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easy, DIY solar powered AC for small room??

Discussion in 'Electronic Design' started by AA, Aug 6, 2012.

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  1. AA

    AA Guest

    I have a small 10x10 foot room I am considering cooling with a small 12000
    btu AC, but I am wondering if it's possible to set this up to be powered by
    solar? The AC would not be run continuously, just long enough to cool the
    room off for sleeping, maybe 2 hours per day. I'm thinking a car battery
    with an inverter and the battery charged by solar cells, but I don't know if
    this would work or even how to begin. I also want to keep this as low cost
    as possible and stick with a simple design that works.

    Any suggestions would be appreciated.

  2. [This followup was posted to and a copy was sent
    to the cited author.]

    Some considerations, The 12000 BTU AC unit listed here:

    ...shows that it has a rating of 115VAC/60Hz with a draw of 10.2 Amps.
    They also quote a rating of 1110 Watts.

    If you were to use an inverter such as the following:

    ...the "optimum" efficiency is listed at ~90%. It would be more likely
    that you may see 85% efficiency. That would take an input power of
    1110 Watts / 0.85 = 1306 Watts.

    Converting the wattage requirement to Amps at a nominal 12VDC would be
    1306W / 12V ~= 110 Amps. This amperage delivered for the two hours that
    you specified is 220 AmpHour.

    A possible battery may be the Trojan model T-145 shown in this brochure
    on page 11 of the PDF. The quoted 5-Hr rate is 215 AH.

    To re-charge the battery from a solar array will require an array
    capable of restoring the 220 AmpHour of energy during one summy day
    period of time.

    The solar panel showen here:
    280_watt_panel.htm good for 280 Watts on a good sunny day. One could estimate that you
    have say 8 hours of sunlight on days when you would like the air
    conditioner to function. Also let us guess at solar battery charger
    efficiency of 85%. Some simple math shows that:

    (1306W / 0.85) / 8 hr = 192

    It looks like one of the above panels could keep your battery charged.
    Two panels would give you a nice extra capacity in case you wanted to
    swipe some system power for something else besides the AC unit.
  3. Wond

    Wond Guest

    I would look at direct solar absorption cooling, like the kerosene or
    propane refrigerators use. The solar heat collector would supply the
    There used to be a clever little 12V 'fridge available, "Norcold" I
    think. It used a "vibrator compressor" and could be used on a small boat.
    Perhaps something could be adapted from that.
  4. hamilton

    hamilton Guest

    WOW, this looks doable.

    What would be used to connect the solar panel too the battery ?

    I would think there would be some sort of a protection circuit between
    the solar panel and battery.

  5. AA

    AA Guest

    Thanks. Although this design looks interesting, it far exceeds the
    budgetary amount I had in mind ($500 max, sorry I didn't specify initially).
    I thought maybe I could put something together quick, simply, and cheaply
    but doesn't really look that way. Unless someone else has some other


  6. [This followup was posted to and a copy was sent
    to the cited author.]

    Adapting the solar panel to the battery requires the use of a high
    efficiency switch mode converter/charger unit. Those are available from
    various solar equipment suppliers.

    It may very well be more cost effective to find an integrated solution
    that combines the solar cell / battery management with the sine wave
    inverter to the 120 or 240 VAC.

    The OP's idea to try to support his 12000 btu AC unit for $500 is just
    not in the game plan. There is a portable 1800 Watt solar power
    gererator available at Northern Tool for ~$1800 that may be of interest
    for those that want a more turnkey solution.

    Do check carefully as to your application with the above unit as it is
    not clear if it even produces a simulated sine wave output or a full
    sine wave output. Some loads will requre full sine output.
  7. nospam

    nospam Guest


    The first (discounted) price I find for a T-145 battery is $209 and it is
    6v so you need 2 of them for $418.

    Cycled to 80% discharge they claim a life of around 700 cycles.

    Replacing around 2.2kWh of mains electricity with a cycle of these
    batteries gives a kWh life of around 1540kWh.

    Where I live 1540kWh of mains electricity costs around $300 (and that is
    after political interference with the market to heavily subsidise eco green
    bollocks power generation)..

    Is constantly loosing money on just the batteries never mind the capital
    expense and depreciation of the panels, control electronics and
    installation doable?

    Only if mains electricity is really expensive where you live or if you can
    steal the equipment.
  8. Guest

    A *big* assumption here is that the inverter can supply the compressor
    starting current.
    Assuming PF=1.
    Double that for 2 hours per day.
    It's not going to work.
  9. Tom Kupp

    Tom Kupp Guest

    Are you sure you need 12,000 btu's for a 10x10 room? I'm pretty sure my
    bedroom is bigger and uses only about 5,900.

  10. Not one net watthour of PV solar has ever been generated.
    To date, pv anything remains a gasoline destroying net energy sink.

    Net pv energy can be expected eight to ten years after the fully
    burdened, subsidy free costs drop below twenty five cents per peak panel

    more at < >

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  11. mike

    mike Guest

    Give it up.
    If you have access to the power grid, there is no, zero, none, not any
    cost-effective PV generation scheme.
    Unless you can steal the components, that is.
    There were schemes where the government stole money from me to subsidize
    your PV scheme, but those programs are pretty much exposed and defunct.

    PV is great for school crossing signs where the high cost of the solar
    is still WAY less than the cost to dig up the street to get power from
    100 feet away.

    Put your budget into better insulation.
    Needing a 12000 BTU unit to cool 100 square feet at night suggests that your
    insulation is deficient.
    Consider ground-source cooling.
    Depends a lot on where you live.
    Right now, it's 86F outside, 107F in the attic and 65F under the house.
    Stays about that temperature under the house year-round.

    I once had a fantasy about reworking an electric blanket to pipe
    chilled water thru it. Never got past the concept stage. Maybe with a
    heat exchanger under the house...
    If you've ever tried to share a waterbed, you're aware how tiny differences
    in water temperature make a huge difference in comfort level. Wouldn't
    take much energy to keep the water cool enough.

    RE the other suggestions... if you do attempt this, consider that it
    can take a LOT more power to start an AC compressor than to run it.
    Make sure your inverter can handle the surge.
  12. miso

    miso Guest

    I have some 95Ahr AGMs and they are ball busters. The biggest Odyssey
    battery is 125Ahr. You can always put batteries in series like they do
    in data centers, but that won't drive the repeater directly.

    It isn't clear from your spreadsheet if you are taking into account both
    the loss of the charger and the charge efficiency of the battery itself.

    Based on an APU battery evaluation project for a company I can't name,
    the Odyssey batteries were the best, with Dekka a close second. They
    are selling APUs for military use, basically to remove generators from
    sensitive locations. An additional product line is to replace APUs in
    trucks with battery storage.

    For example:
  13. amdx

    amdx Guest

    Ya, I did the calculations using my cost per KWh of 12.1 cents.
    We recently had a cost reduction of about 1 cent per KWh. Yea!
    I made the assumption you would use the air conditioner 2 hrs a day for
    6 months of the year.
    I got a yearly expense of $48.89.
    It's a shame electricity is so cheap, or not!
  14. amdx

    amdx Guest

    Build your bedroom in the form of this fridge.

  15. John S

    John S Guest

    Are you sure? Do you mean "power", or do you mean VA? Lots of VA at low
    RPM (except for losses), but less power. And, it doesn't last long. I
    doubt it takes more power to start than to run.
  16. mike

    mike Guest

    Yes, you're right. Takes more amps to start a compressor than to run it.
    I've never tried to measure the starting VA vs starting Power.
    And we've not discussed the potentially significant (power) factor that
    inverters are likely rated in power into a resistive load. Running
    a motor ain't resistive.

    But, your nitpicking is missing the point of the exercise.
    It's gonna take an inverter (system) with a LOT more peak capability
    to start a compressor than to run it. Matching what it says on the
    air conditioner name plate is likely to result in failure.

    An inverter that switches off to protect itself from the start transient
    or worse, lets the smoke out, isn't gonna be very useful in this,
    or any application.

  17. Almost all home sized synchronous inverters consume at least 150 percent
    of the value of the electricity sent through them.

    More at < >

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  18. mike

    mike Guest

    Can you explain how this works?
    I can understand how a breeze can cause evaporation cooling the contents
    as low as the dew point.
    But the article describes how the sun's rays cause the evaporation.
    Can't see how the "rays" do anything helpful?????
  19. Winston

    Winston Guest

    Consider an evaporative cooler.

  20. P E Schoen

    P E Schoen Guest

    "AA" wrote in message
    Check this out:

    He says the small A/C uses just 500-600 watts, so the 280W solar panel could
    produce about 1200-1500 W-Hr per day, and that's enough to run the A/C for
    2-3 hours.
    Or this:

    There were some other ideas:

    That's a 24VDC air conditioner running on about 20A, which is 480W. But 6000
    BTU/hr is about 1800W. It was probably measured with the compressor off or
    under minimum load. But it may be possible to replace the AC motor of an A/C
    unit with an efficient DC motor, eliminating most of the conversion losses.

    Here is a homemade swamp cooler:

    Here's a complete off-grid cabin with 12V appliances:

    You could use a Peltier effect cooler. Here is a 150 BTU/hr (12V 4A 48W)
    unit for $30 plus $13 shipping. Get 10 of them for 1500 BTU/hr. You might be
    able to combine these with a swamp cooler by thermally connecting the hot
    plate to the water. You pay for the difference in temperature, so with cold
    water you'll have a lot colder air (or better heat removal, actually) for
    the same cost.

    Actually you might be able to use a dorm size refrigerator or a 12V camping
    fridge and stick it in a window with the door open and a small fan running.

    Finally, it might be possible to get a car or truck air conditioning unit
    and drive the compressor with a DC motor.

    Sometimes you just gotta think outside the (ice) box! :)

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