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LED's Efficiency Exceeds 100%
New submitter Paul Fernhout writes "Physicists from MIT claim to have demonstrated that an LED can emit more optical power than the electrical power it consumes. Researchers suggest this LED acts like a heat pump somehow (abstract). Is it true that 230% efficient LEDs seem to violate first law of thermodynamics?"
11 comments down and finally someone has actually understood enough of the summary to know that they aren't claiming that conservation of energy is dead.
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The LED is "consuming" external heat to produce the additional light. The article is pretty clear and an enjoyable read.
Yeah, but man, a it's a completely solid state heat pump that dumps waste heat as usable light - now that's something. Just imagine: every server, instead of needing cooling, can have this stuck to the heatsink and mounted on a tall pole. No more datacenter, we'll have datapoles, and our streets will be full of them :)
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Why? Entropic heat is energy, and so is light. What's so broken with converting one to the other?
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Why? It's conceptually equivalent to operate a thermoelectric module in reverse to get electrical energy and feed it into an LED to make light. In essence, you have made a heat pump with the LED.
Yes but taking advantage of entropic heat to generate coherent light would appear to violate the second law.
No, it doesn't, as long as the entropic heat being exploited costs more to organize than to disperse in the first place.
Namely: it would take more than the 39 picowatts of energy being generated to produce the heat to provide the additional 39 picowatts of energy.
The world is full of things that naively contradict the 2nd law of thermodynamics, because people misunderstand that you can have a localized violation of the 2nd law of thermodynamics, as long as the entire closed system that it is in counters that localized violation.
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People seem to forget that heat is power too. The room has heat either from the sun or because something else is using power to heat it (eg. furnance). Either way the energy isn't free it's coming from somewhere (cooling room, sun etc).
Now, some people might still be bothered by this, because the idea of using ambient heat to do useful work is another one of those "perpetual motion machine" kind of claims. Heat represents a disordered (high-entropy) state, from which you cannot extract useful work. The relevant thought experiment here is the Brownian ratchet: the idea being that you have a ratchet that gets bombarded by random molecular collisions (in water or air, say). The ratchet will turn foreward when a random collision is strong enough, and so over time you can use this turning motion to wind a spring and thus convert random thermal motion into stored energy. The reason this doesn't work in real life is because if random thermal motion is enough to overcome the pawl on the ratchet, then the pawl will be 'hot' enough that it will randomly and spontaneously lift up, turning the wheel backwards. The only way to avoid this is to have the pawl at a lower temperature than the rest of the mechanism: this works, but it's well-known that you can extract useful work from a thermal gradient, so the laws of thermodynamics remain intact.
Coming back to this present result, how does this device use ambient heat to generate useful photons? Sure, it acts as a thermoelectric cooler, establishing a local thermal gradient, but this sounds like 'cheating' in that it's a way to extract energy from the entropy of the surroundings! The very first sentence of the scientific paper addresses this:
Basically, the device is converting high-entropy thermal energy into even higher entropy incoherent electromagnetic radiation (light output). So, the second law of thermodynamics is not violated. Essentially, this device is acting as a way to connect thermal degrees of freedom to E&M degrees of freedom. The system, wanting to increase entropy as much as possible, tries to spread energy through all these degrees of freedom, which means creating some photons at the expense of some of the heat in the material.
It's a neat bit of physics, and will probably have implications for device efficiency and other applications.
The semiconductor PN junction is amazing. That's what's fundamentally inside LEDs. When appropriately tuned, PN junctions (a) permit electron flow in only one direction, demonstrating their diode nature, (b) convert current into light, like an LED, (c) convert current into a heat differential, like a Peltier junction cooler, (d) convert light into current, like a photo cell, (e) convert heat differential into current, like a solid-state thermionic energy converter, (f) act like a voltage-tunable capacitor, like a varactor, and more. In fact, to a very coarse first approximation, all PN junctions exhibit each of these characteristics to a greater or lesser degree.
So what's this group done? Shown that an appropriately tuned PN junction (or stack of them, I'd imagine) can be used to simultaneously act as a solid-state thermionic energy converter *and* an LED. Thus, it converts applied electricity to photons, but also converts a heat differential to electricity, which gets converted to photons as well, meaning it's sucking heat out of its immediate evironment. Cool stuff, if you'll pardon the pun.
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So far... only good enough for cooling a few molecules of beer.
The average man could outrun the first combustion-engine powered vehicles. The first modern computers took up entire rooms, were programmed with punchcards and were much less powerfull than the average 1990's cell phone.
We've got our foot in the door. What if we can improve on this the way we have with computers... and then put thousands of them in an array.
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The important thought most people seem to be missing is that your house gets warmer. People are forgetting that while the LEDs are getting cooler, somewhere is a battery or power plant on the other side of the circuit that must be getting hotter. And it must be getting hotter by more than the LEDs are cooled.
People seem to forget that heat is power too.
People seem to forget that energy is not power.
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Heat is only an energy store if there's some cooler place for that heat to flow to. If you can make use of the heat in a room in such a way that it maks part of the room cooler, and another part warmer, thne you're producing energy. If that exceeds the energy you have to feed in to make it happen, you have a perpetual motion machine.
It's not obvious how this LED works in this regard - it's quite unlikely that it's a net energy gain, but it doesn't seem to be the same transfer as a cooling laser.
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Oh, and do you really think that physicists from MIT didn't consider the blackbody effect when measuring the emissions??? Or their reviewers?? Please.
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Heat differentials have been used as power sources throughout history. This however seems to be extracting ambient heat energy directly, rather than using a differential. Very different animal.