> Greeting all
Hi Peter!
> Great to have the list back online.
Isn't it? It's wonderful.
> I remembered a heat pad that my folks used to use to keep their
> hands warm while running sailing races during the winter. The
> pad was a sealed plastic bag containing a substance which was
> either liquid/charged or solid/discharged.
Ah, yes, those heat pads...
> When it was discharged the pad was placed in a pot of boiling water
> until the white solid became completely liquid (as I remember the
> liquid was light brown like oil). When removed from the water the pad
> would cool to room temprature and the contents would remain liquid,
> the pad could be stored like this for weeks (indefinitly??).
Nope, not indefinitely. I'll explain why in a moment...
> When heat was required from the pad, a plastic element inside the bag
> was bent, this triggered a reaction in the liquid which would revert
> to it's solid state releasing heat over a period of time (about an
> hour I think).
>
> This pad was storing heat without a need for insulation, the physics
> seems ideal for packing away summer heat for winter use, all that is
> needed is space like the roughly 100m cubed under the floors of my
> house.
>
> Questions:
> Do you know what the substance in the bag is?
It's a super-saturated solution of something like Sodium Acetate.
(At least that's what they're using here in the States for the
things...) The switch to solid causes a dump of the heat of
fusion for the formation of crystals, all at once, into the
environment. Because it's a super-saturated solution, sudden
shocks at room (or lower) temperature cause it to go from solution
into crystalization. Over time, the crystals start forming anyway
because of the fact that it's in an unstable state to begin with.
Stoarage lifespans of these things in their charged state of a week
or a couple thereof are not uncommon- but they're no long-term
storage scheme for thermal energy as they currently are.
> What volume is required to produce a kWhr of heat?
Quite a bit- something like a quart to a gallon, I would surmise.
> What temprature is required to 'charge' the substance?
Something hot enough to cause the Sodium Acetate to go into solution.
Somewhere around the boiling point of water will provide the
sufficient heat input to charge them up. Again, not something that
is, generally speaking, overly useful in the application you're
looking for. However, if you CAN find a way that works, is efficient,
and isn't expensive, we'd like to know about it.
-- Frank C. EarlEarl Consulting Services