**5. Conclusions**

Treating entropy and electrical charge as basic quantities allows for a concise description of thermoelectric transport phenomena (entropy, charge, thermal energy, and electrical energy) and it is the key to comprehensibility. The basic transport equation involves classical thermodynamic potentials (temperature and electrical potential) and enables the identification of a thermoelectric material tensor. On the material's voltage–electrical current cure, distinct working points can be identified, which allow for consideration of the power conversion and its efficiency of the thermoelectric material apart from a device. The power depends on the power factor, and the conversion efficiency depends on the figure-of-merit *zT*. A clear physical meaning is given to the power factor as the part of the entropy conductivity that couples to the electrical current. The thermal conductivity, expressed here as entropy conductivity, depends on the electrical current and becomes negative when the thermoelectric material is operated in entropy pump mode. The dimensionless figure-of-merit *zT* is the ratio of two entropy conductivities, the one under electrically open-circuited conditions and the one that couples to the electrical current. The performance of the thermoelectric material in generator mode and entropy pump mode are related to each other and they can be considered on a single voltage–electrical current curve apart from a device.

**Funding:** This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – project number 325156807. The publication of this article was funded by the Open Access Fund of the Leibniz Universität Hannover.

**Acknowledgments:** The author is grateful to Jürgen Caro for his continuous encouragemen<sup>t</sup> and sustainable support. The author is grateful to Mario Wolf and Richard Hinterding for critical reading of the manuscript and their suggestions.

**Conflicts of Interest:** The author declares no conflicts of interest.
