Their main concern was to reduce the amount of manual labor that was required. They decided to grind the semiconductors for the generators into powder and then to pour this powder into a mold for tablets. A cold compression process is used to form them into legs. “Here you can already see the first advantage of our process,” explains Patrik Stenner, Head of Exploration & Electrochemistry in Process Technology. “We no longer have to cut them to fit out of solid material. As a result, we don’t lose any material.”

After that, a robot fully automatically inserts the legs into the previously bored holes in a special carrier material. Thus the module’s basic framework is created in a short period of time. This framework is then sintered in a furnace. This process “bakes together”—i.e. firmly connects—the legs and the carrier material. After that, the module is polished so that no impurities can decrease its efficiency. “Next, the component has to be protected and rendered conductive,” Stenner explains. This is done by metallizing the generator—coating the legs with thin layers of metal. Nickel protects the semiconductors from destruction due to foreign metals diffusing into the material. Copper serves as a contact layer for the electrical connection. In addition, bridges are added so that the legs are connected in series. A black top layer insulates the module and protects it from corrosion. In the last step, wires are connected to the module’s contact points.

Although 64 legs are connected in series in a module, the operational generator is only half as big as a five-euro bill. That makes it flexibly applicable. If a large amount of electricity is needed, several generators can be connected together. They can also be combined with other systems for using waste heat. Thanks to the new production process and the special carrier material, thermoelectric generators are extremely robust and can operate at temperatures up to 260°C. Conventional TEGs can be operated only to a limit of 200°C. In processes using higher temperatures, such as those needed for producing and processing cement, glass, ceramics or metal, the TEG is used in the parts of the plant that are less hot.

According to Dirk Lehmann, the biggest advantage of the new production process is its higher speed: “Because our process is fully automated, it speeds up the production of the generators.” It also helps to lower their price, because it reduces the production costs by as much as two thirds.

The researchers from Creavis and the experts from Process Technology invested a great deal of work in this successful project. Firstly, the process had to be developed and perfected. Secondly, the researchers had to prove that not only the process but also the generators produced by it would function reliably. In order to provide this proof, they created a miniseries consisting of 1,000 prototypes and subjected it to field tests. One of the challenges they had to face was a glitch in the process of inserting the legs into the carrier. Initially, this process step ran smoothly. But during the assembly of the 300th generator, the legs suddenly fell right through the previously bored holes. This happened because the drill had become blunt over time and had therefore made the micrometer-precision holes too big. The researchers then had to find a new material for this tool. “We now have an optimal process that works for the production of large runs,” says Lehmann. The team was honored for its work in 2016, when it received the German Sustainability Award in the “Research” category.

The team from Process Technology at Evonik is now setting up a process for testing the generators in various plants throughout the Group—for example, in a production plant in Rheinfelden where plans call for the use of waste heat. For this purpose, individual TEGs are being combined to form a huge module. Using a temperature difference of about 70°C compared to their surroundings, the TEGs will be able to generate enough electricity to power on-site measuring equipment. “The generators will play an important role in making our production machinery resource-efficient,” says Patrik Stenner.

But the TEGs’ potential has still not been completely exploited. Because the starting modules are so small, the generators could also be used in autonomous energy systems—for example, in natural gas-driven heater fans in the large tents that are used as emergency shelters. That would make it possible to generate electricity in locations far from power grids.

In the future, this simplified production process could also be used for modules that reverse the principle behind thermoelectric generators by using electricity to produce heat (or cold). That would open additional markets, such as the one for electric vehicles. “The automotive industry is facing the challenge of boosting the efficiency of lithium-ion batteries in vehicles by means of temperature control or, for example, by using a seatheating system,” Lehmann says.

Evonik has produced several thousand TEGs to date. But it’s now looking for a buyer for the manufacturing process so that TEGs can be produced in large quantities and the technology can be introduced into as many fields of application as possible. “Evonik is a specialty chemicals company, and its core competencies do not include the production and marketing of thermoelectric generators,” Lehmann explains. “However, we believe in our process, and we want to give other companies an opportunity to benefit from our work.”

The Seebeck effect, which is used in thermoelectric generators (TEGs) to produce electricity, was discovered by the physicist and physician Thomas Johann Seebeck in 1821. A thermoelectric generator consists of two semiconductors that are connected by a circuit. If one side of the TEG is heated, the electrons in the n-leg move to the cold end of the leg. In the p-leg the electrons move to the hot end. As a result, a negative charge and a positive charge accumulate at opposite ends of each semiconductor. That creates an electric voltage between the cold ends of the two legs of the thermoelectric generator. The components of the generators are connected to one another by means of a soldered contact bridge. A diffusion barrier prevents the materials in the different components from mixing over time.

Photos: iStockphoto (3), Evonik (11)