Ultra-efficient LED puts out more power than is pumped in

[someone_else]

Article

The paper, Thermoelectrically Pumped Light-Emitting Diodes Operating above Unity Efficiency.

MIT physicists have managed to build a light-emitting diode that has an electrical efficiency of more than 100 percent. You may ask, "Wouldn't that mean it breaks the first law of thermodynamics?" The answer, happily, is no.

The LED produces 69 picowatts of light using 30 picowatts of power, giving it an efficiency of 230 percent. That means it operates above "unity efficiency" -- putting it into a category normally occupied by perpetual motion machines.

However, while MIT's diode puts out more than twice as much energy in photons as it's fed in electrons, it doesn't violate the conservation of energy because it appears to draw in heat energy from its surroundings instead. When it gets more than 100 percent electrically-efficient, it begins to cool down, stealing energy from its environment to convert into more photons.

In slightly more detail, the researchers chose an LED with a small band gap, and applied smaller and smaller voltages. Every time the voltage was halved, the electrical power was reduced by a factor of four, but the light power emitted only dropped by a factor of two. The extra energy came instead from lattice vibrations.

The scientists involved have detailed their discovery in a paper published in Physical Review Letters, saying: "Experiments directly confirm for the first time that this behaviour continues beyond the conventional limit of unity electrical-to-optical power conversion efficiency."

69 picowatts of light, of course, is a very small amount -- so you're not likely to be able to read in bed with one of these LEDs. However, it could have applications in low-power electronics, acting as a thermodynamic heat engine but with fast electrical control.

I've seen (random) people claim and hope that this will solve the problem of radiating away heat in space.
Is it true?

Also, what other uses has this trick?

[Dass.Kapital]

So...as a side note/adding to the question...could such an LED be worked into a laser of some sort and hence provide thrust (Yes, I i know it wouldn't be much) as well as cooling?

Also...will such a small thing be able to be 'scaled up'..or is this as big as it gets?

Much cheers to you and yours.

[Dominus Atheos]

One Thing I thought of when I heard about this was power generation from heat. If we pair this with photovoltics, we can create entire powerplants that draw power from the air around them.

[Rabid]

Maybe not whole powerplants, but one usage I could see is coupling it with the heat sinks of Air Conditioners systems, as a mean to reduce the electricity bill.

The fun thing is that, if I understand it right, it could allow us to create "cold spots" in situations where otherwise the heat would tend to become homogeneously distributed => associated with stirling engines (to run a generator), geothermal pipes (as a source heat) and betavoltaics batteries (as a source of electricity) for extra-durable power supplies in a deep-underground Vault where dumping heat would otherwise be a hard task ?

[Sea Skimmer]

I’d be very interested to know the operating range at which this can work. It might not like space very much

[Winston Blake]

Oh wow. I went to the paper because I couldn't fathom this and I thought maybe I was a little bit retarded. First sentence:

A heated semiconductor light-emitting diode at low forward bias voltage V<kBT/q is shown to use electrical work to pump heat from the lattice to the photon field.

A heated semiconductor light-emitting diode

heated

So... it needs to be heated on purpose to make it hotter than its environment?

Doesn't this mean, overall, that it's like an engine that draws power from the process of heat moving from a source to a sink? Except instead of producing mechanical power as usual it produces light?

• OK, taking Destructionator's air conditioner idea to the extreme, let's say I unplug my fridge and leave the door open. Now the temperature inside is the same as outside. I put one end of a thermocouple inside the fridge and close the door, and leave the other end hanging outside. The thermocouple is connected to an LED on a nearby table, which has a second connection to a DC adaptor (which is connected to the same powerpoint as the fridge). One LED, one powerpoint, two routes.
  
  When I flick the powerpoint on, the LED lights up and the fridge turns on. As the fridge cools down, it cools one end of the thermocouple, and the LED brightens slowly, reaching a maximum brightness. The other end also gets warmer because the fridge warms up the whole room. So far so good.
  
  I now get a second thermocouple, with one end hanging out a crack in the window and the other end lying inside the room. Since the room is now warmer than the outside of the house, power is produced. This is connected to the LED, but although it gets a little brighter, its output power is still less than the power from the powerpoint, because the energy to heat the room originally came from the powerpoint. One LED, one powerpoint, three routes. So far so good.
  
  I then turn on an electrical furnace in the room, powered by a different powerpoint. The temperature inside the room rises. This reduces the power produced by the first thermocouple, but increases the power produced by the second thermocouple. Theoretically I could fill the room with so much heat that I can draw any amount of power from the second thermocouple.
  
  Now, when I measure the input power to the LED, I measure it at the first powerpoint, and neglect the second powerpoint. I find that the 'input power' is lower than the output power.

I then write a paper and put 'ABOVE UNITY EFFICIENCY' in the title. I say in the abstract: 'this system is shown to use electrical work to pump heat from the room to the LED' and "Experiments directly confirm for the first time that this behavior continues beyond the conventional limit of unity electrical-to-optical power conversion efficiency".

Is this what is actually happening? They invented an integrated LED-thermocouple device, and put a false & sensationalist title on it?

I would appreciate input from someone who actually studied thermodynamics properly. I didn't have to study thermodynamics in my degree (or if I did I can't remember it) but I knew there was something fishy here.

[aerius]

Sounds like some sort of Peltier effect LED to me. It sets up a temperature differential with its surroundings to get the Peltier effect going which then sucks in heat that's then converted into light.

[madd0ct0r]

still intresting - turning heat into photons sounds a very good way of radiating them off.

[Grumman]

Is this what is actually happening? They invented an integrated LED-thermocouple device, and put a false & sensationalist title on it?

I would appreciate input from someone who actually studied thermodynamics properly. I didn't have to study thermodynamics in my degree (or if I did I can't remember it) but I knew there was something fishy here.

I think you're right. Very disappointing, as my first thoughts were "Holy ****! They've broken the first law of thermodynamics?" and then "Holy ****! They've broken the second law of thermodynamics?"

But no, it sounds like at best it's an easier/simpler way to do something we can already do.

[someone_else]

Doesn't this mean, overall, that it's like an engine that draws power from the process of heat moving from a source to a sink? Except instead of producing mechanical power as usual it produces light?

Yes, but there is a significant point to be excited about. There is no sink. Heat is sucked in but not dumped in a sink. It goes away as photons.

Not something you can do with the average heat pump or engine.

But no, it sounds like at best it's an easier/simpler way to do something we can already do.

Yeah, because everyone can turn heat into photons without obeying the blackbody radiation laws.

IF you scale this stuff up to decent levels (a moon-sized IF, mind me), you can get Sundiver's refrigeration lasers.

But it's much more likely that this will see use in optic circuitry, the "electronics of tomorrow" so to speak.

[Winston Blake]

Doesn't this mean, overall, that it's like an engine that draws power from the process of heat moving from a source to a sink? Except instead of producing mechanical power as usual it produces light?

Yes, but there is a significant point to be excited about. There is no sink. Heat is sucked in but not dumped in a sink. It goes away as photons.

Not something you can do with the average heat pump or engine.

I accept that some heat energy is converted to light. I don't currently accept that the system, as a whole, violates the second law of thermodynamics.

I'm going to wait and see.