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Thermal management influence on the performance of edge-cooled 1 kW PEM fuel cell stack without external humidification of reactant gases was studied by means of advanced numerical analysis. A developed computational fluid dynamics model was used to study the influence of bipolar plate materials, operating pressure along the reactant channels and different cooling fin configurations on the water and heat balance in the stack. For this purpose, two numerical approaches were considered and compared for a prescribed load profile: lumped model and novel real time transient model. Results indicate that maximal and average temperatures of the stack are linearly correlated to the thermal conductivity of bipolar plate materials and maximal temperatures can be significantly higher. It was found that the operating delta pressure can be manipulated in order to increase the stack performance and that cooling fin redesign plays a major role in the overall temperature uniformity across the stack.

Copper sorption on NaX zeolite using a pitched blade turbine (PBT) was carried out in a baffled and unbaffled batch laboratory reactor. The impeller to reactor diameter ratio (D/dT=0.32), the impeller clearance to impeller diameter ratio (C/D=1.03) and the impeller off-bottom clearance (C/H=0.33) were kept constant. Impeller speed was equal to the lowest impeller speed for complete off-bottom suspension i.e. just suspended impeller speed (NJS) which was determined according to Zwietering 1s criterion for three different zeolite masses in both, baffled and unbaffled, batch reactors. Sorption is regularly carried out in stirred batch reactors without evaluating which mixing speed is necessary to avoid compromises in yield and cost. Mixing in the batch reactor at the NJS is a compromise between these conflicting objectives. Besides kinetic sorption experiments and kinetic analyses of gained data, to get insight into the hydrodynamic behaviour of the system, transient multiphase computational fluid dynamics simulations were performed. The NJS speed is related to the particle settling speed, i.e. the suspension mass concentration, and for that reason it changes depending on the zeolite suspension mass concentration. At higher zeolite suspension mass concentration, a higher NJS speed must be applied to ensure a complete suspension state, i.e. more energy must be applied. The differences in NJS speeds for the zeolite suspension mass concentrations investigated are not substantial, and for this reason, the increase in mixing intensity for the tested systems is not significant. From the results can be concluded that the hydrodynamic conditions used do not significantly affect the maximum amount of copper sorbed, but it is significantly affected by zeolite mass. The results also showed that the reaction is generally faster in the unbaffled reactor, and that the values of NJS and PJS were higher in an unbaffled reactor regardless of the zeolite mass used.

To gain a better understanding of the solidification process the temperatures were recorded using a high number of thermocouples, while the temperature profiles were precisely controlled using PID regulation. Computer simulations were developed for the cases which show the best performance and optimization was carried out.

Mehaničko miješanje se primjenjuje u čitavom nizu proizvodnih procesa. Potpuno razumijevanje ove operacije počiva na poznavanju hidrodinamičkih uvjeta koji vladaju u miješalici temeljem kojih je moguće provedbu miješanja i optimirati. Ukoliko se miješanje provodi dvama miješalima, hidrodinamika u promatranom sustavu postaje znatno kompleksnija u odnosu na sustave s jednim miješalom. Cilj ovog rada je bio sagledati utjecaj udaljenosti dvaju miješala od dna posude (C/D) na karakteristične parametre miješanja u miješalici volumena 14.15 dm3. Miješanje se provodilo korištenjem dvaju turbinskih miješala s lopaticama nagnutim pod kutom od 45° (PBT) čija je međusobna udaljenost (S/D) zadržana konstantnom. U radu je eksperimentalno određeno vrijeme homogenizacije, Ntm, mjerenjem vremena do ustaljenja potencijala otopine nakon injektiranja trasera. Pri odabranim brzinama vrtnje miješala mjeren je i zakretni moment na osovini miješala kako bi se odredio utrošak snage. Numerički model je razvijen u računalnom programu ANSYS™ Fluent v17.2. Isti je korišten za prikaz specifičnih tokova fluida, kao i kontura rasipanja privedene energije unutar miješalice. Ovakav kvantitativan i kvalitativan opis miješanja omogućava potpuniju analizu utjecaja miješanja na pojedine stupnjeve provedbe separacijskih procesa, prije svega šaržne kristalizacije za koju je i razvijen.

The porous electrode under the rib area suffers from lower local oxygen concentration and more severe water flooding than that under the channel, which significantly affect the performance of PEMFCs. To improve the oxygen concentration and water drainage under the rib, a series of novel flow fields with auxiliary channels equipped with through-plane arrayed holes were manufactured by 3D metal printing, and the cell performance, ohmic resistance and pressure drop were experimentally studied, respectively. The novel fields were based on the sophisticated modification of traditional serpentine and parallel flow fields, that significantly improved the cell performance at high current density with an optimal number or length of the auxiliary channels, owing to the tradeoff between the electric resistance and mass transfer under the rib. This novel flow field design solved the trilemma of performance, pressure drop and manufacture feasibility through the implementation of 3D printing technology.

The main objectives for the design of modern PEM fuel cells are to ensure minimize costs and size and ensure operation at elevated power densities with minimized accumulation of water inside the flow fields. Currently, number of authors are dedicated to increasing the flow field complexity, resulting in higher performance but also elevated manufacturing costs. By minimizing the complexity of the flow field and avoiding complex 3D structures, the main candidate still remains the serpentine flow field due to favorable characteristics of low pressure drop and good water removal. Due to discrepancies found in literature, in this work, the focus is on investigation of the liquid water accumulation inside the flow field of PEM fuel cell using up- to-date numerical model and full-scale single cell geometry with active area of 25 cm2, experimentally and numerically using computational fluid dynamics modeling. The initial simulation model is calibrated using experimental data and later modified according to literature recommendations, outlining the influence of designs on water accumulation, resulting in significant performance improvement using simple geometry. The future goal of the research is upscaling the developed geometry for 50 cm2 active area and comparing the numerical results with experimentally obtained locally resolved current density distribution.

Start-up and variable load operation of a 1 kW air-cooled stack is investigated via computational fluid dynamics (CFD) analysis. The results of the transient analysis indicate that the temperatures inside the air-cooled stack are up to 27 °C higher when compared to the temperatures on the outside walls of the bipolar plates and this indicates a requirement for development of a new method for PEM fuel cell operation to increase the relative humidity of the reactants internally and prevent membrane dehydration. The novelty of this work is the method for increasing the relative humidity along the active area of the cell by intentionally increasing the pressure drop across the flow field, resulting in higher performance of the cell.

Air-cooled proton exchange membrane (PEM) fuel cell stacks are gaining momentum as power sources for portable applications such as electric bikes, unmanned aerial vehicles (UAVs), forklifts, etc. Usually the power of portable stacks (PS) is up to several kW. Most of the heat generated during PS operation is dissipated into the surrounding atmosphere via forced air convection along the cooling channels or edge-cooling fins, while the higher power stacks require liquid cooling. The emphasis during the design phase of PS is primarily directed towards determining the most suitable material for bipolar plates and the accompanying thermal management.

A novel flow field was designed by deploying auxiliary channels inside the partially hollow ribs and drilling a series of arrayed holes on the auxiliary channels. This novel design rationally utilizes the ribs of the current collector and improves the volumetric efficiency of the parallel channels, leading to improved cell performance and homogeneity of current distribution. A three-dimensional, two- phase flow model was developed to analyze the influence of a variety of parameters on the oxygen and water saturation profiles, cell performance, and current uniformity. It was found that the combination of auxiliary channels and hole array provides an extra pathway for reacQ4 tant transport and water removal. A reasonable optimization of the flow field geometry, for example, the hole size, the area ratio of arrayed holes and auxiliary channels, nonuniform distribution of arrayed holes, could further improve the cell performance and current uniformity at an extremely low pressure drop.

In this work, the focus is on reducing the need for electricity per mole of hydrogen, and on the investigation of the effect of the magnetic field on green hydrogen production, i.e. hzdrogen production via water electrolysis using solar energy, and current density, depending on the different Lorentz force direction. Energy consumption analyzes will be conducted for different types of porosity of the electrode material. The new design of an electrolyzer is to be constructed without a membrane with the additional introduction of a magnetic (and optical) field, which should ensure higher hydrogen production efficiency. Along with improving the efficiency of hydrogen production processes, processes involving hydrogen transportation need to be addressed. Hydrogen transport is a crucial factor in the costs, emissions, and energy consumption associated with hydrogen routes involving the central production at the production plant. At present, transportation to existing pipelines seems to be the most economical solution for transporting large quantities of hydrogen over long distances. All these would allow a drastic reduction in the cost of mass production of hydrogen, which, along with transportation through existing gas pipelines, would make technology in the price range with today’s conventional fuels/energy sources. This would open opportunities for the industry to become involved in the production of the electrolyzers, which would enable job creation and education of experts in this specialized field of hydrogen technology, which would have a direct impact on the economy.

Various methods for improving the performance of PEM fuel cell without external humidification are shown as well as novel flow fields for improved liquid water removal at higher currents.

Start-up and variable load operation of a 1 kW air-cooled stack is investigated via computational fluid dynamics (CFD) analysis. The results of the transient analysis indicate that the temperatures inside the air-cooled stack are up to 27 °C higher when compared to the temperatures on the outside walls of the bipolar plates and this indicates a requirement for development of a new method for PEM fuel cell operation to increase the relative humidity of the reactants internally and prevent membrane dehydration. The novelty of this work is the method for increasing the relative humidity along the active area of the cell by intentionally increasing the pressure drop across the flow field, resulting in higher performance of the cell. The CFD model also investigates the influence of altering the bipolar plate material on the overall heat transfer inside the cell in order to achieve a similar effect with success in both cases

The occurrence of liquid water inside Proton Exchange Membrane (PEM) fuel cell can have detrimental effects on the performance of the cell, resulting from non-uniform reactant supply along the active area of the cell or neighbouring channels and the accompanying non- uniform heat transfer, to severe flooding and starvation of the cell. Liquid water transport inside the cell is investigated by developing a transient 3D numerical model using Computational Fluid Dynamics (CFD) analysis and Volume of Fluid (VoF) methodology. Several different commercially used bipolar plate materials, with different hydrophobicity, are mutually compared in regard to the dynamic water transport and the transient information on the gas diffusion layer substrate surface coverage. The effect of gas diffusion layer aging is also investigated in conjunction with different bipolar plate materials. The numerical model is developed for single-channel serpentines of a commercial 100 cm2 single- cell. Numerical analysis is also conducted to correlate the required pressure potential between the inlet and outlet of the cell to expel the liquid water plug for a rectangular channel with uniform cross-section and trapezoid channel with two reductions in the cross-section in the downstream direction. The influence of the cell positioning, i.e. gravity, on the net liquid water transport is also investigated and the results also indicate the potential for development of a diagnostic strategy for determining the predominant water transport mechanism inside the channel based on the net pressure drop. This work represents the upgrade vs. common approach where only a small portion of the channel is observed and gives indication of the requirement for more thorough analysis of nowadays commonly investigated channels with different modifications of the channel geometry, and also gives insight in the significance of surface coating of the channels on long term performance of the cell by comparing the CFD data with new and aged substrates for different configurations for the first time.

The performance and lifetime of polymer electrolyte fuel cells are very much dependent on internal conditions and modes of operation. During steady state operation of the fuel cell, the conditioning parameters like stoichiometry, pressure, temperature and humidity can be continuously adjusted to achieve a constant high performance of the fuel cell. In dynamic operation of the fuel cell, it is very challenging to monitor and control the fast changing conditions inside the cell. Fast and disruptive changes are performed in these experiments either by load or voltage steps. By observing the voltage response of a load change it is possible to get fundamental details on the mass transport mechanisms inside the fuel cell.

— Numerical analysis is conducted using Computational Fluid Dynamics (CFD) modelling to investigate different strategies for sea water mixing and species homogenization via bubbling for a commercial-scale sea vessel ballast tank. Mixing of the ballast sea water is achieved by incorporating a novel approach, the internal combustion engine exhaust gases are used instead of inert gases, supplied by the compressors, to suppress the genesis and multiplication of invasive biological organisms inside the ballast sea water tanks. Additionally, the effect of mixing is also investigated as means to minimize the occurrence of sea water freezing inside the ballast tanks while the vessel is cruising in sub-zero atmospheres, achieved by disturbing the free surface of the sea water inside the tanks. One commercially available configuration of the supply pipes is chosen as a reference, and 2D CFD model incorporating Volume of Fluid (VoF) method is developed to study the transient formation and transport of the exhaust gas bubbles inside the ballast tank. Several different design configurations are analysed as well as different bubble mass flow rates to find the most efficient and economically viable solution.

— Archimedes screw turbine (AST) power output and efficiency are investigated in this work via an interactive combination of experimental testing and numerical analysis. The experimental approach considers different configurations of the AT inclination angle (21°, 25° and 30°) coupled with different angular velocities (from 60 to 120 RPM) and water flow rates (10 l l/ss-1 and 12.81 l/s s- 1), while the numerical model considers single flow rate with other variable parameters. The numerical analysis is conducted using Computational Fluid Dynamics (CFD) modelingmodelling. The transient three- dimensional multiphase model incorporating Volume of Fluid (VoF) method was developed for the simulation of the two distinct phases, air and water and their interface, i.e. free surface of water. The sliding mesh methodology was employed to simulate the rotational movement of the AT to ensure the most accurate and realistic simulation environment as possible, when compared to the experimental readings. Transient numerical results are compared with experimental data with reasonable agreement and outlined the influence of inclination angle and rotational speed of the Archimedes screw on the net performance of the hydro turbine. The numerical results also gave insights for in further possibility focused on optimization of the hydro turbine design parameters as well as operating conditions and indicate that the future numerical model must also include the accumulation tanks because they have significant effect on the net results.

—The Sun is the most prominent renewable energy source, while photovoltaics (PV) are the most popular solar energy harvesting product. A small part of solar radiation is converted to electrical energy, with the rest reflected, transmitted and converted to heat, respectively resulting in heat generation in the PV module. The efficiency of the photovoltaic module decreases due to higher operating temperatures. In order to increase the overall efficiency of the PV module, the active cooling technique is incorporated. On the back of the photovoltaic, there is a fixed tank through which the water coolant flows. Three-dimensional computational fluid dynamics (CFD) model for steady-state heat transfer is developed and tested. This numerical analysis deals with the heat transfer through the PV module and provides insight into the intensity of heat extraction from the module thanks to the active cooling system. The developed numerical model predicts output parameters based on three input parameters that can be varied as desired without triggering CFD simulation. The heat source, the coolant inlet velocity, and the ambient temperature are the input parameters that are varied while the coolant inlet temperature is fixed at 290 K. The average PV-module temperature, the pressure drop in the tank and the coolant outlet temperature are output parameters. To create a Response Surface, 30 design points are created, each point representing a unique combination of the input parameters. The predictions of the output parameters are made from the generated Response Surface and compared to the simulation results for 30 design points. The average deviation of the predictions, generated from the Response Surface, is less than 1% when compared to the simulation results for the design points.

Polymer membrane fuel cells are of high interest for automotive applications because of their high efficiency and zero emission propulsion system. The main challenge to bring these systems into the commercial market is the lifetime and the costs. The permanent fluctuation in energy demand leads to harmful conditions inside the fuel cell. Situations like sudden high load changes, start up and shut down procedures as well as fuel starvation and improper humidification of the cell can cause either very high or low voltage levels. This undesired voltage levels accelerate the degradation of the cell in terms of carbon corrosion, platinum oxidation and membrane decomposition. Experiments with single cells are a good indicator which condition is harmful to the cell but do not reflect the behavior of each cell inside a stack. Commercial fuel cell stacks are built up of several hundred single cells and not all effects during operation can be simulated by single cell experiment e.g. the pressure drop and the humidity increase within the gas flow channels. Also the system influence in terms of thermal and dynamic response velocity cannot be done with single cells.

Numerical model is developed using CFD methodology and the results are compared with experimentally obtained data on a segmented cell to estimate the maximal temperature inside the catalyst layers of PEM fuel cell during operation.

Fakultet elektrotehnike, strojarstva i brodogradnje Sveučilišta u Splitu sudjeluje u europskom projektu Giantleap (iz programa 'Obzor 2020.'). Voditelj projekta je institut SINTEF iz Norveške, a partneri su proizvođač autobusa tvrtka VDL iz Nizozemske, tvrtke Elring Klinger i Bosch Engineering iz Njemačke i FCLab Sveučilišta Franche-Comte iz Francuske. Cilj projekta je povećati doseg električnog autobusa s baterijama ugradnjom modula s gorivnim člancima i spremnicima vodika. ElringKlinger je isporučio gorivne članke, Bosch je odgovoran za dizajn cijelog sustava, odabir i isporuku ostatka komponenti i njihovu integraciju i za ispitivanje cijelog sustava, VDL integrira sustav u autobus, dok je FESB zadužen za dijagnostiku degradacije gorivnih članaka, a FCLab za prognostiku preostalog vijeka trajanja gorivnog članka i cijelog sustava. Projekt se uspješno privodi kraju i integracija modula je upravo u tijeku, nakon čega će autobus biti ispitan. Očekuje se da će modul udvostručiti doseg autobusa s 200 na 400 km.

The presentation illustrates the results of accelerated stress tests on the degradation of PEM fuel cells

Performance of proton exchange membrane (PEM) fuel cells depends on water and heat management of the cell. High efficiency is only achieved if the proton conductive membrane is characterized by high membrane water content, which can be accomplished only if the relative humidity of the reactants is close to 100% along the anode and cathode side of the cell. Commercial PEM fuel cell stacks are operated in isothermal mode, i.e. the temperature gradient along the active area of the cell is minimized by relatively high mass flow rate of the coolant, therefore they require the addition of external humidifiers in order to achieve high efficiency. However, the implementation of external humidifiers results in occurrence of higher quantities of liquid water under transient operation and at higher currents in general, hindering the performance and limiting the operating range of the cell. Newly developed concept presented in previous studies termed ”variable temperature flow field” (VTFF) is capable of achieving high efficiency of the cell without the requirement for external humidification. The concept is based on prescribing and maintaining the temperature profile along the cell in close proximity to the water vapor saturation profile extracted from Mollier’s h-x chart. The desired temperature profile is established and maintained via coolant mass flow rate control. This work outlines the results of the research through interactive combination of computational fluid dynamics (CFD) analysis and experimental verification of the variable temperature flow field concept for 2 different MEAs at 3 different current densities.