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Heat capture and concentration.

Quarter Baked and looking to the halfbakery for help.
 
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Ok, we all know how difficult it is to capture heat from low temperature and low density sources (Solar, Geothermal, industrial waste heat) efficiently.

My idea is that instead of using water as a heat transfer medium, which will cool and cannot be stored for a long time or be concentrated use a chemical reaction instead.

For instance, heat is transferred to a gas circulating through pipes, radiator style.

This gas then decomposes into different components, having absorbed energy. These components are then separated and removed to prevent back reaction, accumulated, concentrated and then re reacted in some sort of power plant (possibly a fuel cell, or combustion). The gas is then collected and returned to the heat source.

My first thought was ammonia, NH3 splits into N2 and H2 after absorbing energy, they are then recombined to release energy. However, the energy barriers in both directions are too high.

My second thought was N202, which splits to form NO2, quite easily. However, as there is only one product, there would be no way to prevent back reaction, the mixture would essentially 'cool', losing energy.

My third, carbon dioxide decomposes into carbon monoxide and oxygen; the carbon monoxide could be stored and burnt. Once again, co2 is far too stable to decompose when exposed to these low density heat sources.

So I turn to the halfbakery community to help me find a gas (or alternative), which will readily absorb energy and decompose into heterogeneous products which can be separated easily. Preferably, it should be cheap and non toxic too...

Domser, Feb 05 2009

Einstein Szilard Refrigerator http://en.wikipedia...nstein_refrigerator
Refrigerator did some Baking on this idea, otherwise worked... [loonquawl, Feb 05 2009]

Belousov-Zhaobtinski Reaction http://en.wikipedia...habotinsky_reaction
I'm not sure about the heat potential, but if you're thinking about reactions that oscilate between two semi-stable points, it's worth having a look at this... [zen_tom, Feb 05 2009]

Briggs Rauscher Reaction http://en.wikipedia...s-Rauscher_reaction
...and this - as examples of chemical reactions that are famous for their fickle nature. [zen_tom, Feb 05 2009]

Mag Sulphate http://en.wikipedia...i/Magnesium_sulfate
[afinehowdoyoudo, Feb 06 2009]

Various dissociation reactions for heat storage, considered http://www.ecn.nl/p...x?nr=ECN-RX--06-017
[afinehowdoyoudo, Feb 06 2009]

[link]






       There exist photocells (currently under development) designed to directly split water into H2 and O2, without using an intermediate electricity stage.   

       Of course, this isn't capturing the *heat*, so it's not quite the same thing.
goldbb, Feb 05 2009
  

       do something mildly useful with it, like (in the case of a mild temperature differential) pre-heat a hot-water tank or do that "long garden hose under the lawn" thing which I still don't understand how that's supposed to be preferable to a single large tank.
FlyingToaster, Feb 05 2009
  

       Make anhydrous mag sulphate: heat epsom salts to dehydrate, rehydrate them to recover heat.   

       MgSO4*7(H2O) + heat -> MgSO4 + 7H2O   

       It is an example of a dissociation reaction. Soaks up gobs of heat
afinehowdoyoudo, Feb 06 2009
  

       Flying toaster, I'm thinking on a slightly larger scale than hot water tanks and garden hoses. For instance, Geothermal plants which heat water directly to produce power are only ten percent efficient. If the heat could be used to cause a chemical reaction, the products could be concentrated and used to generate power more efficiently.   

       Iron Horse, would the water driven off the Epsom salts be gaseous? Because that would be a fair waste of energy.   

       Thanks for the links everyone.   

       And why am I getting boned for this ?
Domser, Feb 06 2009
  

       Yes, the latent heat of water vapor is a loss, but it is less than what is retained in the anhydrous... just the way it goes.   

       //And why.. ?// Boning is what we do. If you find it uncomfortable my only advice is relax.. and bend over a little more
afinehowdoyoudo, Feb 06 2009
  

       //And why am I getting boned for this ?//   

       So far, I see only two bones. One of those probably comes from the legendary auto-boner who (according to folklore) bones virtually everything. So only one person out there actually dislikes your idea. It could be a lot worse. ;)
pertinax, Feb 07 2009
  

       //Auto-boner//   

       What an awesome name for a brand of Viagra...   

       Iron Horse (Ironically, another appropriate Viagra brand-name) Do you know what the energy retained in the anhydrous magnesium sulfate is?
Domser, Feb 07 2009
  

       You would be better off using a substance which separates into a liquid and a gas; you would be very unlikely to be able to separate two gaseous products easily. Maybe H2S2O7(l) <--> H2SO4(l) + SO3(g).
spidermother, Feb 07 2009
  

       The most efficient storage medium I've seen in graphite, which holds enormous amounts of heat energy.   

       A series of thermal reactions could be used to create Hydrogen (the Sulfur-Iodine cycle for example) but these require temperatures of 500-1000C and are usually less than 50% efficient.
marklar, Feb 07 2009
  

       Spidermother, you're on the right track there, and separating a liquid into a liquid and gas would be ideal. However, we're talking about pretty dangerous substances being used and re-used as part of a fuel cycle. Could this process be made safe and environmentally friendly ?   

       Marklar, the problem with the graphite is that it is not a long term energy store. The Graphite would simply cool in the same way as water does, though I do grant that it would hold more energy for longer.   

       Also, the 500-1000 C is a little too hot and a series of reactions is not desirable.   

       Spidermother, at what temperature does Oleum dissociate ?
Domser, Feb 07 2009
  

       [Domser], it is 2.8 GJ/m^3. Compared to water, thats about 14 times the energy storage per volume (assuming a 50 degree C temperature swing in the water, between 'fully charged' and 'expended' states.. e.g 20C & 70C)
afinehowdoyoudo, Feb 08 2009
  

       I don't know the specifics of oleum's dissociation; it was simply the first example of a reversible, highly exo/endothermic reaction I could think of. Hopefully there are others that involve less unpleasant substances.
spidermother, Feb 08 2009
  

       How about using substances which dissolve in water? Especially those whose solubility rapidly increases with tempreature. The Ions could be seperated, concentrated and re-reacted, couldn't they?
Domser, Feb 11 2009
  
      
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