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I am imagining a story where people store heat in the summer for use in the winter and store cold in the winter for use in the summer, unleashing the appropriate temperature when needed.

But without hand-waving it away, how would such a thing work? I care not whether this device is magic or technology. (Keep in mind I'm not talking about currently known heat pump technology, which takes a lot of space, and transfers current temperatures from the ground and using the differential to change the building's comfort level.)

Whatever it is I want it to be about the size of a good sized urn, maybe working along the idea that if sat in a sunny place it would absorb heat and be able to release it at a later time, and if set in a cold / snowy place it can absorb cold, and have some sort of release valve. The amount of heat / cold stored would be proportional to the size of the vase.

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    $\begingroup$ what is bad about heat pump, why they are not good - is that only space/mass requirements? How about wood(tree)?)) $\endgroup$
    – MolbOrg
    Dec 25, 2016 at 23:04
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    $\begingroup$ I think this question is too broad, because you say magic is a possibility, but then don't specify the limits of the magic. $\endgroup$
    – Xandar The Zenon
    Dec 25, 2016 at 23:42
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    $\begingroup$ @JesseCohoon about wood? if yes, it was just kinda joke, threes they store sun energy and they can be burned with heat emission. I was just thought about how much it can be stored(heat) - basically limitations are just burning the stuff. I do not get you premise and what you like to have. We may possible imagine a heat machine/AC unit - which stores heat in compact form in magically technology advanced compact device in form of plasma but this will be not for free and will cost energy. Same way we can imagine compact energy storage + AC/heater - but how it differs from what we already have. $\endgroup$
    – MolbOrg
    Dec 26, 2016 at 0:45
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    $\begingroup$ @JesseCohoon Where I live once it gets cold, it won't be warm till summer. Other than solar, or geothermal (not near my location) there would be no natural way to get heat in the winter. If you have dozens of them you are going to use the same space as a heat pump system. Except these thing are going to be "in your way", and heat storage can be under your basement, out of sight out of mind. $\endgroup$
    – cybernard
    Dec 26, 2016 at 3:55
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    $\begingroup$ It seems obvious to me that this would violate the laws of thermodynamics. So you need magic. But with magic, we need you to define how the magic works. Then we tell you consequences of that. You can narrow this either by making it a reality-check. Then people can explain at more length why this is scientifically impossible. Or you can define how your magic works and specify an area of consequences that are of concern. And you don't absorb cold; you release heat. $\endgroup$
    – Brythan
    Dec 26, 2016 at 13:33

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A magic material with near infinite heat capacity for the two urns.

Two boxes for the urns that have perfect insulation and a sliding door to regulate heat transfer

In the summer put the summer urn outside and when it get to the hottest day put it in the insulated box.

When you need to heat:

  • Bring in the summer urn and crack the door
  • Take the winter urn of the insulated box and place it outside. Wait for the coldest day of winter to put it back in the insulated box.

Reverse to cool.

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    $\begingroup$ If you have something with near infinite heat capacity, you only have to worry about having enough heat to pull out of it. If you want it cooler, just move the heat in the room into storage. That's just one urn to worry about. $\endgroup$
    – House
    Mar 22, 2017 at 18:40
  • $\begingroup$ @Byte56 Just how would that cool the room? $\endgroup$
    – paparazzo
    Mar 22, 2017 at 18:43
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    $\begingroup$ Removing heat from the room, cools the room. The heat goes into storage, and is no longer in the room. $\endgroup$
    – House
    Mar 22, 2017 at 18:44
  • $\begingroup$ @Byte56 Don't follow. You are free to post an answer if you have a better solution. $\endgroup$
    – paparazzo
    Mar 22, 2017 at 18:46
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    $\begingroup$ It's like if there was a light in the room and you wanted to make it dark. Would you "add dark to the room" or would you remove the light? You'd remove the light, just like you'd remove the heat to make it colder. $\endgroup$
    – House
    Mar 22, 2017 at 18:48
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Well as you haven't said much about size constraints, I'd suggest storing it in much the same way we do now, in something unimaginatively called Thermal Storage. Large mass of something with good insulation. When it gets hot, it stays hot and vice versa for the cold. I know of at least one molten salt heat storage system (I forget where), they used solar power to heat up the salt and the use the heat at night, I think it stored several Gigawatt-hours of energy and could remain hot for ages. For a 6 month shelf life, just beef up the insulation (there used to be a rumor that the insulation on the space shuttle's liquid hydrogen tanks was sooo good it could keep ice frozen for 12 years)

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  • $\begingroup$ probably this one you are talking about solarreserve.com/en/technology/molten-salt-energy-storage $\endgroup$
    – MolbOrg
    Dec 25, 2016 at 23:06
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    $\begingroup$ Yeah, that's probably it. 1'F heat loss per day, that's pretty impressive, it'd still be good even after a whole year! $\endgroup$
    – Samwise
    Dec 25, 2016 at 23:10
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    $\begingroup$ You don't need space age tech for insulating things - historically, we have shipped ice blocks from Canada to India with sail ships around Africa, taking ~4 months to arrive - en.wikipedia.org/wiki/Ice_trade has a short overview. You just need a lot of the stuff (more volume per surface), and thick cheap insulation. $\endgroup$
    – Peteris
    Dec 26, 2016 at 10:09
  • $\begingroup$ @Peteris I didn't mean to imply that space age insulation was needed, I just thought it was an interesting bit of info, to show what's possible, to say "hey, not only can this certainly work, but it can work really, really well" $\endgroup$
    – Samwise
    Dec 26, 2016 at 20:02
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Energy storage

If you cool a rock to exactly one degree below freezing, then drop it in a nice beverage, the brick will cool the beverage... a little. But not very much, in the grand scene of things. However, if you drop a simple ice cube into the beverage, it will lower the temperature considerably! Same mass of object, but a lot more energy. The reason is the state change - the brick stays a brick, but the ice cube melts, and it takes a lot of energy to go from solid to liquid (and from liquid to gas), and vice versa. Incidentally, this is why those drink-rocks just don't work as well as regular ice.

Granted, that's just water. It can hold a significant amount of energy, but you're not going to get it as small as an urn and still be able to do anything but act as a personal cooling device for a few hours, at best. What you really want, therefore, it a new supermaterial. Something that takes a lot of energy to go from solid to liquid. Then, all you need to do is find a way to get the energy into the substance (to make it warmer), or let the energy out (to make it cooler).

The awesome part is that the temperature is exact. Not "vaguely room temperature", but a specific temperature with 0% fluctuation.

Temperature control

Ok, so I lied. There's a little bit more to controlling temperature than the material. However, this works to our advantage! You see, if you increase pressure, water no longer freezes at the same temperature. Increase it enough, and water starts freezing at room temperature (or hotter). Decrease it enough, and water boils at room temperature (or colder).

Why is this to our advantage? Simple! Your supermaterial is at the solid/liquid state change at room temperature and atmospheric pressure. But, by increasing or decreasing the pressure, you can easily change the melting/freezing point by a few degrees up or down. Which means your supermaterial can be regulated at the twist of a dial! As you crank it one way, it increases pressure, the other way, decreases. Turn it up or down to regulate how much energy is released.

Recharging

As energy-dense as this material is, it will take significant energy to recharge - exactly the energy it lets out, actually. However, with the right material and a good source of energy, a chunk of material could heat an insulated house for a full day, or longer! And since it isn't wasting energy trying to make a house colder than it should be (like a giant block of ice), or warmer than it should be (like a vat of molten salt), it will regulate much more easily.

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  • $\begingroup$ If I could have selected 2 answers to approve (with the checkmark) yours would have been it $\endgroup$
    – Jesse Cohoon
    Dec 26, 2016 at 19:39
  • $\begingroup$ And as an added bonus, because of water's near in-compressible nature, it takes relatively little effort to ramp up the pressure compared with say a gas. $\endgroup$
    – Samwise
    Dec 26, 2016 at 20:05
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Magic

You said you don't care if it is magic or not. So the simple thing to do is use magic. Have some kind of magical storage device that takes charging. In the summer it absorbs heat and in the winter it releases it.

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We used to do this all the time, at least in the storing "cold" way. But it only worked on a very large scale. We used to have ice houses that would store huge quantities of ice. the buildings were well insulated and the ice that would last through to the next winter. they could be used for food storage because they stayed cold all year. https://en.wikipedia.org/wiki/Ice_house_(building)

lots of buildings work by storing the "cold" of night into the day and the heat of the day into the night to keep a mild temperature. there are several ways to do this but all require lots of mass. The simplest is just building really thick stone or earth walls.

the thermal inertia of something as small as an urn is just too small to work so you really will have to use hand waving magic. If you limit it to magic that stores or release energy on demand you can do this just fine.

I seem to remember a story where they did just that with specially carved stones of various sizes. one type could be prevented from warming (thus storing cold) and another could be prevented from radiating heat (thus storing heat) the cold ones were somewhat limited becasue they could not get colder than the coldest winter night, but the hot ones could reach some real extremes in temperature (they are warmed by sunlight so they can eventually reach the temperature of the surface of the sun, except they would melt first) to the point they were usually housed in metal frames to keep people from getting too close while active. I wish I could remember where I read that, it might have been a campaign setting. This does mean you would need more cold stones than hot ones, and not just because it is easier to heat a room than cool it.

I have also seen a variation on this with a magical artifact that is always the same temprature, which just due to how temprature works will tend to bring any room it is in to the same temprature over time as long as it is not to big. if the temprature was set when you made each one you can get a lot of mileage out of the idea. make refrigerators in winter and bread ovens in summer. Or create your house climate control on a nice spring day.

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  • $\begingroup$ Ice houses are common round the Scottish coast, by piers and small harbours where fishing boats used to operate, and especially at salmon fisheries. $\endgroup$
    – user_1818839
    Dec 26, 2016 at 16:50
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I agree with the answers/comments that the question is a bit too broad. If you want actually working stuff, then the molten salt storage or a heat pump (as suggested by others) would be the way to go. If you want it in a (small) urn, then you have to resort to magic. So, how to do it magically with an explanation?

Maxwell's Demon

If you don't want to just say "it's a magical urn [cough, cough, mumble, mumble]", then I suggest you make it contain a Maxwellian demon (well, I guess imp if it should fit a small urn).

Maxwell's demon is a thought experiment by Maxwell to show how the second law of thermodynamics (which say that the total amount of disorder (entropy) in a system has to increase) can be violated. The deamon will, in the thought experiment, choose which gas molecules which get to pass a barrier and, thus, over time the order of the system would increase since the demon would organize the molecules. This is disproven by noting that the demon need to spend energy to determine which molecule to put where and, thus, the global entropy will increse anyway even if some molecules get ordered. This is at least the case for a world without magic.

With magic allowed, the demon (or imp) would live in the urn and selectively choose which molecules get to interact with the urn. This would mean that on a warm day only high energetic molecules (i.e. the warm ones) get to interact with the urn, they lose energy during the interaction and cool down. On a cold day, the demon would do the opposite and only allow "cold" molecules to interact and they would thus gain energy from the storage in the urn. The urn itself can be of any outer material and contains an inner storage cage (to keep the demon/imp in), which is made from some enchanted metal. Inside the storage cage is the demon/imp (obviously) and a semi liquid or plasma-like magical goo which stores the energy. You could use some real compound with high heat capacity (e.g., water), but if you already went for a deamon, then a purple glowing magical goo would fit better. I would personally call the goo Thermoglium, Magmalite, or perhaps Maxwellate.

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A micro-blackhole, -sun, -star the size of large urn. Contained with electro-magnetic fields. Assuming you can draw energy from it, you can power all the heaters or air conditioners you want.

Even better throwing your trash into it, will cause it to compress and release the trashes matter as energy eventually.

Only, 1 problem, getting one of the above, I guess here is where the magic comes in.

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Either go with current technology or tweak it with a teeny bit of magic.

We can store lots of heat in latent form with special substances such as Glauber salts or some waxes. You can use handwavium chloride, with a very high latent heat and a metastable form (sort of super Glauber) that can be ignited in winter. Never travel in the cold months without several packets.

Or you can have some plant use it to build its own substance taking advantage of summer heat (lowering the temperature while doing so) and burn said plant in winter.

Otherwise, you will need some nigh perfect insulator and store heat in some thermal reservoir - underground, or in water tanks.

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Fermilab once built a house over a 10 foot by 10 foot by 10 foot pool of water. During the winter, they pulled heat out of the water, making ice. During the summer, they pumped heat out of the house into the ice, air conditioning the house and melting the ice.

They said that this system worked surprising well.

Now, do you allow a magical heat pumps? Because making a heat pump will take some good manufacturing knowledge, such as making metal tubing.

Keep in mind, as well, that old houses allowed a lot of outside air in which greatly increases the load on the HVAC. Very well insulated homes need to cool they sweaty hot humans during the winter. Do you allow magical foam insulation?

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  • $\begingroup$ You don't need magical foam insulation: straw bale houses work quite well, or so I've read. $\endgroup$
    – jamesqf
    Dec 26, 2016 at 19:59
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huh... isn't the key to store cold/heat, the insulation of the vessel? Lower the temperature of a fluid as much as you can - nitrogen for example - and store in a container made of a ferromagnetic material .

Keep the container in a vacuum chamber and magnetically levitate it.

You have the cold storage. Do the same for a fluid heated as much as you can and you have heat storage. The magic here would be to find ferromagnetic materials that can endure both very low and very high temperatures.

If the purpose is to heat/cool a room to a comfortable temperature for a human being, I think not much magic is needed.

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  • $\begingroup$ Vacuum and levitation won't prevent radiative heat exchange. $\endgroup$
    – L.Dutch ♦
    Mar 26, 2019 at 13:34
  • $\begingroup$ what would you recommend to mitigate the loss of heat due to radiation? $\endgroup$
    – Joao Balesteros
    Apr 18, 2019 at 13:59
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You can base everything around buildings with high thermal mass coupled with High R-value insulation. You can look into things like Earthbag houses or even primitive Adobe, if you make the walls thick enough. line the exterior walls with bailed hay, which should give you an R-value of around 2 per inch, if I remember right, not counting the inherent properties of the earthbags or adobe.

Sure you end up with walls 3 foot thick, but they mean you have a very thermally efficient house, so long as you figure out how to insulate the roof well. There are even earthbag like constructions in a beehive shape that turn the wall into a roof, more or less.

Your vase of handwavium would have less heat to move around, allowing more flexibility with how you could do it.

Finally, you could just put everyone in hobbit holes, or dwarf mines. Underground temperatures require very little thermal interference to maintain the perfect temperature, so long as you get below the frost line.

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Maybe have a material that either captures or gives out heat by changing the thermal properties like changing polarity of a magnet. During summers the material absorbs heat and cools the room. And during winter we change the "polarity" So that it gives off the heat stored inside

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    $\begingroup$ The question requires science based reasoning. Can you provide it? $\endgroup$
    – L.Dutch ♦
    Jul 7, 2019 at 9:08
  • $\begingroup$ I think this actually might work, in the sense of a superconducting material, but the properties would have to be such that it doesn't need the typical types of things such material(s) would normally. $\endgroup$
    – Jesse Cohoon
    Jul 22, 2019 at 23:08

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