h a l f b a k e r yThis is what happens when one confuses "random" with "profound."
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Recently, while travelling, I noticed that some hotel rooms have little fridges, and others do not. The latter variety of hotel room is somewhat irritating but if you want a nice view of the Thames at Henley, your options are limited. Large corporate hotels feature ice machines in a sizable nook that
has inexplicably been within 30ft of my door for the last decade's worth of stays. This feature is also absent in most hotels with character/niche locations.
Anyhow, fridges in hotel rooms are nice if you want to keep things cold, and I do. Brief examination of a fridge will reveal it's mostly an empty box*, some kind of door** and a cooling mechanism***. For this invention, the cooling mechanism we're using is a Peltier element. They're horribly inefficient at turning electricity into cold**** but typically are only about 45mmx45mmx~6mm plus a couple of heatsinks, a small fan and a power supply to get ~12VDC to the aforementioned components. The rest of the fridge is then inflated. Not entirely dissimilar to a large better structured swimming arm band with a door - bonded to the fridge mechanism back. There, now we have an insulated box with enough room inside for a few drinks and a cooling mechanism.
*for the things
**to access the things
***to make the things cold
****not our electricity bill to worry about in this scenario.
A nice view of the Thames at Henley:
https://duckduckgo....a=images&iax=images
not that hard to find. [pertinax, Aug 18 2026]
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I have often thought of a small suitcase doubling up as a mains powered fridge, with the compact compressor etc housed in back of the case. When you take out all your clothes etc, you simply plug it in and it becomes a mini fridge. |
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I did think of that too... |
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Croissant for identifying the problem, but I fear your inflatable solution won't work as you will get convection within the inflatable bit. A foam layer is much better than an air gap at insulating. An inflatable might work if you had a very fine sponge that could be squashed flat and would bounce back. Or you could put your cooling element in a silvered bag with the stuff to be kept cold, and suspend that in an evacuated chamber. |
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A more practical solution might be something like a small chest freezer, but instead of a hinged lid, you have an insulated square with a handle that would slide down inside the body of the fridge like a piston within a cylinder. This way, you would only be refrigerating the volume you needed. Of course, whatever you want is now *right at the bottom*. |
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If I have a mostly empty freezer, I put in ice-cream tubs filled with water. This gives some thermal inertia which keeps the internal temperature stable, so the motor is not forever starting and stopping. |
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//I fear your inflatable solution won't work as you will get convection within the inflatable bit.// |
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It'll work just fine for these purposes. You're still trapping air in an enclosed space preventing convection at the room scale and limiting it to a few 100ml of volume. |
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//If I have a mostly empty freezer, I put in ice-cream tubs filled with water. This gives some thermal inertia which keeps the internal temperature stable, so the motor is not forever starting and stopping.// |
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I have a standard domestic style freezer in the lab, I have a temperature monitor on it. Surprisingly, they swing around in temperature quite a lot, this is intentional to stop cycling the motor rapidly. The one I monitor runs the motor down to -26C or so, then stays off all the way to -16C, a 10C gap between the off signal and on signal. Pretty huge. |
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Water is OK, but it's just acting as a reasonably dense solid at that level. If you want to be much smarter about it, mix up around a 22% salt (NaCl) solution. This freezes at about -20C. So, then you'll have a phase change in the middle (-16>-26C) of your freezer's temp range. The energy required to freeze/melt that is about 10x that required to change the temp of water. Your biggest problem then becomes conducting the energy into/out of the salty ice. I'd make up some thin aluminum panels and line the freezer. That's on my to-do list once I get a band saw and some free aluminum sheet. |
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Soda cans could also be good candidates here. |
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[RichardK] In the tropics I keep several cubic plastic jugs of water in my full size bottom-freezer fridge. Exactly as you say, in power outages or heavy use (company's coming!) it maintains temp much better. It uses space but how much ice cream can humans eat? |
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[bs0u] Please report on the saline experiment! Electric is 44¢/kw here. |
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//Please report on the saline experiment!// |
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Don't need to. Phase change & heat are as baked as it gets. |
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//Electric is 44¢/kw here// |
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Total cooling requirement won't change, it will just be more stable and the compressor will last longer. |
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//convection within the inflatable bit//
//an evacuated chamber//
Hmm... use an inflated outer shell (for easy structural support) with an inner evacuated shell; so 3 layers of plastic/whatever.
( ) ( food ) ( )
As always with a fridge, there's that pesky door messing with the system, but (given the partial flexibility of an air-filled structure) perhaps not have a 'door' as such, but a pinched-shut part that is stretched open when necessary (with a simple mechanism). |
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//As always with a fridge, there's that pesky door messing with the system,// |
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A chest style fridge or freezer will always be dramatically more efficient in use because of all the cold air that spills out every time we open a door in the vertical types. |
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