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Phase change material steady melt temp, like ice water?

  1. Sep 24, 2016 #1
    Ice melting in a box has the water coming off it at a temp just above freezing until all the ice is gone.

    Are all phase change materials pretty much the same, with their immediate containment container exterior staying pretty much just above their phase change temp until all the PCM inside has melted? Or, are there additional considerations with PCM, like that it does not conduct internally as well and thus needs to be in flat thinner panels rather than just a big barrel of it?

    I'm asking because I'll have a 65 gallon tank of hydroponic nutrient solution inside a very well insulated box (4" freezer panels) starting off at an ideal temp of 60F degrees. The nutrient fluid comes and goes via pipes and is cooled back down to a little less than 60F before returning to keep tank contents at 60F.

    I have just enough room in that box for another identical 65 gallon tank that could be filled with a phase change material to buffer temp inside box in case I ever had exterior temps exceed my ability to chill returning fluid sufficiently. That excess heat would be from both externally rising ambient temps conducting into the box and also the fluid coming back into box much hotter than usual, too.

    Would the best strategy be to have the 65 gallons of PCM, located in box next to nutrient 65 gallon tank, be designed to melt and absorb heat at, lets say, 61F, thus tempering and slowing down additional excess heat gain in nutrient tank during those extra hot periods that exceeded my usual ability to keep a cap on nutrient tank temps rising via my chilling incoming fluid?

    What am I missing here or additional considerations to address to maximize efficiency of PCM helping maintain desired 60F temp?

    Thank you for any comments.
     
  2. jcsd
  3. Sep 24, 2016 #2

    Baluncore

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    If you can arrange to cool the PCM to say a couple of degrees below the phase change temperature, then you can use the PCM to cool all your returning solution. Failure of the cooling will then allow the PCM to maintain low temperatures without having to change the solution flow path. Consider using a coiled tube in the top of the PCM tank to chill the bulk PCM, with a separate coiled tube in the bottom of the PCM tank that cools the solution as it returns to the reservoir.

    Have you identified a 61° PCM yet? Do solids float or sink, what is the density of the solid and of the liquid phases? Some PCMs have varying composition and so, unlike pure water, do not have a narrow phase change temperature. For example, fats and waxes will have transitions over about 10° or more depending on molecular weight distribution and chemistry. Some compounds may change chemically over time and so change transition temperature.

    You are using the solid to liquid phase change so your heat sink will be into the solid phase. Solids obstruct thermal siphoning and so you need a coiled tube passing through the PCM in the tank. You need to consider the forces due to freeze-thaw cycles that might damage the heat exchange coils or the reservoir.

    Another possibility would be to fill lengths of flat rubber tube, or “hot-water-bottles”, with PCM, then immerse them in the solution reservoir.
     
  4. Sep 24, 2016 #3
    Balumcore,
    Yes, of course, running my return line through the PCM would be very efficient, thank you for that.

    Also, I can easily bypass it or not right there, always like retaining options in testing new systems.

    I'd found a 59F PCM here...
    http://www.rgees.com/products.php
    ...and am talking with them, but also looking and learning from others, too, like at...
    http://www.puretemp.com/

    - Shane
     
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