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

Abstract

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.

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Acknowledgements

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.

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Authors

Contributions

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 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). https://doi.org/10.1038/s41560-018-0306-x

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