Rippling graphene 'harvests thermal energy'

Discussion in 'Physics & Math' started by Q-reeus, Oct 23, 2020.

  1. Q-reeus Valued Senior Member

    Messages:
    4,167
    A mainstream physics article claiming a tech breakthrough that allows harvesting (i.e. converting to electrical) thermal energy from a uniform temp environment, yet without defying the 2nd Law:
    https://physicsworld.com/a/rippling-graphene-harvests-thermal-energy
    Short YouTube animation linked to within above:
    Physical Review E (paywalled) article:
    https://journals.aps.org/pre/abstract/10.1103/PhysRevE.102.042101
    Unpaywalled technical article: https://arxiv.org/abs/2002.09947

    Their claim seems blatantly contradictory. There is a dearth of technical talent here but maybe someone can point to a harmonization I cannot see possible.
     
    Last edited: Oct 23, 2020
  2. Google AdSense Guest Advertisement



    to hide all adverts.
  3. Q-reeus Valued Senior Member

    Messages:
    4,167
    No takers. A dearth of technical proficiency indeed. To pad it out a bit, here is a key passage from the first linked to article in #1:
    "But how does this setup work when the Brownian ratchet fails? The researchers explain that success lies in how the graphene and the circuit share a “symbiotic” relationship. Even though the circuit allows the thermal environment to do work on the load resistor, the circuit and the graphene operate at the same temperature, meaning that no heat flows between the two.

    “This means that the second law of thermodynamics is not violated, nor is there any need to argue that ‘Maxwell’s Demon’ is separating hot and cold electrons,” Thibado explained."

    BS to that flimsy explanation I say. Variant definitions of the 2nd Law all say the same thing - without a preexisting temp differential, thermal energy cannot be converted to useful work. Period. So they are using mumbo jumbo jargon to hide that they really believe their system does indeed violate the 2nd Law.

    Whence then the mysterious intrinsic undulating nature of graphene film - the purported primary 'engine' that drives the system to light a bulb every so often? Supposedly Brownian motion of the fluid molecules the graphene is immersed in. But at room temp the characteristic Brownian mean collision frequency should be ~ 10^13Hz, with a presumed MB distribution. Mechanical oscillations of an atomically thick graphene film will be many orders of magnitude lower in frequency. Hence coupling between the two is expected to be extremely weak. Even that much assumes coherent impulses from molecular collisions, contrary to the random nature of Brownian motion.
    If the article were published on April 1st, I could suspect a silly gag article. But that's not the case, and Physical Review editors surely wouldn't stand for it. So strange.
     
    exchemist likes this.
  4. Google AdSense Guest Advertisement



    to hide all adverts.
  5. exchemist Valued Senior Member

    Messages:
    10,271
    There's already been a thread on this, actually: http://www.sciforums.com/threads/perpetual-motion-mainly-debunking.163626/#post-3650441

    As you will see, it bothered me too, for much the same reason. My conclusion was this:

    "What I can't follow however, is his assertion that they have "re-routed" random thermal current flows into an ordered form, allowing energy to be extracted. That does seem to me to violate the 2nd Law, as it seem to involve entropy reduction. By the same token, it seems to violate it because if you can extract energy from thermal motion in this way you have a heat engine without any rejection of waste heat to a heat sink. Hmm - itchy beard time.

    I look forward to someone publishing a more detailed analysis of what is going on here. As things stand, I'm afraid I do not believe in the promise of limitless power from a heat source with no heat sink. There is something wrong".
     
  6. Google AdSense Guest Advertisement



    to hide all adverts.
  7. dumbest man on earth Real Eyes Realize Real Lies Valued Senior Member

    Messages:
    3,454
    From my limited understanding, graphene seems to exhibit numerous "odd" properties.

    ...many someones with many analyses, hopefully!

    Could the combination of a few of the "odd" properties of graphene , the fact that it has no band gap and is such an efficient thermal and electrical conductor, in that circuit at least, possibly allow it to be acting as an antenna/receiver/amplifier of some aspect that has yet to be identified?
     
    Last edited: Oct 24, 2020
  8. Q-reeus Valued Senior Member

    Messages:
    4,167
    Ah OK I do not visit Pseusoscience as a rule. It begins there as an extra in post #8, I agree with your #9 which as you say concurs with my own judgement, but not the last para in #11.
    They emphasize the net DC current passes through a resistor e.g. light-bulb as in animation. Hence must be doing actual VI = (I^2)*R work.
    If true the 'active' system gradually cools as thermal energy is pumped to the outer environment. In clear violation of 2nd Law. That or the 1st Law is being violated, or both?! Hugely unlikely.

    My bet is independent attempts to experimentally replicate the claimed 'thermal energy harvesting' will come up null.
     
    Last edited: Oct 25, 2020
  9. exchemist Valued Senior Member

    Messages:
    10,271
    I don't think anyone will need to repeat it. I think a proper thermodynamic analysis will be sufficient to show it's a curiosity but no work can be extracted from the circuit. Does the paper (not the pop-sci article) really claim to have actually put an electrical load in this circuit and extracted energy? I suspect that animation takes considerable liberties. But it is an interesting puzzle and I'd like to see more about it.
     
  10. Q-reeus Valued Senior Member

    Messages:
    4,167
    Indeed. Their summary in arXiv article:
    "In summary, we have studied the thermal fluctuations in freestanding graphene membranes using point-mode scanning tunneling microscopy. After disabling the STM feedback circuit, a displacement current was measured. We modeled the ripple closest to the STM tip as a Brownian particle in a double well potential. When the graphene moves, charge must flow through the circuit and perform electrical work. Our model provides a rigorous demonstration that continuous thermal power can be supplied by a Brownian particle at a single temperature while in thermodynamic equilibrium, provided the same amount of power is continuously dissipated in a resistor. Here, coupling to the circuit allows electrical work to be carried out on the load resistor without violating the second law of thermodynamics.

    Nonequilibrium fluctuations due to extra noises [34, 35] or to different temperatures in electrical circuits [36] will produce entropy and measurable deviations from detailed balance [35, 36], and are worth investigating in freestanding graphene."

    Ignoring the last para as irrelevant to the main claim, it seems they have not performed an actual experiment unequivocally demonstrating energy conversion, but consider a measured displacement current owing to rippling of graphene to be 'as good as' a full blown demonstration because said displacement current 'must' result in a net current through external load resistor.
    Time will likely not be gentle to that assumption.
     
    exchemist likes this.
  11. exchemist Valued Senior Member

    Messages:
    10,271
    Yes, I'm afraid I find this line that the "coupling to the circuit allows electrical work to be carried on on the load resistor without violating" the 2nd law of TD seems to sweep the whole problem under the carpet, suspiciously glibly. What do they mean by"coupling", in this context? And coupling of what to what exactly?

    I think an analysis of what happens with Brownian motion could be the key. When you see smoke particles jiggling about under a microscope, there is a variation in the kinetic energy of each smoke particle, as it exchanges kinetic energy randomly with molecules of air that continually buffet it. So these molecules sometimes do work on the smoke particle and sometimes the smoke particle does work "back" on molecules. Temperature - and thermodynamics more generally - describes the bulk behaviour of ensembles, not individual molecules. Smoke particles are giant molecules in this sense, close to the boundary between behaviour governed by the statistics of kinetic theory and that governed by bulk properties. The bigger the smoke particle, the less it moves, and the more it can be said to have a temperature. So I feel one has to be very careful in analysing systems like this.

    This setup seems to me to be what it would be like if there were some kind of ratchet possible, whereby a smoke particle could move from left to right under the influence of collisions with the air molecules, but was physically blocked from moving from right to left. (In this case the physical block is the one-way nature of the diodes in the circuit.) But could one extract work from a smoke particle in this way? I have difficulty in seeing how. It would just be drift in one direction, rather than a random walk. That's what I meant about there maybe being no voltage, just a unidirectional drift of electrons.

    The practical problem in any case would be the impossibility of scaling it up. The bigger the smoke particle (or graphene sheet) the less it would move under the action of Brownian motion. So the breathless claims of "harvesting" energy in any useful sense must be bullshit, no question.

    Please Register or Log in to view the hidden image!

     
    Q-reeus likes this.
  12. Q-reeus Valued Senior Member

    Messages:
    4,167
    Well they do say millions of such 'engines' on a single chip might allow doing away with a battery. He he. Anyway decided to go back to earlier articles exploring just the graphene rippling itself. In particular this one: https://www.nature.com/articles/ncomms4720
    See fig.2, where a mean period of (stochastic or semi-stochastic) oscillation T ~ 10s, i.e. ~ 0.1Hz seems characteristic. That is certainly plenty low enough to obviate Feynman's ratchet no-go argument, but then the power level must be extraordinarily weak. Probably too weak to raise enough voltage to allow effective operation of any real-world diode (or diode pair). This may be the true Achille's heel.

    I still find it difficult to understand how room temp Brownian motion of say air molecules, with peak intensity of stochastic collisions ~ <10^13Hz> could at all appreciably couple to graphene film to give those ~ <0.1Hz> semi-coherent transverse to the plane oscillations. Magic material.
     
    exchemist likes this.
  13. Q-reeus Valued Senior Member

    Messages:
    4,167
    Addendum: In #2 & #9 I incorrectly gave the mean collision frequency for say air at STP of ~ 10^13Hz, but a check here http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/frecol.html
    amends that to a mere ~ <4.4 x 10^9Hz>. Still vastly greater than graphene oscillations and the general argument made back there remains valid.
     
  14. exchemist Valued Senior Member

    Messages:
    10,271
    YES! I had not heard of the Brownian, or Feynman-Smoluchowski ratchet but I've looked it up. Thanks for putting me onto that.

    This is exactly the way we need to think about this. What they've done is develop an electrical version of this ratchet. And it will suffer similar snags, I have no doubt.
     
    Q-reeus likes this.
  15. arfa brane call me arf Valued Senior Member

    Messages:
    7,159
    One caveat about the discussion here: there is a difference between electrons in a graphene sheet and electrons moving through a resistor.

    Although you might think electrons are always electrons, that isn't actually true; it depends on how they interact with a medium (they are always quasiparticles when they do this). The arxiv paper talks about electron-phonon coupling in the graphene lattice (i.e. a spin coupling); moreover a current in a resistor produces heat, period. The graphs shown in the article are also given by measurements, not theory.

    If the article and the research do have some errors, I'm sure someone will publish a refutation of some kind; but it will have to be based on measurements or demonstrate the authors of the article in question made an analytical mistake.
     
  16. Q-reeus Valued Senior Member

    Messages:
    4,167
    That conduction electrons don't behave as particles but more as wavelets is not germane to the real issue. I do have to rethink where the flaw may ly given they provide measurements of diode currents indicating they do work as on-off switches allowing rectification. An essential component is a battery providing bias voltage and obviously power, which according to animation averages to zero over a complete cycle. But does it?
    If after all is said and done purely thermal origin net output power occurs, their summary below cannot be correct, as covered from the start:
    "Our model provides a rigorous demonstration that continuous thermal power can be supplied by a Brownian particle at a single temperature while in thermodynamic equilibrium, provided the same amount of power is continuously dissipated in a resistor. Here, coupling to the circuit allows electrical work to be carried out on the load resistor without violating the second law of thermodynamics."
    Nonsense.
     

Share This Page