Anyone tried using a thermal reservoir?

[Charlie]

I remember from a university heat and mass transfer module that the thermal conductivities associated with phase changes are about 10x higher than for sensible heating. Has anyone tried constructing a hollow heat sink filled with material that melts at a moderate temperature (i.e. beeswax melts at about 52c) as a way of cramming more output (briefly ) into a small space?

Just an idea that has been bugging me, thought I would let it off the leash for comments/suggestions/withering scorn. :tinfoil:

Cheers
Charlie

 

[joe1512]

It is a really inventive idea. Taking a gram of 31 degree ice to 32 degrees takes 1 Joule.
Taking 1 gram of 32 degree ice to 32 degree water takes a whopping 334 Joules!
(as an example)

One obvious concern is that you couldnt fully fill the reservoir for concerns about potential expansion/contraction. A bit of air inside would alleviate this concern.

I guess we need a thermodynamics person to calculate how much energy it would take to transform say a 8 cubic centimer cube of something from say 80 degrees F to 120F, including phase change.


Mostly, we'd have to figure out the most effective solid that has the highest phase change energy per gram. Since space is the limiting factor (vs weight), we'd want something pretty dense ideally.
No telling what that could be? Beeswax? How about poop!?

Then you could totally patent the "Sheit-Sink"(tm) and make millions!

 

[Charlie]

As well as the higher latent heats there is also the possibility of thermal cycling, heat source is adding heat to liquid medium (no benefits from latent heat), cold sink is accepting heat from liquid medium and solidifying the medium, result, greater heat rejection from the reservoir and increased internal heat transfer.

Likely to get seriously complicated though, somebody would have to do some in depth mathematical modelling :shakehead or just build one and find out :devil:

Probably a good idea to retain a thermal protection circuit though...

Charlie

 

[bshanahan14rulz]

what about a heatpipe that helps send heat down the body tube, to a ginormous heatsink on the butt of the flashlight

 

[Ksailork]

Salts, via phase changes, make for excellent thermal storage but volume changes would have to be considered in a confined system. With any thermal reservoir, you'd have to consider what is going to happen when the reservoir has to, or does, give up it's heat. As Charlie stated, their use may be confined to "briefly". Long term, it may be better to dump the heat to the surroundings as is common with most current designs.

How about poop!?

I'm not a physicist, but I thought the phase of said material was affected by what you ate and the quality of tequila you consumed, not temperature.

 

[Charlie]

Salts, via phase changes, make for excellent thermal storage but volume changes would have to be considered in a confined system. With any thermal reservoir, you'd have to consider what is going to happen when the reservoir has to, or does, give up it's heat. As Charlie stated, their use may be confined to "briefly". Long term, it may be better to dump the heat to the surroundings as is common with most current designs.

I had been thinking that by using a thermal reservoir (and thick walls) it may be possible to get something 9P size to run an SST-90 at full power off 1x18650 for the life of the battery. Given the required cool off period it would then need to be left off for some time (hours?) for the material to change phase again.

The issue of volume changes could be solved by use of a convoluted metal diaphragm like the ones found in pressure transducers (and to a lesser extent, jam jar lids).

Charlie

 

[Apollo Cree]

That's an excellent idea. The only drawback is you have to remember it's a "surge" solution. It keeps your device cool for a while, but if you leave the device on too long, it will heat up.

You could use this to have a short term "turbo mode" where you have a higher power mode, but you can only use it at that mode for a while. The phase-change heat sink would increase the time before it overheats. Some devices have this "turbo" mode already, even if they don't admit it, since they will overheat if left on for too long.

The other advantage would be to reduce the junction temp on a semiconductor device in normal usage to extend the lifetime. Even if you don't immediately burn the device out from the temperature reached, reducing the max temperature most of the time you use the device may extend the lifetime due to "degree-hours." You might also get some increased lifetime due to mechanical stress from thermal cycling because the peak temperature is lower for most of the times you turn the device on.

You'd need to consider thermal expansion, but that shouldn't be too hard. You could have a mechanically expandable reservoir with something like a dome that flexes like the bottom of an old style oil can. You could also simply have some air space in the reservoir so that a gas could take up the expansion of the phase change material.

Presumably, you could tailor the transition temperature by having a material that's a mix of two materials with different melting points. You'd have to use materials that didn't separate with the melt/freeze cycle.

One thing I wonder is if you can get enough material in a small enough space to be worth the effort from a practical sense.