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Reversible exothermic reaction

  1. Feb 5, 2010 #1
    hello guys.
    first, i must say that chemistry never was my beloved subject, that's why I don't know much about it, so please be patient. o.0

    well, i need a heat reservoir*. it should have high energy density and should not lose stored energy over time. the heat should be released on demand, but not all at once. the process needs to be reversible since the only energy source available is heat. i'm speaking of temperatures from 150 - 200 °C.

    here's an example (this is all i've found so far):
    http://en.wikipedia.org/wiki/Sodium_acetate" [Broken] (supersaturation). it would be heated with my heat source, and then cooled down. at the time i needed to release the heat stored, i'd just press teh button, getting around 280 kJ/kg of energy at 58°C.
    this is nice, but maybe there is a better solution. the problem is that the temperature is a bit low and it's a bit awkward once the reaction starts, since it cannot be stopped.
    a more elegant solution would be something like dynamic equilibrium - a mixture that would be heated, its components would separate, i'd store them and mix them on demand, rejecting heat.

    any ideas?

    * this is not a homework neither i want you to solve problems for me, i'm just asking for some ideas/guidelines
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 8, 2010 #2


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    Read Wiki's review on http://en.wikipedia.org/wiki/Phase_change_material" [Broken] for some ideas.
    Last edited by a moderator: May 4, 2017
  4. Feb 8, 2010 #3
    yeah, i've seen this, but this should be ridiculously well insulated to keep from cooling down in a matter of maybe days or weeks.
  5. Feb 8, 2010 #4


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    Trying to find out what PCM's PCM LatestTM 58 is?
    Why don't you ask PCM what it is?
  6. Feb 8, 2010 #5
    sorry, i don't get that.
    this post looks like written by a spambot.

    PCMs contain energy only when hot, but i can't provide enough insulation for that reservoir.
  7. Feb 8, 2010 #6


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    PCM's contain energy when a phase change is possible. The sodium acetate trihydrate example you mention can cool to fairly low temps and still release a suprising amount of the thermal energy when it crystallizes.

    You want something to stay hot without insulation? Not likely.

    Oh, I am no spambot!
  8. Feb 8, 2010 #7
    that's true, but you can cool sodium acetate down to room temperature and it will warm up when crystallized, and that you can't do with PCMs.
    i hope you know what i mean, but i can try to explain in some other way, ...

    yes, of course you're not a spambot. i hope i didn't insult you. :)
  9. Feb 13, 2010 #8
  10. Feb 13, 2010 #9


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    You'd like to cook water from silica gel, drierite, activated carbon, sodium sulfate, molecular sieves, or some other such, store it separately from that agent, and add it back on demand? Not a fixed temperature for a phase change. Various salt hydrates? Fixed, but generally "low" (for your purposes) temperatures.

    Molecular sieves might be something for you to play with a bit --- not too expensive, reusable, not messy --- you'll want to be a bit careful you don't start a fire adding water to them in plastic containers. Activated carbon is cheaper, messier, and same cautions apply.
  11. Feb 14, 2010 #10
    yes, that's what i mean.
    would you have an idea where could i get some more information on such reactions? or about thermochemistry in general?
    where could i get a list of reactions and/or standard enthalpies or something like that?

  12. Feb 14, 2010 #11


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    Let's see if we can put some constraints on the 5-10 years of work you've just outlined: 150-200 C phase changes aren't terribly prone to supercooling, so we'll delete searching enthalpies of fusion in that temperature range; molecular sieves and activated carbon are easily regenerated adsorbents which maintain their properties reasonably well from regeneration to regeneration (silica gels, drierite, and other classical dessicants don't); water is cheap and ubiquitous, so a regenerated adsorbent yields more heat per portable unit mass than hydrates (you don't have to haul the water around).

    Can you nail down some other parameters? Minimum quantity of heat, maximum mass, minimum and maximum temperatures?
  13. Feb 15, 2010 #12
    huh, i didn't know there's so much work to be spent on this topic.
    why is temperature such a problem? i thought of that more like a combustion reaction - the temperature increases regardless of initial temperature. looks like that's not the case. :(

    the context of the problem is the following:
    i'm developing a compressed air car and i need to store huge amounts of energy into a small reservoir. increasing pressure is one way to do that, but the relation is logarithmic, so above 450 bar or so, nothing significant is gained. however, the energy-temperature relation is linear, which means increasing the temperature will always result in a lot of additional energy.
    but: combustion is not an option, since there really is not point in designing a zero-emissions vehicle which is not that.
    the idea of reversible reaction is that there is a lot of heat available while compressing the air and it would be really cool if that heat could be stored and released during the expansion of the air.

    actually, 150°C is not the minimum temperature - anything above 100°C would be great, but keep in mind that every degree higher saves a lot of space and pressure... :)
    there is no minimum temperature, this reservoir could be cooled down to the temperature of the surrounding air or even lower. also, with the right compressor, the heat source temperature could be high enough for just about anything.

    the amount of heat we're speaking of is, say (that's an approximate again) 30-50 MJ.
    I cannot calculate that precisely until i know what the maximum temperature will be.

    so, again: i just need something that would store some heat and release it at the highest possible temperature and that process should go on for a number of times.

    there is a topic about the car: https://www.physicsforums.com/showthread.php?t=310462
  14. Feb 15, 2010 #13


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    Phase changes take place at "fixed" temperatures (can push 'em around by changing pressure, but it ain't practical). Supercool a salt hydrate, nucleate it by shaking, and the bag "warms up" to its freezing temperature while giving off the enthalpy associated with the phase change. Reversing the process, the hydrate takes up heat at that same temperature until it's all melted, then starts warming up at whatever rate its heat capacity dictates.

    Hydrocarbon fuels yield around 40 MJ/kg, cornahol around 25 MJ/kg (noticeable loss in engine efficiencies), and the TV ad is "43 ton miles per gallon of fuel" for whichever railroad is going to be 2kJ/kgkm --- and your range is 300 km, so break-even energy density is 600kJ/kg for your storage, and to be worth dragging around, you're going to want more in the neighborhood of 2-3 times that, is getting into the neighborhood where we boil water and condense steam.

    Save the waste heat from the air compressor and use it to boost performance later? Great idea --- unfortunately, it takes more energy to haul the mass of the storage media around than can actually be stored.
  15. Feb 16, 2010 #14
    "not possible" is not the correct answer.
    i can store 100 kg of stuff with no problem. sodium acetate can store up to 289 kJ/kg, which is almost 30 MJ.
    i know that the burning fuel numbers are an order of magnitude higher, but you don't have to worry about that.

    anyway, thanks for your help.
  16. Mar 22, 2010 #15
    so, after a lot of searching and reading:
    i could use zeolites, that's here:
    the problem is that i cannot find any useful information on these (at least not for me) - there's only a lot of some kind of coordinates of atoms and two theta angles. i simply need heat of sorption and some temperatures that are happening in the process.
    would you know where could i get those? i'm not sure i'm searching with the right keywords.

    the second option is metal oxides, like MgO. the problem here is the same, i can't find any useful data on energies; and temperature of decomposition is a little high.
    can i calculate how much energy can i get from this reaction
    MgO + H2O -> Mg(OH)2
    using standard enthalpy change of formation for each of these substances?
    btw., i didn't find those either.
    are there any similar metal oxides that decompose at lower temperatures? 130°C is the ideal temperature...
  17. Sep 7, 2010 #16
    Well, after a few months of searching, I've come across these two patents:

    Unbelievably, these seem suitable.

    I would like somebody with some knowledge on this topic to confirm that these numbers are true; there are still new patents describing perpetual motion machines in the 21st century, so I somehow can't rely on these documents.

    And next, I want to know more about these inventions. What's the procedure to get to know these patents better?

  18. Sep 8, 2010 #17


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    Glauber's salt can be used for the same purposes.
  19. Sep 8, 2010 #18


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    Do some more research. Look at the author and see where that leads.

    Stop when you get to Wally Minto!
  20. Sep 9, 2010 #19
    Yeah, I guess you're right.
    I hoped there is a better way - this thing is from 1981... :)
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