A thermoelectric (TE) device can directly convert heat emanating from the Sun, radioisotopes, automobiles, industrial sectors, or even the human body to electricity. Many electrical and mechanical devices, such as car engines, produce heat as a byproduct of their normal operation. It’s called “waste heat,” and its existence is required by the fundamental laws of thermodynamics.
“Over half of the energy we use is wasted and enters the atmosphere as heat,” said Boona, a postdoctoral researcher at Ohio State. “Solid-state thermoelectrics can help us recover some of that energy. These devices have no moving parts, don’t wear out, are robust and require no maintenance. Unfortunately, to date, they are also too expensive and not quite efficient enough to warrant widespread use. We’re working to change that.”
Today, thermoelectric generators allow lost thermal energy to be recovered, energy to be produced in extreme environments, electric power to be generated in remote areas and microsensors to be powered. Direct solar thermal energy can also be used to produce electricity, writes Daniel Champie. Thermoelectric generators are expected to fulfil significant market needs including individual cars, transportation trucks and distant sensors in energy intensive industries, e.g. metal or glass production.
Thermo-electric generators allow waste heat to be recovered and used productively to improve fuel economy and reduce CO2 emissions. According to US Military, the reductions in the Department’s need for energy can improve warfighting capabilities, such as increased range, better endurance, longer time on station, and reduced requirements for resupply. Improved energy performance also can reduce the risk and effects of attacks on supply lines and enable tactical and operational superiority.
Military is interested in thermoelectrics for Energy transfer, energy harvesting, thermal management, and refrigeration. DARPA’s Materials for Transduction (MATRIX) program is seeking new materials for energy transduction ( conversion of energy from one form into another) such as communications antennas (radio waves to electrical signals), thermoelectric generators (heat to electricity) and electric motors (electromagnetic to kinetic energy) that would result in new capabilities or significant size, weight, and power (SWAP) reduction for military devices and systems.
Although a number of materials with thermoelectric properties have been discovered, most produce too little power for practical applications. In spite of increased research and development, the thermoelectric power-generating efficiency has been relatively small, with efficiencies of not much more than 10 percent by the late 1980s. Researchers are developing better thermoelectric materials in order to go much beyond this performance level.
Thermoelectric generators utilize thermoelectric effect like Seebeck effect, Peltier effect and the Thomson effect for energy conversion, in which an electric current is produced at the junction between two wires of different materials if they are at different temperatures. The voltage produced by TEGs or Seebeck generators is proportional to the temperature distance across between the two metal junctions.
TEGs are made of pairs of p-type and n-type elements. The p-type elements are made of semiconductor materials doped such that the charge carriers are positive (holes) and Seebeck coefficient is positive. The n-type elements are made of semiconductor material doped such that the charge carriers are negative (electrons) and the Seebeck coefficient is negative.
Most current thermoelectric materials are based on rare or toxic elements, including cadmium-, telluride- or mercury-based materials, which preclude their implementation at large scale. More sustainable materials have been extensively investigated over the years, but mostly at laboratory scale. Furthermore, they failed so far to achieve sufficient performance levels to justify heavy industrial investments towards full scale production and market introduction.
Radioisotope Thermoelectric generators for Deep Space Missions
Radioisotope power systems are generators that produce electricity from the natural decay of plutonium-238, which is a non-weapons-grade form of that radioisotope used in power systems for NASA spacecraft. Heat given off by the natural decay of this isotope is converted into electricity, providing constant power during all seasons and through the day and night. RTG can generate hundreds of watts to power multiple spacefaring instruments.
Radioisotope thermoelectric generator or RTG have been used to power many deep space missions from the Cassini orbiter around Saturn, the New Horizon probe to the outer Solar System, the Curiosity rover on Mars and the veteran Voyager probes. Because an RTG has no moving parts and doesn’t require regular maintenance, it is well suited for powering gadgets that can’t be attended to for long durations.
They offer the key advantage of operating continuously, independent of sunlight, for a long time. They have little or no sensitivity to cold, radiation or other effects of the space environment. Radioisotope electrical power and heating systems enable science missions that require greater longevity, more diverse landing locations or more power or heat than missions limited to solar power systems, says NASA.
Thermoelectric generators for Military
In 2014, GMZ Energy successfully demonstrated a 1,000W TEG designed for diesel engine exhaust heat recapture. With the effort involved in transporting fuel to a battle site, diesel can cost the U.S. military upwards of $10.50 per liter ($40 per gallon). So using that fuel more efficiently will save the Department of Defense significant amounts of money, says Scott Rackey, GMZ’s vice president of business development.
Cheryl A. Diuguid, CEO of GMZ, said: “With the successful demonstration of GMZ’s 1,000W TEG solution, we are excited to move to the next phase of this program and begin testing in a Bradley Fighting Vehicle. In addition to saving money and adding silent-power functionality for the U.S. Military, this TEG can increase fuel efficiency for most gasoline and diesel engines. We look forward to implementing our low-cost TEG technology into a broad array of commercial markets, including long-haul trucking, heavy equipment, and light automotive.”
“GMZ’s patented half-Heusler material is uniquely well suited for military applications. The 1000W TEG features enhanced mechanical integrity and high-temperature stability thanks to GMZ’s patented nanostructuring approach. GMZ’s TEG also enables silent generation, muffles engine noise, and reduces thermal structure,” claims GMZ.
Thermoelectric materials for TEG
The conversion of heat to electricity by thermoelectric devices may play a key role in the future for energy production and utilization. However, in order to meet that role, more efficient thermoelectric materials are needed that are suitable for high-temperature application.
New approach boosts performance in thermoelectric materials
A team of researchers – from universities across the United States and China, as well as Oak Ridge National Laboratory – is reporting a new mechanism to boost performance through higher carrier mobility, increasing how quickly charge-carrying electrons can move across the material. The work, reported this week in the Proceedings of the National Academy of Science, focused on a recently discovered n-type magnesium-antimony material with a relatively high thermoelectric figure of merit, but lead author Zhifeng Ren said the concept could also apply to other materials.
“When you improve mobility, you improve electron transport and overall performance,” said Ren, M.D. Anderson Chair professor of physics at the University of Houston and principal investigator at the Texas Center for Superconductivity at UH.
The material’s power factor can be boosted by increasing carrier mobility, the researchers said. “Here we report a substantial enhancement in carrier mobility by tuning the carrier scattering mechanism in n-type Mg3Sb2-based materials,” they wrote. “… Our results clearly demonstrate that the strategy of tuning the carrier scattering mechanism is quite effective for improving the mobility and should also be applicable to other material systems.
Composite material yields 10 times—or higher—voltage output
In Nature Communications, engineers from The Ohio State University describe how they used magnetism on a composite of nickel and platinum to amplify the voltage output 10 times or more—not in a thin film, as they had done previously, but in a thicker piece of material that more closely resembles components for future electronic devices.
Instead of applying a thin film of platinum on top of a magnetic material as they might have done before, the researchers distributed a very small amount of platinum nanoparticles randomly throughout a magnetic material—in this case, nickel. The resulting composite produced enhanced voltage output due to the spin Seebeck effect. This means that for a given amount of heat, the composite material generated more electrical power than either material could on its own. Since the entire piece of composite is electrically conducting, other electrical components can draw the voltage from it with increased efficiency compared to a film.
While the composite is not yet part of a real-world device, Heremans is confident the proof-of-principle established by this study will inspire further research that may lead to applications for common waste heat generators, including car and jet engines. The idea is very general, he added, and can be applied to a variety of material combinations, enabling entirely new approaches that don’t require expensive metals like platinum or delicate processing procedures like thin-film growth.
Efficient, inexpensive and bio-friendly thermoelectric material
Now the team, led by University of Utah materials science and engineering professor Ashutosh Tiwari, has found that a combination of the chemical elements calcium, cobalt and terbium can create an efficient, inexpensive and bio-friendly material that can generate electricity through a thermoelectric process involving heat and cold air. The material needs less than a one-degree difference in temperature to produce a detectable voltage. “There are no toxic chemicals involved,” he says. “It’s very efficient and can be used for a lot of day-to-day applications.”
The applications for this new material are endless, Tiwari says. It could be built into jewelry that uses body heat to power implantable medical devices such as blood-glucose monitors or heart monitors. It could be used to charge mobile devices through cooking pans, or in cars where it draws from the heat of the engine. Airplanes could generate extra power by using heat from within the cabin versus the cold air outside. Power plants also could use the material to produce more electricity from the escaped heat the plant generates.
Thermoelectric generators (TEGs) based on polymers
Shannon Yee, an assistant professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering is pioneering the use of polymers in thermoelectric generators (TEGs). TEGs are typically made from inorganic semiconductors. Yet polymers are attractive materials due to their flexibility and low thermal conductivity. These qualities enable clever designs for high-performance devices that can operate without active cooling, which would dramatically reduce production costs.
The researchers have developed P- and N-type semiconducting polymers with high performing ZT values (an efficiency metric for thermoelectric materials). “We’d like to get to ZT values of 0.5, and we’re currently around 0.1, so we’re not far off,” Yee said.
In one project funded by the Air Force Office of Scientific Research, the team has developed a radial TEG that can be wrapped around any hot water pipe to generate electricity from waste heat. Such generators could be used to power light sources or wireless sensor networks that monitor environmental or physical conditions, including temperature and air quality.
“Thermoelectrics are still limited to niche applications, but they could displace batteries in some situations,” Yee said. “And the great thing about polymers, we can literally paint or spray material that will generate electricity.”
This opens opportunities in wearable devices, including clothing or jewelry that could act as a personal thermostat and send a hot or cold pulse to your body. Granted, this can be done now with inorganic thermoelectrics, but this technology results in bulky ceramic shapes, Yee said. “Plastics and polymers would enable more comfortable, stylish options.”
Although not suitable for grid-scale application, such devices could provide significant savings, he added.
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.
The market for thermoelectric energy harvesters will reach over $1.1 billion by 2026, according to report by idtech. A large number of car companies, including Volkswagen, VOLVO, FORD and BMW in collaboration with NASA have been developing thermoelectric waste heat recovery systems in-house, each achieving different types of performance but all of them expecting to lead to improvements of 3-5% in fuel economy while the power generated out of these devices could potentially reach up to 1200W.
Wireless sensors powered by thermogenerators in environments where temperature differentials exist would lead to avoiding issues with battery lifetime and reliability. It could be related to saving energy when cooking by utilising thermo-powered cooking sensors, powering mobile phones, watches or other consumer electronics, even body sensing could become more widespread with sensory wristbands, clothing or athletic apparel that monitor vitals such as heart rate, body temperature, etc.
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