Karlsruher Institut für Technologie (KIT) scientists have now come much closer to their goal of converting waste heat into electrical power at small temperature differences. Electrical power per footprint of thermomagnetic generators based on Heusler alloy films has been increased by a factor of 3.4.

The use of waste heat would contribute in a large way to a sustainable energy supply.

Collaborative scientists from Karlsruhe Institute of Technology and Tōhoku University in Japan reported their results as a cover story in the journal Joule.

Thermomagnetic generator image and cycle graph. Image Credit: Karlsruhe Institute of Technology, Institute of Microstructure Technology. Click image for the largest view.

Many technical processes only use part of the energy consumed. The remaining fraction leaves the system in the form of waste heat. Frequently, this heat is released into the environment unused. However, it can also be used for a heat supply or power generation. The higher the temperature of the waste heat is, the easier and cheaper is its reuse.

Thermoelectric generators can use waste heat of low temperature for direct conversion into electrical power. Thermoelectric materials used so far, however, have been expensive and sometimes even toxic. Moreover, thermoelectric generators require large temperature differences for reaching efficiencies of just a few percent.

Thermomagnetic generators represent a promising alternative. They are based on alloys, whose magnetic properties are highly temperature-dependent. Alternating magnetization induces an electrical voltage when a coil is applied. Researchers first presented concepts of thermomagnetic generators in the 19th century. Since then, research has covered a variety of materials. Electrical power, however, has left a lot to be desired.

Scientists of KIT’s Institute of Microstructure Technology (IMT) and Tōhoku University in Japan have now succeeded in largely increasing the electrical power per footprint of thermomagnetic generators.

Professor Manfred Kohl, Head of the Smart Materials and Devices Group of IMT said, “Based on the results of our work, thermomagnetic generators are now competitive with established thermoelectric generators for the first time. With this, we have come a lot closer to the goal of converting waste heat into electrical power at small temperature differences.”

The so-called Heusler alloys – magnetic intermetallic compounds – are applied in the form of thin films in thermomagnetic generators and provide for a big temperature-dependent change of magnetization and quick heat transfer. This is the basis of the new concept of resonant self-actuation. Even at small temperature differences, resonant vibrations are induced in devices and can be converted efficiently into electrical power. Still, electrical power of single devices is low and upscaling will depend on material development and engineering.

Thermomagnetic generator graph scaling material thickness to electrical output. Image Credit: Karlsruhe Institute of Technology, Institute of Microstructure Technology. Click image for the largest view.

The researchers of KIT and Tōhoku University used a nickel-manganese-gallium alloy and found that alloy film thickness and the device footprint influence electrical power in opposite directions. Based on this finding, they succeeded in improving electrical power per footprint by a factor of 3.4 by increasing the thickness of the alloy film from five to 40 micrometers.

The thermomagnetic generators reached a maximum electrical power of 50 microwatts per square centimeter at a temperature change of just three degrees Celsius.

Kohl explained, “These results pave the way to the development of customized thermomagnetic generators connected in parallel for potential use of waste heat close to room temperature.”

These are quite impressive results and quite good news. Notice the area is about 0.4 inch by 0.4 inch, less than a sixth of a square inch. And the temperature difference is less than 6° F. This looks like celebration breakthrough level news. Congratulations to this team, this is progress!


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