Researchers Achieve Major Increase in Thermoelectric Efficiency of Bismuth Antimony Telluride
20 March 2008
 |
| A cross-section of nano-crystalline bismuth antimony telluride grains, as viewed through a transmission electron microscope. Colors highlight the features of each grain of the semiconductor alloy in bulk form. Click to enlarge. Image: Boston College, MIT, and GMZ Inc. |
Researchers at Boston College and MIT have achieved a major increase in the thermoelectric efficiency of bismuth antimony telluride—a semiconductor alloy that has been commonly used in commercial devices since the 1950s—in bulk form. Specifically, the team realized a 40% increase in the alloy’s figure of merit—a dimensionless term calculated to indicate a material’s relative performance—from 1 to a peak of 1.4.
The team’s low-cost approach, details of which are published in the online version of the journal Science, involves building tiny alloy nanostructures. The discovery sets the stage for broader use of this new nanocomposite approach in developing high-performance, low-cost bulk thermoelectric materials.
Among their other potential applications, thermoelectric materials could enable more efficient recovery of waste heat from combustion engines, using the resulting electric power to improve the overall fuel economy of the vehicle. (Earlier post.)
The achievement marks the first such gain in a half-century using the cost-effective material that functions at room temperatures and up to 250° C. The success using the relatively inexpensive and environmentally friendly alloy in bulk form means the discovery can quickly be applied to a range of uses, leading to higher cooling and power generation efficiency.
By using nanotechnology, we have found a way to improve an old material by breaking it up and then rebuilding it in a composite of nanostructures in bulk form. This method is low cost and can be scaled for mass production. This represents an exciting opportunity to improve the performance of thermoelectric materials in a cost-effective manner.
—Zhifeng Ren, Boston College
The researchers crushed bismuth antimony telluride into a nanoscopic dust and then reconstituted it in bulk form, albeit with nanoscale constituents. The grains and irregularities of the reconstituted alloy increased phonon scattering, radically transforming the thermoelectric performance by blocking heat flow while allowing the electrical flow. Phonons, a quantum mode of vibration, play a key role because they are the primary means by which heat conduction takes place in insulating solids.
In addition to Ren and six researchers at his BC lab, the international team involved MIT researchers, including Chen and Institute Professor Mildred S. Dresselhaus; research scientist Bed Poudel at GMZ Energy, Inc, a Newton, Mass.-based company formed by Ren, Chen, and CEO Mike Clary; as well as BC visiting Professor Junming Liu, a physicist from Nanjing University in China.
The research was supported by the Department of Energy and by the National Science Foundation.
Resources
Bed Poudel, et. al, High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys, Science (20 March 2008) DOI: 10.1126/science.1156446
Vehicular Thermoelectrics Applications Overview (John Fairbanks, DOE, DEER 2007)
I do not know if the company Power Chips is a serious company, but this article is interesting:
http://nanotechweb.org/cws/article/tech/9965
Posted by: Jorge | 20 March 2008 at 04:04 PM
A stability limit of 250 degC implies a temperature differential of around 170 degC if fresh engine coolant is used as the heat sink. Combined with a figure of merit of 1.4, this translates to a conversion efficiency of around 12% of the heat remaining in the already-cooled exhaust gas.
http://www.eere.energy.gov/vehiclesandfuels/pdfs/deer_2003/session4/2003_deer_crane.pdf
Unfortunately, it is not possible to fit a thermoelectric device directly into the exhaust such that it will produce useful amounts of electric power but never get exposed to temperatures above 250 degC. One option would be to include a variable volume bypass that becomes active at medium to high engine output levels. Flaps in the exhaust are always a reliability issue and also affect engine sound, but let's assume such a setup is chosen.
The air mass flow through a 2.0L four-cylinder four-stroke diesel engine at 2000RPM is 0.039 kg/s (assumes ambient air at 25degC and normal pressure). At 2bar bmep, the engine will produce 13.3kW of shaft power at an efficiency of perhaps 25% (better values are achieved at higher loads). Another ~20% is lost to coolant, the rest (~24kW) to the exhaust. As a first-order approximation, that yields an engine-out exhaust gas temperature of perhaps 550 degC. Perhaps 10% of the exhaust enthalpy is lost to the turbo, resulting in a turbine-out temperature of ~470 degC. This drops to ~400 degC by convective cooling by the time the emissions equipment and the majority of the exhaust pipe has been passed. At this point, the heat enthalpy flow of the exhaust gas is roughly 16kW.
To protect the thermoelectric element against heat damage, about 20% of mass flow are diverted, creating a diffusor. Part of the enthalpy is used to increase pressure, but roughly 8kW remains in the form of heat at 250degC.
At a conversion rate of 12%, this yields ~1kW of electrical power. Under normal circumstances, total hotel loads is no more than 500W, so at least 500W are available to run the alternator in motor mode (special belt setup required).
At higher engine loads, exhaust gas temperatures will be higher and the diffusor action must be stronger to achieve sufficient cooling. That means the mass flow past the thermoelectric element has to be reduced, so less electricity is generated even though engine-out exhaust enthalpy is actually higher. Delivering the same power at higher engine speed increases electricity generation, but at the expense of lower engine efficiency.
Conclusion 1: for ICE-based vehicle applications, thermoelectric materials should tolerate high temperatures, even if their figure of merit isn't all that great
Conclusion 2: engines designed for modest torque but high revs (e.g. Wankel rotary) may be a better match than diesels and other turbocharged engines
Conclusion 3: there is value in combining thermoelectric electricity generation with a mild hybrid setup
Posted by: | 20 March 2008 at 05:45 PM
@anonymous
With regards to Conc 1: I am yet to see a TEC that can tolerate direct heat from a gasoline engines exhasut
With regards to Conc 2: I've always envisioned combining a Wankel engine with some sort of organic Rankine cycle engine. Wankels produce so much waste heat (low CR and TE) it should be relatively easy for a waste heat engine to recover a lot of it.
With regards to Conc 3: Spot on.
Perhaps a heat exchanger could be built into catalyst-back portion of the exhaust (right after the secondary cats) to allow a fluid to take and transport part of the heat to a TEC. This should be easier than using flaps in the exhaust as the fluid could simply be redirected to the radiator. This would also allow for such systems to be retrofitted to existing vehicles.
A heat-pipe type system could be used to distribute heat from the exhaust over a larger surface area therefore reducing effective temperatures. E.g. one square inch of exhaust area could be spread over two square inches of TEC area dropping temperatures from 400C to 200C (not a correct example, I know, but you get my drift). This would effectively make more of the heat in the exhaust available for TEC use.
Posted by: GreenPlease | 20 March 2008 at 06:58 PM
That's quite and interesting results which has potentialy plenty of applications, but I doubt that Thermoelectric element are well suited for waste heat recovery. As you point out, the problem is that they a have limited range of temperature where they can operate properly and their efficiency, even with this reported improvement, is still quite low. Manufactured in thin film and their efficiency will be even less so not sure it will pay off
Posted by: Treehugger | 20 March 2008 at 09:47 PM
How about in a solar-thermal application, either by itself or to boost the efficiency of solar cells?.. solar cells dont use the infrared portion of the solar spectrum.
Posted by: Herm | 20 March 2008 at 09:53 PM
As long as it does not get hotter than 250C in the engine compartment, these can take the long wave IR from the exhaust headers and convert to electricity.
Posted by: sjc | 20 March 2008 at 10:11 PM
I have been watching the developments of the Thermoelectric entities and have found this company as the best for energy conversion at this time. 3 watts per square centimeter at optimum temperature is the potential I see in Thermo generators. There main page: http://www.nextreme.com/home.htm and watch the video at the bottom of the screen, there best one to explain the nuances for application specifics: http://www.nextreme.com/pages/applications/ic_thermal.html
Posted by: C L | 21 March 2008 at 04:45 AM
Global annual production of tellurium is only around 135 tons, so I doubt this technology could make a large difference in global energy production.
Posted by: Paul F. Dietz | 21 March 2008 at 07:28 AM
@Paul
Tellurium is largely a byproduct of smelting aluminum. Your number seems awefully low. What is your source? (genuine curiosity, not trying to be a jerk)
Similar issues were raised regarding FSLR, but they claim to have several tW worth of economically extractable Te.
Posted by: GreenPlease | 21 March 2008 at 12:01 PM
Tellurium is largely a byproduct of smelting aluminum.
Actually, it is mostly a byproduct of copper refining.
What is your source? (genuine curiosity, not trying to be a jerk)
Excellent question, and no offense taken! USGS is the source for information of this kind, for all sorts of mineral commodities. It's a very useful site.
http://minerals.usgs.gov/minerals/pubs/commodity/
http://minerals.usgs.gov/minerals/pubs/commodity/selenium/mcs-2008-tellu.pdf
Posted by: Paul F. Dietz | 22 March 2008 at 06:03 PM
This is why they call internet chat a "flat medium", you can ask a genuine question and sound like you are challenging. This mode contains no inflections.
Posted by: sjc | 31 March 2008 at 09:25 PM