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?"

Comments

Re:No

[ChronoReverse]

Exceeds 100% ELECTRICAL efficiency is the key here. The conservation of energy is still intact because it supposedly uses heat energy to supplement.

LED Cooling

[DarkXale]

So if I get the article right - LED cooling?

Really puts a whole new perspective on LED clad 'gaming'-machines, which as you know - should have blue LEDs for cooling, and red LEDs for superior overclocking.

Re:LED Cooling

[MartinSchou]

Really puts a whole new perspective on LED clad 'gaming'-machines, which as you know - should have blue LEDs for cooling, and red LEDs for superior overclocking.

You've got that the wrong way around.

Red hot objects are comparatively cooler than blue hot objects.

For instance, compare the temperature of a blue star with that of a red one. Or for a simpler approach, compare the blue part of a flame with the red part.

Re:LED Cooling

[Luckyo]

Not really functional. Looking at the chart, this works in fairly high temperatures (the curve that exceeds 100% is at 135C) and exceptionally low power input and light output.

Basically they are stating that at extremely low voltage and very high ambient temperature, LED can convert a small portion of heat around itself into luminescence. While interesting, practical applications are going to be minimal due to temperature, power and output luminescence values.

Re:No.

[nedlohs]

Or you could read the damn links and find out. But I guess easier to make guesses.

Not breaking any laws

[barlevg]

From the article: "The researchers didn’t try to increase this probability, as some previous research has focused on, but instead took advantage of small amounts of excess heat to emit more power than consumed. This heat arises from vibrations in the device’s atomic lattice, which occur due to entropy." The other thing to note is that these LEDs are being run at REALLY low power.

Re:Not breaking any laws

[Omnifarious]

In fact, it is very laser-like, but in solid, not gaseous form.

In a gas laser, you get the electrons in a whole bunch of molecules of gas into an excited state. Then you get a few photons of the desired frequency bouncing around in the gas and whenever they encounter an excited electron, it jumps back to the ground state releasing all of it's energy as a photon that's exactly in phase with the photon that triggered the collapse in the first place. This happens in a cascade effect and results in a massive release of all the energy in the gas at once.

If you could achieve that same effect in a solid, the atoms would be bunch up much closer together, meaning that you'd get many more released photons per volume.

One of the things I loved about Real Genius was how much of the science (aside from the hacking parts) made perfect sense.

In this LED, you have a laser-like situation. But instead of the energy being stored in the excited state of electrons, it's stored as kinetic energy.

An LED normally works by having the incoming electricity dump its energy into an electron that enters an excited state of some kind. I don't fully understand it, as it doesn't seem exactly analogous to the higher-orbital thing that happens in the gas in the laser example. But this electron moves through the material towards the other side, but eventually encounters a 'hole'. An energy gap in which there could be an electron, but there isn't right now. The hole and the electron combine and the electron gives up its energy as a photon.

Apparently, while heat is not normally enough to push the electrons around, and make them combine with holes spontaneously, the electrical current acts like some sort of ratcheting mechanism (like how flagella in bacteria take advantage of brownian motion) that allows the heat to push the electrons into holes. Though this is a guess and very rough analogy on my part.

Re:Not breaking any laws

[quarterbuck]

It is not foolish science. There are two differences that I see.
1) Black body radiation cannot be turned on or off at will at constant temperature. What these guys have figured out is a way to turn it on or off using electric power.
2) Since it is an LED it emits a specific frequency range of (visible) light. Black body radiation emits all frequencies, but peaks at a frequency dependent on the temperature. I doubt the materials used would have any noticeable amount of visible light at 135C. These guys have managed to somehow convert all these varying frequencies into the natural frequency of the LED at 135C.

The Law

[Anonymous Coward]

For those wondering about conservation of energy, it's intact. The extra energy comes from heat / vibration in the system.

For those concerned about the second law of thermodynamics, it's not specifically addressed in the article, but the smart money's on entropy increasing in this experiment. The second "law" is really just statistics though (law of large numbers anyone?), and as with most statistics people are still arguing about what it really means. See http://en.wikipedia.org/wiki/Second_law_of_thermodynamics#Controversies and http://en.wikipedia.org/wiki/Fluctuation_theorem

Re:The Law

[dotancohen]

The second "law" is really just statistics though (law of large numbers anyone?), and as with most statistics people are still arguing about what it really means.

StackExchange now has a physics section, and this issue was very recently addressed:
http://physics.stackexchange.com/questions/21028/second-law-of-thermodynamics-why-is-it-only-almost-always-true-that-entropy-i

Maybe

[MyLongNickName]

"30 picowatts and measured an output of 69 picowatts of light - an efficiency of 230%. The physical mechanisms worked the same as with any LED: when excited by the applied voltage, electrons and holes have a certain probability of generating photons. The researchers didn’t try to increase this probability, as some previous research has focused on, but instead took advantage of small amounts of excess heat to emit more power than consumed. This heat arises from vibrations in the device’s atomic lattice, which occur due to entropy."

They are not claiming more than 100% efficiency in total terms.

Re:Maybe

[Defenestrar]

A peltier with a light dump instead of a heat dump. There have been a lot of variants on this idea, they usually remind me of the hybrid cooling/propulsion laser David Brin used for his Sundiver book.

Re:No.

[Dynedain]

It says in the summary (and in the article) that the LED at very low electrical input levels, acts as a heat pump. It absorbs local heat energy and converts into photons.

So you get more light out than electricity in, because you're stealing heat and converting it to light. It's not more than 100% efficient, it's multiple energy sources being used. No breaking the laws of thermodynamics.

Re:No

[jdgeorge]

According to TFA, they are actually taking advantage of other sources of energy in addition to the electricity provided by the wall plug. So it's not really the LED getting "greater than 100% efficiency", it's really "producing more light than you would get if you only took advantage of the electricity from the wall plug".

And they're talking in the range of 69 picowatts of light output, using only 30 picowatts of "wall plug" energy input. So it's quite believable.

Re:No

[MightyMartian]

Yes, to say 230% efficient is really a false statement. There's no violation of thermodynamics, it's just that the LED has more energy sources than the electrons it's drawing down the wire.

Good time to RFTA

[mykepredko]

Interesting to see the number of posts saying that this is absolutely not possible - reading through the article, it seems possible and maybe there is enough here to study the phenomena enough to warrant more investigations.

The LED seems to be emitting 69 picowatts (pico = 10^-12) when only 30 picowatts of electricity is being pumped in with a measurable decrease in the temperature of the LED. This implies that the LED is acting as a heat pump, converting heat energy into light. If you've ever seen a Peltier cooler in action (or worked through the operation), it seems like to me this is possible.

Note that the power level this phenomenon is observed at is extremely low - the result is maybe good enough for cooling a few molecules of beer - but I think there is something here that should be investigated to see if any usable applications could come out of it.

myke

Re:Can't break a fundamental law of physics

[Anonymous Coward]

A third option is that you didn't read the article.

Re:No

[Maddog Batty]

No it isn't fiddling with numbers. You are missing the heat pump bit.

The device is taking X amount of energy from the electricity supply and X * 1.3 of energy as thermal and converting this to X * 2.3 as light. i.e. it is 230% efficient when comparing light output to electrical input. Equally, it is 100% efficient when comparing light output to electrical and heat energy input combined.

This does take a little bit of thinking to get your head around but I have a more common example in the shed outside. It contains a heat pump which is 350% efficient. It takes 2kW from the electricity supply and outputs 7kW of heat energy to heat my house. The missing 5kW comes from the pipes in the garden as heat energy. The result being that the garden is slowly being cooled. http://en.wikipedia.org/wiki/Heat_pump

Re:Nothing violates the first law in this universe

[camperdave]

No, it is not. The linked article is quite clear: the LEDs are geting colder, so the extra power output comes from the environment.

Now, 230% efficiency suggests that it is operating as a >100% efficiency heat pump, and that's also impossible. It might be decomposing itself in an endothermic(sp?) chemical reaction, or something.

It's not impossible. Heat pumps are routinely of greater than 100 percent efficiency, because they don't measure the input heat, just the output heat and the input electricity. Sure, it's a marketing scam, but what-r-ya gonna do?

This is actually possible

[Sqr(twg)]

This is not as incredible as it sounds. To explain how it works, it is perhaps easiest to start with a simpler device. I could take a brick, connect a battery to it and say "Look! This brick is only consuming one milliwatt of electric power, yet it is emitting one Watt of infrared radiation. That is 100 000 % efficiency!" If I did the same thing at 1 000 degrees Celcius, the brick would even be emitting visible light (wether connected to a battery or not.)

What the people at MIT do is a little more complicated. They don't use the black body radiation directly. Instead they take electrons that would have emitted infrared photons, add some more energy to them, and get visible light. For this to work, they only have to add the difference between the energy of an infrared photon and a visible photon, yet they get the light output of a visible photon. At a temperature of 135 degrees Celcius (that is 275 degrees Farenheit if you happen to live in Belize or the United States) the difference between the black body radiation and visible light was small enough that they managed to get over 100 % efficiency. No laws of thermodynamics were violated.

Re:No

[thsths]

> The fun trick will be to point this at a 45% efficient photovoltaic panel to generate the electricity.

No chance, at 2.5um that is even theoretically impossible. Higher efficiency requires much shorter wavelength.

The second law of thermodynamics

[goombah99]

This is not violating the first law of thermo (energy conservation). It is getting the energy it needs from it's environment.

However it might possibly be violating the second law of thermo. Turning heat into light at high efficiency should not be sustainable. energy in the form of light has more less entropy than energy in the form of heat.

I could imagine that, in burst mode, that some energy is somehow being stored so that it can when triggered temporarily emit more or seemingly defy entropy. For example perhaps the crystal lattice is disorganizing during emission and then self healing to an organized state over time. This would be taking energy from the environment and shedding entropy to the environment and not neccessarily viloating any laws.

So some game is being played and I'm surprised anyone would publish the findings without an explanation for this.

Re:The second law of thermodynamics

[Chris Burke]

It isn't violating the second law for the same reason it's not violating the first -- the system in question is bigger than the LED itself. It includes the environment from which it is obtaining its energy. Local decreases in entropy are not disallowed.