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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?"
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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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.
Put my fist through my alarm clock with its ding-dong death inside my ear. - The Blackjacks.
People seem to forget that heat is power too.
People seem to forget that energy is not power.
Upward mobility is a slippery slope - the higher you climb the more you show your ass.