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setting up solar systems/inverting

Discussion in 'Renewable Energy' started by Thunderchild, Jul 24, 2007.

  1. Thunderchild

    Thunderchild New Member

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    so I'm thinking (as of yet quite hypothetically) of a solar system, now from what i gather the usual design is solar panels - charging circuit - battery - inverter - load. now from what I see most stuff is done on 12 - 24 volts, so the inverter needs a very bulky transformer if we are talking 1-3 KW to convert 12-24 volts into 240, but as to have enough storage available you need many batteries why not connect them all in series so you have 240 volts DC this can be used to directly power things that run on SMPS which is like anything nowadays PC-TV-VCR-monitors-you name it. and just a transistor circuit with no bulky costly transformer can make 240 VDC into 240 VAC ok you'd probably need like 340 VDC so that the peak voltage of a sinusoidal wave with an RMS voltage of 240 VAC can be achieved but you get the point, after all 1 KW at 12 V means huge currents,

    is this currently being done already or am I missing something vital

    Thanks for your patience
     
  2. mneary

    mneary New Member

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    Because batteries of the size which are generally used do not have insulated terminals, and their insulation isn't 340V rated, connecting them in series is incredibly dangerous. To reduce losses, sometimes you'll see 24V or even 48V banks, but that still isn't as bad as 340V.

    The necessary transformer to invert 1-3 kW at 50/60 Hz is large. At 50 kHz, the transformer is not large.

    I haven't seen any inverters using 50 or 60 Hz transformers for sale since about 1985.
     
    Last edited: Jul 24, 2007
  3. Thunderchild

    Thunderchild New Member

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    I sort of understand about the insulation but not entirely can you explain further ? as for the large amount of equipemt that nowadays runs on SMPS 110-160 V is quite usable for such appliances so would this be an isolation issue ?

    as for inverters so they use smaller high frequency transformers, then have a second oscillator that outputs 50 Hz ?
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Todays hybrid electric cars dont use a huge and heavy 12V motor and battery. They use a lightweight high voltage motor and battery.

    Modern AC inverters use pulse-width-modulation with a small high frequency transformer. Then the output is a sine-wave instead of a square-wave.
     
  6. mneary

    mneary New Member

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    1) Ordinary 12V batteries are dangerous when used in 340V banks. Probably the most important issue is the large terminals which aren't usually easy to shield/insulate.

    2) Yes they use high frequency to change and regulate the voltage and a separate oscillator if they need to produce AC. I have an inverter that accomplishes this for 40W in 2 cubic inches and US$20.
     
    Last edited: Jul 24, 2007
  7. Thunderchild

    Thunderchild New Member

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    ok so insulation wise you are talking about people getting their hands on them yes thats an issue but I thought you meant internal failure of the batteries because of the high voltages around, although having seen some inverter prices I suppose might as well just use the inverter
     
  8. Oznog

    Oznog Active Member

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    You are overestimating the trouble in getting an inverter.

    I got the best results ever with a "Solar" brand inverter, the "brick" as this type was sometimes called, about the size of 1 1/2 std construction bricks. 2KW continuous, 6KW peak. Runs very efficiently and draws
    extremely low idle current. That was a 110V but it was only like $600 or so. Incidentally, don't overlood that idle current. Some inverters can take 300 mA or more just to stay on without a load and that's an absolute killer for a solar system if you were to leave it on 24 hrs a day.
     
  9. things

    things New Member

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    if you had a 240 volt battery bank, you would have to input 240 volts to charge them. inverters are cheap(ish), quiet, light(ish) and output ac not dc.
     
  10. Thunderchild

    Thunderchild New Member

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    i was thinking a large scale system mainly if you want to run totally on solar you need at least 20 panels and in series thats 240 V, you would also need a good many batteries and so a bank of say 120 X 2 v 100 Ah batteries would give a good autonomy in case of cloud for a day or two,

    now considering the huge current requirements for say 3 KW at 12 or 24 volts the need for VERY thick cabling and really the fact that an inverter for such a power - well we all know that probably cost wise and certainly efficiency wise it would be better to make 240 VDC into 240 VAC as it would need only one power oscilator stage and no transformer rather than two and a transformer, after all thats all the inverter does: with one oscilator and the transformer plus a rectifier it is making 240 VDC that is them made into AC with a further circuit stage, probably for home instalations it's not worth it I might be jumping the gun but on a larger scale sure it would be best to start out with a higher DC source ?

    of course it goes without saying that many devices that have a SMPS in then can safely run on DC eliminating an inverter completely, it could require just one more household wire to carry DC as well and a lot of things could run on it even electric heating basically anything but a brushless motor and if i remember correctly DC travels through wire better as it does not suffer skin effect making transporting higher power more easy
     
  11. mneary

    mneary New Member

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    Many SMPS used a small transformer to get them started. They wouldn't like DC.

    You have a choice... short thick wire at low voltage or insulating everything, long or short, for high voltage. Remember the solar array can't be turned off and touching it would be deadly. You would have to do all your maintenance at night. The batteries would be deadly all of the time.

    The skin depth of copper at 50 Hz is about a centimeter. At the small diameter of household wiring, skin effect can be ignored.

    All batteries and panels in series would bring the whole system down on a single failure of any of them.
     

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