MIT Researchers Produce Micro-Light that Outputs More Power Than Applied
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It's basically converting the heat difference into light.
So you can't use it just to cool, it doesn't work that way.
From the description given, the diode uses current to exchange ambient heat for emitted light. It cools the device, and heats whatever the light falls on, essentially acting as a remote heat pump. We can convert waste heat into a somewhat larger amount of "waste light", effectively increasing black-body radiation, which is great if light is more useful to us than heat.
Of course you don't get a free ride, and you can't make something cool without making something else hot (eventually), but conventional air conditioning is bound by the same limitation and is still plenty good enough for our purposes.
In principle, what prevents me from covering my roof with these things, radiating my waste heat (less atmospheric absorption) into deep space, and thus cooling my house?
Again, this supposing that the device could be scaled up to produce usable amounts of visible light while retaining the local cooling effect at room temperature, none of which have yet been demonstrated. Apparently their technique is to run the diode at as low of a voltage as possible, which presents some obvious problems.
[0] http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.108....
Actually the abstract states it acts as a "thermodynamic heat engine", which would support what you're saying-- but given that it requires an input of electrical energy, I don't see how that could be right. They also liken it to "thermoelectric coolers". Possibly that was just mistyping or sloppy terminology, or possibly I'm missing something.
No, that's only if you are in outer space. On earth it's the temperature of the air, or the walls.
It doesn't matter if the light eventually gets to outer space - what matters is that there is something hotter near you (namely the air).
i.e. it's not where the photons end up that matters, it's the total of all the photons that are being sent in your (the lamp's) direction that counts.
> it's not where the photons end up that matters, it's the total of all the photons that are being sent in your (the lamp's) direction that counts.
This is what I was referencing when I siad "the local background radiation." The common minimum was supposed convey high frequency when choosing a random spot in the known universe.
> On earth it's the temperature of the air, or the walls.
The temperature of the air only comes into play if it is emiting radiation at the frequency of light produced by the led and at greater intensity. I do not see why this would be required.
Blackbody radiation. Air (everything) always emits light.
> I do not see why this would be required.
It's a fundamental law of thermodynamics that requires a heat source and a cold source in order to convert heat into some other form of energy.
The air around you is the cold source, and the LED lamp is the hot source. If the LED is colder than the air it will not work - the LED must be hotter (in this particular case they put it in an oven).
The thing I think you're not taking into account is that this device has the effect of slightly increasing your black-body radiation. So you're at 1300K, radiating at 1300K, but we apply a little electricity and you start radiating a little more-- maybe you now look like you're at 1301K. Your heat hasn't actually increased, just apparent radiation, and as a consequence you cool down a little faster than your black-body profile would suggest.
So say you're at room temperature, radiating at room temperature, being warmed by the air, and not losing any heat. We apply current again, and you start radiating at a little over ambient temperature. The air is still warming you, but you're cooling a little faster than before, with the extra heat being deposited wherever the radiation is absorbed. To the air, you now feel cool.
Why should the absolute temperature of the ambient air have an effect on the process?
Fridge lights? House lighting in warm regions? Hmmm. On second thought it could have interesting applications depending how much it cools.
I doubt that they'll be able to scale this up too much though, since it probably relies on an effect that only occurs below a certain threshold.
I'd love a computer that glows brighter to release heat when it gets too hot, though :-)
It's like a peltier cooler[1], but it produces light instead of heat.
I'm thinking of the most amazing beer cooler/Christmas light display ever!
It uses that heat to make light, but it is not a cooler (i.e. a heat pump).
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.
The cosmic background radiation is not exactly common. You'd have to be in space.
BTW, I don't think we are actually arguing.
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.
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.
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.
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.
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.
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.)
Are you asking something more specific than that?
From the description given in the article, it seems that the energy input is simply being used to move energy from one location to another – it just so happens that the energy is sourced as heat, and is deposited as light. This is really not much different from a heat pump, so I see no reason it can't move heat up a gradient.
When used as a heat pump, the device might be useful for solid-state cooling applications or even power generation.
http://phys.org/news/2012-03-efficiency.html
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.
http://www.science20.com/cool-links/optical_heat_pump_100_pe...
When an electrical power input as low as 30 pico-watts is applied, the researchers consistently measured a 69 pico-watt optical power output, seemingly defying the law of conservation of energy. However, upon further investigation, the researchers showed that the system was able to derive extra energy from the thermal energy present in the environment. More specifically, the LED was able to convert the heat generated by the vibrations in the LED’s silicon lattice structure into additional emitted photons, increasing the LED efficiency to over twice unity.
Achieves greater than unity efficiency by simultaneously converting thermal and electrical energy into photons....Cools the surrounding environment as it draws thermal energy from it.
All it takes is the ability to channel some of the light (energy output) away from the reservoir of heat to show that the reservoir of heat will decrease.
Ultimately you'd expect a temperature gradient at the system boundary to start transferring heat back into the system. through the first law of thermodynamics.
Tony Stark built it in a cave ... with a box of scraps