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Posts: 2,240
Reply with quote  #21 
The problem is, making the coffee uses more energy than the grounds can provide:
this is called the law of diminishing heat urns...


(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilĂ " unless talking musical instruments...

Julian Jameson

Posts: 152
Reply with quote  #22 
Well, I did receive the stearic acid and mixed it up in various quantities with coffee grounds and some graphite (in various forms and quantities).

I will write up something more formal on this when I get the chance but my initial findings are:

The mixture doesn't transfer heat very well by itself. This means, for the phase change to occur, you really need to conduct the heat inside the mixture using some other method.

The phase change does occur in the region 50-60C, so fan speeds might have to be slowed accordingly. 

Coffee grounds have to be completely dried out before mixing, so they can absorb all the designated liquid acid (and you don't want any water mixed in there).

Mixture of 2 parts grounds to 1 part stearic acid seems to be correct. Any more acid and saturation occurs, any less and some grounds may stay empty of the acid.

Does it hold the heat? Yes, in my initial tests, for at least a couple of hours (and that was not contained in an insulated area, rather in metal canisters, on a metal plate where the heat will conduct out more readily).

Recommendation for the next stage? Forget about graphite, it's not direct enough at getting the heat where you want it. Use small quantities of the mixture held in small aluminum cases, where air can flow freely around the aluminum. Even place aluminum screen, or similar, within the mixture to get the heat in and out.

To store heat using the PCM method, you're really looking for a sustained amount of time where the temperature is >55C around the mixture (at least an hour), if there's not enough time, the mixture will not entirely heat and the phase will not change from solid and the heat storage potential will not be realised.

Stay within the temperature range! Not sure about when it's contained, but by itself, stearic acid has a flashpoint of just over 100C. Meaning flammable vapours might be released above this temperature. Containment will be key, here, if there's a possibility of temperatures above this range.

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