Heat is power

While the pizza on the baking pan is developing a crispy crust, the oven is producing lots of hot air. But as soon as the oven door is opened, this hot air is dissipated into the room. Refrigerators give off heat so that their interiors stay cold, and cars also continuously release heat through their exhaust pipes. In industrial processes, this heat release takes place on a large scale. In engines, pumps, and oil refineries, large amounts of energy are released as waste heat. In Germany alone, 300 terawatt-hours per year could be reused—an amount that would cover the country’s power requirements for about seven months. According to some estimates, US$57 billion could be saved worldwide by recovering the energy from waste heat.

Researchers are investigating a number of solutions for utilizing waste heat more efficiently in the future. One promising possibility is the transformation of heat into electrical energy. Thermoelectric generators, or TEGs, can generate electricity from even small differences in temperature. A team of researchers from Creavis, Evonik’s strategic innovation unit, and the Process Technology unit, which is a proactive technology developer in the Group, has developed a fully automated production process for this technology. In the future, this process could play a larger role in energy recovery efforts.

A thermoelectric generator has a simple structure. It consists of two different semiconductors that are connected to each other by an electric circuit. One of the legs serves as the n-semiconductor (n stands for “negative”), the other one as the p-semiconductor (p stands for “positive”). If one end of the two legs is heated up, the electrons there receive additional energy. As a result, more charge carriers move from the hot end to the cold end of the leg than vice versa. A negative charge builds up at the cold end of the n-leg, a positive one at the cold end of the p-leg. That creates electric voltage between the two legs. However, this voltage amounts to only a few microvolts for each degree Celsius of the difference in temperature. That’s why a large number of semiconductor pairs have to be connected in series on in order to generate usable amounts of electricity.

The process seems simple, but its implementation is complicated. The biggest problem is the process of producing the generators. Until now, the legs of the thermoelectric generators—components that are only a few millimeters long—first had to be attached to a carrier material by hand. Producing TEGs in large quantities was complicated and expensive. Nonetheless, thermoelectric generators have been demonstrating their effectiveness in niche applications for a long time. For example, on board the Voyager 1 space probe—15 billion kilometers from the Earth—they have been reliably transforming the heat of disintegrating radioisotopes into electricity for 20 years.

Researchers from Creavis wanted to use this potential for applications on earth as well, so they began to develop a cost-effective production process for thermoelectric generators. “This idea was born back when our company was still active in the energy sector and wanted the operation of its power plants to be as environmentally friendly as possible,” explains Dirk Lehmann, the Head of Business & Launch Management. After weighing the advantages and disadvantages involved, the researchers opted for the system based on thermoelectric generators. TEGs operate without any mechanical movements, directly transform the flow of heat into electricity, can be combined with other systems, are compact, and operate very reliably. They also have long maintenance intervals. “In order to make the process practicable for large production volumes, we looked at various possible innovations,” Lehmann says.

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.”

In order to produce TEGs cost-effectively, the researchers from Creavis and Process Technology had to focus mainly on decreasing the amount of manual work involved. Among other things, they decided to use robots in the production process, thus reducing production costs by up to two thirds.