Can peltier devices stack?
Yes but each stack layer has to remove the waste heat from the inefficiency of all the previous layers, so usually you see them laid out in a pyramid
Staying cool without refrigerants: Next-generation Peltier cooling
161–170 of 329 posts
Re: Staying cool without refrigerants: Next-generation Peltier cooling
#162Earlier quoted context omitted.
You can get a COP of ~3-4 out of regular TECs, but only at pretty low temperature differences. That's the killer, fundamentally the TEC material itself is thermally conductive and heat really wants to flow back the other way, so no matter how well it moves the heat, it winds up fighting against the heat load generated by itself. A refrigerant based heat pump works much better because the heat basically only moves in…
What's a "low temperature difference"? I want to build a wine cooler in my basement ~20-24c, and I want it at ~16c. Is that low enough to be reasonably efficient?
TECs are wonderful little devices with operating characteristics unlike comparable devices.
They can be designed to move a specific amount of heat or to cool at some delta-T below the hot side (and due to inefficiencies the hot side can climb above ambient temperatures too, raising the “cold side” above ambient!)
I ran through a design exercise with a high quality TEC and at 8°C delta-T for a wine cooler you could expect a COP of around 3.5–4 (theoretically). This is pretty good! But below the 2.5V max to do that you’re only able to exhaust up to around 40W. For a wine cooler this is not so bad. For a refrigerator it’s a harder challenge because the temperature drops when the door opens, and if someone sticks in a pot of hot soup, it’s important to eject that heat before it raises the temperature inside to levels where food safety becomes a problem. For a CPU it’s basically untenable under load because it’s too much heat entering the cold side thus temperatures will rise.
https://fluffyandflakey.blog/2019/08/29/cooling-a-cpu-with-t...
Things often overlooked:
- Most TECs are cheap and small and come without data sheets, so people tend to become disillusioned after running them too hot.
- You have to keep the hot side cool or else the delta-T doesn’t help you. For a wine cooler this is probably no big deal: you can add a sizable fan and heat sink. For CPU cooling it becomes a tighter problem. You basically can’t win by mounting on the CPU; they are best at mediating two independent water-cooling loops.
- Q ratings are useless without performance graphs. It’s meaningless to talk about a “100W” TEC other than to estimate that it has a higher capacity than a “20W” TEC.
- Ratings and data sheets are hypothetical best cases. Reality constrains the efficiency through a thousand cuts.
When I think about TECs I think more about heat transfer than temperature drops. If you open a well-insulated wine cooler once a week then once it cools it will only need to maintain its temperature, and that requires very little heat movement. Since nothing inside is generating heat you basically have zero watts as a first-order approximation. For the same device mentioned above, it stops working below 1V, and at 8° delta-T that’s a drop in COP to around zero but it’s also nearly zero waste. If you were to maintain a constant 2.5V, however, it would continue to try and pull 40W to the hot side. This would cause the internal temperature to drop and your COP would decrease even though the TEC is using constant power. The delta-T would in fact increase until the inefficiencies match the heat transfer and everything stabilizes. In this case that’s around a 20° drop from the hot side, assuming perfect insulation.
Unlike compressors, TECs have this convenient ability to scale up and down and maintain consistent temperatures; they just can’t respond quickly and dump a ton of heat in the same way.
edit: formatting of list
Re: Staying cool without refrigerants: Next-generation Peltier cooling
#163Earlier quoted context omitted.
The idea was just so astonishing that I ordered some from American Science and Surplus. I connected the leads to a battery and poof, one side got hot and the other got cold. Blew my mind. I didn't actually have a use for it. It was just neat that it actually worked. I understand the basic physics of it perfectly well. It's just one of those things where you expect basic physics to be overwhelmed by friction or someth…
Had a similar experience myself. The thinness of Peltier devices bothers me. Amazing temperature differences… without any room to insulate such!
I don't know how people can convince themselves that they can understand these effects.
Unusually effective heat insulation is exactly how they work. (This is tempered by eg radiative losses, so making them thicker doesn't work better.) placing poorer heat (vs electrical) insulators between peltier material is counterproductive, similar to using resistors to improve conduction between copper wires
Don't ask me for a better explanation :)
As to why COP or even Carnot efficiency hasn't been thrown out in favor of temp-difference independent efficiency metrics like exergy.
I can't tell you either
Re: Staying cool without refrigerants: Next-generation Peltier cooling
#164Earlier quoted context omitted.
You can get a COP of ~3-4 out of regular TECs, but only at pretty low temperature differences. That's the killer, fundamentally the TEC material itself is thermally conductive and heat really wants to flow back the other way, so no matter how well it moves the heat, it winds up fighting against the heat load generated by itself. A refrigerant based heat pump works much better because the heat basically only moves in…
What's a "low temperature difference"? I want to build a wine cooler in my basement ~20-24c, and I want it at ~16c. Is that low enough to be reasonably efficient?
[deleted]
Re: Staying cool without refrigerants: Next-generation Peltier cooling
#165This article uses the term AI 5 times to talk about basic microcontroller sensor readings.
My process control theory textbook has a chapter on neural networks and a lot of the language in control theory has an AI like tinge to it. I think this AI language is native to control theory so it might not be as overblown as it first sounds.
I mean yes, that's what reinforcement learning is.
Re: Staying cool without refrigerants: Next-generation Peltier cooling
#166Earlier quoted context omitted.
For this we calculate the dew point with 2 sensors and react accordingly in my temperature controller. I'd really like to try out these better peltiers, our current ones suck. And the fans to remove the heat are huge and loud.
There are other ways of building refrigerant-free refrigerators too https://en.wikipedia.org/wiki/Magnetocaloric_effect and https://en.wikipedia.org/wiki/Elastocaloric_materials of course they also have the dew point problem, but so do ordinary refrigerators and freezers.
I need active CPU and light cooling. Fridges are easy.
Re: Staying cool without refrigerants: Next-generation Peltier cooling
#167This article uses the term AI 5 times to talk about basic microcontroller sensor readings.
I kinda of gave up after the second mention.
Re: Staying cool without refrigerants: Next-generation Peltier cooling
#168So this is basically a one-way heat transfer film?
Can it be made multi-layer?
And can two plys be glued back to back so both are trying to transfer heat from center outwards and act as an insulator?
Re: Staying cool without refrigerants: Next-generation Peltier cooling
#169This article uses the term AI 5 times to talk about basic microcontroller sensor readings.
"We used copilot to hack the code together."
Re: Staying cool without refrigerants: Next-generation Peltier cooling
#170So this is basically a one-way heat transfer film? Can it be made multi-layer? And can two plys be glued back to back so both are trying to transfer heat from center outwards and act as an insulator?
> and act as an insulator?
As a heater in this case.
> Can it be made multi-layer?
Yes, but each layer adds inefficiency and it's own energy.