Skip to comments.Thermoelectrics ‘pass new milestone’
Posted on 09/19/2012 2:29:53 PM PDT by neverdem
Engineering PbTe on the panoscale making it exceptionally efficient at turning waste heat into electricity © Mercouri Kanatzidis
Researchers in the US claim to have passed a new milestone in thermoelectrics with a material that converts heat to electricity more efficiently than ever before. The new thermoelectric material, which employs panoscale structuring to scatter phonons, has a figure of merit (FoM) some 20% better than previously achieved.
Thermoelectrics convert heat to electricity and can, therefore, harvest waste heat from the environment. When one end of a thermoelectric material is heated, electrons flow to the cooler side, creating a voltage across the material that can be tapped for electricity.
The materials are rated according to the FoM, which depends on several properties. One is an ability to create a high voltage; another is a high electrical conductivity to allow a large electric current. But in the field of thermoelectrics scientists have focused on improving a third key property a low thermal conductivity, which maintains the temperature gradient.
Heat passes through a material in the form of waves, or phonons, which have different wavelengths. To prevent the flow of heat, and thereby reduce the thermal conductivity, scientists must find ways to scatter these phonons. In the early days of thermoelectrics, scientists found that they could scatter the smallest-wavelength phonons by switching atoms in the materials lattices; this produced a maximum FoM of about one. Then in the early 2000s the maximum FoM shot up to roughly 1.8, with nanoscale crystals that could scatter the mid-wavelength phonons.
Mercouri Kanatzidis at Northwestern University in Evanston, Illinois, believes an FoM of two has long been considered a psychological milestone. At two, the case for applications becomes much stronger, he says.
Kanatzidis and his colleagues have now succeeded in creating a material with an FoM greater than two by tailoring not just the atomic scale and the nanoscale, but the mesoscale too. They use sodium-doped lead telluride (PbTe) and strontium telluride a mixture that, when frozen, spontaneously develops both atomic and nanoscale phonon-scattering structures. The researchers then grind the material into a powder that has larger grain sizes large enough to scatter the largest-wavelength phonons. This hierarchical or panoscopic approach produces an FoM of about 2.2.
Only a few years ago most of us believed sodium-doped PbTe could only achieve [an FoM of] 0.6, says materials scientist Jeff Snyder at the California Institute of Technology in Pasadena, US. Now, for a number of reasons contributing together, we have [an FoM] of two. Snyder adds that an FoM above two is not a rigid threshold for sudden adoption into applications, but believes it is a nice milestone.
Materials scientist Eric Toberer at the Colorado School of Mines in Golden, US, points out that the new high efficiencies of thermoelectrics could make them competitive with solar cells. One advantage is that solar thermoelectric generators do not require expensive thin-film growth, he says. Rather, the desired nanostructures form spontaneously.
K Biswas et al, Nature, 2012, 489, 414 (DOI: 10.1038/nature11439)
This wouldn’t necessarily have to be limited to ambient atmospheric heat as the source, if combined with geothermal, using a closed cycle heat exchanger that would heat an enclosed chamber, a steady source of heat for energy generation could be realized.
So, how long before the heat generated inside a closed up car sitting in the summer sun can be used to charge the batteries in an electric car?
Just curious if it’s in the realm of the possible or not...
How about a thermoelectric generator using tailpipe heat from a internal combustion motor? No need for an alternator. Combine with catalytic converter?
A likely early adopter of Thermoelectric is stationary gen-sets then automotive applications if it could replace the alternator completely that would be quite beneficial.
The dubious electrical hybrid market would also be of interest as waste heat from the ICE engine could be harvested into the battery stack
Or on my chimmey of my wood stove.
"The future belongs to the efficient."
Thermoelectric generators are already available: http://biolitestove.com/campstove/camp-overview/features/
The above is a stove that has a TEG-powered battery (though it doesn’t use the new technology discussed in the article), which shortly after the fire starts begins to turn a fan in the stove to increase the efficiency of the stove. Shortly thereafter, the TEG is producing enough power to charge a cellphone. Disclosure - I have NO connection to BioLite or any other TEG manufacturer or seller. I do, however, intend to buy one of the BioLite stoves soon.
Gosh! How many goldmines that produce calaverite (gold telluride) will be looking good.
Similar to piezoelectric energy harvesting sensors,, or not.
Just curious if its in the realm of the possible or not...
That wouldn't work - the TEG (thermoelectric generator) needs to have a temperature differential to work (i.e. one hot side and one not-as-hot side. The bigger the difference in the temperature between the 2, the more electricity it can produce). So, leaving the whole thing inside of a car wouldn't work. However, if it could somehow be built into the vehicle such that part of the TEG was inside the hot car, and the other side outside, you'd have electricity generated and, presumably, available to recharge the car's battery.
Better would be a TEG attached to the the tailpipe of a hybrid car (and better diesel than gas, since diesel is inherently more efficient - there are some diesel hybrids making about 70 mpg in Europe). This would allow for the charging of the battery quite well, and might even allow for a larger electric motor and a smaller gas one, thereby increasing efficiency.
I used to think that thermoelectric generation would be useful for off-grid, wood-heated cabins — if only it were a lot more efficient. The more efficient it becomes, the more applications will be found for it.
Although this sounds great, the efficiency of these devices is still VERY low. And, they tend to be fragile, require assembly, and need cooling on the “cool” side to keep efficiency as good as possible. Still they have their place in areas where absolute reliability is important (no moving parts). The recent Mars rover uses these wrapped around the heat from the radioactive core.
Thanks for the explanation. It seems to me you could have as much as a 40-50 degree differential between in-car temp and outdoor temp (say 110 inside in a closed car in the summer vs. 70 outside) and it wouldn’t be that big a problem to engineer something that had the cold side on the outdoor side.
If there were a 40-50 degree F. difference, how much juice could be generated? A practical amount? Again, just curious. After all, this is truly waste heat, and it’s generated all day long while the car is sitting in a parking lot at work. If it worked, people might even start to hoard the sunny spots like they currently do the shady ones.
You two need to get together and market the idea...almost identical.
Green houses could use the electrical generation for heat mass storage that can be utilized later to normalize the green house during cooler nights.
Same can be true for the home, without the need, at least for most of the months, for an additional wood offering to compensate for the reduced ambient heat.
Again....etc, etc.....nice possibilities.
Now that’s cool...so to speak...
With current technology, my understanding is that a 40 - 50 degree difference wouldn’t generate much. However, most of today’s devices are geared toward stoves of some type or exhaust pipes, to get 200 or more degrees differential. This is probably due to the limits of current TEG technology. With new materials, a smaller temp differential would produce more, and trickle-charging a big battery over 8 hours or so would likely make a lot of sense. I guess that we simply have to wait for devices to be made to know for certain.
I am, however, much more excited about this technology than solar - you don’t need the sun to shine to have this work, and as time goes on, this appears to have the ability to increase more in efficiency. The devices are also a lot more mobile, due to not needing sunlight or even any light at all (like using the heat from a truck’s or car’s exhaust at night).
At http://www.tegpower.com/products.html there’s a 350 watt auto exhaust TEG. As others have mentioned, currently-available TEGs aren’t too efficient, but developments like the one described in the article make this a very promising field.
While I have to admit that my first though upon reading this blurb was “Arm the Phonon Cannon, Mr Worf!”, this is progress. When you read about our planetary probes that use “nuclear batteries”, it usually means that they are using high temperature long-life isotopes and a thermo-electric generator (TEG). A breakthrough like this means that the same size installation would have 20% more power and that is no small consideration!
Right now, our most distant probe, Voyager 1 (1977), is at 122AU from Earth and the signal strength of its transmitter is a measly 20 watts. Imagine what a difference a 20% increase in power would have meant to its design, 40 years ago. AMAZING!
Thermoelectrics are not, however, efficient enough to be used everywhere. Existing technologies can turn only 57% of heat energy into electricity, much less than the conversion efficiency of technologies such as solar panels.'Tantalizing' hints of room-temperature superconductivity Doped graphite may superconduct at more than 100 ºC.
The final step was to stop heat flow over longer scales. To do this, the team created a fractured version of their pretty thermoelectric crystal. The fracturing did the trick: the cracks allowed electrons to move but reflected heat vibrations in the crystal. The material had a conversion efficiency of about 15% double that of normal PbTe thermoelectrics.
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Interesting, thanks for excerpting that neverdem.
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