# How would you store heat?

Um .. naive question perhaps but if somebody wanted to store heat, how would they go about it? Can heat be stored?

I'm told that decomposing kitchen waste in a closed vessel results in a rise in temperature on the body of the vessel. I'm just wondering whether it could be stored for later use.

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The trivial thing is to put the warm stuff into an insulated vessel; were you looking for something esoteric that I didn't catch? –  dmckee Sep 22 '11 at 16:57
Er no. It's possible to store electricity in an electric battery/capacitor; I wondered about it's heat equivalent. –  Everyone Sep 22 '11 at 17:01
Some "esoteric" heat store: some years ago this en.wikipedia.org/wiki/Phase-change_material sodium acetate bags were sold in Germany as some "new" gadget. Clic on German version of that Wiki page to see example pics. –  Georg Sep 22 '11 at 17:36
The decomposing process heat increase is of course because of the bacteria doing what they do, consuming sugars and stuff. And thermoses are great, but most bacteria stops working at 60C and heat is more useful the larger the difference to the norm medium. –  Captain Giraffe Sep 22 '11 at 18:03
You could convert it into electricity easily with a Stirling engine, and then store it in a conventional battery. But it would have to be a pretty big temperature difference to be worth the trouble. But if you haven't heard of Stirling engines before, they're pretty cool: en.wikipedia.org/wiki/Stirling_engine –  Chris B. Behrens Sep 22 '11 at 20:30

Building off of the comments to the question.

It might be instructive to think carefully about what is being stored when you store "electricity" in a capacitor or battery.

Note that it is not electrons as I can charge a capacitor either positive or negative relative to a floating ground. The "what" in that case is energy in the form of electric fields (capacitor) or chemical potential (battery).

Heat is also energy, in the form of excitation of microscopic degrees of freedom, and the way you store it is by exciting the microscopic degree of freedom in some material and then don't allow it to transmit that energy to other forms---which you do by insulating the hot stuff. Or you can convert the heat to some more tractable form (as in Dan's answer or Georg's comment) and store that.

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This is closest to what I was looking for. But I think the insulation at the interface would heat up over a while ... so the storage would be very lossy itself too –  Everyone Sep 22 '11 at 18:17
The insulation would heat up (unless you are using vacuum insulation) and would eventually reach thermodynamic equilibrium with the environment so that the amount of heat lost is the same as the amount of heat generated. However, the stable temperature of the composter with insulation is higher than without. This is what I was refering to with my aside "(at least initially)". –  AdamRedwine Sep 22 '11 at 18:24
Certainly the storage is lossy---even vacuum insulation transmits heat by radiation. Batteries and capacitors are lossy too. The question here is one of timescales, the relative inefficiencies of transforming the energy, and expected losses in other choices of storage. –  dmckee Sep 22 '11 at 18:32
If even vacuum radiates heat, then would it be safe to say there is no way to stop radiation? If so, could we control heat radiation by directing it? –  Everyone Sep 24 '11 at 18:08
You can reduce radiative losses by reducing the emissivity of the radiating surface (or of course, by reducing the temperature difference between the two surfaces, but that doesn't accomplish your basic goal). This is why the inside surfaces of the vacuum vessel in a Dewar are silvered. –  dmckee Sep 24 '11 at 20:18

If you want to store heat in a battery-like device, you could use the heat to power a turbine, generate electrical energy, and store it as chemical energy in a battery. This is extremely inefficient, but I think this is most analogous to what you are asking.

You could also find a high-energy chemical reaction in equilibrium. This would store some of the heat as chemical energy, but would have to be kept at the same temperature or the chemical mixture would start producing heat.

Really, though, the best way is probably dmckee's "warm stuff + insulated container".

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Phase transition is pretty good, too, like melting some salts. It absorbs lots of energy without raising the temperature too much. Raising the temp is bad, because then the losses increase rapidly. –  Florin Andrei Sep 22 '11 at 17:57
""Phase transition is pretty good,"" Right, but often storage of a big mass in a container is just the cheapest thing including investment. Very common are isolated tanks with water to store warm water produced by sun on rooftop during daytime. –  Georg Sep 22 '11 at 18:08

Slightly naive, but not an uninteresting question. The previous answers give some good discussion, but I think they are missing what you are really going for.

While the terminology you use is odd, the ability of a substance to "store" heat energy is one of the first things you learn how to calculate in thermodynamics. The value is called the "heat capacity" and is a measure of the amount of heat required to change the temperature of an object.

$$C = \frac{Q}{\Delta T}$$

Of course, a larger object takes more heat energy to raise it's temperature so this value is often divided by the mass of the object to obtain the specific heat capacity of the material (assuming homogeneity and all that good stuff). This value also depends on wether you keep the volume or the pressure constant so there are a number of different forms that you might run across this concept.

The comments made about converting the heat to chemical or electric potential energy are very relevant, but not, practically speaking, relevant to your specific example. The suggestion of insulation is a good one and would reduce the rate of heat dissipation to the environment (at least initially) of the container.

One way to get an intuitive feel for the amount of heat generated and stored by your composter would be to put a marble slab (say a cutting block for example) underneith. The marble has a high heat capacity and would absorb a fair amount of heat from the composter provided that the two objects are in good thermal contact. After a few days, once the system had reached thermal equilibrium, you could remove the marble block and feel the spot where the composter had been; it should be slightly warm.

Another interesting experiment is to take the marble slab to bed with you. When you go to sleep, it will feel very cold for a long time. If you sleep on it, when you wake up in the morning it will feel very warm because it has absorbed a lot of your body heat. People used to sleep with large quartz stones to keep cool in the summer.

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Thank you (+: that was a lucid explanation –  Everyone Sep 22 '11 at 18:24