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Author Topic: Could paraffin wax be used as working fluid in a heat pump?  (Read 8985 times)

Offline peppercorn

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Paraffin wax - as a solid, has a very high heat capacity.

It will melt at 62degC.
In liquid state it could be circulated from a hot source (say 380degC) to a low heat destination (say 80degC)

I believe it boils at 320degC in ambient conditions, but in a closed system could be superheated.

It might need to be pumped slowly as I assume it takes time to absorb & give up heat.

Obviously a wax with similar properties, but non-flammable would be better...


-> Additionally:
If the cold end temperature was below the melting point then latent heat could play a part in the process. Of course half the cycle would need to pump solid wax, but a grease gun can do this...


So, would 'wax' be a good medium to transport heat?


 

Offline daveshorts

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Could paraffin wax be used as working fluid in a heat pump?
« Reply #1 on: 19/09/2008 11:31:56 »
It has been used to power submarine gliders - its expansion and contraction has been used to float and sink and therefore power a gliding submarine.
http://www.thenakedscientists.com/HTML/content/news/news/1302/

However for it to transport heat you have to move it, and if it solidifies it would be hard to pump.
 

Offline peppercorn

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Could paraffin wax be used as working fluid in a heat pump?
« Reply #2 on: 19/09/2008 12:33:39 »
Thanks for that link Dave.
I believe Wax is also used in some day-to-night solar storage systems.

I was thinking that part of the attraction of using a wax is that low value heat that is normally wasted in the exhaust of standard heat engines could be  employed to initiate the solid-to-liquid state change allowing an even lower final exhaust temp.  Further along the heat transport wax system a second thermal energy gain could result as the paraffin changes from liquid to gas.

I would also hazard a guess that waxes heated to just under their melting point are substantially less viscous than ambient.
 

Offline daveshorts

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Could paraffin wax be used as working fluid in a heat pump?
« Reply #3 on: 20/09/2008 09:01:43 »
Oh sorry, I wan't concentrating, you mean in the gas phase... A problem I could see is that because a wax molecule is long and able to wriggle, it has a lot of degrees of freedom.

The specific heat capacity per mole of a gas is
1/2*R*Number of degrees of freedom - where R is a constant
so an ideal gas is 3/2 RT as it can move in x,y, and z directions
A diatomic gas like oxygen at normal temperatures is 5/2 RT as it can rotate in two independent directions

Something like a wax will be a lot higher as it can wriggle. I think this is a bad thing for a working fluid in a heat engine, but a good thing for one used to transfer heat.

The other problem you may run into is the wax thermally decomposing and causing all sorts of havoc chemically


 

Offline peppercorn

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Could paraffin wax be used as working fluid in a heat pump?
« Reply #4 on: 24/09/2008 11:05:05 »
Thanks Dave.
Yes, I hadn't considered that the wax could decompose at higher temperatures - that does seem quite likely.

I suppose I was just intrigued by the idea of working fluid that could make use of both phase change events (solid-liquid & liquid-gas).

On the up side there would be no shortage of lubricant in such a system!!
;)
 

Offline Bored chemist

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Could paraffin wax be used as working fluid in a heat pump?
« Reply #5 on: 24/09/2008 19:57:41 »
According to this
http://en.wikipedia.org/wiki/Specific_heat
parafin wax has a lower heat capacity (by weight) than water. Since it's also less dense the heat capacity by volume would be even worse still.
Why use it?
 

Offline peppercorn

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Could paraffin wax be used as working fluid in a heat pump?
« Reply #6 on: 24/09/2008 20:16:40 »
parafin wax has a lower heat capacity than water.

Thanks , that answers my question!!
I had assumed it had a higher SHC due to its use in solar storage systems, but I suspect that has more to do with state change...

Stay tuned for more insane ideas.... :)
 

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Could paraffin wax be used as working fluid in a heat pump?
« Reply #6 on: 24/09/2008 20:16:40 »

 

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