Thermoelectric energy harvesting mainly depends on the operation of the thermoelectric generator (TEG). A thermoelectric (TE) device can directly convert heat emanating from the Sun, radioisotopes, automobiles, industrial sectors, or even the human body to electricity. A TEG converts heat directly into electrical energy according to the Seebeck effect.
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. 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.
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.
To date, the most well established and successful thermoelectric materials have been based on metal tellurides, including lead telluride and bismuth telluride. These are commercially available and have been harnessed as a power source in space, where they can locally generate electricity to power satellites and space probes. But they only work well at high temperatures, and in space an on-board nuclear isotope is used to generate this heat and to create a high temperature differential. The approach can act as a long-term, local power source, but the potential health risks of nuclear radiation mean it’s not suitable for many terrestrial applications.
Kedar Hippalgaonkar, Jianwei Xu and their co-workers at A*STAR’s Institute of Materials Research and Engineering (IMRE) think they could soon use low-grade waste heat – think car exhaust or body heat – to power devices. A*STAR’s PHAROS project is focused on the materials that will make these thermoelectric generators possible. The five-year project started in 2016 and aims to find a material composition that is non-toxic and, ideally, Earth abundant (making it cheap), efficient, and easy to fabricate. To do this they are developing less toxic hybrid materials combining organic and inorganic elements, and they are pursuing those with potential for low temperature thermoelectric power generation.
Hippalgaonkar agrees that the proliferation of Internet of Things devices now brings with it a demand for non-toxic, portable power sources. Future body sensors and portable devices could be worn constantly if they harnessed body heat to be energy self-sufficient. “But to do that we need to develop suitable new thermoelectric materials that are efficient at lower temperatures, non-toxic and cheap to produce.”
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.
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.
Although the low efficiency is a drawback to the progress of TEGs, researchers’ and manufacturers’ attention is focused on the improvement of the following characteristics: the dimensionless thermoelectric figure-of-merit ZT ; the operating range of thermoelectric materials to work with the ΔT as high as possible; and the use of low-price materials to reduce the negative impact of low efficiency.
The thermoelectric materials must be both stable from the chemical point of view and strong from the mechanical point of view at high temperatures (e.g., for the automotive exhaust waste heat recovery, at specific working conditions, the range of the average exhausts temperature is from 500 to 600°C with values increasing up to 1000°C)
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.
The most popular thermoelectric material is Bismuth Telluride (Bi2Te3). Its utilisation in TEGs is limited (only for industrial modules with an average value of ZT from 0.5 to 0.8) because the maximum temperature at the hot side of the devices is relatively reduced. In the power generation applications, the best commercially available TEGs made of Bi2Te3 have a ZT of about 1 at the temperature 300 K, leading to a low thermal efficiency of the thermoelectric device (less than 4%).
Calcium manganese and lead telluride are the thermoelectric materials used in the TEG legs, because they resist at higher temperatures. The hot side of TEG is made of materials having a high ZT at higher temperatures (e.g., lead telluride). The cold side of the TEG is made of materials having high ZT at reduced temperatures (e.g., Bi2Te3).
At present, even though the research of the thermoelectric materials development is focused on obtaining the high ZT of 2, unfortunately the efficiency of TEG is limited to ηTEG<10% . Significant progress has been made towards increasing the thermoelectric efficiency of different inorganic material classes (e.g., skutterudites , tellurides , half-Heuslers and silicides. The researchers’ attention is focused on the development of organic materials for thermoelectric energy harvesting due to their advantages (e.g., low-cost, reliability, low weight and so on). For this reason, some polymers with different doping levels (like polyaniline (PANI), polyamide (PA), and poly (3,4-ethylenedioxythiophene) or PEDOT) are assessed for future applications.
Thermocells based on phase transition of material
A thermocell is a type of energy-harvesting device that converts environmental heat into electricity through the thermal charging effect. Although thermocells are inexpensive and efficient, so far only low output voltages–just tens of millivolts (mV)–have been achieved and these voltages also depend on temperature. These drawbacks need to be addressed for thermocells to reliably power electronics and contribute to the development of a sustainable society.
A University of Tsukuba-led research team recently improved the energy-harvesting performance of thermocells, bringing this technology a step closer to commercialization. Their findings are published in Scientific Reports . The team developed a thermocell containing a material that exhibited a temperature-induced phase transition of its crystal structure. Just above room temperature, the atoms in this solid material rearranged to form a different crystal structure. This phase transition resulted in an increase in output voltage from zero to around 120 mV, representing a considerable performance improvement over that of existing thermocells.
“The temperature-induced phase transition of our material caused its volume to increase,” explains Professor Yutaka Moritomo, senior author of the study. “This in turn raised the output voltage of the thermocell.” The researchers were able to finely tune the phase transition temperature of their material so that it lay just above room temperature. When a thermocell containing this material was heated above this temperature, the phase transition of the material was induced, which led to a substantial rise of the output voltage from zero at low temperature to around 120 mV at 50 °C.
As well as tackling the problem of low output voltage, the thermocell containing the phase transition material also overcame the issue of a temperature-dependent output voltage. Because the increase of the output voltage of the thermocell induced by the thermal phase transition was much larger than the temperature-dependent fluctuations of output voltage, these fluctuations could be ignored.
“Our results suggest that thermocell performance can be strongly boosted by including a material that exhibits a phase transition at a suitable temperature,” says Professor Moritomo. “This concept is an attractive way to realize more efficient energy-harvesting devices.” The research team’s design combining thermocell technology with an appropriately matched phase transition material leads to increased ability to harvest waste heat to power electronics, which is an environmentally sustainable process. This design has potential for providing independent power supplies for advanced electronics.
A*STAR’s PHAROS project is focused on the materials that will make these thermoelectric generators possible.
The five-year project started in 2016 and aims to find a material composition that is non-toxic and, ideally, Earth abundant (making it cheap), efficient, and easy to fabricate. To do this they are developing less toxic hybrid materials combining organic and inorganic elements, and they are pursuing those with potential for low temperature thermoelectric power generation.
There’s a lack of efficient materials that operate at around room temperature and that’s what we want to address with the PHAROS project,” says Xu. However, it’s a challenging task to identify new candidate thermoelectric materials, fabricate them and then understand what is happening to charge transfers inside them.
To date, the PHAROS team has been exploring a wide variety of conjugated semiconducting polymers (such as Polyaniline, P3HT or PEDOT:PSS) for the organic component of their hybrids, which are then combined with an inorganic component made from, say, tellurium nanowires, silicon nanoparticles or 2-D materials like MoS2, MoS2. With these, they have investigated the use of carbon nanotubes as an additive.
The team has also explored the thermoelectric potential of methylammonium lead iodide perovskites1, an inorganic-organic hybrid material system that has shot to fame in recent years following its successful use in solar cells. This hybrid material rivals silicon in terms of power conversion efficiency. The big advantage of using a part-organic system is that it suits solution processing, which produces large-area, thin, flexible materials that could be cheaply ink-jet printed.
However, for a thermoelectric material to work well it ideally needs to have a large Seebeck coefficient, which is indicative of how large the voltage generated will be for a given temperature difference. And it is also important for the material to have high electrical conductivity to allow a charge to flow easily, along with low thermal conductivity to support the temperature gradient in place.
“It’s very hard to achieve these attributes simultaneously,” says Hippalgaonkar. “You ideally want to find a material that combines the low thermal conductivity of wood with the high electrical conductivity of a metal and that’s not easy to do.”
To make comparisons between materials easier, something called the ‘ZT value’ was developed to take into account the Seebeck coefficient, thermal conductivity, electrical conductivity and temperature. “We really want something that has a ZT of roughly 1,” says Xu, although a ZT number that high isn’t necessary for a lot of uses. At present, a 1 can be achieved in bismuth telluride and lead telluride, but both materials are toxic, expensive to manufacture and rigid.
Recently, the PHAROS team has developed a safer material that is 10–20% of the way to a perfect thermoelectric scorecard. They did this in a collaboration with researchers at US-based Lawrence Berkeley National Laboratory (LBNL) by optimizing a materials system that combines a carefully designed conjugated polymer with tellurium nanowires. Encouragingly, ZT values of roughly 0.1–0.2 have been achieved2.
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.