AGH UST scientists created disruptive thermoelectric modules

To power multifarious technological processes that drive our civilisation, we need energy. In the overwhelming majority, it is still being produced in the process of fossil fuel combustion. And although we pay a hefty price in the form of carbon dioxide emissions into the atmosphere, which increases the greenhouse effect, we have not even been able to harness half of the primary potential of energy resources. Estimates say that we only use 40 per cent of the energy that comes from burning coal and hydrocarbons, whereas the remaining 60% dissipates in the air in the form of waste heat. As a result, to satisfy our electricity demand, we have to fire up more raw materials, simultaneously contributing to the negative consequences of climate change and high fuel costs.

There are technologies that allow us to transform this waste heat energy into electricity. To achieve this, we can use mechanical converters – for example, ORC systems or Stirling engines; however, these often prove to be too problematic in terms of prolonged use. The alternative might be small thermoelectric modules without moving mechanical parts. Similarly, as in the case of solar cells, the modules are made of tiny semiconductor cubes connected in series. Traditionally, thermoelectric elements are placed between ceramic layers that serve as insulators. When one layer is heated up and the other cooled down, an electric field forms which results in the flow of electricity.

Limited use of thermoelectric generators

The space probes and rovers used, for instance, by NASA are good examples of advanced applications of thermoelectric generators. In this case, the heat, which is then converted into electricity, is produced from the natural decay of radioactive isotopes, e.g. plutonium-238. On Earth, we may find our heat sources in a furnace of a cement or smelter plant, a refinery or geothermal installation, or an exhaust pipe of a car. The question is: Why has a technology successfully used in space not yet been widely applied on Earth, making us lose free resources?

The reason might lie in the high cost of thermoelectric components and, as a result, an unfavourable ratio of installation cost to the value of energy produced in the process. During space missions, it is difficult to generate electricity using other methods; therefore, price is not the key element in deciding which technology to use. On Earth, the cost of thermoelectric devices compared to the value of electrical energy obtained is still too high. However, this might soon change thanks to the scientists from the AGH University of Science and Technology.

Inexpensive and efficient modules developed at the AGH UST

A team led by Professor Krzysztof Wojciechowski from the Faculty of Materials Science and Ceramics, in cooperation with partners from the Łukasiewicz Research Network and the Institute of Physics of the Polish Academy of Sciences in Warsaw, developed and created a prototype of a thermoelectric module with a power density of about 2.5 kW/m², which beats commercial photovoltaic cells ten times over.

The creators claim that the technological process and material cost of their solution are similar to those of a single solar cell element of the same size. ‘Considering the significantly improved power density of thermoelectric converters, the price of 1 W of power should also be markedly favourable than in the case of photovoltaic panels’, Prof. Wojciechowski declares.

The cost of the modules could be significantly reduced in comparison to their commercial counterparts by replacing ceramic layers with cheaper aluminium alloys with better heat conductivity. Furthermore, those aluminium alloys are more plastic than ceramics, which allows us to create modules in almost all shapes and adjust them to specific heat recovery systems.

Scientists search for a brave investor

To fully benefit from the advantages of the technology described, we need to reduce the production cost of the converters. The price of a single thermoelectric module manufactured in a laboratory, where every component was practically handmade by the team members, is comparable to that of a top-shelf smartphone. However, according to the creators, automation and mass production would be conducive to bringing the cost down at least to the level of widely available photovoltaic cells. Which is why our scientists need an investor willing to take up the challenge.

‘Companies that manufacture or develop thermoelectric technologies exist currently only in China, the United States, Ukraine, and Russia. There is practically no competition on the European market, which creates an opportunity for significant profits. I wish there were someone in Poland who would see that the goal is worth the risk and build a prototype production line. If we manage to obtain low production cost for thermoelectric modules, we can conquer the world with our products’, Prof. Wojciechowski claims.

A novel approach to thermoelectric materials

For years, Professor Wojciechowski’s team has been successfully developing technologies based on the conversion of waste heat into electricity. In addition to cheap modules, the scientists succeeded in obtaining much more efficient and prospective, albeit more expensive at the moment, thermoelectric materials characterised by record-breaking effectiveness that exceeds 15 per cent. ‘Products currently available on the market reach an efficiency of 4 to 5 per cent; those used in Martian rovers, Curiosity and Perseverance, 8 per cent; we have nearly doubled it’, boasts the scientists.

The excellent efficiency parameters are a result of the attempt to break the paradigm that has dominated the field of thermoelectric material engineering. The effectiveness of energy conversion depends on the thermoelectric figure of merit ZT. Most research teams focus on maximising this particular parameter. However, the problem is that it is susceptible to significant changes, depending on the temperature variations. Meanwhile, the multiannual research work carried out by Professor Wojciechowski’s team proves that materials with good but not bumper ZT values show improved efficiency in a wide range of temperatures.

To obtain a material with the desired qualities, Professor Wojciechowski’s team developed two methods of modifying them to change their structural and microstructural properties. In the first method, the scientists relied on the concept of functionally graded materials, in which, with carefully selected dopes, they can simultaneously regulate two parameters influencing the local value of the ZT parameter: Fermi level and band gap width. In the second method – they developed compsite materials that significantly reduce the heat transfer and retain good electrical conductivity.

‘A good thermoelectric material should not conduct heat, but should very well conduct electricity. However, it is arduous to meet these requirements because these two qualities are tied by certain physical properties. What we can do instead is produce a material that would constitute a barrier to heat transfer similar to a sponge or polystyrene foam, while simultaneously conducting electricity as metals do’, the AGH UST scientist sums up.

The team’s research work has been carried out within the TEAM-TECH and TechmatStrateg 2 projects funded by the Foundation for Polish Science and the National Centre for Research and Development.