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Is a "Heat Capacitor" Possible?

  1. Feb 27, 2015 #1
    Is it possible to make a device that could store all the heat put into it, until it builds up and amounts to something significant? It would basically be a capacitor for heat. I was thinking that since if it touched anything cooler than it, heat would drain out, so it would have to be in a vacuum. Is that correct? Also, would the heat energy escape as light faster than it could build up? I imagine that the source of the heat could be something like the Sun's rays.

    Again, this is only hypothetical and I just want to know why heat can't be contained and gradually built up (I'm assuming that it can't be).
     
  2. jcsd
  3. Feb 27, 2015 #2

    russ_watters

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    You'll have to be more specific. My fridge has an ice machine and my coffee mug is vacuum insulated. Do those fit your description?
     
  4. Feb 27, 2015 #3
    Does the coffee mug retain all of the thermal energy put into it? What I'm asking is whether a device that retains ALL(or nearly all) of the thermal energy(very minuscule) put into it is possible. Because that would mean that it would keep slowly building up, kinda like a capacitor, right? Is there an ultimate reason, that can't be overcome, that I have overlooked as to why this can't be possible?
     
  5. Feb 27, 2015 #4

    Drakkith

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    It's not possible. Increasing the temperature of an object above ambient will result in heat transfer from that object into the surrounding environment, either through conduction, convection, or thermal radiation. Insulation can reduce the rate of heat flow, but it cannot eliminate it completely. It's like trying to fill a bathtub that has a drain that can't be completely plugged. You can slow it down, but you can't stop it.
     
  6. Feb 27, 2015 #5

    russ_watters

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    Nearly all, yes. It keeps coffee hot for hours. We can make insulators as arbitrarily effective as we want. Never perfect, but pretty close. Note, normal electrical capacitors leak too.
    If you keep adding heat to it, yes.
    Every capacitor or other type of energy storage device has a limit to how much energy it can store. Here's my favorite example of a "heat capacitor" exceeding that limit:

     
  7. Feb 27, 2015 #6

    russ_watters

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    Depends on one's definition of "nearly all". :wink:
     
  8. Feb 27, 2015 #7
    OK, thank you very much. But is that still true if the insulation is a vacuum where there can't be any heat transfer? And once it emitts thermal radiation, it is significantly hotter
     
  9. Feb 27, 2015 #8
    Disregard my last comment completely xD

    Again, thank you for answering, my brain's been aching because of this for quite some time.
     
  10. Feb 27, 2015 #9

    russ_watters

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    I'm not sure I understand the relationship between those two sentences: there is always thermal radiation in a vaccum. It exists at any temperature (difference).
     
  11. Feb 27, 2015 #10
    OK, I didn't realize that thermal radiation is created in every temperature difference.. I thought that it is produced once a certain threshold is reached. Could the Sun's rays heat something faster than it would lose energy as thermal radiation(provided that it's completely insulated)? Would it be able to get very hot at the point when the input heat and the emitted thermal radiation balance out?
     
  12. Feb 27, 2015 #11

    Drakkith

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    Sure. There's a limit, but you can easily heat an object up to several hundred degrees just using everyday materials as insulation and a lens/mirror to focus sunlight onto it.
     
  13. Feb 28, 2015 #12
    Good point. I appreciate you guys helping me out, thanks a lot :)
     
  14. Feb 28, 2015 #13
  15. Feb 28, 2015 #14

    CWatters

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    Surprised nobody has mentioned a Thermal Store used in some heating systems. I have one in my house. Basically it's a tank of water. The water is heated by a boiler/furnace. Heat is taken out by a coil that circulates water between the store and the Under Floor Heating (UFH) system. It works a bit like the smoothing capacitor used to turn AC to DC.

    When lit my boiler burns oil at a fixed rate of say 25kW. However my UFH rarely needs that much. So the thermal store acts like a storage capacitor or buffer between the two. The UFH draws heat from the store at whatever rate it needs. That depends on how many room thermostats are calling for heat. When the temperature in the store falls below a pre-set minimum (say 45C) the boiler fires up and runs flat out until the store is at a pre-set maximum (say 80C).

    Using water to store the heat has some limitations. Water has a good but finite specific heat capacity and a low boiling point. This means that such a store needs to be quite big. Ours is 300L plus insulation. Some people use even bigger stores (1000L) to hold energy from solar panels. One alternative is to use a wax with a melting point of around 60C. Then you can also store energy in the latent heat of melting.

    In some countries electricity is cheaper at night (off peak). In those countries you can buy night storage heaters. They typically use cheap off peak electricity to heat up a block of insulated concrete or similar. Then in the daytime thermostat controlled fans are used to draw heat out of the concrete into the room.

    Edit: It occurs to me that a hot water bottle or bed warmer is also a form of heat capacitor. I dare say that stone age man may have used hot rocks for the same purpose.
     
    Last edited: Feb 28, 2015
  16. Feb 28, 2015 #15

    russ_watters

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    CWatters, in the US most people have domestic water heaters with tanks (my understanding is that in Europe most are tankless?), and I did mention them backhandedly (it's the video I posted). 300L is a pretty common size.
     
  17. Mar 1, 2015 #16

    davenn

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    in Australia and NZ we also have hot water heater tanks. Our one here is around 400 litres and is a significant proportion of our electric power bill :rolleyes:

    Dave
     
  18. Mar 1, 2015 #17

    CWatters

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    There is a small difference between a Thermal Store and a Hot Water Tank (at least in the UK).

    Usually a hot water tank takes in cold water that is heated and delivered to the taps/bath/shower.

    With a Thermal Store the water in the store is only used to hold energy, the water itself never leaves the system and it can be dosed with corrosion inhibitors. Cold water from the main supply flows through a heat exchanger I/on the store and onwards to the taps etc. Another heat exchanger delivers energy to the heating system.
     
  19. Mar 1, 2015 #18

    russ_watters

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    My understanding was in Europe they tend to use instant/pont of use water heaters.
     
  20. Mar 1, 2015 #19

    russ_watters

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    True.
     
  21. Mar 1, 2015 #20

    davenn

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    HI Russ
    we have those there too, there one in the kitchen of where I work, not 10 metres away from me
    I can hear it constantly doing its boiling cycles :smile:
     
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