A numerical model is developed in Simulink to simulate transient alkaline electrolyzer operation. The model is built to simulate a large scale hydrogen plant in Rjukan, Norway. The model is based on physical relations where available, some plant-specific data is modeled using empirical equations. The model describes the anodic and cathodic compartments using electrochemical equations, lumped heat, and a perfectly mixed mass model. The balance of plant is modeled using the same heat and mass model. The model can predict the potential, temperature, efficiency, temperature, and mass flows and is validated using 30 days of measured data of the large scale plant. Transient current input, such as sudden current ramp up from 0 to 100% load, is used to simulate dynamic operation. From these simulations flexibility limitations of the electrolyzer are determined. The results suggest that the liquid levels in the stack andgas-liquid separator and temperature are limiting flexibility. The liquid levels form the biggest limitation of dynamic operation due to potential flooding or draining of the gas-liquid separators.

Alkaline water electrolysis holds significant promise for the production of green hydrogen, playing a crucial role in decarbonization across various industrial sectors. The operational effectiveness of this technology hinges on its electrical efficiency. This is intrinsically tied to the formation and behaviour of gas bubbles at the electrode-electrolyte interface, influencing its overall performance. In the Zero-Gap configuration, the electrodes and the membrane are directly pressed against each other to reduce the ohmic resistance between the electrodes. However, this possibly introduces large bubble overpotentials, reduces active surface area, or facilitates gas crossover. Hypotheses for avoiding these shortcomings include introducing a small space between the electrode and membrane to allow bubbles to escape. This study aims to visualise and quantify the behaviour of bubbles in the near-electrode space, presenting numerous experimental findings and images from high-speed photography. The analysis encompasses multiple physical phenomena influencing bubble behaviour, offering comprehensive insights into their dynamics.

The investigation begins by examining the effect of electric fields on electrolytic bubbles. First, an electrowetting experiment was conducted on a polished nickel electrode. Surprisingly, it was discovered that this effect had no observable influence on the contact angle of bubbles with the electrode when subjected to voltages up to 1.7 V. Furthermore, an electrophoresis experiment revealed that the application of an electric field was found to have a minimal impact on the trajectory of both oxygen and hydrogen bubbles. The electric field had a maximum strength of 15 kV/m. This finding implies that electric fields do not significantly alter the behaviour of electrolytic bubbles in this context.

Two distinct cell configurations were employed further to assess bubble behaviour and bubble influence on AWE overpotentials. The classical Zero-Gap cell design was evaluated, equipped with a spacer to introduce a small gap between the electrode and membrane. Interestingly, linear sweep voltammetry experiments conducted on the Zero-Gap cell configuration contradicted previous literature, suggesting that imperfect gap control might explain this discrepancy.

In parallel, a novel cell design allowed for imaging an electrode's membrane side, which was analyzed using chronopotentiometry. Surprisingly, bubble resistances were found to be independent of current density. Additionally, the study revealed that electrode geometry is essential in influencing the potential increases due to double-layer charging and bubble evolution. The analysis of chronopotentiometry data provided insights into the periodic flow of large bubble slugs on the membrane side of the electrode, with the characteristic time of the flow features being inversely proportional to the current density. The utilization of this configuration led to the acquisition of unforeseen images from the electrode's membrane side.

Finally, a Particle Image Velocimetry (PIV) analysis of the recorded data yielded insights into the velocity fields surrounding an expanded metal electrode in a Zero-Gap configuration. The investigation revealed that bubble highways were established owing to the specific electrode geometry, exhibiting a vertical velocity approximately 1.4 times greater than the average vertical velocity.

In summary, this study delivers comprehensive visualizations of bubble dynamics near the electrode and quantifies their influence on the performance of alkaline water electrolysis. Moreover, the utilization of High-Speed - High-Resolution images enables unprecedented insights into the visualization of bubble behaviour on the electrode. These findings significantly contribute to our understanding of bubble behaviour in the vicinity of water-splitting electrodes and provide essential information for validating more comprehensive models of this technology, ultimately aiding in advancing green hydrogen production and the decarbonization of industrial sectors.

The increasing awareness and urgency of climate change have led to an increase in investments and research into power sources not reliant on fossil fuels. Proton exchange membrane (PEM) fuel cells are a promising technology for automotive, maritime, and auxiliary power applications converting chemical energy into electricity. In order for this upcoming technology to compete with well-established alternatives such as diesel generators and combustion engines, it is of vital importance to improve the power density and durability.

PEM fuel cells produce water and heat during operation. The presence of superfluous liquid water in the fuel cell stack gives rise to flooding of the electrodes, which hampers operation and induces degradation processes. On the other hand, it is of great importance to maintain a high membrane humidification to reduce Ohmic losses over the membranes and avoid the formation of cracks. Therefore, water management plays a vital role both in maintaining the power density and guaranteeing durability.

This thesis is written under the auspices of both TU Delft and PowerCell Group, a manufacturer of PEM fuel cell systems located in Gothenburg, Sweden. Currently, a substantial amount of water condenses in the anode subsystem of PowerCell’s PEM fuel cells in certain operating ranges which subsequently enters the fuel cell stack. This study identifies the influence of certain system operating parameters on responses as the water crossover through the membranes, the relative humidity at the stack inlet, the temperature at the inlet of the stack, the condensation rate in the mixing chamber of the recirculation loop, and the mass flow rates of liquid water and water vapor in and out of the stack. Multiphysics simulation software provided by Gamma technologies is used to simulate 5600 operating points in which the system operating parameters are varied according to the Latin Hypercube sampling method. These simulations give a clear overview of the influence of the operating parameters on the aforementioned responses over the entire operating range of PowerCell’s PS-100 system.

The simulated experiments are subsequently used as a basis to construct metamodels. These metamodels predict the behavior of the system based on the operating parameters varied in the 5600 simulations. The metamodels are constructed both as Krigings and as multilayer perceptrons (MLPs). Kriging is a statistical method which produces an output for a certain response based on known input data where input points which resemble the unknown point are given a greater weight. MLPs are neural networks which recognise patterns in input data and use those to predict an output for a certain response. Finally, the operating parameters are optimized using the metamodels to minimize the liquid water mass flow rate into the stack, to prevent condensation in the mixing chamber of the anode subsystem, and to target a certain inlet relative humidity of the hydrogen feed to the stack. Concluding from these optimizations it would be beneficial to preheat the hydrogen before it reaches the mixing chamber. A number of alternative designs for the anode loop are proposed which require further investigation.

Global endeavors to reduce emissions in the shipping industry are accelerating the interest in fuel cell systems. This paper explores the application of different fuel cell types (LT-PEMFC, HT-PEMFC and SOFC) in combination with different fuels (LH2, LNG,MeOH and NH3) in expedition cruise ships. An impact model is developed for the first design phase. The goal of this paper is to evaluate the impact of the combination of fuel cell system implementation and operational profile on expedition cruise vessels. Impact is expressed in ship size, capital cost, operational cost and emissions. The model takes into account: fuel storage, on-board fuel processing, fuel cell system characteristics, balance of plant components, fuel cost over operational lifetime and all onboard emissions. In the research, seven different fuel cell systems and three different hybridization strategies are considered. For the six best performing combinations of fuel cell system and hybridization strategy, the range, endurance and capacity requirements are systematically varied to determine whether the best performing option depends on these requirements. Finally, hybrid option 2 (using diesel generators to support during long transits) combined with a methanol fueled LT-PEMFC system results in the lowest newbuild price. This option does comply with emission regulations and CO2 goals for 2030. Hybrid option 2 combined with an LNG fueled LT-PEMFC system results in the lowest total cost (newbuild price and fuel cost). This option does comply with emission regulations, but does not meet CO2 goals for 2030. When it is desired to reach this CO2 target, hybrid option 2 with methanol fueled LT-PEMFC is also recommended from a total cost perspective.

Due to rising concerns about climate change, a lot of research is currently underway with respect to the development of new technologies which can contribute to the decline of atmospheric CO₂, and will allow further penetration of renewables into the energy mix. A promising technology which is currently actively researched is the electrochemical reduction of CO₂ (ERC). This technology utilizes otherwise polluting and unwanted CO₂ and converts
it into value-added products under the influence of an electrical current. The process can therefore be designed as an energy storage mechanism since electrical energy is stored as chemical bonds. In this research, ERC towards formic acid has been investigated from two perspectives.

First, the feasibility of the commercial production of formic acid compared to other products of ERC was investigated. The electrochemical production of the most common reduction products have been compared based on production costs, energy storage capabilities, toxicity and manageability. Due to the relatively low energy consumption for 2-electron products, namely formic acid, carbon monoxide and oxalic acid, it is found that these products have the most promising business case. Additionally, ERC to formic acid is best studied compared to other products, and high selectivities are commonly reported. Formic acid and methanol are liquid at atmospheric conditions, which is beneficial as relatively large amounts of energy per unit volume can be stored without the need of additional compression or cooling. This will also allow for easy transportation. As hydrogen carrier, formic acid has the advantage that it can be decomposed in H2 and CO₂ near room temperature. In the second part of this research, the use of numerical modeling to study the reduction of CO₂ in an electrochemical cell towards formic acid/formate at elevated CO₂ pressures is presented. The model investigates to impact on the cathodic half-cell of a cell designed for the reduction of CO₂ in aqueous electrolyte solutions at a constant temperature of 25℃, simultaneously assuming non-limiting conditions with respect to the anodic half-cell. The modeled part of the cell has been divided in three main regions, namely the bulk, cathode surface region and the electrode surface, which are discussed separately. The bulk is assumed to be the region of equilibrated concentrations which are constant in time, as they are not dynamically influenced by any mass transport phenomena. Reactants are supplied from the bulk to the electrode surface and products are removed vice versa via the cathode surface region, which is a thin region in the vicinity of the electrode. The transfer of species within this region and the chemical reactions between the species, form a system of diffusion-reaction equations. This system is solved numerically using appropriate boundary conditions. The actual reduction of CO₂ occurs on the electrode surface, and the kinetics of the electrochemical reactions towards HCOO^-, CO and H₂ are described using Tafel-type kinetics.

The electrochemical model has been verified and compared with experimental data, and despite various simplifications has proven to be predictive of the electrochemical reduction of CO₂. It is found that the potentially beneficial effects of an elevated CO₂ pressure on both the production rate and selectivity, as experimentally observed, can be reproduced with reasonable accuracy. The CO₂ concentration at the electrode surface is identified as the main limiting factor for achieving both a high selectivity towards formate and a higher production rate on formate producing metals. The model shows that with an increased CO₂ pressure the amount of CO₂ dissolved into the solution is increased significantly, resulting in a higher concentration of CO₂ at the electrode and less mass transfer limitations.

The primary objective of this work is to provide new analytical models to support the theoretical understanding of multiphase flows in various electrochemical systems. Most of the previous works in this field use either experiments or numerical simulations to understand the hydrodynamics of the multiphase flows. In this thesis, various new analytical models are derived for different cell configurations such as PEM diffusion layer, flow-through electrolysers, parallel plate electrolyzers and zero-gap electrolysers. New design equations are provided that can be readily used as a first estimate for a new electrochemical cell. The novelty of this work lies in providing new analytical approaches to develop a theoretical understanding of electrochemical cells.@en

This thesis addresses the critical issue of gas crossover in small gap alkaline electrolysers, significantly impacting the safety of hydrogen production systems. The research aims to optimize electrolysis cell design by investigating how bubble coverage, gap size, and electrolyte flow influence hydrogen crossover, particularly for integration with renewable energy sources. Computational simulations using Ansys Fluent revealed that higher bubble coverage and controlled electrolyte flow significantly reduce crossover rates, ensuring safe Hydrogen-to-Oxygen levels. The study highlights the delicate balance required between minimizing gap size and managing crossover risks. Additionally, the research validates numerical results with an analytical model, providing a robust predictive tool for practical applications. The findings offer valuable insights and practical recommendations for the design and operation of alkaline electrolysis systems, enhancing their safety for industrial applications. This work supports global efforts to reduce greenhouse gas emissions, facilitate the transition to renewable energy sources, and advance the hydrogen industry, which is pivotal in the evolving energy landscape.

Reducing the concentration of hydrogen and oxygen dissolved in the electrolyte helps to increase the efficiency of alkaline water electrolysis. This thesis provides an account of the evolution of dissolved concentration in the vicinity of the electrode surface for alkaline water electrolysis for the bubble size, the electrode height, the dissolved gas uptake by bubbles, and the bubble generation at the electrode surface. Included are an analytical and numerical model that not often include local electrokinetic effects coupled with a gas-liquid flow model. As is commonly found for experimental data on dissolved hydrogen and oxygen, the dissolved species is either an average concentration measured relatively far from the electrode surface or an average concentration at the electrode surface without any local effect of the electrode kinetics. In this thesis an agreement is found for the analytical derived natural convection up to an height of 0.01 [m] with the numerical model. The fraction of dissolved hydrogen and oxygen taken up by the gas bubbles is enhanced for smaller bubbles, a high frequency of bubbles generated, and an increased mass transfer of dissolved gas at the electrode. Moreover, a clear difference is found for the dissolved hydrogen and oxygen evolution near the electrode for horizontal and vertical electrodes. Horizontal electrodes have more dissolved gas at the electrodes, likely due to the dissolved gas not being able to transfer to the gas bubbles as easily for vertical electrodes. Also, for both vertical and horizontal electrodes the dissolved gas concentration flattens for increased current density, likely due to homogeneous nucleation. For an increasing electrode height a lowering of the dissolved gas was observed, associated with an increased dissolved gas uptake by the bubbles. Most of these local effects are able to be modelled using an analytical and numerically combined model. These simulations greatly improve the understanding of dissolved hydrogen and oxygen evolution in the vicinity of electrodes.

In this research project, multiphase behaviour in zero gap alkaline water electrolyzers is investigated, focusing on the influence of various parameters including current density, electrode-wall distance and electrode height. This involves a cell containing a vertical electrode pair separated by a membrane submerged in an electrolyte that has a liquid-air interface near the top of the electrodes. The study aims to identify and understand turbulent phenomena through different analysis methods. Additionally, the outcomes derived from the conducted experiments serve as validation data for concurrent modeling research.
The analysis begins by examining vortex formation on the cathodic side, where hydrogen bubbles originate, using Particle Image Velocimetry (PIV). The results indicate the presence of bubble vortices, especially near the top of the channel. At electrode-wall distances of 1.0 cm or more, the bubbles that fail to reach the liquid-air interface tend to circulate extensively within the channel without exhibiting any noticeable discrepancies, such as small vortices. It should be noted that some low-velocity vortices, typically on the order of millimeters per second, can be observed within the bulk of the channel. As a general rule, shorter electrode-wall distances lead to increased vortex formation throughout the entire channel. Additionally, higher current densities induce a greater amount of these vortices or chaotic and irregular motion of the bubbles in the bulk region. In modern configurations, elastic elements, such as materials of thin wires with high porosity and low solid volume fraction, are strategically incorporated into the electrolyzer design to enhance mechanical resilience and accommodate variations in pressure and temperature. These elements serve to ensure the stability and reliable performance of the electrolyzer system under diverse operational conditions. Much of the chaotic behaviour of the bubbles seems to be counteracted when the electrode-wall distance is large (1.5 cm) while an elastic element is placed in between the cathode and opposing wall. When the whole channel is filled by the elastic element (0.6 cm electrode-wall distance), this results in more small-velocity vortices than without the element.
Another examined phenomenon is the presence of a bubble mist. This mist arises when the bubbles that originate at the electrode do not escape the channel at the liquid-air interface, but recirculate towards the opposing wall and downwards from there. Light intensity coming from the bubble clouds is used to gain and calibrate results. Increasing the current density and decreasing the electrode-wall distance lead to deeper bubble mist plumes. Surprisingly, elevated electrolyte height directly influences mist depth, with larger electrolyte heights showing the highest mist depths, even when dividing these by the exposed electrode height. For these measurements, diamond shaped apertures on the electrodes are used. For another series of experiments where higher current densities are examined, slotted apertures are used and the effect of lowering the electrode-wall distance on the bubble mist depth is diminished. Whether this is due to the type of electrodes or the different measurement circumstances (for example lighting) can not be concluded from the results. This emphasizes the need for consistent experimental conditions.
Thirdly, the continuous release and upward movement of gas bubbles originating at the cathode, i.e. the bubble plumes, are investigated. The effect of changing the current density is investigated along different heights of the electrodes. The analysis performed employs three different strategies to assess turbulent behaviour. This includes the PIV method used initially to depict vortices, the light intensity coming from the bubble clouds and visual analysis to find discrepancies like bubble bursts and big bubble trajectories. The PIV method shows capabilities in tracking the plume width and depicting turbulent behaviour. The light intensity analysis however proves challenging due to disparities in lighting. Improved lighting and higher-resolution cameras could yield more reliable results, enabling further investigation into correlations between parameters and turbulence, such as bubble bursts.

This research aims to analyse the non-ohmic resistance of multiple ion-exchange membranes (IEM) in series, for a better understanding of membrane resistances. These membranes can be found in series in energy storage systems such as acid-base flow batteries (ABFB).
The effect of different current densities on the ohmic and non-ohmic resistance of bipolar membranes (BPM), under forward and reverse bias were studied using electrochemical impedance spectroscopy (EIS). Hereafter, an anion exchange membrane (AEM), cation exchange membrane (CEM) and BPM were examined individually and in series using EIS, to compose a simplified equivalent circuit for a whole ABFB triplet.
For the effect of different current densities on the BPM, an exponential decline of the non-ohmic resistance of the diffusion boundary layer (DBL) was found as a function of the current density. Under forward bias, the DBL resistance becomes significant. The resistance of the water dissociation reaction (WDR) was minimal under all current densities. Flow experiments validate a depletion and enrichment of the DBL during water association and dissociation, respectively.
To compose a simplified equivalent circuit for an AEM, BPM and CEM in series, it was found that the specific resistances of all membranes can be summed at their respective frequencies. This made it possible to construct a simplified equivalent circuit for a whole triplet containing an AEM, BPM and CEM.

The unprecedented increase in mean ambient temperature due to immoderate CO2 emissions has shifted the energy policy towards the replacement of fossil fuels with renewable energy sources. Nevertheless, the intermittency of these technologies renders them unsuitable for reliable energy supply. A prominent solution is the utilization of renewable energy to produce green hydrogen that can be stored for later use.

The most environmental-friendly viable method for green hydrogen production is the electrolysis of water. Between the already existing types, alkaline electrolyser is the one with the highest technological maturity and lowest cost of hydrogen production. Despite that, the low energy efficiency, and the energy losses associated with the materials and the geometrical configuration make it difficult to produce hydrogen at competitive prices compared to fossil fuels. The connection with renewables exposes the electrolyser to variable loads that negatively affect the life of materials and the purity of hydrogen since the operating conditions like temperature and electrical current change constantly. Previous research work has shown that the energy losses owing to the electrode kinetics, diaphragm, and electrolyte resistivity are responsible for higher energy consumption than the thermodynamic minimum. These losses are a strong function of temperature and current density. This research project aims to investigate the performance of the electrolyser under fluctuating working conditions to get an insight into how energy consumption and efficiency are affected.

The research objectives are encountered by building a physical model that accurately predicts the electrochemical behavior of the cell under various circumstances and simulates the thermal response of the electrolyser. The numerous experiments performed at XINTC’s laboratory enabled the improvement of the model’s accuracy and the consideration of effects that are not easily confronted in a purely theoretical investigation. The results showed that the model accurately describes the experimental data of the cell with less than a 2% discrepancy. The model was also used for simulating the cell behavior under different pressures, electrodes, and diaphragm materials. The elevated temperatures and highly electroactive materials such as Nickel significantly reduce the voltage of the cell. Regarding the thermal model of the theoretical electrolysis system, sensitivity analysis of the current density, ambient conditions, and electrolyte flow was performed. The temperature across the system was uniform at an electrolyte volume flow of 5 Lt/min and the cooling load accounted for 20% of the electrolyser power. The thermal modeling of the real experimental setup was successful (<1.5°C deviation) at high current densities indicating the sound assumptions of the model. On the contrary, at low current densities, the discrepancy between the model and the experiments reached 10% due to the additional heat generation induced by the shunt currents. For that reason, a simplified electrical circuit analysis was formulated and included in the model. This led to reducing the deviation to less than 5% and at the same time calculation of the current efficiency in the absence of measuring devices was achieved.

A water electrolyzer is an electrochemical device that produces hydrogen and oxygen from water via electricity. A sustainable way of producing hydrogen with renewable electricity, also known as green hydrogen, could enable its use in decarbonisation of sectors that depend on fossil fuels. However, to scale up the production of green hydrogen, larger water electrolyzers are required, which presents challenges. Water electrolyzers produce hydrogen in the form of bubbles, which are transported away through outlet channels. The geometry of the outlet channel is important for performance and safety. A channel that is too wide can result in unwanted ionic current leakage, reducing efficiency and increasing the risk of explosion. To reduce the leakage current, the electrical resistance in the channel can be increased to make it difficult for ions to flow through. However, a narrow channel can increase the hydraulic resistance, leading to clogging due to bubble build-up. This study focuses on the characterization of the slug flow regimes that eventually lead to clogging of the outlet channel in a novel electrolyzer design. To characterize the flow when blockage develops, electrochemical tests with industrially relevant 6M KOH were carried out at room temperature and ambient pressure in conjunction with high-speed video recording. The examined channel was 2 mm high, 3 mm wide, and 40 mm long. The experiments showed that clogging occurs for certain flow regimes. The hydraulic configuration caused clogging at the hydrogen side, which caused the liquid flow prefer to travel via the anode rather than the cathode, which was reflected in the temperature plots. Clogging did not occur when the superficial gas velocity was greater than 0.3 cm/s, which corresponds to a current density of 140 mA/cm2. The clogging was also not visible at a superficial liquid velocity greater than 0.25 cm/s. Below these values, clogging is expected to take place for the investigated channel dimensions. The bubble diameters in the bubble layer in front of the channel were measured with a microscopic camera. The mean bubble diameter was 0.4 mm, which is 80% smaller than the height of the channel through which they flow. However, exceptional bubble sizes that were 3 to 4 times the channel height, which ultimately led to clogging of the channel, were also present. The bubble layer thickness and velocity at increasing liquid flow rates and current densities were measured using a high-speed camera. At low current densities, the velocity and thickness of the bubble layer were not affected by the liquid flow rate. At higher current densities, a higher liquid flow rate appeared to reduce the thickness of the bubble layer, but not the velocity of the bubble layer. The bubble layer thickness as well as the velocity strongly depend on the applied current density (and thus the gas flow velocity). This knowledge might be, when the flow regimes are scaled to larger electrolyzer dimensions accordingly, essential for future electrolyzer manufacturers and operators for the operation and design of larger-scale electrolyzers. It will give insight into which flow regimes should be avoided in order to avoid clogging and thus explosion risks during operation.

In an effort to make water electrolysis more efficient and simple, flow-through electrolyzers eliminate the need for a gas-separating membrane between the electrodes. However, replacing the membrane with a forced flow field brings new challenges and considerations. In this research, the design of a flow-through electrolyzer is optimized for minimum voltage losses due to bubble formation, pressure drop, and electrolytic resistance. These parameters are all influenced by the potassium hydroxide concentration, the inter-electrode gap and the feed flow rate. The effect on the power dissipation of a flow-through electrolyzer is analytically modelled and validated using experiments, where the design variables are varied.

Additionally, the effect on the resistance in the electrolyte of keeping the gasses dissolved in the electrolyte is studied using IV curves of degassed electrolyte. The gasses in the electrolyte were purged with a vacuum chamber. This degassed electrolyte showed a reduction in overpotential of 150 mV at max. However, this reduction in overpotential was outweighed by the energy requirements to degas the electrolyte. Besides degassing, the effect of suppressing bubble formation by different flow rates was investigated. It was found that there was no noticeable reduction in overpotential between the state where bubbles are suppressed and bubbles were formed.

In addition to the effect of keeping the gasses dissolved, an analytical model was constructed to describe the necessary flow rates to mitigate the electrical resistance due to bubbles in the electrolyte. In the analytical model, solubility plays a big role in determining the necessary flow rate, as solubility is related to the emergence of bubbles. Contrarily, experiments showed the effect of solubility was found to be rather low. By varying the gap width, it was shown that the shear rate at the wall is a better indicator for bubble removal and therefore reduction in resistance due to bubbles.

Furthermore, the shape of the discharge channel was changed to promote uniform flow across the electrodes. The uniform flow should make the product removal at every part of the electrolyzer equal, such that there are no stagnant zones where gas accumulation could build up. This new discharge channel shape was analysed using COMSOL Multiphysics by comparing it with a conventional straight discharge channel. The variable discharge channel outperformed the straight discharge channel in creating a uniform flow across the electrodes for Euler numbers bigger than 10, meaning that the inertial forces are negligible compared to the pressure drop. During experiments, the electrolyzer with a variable discharge channel was tested. This electrolyzer configuration did not perform as well as expected from the theory and simulations, having a higher pressure drop and electric resistance than the conventional electrolyzer. The reason why the variable discharge channel performed poorly was inconclusive.

Lastly, the performance of the various variables was evaluated using the total power dissipation as a function of the current density. This took both the pressure drop across the system and the electrical power consumption of the electrolyzer into account. It was found that using a high electrolyte of 6M potassium hydroxide (KOH) together with a small inter-electrode gap gave the lowest energy dissipation per kg produced hydrogen gas. However, increasing the KOH concentration increases the viscosity and thereby the pressure drop. The theoretical optimum for KOH concentration was calculated to be 5M for this flow-through electrolyzer.

This study focuses on water electrolysis and aims to provide suggestions for the company XINTC Global, for the design of the electrolyser and the accompanying flow network. The most important parameters for this project are the dimensions of the cell, the geometry of the piping network, the electrode gap, the volumetric flow rate and circulation velocity. These parameters influence the performance of the cell. As a result, the production of hydrogen is optimized by fine-tuning these parameters. In addition, the performance in different electrolyser geometries is maximized assuming free circulation of the electrolyte. The geometries differ in the number of cells and the stacking arrangement and they are the single cell, single module and horizontally stacked geometries. The performance indicators are the flow velocity through the electrode gap and the volumetric velocity through the electrode gap, which is a product of the velocity, electrode gap and electrode depth. Furthermore, suggestions on the other dimensions are given, with the aim to maximize the flow rate or velocity. To study the behaviour of the electrolysis system, I developed a Python code that simulates the flow through the electrolysis cell and channel network. This numerical model is compared to an analytical model and experimental data from the literature. Finally, the simulations predicted that the optimum electrode gap width is dependent on the total number of cells. The results with the turbulent flow model, showed that the optimum electrode gaps were 1.85 mm 0.250 mm and 0.250 mm for a single module configuration with 1, 60, and 240 electrolysis cells, respectively. Moreover, the horizontal stacking arrangement of the modules, resulted in larger optimum electrode gaps, for the same number of cells.