Slashdot Mirror
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."
I used a laser and a maser to extract energy from the waste heat generated by my Athlon. I've been running everything in my house but my computer off that exhaust tap!
They that would sacrifice their
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.
The hot gases belching out of your car's exhaust are not just useless waste. They are a laser waiting to happen, says physicist Marlan Scully
I sure hope this doesn't change the global warming going on or all that beachfront-after-the polar-icecaps-melt property I bought will remain high and dry (scuba diving in downtown LA whoohooooo)
I Am My Own Worst Enemy
You put them on the head of those friggin sharks.
cheers
Can be found here (in PDF form), for all those who like reading physicists physics.
There's no reason for a sig here.
Do Quantum Mechanics work for Maxtor now?
General Relativity: Space-time tells matter where to go; Matter tells space-time what shape to be.
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.
Well, the university I went to had its own electrical power station. They used the waste heat to generate steam that was sent all over campus for heating. Even the dorms' clothes dryers used steam heat exchangers.
They seemed to have so much heat capacity available that they didn't think that proper thermostats were a priority. A lot of people had to regulate the heat on subzero days by opening the windows.
If this makes it into your average car, would you have to take it to a normal mechanic AND a quantum mechanic? The price of the devices used in research had better come down before it happens.
I can see it now...
QM: (Wipes hands on oily rag) Well, if you lookee here, yer muffler wall is causing the maser beam to create destructive interference.
Car owner: uhuh.
QM: That, combined with the alignment of the quantum magnetic dipole is causing yer car to stall.
Car owner: But how much will it cost?
QM: Yer salary fer the next two years.
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. There is no "irreversible" process. (For the jargon-empowered, QM does not have a natural "arrow of time").
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.
So the idea to use QM to improve this "ideal"-ness (classically speaking) is an intersting step towards understanding the *other* big issue in science : which is how the 2nd Law fits into the grand scheme of things. (Grand Unified Theories do not incorporate 2nd law since microscopically are processes are essentially reversible. The 2nd law drove many people nuts, including Roger Penrose.)
So the point of the paper is not "get more $$$" for you engine. It's an interesting gedenken-experiment (sp?) that proves a point.
Mode (3) smart-aleck mode. Press * to return to main menu.
It actually has a big point.
:).
Although what you say may be correct, you have to remember that either using this to cool the intake or even better using it as a below-ambient intercooler on turbos increases the power-to-weight ratio of the engine because you can obviosly get more charge in a cylinder.
Thus you can create a lighter car with the same power and overall the efficiency increases because you have that much less mass to accelerate and that much less rolling resistance on the tires. Granted the efficiciency of the *engine* does not increase, but the efficiency of the entire system [car] does -- and that's the thing in the end that truly matters.
What I'm waiting for is efficient low-temperature thermo-couples to become cheap. That way electricity can be generated from the wasted exhaust heat getting rid of the need for an alternator.
Combine that with regenerative breaking and a few bucks on gas can definately be saved
What you are refering to is VE(Volumetric Efficiency). VE is a major factor in the specific output(PS/Liter or HP/Cubic Inch). Regardless of VE, what we really are looking at is raising BSFC. That is Brake Specific Fuel Consumption. This number shows how much gasoline is required to produce horsepower. New electronically-actuated valves will do much to raise this. On a 4-valve cylinder, over 20% of the engine output is used soley to spin the camshafts and plunge the valves up and down quickly.
Turbocharged engines help by absorbing some of this engine's exhaust and 'reinvest' this kinetic and thermal engergy in the intake. However, it is a losing proposition; even with an intercooler, the more boost you pump, the hotter the intake charge gets. You quickly develop a cycle where you must retard timing to reduce preignition and detonation thus raising exhaust temp's even more. The retardation of the ignition severely reduces power output thus nullifying any boost pressure you are running anyways.
No, turbochargers are good for increasing VE , but do little to alter the fundamental(thermal) efficiency of an engine.
We need a revolution.
You can't legislate goodness. Let each to his own destiny, by will of his freely made choices.
and it is a pretty interesting idea. I'm not sure about the practical feasibility of the concept for reasons I'll get into below. But, it shows that quantum effects might be usefully exploited to make better engines and will probably prompt a fair amount of thought and experiment into the matter.
... thanks lameness filter ... less than signs could never be useful).
... more molecules must be in one of the upper states than in the lower states. However, in a gas at thermal equilibrium, this is usually not the case ... the probabiliy of finding a given quantum state in state with energy E is proportional to exp(-E / kT ). Here, k is Boltmann's constant and T is the ambient temperature. At low temperatures, the ground state will be where most of the molecules are.
... "b" molecules to preferentially transistion into the ground state (state "c"). However, the "a" population won't be able to come to equilbrium that fast (provided the spontaneous emission rate is sufficiently low and the maser cavity isn't tuned to enhance the transition rate out of "a" state). This net impact of the maser is to create a population inversion between the "a" and "b" states. By passing the non-thermal maser cooled gas into a laser cavity tuned to the "a"-"b" transition, this inversion can be extracted as laser energy. This is the quantum afterburner part.
... involving passing the gas back and forth through two pistons. I'm pretty sure that materials and a simplified engine design could be made to validate the claims though.
Warning: Ph.D. punditry follows.
Suppose a molecule has three possible states ("a", "b" and "c") with energies E_c, E_b, E_a respectively (E_c is the ground state and E_b is the between E_a and E_c
Suppose further, microwave (maser) energy transitions are possible from state "b" to "c". Optical (laser) transitions are possible from "a" to "b".
For lasing to occur, you must have a population inversion
If the hot exhaust gas is first passed through a maser cavity tuned to the "b"-"c" transition containing a radiation field at the temperature of the cold reservior, the "b" and "c" populations will quickly come to thermal equilibrium with the low temperature radiation field
From a quantum standpoint, nothing is particularly new here. Using rapid cooling of a selective population to create inversion is pretty unique but nothing that can't be explained with the standard laser rate equations.
From a purely statistical mechanics standpoint, the net effect is to extract extra useful work from internal degrees of freedom of the working fluid. Statistical mechanics is not my forte so I can't really say if this is particularly out there.
From a practical standpoint, it might be hard to find gases at engine temperatures and gas pressures where the low spontaneous emission lifetimes necessary to sustain the inversion is possible. My intuition says that collisional de-excitation (high temp and pressures) would wipe out the inversion. Also, the exact scheme discussed in the paper is more complicated
As a thought experiment, though, this shows that it may be possible to improve the efficiency of an Otto engine. (By the way, the paper notes that a Carnot cycle efficiency doesn't get a boost from the technique.)
Kevin
(And many thanks to all the scientists who publish on arXiv).
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.
Gentlemen, you can't fight in here, this is the War Room!
Doesn't anybody else find it at least slightly funny that someone is proposing putting a Quantum Afterburner (TM) on a piston engine, the essential design of which is 125 years old? After all, there are other ways to recover waste heat in the exhaust that we could be using now, but aren't. Peltier junctions could be used to generate electricity to supplement or replace the function of the alternator once the engine was hot. Someone else here mentioned stirling engines. Maybe that'd be another way to increase the efficiency. Again, maybe you could drive the alternator with it. Of course, the alternator only uses maybe 1 or 2 horsepower anyway, so even eliminating that drag on the engine is only going to be a small improvement.
Than again... how many horsepower does a car use when cruising? Maybe eliminating 1 or 2 horsepower would make a difference. I would assume that this Quantum Afterburner (TM) would be able to recover a much greater amount of the waste heat, too, so maybe it would make quite a difference.
P.S. -- before anybody starts to rant on me for using horsepower, remember, there are metric horsepower too! According to my unit converter, one horsepower equals 1.01387 metric horsepower. Guess the French have different sized horses than the English! Cheers!