Any idea where to find a 'constant' temperature material?

Nuh-uh. A heat pipe takes heat from here where it's hot, and puts it over there, where it's cooler. Melting wax takes heat from here, and keeps it here, but until it's all melted, it keeps the temperature constant.

It was some kind of customized material -- I doubt that you could just melt candle wax into a jar and get the same effect.

Well what the "bleep" are you doing? How about two styrofoam boxes, one filled with dry ice the other a battery, heater and thermometer. (And a vent to connect the two.) Too cold.. turn on heater. Too warm... open vent and circulate/ exchange cool air with the dry ice box.

George H.

Or, between two slightly different solid states; the problem with liquid/gas transition is that it is very sensitive to atmospheric pressure. What this gives is, at best, a transient temperature that goes rampup-flatspot-rampup/rampdown... instead of rampup/rampdown.

But, if you just want to find a constant temperature, ten feet down in the ground is a great place to look.

Sodium Acetate is used in some hand-warmers as a phase change material.

See wikipedia.

kevin

I didn't know that slightly different solid states exist. The EXACT temperature is not as critical as the ability to eat or give up a lot of calories at the transition.

Your ramp up, HOLD, ramp on upward and ramp down, HOLD, ramp on downward are better descriptions.

ten feet down?! LOL! Yeah tried that but it was very difficult to move to the other factory like that!

Thanks. The first hurdle at doing this search was just trying to figure out the 'correct' terms!

reminds me of the danish movie "flickering lights", the two raving alcoholics spend much of their time discussing the depth at which to bury beers to get the perfect temperature :)

-Lasse

These are not inventions but thermodynamic science discoveries. The most us eful phase change materials are all hazardous/toxic almost by definition. I n many applications the heat energy storage density due to the latent heat of transition is not the only advantage, the really big advantage is the ma terial remains at the system working temperatures, or thereabouts, obviatin g the need for insulation and/or eliminating heat conduction losses/gains.

Look a hydrates, vary a "core" metal ion or organic complexity or pressure...

Or melts?

joe

I don't think there's any chance that it boils (liquid to vapor). There's nowhere for the vapor to go. That causes pressure to increase rapidly, stopping the boil.

Ideally you want the triple point since they are well defined but rare.

Airlines would go ballistic if you tried to take it on board.

Here is an old patent from the 1975 oil crisis interest in solar power on a eutectic mix and stabliser. There are better mixtures now but this one is presumably now long since out of patent.

..... Phil

Patent - Yes Maria Telkes was definitely the lead investigator in this area

These freeze-thaw systems are limited in heat capacity, and Telkes systems were a ton of salt for heating a house. There was a demo house at MIT, but the technology never caught on. One poster here (MB?) posted a "Trombe Wall" that contains the Glaubers salts. Trombe (pronounced "Trom-bay") are passive solar, no moving parts. They are tuned so that the sun hits the outside at 2 PM, and the heat pulse arrives inside the house at 2 AM. A glazing is added to enhance the effect.

Like all solar heating inventions, ya gotta have sunlight + cold, so these are popular in New Mexico, Colorado, Utah, Nevada, California. Not so good in Seattle, coastal oregon, coastal California, Miami, New Jersey.

Back to the energy storage, Telkes' recipes are the best that you can do. An improvement would be to engineer a material system that involved liquid-gas heat of vaporization, but don't know if possible. Let's see - you have methanol under a vacuum at STP. It boils, and volume expands 800X, say. Pressure problem. A mole of material occupies 22.4 Liters at STP, as a gas. A mole of methanol is about 35 ml. So you have 700X expansion.

The miracle material, Dreamonium, might solve this!

or both: quite possibly it's a triple point apatatus. That gives the best temperature stability.

It expands zero by definition because it is sealed in a constant volume container.

Pressure varies with temperature, something like exponentially below the conventional boiling point (e.g. at room temp it might be 10 Pa, at 10C higher, 100 Pa, etc.), and something more linear above. The heat capacity of such a system rises gradually with temperature, as liquid is being vaporized *and* vapor is being compressed. But the temperature does not remain constant as constant pressure boiling does.

Heat capacity will ultimately drop when liquid is exhausted, but depending on how much headspace there was over the liquid, it may still be a lot (no headspace at all leads to the condition of a supercritical fluid, on the order of 20-100 bar, which is under enough pressure that, yeah, there's a lot of energy, and dW/dT, in there).

Tim

It's not uncommon; one example is the martensite-austenite transition in iron, which is the reason we can hot-work the metal at a much lower temperature than melting. An interpretation of 'cold fusion' is that a phase transition in oddly crystallized palladium can be triggered (and then runs away, generating a burst of heat). Third is the white tin/gray tin transformation, in which pure tin becomes a brittle semiconductor at low temperature. That might partly explain the NASA reluctance to endorse near-pure tin for solder.

Maybe, but the transition temperature for downgoing and upgoing transitions needn't be the same (hysteresis, it's called). You can supercool water, for example, and get its temperature down below freezing BEFORE it solidifies. Supercooling can get liquid water down to (!!) -48 Celsius.

My bet is they are using some type of Sodium Acetate. That os what is used in this type of thing:

The temperature range also seems to be about right.