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Not sure I quite understand how this works. The heat / infrared emissions it absorbs has to go somewhere right? I haven't seen any mention on that matter in the

by m-p-3 8y ago

Not sure I quite understand how this works. The heat / infrared emissions it absorbs has to go somewhere right? I haven't seen any mention on that matter in the article.

pmiller2 8y ago
An LED converts current into light. Reverse biasing it causes it to convert light into electricity (i.e it acts as a photodetector).
- sebazzz 8y ago
  So how does it work with the law of conservation of energy? [1] [1]: https://en.wikipedia.org/wiki/Conservation_of_energy https://en.wikipedia.org/wiki/Conservation_of_energy
  - saagarjha 8y ago
    It's just converting between two forms of energy: total energy is still conserved. You might be thinking about the second law of thermodynamics, which limits how much can be converted.
  - jfoutz 8y ago
    Like you'd expect? No one is making claims of free energy. It's like putting a stirling engine on the side of an internal combustion engine. You have a chance to grab some of the waste heat, why not use it? Specifically, capturing that waste heat will boost efficiency by some small %, if the additional systems total construction cost is less than the efficiency given over the lifetime of the system, why not?
mmmrtl 8y ago
Presumably the electricity would be used to provide some of the energy requirements of the computer - in effect, the tower would produce less "waste" heat
timmaxw 8y ago
(Disclaimer: I have some background in physics, but I'm not an expert.) I think the heat is transferred from the "computer" to the LED itself. So the LED itself heats up. To go into a bit more detail: All physical objects emit thermal radiation (a.k.a. black-body radiation [edit, not quite correct]). The hotter the object is, the more radiation it emits and the shorter the wavelength of the radiation. For "red hot" objects, the radiation is in the visible part of the spectrum; for room-temperature objects, it's in the infrared, so you can't see it, but it's still being emitted. If you put two objects of different temperatures next to each other, they'll both emit radiation and absorb each others' emitted radiation. The hotter one will emit more radiation than the cooler one, so there will be a net transfer of energy from the hotter one to the cooler one (normally). But in this experiment, when the researchers run the LED in reverse, this somehow suppresses the LED's normal thermal radiation emissions. The "computer" continues to emit thermal radiation, which is absorbed by the LED just like normal. But the LED doesn't emit thermal radiation back to the computer (at least, not as much as it normally would). So even though the LED is hotter than the computer, the LED acts like it's colder for thermal radiation purposes. So there's a net transfer of energy from the cooler "computer" to the hotter LED. Of course, this isn't a complete solution; you still have to do something about the heat once you've transferred it to the LED. The article doesn't address this part of the problem. But I speculate you could just attach a heat-sink to the LED. Heat sinks work better at higher temperatures, so the computer+LED+heat-sink solution would be more effective than a computer+heat-sink alone.
- the8472 8y ago
  > All physical objects emit thermal radiation (a.k.a. black-body radiation) Not all materials are black body emitters. The black body spectrum is in fact an idealized emission spectrum which only few materials approach. An LED most definitely is not a black body emitter. You can engineer materials that absorb/emit a specific part of a spectrum and reflect/don't emit in other parts. This has uses in thermal engineering - e.g. you can shed heat by making things "white" in most of the spectrum but "black" in the infrared windows of the atmosphere. In this case they tune the behavior electrically.
  - 8y ago

by m-p-3 8y ago

Not sure I quite understand how this works. The heat / infrared emissions it absorbs has to go somewhere right? I haven't seen any mention on that matter in the article.

pmiller2 8y ago

An LED converts current into light. Reverse biasing it causes it to convert light into electricity (i.e it acts as a photodetector).

sebazzz 8y ago
So how does it work with the law of conservation of energy? [1] [1]: https://en.wikipedia.org/wiki/Conservation_of_energy https://en.wikipedia.org/wiki/Conservation_of_energy
- saagarjha 8y ago
  It's just converting between two forms of energy: total energy is still conserved. You might be thinking about the second law of thermodynamics, which limits how much can be converted.
- jfoutz 8y ago
  Like you'd expect? No one is making claims of free energy. It's like putting a stirling engine on the side of an internal combustion engine. You have a chance to grab some of the waste heat, why not use it? Specifically, capturing that waste heat will boost efficiency by some small %, if the additional systems total construction cost is less than the efficiency given over the lifetime of the system, why not?

mmmrtl 8y ago

Presumably the electricity would be used to provide some of the energy requirements of the computer - in effect, the tower would produce less "waste" heat

timmaxw 8y ago

(Disclaimer: I have some background in physics, but I'm not an expert.) I think the heat is transferred from the "computer" to the LED itself. So the LED itself heats up. To go into a bit more detail: All physical objects emit thermal radiation (a.k.a. black-body radiation [edit, not quite correct]). The hotter the object is, the more radiation it emits and the shorter the wavelength of the radiation. For "red hot" objects, the radiation is in the visible part of the spectrum; for room-temperature objects, it's in the infrared, so you can't see it, but it's still being emitted. If you put two objects of different temperatures next to each other, they'll both emit radiation and absorb each others' emitted radiation. The hotter one will emit more radiation than the cooler one, so there will be a net transfer of energy from the hotter one to the cooler one (normally). But in this experiment, when the researchers run the LED in reverse, this somehow suppresses the LED's normal thermal radiation emissions. The "computer" continues to emit thermal radiation, which is absorbed by the LED just like normal. But the LED doesn't emit thermal radiation back to the computer (at least, not as much as it normally would). So even though the LED is hotter than the computer, the LED acts like it's colder for thermal radiation purposes. So there's a net transfer of energy from the cooler "computer" to the hotter LED. Of course, this isn't a complete solution; you still have to do something about the heat once you've transferred it to the LED. The article doesn't address this part of the problem. But I speculate you could just attach a heat-sink to the LED. Heat sinks work better at higher temperatures, so the computer+LED+heat-sink solution would be more effective than a computer+heat-sink alone.

the8472 8y ago
> All physical objects emit thermal radiation (a.k.a. black-body radiation) Not all materials are black body emitters. The black body spectrum is in fact an idealized emission spectrum which only few materials approach. An LED most definitely is not a black body emitter. You can engineer materials that absorb/emit a specific part of a spectrum and reflect/don't emit in other parts. This has uses in thermal engineering - e.g. you can shed heat by making things "white" in most of the spectrum but "black" in the infrared windows of the atmosphere. In this case they tune the behavior electrically.
- 8y ago