Analysis of thermal energy harvesting using ferromagnetic materials

• Ferromagnetic materials show a sharp change in their permeability near the Curie temperature.
• A bias magnetic field permits changing the magnetic flux with the temperature.
• The variable magnetic flux can be converted into electrical energy by using a coil.
• Theoretical and experimental measurements show an energy density up to 7.35 μJ cm−3 K−2 cycle−1.
• Optimization issues should focus on coil quality and global magnetic reluctance variation.

This Letter aims at giving a preliminary investigation of the thermal energy harvesting capabilities of a technique using the temperature-dependent permeability of ferromagnetic materials. The principles lie in the modification of the magnetic field caused by the variation of the permeability due to the temperature change, hence generating a voltage across a coil surrounding the circuit. The technique can be made truly passive by the use of magnets for applying bias magnetic field. Theoretical results, validated by experimental measurements, show a voltage output of 1.2 mV at optimal load of 2 Ω under 60 K temperature variation in 5 s (with a maximum slope of 25 K s−1). Further improvements, such as the use of low resistivity coil and magnet with high remnant magnetic field, indicate that it is possible to convert up to 7.35 μJ cm−3 K−2 cycle−1.