6 ms·

For those worried about tiny COPs from these gizmos, trawling through the actual paper -- as well as the PR from JHU APL -- in this HN post [1] shows claims of

For those worried about tiny COPs from these gizmos, trawling through the actual paper -- as well as the PR from JHU APL -- in this HN post [1] shows claims of COPs of ~15 for Delta Ts of 1.3°C. A compressor based cooler gets a COP of about 4 in the real world. I'm pretty sure this is an apples to oranges comparison to an expert (I am not one of those) but a factor of 3+ increase in COP is fairly noteworthy -- if it holds up. [1] https://news.ycombinator.com/item?id=44424087 https://news.ycombinator.com/item?id=44424087

moffkalast 1y ago
15 is absurd, regular peltiers get like 1.05 at most don't they? That's like inventing the warp drive but for cooling.
- 1oooqooq 1y ago
  lol yeah they did not. it's right on the abstract. > is ~15 for temperature differentials of 1.3 °C.
- rcxdude 1y ago
  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 the direction the refrigerant itself is moving.
  - scosman 1y ago
    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?
    - dmsnell 1y ago
      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-tecs/ 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
  - torginus 1y ago
    Physics dictates that the energy required to cool an object by 1C increases linearly, with difference to ambient temps. It's like trying to swim against the current - the faster the river flows, the harder it is to move forward at the same rate.
userbinator 1y ago
Delta Ts of 1.3°C. Might as well not use a refrigerator if your ambient temperature is that low.
- LeifCarrotson 1y ago
  You missed the "delta", meaning change. One side would have been ~23C and the other 24.3C.
  - zdragnar 1y ago
    That's still a lot hotter than I'd like my refrigerator, to be fair to OP.
  - userbinator 1y ago
    That's exactly what I mean --- if your ambient temperature is ~5C then you don't need a refrigerator if it only cools down by another 1.3C.
HPsquared 1y ago
What about a ΔT of say 20°C? I'd reckon most refrigerators and air conditioners are around there (temp difference of refrigerant between evaporator and condenser). Stacking a bunch of these Peltiers to give more temperature difference would give a pretty low CoP. Say, for a 13°C temperature difference you'd have to stack 10 of them and use 10x the power. It's even worse actually as the hotter ones have to also pump the waste heat from the cooler ones.
- sokoloff 1y ago
  The ΔT between evaporator (typically 2-6°C while cooling) and condenser (often 40-50°C in cooling mode) is much higher than 20°C. The condenser is often almost 20°C above ambient outside temperature. The design ΔT of ~10°C is the typical return-to-supply air ΔT.
- lucb1e 1y ago
  Note that a small temperature difference that is sustained very consistently over a long time using a tiny amount of electricity (let's say half of what the parent post cited, so like a COP of 8) could add up to a lot of nearly-free cooling. You'd chill your walls for weeks and when a heat wave comes with hot nights for a week, if you(r home automation) close(s) the blinds during the heat of the day, the more-powerful AC might barely have to do anything Just an idea of course, but I'd not write new tech off as "ok but just 1.3 degrees who cares" when the claimed COP is so insanely good without first trying it out
  - Dylan16807 1y ago
    Chill for weeks? What kind of thermal mass are you thinking of there? A brick wall is on the heavy side and you'd be able to store a whopping 1/8 of a ton of AC in ten feet of brick wall. It'd save about 0.1kWh of AC power use. A whole house will have many lengths of wall but also multiple tons of cooling requirements, so that doesn't help much. And how are you going to distribute those small temperature differences without wasting a ton of power? And that's after you figure out how to trap a temperature difference in your walls for more than a day.
MobiusHorizons 1y ago
I believe the “apples to oranges” is the temperature gradient. AC units would routinely manage 15-20c and are rated for more than that. And some freezers manage up to 50c. The greater the gradient the worse the efficiency in general.
madaxe_again 1y ago
The delta of 1.3C is critical there - peltier cooling drops precipitously in efficiency as the delta increases, and struggles to hit a COP of even 1 in real world scenarios. Their figure works out at about 6.5% Carnot efficiency, whereas a normal heat pump is usually nearer 45% over a much broader range of temperatures, as you can separate the hot and cold sides completely. Not so with a peltier wafer. What they’ve done here is add a point of failure, use additional materials as well as a traditional heat pump, and called it “AI” and “eco friendly”. Never have I seen more prime VC bait.
- deleted 1y ago
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icehawk 1y ago
The COP generally varies with Delta-T, a compressor-based heat pump's isn't any different. An AC unit is more efficient when it is cooler outside. That said, 1.5C is tiny.
binarymax 1y ago
For anyone wondering (like me) what COP is in this context, it’s Coefficient of Performance: https://en.m.wikipedia.org/wiki/Coefficient_of_performance https://en.m.wikipedia.org/wiki/Coefficient_of_performance
- Calwestjobs 1y ago
  and COP 15 is measure at 1.3'C temperature difference towards outside of fridge, so if it is 80f in your home then good luck to your lettuce.
  - deleted 1y ago
    [deleted]
  - falcor84 1y ago
    Why, what would happen to my lettuce if I set the thermostat near the cooling element to e.g. 3°C and have the cabbage be somewhere between that and 4.3°C?
    - user_7832 1y ago
      Unless I'm very mistaken, this 1.5 degree delta is between the hot side (the condenser at almost your room temp) and the cold side (evaporator inside the fridge). That is, if you want to chill your cabbage to 4.3 degrees C, your room temp should be not more than 5.6 degree C. Or... if it's 25 degrees outside, in an ideal case your cabbage is 23.7 degrees cool. Btw, max theoretical COP increases rapidly as T'hot - T'cold shrinks to zero. Your practical AC or fridge delta T is much more than that, depending on the weather.
mousethatroared 1y ago
COP has to be compared at the same delta T. The COP of a loss less refrigerator is infinite at Delta T = 0 And you dont get to stack Peltiers to increase COP, only to increase delta T. Still, Peltiers are super cool and I have some ideas for their use od they get slightly better. Advances are super welcome.
- jfengel 1y ago
  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 something.
  - mousethatroared 1y ago
    Careful! Wothout a heatsink they'll easily self destruct (solder on the PN junctions)
  - BobbyTables2 1y ago
    Had a similar experience myself. The thinness of Peltier devices bothers me. Amazing temperature differences… without any room to insulate such!
    - gsf_emergency_2 1y ago
      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
rcxdude 1y ago
ugh, reading the paper, their methodology is kinda crap. They basically just guess what the thermal resistances in their system are, and use air temperature measurements to figure out the heat flow. This is not how to measure heat flow accurately. It might be OK as a comparison with whatever TEC they tested with, maybe, but it's not at all something I would trust to compare to another test setup. If their box is more insulative than they think it is, their results are gonna look better than reality. This can be validated at least approximately by just putting a heater in the box that's dissipating a known amount of heat and looking at the temperature rise, but it seems they didn't even do this. And in general the regime where you've got small temperature differences is where your systematic error in a system like this can become huge and distort the results by multiples. (This is an area which is really hard and details matter. Heat is basically impossible to measure directly, and the indirect measurements are fraught with peril. Getting it wrong was a large part of why people thought they had demonstrated cold fusion)
scythe 1y ago
>in this HN post [1] shows claims of COPs of ~15 for Delta Ts of 1.3°C. >A compressor based cooler gets a COP of about 4 in the real world. Real life refrigeration usually isn't very interested in a difference of 1.3 C. The Carnot COP for this temperature drop near ambient conditions is, I believe, around 200. When you consider a cooling technology relative to the Carnot efficiency (or COP) you get a better idea of what the efficiency means in practice. For an AC unit blowing 10 C air on a 40 C day, the Carnot COP is about 10, while real units get less than half that. But I think that's still better than the Peltier effect getting less than 10% performance relative to Carnot limits.
tuukkah 1y ago
COP of peltier elements can be large only when the temperature difference is small, such as the measly 1.3 degrees you quoted. When do you want to cool something by only 1.3°C compared to the surrounding temperature?
deleted 1y ago
[deleted]

moffkalast 1y ago

15 is absurd, regular peltiers get like 1.05 at most don't they? That's like inventing the warp drive but for cooling.

1oooqooq 1y ago
lol yeah they did not. it's right on the abstract. > is ~15 for temperature differentials of 1.3 °C.
rcxdude 1y ago
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 the direction the refrigerant itself is moving.
- scosman 1y ago
  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?
  - dmsnell 1y ago
    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-tecs/ 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
- torginus 1y ago
  Physics dictates that the energy required to cool an object by 1C increases linearly, with difference to ambient temps. It's like trying to swim against the current - the faster the river flows, the harder it is to move forward at the same rate.

userbinator 1y ago

Delta Ts of 1.3°C. Might as well not use a refrigerator if your ambient temperature is that low.

LeifCarrotson 1y ago
You missed the "delta", meaning change. One side would have been ~23C and the other 24.3C.
- zdragnar 1y ago
  That's still a lot hotter than I'd like my refrigerator, to be fair to OP.
- userbinator 1y ago
  That's exactly what I mean --- if your ambient temperature is ~5C then you don't need a refrigerator if it only cools down by another 1.3C.

HPsquared 1y ago

What about a ΔT of say 20°C? I'd reckon most refrigerators and air conditioners are around there (temp difference of refrigerant between evaporator and condenser). Stacking a bunch of these Peltiers to give more temperature difference would give a pretty low CoP. Say, for a 13°C temperature difference you'd have to stack 10 of them and use 10x the power. It's even worse actually as the hotter ones have to also pump the waste heat from the cooler ones.

sokoloff 1y ago
The ΔT between evaporator (typically 2-6°C while cooling) and condenser (often 40-50°C in cooling mode) is much higher than 20°C. The condenser is often almost 20°C above ambient outside temperature. The design ΔT of ~10°C is the typical return-to-supply air ΔT.
lucb1e 1y ago
Note that a small temperature difference that is sustained very consistently over a long time using a tiny amount of electricity (let's say half of what the parent post cited, so like a COP of 8) could add up to a lot of nearly-free cooling. You'd chill your walls for weeks and when a heat wave comes with hot nights for a week, if you(r home automation) close(s) the blinds during the heat of the day, the more-powerful AC might barely have to do anything Just an idea of course, but I'd not write new tech off as "ok but just 1.3 degrees who cares" when the claimed COP is so insanely good without first trying it out
- Dylan16807 1y ago
  Chill for weeks? What kind of thermal mass are you thinking of there? A brick wall is on the heavy side and you'd be able to store a whopping 1/8 of a ton of AC in ten feet of brick wall. It'd save about 0.1kWh of AC power use. A whole house will have many lengths of wall but also multiple tons of cooling requirements, so that doesn't help much. And how are you going to distribute those small temperature differences without wasting a ton of power? And that's after you figure out how to trap a temperature difference in your walls for more than a day.

MobiusHorizons 1y ago

I believe the “apples to oranges” is the temperature gradient. AC units would routinely manage 15-20c and are rated for more than that. And some freezers manage up to 50c. The greater the gradient the worse the efficiency in general.

madaxe_again 1y ago

The delta of 1.3C is critical there - peltier cooling drops precipitously in efficiency as the delta increases, and struggles to hit a COP of even 1 in real world scenarios. Their figure works out at about 6.5% Carnot efficiency, whereas a normal heat pump is usually nearer 45% over a much broader range of temperatures, as you can separate the hot and cold sides completely. Not so with a peltier wafer. What they’ve done here is add a point of failure, use additional materials as well as a traditional heat pump, and called it “AI” and “eco friendly”. Never have I seen more prime VC bait.

deleted 1y ago
[deleted]

icehawk 1y ago

The COP generally varies with Delta-T, a compressor-based heat pump's isn't any different. An AC unit is more efficient when it is cooler outside. That said, 1.5C is tiny.

binarymax 1y ago

For anyone wondering (like me) what COP is in this context, it’s Coefficient of Performance: https://en.m.wikipedia.org/wiki/Coefficient_of_performance https://en.m.wikipedia.org/wiki/Coefficient_of_performance

Calwestjobs 1y ago
and COP 15 is measure at 1.3'C temperature difference towards outside of fridge, so if it is 80f in your home then good luck to your lettuce.
- deleted 1y ago
  [deleted]
- falcor84 1y ago
  Why, what would happen to my lettuce if I set the thermostat near the cooling element to e.g. 3°C and have the cabbage be somewhere between that and 4.3°C?
  - user_7832 1y ago
    Unless I'm very mistaken, this 1.5 degree delta is between the hot side (the condenser at almost your room temp) and the cold side (evaporator inside the fridge). That is, if you want to chill your cabbage to 4.3 degrees C, your room temp should be not more than 5.6 degree C. Or... if it's 25 degrees outside, in an ideal case your cabbage is 23.7 degrees cool. Btw, max theoretical COP increases rapidly as T'hot - T'cold shrinks to zero. Your practical AC or fridge delta T is much more than that, depending on the weather.

mousethatroared 1y ago

COP has to be compared at the same delta T. The COP of a loss less refrigerator is infinite at Delta T = 0 And you dont get to stack Peltiers to increase COP, only to increase delta T. Still, Peltiers are super cool and I have some ideas for their use od they get slightly better. Advances are super welcome.

jfengel 1y ago
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 something.
- mousethatroared 1y ago
  Careful! Wothout a heatsink they'll easily self destruct (solder on the PN junctions)
- BobbyTables2 1y ago
  Had a similar experience myself. The thinness of Peltier devices bothers me. Amazing temperature differences… without any room to insulate such!
  - gsf_emergency_2 1y ago
    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

rcxdude 1y ago

ugh, reading the paper, their methodology is kinda crap. They basically just guess what the thermal resistances in their system are, and use air temperature measurements to figure out the heat flow. This is not how to measure heat flow accurately. It might be OK as a comparison with whatever TEC they tested with, maybe, but it's not at all something I would trust to compare to another test setup. If their box is more insulative than they think it is, their results are gonna look better than reality. This can be validated at least approximately by just putting a heater in the box that's dissipating a known amount of heat and looking at the temperature rise, but it seems they didn't even do this. And in general the regime where you've got small temperature differences is where your systematic error in a system like this can become huge and distort the results by multiples. (This is an area which is really hard and details matter. Heat is basically impossible to measure directly, and the indirect measurements are fraught with peril. Getting it wrong was a large part of why people thought they had demonstrated cold fusion)

scythe 1y ago

>in this HN post [1] shows claims of COPs of ~15 for Delta Ts of 1.3°C. >A compressor based cooler gets a COP of about 4 in the real world. Real life refrigeration usually isn't very interested in a difference of 1.3 C. The Carnot COP for this temperature drop near ambient conditions is, I believe, around 200. When you consider a cooling technology relative to the Carnot efficiency (or COP) you get a better idea of what the efficiency means in practice. For an AC unit blowing 10 C air on a 40 C day, the Carnot COP is about 10, while real units get less than half that. But I think that's still better than the Peltier effect getting less than 10% performance relative to Carnot limits.

tuukkah 1y ago

COP of peltier elements can be large only when the temperature difference is small, such as the measly 1.3 degrees you quoted. When do you want to cool something by only 1.3°C compared to the surrounding temperature?

deleted 1y ago

[deleted]