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Is a refrigeration laser thermodynamically possible?

  1. May 11, 2009 #1
    I am NOT referring to laser cooling, the technique for supercooling atoms.

    The 1980 science fiction novel Sundiver by David Brin describes a kind of spaceship that could fly into the sun.

    One way described to keep the inside of the ship cool was to concentrate the heat inside the ship with some kind of heat pump, and then use the heat energy to power a laser which effectively dumps the excess heat energy out into space.

    Thermodynamically, for a refrigerator to work, you need two things, right? One is some place to put the heat energy; and two is some place to put the entropy. Or, since pumping heat against the normal flow takes work, you have to increase the net entropy of the system um, somewhere? The technique seems to explain the former, but not the latter, which brings me to the question, is a refrigeration laser (as described) thermodynamically possible?

    And if so where does the entropy of the system increase? Could you use part of the laser beam to power the concentrator pump in the first place?
     
  2. jcsd
  3. May 11, 2009 #2

    Mapes

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    Hi pcysics, welcome to PF. Laser light has relatively low entropy compared to blackbody radiation. (The extreme would be a polarized, monochromatic beam, which would have very few or one possible microstate.) Therefore, it would violate the second law for the same amount of energy from the Sun's light to be output as a laser beam.*

    However, if there were a large energy source on board, a very high power laser beam might be produced that would carry away the same amount of entropy that is received from the Sun. I can't see any problems with this scenario. It wouldn't be self-renewing, however; eventually the energy source would be depleted.

    *A similar and classic example of a second law violation is the "Party Boat": motor around all day making ice cubes from sea water; use the energy difference to power the engine.
     
  4. May 11, 2009 #3
    So the problem here is the way we output the heat energy? If instead of using a laser, we just used a focused incandescent light source with a higher temperature than the sun, the output would have higher entropy, right? Then could the system be self-renewing? If the temperature difference were high enough, could you extract useful work from it?
    So laser refrigeration is possible in principle, it just takes a lot of power. So you could plug a similar device into your wall, and have a "ventless" air conditioner (point the laser out a window or something)?
    Never heard of that one.
     
  5. May 11, 2009 #4

    Mapes

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    Well, an incandescent light doesn't transfer entropy, it creates it from electrical work. So the incident entropy isn't being transferred in this case. But if parts of the ship are emitting blackbody radiation, then entropy is being transferred, albeit not at perfect efficiency.

    It seems like what you're proposing is analogous to using a solar panel to run a refrigerator on Earth (a very big spaceship). No violations there.

    I don't see any thermodynamic violations here either, assuming that the laser is emitting more entropy than is created by inefficiencies in utilizing the power source.
     
  6. May 11, 2009 #5
    I don't mean an electric light bulb. I mean heating something to incandescence using the concentrator pump. That's the same as "if parts of the ship are emitting blackbody radiation", right? Is it self-renewing now? Can it produce useful work?
    Could you dump the heat in some other way than the laser, like feed it back to the power grid? Would it still be a net drain of power in this case? And If so, since energy is conserved, where does it go?
     
  7. May 11, 2009 #6

    Mapes

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    What is a "concentrator pump," besides a term from this science fiction book?

    You could use the thermal energy to run a heat engine, whose efficiency would be limited by the Carnot limit. Energy would be conserved in the form of increased temperature of the components and their surroundings.
     
  8. May 11, 2009 #7
    I wasn't referring to a term from the book, but to my first post: "to concentrate the heat inside the ship with some kind of heat pump". A heat pump is the same thing as some kind of refrigerator. The exact method used to pump the heat is probably irrelevant to our discussion. A refrigerator has an insulated box which it pumps the heat out of, (into the ambient environment) the concentrator pump is just a refrigerator in reverse; it has an insulated box which it pumps ambient heat into.

    Huh? The whole point of this device is to act as an air conditioner. What components? Does that mean it stops working as an air conditioner? Correct me if I'm wrong, but a heat engine doesn't put as much energy into the cold reservoir as it extracts from the hot reservoir, right? And the difference can be recovered as useful work?

    I'm guessing that the energy generated by the heat engine isn't quite as much energy as is required to run the concentrator pump in the first place, but if you dump the power generated by the engine back onto the grid you can offset this energy cost partially, though it's still a net drain on the grid.

    So we seem to have a contradiction here:
    1. Energy is taken from the grid to run the machine.
    2. The net temperature of the room decreases (heat energy is taken from the room, because that's what the machine is for).
    3. Energy is conserved.
    The energy seems to be disappearing here, in clear violation of the laws of physics. Something's definitely wrong with my reasoning, I'm just not sure what.
     
  9. May 11, 2009 #8

    Mapes

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    Your machine is a laser, yes? Whatever the laser is pointing at will heat up. There's the "disappearing" energy.
     
  10. May 11, 2009 #9
    Nope, not a laser. I said,
     
    Last edited: May 11, 2009
  11. May 11, 2009 #10

    Mapes

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    And I wrote in my post #6 that you could run a heat engine, which would dump thermal energy into its cold reservoir, besides heating itself due to inefficiencies.
     
  12. May 11, 2009 #11
    Right, but the cold reservoir in this case would be the room itself, which the concentrator pump is extracting the heat from in the first place. This machine is supposed to be an air conditioner. Read my post #7 again.
     
  13. May 11, 2009 #12

    Mapes

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    Perhaps I'm confused, but it seems like you're talking about running two heat engines back-to-back. Would you please describe again the system you have in mind? Is it steady-state, and what are the temperatures of the components?
     
  14. May 11, 2009 #13
    Sorry, it seems I have yet to develop an intuitive grasp of the second law of thermodynamics. I'm not sure what you mean by steady-state.

    You've heard of electric "space heaters". They're compact and portable. They generate heat through electric resistance like a toaster, but this takes power from the wall.

    Suppose I want to make a "space cooler" to have the opposite effect. Currently the closest thing to that are those window air conditioners which vent hot air outside. They're not quite as portable as a space heater, since you have to put the whole unit in the window.

    We seem to have come up with a partial solution in this discussion, based on the sundiver. I suppose you could use a heat pump (say a chain of vortex tubes) to concentrate ambient heat into a small vacuum-insulated (as a thermos bottle) box up to say, around 1000 Kelvins, less than a candle flame (the concentrator pump).

    This box could contain a chunk of calcium oxide (as a limelight) which would incandesce from the heat, effectively converting some of the heat energy into light. You could have the inside of the box be reflective and channel the light into an optical fiber, which you could then point out the window to vent the light without having it re-absorbed as heat by something in the house.

    If I understand correctly what you've told me so far, I think this could function as a more portable air conditioner. Instead of having to put the whole unit in the window, you just have to put the end of the fiber (which can be fairly long) in the window. Now you've got a portable space cooler. It's almost as portable as those electric space heaters, but you just have two cords to deal with instead of one, the power cord and the optical fiber.

    The system I just described may not be very high performance (that's an engineering problem) it's just a proof-of-concept to prove this can be done in principle. (and it could work, right?)

    Now can we modify this system to require only one cord? Can we dump the energy out through the power cable into the grid or the ground?
     
  15. May 11, 2009 #14

    Mapes

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    I appreciate the detailed explanation, pcysics. I'll do my best to give constructive comments on this scenario.

    I think you've got a pretty good idea of the key ideas of thermodynamics. "Steady state" just means that none of the parameters in the system changes with time. It's a good constraint to apply to a system because it means the analysis will be valid in the long term.

    We have a region A (one part of your room) that is cooler than the surrounding environment. Spontaneous heat transfer from the surrounding environment results in a positive input energy E1 and a positive input entropy S1 to region A. We also have a power cord from the outside that delivers positive electrical energy E2 with little or no associated entropy. The energy is used to maintain a temperature gradient between region A and region B (the incandescent part of your room) that has a temperature higher than that of the surrounding environment. A positive energy E3 and a positive entropy S3 move up this gradient as a result of the input electrical work. Heat transfer (through an optical fiber) spontaneously transfers positive energy E4 and positive entropy S4 from region B to the surrounding environment. Entropy is generated in both rooms as a result of inefficiencies.

    The steady-state requirement makes it easy to monitor energy and entropy transfers even through we don't know the exact temperatures of regions A and B. By the first law, E3 = E1 and E4 = E3+E2. By the second law, S3 > S1 and S4 > S3.

    Have I got this straight?
     
  16. May 11, 2009 #15
    That looks right.
     
  17. May 11, 2009 #16

    russ_watters

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    The thermodynamic efficiency of any kind of heat pump is dependent on temperature: the larger the temperature difference between the hot and cold side, the lower the efficiency. So yes, you could do what you suggest, but you could do very little cooling with it.
     
  18. Sep 4, 2010 #17
    here is the quote from the book which explains basically what David Brin (he is a astroner with a Master degree in applied physics and a Doctor in Philosofy of space exploration) thought about his "refrigerator laser"

    other parts of the book explain it better.
     
  19. Sep 9, 2010 #18
    I sent David Brin a message pointing out to discussions on the net saying his refrigerator laser was implausible and asking him if he really thought the concept would work, if it would only work with the physics knowledge of the Galactics (in the books), etc

    here is his answer
    "Thanks for your thoughtful and interesting message. It truly is gratifying when people write, and I always try to answer.

    As a matter of fact, I ran the refrigerator laser past a couple of Nobel Prize winning physicists, back in the 1980s. Plasma physicist Hannes Alfven could find nothing wrong with my reasoning... and found it "plausible."

    Remember the comparison to the refrigerator in your kitchen. With an effectively infinite energy source (wall current) your fridge pumps heat from one space (the freezer box) into another space (the surrounding kitchen)... along with the waste heat involved in the process. It simply works.

    If the sun's Chromosphere is the 'kitchen' and the ship is the freezer
    "

    So... two Nobel Prize winning physicists supported the concept, including a plasma physicist.

    but here in the forum, we are saying its impossible...
     
    Last edited: Sep 9, 2010
  20. Sep 9, 2010 #19

    ZapperZ

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    Here's the problem: we NEVER do physics simply via hearsay!

    As far as I can tell, you did not cite a single peer-reviewed citation to back any of such claim. Note that any moron can write a book and have it published. Just browse the net and see a ton of crackpottery being published in books.

    For something like this to be judged as valid, it needs to have very clear description of not only the mechanism, but also the physics involved. The DETAILS are missing here! Just saying "2 Nobel prize winners" can't find anything wrong with it is utterly weak if THAT is all this person have! I can show you several nobel prize winners pushing theories that many of us scratche our heads. Shall I quote you a passage from Robert Laughlin's book calling out a particular Nobel prize winner for an obviously wrong conclusion about superconductivity?

    If you have peer-reviewed publication supporting this, then please cite it. If not, then you don't have much to stand on and you are violating the PF Rules that you had agreed to for advertising such a thing on here.

    Zz.
     
  21. Sep 9, 2010 #20
    sorry, how exactly what I posted falls on the Advertising Rule? I did not link to any website or blog.
     
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