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Clathrates: Tiny cages enclosing single atoms are shown. Credit: TU ViennaClathrates are crystals consisting of tiny cages in which single atoms can be enclosed. These atoms significantly alter the material properties of the crystal. By trapping cerium atoms in a clathrate, scientists at the Vienna University of Technology have created a material which has extremely strong thermoelectric properties. It can be used to turn waste heat into electricity....
By trapping cerium atoms in a clathrate, scientists at the Vienna University of Technology have created a material which has extremely strong thermoelectric properties. It can be used to turn waste heat into electricity....
with a silicon-sort-of, non-mechanical process ?
this even beats fuel cells, eh?
(and yes thats a BIG question)
really?
i'm leaving the techy stuff to the engineers, but huh...
So they say...it would be nice if it was actually true, and soon to be available...then we'd have that cheap energy we all need to live luxuriously on. Until then, though, I'll keep saving my pennies.
Yeah, if by silicon-sort-of you mean "solid state": no moving parts except for electrons and phonons. The article mentions "sophisticated crystal growth techniques" and that generally means "expensive". Thermoelectric generators aren't cost-effective for widespread use, but they have their niche where long-term reliability and compact form factor are more important than power output -- a better thermoelectric material like this could well expand the niche.
Anyway, in a handy-wavy sense, a good thermoelectric material has to conduct electricity with low resistance while also being a thermal insulator. That generally means designing a material in which electrons are unlikely to run into stuff, but lattice vibrations (phonons) are. The article makes reference to the Kondo effect, so apparently trapping those magnetic cerium atoms in the clathrate lattice makes the material's minimum electrical resistance occur at some relatively high temperature, rather than at absolute zero. I guess clathrates are generally poor thermal conductors, but it sounds like the magnetic coupling between the trapped cerium atoms and the conduction electrons in the clathrate improves electrical conductivity.
I think a thermoelectric generator that uses this material would be more directly comparable to a Stirling engine, and would probably not be as efficient as a Stirling engine even with a 50% improvement in its thermopower... just smaller and without moving parts.
Yeah, if by silicon-sort-of you mean "solid state": no moving parts except for electrons and phonons. The article mentions "sophisticated crystal growth techniques" and that generally means "expensive". Thermoelectric generators aren't cost-effective for widespread use, but they have their niche where long-term reliability and compact form factor are more important than power output -- a better thermoelectric material like this could well expand the niche.
Anyway, in a handy-wavy sense, a good thermoelectric material has to conduct electricity with low resistance while also being a thermal insulator. That generally means designing a material in which electrons are unlikely to run into stuff, but lattice vibrations (phonons) are. The article makes reference to the Kondo effect, so apparently trapping those magnetic cerium atoms in the clathrate lattice makes the material's minimum electrical resistance occur at some relatively high temperature, rather than at absolute zero. I guess clathrates are generally poor thermal conductors, but it sounds like the magnetic coupling between the trapped cerium atoms and the conduction electrons in the clathrate improves electrical conductivity.
I think a thermoelectric generator that uses this material would be more directly comparable to a Stirling engine, and would probably not be as efficient as a Stirling engine even with a 50% improvement in its thermopower... just smaller and without moving parts.
Damn! I didn't understand half of what you said, but it was impressive none the less. About six months ago I named my daughter after you. Actually, her name is Ashley... and now that she is giving the walker a try, we often call her Crashley... and on the occasion that her older sister sits on her (now 2 years old), we call her Smashley.... In any event, I was thinking of you when I named her.
Let's not forget that clathrates also refers to trapped methane - another one of the supposed sources of world doom.
The issue - as ASH has noted - is the creation of clathrates is non-trivial.
My friends in the semiconductor/solar business, for example, were able to create fantastically efficient GaAs solar cells in the lab. The process for converting these fantastically efficient, lab created GaAs solar cells into a manufacturing process, however, would at a minimum have required tens if not hundreds of millions of dollars - which was all obviated when the companies managed to blow their stashes on fancy offices and what not.
What we see here is the left side of the equation, linked with the right side of the equation, but missing the 'Then A Miracle Occurs' middle portion:
... Thermoelectric generators aren't cost-effective for widespread use, but they have their niche where long-term reliability and compact form factor are more important than power output -- a better thermoelectric material like this could well expand the niche....
Thanks for those insights, ASH.
The only places I am aware that thermoelectric generators are used now is in very remote unattended telemetry stations, like buoys out at sea sending weather data, or remote arctic pipelines sending flow data. These use a heat source of a burner running on propane, natural gas, or butane. When your alternative is to fly in by expensive helicopter to repair and refuel a diesel generator, you can use one of these instead and install many fuel bottles less often, and at least skip the repair trips for a conventional generator set and not have breakdowns.
Also in spacecraft where where a highly radioactive pellet is the thermal heat source and the package generates a little bit of power for years without interruption. These are normally called "RTG" - Radioisotope Thermoelectric Generators
Actually, her name is Ashley... and now that she is giving the walker a try, we often call her Crashley... and on the occasion that her older sister sits on her (now 2 years old), we call her Smashley.... In any event, I was thinking of you when I named her.
Thanks for being a part of this community.
You do me too much honor; it's a pleasure to be part of this community.
Congratulations on your little Ashley/Crashley/Smashley -- may she grow strong and healthy and curious! My girls came at roughly the same spacing (they're 3 and 5 now)... things got a whole lot easier about the time the youngest turned 2.
You do me too much honor; it's a pleasure to be part of this community.
Congratulations on your little Ashley/Crashley/Smashley --...
+1
heheheh... gotta love it... and _always_ a pleasure to read you, mr ash.
and altho like da i cant unnerstanz 1/2 of it either, its very cool to try to follow along
(with ALL the brainiacs we're lucky to have here on the 'tulip ;)
...and altho like da i cant unnerstanz 1/2 of it either...
I take that as a challenge to write a clearer explanation that depends less on jargon -- or at least explains it better -- if you will indulge the attempt.
It's necessary to begin with a description of the structure of a solid. The main thing to understand is that the chemical bonds which join atoms together are stretchy, like springs. In a solid, each atom is connected to its neighbors by a stretchy chemical bond, and each atom has some weight. You can envision the solid as a whole bunch of ball bearings stuck together with springs. If you tap the assembly, the atoms will vibrate against each other as their bonds expand and contract. Heat is energy manifest in the randomized mechanical motion of an object's parts, and most of a solid's heat is in the random shaking of its atoms vibrating against each other, as their bonds stretch and compress.
With that preliminary, let's assume you have a bar of electrically conductive material, and it's hot at one end and cold at the opposite end. The natural tendency is for temperature to equalize along the bar, so that every part of the bar is at the same temperature. It turns out that while the temperature is equalizing, you can extract other forms of energy, such as electricity, from the heat flow. But the process of moving heat from the hot end to the cold end of the bar requires something to carry the heat across the bar -- to make the hot end shake less and the cold end shake more. The two things which can carry heat in the bar are conduction electrons and phonons.
Electrons form the outer parts of atoms and are responsible for the chemical bonding that assembles atoms into materials, such as the material from which the bar is made. Conduction electrons are electrons in a solid material which are free to wander through the solid rather than being tied up in any individual atom or localized chemical bond. The motion of conduction electrons is what carries DC electrical current through a wire, but conduction electrons can also carry heat. This works in a way similar to how heating a gas causes it to exert pressure on the walls of its vessel if contained, or to expand if it isn't contained. Vibrating atoms can smack into conduction electrons and accelerate them; fast-moving electrons can smack into atoms and start them vibrating. In a piece of solid material at a uniform temperature, energy gets passed back and forth between vibrating atoms and roving electrons rapidly and endlessly, such that an equilibrium is achieved in which the average electron is no more likely to transfer energy to an atom in a collision than to receive energy from the atom. The higher the temperature of the solid, the more heat energy is manifest in random atomic vibrations, and the higher the equilibrium speed of the conduction electrons. Saying that the electrons in a hot solid move faster is the same as saying that gas molecules move faster -- and therefore hit the walls of a balloon harder -- if they're hot, resulting in higher pressure. Anyway, if you now imagine that you've got a bar of material with a hot end and a cold end, you can see that the average speed of the conduction electrons on the hot side is higher than on the cold side. This has two consequences. First, since the conduction electrons are free to wander through the bar, some of the "hot" electrons are bound to move into the cold region of the bar, where their first collisions with the atoms there are more likely to transfer energy from electron to atom than from atom to electron; the converse applies to the "cold" electrons that wander into the hot side of the bar and are more likely to pick up energy from their first collisions than to lose it. That's how the electrons transport heat across the bar. But the second consequence is what permits power generation from the temperature difference. Since the hot electrons have a higher average velocity than the cold electrons, they will wander into the cold side faster than cold electrons wander into the hot side, which means there will be a net electric current! That's basically how a thermoelectric generator produces electricity from a heat differential.
Phonons are harder to understand -- they're "particles of sound" in the same sense that a photon is "a particle of light". The myriad different ways the atoms can collectively rattle against each other in a solid can be broken down mathematically into a set of patterns that are characterized by how far and in what direction each atom displaces from its equilibrium position, and the frequency of its displacement. These patterns are called vibrational modes, and are similar to the pure tone of a plucked guitar string or a rung bell. Quantum mechanics says that energy can only be added to or extracted from these vibrational modes in chunks of a set size; these chunks are called phonons. (Why quantum mechanics says this is another, long conversation.) The connection to heat is this: since heat mainly manifests in a solid as the random vibrational motion of its atoms, phonons are by definition little chunks of heat. Instead of moving energy out of atomic vibrations and into the motion of conduction electrons, heat can also move through the solid by direct mechanical transfer: a chain of atoms smacking into neighboring atoms instead of atoms smacking into conduction electrons. In particular, heat transfer requires moving energy out of some vibrational modes and into others, and can be looked at as the "motion" of phonons.
Now, heat that gets carried by phonons is heat that isn't carried by conduction electrons, and which therefore can't be used to generate electricity. That's why you want thermoelectric materials in which electrons can get around easily (high electrical conductivity) but in which phonons have trouble getting from point A to B (low thermal conductivity). I'm sure I'm missing some subtleties (I don't work on thermoelectrics), but I think that stuff about the Kondo effect is just a detail: a clever way to get better electrical conductivity in a material that's a bad thermal conductor. A lot of the better electrical conductors are also good thermal conductors (think most metals), and so are bad for thermoelectric generators. Separating thermal conductivity from electrical conductivity is tough because the main property responsible for good electrical conductivity (a very regular crystal lattice) also normally results in good thermal conductivity.
I've always been somewhat confused by phonons as they are not fundamental particles like electrons, photons, although they are extremely useful constructs for modeling and prediction. I infer from your overview that phonon characteristics are dependent and therefore "tunable" based on the atomic, stoichiometric, and crystallographic structure, and this is what allows the design of the type of materials under discussion.
In my firm, the opposite solution is sought, i.e., achieving high thermal conductivity of an electric insulator - application in this case is heat sink for LED lamps (we don't want a short to charge the heat sink for example).
Originally posted by thriftyandboringinohioView Post
The only places I am aware that thermoelectric generators are used now is in very remote unattended telemetry stations, like buoys out at sea sending weather data, or remote arctic pipelines sending flow data.
Cool. I didn't know about the buoy and pipeline applications; I'd only ever heard about RTGs. I gather that the great dream for thermoelectric generators is to scavenge waste heat from all sorts of other engines and industrial applications of power, but it sounds like there's a pretty big gap of efficiency and cost yet to cross to get to economic viability.
I infer from your overview that phonon characteristics are dependent and therefore "tunable" based on the atomic, stoichiometric, and crystallographic structure, and this is what allows the design of the type of materials under discussion.
That's right. Phonon characteristics are given by dispersion relations of the sort one normally calculates in classical mechanics for elastic waves, based on geometry, mass, and coupling spring stiffness. Changing the masses, their spatial arrangement, and/or the spring constants coupling them changes the dispersion relation. The fact that these vibrational modes are excited in discrete units (phonons) mostly comes into play when calculating how energy gets transferred into and out of the vibrational modes, or passed between different types of excitations in the crystal. The analogy to photons is pretty close, because in most engineering applications, you can apply classical electrodynamics to calculate optical mode shapes and only appeal to the quantum picture of there being photons when considering how energy gets transferred into and out of the optical modes, or coupled into other physics like charge carrier recombination. Of course, as you point out, photons are an excitation of a wave medium that permeates all space (the electromagnetic field) and so are more fundamental than the excitations of a bounded physical medium like a chunk of crystal.
In my firm, the opposite solution is sought, i.e., achieving high thermal conductivity of an electric insulator - application in this case is heat sink for LED lamps (we don't want a short to charge the heat sink for example).
I see. Since you're doing something practical that needs to make financial sense, I understand what you mean about "way too expensive". Otherwise you'd "just" use something like diamond, eh?
... I gather that the great dream for thermoelectric generators is to scavenge waste heat from all sorts of other engines and industrial applications of power, but it sounds like there's a pretty big gap of efficiency and cost yet to cross to get to economic viability.
That's my understanding too, ASH.
Efficiency is less relevant when we are recovering waste heat, as every single BTU is going in the trash now. I think of that as free fuel.
Thermoelectric generators are typically long lived and maintenance free, so anything gained after we pay off the initial investment is real money in our pocket.
Opportunity cost is the only issue remaining, and as PCO drives up the value of wasted BTUs it all gets more attractive.
Efficiency is less relevant when we are recovering waste heat, as every single BTU is going in the trash now. I think of that as free fuel.
That's not quite true.
The construction cost of the waste heat recovery device must also be taken into consideration.
For cars - weight is also a consideration.
Lastly the use for said recovered BTUs is also worth consideration. How useful would a thermoelectric device - even of perfect efficiency and minimal cost - be in an automobile?
There is definitely drag on a car's engine from the alternator - should demand load increase. Would thermoelectric recovery offset this to a significant degree? If so, this would improve mpg at least to some degree - though now much, is difficult to say.
Other everyday use might be to recover electricity from waste heat in ovens, refrigerators, and the like - however, the low cost of electricity would make the value prop. probably fairly poor, unless the devices were ridiculously cheap.
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