A Breakthrough in Converting Heat To Electricity More Efficiently

Scientists have made a breakthrough in converting heat to electricity much more efficiently with a class of materials called thermoelectric oxides.  One of the main reasons that this is important is that in developed countries nearly two-thirds of total energy production (mostly from fossil fuel power sources, and from nuclear fission power plants) is typically discarded as "waste heat".

This would make it possible, for example, to convert waste heat from electricity generation to electricity, which has been a major problem involved in making nuclear fusion power generation economically viable. It is a break through that is also relevant to some, but not all, renewable energy sources. It would enhance solar thermal plants, biofuel plants, and geothermal plants, but wouldn't have much of an impact on hydropower, tidal power, wind power, or photovoltaic solar cells. (Geothermal plants, by the way, are usually sited where magma is unusually close to the surface of the Earth in places like Iceland, but can, in principle, be used anywhere if you dig deep wells.)

More mundanely, this development might facilitate clothing with thermoelectric converters that would power your smart phone, flashlight, or laptop. Or it might be used in hybrid cars to extend their range by using waste heat from the internal combustion engine to recharge the car's batteries in much the same way the regenerative braking does in existing hybrid and electric cars. It could also reduce the drain that using air conditioning places on electric car batteries, by using the waste heat the air conditioner generates to recharge the car's batteries.

Other materials called chalcogenides are currently still more efficient at converting heat to electricity than even these new enhanced thermal oxides, but chalcogenides have other drawbacks that have prevented them from being used on a large scale. Chalcogenides are unstable in air, require cold temperatures to work well, and are toxic. So, their large scale use has been a problem.

The methods used in the breakthrough, using a Lithium-Titanium oxide, in contrast, lacks those drawbacks. The benefits of the new method arise mostly from the manufacturing process used to create the two layer, film over substrate, material, rather than the specific thermoelectric oxide material used in this proof of concept test. So this method can probably be generalized to other thermoelectric oxides that work even better.

These materials would allow power plants to attain a total energy production efficiency similar to that of co-generation plants that divert waste heat from electricity generation to generate useful heat such as heating greenhouses or buildings. So, power plants would produce much more useable energy with the same amount of fuel. The reduced fuel consumption would also makes the electricity generated at fossil fuel and nuclear fission power plants greener in every respect.

Waste heat itself is an environmental problem called thermal pollution, that contribute to global warming and disrupt ecologies in the water and other habitats near power plants: "Elevated water temperatures decrease oxygen levels, which can kill fish and alter food chain composition, reduce species biodiversity, and foster invasion by new thermophilic species." This breakthrough could greatly reduce thermal pollution.

One of the possible ways to recover this waste heat as electricity is via "thermoelectric conversion" -- a process that uses temperature difference in semiconductors to convert waste heat into electric power. The thermopower (S) is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across the material. The electric power is evaluated by power factor (PF), which is the product of thermopower (S)-squared and electronic conductivity (s). Therefore, the high electric power (PF) is capable of combining large S with high s in thermoelectric materials.

However, the PF is constrained by a trade-off between S and s. The S and s depend on carrier concentration, and thus the PF is usually maximized by tuning the carrier concentration with the addition of impurity elements -- the s increases with increasing impurity concentration but the S decreases. This trade-off limits the PF.

In a recent study published in Advanced Science, a team of scientists led by Associate Professor Takayoshi Katase of the Tokyo Tech have discovered a way to break this trade-off. The scientists grew thin films of the Mott insulator oxide LaTiO3 on different substrates and found a way to introduce epitaxial strain, a strain that is born from a mismatch in the lattice structures of the substrate and the deposited (epitaxial) film. The artificial compressive strain was able to change LaTiO3 from the Mott insulator to metal. In the metallic state, increase in both S and s resulted in a hundred-fold increase in PF. . . .

This discovery promises to advance the field of thermoelectric materials. 

Our experiments suggest that epitaxial strain will be a novel tool to harvest large power factors from thermoelectric oxides that are inconspicuous in their bulk by breaking the trade-off problem. Metal chalcogenides such as Bi2Te3 have been known as high performance thermoelectric materials, but the chalcogenides have problems with toxic elements, and low thermal and chemical stability, which restrict the large-scale use of thermoelectricity. Contrarily, since oxides are stable in air and even at high temperatures, they are ideal for maintenance-free thermoelectric conversion applications. The thermoelectric conversion efficiency is much lower than that of metal chalcogenides at this stage. But, by greatly improving the thermoelectric performance of oxides beyond the trade-off relationship, thermoelectric conversion is expected to become widespread as a general energy source. 

concludes Dr. Katase.

From Science Daily discussing a paper in:

Takayoshi Katase, et al., "Breaking of Thermopower–Conductivity Trade‐Off in LaTiO 3 Film around Mott Insulator to Metal Transition." Advanced Science 2102097  (2021). DOI: 10.1002/advs.202102097