 24v system is that i can add solar to my mppt with solar panel cost only, so 3kW where i run 1.5kW now into my mppts.

    can you expand on your comment

    "That is terrible design and will certainly be improved by reconfiguring them into 3 strings at 24 volts"

    is this from a solar standpoint? ie what improvements? i have some legacy 12 volt systems in my house

    re my generator, i have a fairly efficient diesel running at 1800 rpm - and run only when chewing power in the house (toaster, entertainment), m inverter will only take 30 amps of the 60 the gen puts out, and the other 30 is bypassed to supply house power. I live in an area where temps are -20C or greater for 6 mos of the year, and this units starts cold with no probs, so some inefficinecies are conveniences. I have run gas generators prior which are more costly.

  • vtmapsvtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    hrdrok said:
    can you expand on your comment
    "That is terrible design and will certainly be improved by reconfiguring them into 3 strings at 24 volts"
    The problem with parallel batteries is that the current does not divide equally between the parallel pathways.  Here's a good link to get started on understanding this:
    http://forum.solar-electric.com/discussion/13179/series-rule-of-thumb

    There are many other reasons to have a single string.  For one thing there are stochastic considerations.   The capacity and lifespan of your battery bank is NOT the average capacity and lifespan of the individual cells... rather it is the weakest cell that sets the capacity and lifespan of the entire bank.

    If you have a single cell (2 volt) system, you have a 1 in 2 chance of getting an above average cell.  If you have a 4 volt system (2 cells in series) you have a 1 in 4 chance of getting 2 above average cells.  For a 12 volt (6 cell) system it's 1 chance in 64 of getting 6 above average cells.  Don't count on it!

    A more useful way to look at it is to consider the below average cells... you will get some.  But how far below average?  The cell quality has a Gaussian distribution...  The more cells you have, the greater the chance that you will have a far below average cell, an outlier on the Gaussian curve.   If you have 3 parallel strings you have almost 3 times the chance (compared to a single string) of getting a really weak cell.

    An optimal design for a system has as few cells as is practical.   If you have a 2 volt system, you have a battery that will last longer than most 12 volt batteries.... of course, 2 volts is not practical for the power levels you will use.

    As mentioned in the thread I linked to, each of your parallel strings should have a fuse or non polarized DC circuit breaker.   Fuse size is tricky... if one string (of 3) develops high resistance all of its current will pass through the other two strings... the current is unlikely to divide evenly, causing one of the other fuses to blow, and then the current will be forced into the remaining string and cause its fuse to blow.  

    And then there are points of failure.  Every cable, crimp, bolted connection, circuit breaker, etc. is a potential point of failure.  Three strings of batteries has more potential points of failure than a single string. 

    --vtMaps

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • hrdrokhrdrok Registered Users Posts: 3
    thanks for the all the info,

    you were correct in that i have 2 arrays @ (3x250w) solar for 1500w, 

    just for clarification, we only see max solar potential for 8 months a year and have to rely on diesel generator for the other 4 months of the year (when we have 20 hours of night), meaning my batteries do a lot of work with daily discharges by about 20% and nightly recharges via generator.

    My original question regarding charge time 12v system vs 24 volt system was geared to the generator side of things, not solar. My generator sees 1000hrs+ run time / year which is small air cooled are not the best choice up here.

    My 12V inverter puts 70 amps (not 216.6 max as you point out in original reply) @ 14.5v nom max charge - 
    A 24v inverter puts ~110 amps @ 29v nom will charge the bank in a shorter time (1/3 or so) - I am not missing something here am I?


    With this info in hand I am now looking at converting my system from 12v to 24v-

    Reconfigure exisitng battery bank from 12v to 24v - 6 (2x6v) strings to 3 (4x6v) strings
    Change out 12v 1500w inverter for 24v 2500w inverter
    Upgrade solar panels by up to 750w / array with existing mppt's 

    Is there anything I have failed to consider here?


  • vtmapsvtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    hrdrok said:
    My 12V inverter puts 70 amps (not 216.6 max as you point out in original reply) @ 14.5v nom max charge - 
    A 24v inverter puts ~110 amps @ 29v nom will charge the bank in a shorter time (1/3 or so) - I am not missing something here am I?
    In my previous reply I was pointing out that 3600 watts conveys the same amount of energy per time in a 12 volt or a 24 volt system.   14.5 volts X 70 amps = 1015 watts.   29 volts X 110 amps = 3190 watts.  So yes, bulk charge will be 3 times faster with your new setup.  Absorb charge will probably NOT be 3 times faster in the 24 volt system.

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