Solar Powered Freezer

Discussion in 'Alternative Energy' started by JAK, Jul 9, 2006.

  1. JAK

    JAK Well-Known Member

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    I have this crazy idea for a weekend camp. It is mostly a mental exercise to try and better understand solar power and refrigeration. The idea is to use solar power to freeze ice in a chest freezer while you are away, and then when you show up for the weekend with you usual cooler full of food you use ice from the freezer for your cooler, and put frozen stuff from the cooler into the freezer. Also, if the freezer also has some plastic containers of frozen brine then you can shut the freezer down for the weekend to use the solar power for other stuff. The idea is to reduce the size of your battery bank and still fully utilize the solar panels even when you are away. I would have a rainwater collection system and a shallow spring so I would use manual power for all the water systems. So when away the only work for the panels would be to charge batteries and freeze ice and brine.

    For example, with an 8 cuft freezer if I had it filled while I was away with 4 cu ft of brine with a freezing point of 10F it would store 20,000 BTU of energy as latent heat, or about 6 kwh, or about 500 ah at 12v. If I also gave it 4 cu ft of water to freeze it would freeze that first, which would be another 30,000 BTU, or about 9 kwh, for a total of 15kwh. Such a freezer would normally use about 600 watt-hrs per day, so if the solar panels averaged 1000 watt-hrs per day (200w module in summer) then there would be enough work while away to keep the solar module busy for about 5 weeks. Conversely, there would be enough heat capacity to keep the freezer at 10F for 2 weeks and at 0F for another 3 weeks. Of course this would be reduced by half if the freezer was half full of food.

    Questions:
    Are batteries even needed? Would it be possible to hook the solar panels directly to the freezer so that simply ran the compressor whenever there was enough sun? Would there be some energy savings by bypassing the batteries? Would you burn out the freezer motor and compressor by running it 24/7? What is the best way to maximize the utilization of the solar panels by having it charge batteries at the same time as it runs a freezer? Also, if you buy the freezer, can you set the temperature to 35F and run it as a fridge?

    200w panel = $1000
    8cuft freezer = $1000
    Batteries, Inverter, Charge Controller ???

    http://www.sundanzer.com/
     
  2. Guy_Incognito

    Guy_Incognito Well-Known Member

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    Should be possible to do.

    If you can get a hold of a 12V compressor freezer, it'd be a lot easier - you could simply run it off the panels with a maximiser - with the panels sized right, you'd probably get 6-8 hours a day of continuous running if needed. It'd have to be a true basic 12V compressor, not the ones that are driven by electronics. The electronic ones probably wouldn't appreciate the maximiser.

    You'd still need the thermostat - bad things might happen to your compressor if you run the freezer to far below it's design temp.

    If you were using an inverter + charge controller + batteries, I'd probably go for something like this :

    - Hook panels to batteries via charge controller.
    - Hook inverter to freezer and inverter to batteries via a relay.
    - Get a timer to switch the relay on for a set period of time when it gets a "full battery" signal from your charge controller.

    That way, the freezer would only run after the batteries reach full charge. You could work out the run-time available by how much you want to flatten the batteries and how many panels you have,etc. This setup would stop your freezer wrecking the batteries at night or during extended cloudy periods.

    And yes, you can tinker with the thermostat on your chest freezer and turn it into a fridge. They're very efficient fridges, due to a heap of insulation and the fact that the cold air doesn't drop out of them when you open them.
     

  3. Jim-mi

    Jim-mi Well-Known Member

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    The SunDanzer semses wether its 12 or 24vdc
     
  4. JAK

    JAK Well-Known Member

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    I noticed also the sundanzer compressor can handle up to 17v, so it seems to be designed specifically to work off a solar panel and batteries. The compressor is a danfoss, which I understand is the guts of similar DC fridges and freezers.
    http://www.sundanzer.com/prodinfo.htm

    Of course I would probably still get batteries and an inverter for other loads, but it was interesting in theory to figure out how much energy could be stored in a freezer. The idea might work better with a wind turbine. With a savonious rotor the compressor could even be driven directly off the rotor.
     
  5. Jim-mi

    Jim-mi Well-Known Member

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    Not the best of ideas---wind direct.

    The compressor would cycle on and off far too many times as the wind speed comes and goes.
    A battery is needed.

    If a savonious rotor was more than a tinker's toy than one would, by now, be commerically available.
     
  6. blue gecko

    blue gecko Well-Known Member

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    Around here if you left something like that at a week-end camp someone would "borrow" it (and anything else not nailed down) while you were gone during the week!
     
  7. JAK

    JAK Well-Known Member

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    I think savonious rotors are efficient enough and are not too bulky up to about 1000w, but suffer primarily from a very slow rotation speed and the fact that they are impractical to raise up off the ground where the wind is. Still, I think it would be neat to have one on the peak of a round roofed barn, with a central shaft for pumping water and other stuff, like running a compressor.

    As for the compressor cycling on and off, I am not sure that would be a problem if you had a lot of heat capacity built in. For example, if you had a small 1000w savonius, but only got a capacity factor of 0.10 in summer and 0.20 in winter because of its low height. In the extreme case you might get 24kwh every 20 days in summer and every 10 days in winter. If you had a COP on the compressor of 2.0 you would need a heat capacity in your freezer of about 32,000 BTU, which would be 7 cubic feet of brine with a freezing point of about 10F, or about 3.5 cubic feet of spring water ice. Hey, a Savonious Icemaker. Who wouldn't want one? :)

    Not to useful in winter I suppose. In winter perhaps it could be a heat pump.
     
  8. WisJim

    WisJim Well-Known Member Supporter

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    The problem with short cycling is that it would wear out the compressor much sooner than normal operation. It could be starting and stopping dozens of times an hour, or more, if direct connected.

    Unless you are using the electricity to provide resistance heat, you need a battery to help regulate the voltage in a wind generator or PV system.
     
  9. JAK

    JAK Well-Known Member

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    I see what you mean now. I think that would be particularly true near the ground where you rely on gusts for a lot of the energy.