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Thread: Thermal value

  1. #1 Thermal value 
    Forum Professor marcusclayman's Avatar
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    I need some direction. Please excuse my lack of understanding on the subject.

    I am not preoccupied with the efficiency of this project, it's just an idea that I'd like to do some research on and draw up some designs.

    The idea is this, cooling using Ice from winter, stored in a thermal building so that the ice lasts throughout the summer.

    To state the obvious variables

    x temperatures outside
    y thermal value of the building
    z thermal mass of the ice

    is this correct? are there other things I need to take into account?

    Now I'm assuming that the most efficient shape is a sphere, and for the sake of this thought experiment will be using a thermal dome for the building.

    The idea is this.

    During the winter, a big block of ice is manufactured along side of a water source. The block is made as big as possible and shaped roughly as a dome. As spring approaches, the block is made into the shape of a dome. A thermal dome is then built around the pile of ice.

    for those who don't know how to make a thermal dome
    1: spray closed cell Styrofoam around the form, in this case, on the outside of the ice
    2: stick steel re-bar into the Styrofoam
    3: spray cement onto the steel re-bar and Styrofoam

    A vent would be needed, or a vacuum would be created as the ice melts. BUT a vacuum keeps things cooler right? If so, pressure of a certain amount could be tolerated.

    I want to figure out how long the ice would last.

    Where should I start?


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  3. #2  
    Forum Isotope Bunbury's Avatar
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    Presumably you don't want to just sit there and watch it melt; you want to do something useful with it - right? So the place to start is to decide what the purpose of your giant ice block is. To do anything useful you will have to exchange heat with some other medium - maybe air if you want to cool a building like the early days of air conditioning, or maybe run water pipes through it to produce cold water, or maybe just store food. The rate of melting will depend on what you are exchanging heat with.

    Once you've decide what you're going to do with it, the heat transfer through your well-insulated dome will probably be much smaller than the heat exchange with your process whatever it is. If it isn't you are pretty much wasting your time.


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  4. #3  
    Forum Professor marcusclayman's Avatar
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    I don't have anything planned for this. I just want to learn about thermal dynamics, and investigate whether or not there is a practical use of this.

    Whether it is to run pipes through for cooling, which I doubt would be worth it; or if it's merely to use as a giant walk in refrigerator/freezer.
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  5. #4  
    Forum Isotope Bunbury's Avatar
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    Then you would start by calculating the thermal resistance of the wall of your building, which is the sum of resistances of the styrofoam and the concrete, The rersistance is calculated from the thickness divided by the thermal conductivity. You should correct for the spherical shape, but id the radius is large you can treat it as a flat wall. Then add in a film resistance for air outside the dome.

    Then it starts to get complicated because you now have to look at the mass of ice itself. If it is really cold then you can warm it up without melting it until its surface reaches 0 deg C and it starts to melt. You will have to involve the Biot number for the mass of ice, and if ther ice is in close contact with th dome you should include the dome in your Biot calculation. I''m sure you can google how to do this.

    Then it gets really complicated because once the ice starts to melt you will have an air space between the ice mass and the dome, and you now have to consider the film resitances between the doem and air and the ice and air. The melt water will increase the rate of heat transfer, while the mass of ice decreases so your Biot number is no longer a constant.

    Since this is a transient case, not steady state, you would probably do best to model it in a spreadsheet with a little convergence routine. I did this once for an autoclave sublimating uranium hexafluoride.

    Ice storage is not a new idea by the way. There are ice houses in most US cities dating from the days before mechanical refrigeration.

    http://query.nytimes.com/mem/archive...669D946797D6CF

    Maybe you should start with a simpler steady state model if you are just trying to learn some heat transfer.
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  6. #5  
    Forum Professor marcusclayman's Avatar
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    Indeed, thank you, a little direction goes a long way when you don't know where to start.

    I got stuck trying to figure out how to "reciprocate" to determine R value, or U value, don't remember which.

    I guess it would be easier to find a calculator, and figure out what it's doing.



    Yeah, something similar would be better to start with.
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  7. #6  
    Forum Isotope Bunbury's Avatar
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    Resistances in series are simply additive (as in electrical analogy). Once you've added up all your resistances the U value is the reciprocal.
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  8. #7  
    Forum Professor marcusclayman's Avatar
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    I got that far, but in trying to figure out what "reciprocal" means, I found dozens of things, a few of which were mathematical, all of which were beyond my comprehension.
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  9. #8  
    Forum Isotope Bunbury's Avatar
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    R = r1 + r2 + r3 + r4

    U = 1/R

    1/R is the reciprocal of R

    There is most likely a 1/x function on your calculator.
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  10. #9  
    Forum Professor marcusclayman's Avatar
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    Ah, that's simple enough. I can do that in my head, lol. The formula I found was much more complicated than that.
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