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Industrial water electrolyzers mainly use old thyristor-based rectifiers to obtain the DC current required to run because of the low voltage level and high current requirements of the processes. These rectifiers cause significant ripple in the electrolyzer input current, leading to dynamic operation of the electrolyzer. Even though industrial-scale water electrolyzers are operated under such dynamic conditions, the effect on the electrolyzer performance is not well explored. In this study, current measurements from an industrial alkaline electrolyzer plant were used to define the common current ripple amplitude and frequency caused by the thyristor-based rectification. Based on the parameters obtained, laboratory measurements were conducted using an alkaline water electrolyzer to define the power losses incurred at various ripple amplitudes and frequencies. Additionally, the linearization of the electrolyzer current–voltage behavior as a function of frequency was studied using two electrode sets made of different materials. The laboratory measurements carried out in the study show that the ripple amplitude has a significant effect on increasing the losses, whereas the ripple frequency counteracts this. Thus, dynamic operation can have a large impact on losses, especially at partial loads, where the ripple current amplitudes increase significantly when using thyristor rectifiers. Lastly, the frequencies where the electrolyzer starts to behave linearly were observed to be at 68 Hz with the first electrode set and at 5 Hz with the second one. The considerable difference between the electrode sets indicates that the electrode materials and microstructure play a significant role in defining the electrolyzer frequency behavior. Because common thyristor-based power delivery systems operate at 300 Hz or 600 Hz, the results also imply that when modeling these systems, a linear model can be used for the electrolyzer to simplify the simulation. [Display omitted] • Large amplitude current ripple on alkaline electrolyzers is studied. • Ripple amplitude increases power losses, while frequency counteracts it. • As ripple frequency is increased, the electrolyzer starts to behave linearly. • The linear slope is determined purely based on the impedance. • At high frequencies, power usage can be determined from EIS measurement. [ABSTRACT FROM AUTHOR]

Industrial water electrolyzers apply direct current in kiloampere ranges. Therefore, rectifiers are typically based on thyristors despite the high amplitude current fluctuation excited by the rectifier. This paper studies the effect of thyristor rectifiers on the power quality of the electrolyzer stack and the supplying AC grid. The thyristor bridge excites high current and power fluctuation causing additional losses in the electrolytic cells. Further, the both AC supply side and DC output power factors are low especially under partial load conditions. The reactive power is divided into phase shift and distortion components. In the case of thyristor rectifier without any passive filter components the distortion reactive power component is dominating. The simulated results are supported by the power factor measurements of an industrial alkaline water electrolyzer. It is proposed that electrolyzer stack manufacturers should give requirements for the DC power quality to maximize the stack energy efficiency and lifetime. • DC current harmonics increase the specific energy consumption of water electrolyzers. • Thyristor rectifier has poor input and output power factor at partial loads. • Thyristor rectifier has high distortion reactive power component. • High AC reactive power component indicates poor DC quality. [ABSTRACT FROM AUTHOR]

The ripple of the supplied direct current has a significant effect on the energy efficiency of the water electrolyzer cell. Hydrogen production is defined by the current mean value, but current harmonics generate additional losses in the electrolyzer cell. However, the direct measurement of the DC current in the kiloampere range may be a challenging task compared with the voltage, which is practically in the range of a few hundreds volts in the electrolyzer stack. The effect of current harmonics on the energy efficiency and the voltage mean and root-mean-square (RMS) values is analyzed analytically based on the measured polarization curve of the PEM electrolyzer cell. The results show that both in the case of a theoretical sinusoidal current ripple and a practical thyristor bridge supply, the voltage mean or RMS values do not give a reliable estimate of the power quality. Instead, the voltage waveform as a function of time or the difference between the current mean and RMS values should be monitored. • Current quality has significant effect on the energy efficiency of electrolytic cells. • Sinusoidal and thyristor rectifier-induced current ripples cause additional losses. • Current RMS value differing from the mean value indicates poor power quality. • Cell voltage mean or RMS values do not indicate power quality. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Dynamic mass and energy balance simulation of a 3 MW industrial alkaline water electrolyzer. • Results are verified with measurements of an industrial plant. • Faraday efficiency at nominal operation was estimated at 86 %. • Contribution of each Faraday loss parameter was approximated. • 97 % of the stacks heat is extracted through the electrolyte circulation with heat exchangers. [ABSTRACT FROM AUTHOR]

The demand for green hydrogen is growing as the increasing production of pure hydrogen can no longer be based on fossil hydrocarbons. As water electrolysis would move from niche to a significant consumer of emission-free electricity, the efficiency of electrolytic gas production, lifetime of water electrolysis systems, and balance in the electricity grid become essential. The operational and investment costs of an industrial-scale alkaline water electrolyzer with four practical rectifier topologies are investigated. The results show thyristor-based rectifiers provide poor quality for the AC and DC sides, which leads to nonoptimal specific energy consumption (SEC) of the water electrolyzer and a notable reactive power component. Transistor-based topologies may offer up to 4.5% lower SEC of the electrolyzer than the conventional 6-pulse thyristor bridge. The reactive power results in additional costs in either investments in compensation equipment or regular allowance costs. An additional DC/DC converter can be installed to improve the power quality for the electrolyzer, but the second conversion stage adds to the system complexity and cost without eliminating reactive power. A modular single-stage rectifier based on insulated-gate bipolar transistor (IGBT) bridges is suggested as an alternative to provide improved power quality for both the electrolyzer and the electricity grid. • Four power supply topologies are studied in industrial green hydrogen production. • Thyristor rectifiers have poor AC power factor especially at partial loads. • Transistor-based topologies can minimize the stack SEC at wider partial loads. • A novel modular single-stage IGBT rectifier is presented for electrolysis applications. • The novel rectifier can be the most competitive solution in green hydrogen production. [ABSTRACT FROM AUTHOR]