Experimentelle und theoretische Untersuchungen einer thermogalvanischen Zelle auf Basis einer Polymer-Elektrolyt-Membran

The utilization of low-temperature waste heat, for example from industrial processes and data center, is an important contribution to the increase of energy efficiency and thus the mitigation of global climate change. Besides the already established (Organic-) Rankine cycle processes, ther-mogalvanic cells as a variant of thermoelectric energy converters are a promising technology for the further use of this thermal energy. One possible concept is a hydrogen-powered thermogal-vanic cell based on a polymer electrolyte membrane. The operating behaviour of this energy converter is of particular relevance for evaluating the field of potential applications and is char-acterized in this work in experimental investigations. Therefore, the knowledge of the thermo-dynamic states present at the membrane surfaces is crucial, with the chemical potential of the water and the temperature being particularly important, as they directly influence the open-circuit voltage of the cell. The presence of gradients in the cell causes the formation of mass and heat flows as well as electric fluxes, having a direct impact on locally present thermodynamic states. Since neither the temperature nor the chemical potential of the water can be measured right at the membrane surfaces, it is important to precisely describe the transport processes tak-ing place in the cell and its impact on the resulting thermodynamic properties. In the present work, this is realized by a theoretical model based on the method of non-equilibrium thermodynamics considering monocausal force-flux couplings. The latter allows the evaluation of the singular dependence of a gradient on a resulting flux when other gradients are excluded. The present force-flux couplings are expressed by phenomenological coefficients, which are often approximated by means of classical transport coefficients due to the lack of available data in the literature. A central investigation of this work is therefore the experimental determination of various phenomenological coefficients in a polymer electrolyte membrane. The results show some significant deviations between the phenomenological coefficients approxi-mated using the classical transport coefficients and those determined experimentally. This dis-crepancy may appear due to the inaccurate prediction of the thermodynamic states in the cell and different measurement setups impacting the experimental results. Furthermore, the meas-urement results show an influence of the gravitational force on the transport processes in the cell, which needs to be investigated in subsequent work. In addition to the evaluation of transport coefficients, the influence of individual and coupled gradients on the operating behav-iour of the thermogalvanic cell is analysed in this work. The results show that the combination of a gradient in the temperature and a gradient in the chemical potential of water acting in opposite directions, a high mean membrane temperature and a high mean chemical potential of water, leads to a maximum power output of the cell of approximated 0,38 W/m2. The transfer of the positive influence of coupled gradients on the maximum power output to other thermoelectric energy converters represents a promising possibility for increasing their performance and can thus make a positive contribution to the realization of efficient waste heat utilization.