Experimental study on the external electrical thermal and dynamic power characteristics of alkaline water electrolyzer

As hydrogen production with a water electrolyzer is an effective way for renewable energy consumption, understanding the external electrical characteristics of water electrolyzer is of great significance for the modeling and simulation, system configuration, and control strategy of the system for hydrogen production by renewable energy. However, there are relatively fewer studies in this area. This paper presents the establishment of an experimental platform to conduct an experimental study on the static and dynamic voltage‐current characteristics and analyze the adjustability of the electric power of the traditional alkaline water electrolyzer, the relationship between the electrical characteristics and the electrolyte temperature, and operating point of the alkaline water electrolyzer. In addition, the mathematical fitting problem of the electrical characteristics of the alkaline water electrolyzer is discussed. The work could supply a reference to alkaline water electrolyzer intergrated application in renewable energy.     

Development of hybrid photovoltaic-fuel cell system for stand-alone application

In this paper we present firstly the different hybrid systems with fuel cell. Then, the study is given with a hybrid fuel cell–photovoltaic generator. The role of this system is the production of electricity without interruption in remote areas. It consists generally of a photovoltaic generator (PV), an alkaline water electrolyzer, a storage gas tank, a proton exchange membrane fuel cell (PEMFC), and power conditioning units (PCU) to manage the system operation of the hybrid system. Different topologies are competing for an optimal design of the hybrid photovoltaic–electrolyzer–fuel cell system. The studied system is proposed. PV subsystem work as a primary source, converting solar irradiation into electricity that is given to a DC bus. The second working subsystem is the electrolyzer which produces hydrogen and oxygen from water as a result of an electrochemical process. When there is an excess of solar generation available, the electrolyzer is turned on to begin producing hydrogen which is sent to a storage tank. The produced hydrogen is used by the third working subsystem (the fuel cell stack) which produces electrical energy to supply the DC bus. The modelisation of the global system is given and the obtained results are presented and discussed.     

Technological development of hydrogen production by solid oxide electrolyzer cell (SOEC)

High-temperature solid oxide electrolyzer cell (SOEC) has great potential for efficient and economical production of hydrogen fuel. In this paper, the state-of-the-art SOEC technologies are reviewed. The developments of the important steam electrolyzer components, such as the ionic conducting electrolyte and the electrodes, are summarized and discussed. YSZ and LSGM are promising electrolyte materials for SOEC working at high and intermediate temperatures, respectively. When co-doping or a blocking layer is applied, SDC or GDC are possible electrolyte materials for intermediate-temperature SOEC. Ni–YSZ remains to be the optimal cathode material. Although LSM–YSZ is widely used as SOEC anode, other materials, such as LSF–YSZ, may be better choices and need to be further studied. Considering the cell configuration, planar SOECs are preferred due to their better manufacturability and better electrochemical performance than tubular cells. Anode depolarization is an effective method to reduce the electrical energy consumption of SOEC hydrogen production. Although some electrochemical models and fluid flow models are available, the present literature is lacking detailed modeling analyses of the coupled heat/mass transfer and electrochemical reaction phenomena of the SOEC. Mathematical modeling studies of SOEC with novel structures and anode depolarization processes will be fruitful for the development of SOEC. More works, both experimental and theoretical, are needed to further develop SOEC technology to produce hydrogen more economically and efficiently for the coming hydrogen economy.     

Technico-economic analysis of off grid solar PV/Fuel cell energy system for residential community in desert region

A technico-economic analysis based on integrated modeling, simulation, and optimization approach is used in this study to design an off grid hybrid solar PV/Fuel Cell power system. The main objective is to optimize the design and develop dispatch control strategies of the standalone hybrid renewable power system to meet the desired electric load of a residential community located in a desert region. The effects of temperature and dust accumulation on the solar PV panels on the design and performance of the hybrid power system in a desert region is investigated. The goal of the proposed off-grid hybrid renewable energy system is to increase the penetration of renewable energy in the energy mix, reduce the greenhouse gas emissions from fossil fuel combustion, and lower the cost of energy from the power systems. Simulation, modeling, optimization and dispatch control strategies were used in this study to determine the performance and the cost of the proposed hybrid renewable power system. The simulation results show that the distributed power generation using solar PV and Fuel Cell energy systems integrated with an electrolyzer for hydrogen production and using cycle charging dispatch control strategy (the fuel cell will operate to meet the AC primary load and the surplus of electrical power is used to run the electrolyzer) offers the best performance. The hybrid power system was designed to meet the energy demand of 4500 kWh/day of the residential community (150 houses). The total power production from the distributed hybrid energy system was 52% from the solar PV, and 48% from the fuel cell. From the total electricity generated from the photovoltaic hydrogen fuel cell hybrid system, 80.70% is used to meet all the AC load of the residential community with negligible unmet AC primary load (0.08%), 14.08% is the input DC power for the electrolyzer for hydrogen production, 3.30% are the losses in the DC/AC inverter, and 1.84% is the excess power (dumped energy). The proposed off-grid hybrid renewable power system has 40.2% renewable fraction, is economically viable with a levelized cost of energy of 145 $/MWh and is environmentally friendly (zero carbon dioxide emissions during the electricity generation from the solar PV and Fuel Cell hybrid power system).     

Hydrogen as a battery for a rooftop household solar power generation unit

Decentralization of electrical power generation using rooftop solar units is projected to develop to not only mitigate power losses along transmission and distribution lines, but to control greenhouse gases emissions. Due to intermittency of solar energy, traditional batteries are used to store energy. However, batteries have several drawbacks such as limited lifespan, low storage capacity, uncontrolled discharge when not connected to a load and limited number of charge/discharge cycles. In this paper, the feasibility of using hydrogen as a battery is analyzed where hydrogen is produced by the extra diurnal generated electricity by a rooftop household solar power generation unit and utilized in a fuel cell system to generate the required electrical power at night. In the proposed design, two rooftop concentrated photovoltaic thermal (CPVT) systems coupled with an organic Rankine cycle (ORC) are used to generate electricity during 9.5 h per day and the extra power is utilized in an electrolyzer to produce hydrogen. Various working fluids (Isobutane, R134a, R245fa and R123) are used in the ORC system to analyze the maximum feasible power generation by this section. Under the operating conditions, the generated power by ORC as well as its efficiency are evaluated for various working fluids and the most efficient working fluid is selected. The required power for the compressor in the hydrogen storage process is calculated and the number of electrolyzer cells required for the hydrogen production system is determined. The results indicate that the hybrid CPVT-ORC system produces 2.378 kW of electricity at 160 suns. Supplying 65% of the produced electricity to an electrolyzer, 0.2606 kg of hydrogen is produced and stored for nightly use in a fuel cell system. This amount of hydrogen can generate the required electrical power at night while the efficiency of electrolyzer is more than 70%.     

New multi-physics approach for modelling and design of alkaline electrolyzers

This work presents a multi-physics model used for the design and diagnosis of the alkaline electrolyzers. The model is based on a new approach that allows to choose precisely the design parameters of a new electrolyzer even if it is not commercially available and predicting energy consumption, efficiency and rate of hydrogen production, taking into account to their physical state and various operating conditions. The approach differs from those of conventional models of the following: It allows the characterization of the electrolyzer based on its structural parameters in a relatively short time (few minutes) compared with the conventional approach which need experimental data collected for few weeks (Ulleberg). The approach allows describing a range of alkaline electrolyzers, while semi-empirical models found in literature are inherent to a specific electrolyzer. In addition, the model takes into account the variation of all structural parameters (geometry, materials and their evolution depending on operating conditions) and operational parameters of the electrolyzer (temperature, pressure, concentration, bulk bubbling and recovery rate of electrode surface by the bubble), while the models in the literature involve only the temperature. The developed multi-physics model was programmed in a Matlab Simulink^® environment and an alkaline electrolyzer’s simulation tool was developed. The simulation tool was validated using two industrial (Stuart and Phoebus) electrolyzers with different structures and power rates. Simulation results reproduced experimental data with good accuracy (less than 0.9%). The simulation tool was also used to compare the energy efficiency of two hydrogen production systems. The first one is based on atmospheric electrolyzer with a compressor for hydrogen storage and the second one is a barometric electrolyzer (under pressure) with its auxiliary devices to identify the effective mode of hydrogen production according to the physical state and operating conditions of the electrolyzer. The analysis of results revealed that the second mode of hydrogen production is more efficient and confirms the results of the literature based solely on the thermodynamic approach (K. Onda et al) without the input of the power consumed by power overvoltages.     

Electric round-trip efficiency of hydrogen and oxygen-based energy storage

An electrolyzer and a fuel cell have been integrated in a small-scale stand-alone renewable energy system to demonstrate that hydrogen can be used for long-term stationary energy storage. The economic and environmental performance of such a system is strongly related to the ability of the electrolyzer to convert electrical energy to hydrogen and the ability of the fuel cell to convert hydrogen back to electrical energy, which together define the round-trip efficiency of the hydrogen storage system. One promising way to improve the efficiency as well as to decrease the capital costs of the fuel cell is to recuperate the oxygen from the electrolyzer and use it as the fuel cell oxidant instead of compressed air. This paper presents the modifications made to the system in order to implement oxygen recuperation. The round-trip system efficiency was found to be 18% with oxygen recuperation and 13.5% without it.     

Electrolytic hydrogen based renewable energy system with oxygen recovery and re-utilization

The Hydrogen Research Institute (HRI) has developed a stand-alone renewable energy (RE) system based on energy storage in the form of hydrogen. When the input devices (wind generator and photovoltaic array) produce more energy than is required by the load, the excess energy is converted by an electrolyzer to electrolytic hydrogen, which is then stored after stages of compression, purification and filtration. Conversely, during a time of input energy deficit, this process is reversed and the hydrogen produced earlier is reconverted to electrical energy through a fuel cell. The oxygen which has been produced by the electrolyzer during the hydrogen production is also stored at high pressure, after having gone through a purification and drying process. This stored oxygen can be re-utilized as oxidant in place of compressed air in the fuel cell. The modifications of the electrolyzer for oxygen storage and re-utilization of it as oxidant for the fuel cell are presented. Furthermore, the HRI has designed and developed the control system with power conditioning devices for effective energy management and automatic operation of the RE system. The experimental results show that a reliable autonomous RE system can be realized for such seasonal energy sources, using stored hydrogen as the long-term energy buffer, and that utilizing the electrolyzer oxygen by-product as oxidant in the fuel cell increases system performance significantly.     

Modeling,control and simulation of a photovoltaic /hydrogen/ supercapacitor hybrid power generation system for grid-connected applications

Hybrid renewable energy systems (HRES) should be designed appropriately with an adequate combination of different renewable sources and various energy storage methods to overcome the problem of intermittency of renewable energy resources. Focusing on the inevitable impact on the grid caused by strong randomicity and apparent intermittency of photovoltaic (PV) generation system, modeling and control strategy of pure green and grid-friendly hybrid power generation system based on hydrogen energy storage and supercapacitor (SC) is proposed in this paper. Aiming at smoothing grid-connected power fluctuations of PV and meeting load demand, the alkaline electrolyzer (AE) and proton exchange membrane fuel cell (PEMFC) and SC are connected to DC bus of photovoltaic grid-connected generation system. Through coordinated control and power management of PV, AE, PEMFC and SC, hybrid power generation system friendliness and active grid-connection are realized. The validity and correctness of modeling and control strategies referred in this paper are verified through simulation results based on PSCAD/EMTDC software platform.     

Predicting efficiency of solar powered hydrogen generation using photovoltaic-electrolysis devices

Hydrogen fuel for fuel cell vehicles can be produced by using solar electric energy from photovoltaic (PV) modules for the electrolysis of water without emitting carbon dioxide or requiring fossil fuels. In the past, this renewable means of hydrogen production has suffered from low efficiency (2–6%), which increased the area of the PV array required and therefore, the cost of generating hydrogen. A comprehensive mathematical model was developed that can predict the efficiency of a PV-electrolyzer combination based on operating parameters including voltage, current, temperature, and gas output pressure. This model has been used to design optimized PV-electrolyzer systems with maximum solar energy to hydrogen efficiency. In this research, the electrical efficiency of the PV-electrolysis system was increased by matching the maximum power output and voltage of the photovoltaics to the operating voltage of a proton exchange membrane (PEM) electrolyzer, and optimizing the effects of electrolyzer operating current, and temperature. The operating temperature of the PV modules was also an important factor studied in this research to increase efficiency. The optimized PV-electrolysis system increased the hydrogen generation efficiency to 12.4% for a solar powered PV-PEM electrolyzer that could supply enough hydrogen to operate a fuel cell vehicle.     

Hierarchical energy management for PV/hydrogen/battery island DC microgrid

This research presents an optimum design scheme and a hierarchical energy management strategy for an island PV/hydrogen/battery hybrid DC microgrid (MG). In order to efficiently utilize this DC MG, the optimum structure and sizing scheme are designed by HOMER pro (Hybrid Optimization of Multiple Energy Resources) software. The designed structure of hydrogen MG includes a PV generation, a battery as well as a hydrogen subsystem which composes a fuel cell (FC) system, an electrolyzer and hydrogen tank. To improve the robustness and economy of this DC MG, this study schedules a hierarchical energy management method, including the local control layer and the system control layer. In the local control layer, the subsystems in this DC MG are controlled based on their inherent operating characteristics. And the equivalent consumption minimization strategy (ECMS) is applied in the system control layer, the power flow between the battery and FC is allocated to minimum the fuel consumption. An island DC MG hardware-in-loop (HIL) Simulink platform is established by RT-LAB real-time simulator, and the simulation results are presented to validate the proposed energy management strategy.     

Simulation of a solar-hydrogen-fuel cell system: results for different locations in Mexico

The authors report the results obtained from the simulation of a PV-hydrogen-fuel-cell (PVHFC) hybrid system for different locations in Mexico. The hybrid system consists of photovoltaic arrays coupled with an electrolyzer to produce hydrogen, a fuel cell which converts chemical energy (H₂) to electricity, a hydrogen storage, a battery storage system, and the load. In this kind of system, all components can be connected electrically in parallel. The voltage of the PV arrays the fuel cell must be high enough to charge the battery, and the voltage of the electrolyzer must be low enough for the battery to power it during periods of low insolation. The simulation is based on the electrical component models and variable insolation data depending on the location.     

An electrical modeling and fuzzy logic control of a fuel cellgeneration system

The fuel cell generation system consists of a stack, a reformer, and converters. The stack generates DC power by electrochemical reaction. For system design and analysis, it is necessary to obtain electrical models. Simplified electrical models of a fuel cell generation system for system control are proposed. Then using the electrical models, system performance of a fuel cell generation system in which power is boosted by step-up choppers is analyzed. A fuzzy controller is designed for improved system performance. Simulation and experimental results confirmed the high performance capability of the designed system     

Solar-powered regenerative PEM electrolyzer/fuel cell system

An electrolyzer/fuel cell energy storage system is a promising alternative to batteries for storing energy from solar electric power systems. Such a system was designed, including a proton-exchange membrane (PEM) electrolyzer, high-pressure hydrogen and oxygen storage, and a PEM fuel cell. The system operates in a closed water loop. A prototype system was constructed, including an experimental PEM electrolyzer and combined gas/water storage tanks. Testing goals included general system feasibility, characterization of the electrolyzer performance (target was sustainable 1.0 A/cm² at 2.0 V per cell), performance of the electrolyzer as a compressor, and evaluation of the system for direct-coupled use with a PV array. When integrated with a photovoltaic array, this type of system is expected to provide reliable, environmentally benign power to remote installations. If grid-coupled, this system (without PV array) would provide high-quality backup power to critical systems such as telecommunications and medical facilities.     

Optimization of power allocation for wind-hydrogen system multi-stack PEM water electrolyzer considering degradation conditions

Hydrogen production from wind power has become one of the most important technologies for the large-scale comprehensive development and utilization of wind power, but the randomness of wind power has a large negative impact on the stability and cost of such wind-hydrogen hybrid energy systems. In this work, we initially analyze the relationship between electrolyzer efficiency and degradation with a three-dimensional multi-physics field model of PEMWE single-cell. Optimization of a power allocation strategy for wind-hydrogen system with a multi-stack PEM water electrolyzer (PEMWE) is proposed by considering degradation conditions. The multi-stack PEMWE power allocation strategy consists of the control module and execution module. In the control module, the degradation of PEMWE is quantified using the voltage degradation rate under different operating conditions. By setting the turning power point and external power supply and calculating the power allocation order online to reduce the degradation of PEMWE. In the execution module, the extended duty cycle interleaved buck converter (EDCIBC) based on fuzzy PID control is used to power each PEMWE single-stack. Case studies are carried out via computer simulation based on the configuration and experimental data for a specific wind farm located in Cixi, Zhejiang, China. Our results show that the energy efficiency of the wind-hydrogen system is 61.65% in a one-year operation, the voltage degradation of the PEMWE single-stack is 7.5 V, and the maximum efficiency is 6.29% lower than that when it is not aged. The EDCIBC output current ripple is as low as 0.053%, which rapidly and accurately follows the generated power allocation signal.     

Modeling,control and simulation of a PV/FC/UC based hybrid power generation system for stand-alone applications

Different energy sources and converters need to be integrated to meet sustained load demands while accommodating various natural conditions. This paper focuses on the integration of photovoltaic (PV), fuel cell (FC) and ultra-capacitor (UC) systems for sustained power generation. In the proposed system, during adequate insolation, the PV system feeds the electrolyzer to produce hydrogen for future use and transfers energy to the load side if possible. Whenever the PV system cannot completely meet load demands, the FC system provides power to meet the remaining load. If the rate of load demand increases the outside limits of FC capability, the UC bank meets the load demand above that which is provided by PV and FC systems. The main contribution of this work is the hybridization of alternate energy sources with FC systems using long and short-term storage strategies with appropriate power controllers and control strategies to build an autonomous system, with a pragmatic design and dynamic model proposed for a PV/FC/UC hybrid power generation system. The model is developed and applied in the MATLAB^®, Simulink^® and SimPowerSystems^® environment, based on the mathematical and electrical models developed for the proposed system.     

Modeling of hydrogen production with a stand-alone renewable hybrid power system

Hydrocarbon resources adequately meet today’s energy demands. Due to the environmental impacts, renewable energy sources are high in the agenda. As an energy carrier, hydrogen is considered one of the most promising fuels for its high energy density as compared to hydrocarbon fuels. Therefore, hydrogen has a significant and future use as a sustainable energy system. Conventional methods of hydrogen extraction require heat or electrical energy. The main source of hydrogen is water, but hydrogen extraction from water requires electrical energy. Electricity produced from renewable energy sources has a potential for hydrogen production systems. In this study, an electrolyzer using the electrical energy from the renewable energy system is used to describe a model, which is based on fundamental thermodynamics and empirical electrochemical relationships. In this study, hydrogen production capacity of a stand-alone renewable hybrid power system is evaluated. Results of the proposed model are calculated and compared with experimental data. The MATLAB/Simscape^® model is applied to a stand-alone photovoltaic-wind power system sited in Istanbul, Turkey.     

Monitoring and control of a hydrogen production and storage system consisting of water electrolysis and metal hydrides

Renewable energy sources such as wind turbines and solar photovoltaic are energy sources that cannot generate continuous electric power. The seasonal storage of solar or wind energy in the form of hydrogen can provide the basis for a completely renewable energy system. In this way, water electrolysis is a convenient method for converting electrical energy into a chemical form. The power required for hydrogen generation can be supplied through a photovoltaic array. Hydrogen can be stored as metal hydrides and can be converted back into electricity using a fuel cell. The elements of these systems, i.e. the photovoltaic array, electrolyzer, fuel cell and hydrogen storage system in the form of metal hydrides, need a control and monitoring system for optimal operation. This work has been performed within a Research and Development contract on Hydrogen Production granted by Solar Iniciativas Tecnológicas, S.L. (SITEC), to the Politechnic University of Valencia and to the AIJU, and deals with the development of a system to control and monitor the operation parameters of an electrolyzer and a metal hydride storage system that allow to get a continuous production of hydrogen.     

A data-driven digital-twin model and control of high temperature proton exchange membrane electrolyzer cells

The high temperature proton exchange membrane electrolyzer cells (HT-PEMEC) are promising for hydrogen generation from fluctuating and intermittent renewable energy. In this study, a data-driven method is developed to study the dynamic behavior of HT-PEMEC. This method combines multiphysics simulation and nonlinear system identification, avoiding expensive experimental costs and time-consuming full multiphysics calculations. Dynamic models for predicting the power consumption, hydrogen production and temperature are identified, and the verified fit is 96.31%, 97.87%, 87.73%, respectively, which demonstrated the accuracy of the identification model. Subsequently, the identification model was used to predict the dynamic behavior of HT-PEMEC and design control strategies. Fuzzy control strategy and neural network predictive control strategy are implemented to alleviate overshoot and suppress fluctuations so as to improve the durability of the electrolyzer. Moreover, compared with the fuzzy control strategy, the neural network predictive control strategy reduces the power overshoot by approximately 92%. This data-drive digital-twin model can not only guide dynamic experimental research, but also can be extended to study the dynamic behavior of various fuel cells and electrolyzer cells.     

Thermodynamic and electrochemical analyses of a solid oxide electrolyzer for hydrogen production

In this paper, a modeling of the Solid Oxide Electrolysis Cell (SOEC), through energetic, exergetic and electrochemical modeling approaches, is conducted, and its performance, particularly through exergy efficiency, is analyzed under various operating conditions and state properties for optimum hydrogen production. In a comprehensively performed parametric study, at a single electrolysis cell scale, the effects of varying some operating conditions, such as temperature, pressure, steam molar fraction and the current density on the cell potential and hence the performance are investigated. In addition, at the electrolyzer system scale, the overall electrolyzer performance is investigated through energy and exergy efficiencies, in addition to the system's power density consumption, hydrogen production rate, heat exchange rates and exergy destruction parameters. The present results show that the overall solid oxide electrolyzer energy efficiency is 53%, while the exergy efficiency is 60%. The exergy destruction at a reduced operating temperature increases significantly. This may be overcome by the integration of this system with a source of steam production.