Comment by sandworm101 19 hours ago
Title misses the big story: "power generation ... without a temperature gradient".
Turning heat directly into electricity is one of those Trek-level technologies. Many would debate whether it is even theoretically possible, while others claim practical successes.
If the paper's claim is correct, it would literally be the creation of Maxwell's Daemon.
I am banking on the author not fully understanding their own experiment, rather than a complete overturning of thermodynamics. I would be very excited to be wrong, though.
I wouldn't hold your breath on being wrong. A device that converts heat into electricity without a gradient is a perpetual motion machine. Consider what would happen if you popped such a device on one side of a thermoelectric plate, and used its output to power a heater on the other side of the plate. The arrangement would permanently maintain a gradient across the thermoelectric plate - free electricity forever!
Not if it produces less heat than it converts, the rest being lost as ... ah, wait a minute. Well anyway it would only produce as much heat as it converts, so this perpetual cycle couldn't do any useful work. Perpetual useless motion is allowed, isn't it?
Oh, I see it now, you're suggesting that it could just passively create a gradient that could produce power in a more conventional way, like via an engine, until I guess the device kicks in again and re-establishes the gradient (producing more power of its own in the process). Yes. That does sound like impossible free energy.
I suppose if we’re assuming that overturning thermodynamics is the byline, such a machine could exist without being a perpetual motion machine if somehow the energy that produces the machine’s work was lost as say… nuclear radiation instead of heat?
Or perhaps requiring some kind of catalyst that is rendered into a higher energy state which is functionally useless for its purpose as said catalyst?
Either way, we’ve got some interesting sci-fi concepts to work with!
But so do solar panels. A PV system turns a moving particle (a photon) into an energy gradient, a voltage. So the concept of using another moving particle, this time a hot molecule, doesn't seem totally impossible. If one understands heat as being molecules moving at variety of speeds, harvesting energy only from the fastest of them wouldn't violate thermodynamics. It can be understood as taking advantage of the temperature gradients across the gaps between individual molecules.
It sounds like you have basically given a formulation of Maxwell’s Demon https://en.wikipedia.org/wiki/Maxwell%27s_demon
“All you have to do is harvest the hottest/fastest molecules” pretty much describes exactly what Maxwell’s Daemon is.
Now, you can see from the Wikipedia page that there is at least some debate about the topic. But, I think that generally, most physicists think that Maxwell’s Demon would violate the 2nd law, and that the 2nd law is a real limitation.
It certainly sounds like this device would violate the “metaphysics” definition of the 2nd law, “entropy always increases”. Because it sounds like it’s harvesting energy from heat for “free” (not actually energy from nothing, not violating conservation, but free in that you are not paying for the loss of entropy by increasing entropy somewhere else, which is typically “required”).
I am not a physicist, and in my layman’s understanding, I have always felt that the 2nd law seemed fishy. A lot of other people do, too. It’s called “a scientific law”, but it feels more like a philosophical “principle”. It feels different than, for example, “the law of gravity”.
But, this invention, if it is not a hoax or error, sounds world changing. 94 nW/cm2 means a milliwat for 2 square inches. That’s already usable amounts of power for tiny devices. Can you roll up a couple square meters of this stuff into something like a capacitor and get a usable “free” AA battery?
Is heat about to be free energy? No more problems with global warming? Are our refrigerators and air conditioners soon going to be a source of energy rather than a sink?
If this claim is true, then this is world changing technology available to anyone with pretty basic thin film technology…. And there are YouTubers who will be replicating this soon. Let alone real fabrication labs.
You can see why it’s reasonable to be skeptical…
The laws of thermodynamics are empirical: all of our physics experiments and observations thus far support and agree with them. We have no external confirmation, e.g. "god's source code for the universe" or some such that proves them to be absolute laws, but given that they hold up consistently across a huge number of observations, in a huge number of domains- we are nearly certain that they are absolute. Gravity itself (e.g. general relativity) is also on the same footing- like thermodynamics it has proven itself to reliably predict observations across a huge number of different new experiments.
This reasoning isn't convincing enough for some (non-physicist) people- there are whole forums of people working on perpetual motion machine designs, etc. that think thermodynamics is nonsense. However, thus far none of their machines have ever worked.
Although I am not a working physicist, I was trained as a physicist (a long time ago!), and assign a prior of essentially 100% to the laws of thermodynamics, from a huge amount of evidence. E.g. I am certain that either I am misunderstanding this paper, or it is simply wrong. I'm hoping someone whose physics isn't as rusty as mine will step in and explain what I am missing here.
I agree. In my opinion this is direct Nobel Price or big honest error [1]. I'm really surprised they didn't add a discussion about the obvious violation of the second law and that the referee didn't ask for one neither. Anyway, I'm almost sure (100-10^15)% it's just an experimental error.
[1] or fraud or bullshit, but let's be assume good intentions.
This is false. Solar panels do not violate the laws of thermodynamics. Generally the sun is quite a bit hotter than the environment in which the solar panels operate, which are the correct temperatures to compare in this case.
"harvesting energy only from the fastest of them" actually does violate thermodynamics and, if possible, would constitute the biggest revolution in physics in all of history.
They don't, but depending on one's understanding an perspective they can be described as violating. The point is that just because something can possibly be described as a violation does not mean that there are perhaps other perspectives that may understand them without such violation.
>But so do solar panels. A PV system turns a moving particle (a photon) into an energy gradient, a voltage.
Solar panels are not an example of a Carnot violation. As far as we know, no such examples exist.
https://en.wikipedia.org/wiki/Solar_cell_efficiency#Thermody...
> Solar panels only work because there's a temperature difference between the panel and the optical surface of the Sun.
I believe this isn't quite true (even though it's often repeated).
Consider that you could have an LED, at the same temperature as the PV cell, emitting light onto the PC cell, and so the PV cell would "generate" power without a temperature differential.
The Carnot cycle applies (in its initial form, at least), ONLY to generating power from temperature differentials, but PV uses the photoelectric effect, not temperature differentials.
This is not my area of expertise, but I think you can generalize the Carnot efficiency to talk about low-entropy energy sources instead of temperature differentials, but it's not quite as simple as associating a light wavelength with a temperature, because that doesn't work for radiation from something that's not a black body (like a laser or an LED).
I think here there is a misunderstanding on the meaning of 'work'. It's most likely clear to all authors in this chain that if you take an LED and shine it onto an appropriate PV you will get voltage out of it. That much isn't governed by thermodynamic limits.
Likely the word 'work' here is in the context of collecting or generating energy in some manner. Your example here is essentially energy transfer. In analogy what you're describing is the transfer of power from driveshaft to axle, not what is happening in the engine itself.
You still need the photons to hit the surface of the object and deposit their energy. Talking about the temperature of the surface of the sun is just a simpler way to refer to the energy output. What you are saying is a very high level view of the actual physics involved. The physics just works out that we can use temperature to talk about this.
This is the same argument the authors make to reviewer #3:
"At this moment, we think that the thermal energy can be supplied to the devices from the surrounding air which possesses thermal energy such as 25 meV at room temperature. The air surrounding the devices can be considered as an infinite heat bath, and the analogized conditions of steady light irradiation in a solar cell, i.e., “photon absorption,” are considered to be formed by “phonon absorption.”"
This sounds like nonsense to me- I don't think a solar cell would function without a gradient either, e.g. if the cell itself were the same temperature as the light source.
Does a photon carry information about the temperature of it's source?
What about photons from non-thermal sources? A PV panel is able to generate electrical current from photons generated by LEDs, bioluminescence, and indirect light reflected off a cold surface.
No, that still violates thermodynamics.
Thermodynamics says "no", at least on macroscopic scales.
My thermodynamics lectures are some 20 years in the past, so my memory is a bit rusty, but iirc you need "free energy", that is, usable energy in the form of a low-entropy energy source, which is usually a temperature gradient (but could also be a radiation source or something else).
To get to these results, you have to do statistics of large numbers, so it's theoretically possible to violate this on the nano scale.
The optimist in me hopes that we've found a scalable way to exploit this, the pessimist/skeptic in me says it's another cold fusion moment.
Indeed, I am really skeptical this is possible- it would seem to violate the laws of thermodynamics…