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Thermoelectrics’ Potential for Material Impact on Climate Crisis Questioned, Even in Vehicle Waste Heat Recovery
4 May 2008
While the science, technology and business of thermoelectric (TE) technology—solid state “heat engines” capable of converting heat to electricity or alternatively converting electricity into cooling—has never been stronger than today, the technology’s potential for a material impact on the climate crisis appears limited, according to Cronin Vining, president of the thermoelectric consulting company ZT Services (ZTS) and publisher of Thermoelectric News.
Only a single application, recovery of vehicle waste heat, appears plausible in this respect, and even that application faces stiff barriers, he said, in an invited presentation to a roundtable on Nanotechnology and New Materials chaired by former Vice President Al Gore held on 1 May in New York City.
While the discussions at the roundtable were off the record, Vining is making his contribution public via the ZTS website to stimulate discussion. Brief comments should be posted online there.
Despite substantive improvements in the science of and increases in funding for thermoelectrics, Vining notes,
After 15 years of intense R&D only three efforts have produced ZT values in excess of 2: Harman’s quantum-dot superlattice, Venkatsubramanian’s superlattice and Kanatizidis’s ‘LAST’ bulk/‘nanodot’ material. In each study, only one ‘type’ (either n- or p-type, but not both) was reported improved. And predictive models to quantitatively explain the results have yet to emerge. Translation of these high ZT laboratory results to commercial quantities of materials and/or efficient devices does not appear imminent.
(ZT is a dimensionless figure of merit used to designate thermoelectric efficiency. ZT is a combination of three properties of a material: thermal conductivity (k), electrical resistivity (r) and Seebeck coefficient (S).)
Challenges to migrate the new, more efficient TE materials to products include understanding exactly why high ZT happens, developing predictive models, capturing the essential, underlying nanophysics in cost-effective materials, extending the temperature range of high ZT, and establishing both n- and p-type materials with high ZT.
Even for automotive waste heat recovery, an application area that, according to Vining, is “the most promising thermoelectric application with Greentech implications,” TE faces a significant challenge from mechanical engines.
Honda, for example, has tested a system using a Rankine steam engine to generate electricity from waste heat in a Honda Stream hybrid vehicle, increasing overall engine efficiency by 3.8%. BMW has for some years had a similar effort called Turbosteamer, but in their effort the added device is used to supplement the power train (rather than generate electricity) improving fuel efficiency 15%. Either project appears to beat the FreedomCar goal of 10% fuel savings today.
The Department of Energy FreedomCar project is sponsoring research by four teams to develop a thermoelectric generator that can improve overall fuel economy through waste heat recovery by 10% and can be in production in the 2011-2014 timeframe.
Moreover, even if this future R&D achieves a full-fledged, device level average ZT=4 it is still probably insufficient to displace mechanical engines for large-scale applications. Of course, ZT=4 should greatly enhance the range and performance of niche applications TE technology serves so well today. But the impact on the climate crisis, even with ZT=4, seems limited to smaller scale, decentralized applications the most promising of which appears to be vehicle exhaust heat recovery. Even there, the benefit is potentially about 10% improved fuel economy assuming all the hurdles to market penetration are overcome. The opportunity for thermoelectric technology to help in the climate crisis appears limited.
Resources
Cronin Vining (2008) The Limited Role for Thermoelectrics in the Climate Crisis
May 4, 2008 in Climate Change, Thermoelectrics | Permalink | Comments (6) | TrackBack (0)
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Comments
Would there not be an application for thermoelectrics in large arrays of concentrating photovoltaics (CPV) cells? CPV concentrates several hundred suns onto very efficient (> 30%) and very expensive PV cells and requires direct sunlight, which favors desert locations where the uses for the excess heat, e.g., hot water heating, are limited.
Posted by: NorthernPiker | May 4, 2008 8:12:42 AM
A concentrator cell company has done a cell with an IR cell company. They used one of their 30%+ cells with an IR cell. They got more power, but the cost and the small amount more that they got was not worth it.
If they increased efficiency of IR cells then a lot of things are possible. They have quantum well designs that have a higher figure of merit. The one posted on here went from 1 to 1.4 figure of merit. QW has about a 4 figure of merit. It is estimated that you would need a 3 to 4 figure of merit for make it practical for engine heat recovery.
Posted by: sjc | May 4, 2008 8:56:44 AM
Don't thermocouples generate a lot of current? I remember a high school physics demonstration consisting of a metal horseshoe with its two ends in ice water and a burner, and a different metal bridge across the horseshoe mouth, and the thermocouple current around that loop was large enough to support something like 100 kg of (non-magnetized) iron.
Posted by: Adam | May 6, 2008 7:15:23 AM
An immediate application of this technology would be to recover even a small fraction of the waste heat generated by Al Gore...
Posted by: sulleny | May 6, 2008 7:24:18 AM
Even in automotive applications, there's a limit to how much heat you can extract from the exhaust gas. First, you need to keep the catalytic converter above its light-off temperature. Second, the waste head from any secondary process has to be shed via a larger radiator, which may create packaging problems.
One potentially useful idea would be a secondary turbine for a large diesel engine or small gas turbine. Scania uses a hydraulic clutch and fixed gear ratios to connect one to the crankshaft if its HDV diesel. Alternatively, shaft work from the secondary turbine could be used to pump low-grade heat from the engine coolant into a secondary closed-loop organic Rankine cycle. The waste heat from that would go back into the engine coolant via a passive heat exchanger.
Unfortunately, the resulting system would be complex, involving an engine, secondary turbine plus heat pump and, an organic Rankine cycle. Nevertheless, it could be useful for class 8 HDVs, locomotives and ships.
Posted by: Rafael Seidl | May 6, 2008 10:18:32 AM
A solid state solution would be preferable, but until they get there, the turbo alternators and vapor turbines will be more useful. On large trucks that get 5 mpg on the highway, both would be very nice. You might get 6-7 mpg.
A long haul truck can put on 200k miles per year and burn 35-40k gallons of diesel. Each truck could save 5k gallons at almost $5 per gallon for a savings of $25,000 per year. That is a lot of money and fuel saved.
I see this as part of a bigger program. Streamlining, cooling, APUs and other methods will be used as well. The truckers are hurting right now from fuel costs and need all the help that they can get. We also need to look more at truck to rail to reduce long haul traffic in the west.
Posted by: SJC | May 6, 2008 10:58:45 AM
