December 21, 2010

Thermoelectrics could be improved from 5% to 20% effciency for heat to electricity conversion

Certain configurations of the filler element barium will be very effective in lowering the materials' thermal conductivity and thus increasing their conversion efficiency.

Today's state-of-the-art thermoelectric materials are only five percent efficient. Skutterudites, and this new knowledge about how best to arrange their atoms, could help improve their performance to 15- or 20-percent, at which point they become useful in many practical applications, said Massoud Kaviany, professor in the Department of Mechanical Engineering.

"We explained the physics of these materials for the first time. This will help to advance the development of these materials. If you are designing them based on fundamental physics and materials and not just by trial and error, then you need to know how they work so you can avoid haphazard experimentation," Kaviany said.

ZT for the temperature ranges (700K) for best TE materials (mid-2010) are about 1.4 and need to be closer to 3.0 to reach the efficiency goals requested by DOE.

The conversion efficiency changes with ZT (figure of merit) and temperature difference. I have included the chart of ZT and temperature differential below to show how percentage of efficiency is effected by those two values. The researchers are claiming that their barium alloys can get ZT into the 2 range up from current lab work in the 1.4 range and commercially at the 1.0 range. The temperature differential varies according to the heat source that you are attempting to convert. (like steam from a powerplant or exhaust from a car)

Being able to get to a ZT of 2 at higher temperatures would achieve 20% efficiency.

Physical Review Letters - Structural Order-Disorder Transitions and Phonon Conductivity of Partially Filled Skutterudites

Filled skutterudites are high-performance thermoelectric materials and we show how their phonon conductivity is greatly influenced by the topology of the filler species. We predict (ab initio) the phase diagram of BaxCo4Sb12 and find several stable configurations of Ba ordering over the intrinsic voids. The phonon conductivity predicted using molecular dynamics shows a minimum in the two-phase mixture regime, dominated by significantly reduced long-range acoustic phonon transport.

There is a discussion of what could be with ZT= 4 thermoelectrics

Potential Impact of ZT = 4 Thermoelectric Materials on Solar Thermal Energy Conversion Technologies

State-of-the-art methodologies for the conversion of solar thermal power to electricity are based on conventional electromagnetic induction techniques. If appropriate ZT = 4 thermoelectric materials were available, it is likely that conversion efficiencies of 30−40% could be achieved. The availability of all solid state electricity generation would be a long awaited development in part because of the elimination of moving parts. This paper presents a preliminary examination of the potential performance of ZT = 4 power generators in comparison with Stirling engines taking into account specific mass, volume and cost as well as system reliability. High-performance thermoelectrics appear to have distinct advantages over magnetic induction technologies.

The status of thermoelectrics in March 2010

About 90 percent of the world's useful power (approximately 10 TW) is generated by heat engines that convert heat to electrical or mechanical power, but they also dissipate about ~15 TW of heat to the environment. If even a modest fraction of this low-grade thermal waste can be converted to electricity, the potential impact on energy efficiency could be enormous, leading to savings in fuel and reductions in carbon dioxide emissions.

Thermoelectric energy converters can directly convert heat to electricity using semiconducting materials via the Seebeck effect; they are all solid-state, robust, and have a high power density. Their efficiency depends on the thermoelectric figure of merit (ZT) of the material, which is defined as ZT = S2 T/ r k where S, r, k, and T are the Seebeck coefficient, electrical resistivity, thermal conductivity, and absolute temperature, respectively. Thermoelectric technology is scalable to various power levels such as those encountered in automotive exhaust heat recovery and residential distributed solar-thermal electrical generators. In these applications, it can become competitive with current technologies provided one can develop materials with ZT over 1.5 at the appropriate temperatures; higher ZT values would open many more applications. Although there is no fundamental upper limit to ZT, over the last 50 years the ZT of commercially available materials has increased only marginally, from about 0.6 to 1, resulting in performance less than 10 percent of the Carnot limit. However, recent research results have used various strategies to enhance ZT by reducing the phonon k and/or enhancing the electron power factor (S2/ r).

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