Capturing Waste Heat with Quantum Mechanics

TheMatt writes: "There is a summary of a Phys.Rev.Lett. article up at Nature Science Update that describes a design for a 'quantum afterburner' that would improve the efficiency of an Otto engine. It improves the efficiency by using a laser and maser to extract energy from the hot exhaust of the engine. In fact, the process could enhance performance beyond that of the "ideal" Otto engine."

Comments

But what do you do with the light?

[Anonymous Coward]

So I get that you convert waste heat into light, first with the maser to get microwaves, and then with the laser to get some other wavelength. What then?

Re:But what do you do with the light?

[Anonymous Coward]

The author doesn't mention anything about this. I reread the article several times and still don't understand what is going to be done with the extracted energy. Do you use it to increase the density of the air entering the engine, much like a turbocharger does now? I suspect it would be better to turn the extracted energy into something that could drive another engine, like an electric assist motor. As for increasing the thermo efficiency higher than the ideal otto cycle, I believe this is impossible based on the second law of thermodynamics. They might be able to approach the ideal efficiency, but noone has ever built anything better.

Re:But what do you do with the light?

[CanadaDave]

This is an extremely inefficient method, and solar cells are extremely expensive for what you get out of them. Which is exactly why this whole crazy idea will never happen until the economics work out....and until there is a good use for a laser in a car. It's a waste to convert the energy to another form, every time you do that, you lose tremendous amounts of energy. The heat in the exhaust should be used directly (for heating) or the laser sould be used directly if possible, and economical. Somehow I don't think any useful laser can be powered from exhaust, and a battery would surely be needed anyways.

I think that the laser should be used in some way for a laser guidance system. Although this is more suited to rockets, and projectiles, but who knows. Based on what happened in 20th century, the future is almost impossible to predict.

Adsorption refrigerator to cool intake charge.

[Nick Driver]

Why not simply use an adsorption type "refrigeration" (ammonium hydroxide & water) system to cool the air/fuel intake charge to make it more dense and get some more efficiency out of the internal combustion engine? The waste heat going out the exhaust and radiator could run the adsorption-cycle cooling system.

Re:Adsorption refrigerator to cool intake charge.

[cgleba]

I've been thinking about that for years. There are a few papers on that, too. . .I can't find them off-hand.

Two useful things that I thought of for that are:

1) Free air-conditioning
2) Use as an intercooler for turbocharged engines.

The problem with ammonia, however (and the reason why they stopped making propane-run absorption refrigerators a long time ago) is that under certain conditions I guess ammonia is explosive and not to mention not too good for you :).

But I don't know much for IANAC (I am not a chemist). If anyone knows anything about this (even though it is edging on off-topic) I would love to hear a discussion as I have pondered this a LOT. . .

Re:Adsorption refrigerator to cool intake charge.

[dfenstrate]

Because for Otto combustion engines, the efficiency is primarily a factor of compression ratios, not temperature or density. You could get a very efficient engine if you could compress the fuel infinetly (of course, gasoline won't stand for much compression) before igniting it.

If you increased the density, then you could compress it less, probably resulting in less efficiency. Increasing air flow is usually a good thing though.

Not quite

[Anonymous Coward]

First off, so-called gas-dynamic lasers have been known for years if not decades, so this is old news. Secondly, the energy efficiency is not increased beyond the theoretical limit of an Otto engine.

Re:Not quite

[nusuth]

Ideal otto engine efficiency!=ideal heat engine efficiency. No heat engine can beat Carnot cycle in terms of thermal enegry->work conversion, but an "enhanced" otto cycle engine can beat usual otto cycle, without violating any thermodynamic laws provided that it doesn't beat carnot cycle's efficiency.

On a related note, heat engines are much less efficient that 100% you seem to imply with "it should give same amount blah blah." The reason is second law of thermodynamics. You can convert all heat energy you put in the engine to work, since doing so would require heat transfer with no temperature gradient.

Efficiency

[AntipodesTroll]

After reading the article, its not as far fetched as it sounds, atleast at a low efficiency.

The problem as I see it though is this, what is the engine going to do with the laser light anyway? Laser light isnt that usefull in a car as an energy source. And I cant see the intensity of light being enough to do something cool, like dissasociate water to H2 and O. This is probably in the "neat-things" file for quite some time. Though maybe they can use this technology for fixed poer generation (coal, nuclear) where the gasses temperatures are higher and there is more volume.

Re:The importance of the paper is more than just $

[Soft]

Quantum Mechanics has been known to be a time-trasnlation invariant theory. In layman's term, it means that you can run the clock backwards and everything is fine.

Same with classical mechanics, and more so, as QM has the "destructive measurement" hypothesis, that by merely measuring that an object is in a given state, you collapse any state superposition in which it might have been. Besides, Statistical Physics and Thermodynamics have borrowed quite a lot from QM (particles being in given states among a number of possible ones, etc.)

However, we know the Thermodynamics 2nd law tells us that even *ideal* processes are essentially irreversible if we do work, i.e. waste heat is inevitable.

Yes, this comes from the fact that there exists a great many more possibilities that waste heat will be irrecoverably produced. It might stay in a usable form, just as you might open a bottle of ink under water, and the ink might flow out and then all crawl back into said bottle. It is just highly unlikely.

There is no need for an arrow of time at the microscopic level for that.

As for the paper itself, if I understand the summary correctly, it is ingenious but I'd look for a catch, such as the maser requiring at least as much power to function as that you can extract from the waste heat... Wouldn't that be annoying?