4 ms·
The led itself should cool down if it is producing more power in light then is being put in electrically. The energy is coming from the the lattice phonons and
by davorak 14y ago
The led itself should cool down if it is producing more power in light then is being put in electrically. The energy is coming from the the lattice phonons and as a result the lattice cools down.
The paper[1] behind a pay wall show has a graph showing its cooling power vs current.
I think where the light is shining into is open space so the source temperature has to exceed what ever temperature the local background radiation is at; a common local minimum being the cosmic background radiation.
[1] http://prl.aps.org/abstract/PRL/v108/i9/e097403 http://prl.aps.org/abstract/PRL/v108/i9/e097403
- ars 14y agoYes, the LED is cooling down, but it will not cool down any lower than the ambient air. i.e. you can not use this as an A/C - you must already have a cold source in order to use it. The cosmic background radiation is not exactly common. You'd have to be in space. BTW, I don't think we are actually arguing.
- electrograv 14y agoI don't understand. Why can't it cool down more than ambient air? You put in X amount of power, and you get out 2X amount of power as light -- therefore additional energy is being taken in as heat and converted to light. Say you direct that light AWAY from the area through mirrors or something so it doesn't heat anything nearby up. Then, by simple energy thermodynamics, you can show that X amount of energy IS being taken away from this local system and deposited elsewhere as light. Unless you claim that the LED stops producing the >100% efficiency levels at room temperature, I don't see why you're claiming it can't cool anything down.
- aqme28 14y agoSecond law of Thermodynamics. It takes additional energy to move heat up a temperature gradient, and you're not accounting for that. You can't move heat from a cold source to a hot sink without paying for it somewhere. edit: I don't mind a downvote now and then, but I would really prefer a discussion or explanation to a fire-and-forget downvote.
- khafra 14y ago> Why can't it cool down more than ambient air? You put in X amount of power, and you get out 2X amount of power as light -- therefore additional energy is being taken in as heat and converted to light. Light is a more highly ordered form of energy than thermal noise; it can be used to do work. If you could convert thermal noise into light and beam it away, you could use that as the cool side for a heat engine and have a perpetual motion machine.
- Cushman 14y ago> If you could convert thermal noise into light and beam it away, you could use that as the cool side for a heat engine and have a perpetual motion machine. By "beam it away", I assume you mean into space or somewhere else you don't care about heat being. In this case, the cold side of the device isn't the real cold sink, it's just a proxy for the several million cubic lightyears of background-temperature near vacuum which will eventually wind up absorbing the light. Aliens on a distant planet may be able to extract power from the light; bully for them, but I already had to pay extra for shipping, so there's no funny business going on. What makes you think that violates thermodynamics? (For what it's worth, "perpetual motion machines" are not outlawed by classical thermodynamics, only those which do additional work. There's nothing in theory which prevents you hooking up a 100% efficient emitter and a 100% efficient receptor up with superconducting wire and letting it run forever, so long as you don't extract any energy from the cycle, for example by observing that it is taking place. There may be something in quantum physics which states that such a system would have to leak information about itself somehow, but I don't know enough to say.)
- frugalfirbolg 14y agoI can still see uses for it only cooling down to ambient air temperature. I'm thinking of high-frequency transistors / ICs with heatsinks and a bank of these lights being attached. It wouldn't be a primary cooling method, but if you needed a light on your device anyways then you could make use of this mechanism as well. Imagine backlights for LCDs in mobile devices. An iPad or Android tablet can easily reach 100 F when playing games, and for most cases that's warmer than ambient air temperature. I don't have access to the original paper (haven't checked arxiv yet). Do the lights need to be at a certain temperature for this to work at all? I know that in similar experiments (condensates) the lattice vibrations only propagate at temperatures much lower than 70 F. At room temperature there is too much chaotic motion in the structure. I'd see that as a major limitation on scale and real-world application.
- nl 14y agoYes, the LED is cooling down, but it will not cool down any lower than the ambient air. i.e. you can not use this as an A/C - you must already have a cold source in order to use it. Where are you getting that? A Peltier-effect refrigerator can certainly cool things lower than the ambient temperature. It does that by moving the heat from one place to another. I'm not aware of anything fundamental that says that the heat energy has to be radiated as heat - it could just as easily be radiated as light. So why couldn't this cool things? (Your argument that it could somehow work if you shone it on something that was cooler than the device implies time travel - the photons would need to now the destination is cooler before they are emitted.