48V PV array with various load voltage

System
System Posts: 2,511 admin
in General Solar Power Topics #1
Hi,
I need to power different types of Communication Eq. from the Solar System.
I'm pulling about 3000w/day. The equipments are fixed voltage of DC 48V, 24V, 12V AND AC 110V.
I'm planning to install 1KW PV Array, 8 GEL 12V225Ah, but here comes my problem: I've been told by many due to some kind of interference and feedback, I cannot install the 800W Trace/Inverter AFTER my converters.

. PV Array---Chg/Load cont.
48V Comm. Eq. Load
..............................|............|
..............................|............
24V DC Conv.----24V Comm. Eq. Load
..............................|............|
..................48V Batt. Bank......
12V DC Conv.-- --12V Comm. Eq. Load
.......................................................................|
....................................................................... ---800W Trace/Inv.

I googled everywhere I could, but never located my answer.
Is that TRUE? What kind of problem would it be for the Load Controller and the Converter because of the Inverter?
Your replies and experience would be greatly appreciated.

Truly...
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Comments

  • boB
    boB Solar Expert Posts: 1,032 ✭✭✭✭
    #2
    Re: 48V PV array with various load voltage

    I'm not sure I understand what you mean by after your "converters" (charge controllers maybe ?)

    The only problem that I can think of is that it is not normally advisable to tap off battery voltages like this for different voltage loads. It has to do with charge balancing of the different levels of voltage and battery life
    since they don't have an even load.

    It might be OK if you are using separate charge controllers to make sure the different battery sections are well balanced, voltage wise.

    A small battery-balancer would be fine for a small 12 volt load, but I'm not sure about this communications system. Another alternative is to use a voltage down converter off the whole 48 volt string, which would make them all balanced.

    Is this going to be up on a mountain top for repeaters etc. per chance ?

    On "interference", do you mean RFI radio interference ?? The older Trace Engineering mod-square-wave 800 Watt inverter would more likely be a problem at lower AM frequencies and "normally" less of a problem on VHF/UHF frequencies.
    boB
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  • GreenerPower
    GreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    #3
    Re: 48V PV array with various load voltage

    First, I don't know what 800W inverter you have in mind but pick a true-sine wave one. The modified sine wave is likely to have RF interference with your comm. equipments.
    The first stage of the inverter is a boost DC-DC converter. So, essentially the input is shorted to ground through an inductor for a brief moment. That's propably bad for using the output of a DC-DC converter to drive the inverter (unless the DC-DC has significantly more wattage 2X maybe ?).
    Instead of a 48V-12V DC-DC converter for the inverter, you can use a CC such as the C40 and a small 12V batttery (gelcell maybe ?). The CC would keep the battery healthy and the battery would "assist" the CC for occasional "surge" current needed by the inverter.
    You might need 2 CCs since 800W/12V = 67A. The inverter would draw more because of efficiency.

    48V
    CC
    +
    inverter
    .............................|
    ...........................12 V Batt.

    Good luck
    GP
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: 48V PV array with various load voltage

    My suggestions...

    At what voltage is most of your power (P*I) going to be used... It might be difficult to find a smaller (800 watt) 48 VDC inverter, but if you can find a 24 VDC inverter that meets your needs, you might be more efficient to pick 24 VDC as your base voltage and up/down converter the other voltages.

    In any case, you probably would want to avoid double conversions (48 to 12 to 120 VAC) if you can avoid it... Each conversion will probably cost you 20% in losses.

    On the other hand, for small amounts of current at voltages you need to convert--You have several options for converting Batt voltage to device voltage (48 vdc to 24, or 24 to 48/12, etc.).

    1. Using a DC to DC power supply (battery voltage in, device voltage out).
    2. Using 48 VDC bank and using a solar battery charge controller to down convert to a second battery bank at the device voltage.
    3. Using battery voltage to a 120 VAC inverter, and usie small AC to DC converters to supply device voltage(s) for non-battery voltages.

    All of these options have their advantages and disadvantages. What your your exact power requirements for each voltage?

    Depending on what Trace/Xantrex inverter you want, there are several issues with Xantrex. The SW product line is being discontinued and the Prosine series are difficult to get right now.

    Using a solar charge controller to charge a lower voltage battery bank sounds interesting--but this would require more batteries and, ideally, you would need to control the controllers so that they only take power when the sun is up and the main battery bank is >50% charge level (to avoid a. double battery losses of 10-20% and b. avoid killing the main battery bank when it is below 50% state of charge).

    You did not say anything, but are you also planning on alternative sources of energy (wind, generator, etc.)?

    If wind, you might look at a lower voltage battery bank--at least some wind generators don't generate any power in low wind conditions when connected to a high(er) voltage battery bank--you would need very windy conditions for some of these turbines to work well on a 48 VDC bank.

    If you are using a generator, an AC output generator can power your 120 VAC equipment directly during stormy conditions and other equipment/battery failure conditions.

    Also, if using 120 VAC as your base voltage--it might be easier for you to design and build a load shedding system for the times when you should turn off non-critical equipment when the battery charge is low (to protect the batteries from damage). Using multiple controllers and battery voltages can be done--but it would need more work on your part to make everything compatible.

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