Is a refrigeration laser thermodynamically possible?

  • Thread starter pcysics
  • Start date
9
0
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?
 

Mapes

Science Advisor
Homework Helper
Gold Member
2,592
17
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.
 
9
0
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.*
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?
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.
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)?
*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.
Never heard of that one.
 

Mapes

Science Advisor
Homework Helper
Gold Member
2,592
17
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?
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.

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)?
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.
 
9
0
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 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?
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.
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?
 

Mapes

Science Advisor
Homework Helper
Gold Member
2,592
17
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?
What is a "concentrator pump," besides a term from this science fiction book?

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?
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.
 
9
0
What is a "concentrator pump," besides a term from this science fiction book?
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.

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.
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.
 

Mapes

Science Advisor
Homework Helper
Gold Member
2,592
17
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.
Your machine is a laser, yes? Whatever the laser is pointing at will heat up. There's the "disappearing" energy.
 
9
0
Nope, not a laser. I said,
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?
 
Last edited:

Mapes

Science Advisor
Homework Helper
Gold Member
2,592
17
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.
 
9
0
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.
 

Mapes

Science Advisor
Homework Helper
Gold Member
2,592
17
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?
 
9
0
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?
 

Mapes

Science Advisor
Homework Helper
Gold Member
2,592
17
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?
 
9
0
That looks right.
 

russ_watters

Mentor
18,702
4,923
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.
 
15
0
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"

Project Icarus it was called, the fourth space program of that name and the first for which it was appropriate. Long before Jacob's parents were born—before the Overturn and the Covenant, before the Power Satellite League, before even the full flower of the old Bureaucracy—old grandfather NASA decided that it would be interesting to drop expendable probes into the Sun to see what happened.

They discovered that the probes did a quaint thing when they got close. They burned up.

In America's "Indian Summer" nothing was thought impossible. Americans were building cities in space—a more durable probe couldn't be much of a challenge!

Shells were made, with materials that could take unheard of stress and whose surfaces reflected almost anything. Magnetic fields guided the diffuse but tremendously hot plasmas of corona and chromosphere around and away from those hulls. Powerful communications lasers pierced the solar atmosphere with two-way streams of commands and data.

Still, the robot ships burned. However good the mirrors and insulation, however evenly the superconductors distributed heat, the laws of thermodynamics still held. Heat will pass from a higher temperature to a zone where the temperature is lower, sooner or later.

The solar physicists might have gone on resignedly burning up probes in exchange for fleeting bursts of information had Tina Merchant not offered another way. "Why don't you refrigerate?" she asked. "You have all the power you want. You can run refrigerators to push heat from one part of the probe to another."

Her colleagues answered that, with superconductors, equalizing heat throughout was no problem.

"Who said anything about equalizing?" the Belle of Cambridge replied. "You should take all excess heat from the part of the ship were the instruments are and pump it into another part where the instruments aren't."

"And that part will burn up!" one colleague said. "Yes, but we can make a chain of these 'heat dumps,'" said another engineer, slightly more bright. "And then we can drop them off, one by one ..."

"No, no you don't quite understand." The triple Nobel Laureate strode to the chalkboard and drew a circle, then another circle within.

'Here!" She pointed to the inner circle. "You pump your heat into here until it is, for a short time, hotter than the ambient plasma outside of the ship. Then, before it can do harm there, you dump it out into the chromosphere."

"And how," asked a renowned physicist, "do you expect to do that?"

Tina Merchant had smiled as if she could almost see the Astronautics Prize held out to her. "Why I'm surprised at all of you!" she said. "You have onboard a communications laser with a brightness temperature of millions of degrees! Use it!"

Enter the age of the Solar Bathysphere. Floating in part by buoyancy and also by balancing atop the thrust of their refrigerator lasers, probes lingered for days, weeks, monitoring the subtle variations at the Sun, that wrought weather on the Earth.
other parts of the book explain it better.
 
15
0
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:

ZapperZ

Staff Emeritus
Science Advisor
Education Advisor
Insights Author
2018 Award
35,045
3,879
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.
 
15
0
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.
sorry, how exactly what I posted falls on the Advertising Rule? I did not link to any website or blog.
 

ZapperZ

Staff Emeritus
Science Advisor
Education Advisor
Insights Author
2018 Award
35,045
3,879
sorry, how exactly what I posted falls on the Advertising Rule? I did not link to any website or blog.
You are advertising something that only gets mentioned in a book. Advertising isn't necessarily related to something commercial. Many crackpots try to advertise their theory on here, and that's the only means for such things to see the light of day.

I'm a bit concerned that you're not at all worried about my query if something like this has been published in peer-reviewed journals. This should have been the #1 issue that you should have addressed, not my "advertising" prohibition.

Zz.
 
15
0
I did not create the thread, nor I am pushing the issue any further. Also, I did not advertise the book, for it was the creator of the thread who said it was a David Brin book. I only posted a passage of the book, so maybe we could better understand the concept.

When I said two Nobel laureates had thought the concept plausible, you answered by saying there are tons of Nobel laureates that commit stupid mistakes in other subjects. Thats a fine answer for me.

You worry that I did not care about the issue of the peer reviewed journals. But why should I worry, if I am not the one that created the thread about refrigerator lasers? I only continue a subject created by another but I am the one called the attention by a moderator? You should ask the thread creator if a refrigerator laser has been published in a peer reviewed journal.

The only "rule" I broke was "advertising", and that again, by a stretch of imagination. Btw, refrigerator lasers are NOT a theory. They are a piece of sci-fi, and the book author is not advertising here. Nor am I advertising a theory, since its not even a theory, just a cool concept used in a sci-fi book. I dont remember Clarke submitting to peer review geosynchronous sattelites or space elevators.
 

Andrew Mason

Science Advisor
Homework Helper
7,512
298
The problem with the concept of using a laser to eject heat, using heat as the power source for the laser is that in order to create a laser from a heat source, the heat source must do work. In order to do that, you must have heat flow. In order to do that, you need a temperature difference from a hot to a cold reservoir.

The problem is that the amount of energy you can eject with a laser is no greater than the work done in creating the laser light. That work is limited by the temperature difference between the hot and cold reservoir.

Essentially, you are trying to use a heat engine to run a heat pump and using the heat pump to maintain the temperature difference that enables the heat engine to run. The best you can do is break even. Otherwise, you have a net flow of heat from cold to hot which violates the second law.

So in answer to your question, this concept is thermodynamically impossible.

AM
 

ZapperZ

Staff Emeritus
Science Advisor
Education Advisor
Insights Author
2018 Award
35,045
3,879
I did not create the thread, nor I am pushing the issue any further. Also, I did not advertise the book, for it was the creator of the thread who said it was a David Brin book. I only posted a passage of the book, so maybe we could better understand the concept.

When I said two Nobel laureates had thought the concept plausible, you answered by saying there are tons of Nobel laureates that commit stupid mistakes in other subjects. Thats a fine answer for me.

You worry that I did not care about the issue of the peer reviewed journals. But why should I worry, if I am not the one that created the thread about refrigerator lasers? I only continue a subject created by another but I am the one called the attention by a moderator? You should ask the thread creator if a refrigerator laser has been published in a peer reviewed journal.

The only "rule" I broke was "advertising", and that again, by a stretch of imagination. Btw, refrigerator lasers are NOT a theory. They are a piece of sci-fi, and the book author is not advertising here. Nor am I advertising a theory, since its not even a theory, just a cool concept used in a sci-fi book. I dont remember Clarke submitting to peer review geosynchronous sattelites or space elevators.
The difference here is that one is asking for clarification, the other is providing answers AND challenging the fact that on "here", it has been claimed to not work. Which one do you think your post falls under? When you make such statements AND, the only criteria you used is some hand-waving claim made by someone that Nobel Laureates can't find anything wrong with that, then the SOURCE that you are using to justify your claim is dubious!

If Arthur C Clarke were to come here AND made such dubious claim, we would haul him off as well! There is a reason why we call this "Physics Forums" and not "Science-Fiction Forums".

Zz.
 
15
0
The problem with the concept of using a laser to eject heat, using heat as the power source for the laser is that in order to create a laser from a heat source, the heat source must do work. In order to do that, you must have heat flow. In order to do that, you need a temperature difference from a hot to a cold reservoir.

The problem is that the amount of energy you can eject with a laser is no greater than the work done in creating the laser light. That work is limited by the temperature difference between the hot and cold reservoir.

Essentially, you are trying to use a heat engine to run a heat pump and using the heat pump to maintain the temperature difference that enables the heat engine to run. The best you can do is break even. Otherwise, you have a net flow of heat from cold to hot which violates the second law.

So in answer to your question, this concept is thermodynamically impossible.

AM

thanks, a very good and clear explanation.
 

Related Threads for: Is a refrigeration laser thermodynamically possible?

Replies
6
Views
358
Replies
0
Views
2K
Replies
1
Views
2K
  • Posted
Replies
7
Views
3K
Replies
37
Views
4K
Replies
7
Views
2K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top