Many technical processes only use part of the energy consumed. The remaining fraction leaves the system as waste heat. Frequently, this heat is released into the environment without being used; however, it can also be used for heat supply or power generation. The higher the temperature of the waste heat, the easier and cheaper it is to reuse.
Thermoelectric generators can use low temperature waste heat for direct conversion into electrical energy. However, the thermoelectric materials used so far were expensive and sometimes even toxic. In addition, thermoelectric generators require large temperature differences to achieve efficiencies of only a few percent.
Thermomagnetic generators represent a promising alternative. They are based on alloys whose magnetic properties are strongly dependent on temperature. Alternating magnetization induces an electrical voltage in an applied coil. Scientists succeeded in increasing the electric power per footprint of thermomagnetic generators, making them competitive for the first time with established thermoelectric generators.
The so-called Heusler alloys – magnetic intermetallic compounds – are applied in the form of thin layers in thermomagnetic generators and allow a large change in magnetization depending on the temperature and a rapid 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 efficiently converted into electrical energy. Still, the electrical power of individual devices is low, and scaling up will depend on materials development and engineering.
The researchers used a nickel-manganese-gallium alloy and found that the thickness of the alloy film and the footprint of the device influence electrical power in opposite directions. Based on this finding, they succeeded in improving the electrical power per footprint by a factor of 3.4 by increasing the thickness of the alloy film from 5 to 40 micrometers. The thermomagnetic generators reached a maximum electrical output of 50 microwatts per square centimeter at a temperature change of only 3°C.
These results pave the way for the development of custom parallel-connected thermomagnetic generators for potential near-ambient waste heat utilization.