Electric heat: A small sample of a new material for converting heat into electricity is attached to electronic leads and a tiny heater for testing.
Emily Burkhard and Vladimir Jovovic
An advance makes the conversion of heat to electricity practical.
By improving the electronic properties of a common thermoelectric material--a type of semiconductor that converts heat into electricity--researchers have doubled its performance, making it more practical for generating electricity from waste heat such as that produced in power plants and car engines.
Thermoelectrics haven't been widely used to generate electricity because they are expensive and inefficient. To increase the efficiency, the researchers, including Joseph Heremans, a professor of mechanical engineering and physic at Ohio State University, added trace amounts of thallium to lead telluride, a thermoelectric material that's been generating electricity onboard deep space probes for decades. The added thallium doubled the material's ability to convert heat into electricity by increasing the voltage that it produces. Heremans says that the improved efficiency could translate into a 10 percent increase in the fuel economy of cars if the devices are used to replace alternators in automobiles by generating electricity from the heat in exhaust. The new materials are described in this week's issue of the journal Science.
The new work is important for several reasons, says Gang Chen, a professor of mechanical engineering at MIT who was not involved in the work. First, it's a "quite impressive" increase in the efficiency of one kind of thermoelectric material, he says. Conventional lead telluride thermoelectrics convert about 6 percent of the energy in heat into electricity. Once it's incorporated into a thermoelectric generator, the more efficient thallium-enhanced material could increase this to 10 percent, once losses, such as those from making electrical connections, are taken into account.
More important, Chen says, Heremans's work gives researchers a new way to improve thermoelectric materials that could increase the efficiency of a wide variety of experimental materials. Thermoelectric materials are good electronic conductors but poor thermal conductors: the heat difference within the material largely accounts for the thermoelectric properties. Almost all the recent improvements to thermoelectric materials--and there have been significant improvements in the past few years--have come with a decrease in their thermal conductivity. Heremans and his colleagues have tried a different approach, increasing the voltage that the materials create. "That is," Heremans says, "we get the electrons to do more work."
Techniques employed to cut the thermal conductivity could be used to complement the new techniques developed by Heremans and his colleagues. That would allow the researchers to double the performance of the materials yet again, suggests Heremans. And that, in turn, would start to make thermoelectric devices competitive with conventional generators, says Jeffrey Snyder, a materials-science researcher at Caltech and one of the other researchers involved with the Science paper.
One drawback to the new materials is that thallium is extremely toxic, so it would require safeguards during manufacturing and disposal. (During use, the materials are encapsulated and therefore pose less of a danger.) However, Heremans says that the devices could be removed from old cars and put on new ones since they could easily last the lifetime of several vehicles, decreasing waste-disposal problems.
Heremans is optimistic that the new materials can be quickly commercialized, since engineers already have years of experience working with lead telluride. He says that the first products, likely thermoelectric generators that convert automobile exhaust into electricity, could be ready in three to four years.
New thermoelectric materials will be tested in BMW, Ford, and Chevrolet vehicles by the end of summer.
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I need to do more research on this. How much power can one convert with this at a reasonable price? Would it be cost effective for use in concentrating solar thermal systems instead of PV cells? What deltaT is required for effective power generation?
I don't think you're looking at it the right way. I don't think 10% efficiency, or even the potential 20% the article mentions, would be good enough to compete with solar PV or solar thermal.
Instead, you should think of it as a way to make traditional internal combustion engines and generators more efficient by turning part of a waste stream; their exhaust; into something useful. All traditional power plants, Coal, Nuclear, etc, produce heat to turn a turbine which spins a generator and generates the electricity. The steam, after going through the generator, is useless, hence those massive cooling towers. The combustion exhaust is also just sent up a chimney. You could use these thermoelectric cells to line the pipes leading to the cooling/exhaust towers generating more energy from that waste. The same thing could be done on a smaller scale, as said in the article, in any device that uses an ICE and needs electricity.
Honestly, I'm surprised they didn't think of power plants in the article, which would seem to be the best usage to me since they do last 50+ years in most cases.
People are so fixated on research to make thermoelectric more efficient for waste heat applications, when in fact there are far more promising technologies that can be used for this, that don't get much attention.
For example, the article on this site about infrared nano-antennas. Hot objects radiate infrared into the colder ambient. Use this principle in conjunction with infrared nano-antennas and very high efficiency can be achieved, higher than thermo-electrics could ever do.
I'm not sure how using heat from exhaustfumes would effectively eliminate the fumes themselves, the technology if used as described above will still be harmfull to the environment. I agree with others though that this technology would be a great asset in conjuction with other technologies that are already in use, increasing overal 'perfomance' of a system.
Instead of replacing PV, what about using this technology to convert the heat resulting from the use of solar concentrators in existing PV systems. Using two methods to produce electricity in the same system might lower the overall cost of the installation and raise the total energy conversion ratio.
browni gets the point. Its is not necessarily about replacing one form of generation with another. It is about increasing overall efficiency. TEs can be used as a bottomin cycle just about anywhere... Residual heat from an ICE, downstream of a steam turbine, heat on the backside of a PV panel,and if your really want a "green" house... how about heat from your roof shingles, waste heat from the back of your refigerator, and even the heat from the hot water going down the drian. I'm thinking TEs on the waste heat side of a 5kW SO Fuel Cell generating electricity for the independant home.
if you put this on the back of your fridge you will be insulating its heat sink, reducing its efficiency. you won't get as much power back as you'll need to add to keep the fridge as cool as before.
you missed the point--and the opportunity--with respect to refrigerators.
The cooling coils are still the part of the fridge that extends farthest out. The TEs (or TVs for consistency w/ PhotoVoltaics) go between the coils and the insulation at the back of the cold compartment. This prevents heat extracted from the CC returning to it. Perhaps a certain amount of insulation could even be replaced by the TVs, but at the very least we could now have some of the cooling energy "recycled," lowering the overall electric use of the device.
Peak efficiency is reached at a temperature delta of 900F. Very doable in the exhaust of a car, though not so much for a NG or coal fired plant.
Also note that TECs in order to function properly must act as an insulator which makes them a no no for concentrated photovoltaics.
Using them on spent uranium fuel rods would be an excellent idea.
I'm not sure if this would be suitable for spent fuel. The reason is safety: economical use of thermo-electrics requires high temperatures, and when the thermo-electrics fail, the container could melt and catch fire, releasing dangerous amounts of radioactivity.
It's much better to just use dry cask storage with passive cooling that never fails. The safety risk just isn't worth the relatively small amount of electricity that can be generated from the spent fuel.
Unless of course, we're talking about the spent fuel ponds, where the water acts as passive cooling (and Tlow for the thermo-electrics). That could work I think. That's also the phase where the fuel releases the most heat (short half life isotopes decay) so on second thought it could actually work quite well. The temperature difference would be low though (we don't want the water to boil!), so forget about high efficiency with thermoelectrics. We're still talking about relatively small amounts of power compared to the output of nuclear plants themselves. Nuclear powerplants are inefficient because of the low temperature heat engines (saturated rankines). So there should be a lot of electricity to be had from waste heat to electric conversion from the turbine exhaust itself rather than from the spent fuel.
they are wonderful for CPV...Just use convection water cooling - long mastered by the navy for sub reactors - to bring the heat to a hot water jar lined with TV materials.
And as for "what if they break down..." Heavens Forfend! These circuits simply cannot break down unless the material physically breaks in half so as to prevent circuit completion - in which case the water bottle will be broken as well.
Just a moment here! Why "instead of PV cells"??
PV cells operate better at lower temperatures, right? Why not simply use such thermoelectrics as a heatsink to cool PV-cell temperature on the one side (increasing PV-cell efficiency), while generating their own additional energy at the same time?
Then, place these "hybrid" thermoelectric-PV-arrays over parking lots, thus creating shade (& lots of solar energy, either for charging elec. cars, or for selling back to the grid).
This would surely work as a magnet for shopping centers. After all, where would you prefer to park your car? In the shade? Where (if elec./hybrid) it could be recharged for free while shopping?
dbd
The technology is most likely to be used in places where other heat recovery technology is impractical. Automobiles have relatively huge possibilities for alteration in dimension and function. This makes the automobile the perennial favorite for adding new gadgets. Always be critical of altering automobiles because their design is market driven.
How about generating some energy from the high heat available from a home heating appliance. It would be nice to use some of the energy as electricity before it's used as space heating, using it twice in the home or work place.
I wonder how much electricity could be harvested from the heat in an attic.
if your insulation is good, not much. if it is bad, then fiberglass is cheaper. and remember, you can expect to get back at most 20% of that waste heat; better not to waste it in the first place.
hello? If your insulation is good then all summer long you have tremendous temperatures in your attic space from sun on your roof eventually contaminating the attic space but then being unable to pass into the living space.
As for PV being more efficient...PV efficiency is low enough that the vast majority of light will still become heat. Most of that heat will be passed on to the attic space. And, of course, in few cases will all the available roof space be covered in PV anyway. So your attic continues to overheat from the uncoverted solar radiation. That heat is then easily converted to electricity through TV.
We're talking large arrays then, I'm not sure if that can in any way compete with current solutions like PV, that even generate current without big temperature difference in terms of costs. Seeing as the thermoelectric item under discussion here can last several car lifetimes, I'd say it'd well last my lifetime as well (I'm talking european cars here ofc ;)).
For the long run, this kind of product seems well more suited for longterm additions to existing means of powerconversion.
Guest (DrZook)
Two things to add. 1) How about using heat from from waste nuclear fuel rods. Thermocouples could be attached to a metallic heat transfer structure that surrounds the used uranium and waste product. The whole device then could be encapsulated in concrete as a barrier with two wires that would supply a steady stream of electricity forever.
2)Look up a company called Borealis; they have an electron tunneling device that they are developing that attains better than 90% thermal efficiency. It uses what they call the Avto effect.
I think there is a problem about this following aspect of what you said: "The whole device then could be encapsulated in concrete as a barrier with two wires that would supply a steady stream of electricity forever."
That concrete barrier would contain the heat within the concrete, thereby leaving the entire thermoelectric device at the same temperature, that of its concrete walls.
In order to extract power there must be a heat source and a heat sink at two different temperatures. One of the terminals of the device must be colder than the other. So to get energy out of the concrete tomb that you envision, a cooling liquid would have to be pumped in. The comments submitted by others all assume an open environment wherein the atmosphere would serve as the heat sink.
This is not to say your idea can't be made to work, once a cooling scheme is designed into the idea. But an entombment in concrete just makes this problem worse. Without active cooling there is a limit to the rate at which electrical power can be extracted. And that limit is dependent upon the surface area of the containment vessel, which for radioactive waste would be as small as possible.
And nuclear waste has to be kept reasonably cool for safety reasons. If we're talking about spent fuel ponds then water is the primary heat sink (secondary probably atmospheric). High temperature waste is highly undesirable in this application. As such, high efficiency isn't possible (and these thermo-electrics arent' efficient even at big delta Ts in the first place).
The potential impact for turbine exhaust heat to electricity could be really big though. Nuclear, coal, solar thermal...
It would have to be cheaper and more efficient than organic Rankine cycles to be competitive in this application.
Darn, I keep forgetting that we're basically talking about inverted peltier elements, thanks for the heads up in your article :)
The greatest fraction of heat energy you can turn into electricity is given by the temperature difference divided by the higher [absolute] temperature. (Example: moving heat from 25C to 0C, you'd be able to convert about 25/298 (about 8%) to electricity.) When we speak of efficiency, are we talking about the fraction of this maximum fraction, or something else?
Guest (rhapsodyinglue)
Every time I read about waste heat harvesting, I hope the article is honest/rigorous enough to define the use of the "efficiency" metric they invariably cite. None ever seem to. I suspect you are correct that they are stating an efficiency relative to the thermodynamic limit.
Far more impressive to talk about double digit efficiencies than to admit what small portion of the actual energy contained in the heat is getting converting into something useful.
Perhaps the author could clarify?
The author makes it clear that it's about heat-electric efficiency. What's important to realize is what the other guy said, that thermo-electrics efficiency varies over the temperature difference. What's needed is a heat to electrical converter that is inexpensive and operates efficiently at relatively low temperature differences, say 200 or less degrees Kelvin delta T. Thermo-electrics are not the best contender here, and won't likely be for many years to come.
Any idea how it's efficiency (present and speculative) compares to the Stirling engine? I know they operate at lower temperature differentials, but with moving parts, albeit far fewer than ICE generators.
The efficiency of the solar-Stirling engine is 31% while the current efficiencty of the best TE device is 3-5%. Even this paper would double that to about 6-10% that is far less than the Stirling engine.
Could this material be inexpensively added to existing brake discs/drums or built into replacement brakes to extract some electricity from the plethora of waste heat? That sounds like an opportunity, a "partial hybrid retrofit" for existing vehicles..although no electric motor drive would be present, the alternator could perhaps be disconnected or removed, lightening engine load and improving efficiency, if the brakes can charge the battery... : ) S
So what is the weight of these on an exhaust system where they could replace an alternator? Small aircraft recip. engine plugs are energized by magnetoes but today's instruments do require a reasonable amount of power. Aircraft engines are generally run at 60% power or more so there is lots of excess heat and at any reasonable altitude Delta T with ambient air temp. is even greater. Another bonus would be improved reliability, something obviously more important for pilots than auto drivers. These, coupled with a Jumo junkers type diesel could be an aviators dream for reliability and efficiency. Call me anytime for a test aircraft.
I agree, like brake shoes, any internal combustion engine's exhaust system is another source of waste heat that's just "lying around"... even a small amount of electricity would be great, when obtained for free...
If these thermoelectrics could be used to fuse with current energy sources, could they lower necessarily fuel needs by around 35%?
Manufacturing in the United States is in trouble. That's bad news not just for the country's economy but for the future of innovation.
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lkrndu
36 Comments
Side Benefits
Intriguing. If it can pan out, not only does it reduce the mechanical complexity of cars or other machinery, the whole notion of a component that has several lifetimes - re-useable across several useful lives of the machine that employs it - multiplies the savings. Less stuff manufactured, fewer replacements, not so many, one hopes, of calls for help when a cranky alternator goes fffft.
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boz_hobbs
4 Comments
Re: Side Benefits
O'yeah. We need this like we need another nose. Another toxic ditty to carry around at freeway speeds (in the millions of numbers) to replace an item that is proven and not that toxic. Nope. The ICE-driven vehicle is dead; in the thousands of days, not ten thousands as most the writers here seem to imply.
I do like the concrete encasement idea; although, since the government is running the potential disposal site, it may prove explosive.
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