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Comment by leguminous | Hacker News Reader

Scanning the original paper linked by _Microft and rmbryan, it looks like the pressures the researchers used were very high, 0.25-0.5GPa. Maybe someone with more domain-specific knowledge can answer this: are such high pressures actually practical? My understanding is that one of the reasons R-744 (CO2) is not more common is that it requires high pressures, which means specialized equipment. But the pressures involved here seem to be an order of magnitude higher even than required with R-744.

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You can get hydraulic pressures as high as this. Usually they are very low volume though. You don't store much energy in liquids and solids due to their low compressibility so the chance of explosions is a lot less.

Gasses can store tons of energy if you get them up to these pressures so it's a hazard to anyone near the thing. If you have gasses in your high pressure system and something fails it can result in shrapnel. Hydraulics can do something similar with spring forces in the casing of the machine, but it isn't nearly as energetic!

In this case they are compressing a solid, which is much safer than compressing a gas. There is no explosion risk if the high "pressure" solid is suddenly released from its container because its volume has not changed by much. Looks like this solid changed its volume by ~2x (fig 2e). For CO2 the change is about 28x. If your high pressure CO2 suddenly gets out of its container, it will immediately expand to 28x its volume.

There is of course a question about how exactly you build a mechanical device to implement this cycle. In current devices the material which changed temperature physically moves around a circuit and removes and deposits heat energy at different places in the circuit as it moves. They definitely don't talk much about how a practical device could be constructed.

I'm no refrigeration engineer, but I figure you might need some sort of pulsed cooling. Compress the solid so it gets hotter, run liquid coolant through it to extract the heat to a radiator. Then let the solid decompress, and switch to a coolant circuit that extracts heat from the interior of the fridge and dumps it in the solid. If you needed continuous operation of both coolant circuits, you could add a second block of solid material, and switch the cooling and heat-exhaust circuits between the two.

Thanks! I think the idea you outlined would definitely work, but it is different than current refrigerants a few ways. For example: the secondary liquid loop you mention would probably vary in temperature with time. Right after the pulse starts the fluid would be very hot (or cold). As the solid material changes trends back toward the mean temperature, the secondary loop would follow. The secondary loop would need to then exchange heat with the system being heated or cooled. This would require the heat exchanger to work over a variety of temperatures for the fluid loop. This in itself is obviously not impossible but it does prohibit important optimizations to efficiency that you can get by assuming more steady operating conditions.

There is an interesting similar concept using magnetic fields instead of compression - some materials rise considerably in temperature when immersed in a strong magnetic field.

Extract heat to a radiator, turn off the electromagnet, open the coolant circuit valve. It's called Magnetocaloric effect and they were considering it for in-car AC a few years ago, dunno what came of it.

https://en.wikipedia.org/wiki/Magnetic_refrigeration https://www.sciencedirect.com/topics/chemistry/magnetocalori...

I'm having a hard time confirming it (can't find the original paper w/o a link), but you are probably referring to mechanical stress instead of fluid pressure.

if you hang a tensile load of 1000 Newtons from a 1mm x 1mm rod, the rod is under 1 GPa of stress.

Here's the original paper: https://www.nature.com/articles/s41467-019-09730-9

This is all I saw to address the pressures required:

> Our higher operating pressures do not represent a barrier for applications because they can be generated by a small load in a large volume of material via a pressure-transmitting medium, e.g., using a vessel with a neck containing a driving piston, whose small area is compensated by its distance of travel.

CO2 critical temperature is just 31C, almost a room temperature

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