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Building a portable AC for my car

  1. Jun 30, 2016 #1
    Hello everyone. This is a very simple project that I intend to build. It involves a cooler full of ice and water, two 120mm computer fans, a 240mm computer radiator and a 600 gallons per hour water pump. All these will run off the cars cigarette lighter rated at 12 Volts. I need your help to determine how to wire all of this up.

    Pump 12v rated at 3 Amps
    Fans 7-12V rated at 0.5 Amps

    I'm guessing i need to wire this up in parallel? And if the pump is running too fast and splashing water everywhere can I add a resistor to slow it down? Let me know thanks!
     
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  3. Jun 30, 2016 #2

    sophiecentaur

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    If you have a good sized heat exchanger (say an old car heater matrix), then there will probably be enough circulation (thermo-syphon - look it up) if you just connect it directly to your ice tank (a big hose top to top and a big hose bottom to bottom. Your problem here will probably be that a useful AC system uses a lot of power (total energy to shift = power times time) - which corresponds to a lot of Ice to carry for a long journey or to cool down a car, initially. Be prepared to carry an awful lot (gallons and gallons) of iced water.
    It could be an interesting experiment in proving that the numbers count in engineering.
     
  4. Jun 30, 2016 #3
    I don't think thermosiphoning is going to work for my application. This is meant to be done on the cheap <$100. And yes I only expect it last for an hour maximum. Those are my longest journeys in the car anyway. I need help wiring it as I am unsure how I should have it wired and if I need a fuse or not.

    To give you an idea, the water pump will sit in the bottom of the cooler, I will cut out an opening in the lid for the radiator to be glued in and the fans will sit on top of the lid where the radiator is integrated. The cooler holds about 3 gallons.
     
    Last edited: Jun 30, 2016
  5. Jun 30, 2016 #4

    sophiecentaur

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    I can't think of a cheaper system than a scrap radiator and its hoses and some tape with (cheap plastic) plumbing fittings top and bottom of your cooler tank. A pump involves another electric motor and another pair of waterproof joints.
    I am wondering just how much heat transfer your computer radiator is designed to transfer with a temperature difference of 20C. Does it have adequate total area to make a difference to the volume of air in a car? I imagine there will be a spec somewhere to give you some idea. But remember, a computer probably dissipates around 100W, whilst the windows of a car will be admitting several hundred Watts and the initial heat content of the vehicle must be significant (i.e. for initial cooling down)
    How many kW of power do you think a regular AC unit produces? (I just don't want you to be disappointed if the performance of your system is too low.
     
  6. Jun 30, 2016 #5

    sophiecentaur

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    And YES!!!! you must have a fuse, right next to where you take the power off. Changing that fuse could be a lot easier than getting under the bonnet (hood) to change the fuse in the car's electrics. :smile:
     
  7. Jun 30, 2016 #6
    Yeah, I understand what you mean. The radiator is 11x6 inches. The two fans will cover than entire area. And the water pump is submersible. Literally just going to take vinyl tubing and hose clamp. It's not like its blasting 2000psi through the lines. It'll work. But say the car is 90F and id like it to be near 75F in the car. Cooler can only hold 3 gallons of ice water. This radiator that was in my computer on full load on the cpu was about 45F. While the regular heatsink and fan was near 65F on full load. Also the car is very small. A vw golf.
     
  8. Jun 30, 2016 #7

    sophiecentaur

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    You have clearly given this some thought, which is good. However, it is the Heat (Joules) and not temperature that counts when you want to cool something.

    Where did you plan to put the radiator? A pump would give you more flexibility in the position of the Cold Sink than a thermo-syphon.
     
  9. Jun 30, 2016 #8
    See the attached picture. That's exactly what I'm imagining. Minus the battery because those pos and neg will be connected to the 12v adapter for the cigarette. My cooler is a bit larger than that one. And oops correction: the rad on the cpu was 45C and heatsink with fan was 65C
     

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    Last edited: Jun 30, 2016
  10. Jun 30, 2016 #9
    Just bought an 8A fuse, that should be sufficient as I don't see the system going over 4 amps. Plus car fuse on the 12v line is 10A or 15A i think.
     
  11. Jun 30, 2016 #10

    rbelli1

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    This is all well and good but a typical cooing system in a car will be the equivalent of melting several thousand pounds of ice per hour. Your cooler will only hold several tens of pounds of ice. Your order of magnitude is off by 2.

    You have to realize you are driving around an uninsulated greenhouse. The typical calculations for room size assume that you have a well insulated space with only a small percent of windows.

    BoB
     
  12. Jul 1, 2016 #11

    billy_joule

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    I agree. For similar time and cost to this ice contraption you could probably find a complete AC unit at an auto dismantler and bolt it straight in to your car.
     
  13. Jul 1, 2016 #12

    sophiecentaur

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    Hmmm. That's a bit of an overstatement, isn't it? Where would you fit the front heat exchanger without getting all new radiator fittings and is there a suitable fixing bracket for the pump?
    However, I do agree that the project needs many more sums done on it than the OP has done. Auto AC involves a lot of Power (from the engine) and there is loads to spare (you pay in mpg but no one cares about that). As BoB says, you could be out by two orders of magnitude. These sort of projects do not work on the strength of enthusiasm but on hard Engineering figures.
    PS, if your ambient humidity is low, a system based on a Swamp Cooler could be worth considering. Blowing air over a wet sponge will utilise the Latent Heat of Vaporisation, which is much greater than the Latent Heat of Melting.
     
    Last edited: Jul 1, 2016
  14. Jul 1, 2016 #13

    billy_joule

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    If a higher spec model came with AC then lower spec models generally will have all the mounts and space etc required, down to the unused plugs on the loom and the blank in the dash where the ac button fits, every thing just bolts straight on. At least that's my experience on a 97 corolla.
     
  15. Jul 1, 2016 #14
    Well, I suppose you're all correct. I have limited knowledge of thermodynamics which is why I consulted you guys haha. Retro fitting an AC in my car is way too much work and expensive $$$. I've already looked into it so that's a no no. I appreciate all of your responses. I'm going to proceed with this project, purely out of curiosity now as it mostly likely will not meet my expectations anyway but i will let you know how it works in the next coming days!!
     
  16. Jul 1, 2016 #15

    NascentOxygen

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    As a compromise, maybe water-cool a cushioning backrest on the driver's seat?
     
  17. Jul 1, 2016 #16

    jim hardy

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    I like your spirit
    keep us posted

    Well....

    a car air conditioner is probably between 10,000 and 20,000 BTU's per hour
    144 BTU's will melt one pound of ice

    so you can estimate how many pounds of ice you'd need to provide that much cooling for the duration of the trip you have in mind.
     
  18. Jul 1, 2016 #17

    billy_joule

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    Did you look at self help auto wreckers? I got an entire AC unit, every thing required, for 76 NZD, less what the parts listed in the OP would cost me.
     
  19. Jul 1, 2016 #18

    OmCheeto

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    From the comments, it sounds like iduhfuse is my next door neighbor, and needs A/C about once a year.
    That's generally how often I need it.
    It strikes me as a bit silly, to spend hundreds of dollars, and countless hours, on something, that you need, once a year.

    Personally, I've traditionally gone for the most scientifically & economically logical solution.

    Though, I've recently been investing in all of the hardware iduhfuse seems to have collected, and am interested in the outcome of the experiment.

    Yes.
     
  20. Jul 2, 2016 #19

    Tom.G

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    If you can direct the cool fan exhaust directly on the human cargo, it could be almost practical (for short trips).:smile:
     
  21. Jul 3, 2016 #20

    OmCheeto

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    And for that reason, I will be building one this afternoon.
    Weather permitting, of course. The morning clouds are predicted to burn off my 2 pm.
    And my vehicle is black! This should be a good test.

    I've also done some of the preliminary thermodynamic calculations.
    3 gallons of ice water raised to 65°F will cool 896 m^3 of air from 90°F to 75°F
    896 m^3 is equivalent to 338 interior volumes of a 2015 VW Golf
    Unknowns:
    • thermal heat capacity of the vehicle interior
    • thermal transfer rate into the vehicle
    • interior volume of my truck (2009 Ford Ranger, standard cab. Just in case anyone knows this number off the top of their head.)
    • whether or not the little radiator from the junk yard leaks or not
    • other things I'm probably forgetting


    ps. I don't think I'll be needing it this year, as my only remaining scheduled trip of 60 minutes or more is in 4 days, and the forecast calls rain. But it's a miserable drive when it's hot out, and I've got all this junk lying around to make it. So my total cost will be about 2 hours, 2 bags of ice, and some water to top off the cooler.
     
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