Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Renewable energy technologies are indispensable in order to limit the CO2 emissions while the worldwide energy demand is steadily growing. Efficient energy storage systems will be required for renewable energy sources which are not available continuously, e.g. wind and solar energy. Chemical energy storage in the form of hydrogen offers a very high energy density in contrast to other systems such as thermal or mechanical energy storage and is therefore one of the most promising candidates for a long-term energy storage. An important technique to convert electric energy from a wind turbine or solar panel into chemical energy is the electrolysis of water, where hydrogen and oxygen gas is generated. However, low efficiencies of about 60 % for typical water electrolyzers are a significant drawback of this technique (Pletcher and Li (2011)). These low efficiencies are partly the result of hydrogen and oxygen gas bubbles, which evolve at the electrodes of the electrolyzer and reduce its free area available for the chemical reaction and increase the ohmic resistance of the electrolyte. Therefore, the efficiency of water electrolysis can be improved by an accelerated removal of the hydrogen gas bubbles from the electrode surface (Fern ́ andez et al (2014), Koza et al (2011)). One method that has been found to be effective in this regard is the use of forced convection.
Forced convection can be generated by simple stirring or more elegantly by body forces. A method that has been a subject of recent studies is the application of magnetic fields (Koza et al (2011), Monzon and Coey (2014), Weier and Landgraf (2013)). The superposition of a magnetic field with the electric field, which is is already present during the electrolysis of water, gives rise to body forces acting directly in the electrolyte, i.e. the so-called Lorentz force. The Lorentz force has its largest value at the wall and decreases exponentially with the distance from the electrode. The advantage of this force distribution is the large shear generated directly in the vicinity of the wall. Due to the viscosity of the fluid also the bulk flow is strongly influenced. At present, there have been only few experimental studies on the effect of Lorentz forces on bubble growth and on the fluid dynamics of the two-phase flow at gas-evolving electrodes (Weier and Landgraf (2013)).
Especially the characterization of the near-wall region is essential to understand the influence of the strong shear on the bubble detachment and the interaction of the bubble driven and Lorentz force driven flow.
In order to investigate both the near-wall and the bulk flow, experiments with advanced measuring and evaluation techniques have been carried out in an undivided electrolysis cell with and without the application of magnetic fields. Bubble trajectories and the velocity field of the surrounding electrolyte were measured simultaneously for this purpose. Fluorescent tracer particles and a laser light illumination were used to measure fluid velocities in the electrolyte. A background illumination with a different wavelength was used to measure the size and trajectory of bubbles by means of shadowgraphy. Bubble shadow and tracer particle images were captured simultaneously by two sCMOS cameras with two different wavelength filters to separate both signals. Fluid velocities were evaluated using particle image velocimetry (PIV) as well as particle tracking velocimetry (PTV). It turned out that the PIV results obtained near the electrode, where high velocity gradients occur, were substantially biased as the result of the high void fraction and the associated light absorption in this region.
However, the knowledge of the near-wall fluid velocities and its gradients is essential as they influence the detachment of bubbles from the electrode. In contrast to correlation based evaluation methods as PIV, particle tracking techniques do not depend on the particle image intensity itself but on the ability to detect particles and track them correctly. For this reason, different image filters were applied to enhance the particle images for a more reliable particle detection. More importantly, a sophisticated tracking algorithm proposed by Ohmi and Li (2000) and adapted by Cierpka et al (2013) was used, which takes the similar movement of neighboring particles into account and thus helps to avoid spurious vectors resulting from particle images which were undetectable in one of two consecutive frames. This approach allows for more precise measurements closer to the electrode as exemplified in Fig.1a). This way, the influence of Lorentz forces on the near-wall velocity distribution was investigated. It will be shown in the final paper that Lorentz forces lead to a significant acceleration close the electrode, especially for higher electric current densities (see Fig.1b)). In addition to the velocity fields, the bubble trajectories were of interest. Therefore, multiple image filters were applied on the bubble shadow images to obtain discernible bubble images. A simple PTV algorithm was used to determine the size and trajectory of bubbles. It will be shown that most of the large rising bubbles have path oscillations and can have a significant impact on the near-wall fluid velocities and mass transfer to the electrode.
The full article will give detailed information on how to extract proper data for the presented experiment with the help of the aforementioned tracking algorithm and will discuss the limits of this approach. The velocity fields obtained with PTV will be compared to those determined with PIV for different current densities with and without the application of Lorentz forces. Moreover, the size and trajectrories of bubbles with path oscillations and their impact on the velocity field and velocity fluctuations will be shown for some exemplary cases.