New thermoelectric materials could provide a low-cost option for converting heat energy into electricity

Diagram showing how thermoelectric materials use differences in temperature to generate electricity. Credit: Tianjun Liu.

Researchers from Queen Mary University of London have developed new thermoelectric materials, which could provide a low-cost option for converting heat energy into electricity.

Materials known as halide perovskites have been proposed as affordable alternatives to existing thermoelectric materials, however so far research into their suitability for thermoelectric applications has been limited.

In this study, published in Nature Communications, scientists conducted a series of experiments on thin films of the halide perovskite, caesium tin iodide, to test its ability to create electrical current from heat. The researchers found they were able to improve the materials’ thermoelectric properties through a combination of methods, which involved partial oxidation and the introduction of additional elements into the material.

Improving thermoelectric performance

Dr Oliver Fenwick, lead Royal Society University Research Fellow and Lecturer in Materials Science at Queen Mary University of London, said: “For many years halide perovskites have been suggested as promising thermoelectric materials, but whilst simulations have suggested good thermoelectric properties real experimental data hasn’t met these expectations.

“In this study, we successfully used ‘doping’ techniques, where we intentionally introduce impurities into the material, to tweak and improve the thermoelectric properties of caesium tin iodide, opening up options for its use in thermoelectric applications.”

This work was conducted in collaboration with researchers from University College London and CNR Bologna, Italy

Sustainable approach to energy production

Thermoelectric materials use temperature differences to generate electrical energy. They have been suggested as a promising sustainable approach to both energy production and recycling, as they can be used to convert waste heat into useful electricity. However, current widely-used thermoelectric materials are costly to produce and process, which has limited the uptake of this greener technology.

Dr Fenwick, said: “With the heightened global awareness of climate change and realisation that a number of renewable energy solutions will be needed to meet our energy demands, thermoelectric generators are now at the centre stage in today’s “green technology” debate.”

“The thermoelectric materials we currently have are expensive, and some even contain toxic components. One of the largest growth areas for thermoelectric technology is for domestic, commercial or wearable applications, so there’s a need to find cheaper, non-toxic thermoelectric materials that can also operate well at low temperatures, for these applications to be fully realised. Our research suggests the halide perovskites could, with some fine-tuning, fill this void.”

Learn more: Fine-tuning thermoelectric materials for cheaper renewable energy

 

 

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A new effective approach to converting heat to electricity has huge potential

Dr Tony Shien-Ping Feng of the Department of Mechanical Engineering at the University of Hong Kong (HKU) and his team (from left to right: Wang Xun, Huang Yu-ting and Mu Kai-yu), invented the Direct Thermal Charging Cell (DTCC), which can convert low-grade heat to electricity.

Dr Tony Shien-Ping Feng of the Department of Mechanical Engineering at the University of Hong Kong (HKU) and his team invented a Direct Thermal Charging Cell (DTCC) which can effectively convert heat to electricity, creating a huge potential to reduce greenhouse effects by capturing exhaust heat and cutting down primary energy wastage.

The new invention is recently published in the prestigious journal Nature Communications (http://www.i-nanoeng.com/upload/2019/09/20190918160051.pdf), and the research has been featured in the Nature Communications Editors’ Highlights webpage. HKU’s Technology Transfer Office has filed for the invention’s US provisional patent and PCT (Patent Cooperation Treaty) patent.

Low grade heat is abundantly available in industrial processes (80 to 150°C), as well as in the environment, living things, solar-thermal (50 to 60°C) and geothermal energy. Over 60% of the world’s primary energy input, whether it is in the industrial process or domestic energy consumption, is wasted as heat. A majority of this loss as waste heat is regarded as low-grade heat.

The newly designed DTCC is a game-changing electrochemical technology which can open new horizons for applications to convert low-grade heat to electricity efficiently. It is a simple system with the basic unit sized only 1.5 sq.cm and thickness 1 to 1.5 mm. The cell is bendable, stackable and low cost.

DTCC can be used in HVAC (heating, ventilation, and air conditioning) system to recycle low-grade heat from the compressor and condenser into electricity for use in electrical devices. It can be integrated with the window frame to harvest solar thermal energy to power electrochromic windows, or used as portable devices to power iphones or life-saving equipment in the wilderness. With the increasing popularity of wearable technology, this system may one day harness body heat to power wearable electronic devices or medical devices for monitoring body health conditions like blood sugar levels and blood pressure.

Dr Feng said: “Efficient low-grade heat recovery can help to reduce greenhouse gas emission but current technologies to convert this heat to electricity is still far from optimum. DTCC yields a conversion efficiency of over 3.5%, surpassing all existing thermo-electrochemical and thermo-electric systems, which is either too costly or complicated, or too low in efficiency for everyday applications. DTCC is a revolutionary design with great potentials in smart and sustainable energy devices.”

The new thermal charging cell uses asymmetric electrodes: a graphene oxide/platinum (GO/Pt) cathode and a polyaniline (PANI) anode in Fe2+/Fe3+ redox electrolyte via isothermal heating operation without building thermal gradient or thermal cycle. When heated, the cell generates voltage via a thermo-pseudocapacitive effect of GO and then discharges continuously by oxidizing the PANI anode and reducing Fe3+ to Fe2+ under isothermal heating on cathode side till Fe3+ depletion. The energy conversion works continuously under isothermal heating during the entire charge and discharge process. The system can be self-regenerated when cooled down. This synergistic chemical regeneration mechanism allows the device cyclability.

Learn more: HKU Engineering team invents novel Direct Thermal Charging Cell for Converting low-grade waste heat to usable electricity

 

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A new type of material generates electrical current very efficiently from temperature differences

Prof. Ernst Bauer in the lab
Credit TU Wein

A new type of material generates electrical current very efficiently from temperature differences. This allows sensors and small processors to supply themselves with energy wirelessly.

Thermoelectric materials can convert heat into electrical energy. This is due to the so-called Seebeck effect: If there is a temperature difference between the two ends of such a material, electrical voltage can be generated and current can start to flow. The amount of electrical energy that can be generated at a given temperature difference is measured by the so-called ZT value: The higher the ZT value of a material, the better its thermoelectric properties.

The best thermoelectrics to date were measured at ZT values of around 2.5 to 2.8. Scientists at TU Wien (Vienna) have now succeeded in developing a completely new material with a ZT value of 5 to 6. It is a thin layer of iron, vanadium, tungsten and aluminium applied to a silicon crystal.

The new material is so effective that it could be used to provide energy for sensors or even small computer processors. Instead of connecting small electrical devices to cables, they could generate their own electricity from temperature differences. The new material has now been presented in the journal “Nature”.

Electricity and Temperature

“A good thermoelectric material must show a strong Seebeck effect, and it has to meet two important requirements that are difficult to reconcile,” says Prof. Ernst Bauer from the Institute of Solid State Physics at TU Wien. “On the one hand, it should conduct electricity as well as possible; on the other hand, it should transport heat as poorly as possible. This is a challenge because electrical conductivity and thermal conductivity are usually closely related.”

At the Christian Doppler Laboratory for Thermoelectricity, which Ernst Bauer established at TU Wien in 2013, different thermoelectric materials for different applications have been studied over the last few years. This research has now led to the discovery of a particularly remarkable material – a combination of iron, vanadium, tungsten and aluminium.

“The atoms in this material are usually arranged in a strictly regular pattern in a so-called face-centered cubic lattice,” says Ernst Bauer. “The distance between two iron atoms is always the same, and the same is true for the other types of atoms. The whole crystal is therefore completely regular”.

However, when a thin layer of the material is applied to silicon, something amazing happens: the structure changes radically. Although the atoms still form a cubic pattern, they are now arranged in a space-centered structure, and the distribution of the different types of atoms becomes completely random. “Two iron atoms may sit next to each other, the places next to them may be occupied by vanadium or aluminum, and there is no longer any rule that dictates where the next iron atom is to be found in the crystal,” explains Bauer.

This mixture of regularity and irregularity of the atomic arrangement also changes the electronic structure, which determines how electrons move in the solid. “The electrical charge moves through the material in a special way, so that it is protected from scattering processes. The portions of charge travelling through the material are referred to as Weyl Fermions,” says Ernst Bauer. In this way, a very low electrical resistance is achieved.

Lattice vibrations, on the other hand, which transport heat from places of high temperature to places of low temperature, are inhibited by the irregularities in the crystal structure. Therefore, thermal conductivity decreases. This is important if electrical energy is to be generated permanently from a temperature difference – because if temperature differences could equilibrate very quickly and the entire material would soon have the same temperature everywhere, the thermoelectric effect would come to a standstill.

Electricity for the Internet of Things

“Of course, such a thin layer cannot generate a particularly large amount of energy, but it has the advantage of being extremely compact and adaptable,” says Ernst Bauer. “We want to use it to provide energy for sensors and small electronic applications.” The demand for such small-scale generators is growing quickly: In the “Internet of Things”, more and more devices are linked together online so that they automatically coordinate their behavior with each other. This is particularly promising for future production plants, where one machine has to react dynamically to another.

“If you need a large number of sensors in a factory, you can’t wire all of them together. It’s much smarter for the sensors to be able to generate their own power using a small thermoelectric device,” says Bauer.

Learn more: New Material Breaks World Record Turning Heat into Electricity

 

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Fighting climate change with thermoelectric generators

An artist’s conception of how applying pressure in the diamond anvil cell changes the electronic structure of lead selenide, courtesy of Xiao-Jia Chen.

Pressure improves the ability of materials to turn heat into electricity and could potentially be used to create clean generators, according to new work from a team that includes Carnegie’s Alexander Goncharov and Viktor Struzhkin published in Nature Materials.

Alternative energy sources are key to combating climate change caused by carbon emissions. Compounds with thermoelectric capabilities can convert thermal energy’s innate, physical need to spread from a hot place into a cold place into energy—harvesting electricity from the temperature differential. In theory, generators built from these materials could be used to recover electricity from “wasted” heat given off by other processes, making major contributions to the nation’s energy budget.

However, engineers have been unable to improve the room-temperature performance of any thermoelectric materials in 60 years, meaning that devices built to take advantage of this capability are only practical for some very specific applications, including remote gas pipelines and spacecraft.

“Our measurement of the efficiency of room-temperature thermoelectricity has not budged in more than half a century,” said Goncharov. “Thermoelectric compounds have demonstrated improved performance at high temperatures, but we really need them to work well at room temperature to make the most of their potential for green energy.”

This is precisely the kind of problem that material science is suited to address.

The research team—led by Liu-Cheng Chen of the Center for High Pressure Science and Technology Advanced Research—found that they could improve the thermoelectric capability of lead selenide by applying pressure and mixing in charged particles of chromium.

By squeezing the material in the diamond anvil cell —which acted as a sort of “chemical pressure”— and adding the chromium, the lead selenide was encouraged to undertake a structural rearrangement  at the atomic level, enabling the most-efficient demonstration of room-temperature thermoelectric generation ever recorded.

Under 30,000 times normal atmospheric pressure, the chromium-doped lead selenide was able to produce electricity with the same efficiency that the top-performing thermoelectric materials do at 27 degrees Celsius (80 degrees Fahrenheit).

“Our work presents a new way to use compression techniques to improve the thermoelectric performance, bringing us closer to practical applications that could help fight climate change,” concluded Xiao-Jia Chen of the Center for High Pressure Science and Technology Advanced Research (formerly of Carnegie).

Learn more: Pressure May Be Key To Fighting Climate Change With Thermoelectric Generators

 

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A new thermoelectric material works efficiently at room temperature at much lower cost

Texas Center for Superconductivity at UH Director Zhifeng Ren, right, and post-doctoral researcher Jun Mao say a new thermoelectric cooling material is inexpensive to produce and works efficiently at room temperature.

Has your steering wheel been too hot to touch this summer? A new thermoelectric material reported in the journal Science could offer relief.

The widespread adoption of thermoelectric devices that can directly convert electricity into thermal energy for cooling and heating has been hindered, in part, by the lack of materials that are both inexpensive and highly efficient at room temperature.

Now researchers from the University of Houston and the Massachusetts Institute of Technology have reported the discovery of a new material that works efficiently at room temperature while requiring almost no costly tellurium, a major component of the current state-of-the-art material.

The work, described in a paper published online by Science Thursday, July 18, has potential applications for keeping electronic devices, vehicles and other components from overheating, said Zhifeng Ren, corresponding author on the work and director of the Texas Center for Superconductivity at UH, where he is also M.D. Anderson Professor of Physics.

“We have produced a new material, which is inexpensive but still performs almost as well as the traditional, more expensive material,” Ren said. The researchers say future work could close the slight performance gap between their new material and the traditional material, a bismuth-tellurium based alloy.

Thermoelectric materials work by exploiting the flow of heat current from a warmer area to a cooler area, and thermoelectric cooling modules operate according to the Peltier effect, which describes the transfer of heat between two electrical junctions.

Thermoelectric materials can also be used to turn waste heat – from power plants, automobile tailpipes and other sources – into electricity, and a number of new materials have been reported for that application, which requires materials to perform at far higher temperatures.

Thermoelectric cooling modules have posed a great challenge because they have to work at cooler temperatures, where the thermoelectric figure-of-merit, or ZT, is low because it is dependent on temperature. The figure-of-merit is a metric used to determine how efficiently a thermoelectric material works.

Despite the challenge, thermoelectric cooling modules also, at least for now, offer more commercial potential, in part because they can operate for a long lifespan at cooler temperatures; thermoelectric power generation is complicated by issues related to the high temperatures at which it operates, including oxidation and thermal instability.

The market for thermoelectric cooling is growing. “The global thermoelectric module market was worth ~0.6 billion US dollars in 2018 and it is anticipated to reach ~1.7 billion US dollars by 2027,” the researchers wrote.

Bismuth-tellurium alloys have been considered the best-performing material for thermal cooling for decades, but the researchers said the high cost of tellurium has limited widespread use. Jun Mao, a post-doctoral researcher at UH and first author on the paper, said the cost has recently dropped but remains about $50/kilogram. That compares to about $6/kilogram for magnesium, a primary component of the new material.

In addition to Ren and Mao, additional authors on the paper include Hangtian Zhu, Zihang Liu and Geethal Amila Gamage, all of the UH Department of Physics and TcSUH, and Zhiwei Ding and Gang Chen of the Department of Mechanical Engineering at the Massachusetts Institute of Technology.

They reported that the new material, comprised of magnesium and bismuth and created in a form carrying a negative charge, known as n-type, was almost as efficient as the traditional bismuth-tellurium material. That, combined with the lower cost, should expand the use of thermoelectric modules for cooling, they said.

To produce a thermoelectric module using the new material, researchers combined it with a positive-charge carrying, or p-type, version of the traditional bismuth-tellurium alloy. Mao said that allowed them to use just half as much tellurium as most current modules.

Because the cost of materials accounts for about one-third of the cost of the device, that savings adds up, he said.

The new material also more successfully maintains electrical contact than most nanostructured materials, the researchers reported.

Learn more: New Low-Cost Thermoelectric Material Works at Room Temperature

 

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