Dynamic modelling of alkaline self-pressurized electrolyzers: a phenomenological-based semiphysical approach

This paper proposes a phenomenological based semiphysical model (PBSM) for a self-pressurized alkaline electrolyzer. The model, based on mass and energy balances, represents the dynamic behaviour of hydrogen and oxygen production using electrolysis. The model allows to anticipate operational variables as dynamic responses in the concentrations of the electrolytic cell, and variations in both, level and pressure, at the gas separation chambers due to the change in electric current. The model parameters have been adjusted based on experimental measurements taken from an available prototype and through a suitable identification process. Simulation results replicate the current dynamic response of the experimental self-pressurized electrolyzer assembly. This model proves to be useful in the improvement of the control of gas production rate in this kind of assemblies, both as a validated simulation platform and as a source of reduced order models for model-based control design.     

Graphene oxide modified non-noble metal electrode for alkaline anion exchange membrane water electrolyzers

This paper reports the performance of a graphene oxide modified non noble metal based electrode in alkaline anion exchange water electrolyzer. The electrolytic cell was fabricated using a polystyrene based anion exchange membrane and a ternary alloy electrode of Ni as cathode and oxidized Ni electrode coated with graphene oxide as anode. The electrochemical activity of the graphene oxide modified electrode was higher than the uncoated electrode. The anion exchange membrane water electrolyzer (AEMWE) with the modified electrode gave 50% higher current density at 30 °C with deionised water compared to that of an uncoated electrode at 2 V. Performance was found to increase with increase in temperature and with the use of alkaline solutions. The results of the solid state water electrolysis cell are promising method of producing low cost hydrogen.     

Valorization of the waste heat given off in a system alkaline electrolyzer-photovoltaic array to improve hydrogen production performance: Case study Antofagasta,Chile

The efficiency of an electrolyzer can be improved by preheating the water consumed, which is generally done by means of solar energy in PVT panels. In this research, the first objective is to determine whether it is possible to preheat the consumed water by using the residual heat given off by the electrolyzer itself fed by a PV array, and if the above is met, the second objective consists of quantify the benefits obtained in the performance of the system. The simulation is carried out over a period of one year, considering the meteorological conditions of the city of Antofagasta, Chile. The results indicate that it is possible to constantly maintain the water temperature consumed by the electrolyzer at its nominal value of 80 °C, since the energy contained in the waste heat is about 30 times higher than this hot water demand. Continuous operation at 80 °C compared to operation at variable temperature achieves an annual increase of 0.22% in hydrogen production and an average of 0.33% in electrolyzer efficiency. Moreover, by considering the thermal energy given off by the electrolyzer as useful output of the system, the overall energy efficiency increases by a relative percentage of 13%.     

Process modelling of an alkaline water electrolyzer

In this paper a model for the prediction of the product gas purity in alkaline water electrolysis is proposed. For the estimation of the exhaust gas compositions the operating conditions, such as current density, electrolyte flow rate, concentration and temperature as well as process management possibilities are considered. The development of the model relies on a classical process engineering approach and depicts the electrolysis cell through coupled continuously stirred-tank reactors. Furthermore, the mass transport phenomena between the phases are considered through the application of Reynolds and Sherwood correlations. Finally, the validation of the model is performed through experiments, which are carried out in a lab-scale electrolyzer with a 150 cm² zero-gap cell and KOH electrolyte at atmospheric pressure. This investigation reveals that gas purity in alkaline water electrolysis is mainly affected by mixing the anodic and cathodic electrolyte cycles, which transport dissolved electrolysis products into the opposite half cell compartments. However, this transport mechanism can be significantly reduced by adjustment of the operating conditions of the electrolyzer.     

Automized parametrization of PEM and alkaline water electrolyzer polarisation curves

A comprehensive literature review of current water electrolyzer modelling research was conducted and presented models critically evaluated. Based on the literature review this paper presents an open-source MATLAB toolbox for water electrolyzer polarisation curve parametrization and modelling. The modelling capabilities of the tooling were verified using measured PEM and alkaline water electrolyzer polarisation data. As real-world measurement data is rarely ideal, tests were also conducted using suboptimal data, first with data sets that have a low number of measurement points and secondly with data sets that have low or high current densities missing. The tooling is shown to work with a wide variety of use cases and provides an automated method for modelling and parametrization of electrolyzer polarisation curves.     

Thermodynamic modeling and assessment of a combined coal gasification and alkaline water electrolysis system for hydrogen production

The performance of a clean energy system that combines the coal gasification and alkaline water electrolyzer concepts to produce hydrogen is evaluated through thermodynamic modeling and simulations. A parametric study is conducted to determine the effect of water ratio in coal slurry, gasifier temperature, effectiveness of carbon dioxide removal, and hydrogen recovery efficiency of the pressure swing adsorption unit on the system hydrogen production. The exergy efficiency and exergy destruction in each system component are also evaluated. The results reveal that the overall energy and exergy efficiencies of this system are ∼58% and ∼55%, respectively. The weight ratio of the hydrogen yielded to the coal fed to this system is ∼0.126. Although this system produces hydrogen from coal, the greenhouse gases emitted from this system are fairly low.     

Challenges and important considerations when benchmarking single-cell alkaline electrolyzers

This study outlines an approach to identifying the difficulties associated with the benchmarking of alkaline single cells under real electrolyzer conditions. A challenging task in the testing and comparison of different catalysts is obtaining reliable and meaningful benchmarks for these conditions. Negative effects on reproducibility were observed due to the reduction in conditioning time. On the anode side, a stable passivation layer of NiO can be formed by annealing of the Ni foams, which is even stable during long-term operation. Electrical contact resistance and impedance measurements showed that most of the contact resistance derived from the annealed Ni foam. Additionally, analysis of various overvoltages indicated that most of the total overvoltage comes from the anode and cathode activation overpotential. Different morphologies of the substrate material exhibited an influence on the performance of the alkaline single cell, based on an increase in the ohmic resistance.     

Experimental studies and modeling of advanced alkaline water electrolyser with porous nickel electrodes for hydrogen production

The commercial hydrogen production by water electrolysis is limited by the high cost of electricity. The production cost can be minimized, if the cell module is operated with the minimum voltage at maximum current density. In the present study, porous nickel electrodes were developed indigenously on an engineering scale and used in an advanced zero gap filter press type bipolar electrolyser to minimize the cell voltage. As the cell voltage–current density characteristic of the cell module is unique feature of its design and the operating parameters, the polarization experiments were carried out using this cell module and the cell voltage–current density characteristics were generated at different operating temperatures. Further, the system is modelled for its electrochemical performance and the parameters accounting for different losses such as Ohmic and activation over potential, were estimated at different temperatures. These different parameters were compared with the data existing in literature and based on the analysis, the present cell module is found to be superior to the existing commercial electrolyzers in terms of energy efficiency.     

Influence of operation parameters in the modeling of alkaline water electrolyzers for hydrogen production

In order to improve the production of environmental friendly hydrogen, new advanced alkaline electrolyzers must be designed to optimize their combination with renewable energies. In pursuing this goal, modeling of alkaline electrolyzers becomes a powerful design tool. In this paper is presented a mathematical model that describes the behavior of an alkaline electrolysis cell. Unlike most of the existing models in literature, the proposed model simulates the influence of both electrode/diaphragm distance and electrolyte concentration in regular operation. The role of these aspects is crucial when designing alkaline electrolyzers since the process efficiency is found to be very sensitive to them. The computations done with the model presented here were validated with in situ experimental data, reporting a great accuracy: the maximum error was around 1% from all polarization curves studied. Combination with renewable energies was also studied by introducing a solar PV profile and the error reported never exceeds 3%. The influence of the considered variables (temperature, electrode/diaphragm distance and electrolyte concentration) was quantified using sensitivity analysis.     

Modeling and energy demand analysis of a scalable green hydrogen production system

Models based on too many parameters are complex and burdensome, difficult to be adopted as a tool for sizing these technologies, especially when the goal is not the improvement of electrochemical technology, but the study of the overall energy flows.The novelty of this work is to model an electrolysis hydrogen production process, with analysis and prevision of its electrical and thermal energy expenditure, focusing on the energy flows of the whole system. The paper additionally includes investigation on auxiliary power consumption and on thermal capacity and resistance as functions of the stack power. The electrolysis production phase is modeled, with a zero-dimensional, multi-physics and dynamic approach, both with alkaline and polymer membrane electrolyzers.Models are validated with experimental data, showing a good match with a root-mean-square percentage error under 0.10. Results are scaled-up for 180 kg/day of hydrogen, performing a comparison with both technologies.     

Aspen Plus model of an alkaline electrolysis system for hydrogen production

A model of an alkaline electrolysis plant is proposed in this paper, including both stack and balance of plant, with the objective of analyzing the performance of a complete electrolysis system. For this purpose, Aspen Plus has been used in this work due to its great potential and flexibility. Since this software does not include codes for modelling the electrolysis cells, a custom model for the stack has been integrated as a subroutine, using a tool called Aspen Custom Modeler. This stack model is based on semi-empirical equations which describe the voltage cell, Faraday efficiency and gas purity as a function of the current. The rest of the components in the electrolysis plant have been modelled with standard operation units included in Aspen Plus. Simulations have been carried out in order to evaluate and optimize the balance of the plant of an alkaline electrolysis system for hydrogen production. Also, a parametric study has been conducted. The results show that increasing the operation temperature and reducing the pressure can improve the overall performance of the system. The proposed model in this work for the alkaline electrolyzer can be used in the future to develop a useful tool to carry out techno-economic studies of alkaline electrolysis systems integrated with other process.     

Simulation of proton exchange membrane electrolyzer: Influence of bubble covering

The bubble covering phenomenon has been considered one of the most critical factors affecting Proton Exchange Membrane Electrolysis Cell (PEMEC) at high current densities. However, the relationship between bubble dynamics and electrochemistry has not been clearly defined. This study analyzes the bubble coverage and PEMEC performance under different input conditions and develops a mathematical model of PEMEC incorporating bubble dynamics. The model successfully predicted the polarization curves and coincided with the experimental data. The results show that bubble coverage increases with increasing current density, bubble detachment radius, and temperature. It decreases with increasing pressure and water inlet velocity. Bubble coverage is influenced by temperature, pressure, wettability, current density, and water inlet velocity. Meanwhile, bubbles covering the electrode deteriorate the performance of the PEMEC, leading to higher overpotentials and lower efficiencies, which becomes more apparent with increasing current density. This paper elucidates the relationship between bubble growth/detachment, bubble coverage, and electrochemistry for the first time, and the results can provide a reference for the development and optimization of high-performance PEMEC.     

One-dimensional dynamic modeling of a high-pressure water electrolysis system for hydrogen production

Research on high-pressure water electrolyzers is under way worldwide as the economic production of hydrogen from renewable energy sources becomes more important. With increases in operating pressures, new safety issues have emerged, for which a reliable dynamic model of the electrolyzers is important for predicting their behavior. In this paper, a one-dimensional dynamic model of a high-pressure proton exchange membrane water electrolyzer is proposed. The model integrates various important physico-chemical phenomena inside the electrochemical cell that have been investigated individually into a dynamic model framework. Water transport, gas permeation, gas volume variation in anode/cathode channels, gas compressibility, and water vaporization are considered to formulate the model. Numerical procedures to handle and solve the model and the model performance for the prediction of steady and dynamic state behaviors are also presented.     

Chlorine-free alkaline seawater electrolysis for hydrogen production

A new process for chlorine-free seawater electrolysis is proposed in this study. The first step of the process is separation of Mg²⁺ and Ca²⁺ ions from seawater by nanofiltration. Next, the NF permeate is dosed into the electrochemical system. There it is completely split into hydrogen and oxygen gases and NaCl precipitate. The electrochemical system comprises an electrochemical cell operated at elevated temperatures (e.g. ≥ 50 °C) and a settling tank filled with aqueous NaOH solution (20–40 %wt) that operates at lower temperatures (e.g. 20–30 °C). High concentration of hydroxide ions in the electrolyzed solution prevents anodic chlorine evolution, while the accumulated NaCl precipitates in the settling tank. Batch electrolysis tests, performed in NaCl-saturated NaOH solutions, showed absolutely no chlorine formation on Ni200 and Ti/IrO₂RuO₂TiO₂ anodes at [NaOH] > 100 g/kgH₂O. Three long-term operations (9, 12 and 30 days) of the electrochemical system showed no Cl₂ or chlorate (ClO₃^?) production on both electrodes operated at current densities of 93–467 mA/cm². The Ni200 anode was corroded in the continuous operation that resulted in formation of nickel oxide on the anode surface. On the other hand, the system was successfully operated at 467 mA/cm² with Ti/IrO₂RuO₂TiO₂ electrodes in NaCl-saturated solution of NaOH (30 %wt) for 12 days. During this period no formation of Cl₂ and ClO₃^? has been observed and precipitation of NaCl occurred only in the settling tank. The performance of the system was stable during the operation as indicated by the insignificant fluctuations in the applied cell potentials and measured constant concentrations of NaOH_(aq) and NaCl_(aq) in the electrolyte solution. During 12 days of operation at ≈ 470 mA/cm² about 1.2 m³ of H₂ and ≈150 g of solid NaCl were produced in the system. Electrical energy demand of the electrolysis cell was 5.6–6.7 kWh/m³H₂ for the current density range of 187–467 mA/cm².     

Development of high pressure membraneless alkaline electrolyzer

A technology for cyclic generation of hydrogen and oxygen using electrodes made of variable valency material that does not need the use of separating ion-exchange membranes is presented. The technological solution enables to fabricate electrolyzers for uninterrupted producing high-pressure hydrogen with reduced energy intensity of the production. The total work for compressing 1 m³ of hydrogen and 0.5 m³ of oxygen has been estimated. Results of investigation of influence of discrete supply of DC current to the electrolysis cell, in order to improve the processes of gas evolution and to simplify the power systems of the electrolysis plant, have been considered. There is also considered an electrolysis installation equipped with a thermosorption compressor in which LaNi₅ is used as a hydride-forming compound. The comparative characteristics of the developed electrolyzer and the currently used hydrogen generators are given.     

Development and performance evaluation of Proton Exchange Membrane (PEM) based hydrogen generator for portable applications

This paper presents the development of key components, specifications, configuration and operation characteristics of an 80 l/h Proton Exchange Membrane (PEM) water electrolyzer system for portable application. The developed PEM water electrolyzer can produce 80 l/h hydrogen (purity > 99.99%) with moderate pressure range up to 500 kPa (73 psi) at an operating current of 100 A with energy efficiency of 77.48%. The reliability in operation of developed PEM water electrolyzer system is tested for running the stack about 3000 h with 100 A current. The results indicate the reasonable stability of MEA fabrication and cell design method.     

A tank in series model for alkaline fuel cell in cogeneration of hydrogen peroxide and electricity

This work presents a Tank in Series Reactor (TSR) model for the alkaline fuel cell operating in potentiostatic mode in cogeneration of H₂O₂ and electricity. The developed TSR model accounts for the component and the energy balances in gas channels, liquid alkaline and catalyst layers together with charge balances at electrode/electrolyte interfaces. The TSR model is able to predict the limiting two-dimensional profiles in alkaline fuel cell. The simulation results indicate the influence of mass transfer on the distribution of concentration, temperature and current density.     

Dynamic energy and mass balance model for an industrial alkaline water electrolyzer plant process

This paper proposes a parameter adjustable dynamic mass and energy balance simulation model for an industrial alkaline water electrolyzer plant that enables cost and energy efficiency optimization by means of system dimensioning and control. Thus, the simulation model is based on mathematical models and white box coding, and it uses a practicable number of fixed parameters. Zero-dimensional energy and mass balances of each unit operation of a 3 MW, and 16 bar plant process were solved in MATLAB functions connected via a Simulink environment. Verification of the model was accomplished using an analogous industrial plant of the same power and pressure range having the same operational systems design. The electrochemical, mass flow and thermal behavior of the simulation and the industrial plant were compared to ascertain the accuracy of the model and to enable modification and detailed representation of real case scenarios so that the model is suitable for use in future plant optimization studies. The thermal model dynamically predicted the real case with 98.7 % accuracy. Shunt currents were the main contributor to relative low Faraday efficiency of 86 % at nominal load and steady-state operation and heat loss to ambient from stack was only 2.6 % of the total power loss.     

Hybrid energy system for hydrogen production in the Adrar region (Algeria): Production rate and purity level

The following work treat the prediction of the production rate and purity level of hydrogen produced by an alkaline electrolyzer fed by a renewable source in a hybrid energy system HES in the locality of Adrar in the south of Algeria. This work is made for different renewable energy penetration rate from 0% to 60% of conventional power (Genset generator). The cell electrolyzer model permits to predict the production rate of hydrogen with accuracy, according to operating parameters, climatic conditions and the load of the site of Adrar. The study permits to introduce a model of hydrogen purity level based on the operating parameters and the power supplying the alkaline electrolyzer. It also shows that the great influence of the intermittent energy supplying the electrolyzer on the production rate and purity level of hydrogen. The prediction of production rate and purity level by the models allow to obtain a distribution and storage of hydrogen produced according to predetermined selection criteria imposed by the operator.In the process of electrolysis, the oxygen is considered as by-product of the hydrogen production. The amount and purity level were estimated jointly.An HES-H₂ production program under MATLAB^®/SIMULINK^® has been developed to simulate the hourly evolution of the production rate and purity level of hydrogen and oxygen produced by an electrolyzer for different penetration rate of renewable energies in an HES.