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Energy harvesting near room temperature using a thermomagnetic generator with a pretzel-like magnetic flux topology


To date, there are very few technologies available for the conversion of low-temperature waste heat into electricity. Thermomagnetic generators are one approach proposed more than a century ago. Such devices are based on a cyclic change of magnetization with temperature. This switches a magnetic flux and, according to Faraday’s law, induces a voltage. Here we demonstrate that guiding the magnetic flux with an appropriate topology of the magnetic circuit improves the performance of thermomagnetic generators by orders of magnitude. Through a combination of experiments and simulations, we show that a pretzel-like topology results in a sign reversal of the magnetic flux. This avoids the drawbacks of previous designs, namely, magnetic stray fields, hysteresis and complex geometries of the thermomagnetic material. Our demonstrator, which is based on magnetocaloric plates, illustrates that this solid-state energy conversion technology presents a key step towards becoming competitive with thermoelectrics for energy harvesting near room temperature.

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Fig. 1: Photo of a TMG, and the key functional properties of the thermomagnetic material used within it.
Fig. 2: Topologies of magnetic circuits of TMGs for different genera.
Fig. 3: TMG with flux reversal using a magnetic flux topology of genus = 3.
Fig. 4: Characterization of a TMG with genus = 3.
Fig. 5: Measured electrical power output of a TMG with genus = 3 depending on the key operating parameters.
Fig. 6: Comparison of the theoretical and experimentally obtained magnetic flux changes through the La–Fe–Co–Si material used within the TMG.
Fig. 7: Comparison of key properties of TMGs with different topologies.

Data availability

The data sets generated and analysed during the current study are available from the corresponding author upon reasonable request.


  1. 1.

    Schierning, G. Bring on the heat. Nat. Energy 3, 92–93 (2018).

    Article  Google Scholar 

  2. 2.

    Champier, D. Thermoelectric generators: a review of applications. Energy Convers. Manag. 140, 167–181 (2017).

    Article  Google Scholar 

  3. 3.

    Tesla, N. Pyromagneto electric generator. US patent 428,057 (1890).

  4. 4.

    Edison, T. A. Pyromagnetic generator. US patent 476,983 (1892).

  5. 5.

    Srivastava, V., Song, Y., Bhatti, K. & James, R. D. The direct conversion of heat to electricity using multiferroic alloys. Adv. Energy Mat. 1(1), 97–104 (2011).

    Article  Google Scholar 

  6. 6.

    Pecharsky, V. K. & Gschneidner, K. A.Jr Giant magnetocaloric effect in Gd5(Si2Ge2). Phys. Rev. Lett. 78, 4494–4497 (1997).

    Article  Google Scholar 

  7. 7.

    Waske, A., Gruner, M. E., Gottschall, T. & Gutfleisch, O. Magnetocaloric materials for refrigeration near room temperature. MRS. Bull. 43, 269–273 (2018).

    Article  Google Scholar 

  8. 8.

    Christiaanse, T. & Brück, E. Proof-of-concept static thermomagnetic generator experimental device. Met. Mat. Trans. E 1(1), 36–40 (2014).

    Google Scholar 

  9. 9.

    Brillouin, L. & Iskenderian, H. P. Thermomagnetic generator. El. Com. 25(3), 300–311 (1948).

    Google Scholar 

  10. 10.

    Elliott, J. F. Thermomagnetic generator. J. Appl. Phys. 30(11), 1774–1777 (1959).

    Article  Google Scholar 

  11. 11.

    Kirol, L. D. & Mills, J. I. Numerical analysis of thermomagnetic generators. J. Appl. Phys. 56, 824–828 (1984).

    Article  Google Scholar 

  12. 12.

    Hsu, C.-J., Sandoval, S. M., Wetzlar, K. P. & Carman, G. P. Thermomagnetic conversion efficiencies for ferromagnetic materials. J. Appl. Phys. 110, 123923 (2011).

    Article  Google Scholar 

  13. 13.

    Ohkoshi, M., Kobayashi, H., Katayama, T., Hirano, M. & Tsushima, T. A proposal of application of spin reorientation phenomenon to the thermomagnetic power generation. Jpn J. Appl. Phys. 15(10), 2019 (1967).

    Article  Google Scholar 

  14. 14.

    Brungsberg, H. Vorrichtung zur umwandlung von thermischer in elektrische oder mechanische energie mittels eines magnetischen systems. DE Patent DE 3106520 (1981).

  15. 15.

    Kazumasa, S. Thermomagnetic power generation apperatus using thermosensitive magnetic substance. J Patent 07107764 A (1995).

  16. 16.

    Herzig, A., Herzig, K. & Herzig, Y. Vorrichtung und verfahren zur gewinnung elektrischer energie aus wärmeenergie. DE Patent DE 10 2007 023 505 A1 (2007).

  17. 17.

    Edison, T. A. Pyromagnetic motor. US Patent 380,100 (1888).

  18. 18.

    Tesla, N. Thermomagnetic motor. US Patent 396,121 (1889).

  19. 19.

    Kishore, R. A. & Priya, S. A review on design and performance of thermomagnetic devices. Renew. Sustain. Energy Rev. 81, 33–44 (2018).

    Article  Google Scholar 

  20. 20.

    Swiss Blue Energy AG, Thermo-magnetic motor; (accessed 2 March 2018).

  21. 21.

    Gueltig, M. et al. High-performance thermomagnetic generators based on Heusler alloy films. Adv. Energy Mater. 7, 1601879 (2017).

    Article  Google Scholar 

  22. 22.

    Chun, J. et al. Thermo-magneto-electric generator arrays for active heat recovery system. Sci. Rep. 7, 41383 (2017).

    Article  Google Scholar 

  23. 23.

    O’Handley, R. C. Modern Magnetic Materials: Principles and Applications (John Wiley & Sons, New York, 2000).

    Google Scholar 

  24. 24.

    Advanced Materials—The Key to Progress (VACUUMSCHMELZE, 2015);

  25. 25.

    Kitanovski, A. et al. Magnetocaloric Energy Conversion (Springer, Heidelberg, 2015).

  26. 26.

    Waske, A. et al. Asymmetric first-order transition and interlocked particle state in magnetocaloric La(Fe,Si)13. Phys. Stat. Sol. RRL 9, 136–140 (2015).

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We thank S. Grasemann for assistance with the technical realization of the TMG and the technical drawing, A. Chirkova for the photo and U. K. Rößler and M. Kohl for discussions.

Author information




A.W. and S.F. conceived the experiments and wrote the outline of the paper. D.D. characterized the TMG. K.S. proposed the topology with genus = 3 and optimized the design. D.B. conducted most of the finite element calculations. A.S. characterized the thermomagnetic material. K.N. added the discussion on the impact and thermoelectric generators. S.F. wrote the first version of the manuscript, and all the authors contributed to the final version.

Corresponding author

Correspondence to Sebastian Fähler.

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Competing interests

K.S., S.F. and A.W. filed a patent (DE patent application no. DE 10 2016 122 274.7) on this topology.

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Supplementary Information

Supplementary Figures 1–17, Supplementary Tables 1–2, Supplementary Notes 1–14 and Supplementary References

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Waske, A., Dzekan, D., Sellschopp, K. et al. Energy harvesting near room temperature using a thermomagnetic generator with a pretzel-like magnetic flux topology. Nat Energy 4, 68–74 (2019).

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