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 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 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 in a coil applied. Scientists have successfully increased the electrical power per footprint of thermo-magnetic generators, making them competitive with established thermoelectric generators for the first time.
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 up-scaling will depend on material development and engineering.
The researchers 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 5 to 40 micrometers. The thermo-magnetic generators reached a maximum electrical power of 50 microwatts per square centimeter at a temperature change of just 3 °C.
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.