We present fundamental calculations to identify operating conditions of a proton exchange membrane electrolyzer such that the product gases are exactly saturated with water vapor. The required stoichiometric flow ratios depend strongly on the electrolyzer temperature and reactant pressures, and they are below xi = 3 which necessitates a symmetric electrolyzer design and uniform water feeding. Preheating the incoming water leads to a voltage gain in the order of 50 mV, and it is shown, how the electrolyzer temperature can conceivably be controlled via the water flow rate. The analysis results in diagrams to determine suitable operating conditions for three different electrolyzer operation modes: standby operation to reduce the startup time, normal operation and high-power operation with efficiencies of 96%, 91%, and 86%, respectively. A comparison with literature data gives indications about the expected current densities at the respective voltages. Finally, it is suggested that electrolyzer operation where the anode side pressure is at a partial vacuum can facilitate the proposed operation mode as well as reduce the iridium loading.

A computational fluid dynamics analysis of the anode side of a proton exchange membrane (PEM) electrolyzer cell has been conducted. The geometry is symmetrical and allows for the investigation of a single feed channel and a single exhaust channel in an interdigitated flow field. The model utilizes the Eulerian approach and thus solves a full set of conservation equations for both gas and liquid phase. Moreover, it is non-isothermal and it includes phase change of water. The operating stoichiometric flow ratio results in segregated flow in the horizontal flow channels. At a current density of 1.0 A/cm2, a local hot spot with a temperature increase of 7 °C is predicted. A reduction in the operating pressure below atmospheric pressure results in a more favorable concentration ratio of water vapor to oxygen at the PTL/CL interface, and the temperature distribution is more even. However, when the outlet pressure is too low, the outlet temperature is below the inlet temperature which makes this operation mode unfeasible. Adjustment of the back pressure can generally be used to control the temperature of the electrolyzer.

A novel design of a proton exchange membrane electrolyzer is presented. In contrast to previous designs, the flow field plates are round and oriented horizontally with the feed water entering from a central hole and spreading evenly outward over the anode flow field in radial, interdigitated flow channels. The cathode flow field consists of a spiral channel with an outlet hole near the outside of the bipolar plate. This results in anode and cathode flow channels that run perpendicular to avoid shear stresses. The novel sealing concept requires only o-rings, which press against the electrolyte membrane and are countered by circular gaskets that are placed over the flow channels to prevent the membrane from penetrating the channels, which makes for a much more economical sealing concept compared to prior designs using custom-made gaskets. Hydrogen leaves the electrolyzer through a vertical outward pipe placed off-center on top of the electrolyzer. The electrolyzer stack is housed in a cylinder to capture the oxygen and water vapor, which is then guided into a heat exchanger section, located underneath the electrolyzer partition. The function of the heat exchanger is to preheat the incoming fresh water and condense the escape water, thus improving the efficiency. It also serves as internal phase separator in that a level sensor controls the water level and triggers a recirculation pump for the condensate, while the oxygen outlet is located above the water level and can be connected to a vacuum pump to allow for electrolyzer operation at sub-ambient pressure to further increase efficiency and/or reduce the iridium loading.

This work describes an innovative three-dimensional model of a proton exchange membrane electrolyzer. For the first time, a multi-phase model has captured segregated channel flow together with multiphase flow in a porous medium, as well as heat transfer and phase change employing an Eulerian multiphase model. The novel electrolyzer design investigated employs a symmetrical, interdigitated flow field to facilitate even water distribution. In the current case, a hot spot is predicted with a temperature increase of 7 °C at a current density of 1.0 A/cm2. The flow field plates are horizontally oriented, and it is shown that gravity plays an important role in the electrolyzer design and orientation. A parametric study shows, for the first time, the effect of operating a PEM electrolyzer at sub-ambient anode pressure to favorably adjust the concentration ratio between water vapor and oxygen in the anode compartment. This ratio is increased by a factor of 5.6 when the pressure is decreased from one bar to 500 mbar.

The operation of proton exchange membrane fuel cells (PEMFC) at current densities in the order of 10 A cm-2 is investigated. Such high current densities are conceivable when the fuel cell employs cathode flow microchannels with dimensions in the order of 150-200 mu m to obtain a high pressure drop in conjunction with the perforated metal transport layers to enhance waste heat removal. Employing a state-of-the-art computational fluid dynamics model that is based on the Eulerian multiphase approach, high current density fuel cell operation is studied, and the interplay between the operational pressure, temperature and stoichiometric flow ratio is investigated in detail. The results suggest that a combination of a current density in the order of 8-10 A cm-2 together with a low stoichiometric flow ratio around xi=1.2-1.5 is feasible, while the air only needs to be minimally humidified because the high pressure at the cathode inlet leads to immediate self-humidification of the membrane. If indeed practical, this mode of operation can lead to unprecedented high power densities. Finally, the molar ratio of the inlet water to the product water Theta is calculated, and its importance for the sizing of the humidifier is highlighted.

The crossover of the product gases hydrogen and oxygen in alkaline electrolyzer operation is a critical factor, severely limiting the operational window in terms of current density and pressure. In prior experiments, it was found that a large degree of oversaturation of the reaction products in the liquid electrolyte phase leads to high amounts of crossover. We are proposing to reduce this amount of oversaturation by introducing micro-cracks in the Zirfon diaphragm. These cracks are meant to induce the formation of hydrogen and oxygen bubbles on the respective sides, and thereby reduce the oversaturation and amount of crossover. In theory, the size of the bubble corresponds to the size of the cracks, and from our computational fluid dynamics simulations, we conclude that the bubbles should be as large as possible to minimize the ohmic resistance in the electrolyte phase. The results suggest that an increase in bubble diameter from 50 microns to 150 microns results in a 10% higher current density at a cell voltage of 2.1 V.

Indirect evaporative coolers (IECs) are becoming a viable alternative to the more energy-intensive traditional HVAC systems for space cooling, especially in arid regions. In this work, a recently developed computational model of an IEC was used to conduct a parametric study. The model employs a spray dryer model to track the flow path and evaporation rate of droplets. The key parameters investigated were the temperature of the droplets, a bypass effect where the amount of exhaust air and water was reduced to as low as 10%, and the length of the heat exchanger. The results suggest that the wet bulb efficiency could be increased from the previously observed 35% to 72.5% if the water temperature is decreased to 16 °C. In order to drastically increase the performance, the heat exchanger length should be increased from 50 cm to 100 cm, which could still end up in a more compact design overall as fewer plates are required. The bypass study resulted in peak performance when 40% of the secondary air flow was used as working air in conjunction with a proportional reduction in water usage. Overall, the computational model has been employed in an attempt to reduce the bulkiness, increase the efficiency and reduce the water consumption of such a system.

This paper focuses on proton exchange membrane fuel cell (PEMFC) operation at current densities in the order of 6 A/cm2. Such high current densities are conceivable when the traditional carbon fiber papers are replaced with perforated metal plates as the gas diffusion layer to enhance waste heat removal, and at the same time the relative humidity inside the fuel cell is kept below 100% by applying appropriate operating conditions as was found in previous one-dimensional modeling work. In the current paper, we applied a three-dimensional, multi-phase computational fluid dynamics model based on Ansys-CFX to obtain additional insight into the underlying physics. The calculated pressure drops are in very good agreement with previous one-dimensional modeling work, and the current densities in all case studies are in the order of 5–6 A/cm2, but different from the previous one-dimensional study, the results suggest that the relative humidity is very close to 100% throughout the entire channel length when the inlet relative humidity is 100%, ensuring best hydration cell conditions and hence best performance. Importantly, the model results suggest that fuel cell performance at a high current density in conjunction with relatively low stoichiometric flow ratios around 1.5–2 is possible.

A one-dimensional computational model has been developed that can be used to identify operating conditions for the cathode side of a proton exchange membrane fuel cell such that both the inlet and outlet relative humidity is equal to 100%. By balancing the calculated pressure drop along the cathode side flow channel with the change in molar composition, inlet conditions for the cathode side can be identified with the goal of avoiding channel flooding. The channel length, height, width and the land-to-channel width ratio are input parameters for the model so that it might be used to dimension the cathode flow field. The model can be used to calculate the limiting current density, and we are presenting unprecedented high values as a result of the high pressure drop along the flow channels. Such high current densities can ultimately result in a fuel cell power density beyond the typical value of 1.0–2.0 W/cm2 for automotive fuel cells.

A thermodynamic, first law analysis was conducted to understand the temperature of a proton exchange membrane electrolyzer when operated at a constant voltage and feed water flow rate. When using the electrolyzer voltage, current, liquid water feed rate and temperature, the electrolyzer operating temperature can be calculated. Heat losses can also be accounted for but are currently neglected. Carpet plots show the resulting electrolyzer temperature under varying conditions. The calculated temperatures are in very good agreement with measurements conducted in our laboratories.

Indirect evaporative coolers (IECs) for air conditioning rely on liquid water being sprayed into the exhaust stream of used air to induce evaporation and cool down the incoming stream of fresh air in an indirect heat exchanger. This paper describes a computational fluid dynamics analysis that makes use of the particle transport model to simulate the evaporation of the water droplets at the exhaust side of an IEC using a pre-implemented spray dryer model. Critical parameters include the average size of the droplets and the amount of water sprayed into the system. In addition to droplet evaporation, the evaporation of water from the wet wall on the exhaust side is accounted for. The results show the calculated temperature field in both air streams, the pressure distribution, the relative humidity distribution at the exhaust side and the particle tracks. The predicted wet bulb efficiency of around 30–35% is moderate but in agreement with the literature to date, and it can be attributed to the small heat exchanger size. A parametric study investigated the effect of the droplet size and mass flow rate. At an average size of 50 microns and below, the effect of the mass flow rate is quite strong, while at a higher droplet size the mass flow effect is small. Overall, the model can be used to shed fundamental understanding in order to increase the performance of the IEC while maintaining its compactness.

A single-channel proton exchange membrane fuel cell model (anode side) based on computational fluid dynamics is used to investigate the possibility of operating a fuel cell at low stoichiometric flow ratios using completely dry inlet hydrogen. A case study of three different stoichiometric flow ratios (x = 1.01, x = 1.03, x = 1.05), three different operating temperatures (343.15 K, 347.15 K, 353.15 K), and three different operating pressures (1 atm, 1.2 atm and 1.5 atm) are presented. It is found that the predicted hydrogen concentration and relative humidity (RH) in the catalyst layer (CL) have opposite trends: the RH in CL decreases with increasing stoichiometric flow ratios, but it reaches 100% at the outlet. While hydrogen concentration in CL increases with increasing stoichiometric flow ratios and the largest difference is at the inlet, with a maximum of 3.6%. The results also suggest that PEM fuel cells may be operated in a stoichiometric flow ratio as low as x = 1.01 at the anode side. This cell operation would allow open-ended anode operation without a recir-culation system, thus significantly reducing system complexity and cost. The CFD code is disclosed to provide a starting point for more complex model development.(c) 2023 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/).

Green hydrogen has emerged as a key solution for high-intensity energy storage, with various technologies introduced to facilitate its production, including alkaline electrolyzers (AWE), proton exchange membrane electrolyzers, and solid oxide electrolyzers. While each of these technologies presents unique strengths and weaknesses, alkaline electrolyzers are particularly noted for their cost-effectiveness and robust structure in hydrogen production. However, a comprehensive understanding of the internal dynamics of the electrolyzer during operation requires detailed modeling. This study aims to delve into the dynamics of AWE, optimizing operational parameters and cell design to enable higher current operation without increasing voltage losses, thereby enhancing overall efficiency. Computational fluid dynamic modeling is employed to achieve this goal, focusing on gas bubble behavior within the electrolyzer, their impact on cell performance, and the dynamics of mass transfer between different phases. One drawback of alkaline electrolyzers is the crossover of hydrogen and oxygen through the membrane, resulting in reduced hydrogen purity and increased hydrogen loss. The preset study investigates the effect of electrolyte saturation levels on crossover, highlighting its significance. Additionally, a parametric study is conducted, varying bubble diameter across simulations to further understand its implications. Figure 1

To further reduce the size and cost of proton exchange membrane fuel cells, it is desired to operate at high current densities exceeding 2 A/cm2, and consequently achieve power densities above 1 W/cm2. In a prior one-dimensional modeling study that employed the Engineering Equation Solver (EES), it was suggested that proton exchange membrane fuel cells might operate at current densities as high as 10 A/cm2 and even more when appropriate operating and channel dimensions are chosen, and perforated metal plates are used as porous transport layers instead of carbon fiber papers. The current study verifies the general findings of the previous 1-D model by a 3-D multiphase computational fluid dynamics model in Ansys CFX to obtain additional insights. Operating a proton exchange membrane fuel cell at a constant humidity (100%, 80%, 60%) from the inlet to the outlet at the cathode side appears feasible, and results for a current density above 6 A/cm2 were obtained. This mode of operation is promising also for automotive applications.

Numerical modeling is a powerful tool for the virtual design and optimization of the next-generation proton exchange membrane fuel cells (PEMFC). While almost all of them are not open-source, which causes inconvenience and limits. In this paper, a three-dimensional, multi-component, multi-fluid, open-source model developed in the commercial CFD package (CFX) is used to investigate the species’ transport in a proton exchange membrane fuel cell. The model includes a complete fuel cell with both the seven-layer membrane-electrode-assembly and the gas flow channels and bipolar plates, which account for all major transport phenomena. In the porous medium, liquid water transport is dominant by the capillary pressure gradient; momentum loss describes the Darcy equation; and mass transfer between phases by a nonequilibrium phase change model. Multi-component gas phases are governed by convection and diffusion. Moreover, diffusion is the predominant transport mechanism for dissolved water between the membrane and catalyst layers. Consequently, the code of this model is shared on GitHub to provide a starting point and inspire further development and optimization.

We are proposing a conceptual membrane electrode assembly (MEA) of a proton exchange membrane water electrolyzer that includes a layer of graphene oxide (GO) at the cathode side. This GO layer primarily reinforces the MEA to allow operation at a higher pressure difference between the cathode and anode side. Additional benefits would be that a perfect GO layer would prevent both water and hydrogen crossover and thus would allow for pure, dry hydrogen escaping directly from the electrolyzer without losses due to hydrogen crossover, thus eliminating the need for hydrogen clean-up steps. The mechanical strength of graphene will also allow for a thinner polymer electrolyte membrane and could thus save cost. Finally, the effect of electro–osmotic drag on the water content in such an MEA is discussed, and it is argued that it could lead to an oversaturated membrane, which is highly desirable.

A computational study of the nitrogen purging of a solid oxide fuel cell stack enclosed in a hot box is presented. The stack operates on ammonia as a fuel, and in the case of a hydrogen leakage, the entire compartment is immediately purged with nitrogen to ensure that there are no regions with high oxygen concentrations. In addition to this, the speed at which a hydrogen leak can be detected is determined. The results are then compared to a case with a relocated nitrogen inlet. A computational fluid dynamics (CFD) model is developed using the Reynolds-averaged Navier–Stokes equations for compressible flow in combination with conservation of energy and species equations in OpenFOAM. The results suggest that for the maximum concentration of oxygen to be below 5%, the hot box should be purged for 35 s, corresponding to 1.1 kg of nitrogen, if the hot box was already heated. If the hot box was at T = 300 K, it should be purged for 95 s, corresponding to 3.0 kg of nitrogen. The purge of the heated hot box results in a heat loss of 18 kW on average. A leak could be detected in 3.2 s during open circuit voltage tests. Changing the location of the outlet does not affect the cold purge, but results in a minimum purge period of 48 s during the hot purge, and the leak could be detected in 2 s. This paper demonstrates how CFD methods can be employed in order to address questions related to hydrogen safety.

During the previous years, increasing awareness of the detrimental effects of greenhouse gas emissions along with the need to supply an increasing world population with electricity has given rise to investments in the field of green energy technology. In particular, research and development has focused on the production of “green hydrogen” which can be used as a source for a sustainable energy system. Green hydrogen can be made from water electrolysis provided the electricity stems from a renewable energy source. Among the different types of water electrolysers, the alkaline electrolyzer cell (AEC) is the most mature technology. Among its advantages compared to other technologies are the low capital expenditure and the simplicity of the system with proven components. However, the detailed heat and mass transfer mechanisms that occur in an AEC are far from completely understood. It is expected that further improvements of the technology and reduction in cost can be attained through a fundamental understanding of above-mentioned phenomena. In order to better understand the phenomena and the physics of such system, a numerical model is developed in this project. Using ANSYS Fluent 2021 R1, an isothermal, single phase, three-dimensional model is developed to replicate the phenomena in a small single cell. Equations such as Butler-Volmer and Nernst Equation are considered to describe the electrochemical part of the system. A simplified Nernst-Planck equation is also modelled to account with the diffusion and migration of charged species (in this case, the ion OH-). The flow is considered laminar, and the species conservation equation are written to solve the molar concentration of each species. The porosity of the electrodes is changed along the simulations to study its influence on the performance of the electrolyzer. Also, the initial concentration of the species is changed in order to evaluate its effect on the output of the system. First results suggest that the polarization curve acquired shows a strong consistency with prior experimental findings and outcomes from other simulation models. Figure 1

As the most mature technology within the electrolysis field, alkaline electrolyzers are expected to play an important role in the energetic transition. Advantages as their simplicity, modularity and low-cost components make it an interesting technology for the near future. However, the fundamental physics are complex as it includes multicomponent, multiphase flow, porous media, electrochemistry and heat transfer. To have a better understanding of these phenomena, the presented work is devoted to the development of a computational fluid dynamic model to better understand the behavior of all the species that are part of this technology.

To better accommodate changes in municipal solid waste (MSW) properties due to waste classification, advanced computational fluid dynamics (CFD) simulations are carried out for a 750 t/d MSW moving-grate boiler. A moving-grate bed model is developed and iteratively coupled to the freeboard simulation which is performed in Ansys Fluent. The model is first validated by the measurement data for the daily operation case incinerating current feedstock. Then, the model is deployed to investigate the impacts of feedstock change and adjust boiler operation for better accommodating the new MSW. The results indicate incineration of the new MSW leads to irrational utilization of oxygen, non-uniform temperature distribution and low mixing, while maintaining current operation conditions. Subsequently, adjustments of air supply and thermal input are proposed and conducted by the model, which address the potential issues and benefit boiler operation and energy recycling. Finally, the uniformity of velocity and turbulent kinetic energy which indicates mixing, are compared for different cases. The latter is increased by 51.39% and 81.04% after the adjustments of air supply and thermal input. The investigation provides solid references for incinerating new MSW in the current boiler. (C) 2022 Elsevier Ltd. All rights reserved.