Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
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The best commercial materials are alloys <strong>of</strong> Bi2Te3 with Bi2Se3 (n-type) and with Sb2Te3<br />
(p-type). The alloys are used because phonon thermal conductivity can be reduced significantly<br />
with only a small reduction in the electronic power factor (S 2 σ). Bi2Te3-based alloys have a<br />
maximum ZT around 1 near room temperature. Thus these materials are used <strong>for</strong> refrigerators.<br />
Thermoelectric refrigerators based on these materials have a COP ~ 1 in the same operational<br />
temperature range <strong>of</strong> compressor-based household refrigerators, which have a COP ~3–4. Thus,<br />
current commercial materials are not competitive. However, thermoelectric refrigerators have<br />
advantages <strong>for</strong> small refrigeration applications and have found niche markets in a variety <strong>of</strong><br />
applications, such as picnic coolers, automobile car seats (luxury models), medical equipment,<br />
and laser-diode temperature control. Bi2Te3-based materials are also used in some power<br />
generation applications; however, the module efficiency is limited to 5%. The U.S. National<br />
Aeronautics and Space Agency used SiGe alloys (and PbTe-based alloys) to make radioisotopepowered<br />
thermoelectric power generators operating in the temperature range <strong>of</strong> 300–900°C (and<br />
300–600°C <strong>for</strong> PbTe-based alloys), with a system conversion efficiency ~6–7%. These materials<br />
all have a maximum ZT less than but close to 1. Commercial PbTe-based power generation<br />
systems using fossil fuel have a fuel-to-electricity efficiency <strong>of</strong> ~2.5%. The lower efficiency is<br />
due to heat loss carried by combustion gas.<br />
The commercial materials discussed above, with a maximum ZT ~ 1, were mostly discovered in<br />
the 1950s. Little progress was made in the subsequent years. In the 1990s, the possibility <strong>of</strong><br />
improving the thermoelectric figure <strong>of</strong> merit based on electron band-gap engineering and phonon<br />
engineering in nanostructures was investigated. These ideas have led to resurgence in<br />
thermoelectric research and significant progress in improving ZT, particularly based on<br />
nanostructured materials (Chen 2003). Venkatasubramanian et al. (2001) reported that<br />
Bi2Te3/Sb2Te3-based p-type superlattices have a room-temperature ZT <strong>of</strong> 2.4. Harman et al.<br />
(2002) reported that PbTe/PbTeSe superlattices with nanodots <strong>for</strong>med by strain have a roomtemperature<br />
ZT <strong>of</strong> 1.6 and a ZT ~ 3.5 around 300°C. Hsu et al. (2004) reported bulk<br />
nanostructures <strong>of</strong> AgPb2SbTe2+m. with a ZT <strong>of</strong> 2.2 at 527°C. These results suggest that<br />
thermoelectric materials can have major impacts in energy conversion technology. Meanwhile,<br />
several research projects aiming at improving device efficiency based on more mature materials<br />
are underway. The Jet Propulsion Laboratory reported a segmented thermoelectric unicouple<br />
with an efficiency <strong>of</strong> ~14% with the hot side at 975K and cold side at 300K.<br />
Current commercial thermoelectric modules based on Bi2Te3 are at ~$4.60/W because the<br />
market is very small. With current efficiency, it has been projected that the cost can reach<br />
$0.74/W if the annual consumption is more than 2 million modules. If the efficiency can be<br />
improved, significant cost reduction is possible. It was projected that $0.3/W (electrical power)<br />
could be realized by using nanostructured materials. Assuming a 35% energy conversion<br />
efficiency <strong>of</strong> thermoelectric devices and a concentrator cost <strong>of</strong> $0.24/W (solar input power), the<br />
cost <strong>of</strong> solar-based thermoelectric power generator systems has the potential to reach the<br />
$1–1.5/W (electric) range.<br />
The enabler <strong>for</strong> the cost/per<strong>for</strong>mance improvement is cost-effective materials with high ZT<br />
values. Nanostructured materials have broken the ZT ~ 1 barrier, and it seems that ZT ~ 4 is<br />
reachable. A major ef<strong>for</strong>t should be aimed at mass-producible nanostructures with high ZT and<br />
further understanding <strong>of</strong> the scientific underpinnings <strong>of</strong> high-ZT values. Other important factors<br />
are material reliability, system design, and thermal management.<br />
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