Efficient PEM Electrolyzer Simulation with AVL FIRE?

by?Clemens Fink & Matija Mlakar

Why is the production of green hydrogen important?

The global demand for hydrogen is currently growing in all sectors (e.g. refining, industry, transportation). The hydrogen demand is estimated to double in this decade (180 Mt by 2030 compared to 94 Mt in 2021). Green hydrogen is considered a cleaner and more sustainable option compared to blue and grey hydrogen. Here are some key differences:

• Emissions: Green hydrogen is produced using renewable energy sources and does not generate any carbon emissions. Blue hydrogen is produced using fossil fuels, typically natural gas, but the carbon emissions are captured and stored using carbon capture and storage (CCS) technology. Grey hydrogen is also produced using fossil fuels, but without any carbon capture or storage, resulting in significant carbon emissions.
• Environmental impact: Green hydrogen has a minimal environmental impact because it does not generate any carbon emissions or pollutants. Blue hydrogen can significantly reduce emissions compared to traditional fossil fuels, but the CCS process is not entirely emissions-free, and the environmental impact can vary. Grey hydrogen has significant carbon emissions and is considered a traditional, non-sustainable source of energy.
• Cost: Currently, green hydrogen is more expensive to produce compared to blue or grey hydrogen. However, as the cost of renewable energy sources continues to decrease and technology improves, the cost of green hydrogen is expected to decrease, making it more competitive with other hydrogen production methods.

Overall, green hydrogen is considered the most sustainable and environmentally friendly option for hydrogen production. While blue and grey hydrogen may still have a role to play in the transition to a low-carbon economy, the ultimate goal is the transition to a fully renewable energy system, where green hydrogen will play a critical role. One of the most common ways of producing green hydrogen is through electrolysis of water using renewable electricity.

Which types of electrolyzers exist?

There are basically three types of water electrolyzers:

• Alkaline electrolyzer
• Proton exchange membrane (PEM) electrolyzer
• Solid oxide electrolyzer

Alkaline and PEM electrolyzers are operated at low temperatures (30 – 80 °C) whereas solid oxide electrolyzers are operated at high temperatures (500 – 850 °C). The operating principles of the three electrolyzer technologies are shown in Figure 1 and their pros and cons are listed in Table 1. New developments in the field of low temperature electrolysis move towards combining alkaline and PEM technologies in the form of anion exchange membrane (AEM) electrolyzers. AEM electrolyzers are basically alkaline electrolyzers in which the diaphragm located between the electrodes is replaced with an ion exchange membrane. They combine the advantages of alkaline electrolyzers (low cost & non-noble catalyst material) with those of PEM electrolyzers (high power density & high operating pressure).

Table 1: Pros and cons of the water electrolyzer technologies [1, 2].

?[1]? S.S. Kumar and V. Himabindu,?Materials Science for Energy Technologies?2, 442, 2019.

[2]? M. Carmo et al.,?Int. J. Hydrog.?Energy?38, 4901, 2013.

How does the type of electrolyzers impact simulations with AVL FIRE? M?

Because solid oxide electrolyzers are in principle 100% reversible solid oxide fuel cells, no extra implementation effort for the electrolysis mode is necessary here, as the physical and numerical implementations are the same as for the fuel cell mode. For PEM electrolyzers the situation is different, mainly because of the aggregate state of the inlet fluids: In PEM fuel cells humid?gases??are fed to the cells and, generally, liquid water exists only in small amounts. The flow channels contain portions of liquid water (droplets, slugs, films) and the gas phase drags these portions more or less efficiently out of the cell keeping the channels basically free of liquid water accumulations. In contrast, PEM electrolyzers are usually fed with 100%?liquid?water and the produced gases (H2, O2?) have to find their way through the liquid water towards the channel outlet in the form of gas bubbles. Depending on the specified inlet mass flow rate, a variety of gas bubble sizes and shapes can occur – see Figure 2. Obviously, the type of multiphase flow is strongly different between fuel cells and electrolyzers. Apart from the different physical models, this implies also a different numerical implementation. The related challenges are explained in the subsequent section.

[3]? O. Panchenko et al.,?Energies?12, 350, 2019.

What are the numerical challenges in the PEM electrolyzer modeling?

As mentioned in the previous section, PEM electrolyzer modeling implies challenges in conjunction with the multiphase flow. The main challenge is the modeling of the liquid water transport in the porous media (catalyst layers, diffusion layers) where the liquid water exhibits very small flow velocities (about 1e-5 m/s) under large capillary pressure gradients. From a physical point of view, this liquid water transport is, therefore, of diffusive rather than of convective nature (not be confused with friction forces which are also diffusive). With no special numerical treatment such a flow problem tends to be numerically unstable. FIRE M uses a stabilization trick to overcome this that only changes the way?how?the solution is obtained but not the solution?itself.

How does the model perform when applied to industrial cells?

Simulation capabilities for PEM electrolyzers were extensively tested in a project for a customer. As part of this project, the customer designed and experimentally tested a complex unit cell for a PEM electrolyzer, aiming to assess the capabilities of FIRE M as a reliable CFD tool for R&D of new-generation PEM electrolyzers. The simulation provided detailed insights into the thin structure of the unit cell, which would be difficult and costly to test in a physical experiment. The results from the model showed a very good match with experimental data, as illustrated in Figure 1, with key mean results and temperature distribution visible on a small segment of the complete unit cell (details are confidential). The success of the FIRE M model led to expanded cooperation between the customers electrolyzer department and AVL, with two additional projects commissioned for an Electrochemical Hydrogen Compressor and Alkaline Oxygen Compressor. Furthermore, the customers fuel cell department was impressed with FIRE M's capabilities and purchased a FIRE M license after a short benchmark.

How can 3D results be interpreted?

The analysis of 3D results enables the optimization of cell design and operating conditions. Inhomogeneities in the distributions of current density, liquid water, gas species and temperature can be detected and changes in design and operation can be suggested. Additionally, the impact of material parameters can be assessed. In this section, 3D results are investigated on a cell with interdigitated anode flow channels. The bipolar plates and flow channels as well as the inlet and outlet fluids are shown in Figure 4.

In the interdigitated channel design, the inlet and outlet channels are not directly connected but separated by the liquid/gas diffusion layer (LGDL). Such a design has the advantage that the produced gas (O2?) can be transported easily out of the cell through the outlet channels while hardly disturbing the liquid water flow in the inlet channels. The Image of the Day below shows the liquid water distribution in the channels and diffusion layers. In the anode channels, the mentioned separation of gas and liquid can be seen clearly. Figure 5 shows results in the membrane: ionic current density, membrane water content and temperature. The current density increases with higher temperature and higher humidity. On the other hand, a higher current density also increases the temperature and lowers the humidity, since water is consumed in the anode catalyst layer. In the steady state, thus, an overall equilibrium is established.

Figure 6 shows the O2?and H2?mole fraction in the channels and the diffusion layers. O2?/ H2?are produced in the anodic / cathodic catalyst layers and transported towards the channel outlets. Thanks to the interdigitated flow channel design at the anode, the O2?removal is quite efficient and no major O2?accumulations occur. On the other hand, at the cathode, due to long transport paths, H2?is accumulated below the bipolar plate ribs.

?Finally, the surface temperatures of the bipolar plates are shown in Figure 7. The cathodic bipolar plate exhibits a much higher temperature than the anodic bipolar plate. The reason for that is the entropic contribution (reversible heat) of the electrochemical reaction which causes cooling of the anode. Such a cooling effect is typical for electrolyzers and does not exist in fuel cells, because here the entropic reaction heat is always positive.

?As much as we love to nerd out about simulation and read lengthy articles about it, we have to cut it short at this point.

We want to thank Clemens Fink & Matija Mlakar for the insights and the impressive work that is performed day to day behind the scenes.

Real-world activities and their real-time limitations bring this Simulation Saturday to an end, but stay tuned for another one soon!

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