In the current environmental climate, energy needs to be conserved and recycled as much as possible to prevent wastage – every joule saved means less fossil fuel burnt. Harvesting waste energy has become a popular target for researchers and scientists, who are coming up with innovative techniques to achieve this. One of the most promising is nanotechnology-based thermoelectric devices, which look to be a promising approach to save energy.
The direct conversion of heat into electricity, or electricity into heat, is known as thermoelectricity. This conversion process is referred to as the Peltier-Seebeck effect. This effect is name after two physicists – Jean Peltier and Thomas Seebeck. Peltier discover that current sent into two different electrical conductors that are connected at two junctions will result in heating up of one junction while the other junction cools down. Peltier went on to demonstrate that a drop of water can be made to freeze at a bismuth-antimony (BiSb) junction by just reversing the current. Peltier also discovered that electric current can be made to flow when a temperature difference is placed across a junction of different conductors.
Thermoelectricity is an extremely interesting source of electric power because of its ability to convert heat flow directly into electricity. Thermoelectric devices are energy converters that are easily scalable and have no moving parts or liquid fuels, making them applicable in almost any situation where large quantities of heat tend to go to waste, from clothing to large industrial facilities.
A number of research groups have been investing their efforts in finding the perfect material with appropriate properties to create an efficient thermoelectric effect, and nanomaterials seem to fit the bill. Whilst thermoelectric materials have been known and understood for quite some time, they have so far not been efficient enough to use commerically. With the advent of nanotechnology, however, that is changing.
Nanostructures used in materials will help maintain good electrical conductivity and reduce thermal conductivity. The performance of thermoelectric devices can thus be enhanced with the use of nanotechnology-based materials that have improved thermoelectric properties and good solar energy absorption abilities.
One of the proposed uses of these efficient, nanotechnology-enhanced thermoelectric devices focuses on reducing the load of batteries carried by soldiers in the battlefield. This will help increasing the range of the soldiers and allow them to carry more ammunition and food and stay away from base for longer time periods.
Thermoelectric materials based on bismuth telluride stand out as perfect examples highlighting the role of nanomaterials for thermoelectric devices. Bismuth telluride materials are best suited for room temperature applications with ZT (figure of merit) of ~1. The usage of nanostructured materials and low dimensional superlattice structures has lead to improvements in ZT. The usage of nanostructures smaller than the wavelength of light enhances the scattering of photons and thus decreases thermal conductivity. This decrease in thermal conductivity seems to be the most vital benefit of nanostructuring for thermoelectric materials.
A good thermoelectric material will thus have very poor thermal conductivity, but a very good electrical conductivity. Carbon nanotubes and graphene as thermoelectric materials exhibit improved thermoelectric properties. Further applications of nanostructuring can help improve scaling and optimization of thermoelectric devices that can be used to decrease carbon dioxide emissions and improve energy efficiency.
Carbon Nanotubes Boost Thermoelectric Performance
In a report published in October, scientists from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) used single-walled carbon nanotubes (SWNCTs) to advance the thermoelectric performance of organic semiconductors. The carbon nanotube thin films, they said, could ultimately be integrated into fabrics to convert waste heat into electricity or serve as a small power source.
In organic thermoelectric materials, carbon nanotubes are often an electrically conductive “filler” – one part of a polymer-based composite. The NREL researchers believe that carbon nanotubes could be a thermoelectric material in their own right, and a primary material for efficient thermoelectric generators.
The NREL researchers demonstrated that the same SWCNT thin films achieve equivalent thermoelectric performance when doped with either positive or negative charge carriers – an important finding, says Ferguson. The identical performance, he said, suggests that carbon nanotube networks have the potential to be used for both the p-type and n-type legs in a thermoelectric device. P-type and n-type legs can be made from the same SWCNT material, inherently balancing the electrical current in each and simplifying device manufacturing.
“That opens up the possibility of fabricating a device that is essentially a single semiconductor material, and then creating p- and n-type regions in that semiconductor,” said Ferguson. The same cannot be said of almost all inorganic semiconductor materials, said the senior scientist, which are typically n-type or p-type, but rarely both.
According to the team’s report, NREL’s combination of ink chemistry, solid-state polymer removal, and charge-transfer doping strategies enable n-type and p-type TE power factors, in the range of 700 μW m−1 K−2 at 298 K, for the thin films containing 100% s-SWCNTs.
“Our results indicate that the TE performance of s-SWCNT-only material systems is approaching that of traditional inorganic semiconductors, paving the way for these materials to be used as the primary components for efficient, all-organic TE generators,” said the authors in their Energy & Environmental Science abstract.
Graphene an Ultra-Efficient Thermionic Generator
Thermionic energy converters (TEC) traditionally used bimetallic junctions to convert heat into electricity, now Researchers at Stanford University have built a new prototype that uses graphene in the place of metal to make it nearly seven times more efficient than the original.
“TEC technology is very exciting. With improvement in the efficiency, we expect to see an enormous market for it,” said Stanford researcher and lead author of the paper, Hongyuan Yuan. “TECs could not only help make power stations more efficient, and therefore have a lower environmental impact, but they could be also applied in distributed systems like solar cells. In the future, we envisage it being possible to generate 1-2 kilowatts of electricity from water boilers, which could partially power your house.”
Stanford’s TEC prototype uses two electrodes, the emitter and collector, which are separated by a small vacuum gap. The researchers tested their prototype using a single sheet of graphene in place of tungsten as a collector material. Their results revealed that the new carbon-based collector material improved the efficiency by 6.7 times when converting heat into electricity at 1,000° C (1,832° F).
The technology is still not ready to be applied to practical uses such as powering homes, as it still works only in a vacuum chamber. But researchers are working on a vacuum packaged TEC that will allow them to test the reliability and efficiency of the generator in real-world situations, as reported by Colin Payne.
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