If photon upconverting materials exist, how come I can't

>>1041313
Because the useful energy that can be extracted by any process is thermodynamically limited to at most Carnot efficiency, which depends on temperature. Though temperature isn't usually associated with individual particles like photons, temperature is a measure of kinetic energy per particle, so it's calculable. Visible-range photons have a thermodynamic temperature of multiple tens of thousands of degrees, and so energy manipulation involving them can in principle be rather efficient. Low-level heat sources have a much lower maximum efficiency. Additionally, the "temperature" of electricity is also very high, with one Volt representing about 11,000 degrees, so "upconverting" thermal photons into a useful Voltage would be much more involved than visible or near-visible upconversion. That said, some people are working on this as a way to make solar-thermal electricity more efficient.

>>1041313

Because the Seebeck effect works well enough on the small scale, sterling engines work well enough on the medium scale and turbines work well enough on the large scale. There is no point in that continuum where infra red photonic capture is best. Also capturing radiated infra red energy and fucking around with it is less effective than capturing kinetic heat directly.

Even if the use of radiated heat was desirable you could simply use infra red tuned photoelectric cells directly without up converting the photons.

There is some interesting work on piezo based thermal electric generators.

>>1041329
>>1041335
So it's not that it's impossible, it's that no one does it because it has an extremely low yield?

Still, the solution sounds like it's just to apply the effect to something that has an extremely large surface area.
e.g. a sponge that absorbs its own thermal radiation.

>>1041361
what a bunch of retards who dont know what a thermocouple is

>>1041373
Thermocouples require a heat disparity.
My question pertains to objects at room temperature.
>inb4 "because it breaks the laws of thermodynamics"
Saying "because it breaks the laws of thermodynamics" isn't a valid explanation, because the laws of thermodynamics relies on other phenomena to uphold it.

>>1041377
>other phenomena to uphold it
Without a temperature differential to drive it, energy-extracing processes typically operate at least as well in reverse as forwards. So due to the law of large numbers, there is at best no net electricity generation.

In this case, the ability to absorb photons and generate electricity requires the inherent ability to absorb electricity and release photons. Without an energy difference in the photons and electrons at hand, there is nothing to bias the process one way or the other.

Note that these >>1041329 >>1041335 involve extracting energy from a heat gradient. Photovoltaics only work if the photons being harvested are hotter than the material of the collector, and there are other techniques that can extract energy if that's the case.

>>1041384
But the whole point of the OP was that if you use a photon upconverter, however it may work, then the photons ARE hotter than the material of the collector.

>>1041389
Photon upconverters only work on incoming photons that are also hotter than the collector. It's a bit like using a transformer to increase electrical Voltage. Without an energy input, it won't do anything.

>>1041393
I've heard of the idea in the OP being studied though; if it was as simple as "this physics law denies this being possible" then surely it wouldn't be studied?
And also, heat can't be perfectly uniform on the extremely small scale, which just so happens to be the scale on which this concept operates.

>>1041397
>I've heard of the idea in the OP being studied though
Yes. They can turn heat into electricity, as mentioned in the OP. That is, they operate on thermal gradients, same as every other heat engine ever built or seriously proposed. Not reversing the entropy of ambient heat as clarified here >>1041377. If photon upconverters could do that, it would be bigger news than I can think of a suitable example for at the moment. There is also such a thing as basic research, where scientists poke at the universe to figure out how it works without having a particular application in mind.

>if it was as simple as "this physics law denies this being possible" then surely it wouldn't be studied
It's studied for use in situations that involve heat gradients, like enhancing solar energy collectors. Photon upconversion also has various other uses for which it is studied.

>heat can't be perfectly uniform on the extremely small scale
True, and nothing at all prevents an individual atom of a photon upconverter from turning a small amount of ambient heat into a visible photon suitable for photovoltaics, resulting in a small reduction in entropy. It's just unlikely to occur without a power source, and producing usable electricity means that it has to happen a lot, with a statistical bias that it has no reason to have in the absence of a power source. See >>1041384. This is like relying on a flipped coin to land on heads quintillions of times per second more often than tails (one amp involves over 6E18 electrons per second of flow). Only it's worse, since spontaneous reductions in entropy are less likely than spontaneous increases in entropy, so it's more like relying on the coin to land on its edge more often than tails.

>>1041419
I'm talking about study of systems without heat gradients.
Though it's by word of mouth from an undergraduate professor I was talking to.

And since we're not concerned with the rest of our universe, but only the rare cases in which a passive system produces a high energy photon we can use to bump an electron, if we can get even a single high energy photon to come out by sheer luck then the basic idea is a success and the rest is nanotechnology we're not capable of yet.

>>1041422
It's the same problem as a Brownian ratchet. If the mechanism for making use of that high-energy photon allows the energy to move from the point of spontaneous generation to some point of storage (it has to to produce useful power), then it must inherently allow the stored energy to flow back out into thermal noise. And the very best that such a system can do without an external power source is to break even. And it can only do that on a cold day. Except it doesn't get that cold.

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143KB, 499x499px

>>1041377
>Saying "because it breaks the laws of thermodynamics" isn't a valid explanation

oh boy here we go

>>1041428
I was just going to post this.

>>1041428
That's basically just saying "solar panels don't work" if you're now talking about the photovoltaic component.

>>1041454
And solar panels don't work. That is, they cannot produce electric current from ambient heat / thermal photons. Alas, if only solar panels had some kind of external power source, they might be able to do some interesting things.

>>1041458
They're not all thermal photons though, due to the possibly-very-weak-but-not-zero rate of upconversion.

>>1041454
>>1041630
So you're agreeing that photovoltaics can't produce electrical current without an input of high-energy photons? Then we're back to why photon upconverters also need an input of high-energy rather than ambient-thermal photons to work. Have you read the thread?

>>1041653
But photon upconverters operate on the extremely small scale, where heat is not uniform; for something of a given temperature, half the atoms will be below that temperature.
So those atoms can be hit by thermal photons corresponding to higher temperatures than them, because at that moment the atom could be at an unusually low temperature.

>>1041683
And? Do you believe I didn't know that? That's part of the background that thermodynamics operates in. My post here >>1041384 explains why that doesn't work for producing usable energy from ambient heat.

To repeat, thermodynamic reactions are reversible, with the direction and equilibrium of the reaction depending on conditions like the temperatures of the different components and the changes in entropy involved. A device that can upconvert photons can also downconvert photons, and without an external input of energy (as would be lacking in a thermal equilibrium state), the downconversion is favored due to its increase in entropy. The mechanism for extracting power from upconverted photons is also subject to these considerations. Since energy can flow from thermal fluctuations to a useful task, energy can also flow from the embodied energy associated with that task back to thermal fluctuations. In the absence of a heat gradient to power the export of entropy, the latter is favored.

1/2

>>1041683
2/2

As I mentioned here >>1041428 the situation is akin to a Brownian ratchet https://en.wikipedia.org/wiki/Brownian_ratchet in that without a thermal gradient for power, all parts of the system are subject to thermal fluctuations of the same scale, which messes up attempts to get energy to flow in one direction only. The best such a system can do is to maintain the status quo, but that is only possible in idealized conditions (the unrealistically cold day mentioned there). In even optimal realistic conditions, any free energy at the work area will flow backwards through the system to reach a more entropic state. A capacitor will discharge rather than produce power. A balloon will deflate rather than inflate. A heap will settle rather than grow taller.

The possibility of random chance lining up to produce any (or arbitrarily many) steps away from increased entropy isn't useful for producing usable power because it takes a sustained bias against entropy for any such steps to last (law of large numbers), and the chances of that become fantastically unlikely well before anything even close to macroscopic scales (and at quantum scales, reaction reversibility would cause the decay of ordered patterns faster than random fluctuations would re-order them). That is, unless there is a thermal gradient (an input of energy), which is capable of biasing the reversible reactions to produce useful, sustained changes. Any questions?

>>1041798
The mechanism for extracting power from upconverted photons is the photoelectric effect, which works perfectly fine with our upconverted photons.

The entropy increase you're looking for comes from the photon downconversion, and a photon being downconverted will not somehow sap energy from the photoelectrics.

>>1041801
You realize that the photoelectric effect is reversible, right?

free electrons <=> high energy photons
high energy photons <=> low energy photons

Photoelectrons can move their energy to low-energy photons just fine. Your explanation doesn't avoid the problem I pointed out above.

>>1041804
Sure, they're reversible, but free electrons contributing to voltage is not reversible with straight wires.

>>1041805
So, to summarize your position, you claim that reversible reactions operating in a condition of thermal equilibrium will spontaneously and consistently progress in their entropy-reducing direction, moving electrons up a Voltage gradient to produce useful electrical energy without any external input. And that despite the presence of a chain of reversible reactions feeding the Voltage gradient, you claim that the energy thereof cannot discharge back to thermal noise.

That scenario is reasonable if you reverse the arrow of time, but I don't think that's what you have in mind. Can you justify any of that?

>>1041829
There are three reactions, with the first two (photon upconversion, high energy photon bumping off an electron) being reversible but the third (the bumped off electron hitting a metal plate to add negative charge) being irreversible.

The only things to question is where does the entropy go, and where does the energy come from.
The entropy probably manifests as very low energy photons as an upconversion inefficiency, while the energy comes from heat.

>>1041855
>the third (the bumped off electron hitting a metal plate to add negative charge) being irreversible.
Why is this irreversible? It's a metal surface in an area with high-energy photons bouncing around. Looks like a plausible setup for the photoelectric effect to send an electron back the other way. Especially since the electric potential between the two surfaces would make the photoelectric effect more active in the reverse direction than in forward.

>The only things to question
I would question why you think a perpetual motion machine of the second kind can be built from processes that obey the second law of thermodynamics. You see how your proposed mechanism would be able to produce unlimited work from finite starting resources, right?

But yes, the energy comes from heat, by way of the spontaneous cooling of part of a single heat reservoir to produce concentrated energy that can be used for work. The "low energy photons" you're invoking would be cooler than the ambient temperature and its thermal photons, when the actual particles involved in upconversion are hotter than the average.

In actual upconversion, the inputs are high-energy photons far more energetic than the material of the upconverter, and some of that energy is upconverted while some is dispersed as heat, thereby increasing entropy as expected. There is investigation of using a heat source to provide thermal photons, but this heat source is necessarily hotter than the material of the upconverter, so again thermodynamics operates as expected. In your scenario, random heat fluctuations spontaneously sort themselves to produce a heat gradient that produces useful power, thereby violating the second law of thermodynamics. See the difference?

>>1041870
Couldn't the surface the electron arrives in just have a higher work function?
Also this would make a lot more sense knowing the exact form and function of these upconverters.

>>1041882
>higher work function
Sure, but that doesn't matter. Whatever the work functions, there will be a particular equilibrium state. You are proposing that this equilibrium state will involve one-way movement of electrons when in an equilibrium electron movement in both directions is equal. Remember that the Voltage to be generated will act against such a difference in work function. Also remember why Brownian ratchets don't work. A work function difference is effectively the pawl in a Brownian ratchet, where our classical intuitions about how things ought to work suggests that it would lead to one-way motion. And it could with an external energy input. But the invocation is of random thermal fluctuations, and that changes things.

For example, a higher work function means more energy is deposited when an electron is absorbed, and that energy can go on to do undesired things. In the scenario where the upconverter is a perpetual motion device that produces unlimited free energy, there is an energy input, so one-way flow is possible. It just violates thermodynamics at the earlier step. If the system is to be analyzed according to real physics, you have to take a careful look at the whole thing and track what thermal fluctuations do everywhere. This close but systemic look is what identifies the problem with the Brownian ratchet (thermal fluctuations turn out to make the pawl ratchet in reverse as often as it ratchets forward).

Form and function can be informative, but they don't actually matter for this consideration. The theoretically optimal material properties can't get around the laws of thermodynamics, so engineered compromises aren't going to either.

>>1042122
Ah, so the problem lies not at what the electron from the photovoltaic plate does, but the upconverter instead?
While I assume reliance on random thermal fluctuations, it doesn't even matter if they're disadvantageous: A photon upconverter doesn't need a pawl unless you're aiming to violate entropy.

Upconverting photons isn't free energy, it's just a way of shedding entropy so that heat can become a useful form of energy again.

>>1042232
>unless you're aiming to violate entropy
But that's exactly what you're proposing will happen. You claim that a photon upconverter can take ambient heat and turn it into a gradient that can be used to perform work. THAT IS A REDUCTION IN ENTROPY. Photon upconverters can't do that BECAUSE DISADVANTAGEOUS THERMAL FLUCTUATIONS WOULD OFFSET ADVANTAGEOUS ONES. The "pawl" is the supposed ability of a photon upconverter to work in the "upconvert" direction consistently without an external power source.

>it's just a way of shedding entropy so that heat can become a useful form of energy again.
That's what free energy is in this context. https://en.wikipedia.org/wiki/Thermodynamic_free_energy Free energy is converted to entropy over time as work is performed, and it cannot be recovered once lost. Producing usable electricity from ambient heat increases free energy without an external input, thereby qualifying as a perpetual motion machine of the second type (the kind which violates the second law of thermodynamics).

To be more explicit, imagine the generator and the things it powers in a box (it is now an isolated system). Turn it on, and it produces electricity from ambient heat, which turns back to heat as it is used. Now turn it off and let it sit for a while. All the recently-generated thermal gradients dissipate, and the box returns to its original state, containing the same amount of matter, free energy, and entropy as it did before things were turned on. Handwaving about low-energy photons carrying the excess entropy away is clearly silly now that it's obvious that any emitted photons can reabsorbed to return things to the initial state.

Handwavey sphysicsy stuff belongs on >>>/sci/, guys

>>1042344
The thought experiment you propose is one I've thought of, but that raises some questions such as the exact definition of entropy.
Some say high entropy means a high ratio of energy to free energy, some say high entropy means more individual particles.
The odd thing though is that if you create free energy through a heat gradient, you also increase the number of individual particles because if you have two medium-temperature objects then you'll get a medium number of photons, but if you have a cold object and a hot object then the cold one will emit a little less but the hot one will emit far, far more.

So in the thought experiment, turning on the device reverses one form of entropy by freeing up energy but increases another by causing more photons to be emitted, and vice-versa when the device is turned off again when the heat gradient is lost but the particle count decreases.

>>1042415
>the exact definition of entropy.
Entropy is thermal energy in a system that cannot be used to perform work. (https://en.wikipedia.org/wiki/Introduction_to_entropy) There is a more technical definition actually used in modern analytics (https://en.wikipedia.org/wiki/Entropy), but that's the practical upshot relevant to the thread topic.

So your conundrum isn't a conundrum at all, but a clear and unambiguous case of different levels of free energy and entropy. There are two isolated systems, identical apart from one having a heat gradient (a cold object and a hot object in B rather than two medium-temperature objects in A). Same amount of matter, energy, and other conserved quantities (let's assume that nothing will produce or absorb heat, so that's conserved too). The heat gradient in system B can be used to perform work via a heat engine, while system A lacks this potential for work. Hence, system B has more free energy than system A, and since they have equal total thermal energy, B therefore has less entropy than A. B can spontaneously become like system A (the heat gradient dissipates), but the second law of thermodynamics prevents system A from becoming like B without an external energy input.

But what of the photons in system B? As you mentioned, the hot object in B will emit photons. Easily more than the other three objects combined if the temperature difference is substantial. And yes, those photons represent an increase in entropy. Here's the thing: The hot object is cooling as it emits photons. The heat gradient is dissipating as the photons mediate the transition of system B to a state more like system A. Note that this is a spontaneous process that generates entropy. The photons are still warm, and their temperature difference with the cold object is another heat gradient that could be used to power work. The absorption of the warm photons by the cold object is another spontaneous entropy-generating process that changes B to be more like A.