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Scientific Reports volume 14, Article number: 24767 (2024 ) Cite this article full wave rectifier
This research article meticulously examines advanced power electronic converters crucial for optimizing electrolyzer perfor- mance in hydrogen production systems. It conducts a thorough review of mature electrolyzer types, detailing their specifications, electric models, manufacturers, and scalability. To meet the high current and stable DC voltage demands of industrial electrolyzers, the study delves into a broad spectrum of AC-DC and DC-DC converter topologies. It explores cutting-edge solutions like 12-pulse and 20-pulse rectifiers, advanced higher multi-pulse rectifiers utilizing conventional and auto-connected transformer units, Multi-Step Auto-connected Transformers (MSAT), polygon autotransformers, and auxiliary filters and circuits such as pulse multiplication circuits and active power filters. These innovations significantly reduce harmonic distortions and enhance power quality, addressing challenges inherent in conventional 6-pulse diode bridge rectifiers. The research also focuses on the efficiency and power factor correction capabilities of Active Front End (AFE) converters and the 3L-DNPC rectifier. Additionally, it investigates various DC-DC converters, including the Continuous Input Current Non-Isolated Bidirec- tional Interleaved Buck-Boost DC-DC Converter, the Interleaved Buck Converter (IBC) with extended duty cycles, the 3-level buck-boost converter with a coupled inductor, Quadratic converters designed for fault tolerance, the three-level interleaved buck converter, among others. Converter designs like the galvanically isolated half-bridge converter with controlled reverse-blocking switches for achieving zero voltage switching (ZVS), the Push–Pull isolated DC-DC converter prioritizing current stability, and the Isolated Full-Bridge Boost Converter (IFBBC) adept at handling fluctuating power outputs from renewable sources are also explored. Moreover, the study scrutinizes a range of converter configurations such as the Two-stage ZVT boost converter with LCL-Type SRC, multiphase interleaved DC-DC stage, and three-port isolated DC/DC converter for efficient integration with multiple energy sources concurrently. Discussing feature trends in power-to-hydrogen systems, the research reviews multi-cell modular rectifiers and multi-stage rectifiers. Overall, the study underscores the critical role of advanced converter topologies in enhancing efficiency, reliability, and power quality in electrolyzer systems, thereby contributing significantly to the progression of sustainable energy technologies.
The current global hydrogen production has reached a significant volume of 94 million tonnes annually1, as depicted in Fig. 1. A majority of this, approximately 59.7%, is obtained through steam methane reforming, followed by 19% from coal gasification, 0.6% from oil reforming, with the remaining 20.7% derived from naphtha reforming by-products2. Despite these figures, a minimal fraction of hydrogen is produced via electrolyzers. Hydrogen, vital in numerous sectors, is predominantly utilized in ammonia production (65%), methanol production (25%), and iron production (10%), with the residual 1% allocated to diverse applications such as E-Mobility, energy storage, oil refining, and power generation, as illustrated in Figs. 2 and 32. The annual growth rate of hydrogen production is estimated at 3–4%, primarily fueled by non-renewable energy sources, leading to significant CO2 emissions—approximately 900 MtCO2 per year, constituting 2.5% of the global output3. With the increasing global focus on achieving net-zero carbon emissions and transitioning towards hydrogen-based e-fuels, the demand for hydrogen is projected to escalate over tenfold by 20504. To counteract the reliance on fossil fuels, the integration of water electrolysis powered by renewable energy sources, such as wind turbines and photovoltaics, emerges as a sustainable alternative. This approach aligns with the objective of developing eco-friendly hydrogen production methods to replace conventional pollutant-based processes5.
Illuminating the percentages of production methods, revealing the pivotal roles played by various methods in shaping the hydrogen production eco-system.
Illustrates holistic power-to-hydrogen system, showcasing the interconnected components—power sources, power electronics converters, electrolyzers, and hydrogen applications—that form the foundation of a versatile and sustainable energy ecosystem.
Mapping the landscape of hydrogen consumption rates across Industries.
Water electrolysis, a technique discovered in 1789 by Jan Rudolph Deiman and Adriaan Paets van Troostwijk, utilized an electrostatic machine to generate electricity, which was then discharged onto gold electrodes in a Leyden jar6,7. This foundational experiment was further advanced in 1800 by William Nicholson and Anthony Carlisle, who successfully decomposed water into hydrogen and oxygen gases using an electric current, a pivotal moment in the history of water electrolysis8,9.Michael Faraday, in 1820, contributed significantly by demonstrating that the energy consumption of this reaction was directly proportional to the quantity of hydrogen produced, thereby establishing a crucial principle of modern water electrolysis8. The inception of the first industrial water electrolyzer in 1847 by Robert Bunsen marked a significant milestone, paving the way for the development of various types of water electrolyzers that now play a critical role across multiple sectors8. The operational principle of water electrolysis is relatively simple but effective. It involves a setup comprising two electrodes—an anode and a cathode—separated by an electrolyte or a separator. When water is introduced into the cell and an electric current is applied, the anode oxidizes water molecules, resulting in the generation of oxygen and hydrogen ions. These ions traverse through the electrolyte towards the cathode, where they recombine to form hydrogen and oxygen gases, a process thoroughly elucidated in references8,9.
Water electrolyzers have earned international prominence for their ability to produce sustainable hydrogen as an alternative to fossil fuels. Countries including Germany, Japan, Korea, and the United States have used water electrolysis technology to manufacture hydrogen for diverse applications8. In Germany, numerous initiatives are ongoing, which are creating hydrogen utilizing water electrolysis technology in renewable energy applications. In Japan, Toyota is utilizing the technology in their fuel cell vehicles produced commercially. Korea is also investing extensively in water electrolysis technology for hydrogen production, and the United States is likewise sponsoring research studies into its practicality8.
There are four main types of electrolyzers that have been manufactured, each with its particular set of features, specifications, and mode of functioning. The first form of electrolyzer is the alkaline electrolyzer, which employs a liquid alkaline electrolyte, generally potassium hydroxide, and nickel electrodes during electrolysis. It was first established in 1939 by Francis T. Bacon and George N. Kestern10. Alkaline electrolysis is an established technology with over 75 years of operational history10. Alkaline electrolyzers are highly efficient. They can handle a variety of water types, including salty and polluted water, although cleanliness is crucial for cathode function. Impurities may reduce efficiency, resulting in increased maintenance expenditures11. Power electronics schemes are essential for the efficient and reliable operation of hydrogen extraction systems. Hydrogen extraction is the process of separating hydrogen from water or other sources using electrolysis, thermochemical, photochemical, or biological methods. Power electronics schemes can provide the necessary voltage and current control, power quality improvement, and energy management for different types of hydrogen extraction systems.
Power electronics schemes play a crucial role in the operation of hydrogen extraction systems, which involve the separation of hydrogen from water or other sources through various methods such as electrolysis, thermochemical, photochemical, or biological processes. These schemes are responsible for providing and conditioning the required voltage and current control, enhancing power quality, and managing energy for different types of hydrogen extraction systems. Since the water electrolyzer functions as a direct current (DC) load, the power supplied to it must be conditioned either as AC/DC or DC/DC. Through precise control of the power conditioning stage, the system can select the appropriate output power. The fundamental concept of power conditioning and control, as elucidated in Fig. 4, is pivotal in ensuring efficient and optimized operation of hydrogen extraction systems.
Main concept of power conditioning and control in power conversion circuits for power-to-hydrogen systems.
The roles of power electronics in hydrogen extraction include:
Optimizing power converter designs to manage the high-current demands of electrolysis processes is essential.
Minimizing harmonic distortions is crucial for maintaining grid stability and ensuring the efficient operation of power electronics in hydrogen extraction.
Achieving galvanic isolation in DC-DC converters for electrolyzer applications, particularly in high-power systems, presents technical challenges that need to be overcome to maintain high efficiency.
Ensuring the reliability and durability of power electronics components and systems is vital for long-term operation in harsh industrial environments associated with hydrogen extraction processes.
Developing advanced energy management systems is crucial for the seamless integration of renewable energy sources with power electronics in hydrogen extraction processes.
However, the effective implementation of these systems is accompanied by several challenges related to power electronics, which play a crucial role in converting and conditioning electrical power for hydrogen production. The following points outline the key challenges faced in this domain:
Efficiency: High energy losses from switching and conduction in devices (diodes, IGBTs, MOSFETs) due to the energy-intensive nature of electrolysis.
Harmonic distortion: Harmonics introduced by converters can degrade power quality, causing voltage and current waveform distortions that destabilize grid operations.
Power factor maintenance: Achieving and maintaining a high power factor can be complex without sophisticated circuit designs or advanced control strategies.
Thermal management: High-power electronic components generate significant heat, necessitating effective cooling solutions to prevent overheating and extend component lifespan.
Voltage and current stress: High voltage and current applications place stress on power converters, requiring careful design to balance performance with size, weight, and cost.
Integration with renewable energy sources: Variability of sources like solar and wind requires power electronics to rapidly adapt to fluctuations without compromising system stability.
Cost and complexity: High costs associated with advanced power converters and their complex designs increase engineering challenges, complicating maintenance and reliability.
Electromagnetic interference (EMI): High-frequency switching generates EMI that can disrupt the operation of nearby equipment.
Scalability: As P2H systems grow in capacity, managing associated costs and efficiency challenges becomes increasingly critical.
Alkaline electrolyzers, a well-established and frequently utilized technology in hydrogen production, operate using an aqueous solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) as the electrolyte. This configuration enhances ion conduction via hydroxide and potassium ions, facilitating the water-splitting process into hydrogen and oxygen through a series of intricate chemical reactions12,13. In this process, at the anode, water undergoes oxidation, leading to the generation of oxygen, hydroxide ions, and electrons, a reaction mechanism detailed in reference14.
Conversely, at the cathode, hydrogen ions react with electrons to form hydrogen gas14:
The hydroxide ions created at the anode mix with the positive potassium or sodium ions in the electrolyte to make either potassium hydroxide (KOH) or sodium hydroxide (NaOH), thus completing the alkaline electrolysis cycle14:
The alkaline electrolyzer, a prominent technology in hydrogen production, exhibits several key characteristics as delineated in Table 1. Notably, it has a compact cell area under 4 m2, optimizing hydrogen production efficiency15. The hydrogen output pressure typically ranges between 10 to 30 bar, without exceeding 200 bar, thereby maintaining safe and controlled operational conditions15,16, this technology is also characterized by its ramp-up capability, smoothly transitioning from a minimum to full load within a range of 10% to 100% of its rated capacity17. In terms of efficiency, the alkaline electrolyzer achieves an electrical efficiency between 62 and 82%, optimizing electrical power utilization in hydrogen production18. It ensures high hydrogen purity, usually between 99.9 to 99.9999%, suitable for diverse applications19,20. The operational lifespan of the electrolyzer’s stack is substantial, ranging from 60,000 to 90,000 hours21. Additionally, it features a rapid cold-start time of less than 60 min, enabling efficient initiation of the electrochemical process, and possesses a system response time measured in seconds, facilitating quick adaptation to fluctuating operating conditions15. Its lower dynamic range, between 10 and 40%, allows for effective operation under various load conditions15,18. The hydrogen production rate of the alkaline electrolyzer can reach up to 3880 Nm3/h, catering to a broad spectrum of applications22. The electricity consumption ranges from 4.5 to 6.6 kWh/Nm3 of hydrogen, inclusive of energy demands from auxiliary components21. The electrolyzer operates with a current density of 0.2 to 0.4 A cm2, indicating its capability to manage electrical current flow during electrolysis18. It functions optimally at a voltage range of 1.8 to 2.4 V, affecting its overall energy efficiency18,21. Finally, the capital cost is estimated between 1000 to 1200 euros per kilowatt of electrical power (kWel), reflecting the investment necessary for its implementation and operation21.
The equivalent electrical circuit model for alkaline electrolyzers, depicted in Fig. 5, is designed to precisely emulate both the static and dynamic behaviors of the system. This model integrates various components that symbolize the electrochemical processes occurring within the cell. It includes a DC source, which represents the reversible voltage at the cathode, and integrates current sources and capacitors to account for the activation overvoltage and the double-layer effect. Additionally, resistors are employed to depict the resistance at the anode, cathode, membrane, and electrolyte13. The model simplifies by omitting the activation branch at the cathode, which is justified by its relatively lower overpotential in comparison to the anode13. The calibration of this circuit model’s parameters necessitates experimental data and advanced modeling techniques, taking into account the effects of various operating variables like current, temperature, and gas pressure13. Utilizing methodologies such as regression analysis or optimization algorithms enables the determination of these parameters, ensuring a robust correlation between the modeled outcomes and experimental data13. This modeling approach is crucial for simulating and understanding the dynamic response of alkaline electrolyzers, thereby offering crucial insights into their operational performance under diverse conditions13.
Equivalent electrical circuit model for both dynamic and static behaviors of the Alkaline electrolyzer.
In the domain of alkaline electrolyzers, a diverse array of manufacturers such as Green Hydrogen, McPhy, NEL A-Series, Sagim S.A, Teledyne Energy System, and AccaGen contribute significantly to the industry. These manufacturers offer electrolyzers with a wide range of characteristics, as comprehensively tabulated in Table 2. The production rates of these systems vary notably, ranging from 1.5 to 3880 Nm3/h. Equally varied is the stack power consumption, which oscillates between 3.8 and 5 kW/Nm3 for some models, while extending up to 74.73 kW/Nm3 for others. The operational ambient temperature spectrum of these electrolyzers spans from – 20 to 80 °C. Furthermore, they demonstrate a capacity factor variation between 0 to 100%, ensuring adaptability to diverse operational demands. The purity of the hydrogen produced by these systems is another critical parameter, typically ranging from 99.9% to 99.9999%. Additionally, these electrolyzers are capable of delivering hydrogen at pressures ranging from 10 to 200 bar, indicating their suitability for a variety of applications.
Proton exchange membrane (PEM) electrolysis represents an electrochemical advancement that operates as the reverse of PEM fuel cells, employing a Perfluorosulfonic Acid (PFSA) polymer electrolyte, commonly identified by the brand Nafion™ from DuPont. This nonaqueous electrolyte facilitates a more compact system design, enabling enhanced electrochemical performance compared to traditional Alkaline Electrolysis14.
PEM electrolyzers incorporate a Proton Exchange Membrane composed of a perfluorinated sulfonic acid polymer, serving as an electrolyte that effectively separates the anode and cathode compartments while permitting proton migration. During operation, water at the anode undergoes oxidation to form oxygen, protons, and electrons14:
The electrons traverse an external circuit to reach the cathode, whereas the protons diffuse through the membrane to the same destination. At the cathode, a reduction reaction occurs where hydrogen ions combine with electrons to produce hydrogen gas14:
Overall, the chemical equation governing the process in PEM electrolyzers parallels that of alkaline electrolyzers14:
The operational features of the PEM electrolyzer, as outlined in Table 3, include a compact cell area of less than 0.3 m215, enabling highly efficient hydrogen production. The system is capable of generating hydrogen at pressures between 10 to 30 bar, with the potential to reach up to 50 bar14,16, ensuring safe and reliable operation. Its dynamic response allows smooth transitions across a load range from 0% to full capacity, with electrical efficiency between 62 to 87%18. Hydrogen purity levels are notably high, typically within 99.998% to 99.9998%, catering to various industrial applications19,20. The electrolyzer’s lifespan extends from 20,000 to 60,000 hours, coupled with a (0% to 10%) provide agility in adapting to fluctuating load demands15,21. The hydrogen production rate can reach up to 2130 Nm3/h, with electricity consumption between 4.2 to 6.6 kWh per Nm3 of hydrogen, inclusive of the energy demands of ancillary components16,21. Current densities range from 0.6 to 2 A/cm2, operating at voltages between 1.8 and 2.2 volts, optimizing energy efficiency18,21. Finally, the capital cost is estimated at 1860 to 2320 euros per kilowatt-electric (kWel), encompassing the investment for system deployment and functionality21.
The electrical behavior of Proton Exchange Membrane (PEM) electrolyzers has been rigorously studied through the development of an electrical model to analyze the dynamics encapsulated within the current–voltage curve27. This model conceptualizes the electrolyzer as an electronic circuit, the intricacies of which are illustrated in Fig. 6. The constructed circuit model integrates dual resistance-capacitor (RC) branches that encapsulate the dynamic responses of the anode and cathode, an additional resistor to represent the membrane’s resistive properties, and a direct current (DC) voltage source that signifies the reversible potential24.
PEM electrolyzer electrical circuit model.
The stack voltage of the electrolyzer, denoted as Vcell, is computed as the aggregate of the reversible potential Erev and the overpotentials at the anode ηact,a, cathode ηact,c, and across the membrane ηohm. Governing the transient behavior of the overpotentials, differential equations are employed where Ca and Cc designate the capacitance at the anode and cathode, respectively, and Ra and Rc signify their respective resistances. The time constants, τa and τc, are intrinsic to the dynamic behavior and are contingent upon the electrolyzer’s operational conditions. The ohmic overpotential is delineated using Ohm’s law with Rmem symbolizing the membrane resistance.
Empirical observations have indicated that PEM electrolyzers manifest comparatively slower dynamics than PEM fuel cells, with a larger equivalent capacitance (37 F) signifying a more gradual response to changes. The anode reaction is predominantly responsible for these dynamics, with the ohmic overpotential significantly influenced by the membrane resistance. This dynamic representation of the electrolyzer’s behavior yields enhanced predictive accuracy and reliability, particularly notable during transient conditions, exhibiting a maximum error approximation of around 4%, a marked improvement over the 15% error observed with static models24.
Parameter determination for the circuit model is conventionally conducted via both static and dynamic identification methods. It is crucial to acknowledge that these parameters are not universally applicable but are instead specifically pertinent to defined current ranges, which reflect the variable dynamics of the electrolyzer under differing operational conditions. The development of adaptive-parameter models is therefore imperative to augment the fidelity and precision of the PEM electrolyzer model24.
The aforementioned model elucidates the sophisticated dynamics inherent to PEM electrolyzers and provides a foundation for future enhancements in electrolyzer control strategies and optimization techniques.
The landscape of Proton Exchange Membrane (PEM) electrolyzer manufacturing is populated by leading entities such as NEL M-series & C-series, Proton On-Site S-series & H-series, H-TEC Systems, Hydrogenics, Areva h2 ge, and Siemens, among others. These manufacturers showcase a diverse array of electrolyzer models, each characterized by distinct operational parameters. According to the data compiled in Table 4, the hydrogen production rates offered by these units span a broad range from 0.22 to 2130 Nm3/h, with stack power consumption figures lying between 4.3 to 7.3 kW/Nm3. Operating in ambient temperatures ranging from –20 to 50 °C, these electrolyzers boast a capacity factor flexibility from 0–100%, ensuring adaptability across various operational demands. The hydrogen gas produced is of high purity, between 99.998 to 99.9998%, and is deliverable at pressures ranging from 10 to 50 bar. This diversity underscores the industry’s capability to cater to a wide spectrum of applications, from small-scale energy solutions to large-scale industrial hydrogen production, highlighting the technological advancements and customization options available within the PEM electrolyzer market.
Given the electrical specifications necessary for the operation of industrial electrolyzers, including a stable DC voltage range of 640 to over 1 kV and a current range of 1 to 10kA, with power ratings reaching up to 10 MW, the employment of a power electronics converter becomes indispensable. Such converters play a crucial role in ensuring the safe and efficient functioning of the electrolyzer, mandated by the need for precise voltage regulation, surge protection, and adaptive power output adjustments in response to fluctuating operational conditions.
As Shown in Fig. 7 to cater to these requirements, when the electrolyzer is powered by the electrical grid or a wind farm, an AC-DC converter is employed to transform alternating current into a direct current that matches the electrolyzer’s voltage specifications. This conversion process is vital for integrating electrolyzers into various power generation contexts, providing the necessary flexibility and compatibility with renewable energy sources.
Exploring power electronics converters for different energy sources in electrolyzer applications.
On the other hand, for electrolyzers that are powered by a solar panel farm, a DC-DC converter is utilized. The primary function of this converter is to enable Maximum Power Point Tracking (MPPT) alongside voltage regulation. MPPT is a crucial feature for photovoltaic systems, optimizing the power output from solar panels by dynamically adjusting the electrical operating point of the modules or array.
Table 5 delineates the detailed specifications and requirements for the power electronics converters, underlining the importance of these technologies in bridging the gap between renewable energy sources and electrolyzer operation. By providing a stable and regulated power supply, power electronics converters ensure the electrolyzer’s performance is maximized, contributing to the overall efficiency and reliability of hydrogen production systems. This underscores the critical role of advanced converter topologies in the integration of renewable energy systems with industrial-scale electrolysis, facilitating a sustainable and resilient energy infrastructure.
In the field of AC-DC conversion, numerous types of converters have been developed, with the 6-pulse diode bridge rectifier (DBR) frequently employed in power-to-hydrogen systems. However, the use of the 6-pulse DBR introduces issues such as a lower power factor and the injection of harmonics into the main line current due to the switching process. These current harmonics can induce voltage distortion in the waveform by flowing through the source’s impedance, causing various difficulties in the power system, affecting equipment, electrical loads, and particularly monitoring and control devices. To combat these issues, standards such as IEEE-519 and DO-160G have been developed to eliminate main current harmonics and limit current and voltage disruptions.
To reduce harmonics, employing filters and multi-pulsing rectifiers (MPRs) has been presented as two effective strategies. Power quality can be improved by using both active and passive filters. However, passive filters come with drawbacks such as high losses, large space requirements, design complexity, and efficiency dependence on frequency changes. Likewise, active filters are criticized for their complexity, high cost, and load rate dependence. Consequently, increasing the number of pulses in the converter has been proposed and employed as an efficient approach to enhance power quality indicators, reduce the current passing through power electronic equipment, and lower the transformer’s kVA rate28,35. Multi-pulse rectifiers have been increasingly used to improve power quality indicators in industrial applications due to their low harmonic distortion, low output voltage ripple, easy configuration, great robustness, and inherent power factor correction36.
Studies have shown that 12-pulse and 18-pulse rectifiers effectively reduce line current harmonics in industrial applications. However, rectifiers with a higher number of pulses require transformers with more windings, larger core dimensions and weights, and, consequently, higher kVA ratings37,38. The increase in parts, design complexity, and cost are disadvantages of multi-pulse rectifiers with a high number of pulses39,40,41,42,43,44. The 40-pulse rectifier, comprising a 10-phase autotransformer with a complex structure and high kVA rating, significantly improves the kVA rating of the 40-pulse rectifier but at the expense of high complexity and cost39. As a result, 12-pulse rectifiers are generally used in industrial applications due to their simpler structure, resulting in lower kVA rates and costs.
However, the input current total harmonic distortion (THD) in ordinary 12-pulse rectifiers is theoretically about 15%, which cannot meet the restrictions of the DO-160G and IEEE-519 standards without additional filters. Various methods based on active or passive auxiliary harmonic reducing circuits in 12-pulse rectifiers have been reported to eliminate harmonics and meet standard requirements by reducing weight, dimensions, and kVA rate45. Active pulse multiplication circuits, such as active inter-phase reactors, current injection circuits, and shunt active power filters, have been presented as efficient methods to reduce harmonics. Nonetheless, these methods come with disadvantages such as computational complexity, sophisticated control procedures, losses, and overall expense46,47,48,49,50,51,52.
Alternatively, a 24-pulse rectifier using a passive harmonic reduction circuit inserted in the direct current (DC) link has been proposed53,54,55,56. This approach replaces the inter-phase reactor with a tapped reactor, simplifying the design and reducing costs. Furthermore, a 20-pulse rectifier has been proposed, with a kVA rate of 35.3% relative to the load, demonstrating the potential for simpler and lighter multi-pulse rectifiers with reduced complexity and kVA rates to meet IEEE-519 standards56.
The harmonic reduction method employing pulse multiplication circuits offers a straightforward, simple, and economical solution57.
Moreover, considering that the magnetic component of the transformer in multi-pulse rectifiers constitutes a significant portion of the dimensions, weight, and cost, the use of autotransformers in non-isolated applications has been suggested. This is due to the reduction of the magnetic component by about 80% compared to traditional transformers, resulting in reduced dimensions, weight, losses, and costs of the multi-pulse rectifier.
The 12-pulse diode rectifier with multi-phase configuration, as illustrated in Fig. 8, emerges as a promising solution for electrolyzer applications, offering enhanced power quality and addressing high-voltage and high-current requirements. Its architecture, comprising diodes, proves particularly suitable for medium to high-power applications. By distributing the load across multiple phases, the rectifier effectively minimizes current ripple and elevates the power factor, consequently reducing current distortion and enhancing overall power quality58,59,60,61.
12-Pulse diode-based rectifier with multi-phase chopper for electrolyzes applications system diagram.
The integration of a multi-phase chopper significantly augments electrolyzer systems by providing precise control over electrolyzer current and power. Constructed using silicon IGBTs and freewheeling diodes in an interleaved manner, the chopper minimizes current ripple across the electrolyzer. By modulating the duty ratios of the chopper IGBTs, efficient adjustment of electrolyzer current and power is achieved, ensuring consistent high-power factor throughout the converter’s operational range. Additionally, the incorporation of a multi-phase buck converter complements the system by mitigating output current ripple and enhancing reliability. The converter’s capability to distribute current among multiple inductors minimizes Joule losses and improves energy efficiency58,59,60,61.
Furthermore, the interleaved buck converter ensures uninterrupted supply to the electrolyzer, even in the event of power switch failures, owing to its static redundancy. This feature enhances system reliability and reduces the likelihood of disruptions in electrolyzer operation. Collectively, the 12-pulse diode rectifier, multi-phase chopper, and three-phase interleaved buck converter synergistically enhance electrolyzer systems. The rectifier optimizes power quality, particularly for high-voltage and high-current applications, while the multi-phase chopper offers precise current and power control, thereby minimizing ripple and improving power quality. Meanwhile, the interleaved buck converter reduces output current ripple, enhances energy efficiency, and maintains system stability, even under adverse conditions. Together, these components elevate system performance, facilitate voltage and current control, and enhance the usability of electrolyzer systems across diverse applications58,59,60,61.
In Ref.62, 12-pulse rectifier based on a polygonal autotransformer, as depicted in Fig. 9, is proposed with the aim of enhancing the elimination of harmonic distortion in the input current. The key innovation lies in the unique pulse multiplication circuit (PMC), which incorporates two tapped inter-phase reactors (TIPRs) and six auxiliary diodes, effectively doubling the number of pulses in the rectifier without necessitating active components. This configuration offers several advantages, including reduced complexity, a lower kilovolt-ampere (kVA) rating, and significantly lower cost compared to conventional rectifiers.
12-Pulse diode rectifier with pulse multiplication circuit.
The introduction of TIPRs eliminates the need for the conventional zero-sequence blocking circuit typically found in autotransformer multi-pulse rectifiers. This not only results in further reductions in the kVA rating and total rectifier cost but also simplifies the overall design. Consequently, the proposed rectifier emerges as an optimal solution in terms of both technical and economic parameters when compared to previous rectifier designs.
Experimental evaluations of the proposed architecture demonstrate its remarkable effectiveness in mitigating harmonic distortion. The total harmonic distortion (THD) of the voltage is measured at an impressively low value of 0.43%, while the THD of the current similarly decreases to 0.70%. These findings underscore the significant efficacy of the proposed rectifier in minimizing harmonic distortion in both the input voltage and current waveforms.
Of particular note is the rectifier’s kVA rating of 30.08%, indicating its substantial power handling capabilities. This attribute renders it suitable for a wide range of applications where power requirements are critical. Overall, this rectifier design holds substantial promise for various applications requiring enhanced power quality and cost-effective solutions.
Table 6 illustrates how the performance of 12-Pulse rectifiers varies based on factors such as converter complexity, filter types, and phase shift transformer designs. Utilizing a Conventional Transformer Unit (CTU) results in the lowest efficiency, power factor (PF), and highest Total Harmonic Distortion (THD %). Replacing CTU with an Autotransformer Unit (ATU) enhances efficiency but yields the lowest PF and reduced THD. Integrating an interleaved buck converter with CTU significantly improves efficiency, achieves satisfactory PF, and maintains natural THD %. Incorporating a pulse tripling circuit elevates efficiency, delivers excellent PF, and ensures superior THD. Additionally, implementing a pulse multiplication circuit within the rectifier can further refine THD levels.
In the realm of electrolyzer applications, the imperative to enhance power quality and minimize current ripple is paramount for operational efficiency and product quality58,64. A notable advancement in addressing these challenges is the implementation of the 12-pulse thyristor rectifier shown in Fig. 10, an evolution from the conventional six-pulse thyristor rectifier design. This enhanced rectifier topology, illustrated in Fig. 10, provides substantial improvements in power quality and current ripple reduction, as evidenced by research findings58,66.
The 12-pulse configuration can be orchestrated either in parallel, to boost the output current, or in series, to increase the output voltage. This versatility is facilitated by a three-winding wye-delta-wye phase shift transformer, which effectively mitigates 5th and 7th order harmonic currents. The transformer’s secondary windings can be configured in either delta or star formations, inducing a natural phase shift of π/6 between the generated voltages, thereby augmenting the system’s harmonic suppression capabilities66.
Compared to its six-pulse counterpart, the 12-pulse thyristor rectifier exhibits a significantly reduced current ripple, thereby elevating power quality. The ripple factor is notably diminished to a range of 3.2% to 4.8%, contingent upon the load conditions66. This reduction not only decreases specific energy consumption but also contributes to improved hydrogen purity, marking a significant leap towards operational efficiency and product quality enhancement.
Despite these advancements, the specific energy consumption of the 12-pulse thyristor rectifier system remains elevated compared to the ideal of pure DC current application. This underscores the necessity for ongoing refinement in design and application strategies to optimize energy utilization66.
A distinguishing feature of the 12-pulse thyristor rectifier is its proficiency in eliminating odd-ranked harmonics, thus decreasing reactive power consumption and enhancing the power factor. Nevertheless, compliance with stringent international standards, such as IEEE 519–2014, often necessitates additional measures. A viable solution, as proposed in the literature, involves the adoption of a hybrid filter comprising a shunt passive filter coupled with a Distribution Static Compensator (DSTATCOM)66. This configuration achieves an input power factor of 0.98 and maintains Total Harmonic Distortion (THD) at a low level of 4.8%66.
For harmonic compensation in Power-to-Hydrogen (P2H) converters employing the 12-pulse thyristor rectifier architecture, passive trap filters tuned to the 11th and 13th harmonics are commonly utilized. Additionally, a shunt-passive high-pass filter may be implemented to divert high-order current harmonics effectively66.
Given the operational characteristics of 12-pulse thyristor rectifiers, particularly with extended firing angles, the integration of a static VAR compensator or static synchronous compensator is essential for providing the requisite reactive power66.
In Ref.67, a sophisticated 12-pulse converter system is presented, designed for the dynamic adjustment of DC voltage through modu- lation of firing angles. The system’s architecture incorporates a controlled rectifier, a three-phase three-winding transformer, an Active Power Filter (APF), and a meticulously devised control scheme. The core of this system, the 12-pulse converter, is ingeniously constructed from two six-pulse converter sets arranged in series, as depicted in Fig. 11. This configuration is instrumental in mitigating the harmonic distortions generated individually by each six-pulse converter, effectively neutralizing the cumulative harmonics with the aid of the APF.
12-Pulse thyristor rectifier with source current detection based SAPF.
A notable innovation of the proposed design is the employment of a three-winding transformer, which ingeniously facilitates a reduction in filter side voltage. This obviates the need for a high-bandwidth step-down transformer, thereby diminishing the voltage demands on the APF’s switches and extending the permissible switching frequency range. Such enhancements contribute significantly to the system’s enhanced efficiency and performance.
The control strategy integral to this system is aimed at the precise regulation of the APF, tasked with counteracting the grid current harmonics engendered by the nonlinear load characteristics of the 12-pulse converter. The documentation outlines two conventional control methodologies: the load current detection technique and an open-loop control method. Despite their utility, these methods exhibit limitations in terms of compensation quality, potential for harmonics mis-cancellation, and dynamic response capabilities.
To surmount these limitations, the proposed control strategy introduces an approach by implementing a source current detecting technique, underpinned by a vector resonant (VR) controller. This method amalgamates the benefits of load current sensing and open-loop control, significantly streamlining the control architecture and eliminating the necessity for a separate harmonics extraction algorithm. The comprehensive control mechanism encompasses a phase-locked loop (PLL) for precise real-time phase alignment, dc link voltage stabilization, and a current control loop dedicated to harmonics filtration.
Experimental evaluations of the system validate the effectiveness of the proposed compensation strategy, demonstrating a substantial reduction in Total Harmonic Distortion (THD) post-correction, thereby markedly improving the grid current waveform quality. The system achieved THD values of 3.03% and 2.76% with specified firing delay angles, showcasing the proposed system’s capability to enhance power quality significantly. This research not only highlights the potential of advanced converter systems in minimizing harmonic distortions but also underscores the effectiveness of the integrated control strategy in ensuring optimal operational performance.
As shown in Table 7, the performance of 12-Pulse rectifier topologies varies according to several factors, including converter complexity and the type of filters used. Employing a conventional 12-Pulse thyristor rectifier with a passive filter result in satisfactory efficiency, power factor (PF), and total harmonic distortion (THD). Adding a hybrid filter to the rectifier leads to a slight increase in efficiency and an improved power factor. Additionally, incorporating a source current detection based on Shunt Active Power Filters (SAPF) or an Auxiliary Circuit with a four-star transformer result in a significant reduction in THD values.
In Ref.69, a topology is introduced, characterized by a 20-pulse asymmetric, non-isolated Multi-Step Auto-connected Transformer (MSAT) system, which exhibits superior performance over traditional designs. As depicted in Fig. 12, this system integrates a front-end asymmetric MSAT, known for its power quality enhancement capabilities, alongside two prime numbered diode bridge converters (PNDBCs) at its output stage. The MSAT ingeniously generates a series of prime numbered phases from a standard three-phase AC input, which can include configurations such as 5-phase, 7-phase, 11-phase, and beyond. For the sake of simplicity and practicality, the proposed topology limits the prime numbered phases to five. A cornerstone advantage of this MSAT design lies in its proficiency in harmonics mitigation, crafting an almost sinusoidal waveform by evenly displacing adjacent phases. This displacement introduces a greater number of steps in the input source current, thereby elevating the overall power quality. By provisioning two sets of prime numbered 5-phase supplies to feed two PNDBCs, the output pulse count is effectively doubled. Moreover, the inclusion of a zig-zag transformer configuration not only facilitates the creation of an effective neutral point but also ensures system balance during single-phasing events and prevents the injection of harmonic currents back into the AC mains. Comparatively, the MSAT topology outlined offers significant improvements over existing multi-pulse autoconnected transformer solutions. It achieves lower voltage and current Total Harmonic Distortion (THD) levels while maintaining a unity Power Factor (PF) across various load conditions, thereby aligning with IEEE standards. Additionally, the magnetic rating of this design is substantially lower than that of traditional 6-pulse configurations and other 20-pulse auto transformer arrangements. This reduction in magnetic rating translates to a more compact, cost-efficient system that demands less space and reduces overall expenditure, boasting an efficiency of 97.65% at full load. Despite its numerous advantages, potential limitations of the MSAT topology warrant consideration. The complexity of the autoconfigured transformer design escalates with an increase in the number of prime numbered phases beyond five. Furthermore, the implementation of the asymmetric MSAT and PNDBCs might necessitate precise design and control strategies, potentially elevating the system’s complexity and associated costs. It is also imperative to acknowledge that the specific benefits and constraints of the MSAT may vary depending on the application and operational requisites.
20-Pulse asymmetric non-isolated multi-step auto-connected transformer configuration.
The proposed 20-pulse asymmetric MSAT architecture offers notable advancements in terms of power quality, reduced THD, and minimized magnetic rating when compared to preceding topologies. Its innovative approach to generating prime numbered phases and leveraging a zig-zag transformer design contributes additional benefits regarding system stability and harmonic suppression. Nonetheless, the intricacies inherent in the design and the requisite precise control mechanisms should be meticulously evaluated during the implementation phase.
In Ref.70, an auto connected-transformer-based 20-pulse AC–DC converter is introduced as an efficient means to enhance power quality metrics, reduce magnetic requirements, and provide a space-saving, economical option for mid-sized SMPS deployments. This 20-pulse AC–DC converter, depicted in Fig. 13, employs a combination of multiphase and phase-staggering techniques to deliver dual five-phase supplies to each ten-pulse diode bridge rectifier. Y − ∆ − Y transformer is utilized for each pair of five-phase supply, generating a 30° phase difference across the voltages of two 6-pulse converters, forming the basis for either 12- or 24-pulse converters. The introduction of an auto connected transformer to power the multi-pulse converter significantly lowers magnetic specifications without sacrificing isolation, effectively reducing harmonics through analogous phase displacement and even phase distribution within a multiphase framework.
Auto connected-transformer-based 20-pulse AC–DC converter.
The assessment of Total Harmonic Distortion (THD) in input line current involves comparing its rms value against the rms value of the fundamental frequency’s component. Remarkably, under both heavy and light load conditions, the THD levels remain well within IEEE standards. Demonstrated through simulations and empirical evaluations, the 20-pulse converter outperforms a traditional six-pulse converter, satisfying all power quality standards as per IEEE Standard 519 across diverse loading scenarios. With only two dc–dc converters required, this proposed system is more economical and compact than an 18-pulse converter system, which necessitates three dc–dc converters and three high-frequency transformers.
Although primarily tailored for medium-capacity SMPS scenarios, the scalability of this topology for higher power settings is noted as a potential limitation. Nonetheless, it presents a feasible option for those seeking improved power quality and reduced magnetic requirements in medium-scale SMPS applications. The effectiveness and applicability of the proposed 20-pulse ac–dc converter is further affirmed through the development and testing of an experimental prototype, validating the design approach and simulation models. This auto connected transformer-based 20-pulse ac–dc converter demonstrates its capability to boost power quality metrics, lower magnetic requirements, and achieve a cost-efficient, compact design for medium-capacity SMPS uses. Its efficacy exceeds that of a six-pulse converter and matches that of an 18-pulse converter with fewer components, suggesting that additional investigation and testing are needed to explore its potential for high-power applications.
Table 8 reveals that multi-pulse rectifiers with a higher number, ranging from 18-Pulse to 72-Pulse configurations, deliver enhanced performance compared to other designs, with efficiencies ranging from 97.65 to 98.3% or more. These rectifiers also boast superior Power Factors (PF) between 0.98 and 0.9995 and maintain acceptable Total Harmonic Distortion (THD) levels between 3.7 and 1.04%. However, these systems come with significant disadvantages, including increased size, complexity, and cost.
Active Front End (AFE) rectifiers, characterized by their high efficiency and reliability, play a crucial role in minimizing disturbances, reducing harmonic distortion, and operating close to Unity Power Factor (UPF). Various AFE types are employed in industrial applications today. According to Ref.76, the three-phase diode rectifier represents the most basic AFE configuration, comprising just six diodes. This simplicity eliminates the need for a control system or gate drivers, streamlining its functionality. However, operating solely at grid frequency, this type lacks current shaping and output voltage control capabilities, potentially introducing a Total Harmonic Distortion (THD) ranging from 40 to 70%. Given its high KVA ratings and substantial THD, this conventional passive rectifier topology is generally unsuitable for high power applications. As detailed in Refs.76,77,78, the Three-Phase Two-Level Six-Switch Rectifier (B6 rectifier) features six active switches, AC side inductors, and a DC side filter capacitor. This two-level rectifier topology is noted for its simplicity, robustness, and widespread recognition, easily constructed using commercial H-bridges. Despite requiring larger input inductors and facing limitations in maximum switching frequency compared to three-level converters, the six active switches enhance THD reduction through improved current shaping and voltage control. However, the topology necessitates limiting the lower boundary of the DC link voltage due to its inherent boosting characteristic.
Figure 14 showcases the structure of a three-phase two-level rectifier tailored for electrolyzer applications. The design necessitates dual converter units; one connected to the wye side and the other to the delta side of the phase shift transformer. Each converter unit is equipped with six active switches, AC-side boost inductors, and a DC-side filter capacitor. The build of the boost-type two-level rectifier is recognized for its simplicity, durability, and familiarity, easily constructed using off-the-shelf H-bridges as indicated in references76,77,78. This rectifier configuration demands larger input inductors and faces limitations in its maximum switching frequency as noted in Refs.76,77,78. The performance of the two-level six-switch rectifier ensures output voltage stability with less than 1% ripple, achieving a grid-side current THD of 4.5% at full load, alongside a power factor of 0.997, and converter efficiency reaching 98.5%, according to reference76.
Three Phase two level rectifier for electrolyzer applications.
The three-level diode neutral-point-clamped (3L-DNPC) rectifier, as shown in Fig. 15, presents unique construction advantages, limitations, and outstanding operational characteristics according to the provided data. Operating based on the DC link voltage setpoint, the 3L-DNPC design ensures a minimal DC link voltage ripple of 1%. It boasts a grid-side current total harmonic distortion (THD) below harmonic distortion (THD) below 2.24% at full load, indicating high power quality. With a power factor of 0.997 and an impressive efficiency of 99.3% at full capacity, it exemplifies effective energy conversion as reported in references76,79. A key advantage of the 3L-DNPC model is its ability to reduce the inductor’s size by 44% relative to other designs84. The rectifier’s switches operate at only half the DC link voltage, reducing stress on the components. Nonetheless, the 3L-DNPC setup is not without drawbacks; it incorporates 18 active switches, elevating its cost and complexity. Furthermore, it necessitates two series capacitors, leading to higher capacitance and lower voltage ratings per capacitor. ABB, a prominent technology corporation, employs the 3L-DNPC rectifier in their electrolyzer systems as documented in references76,79,80,81. ABB leverages the topology’s inductor size reduction and efficient energy conversion advantages, aiming to enhance the performance and dependability of their hydrogen production systems82. The three-level diode neutral-point-clamped (3L-DNPC) rectifier, as shown in Fig. 15, presents unique construction advantages, limitations, and outstanding operational characteristics according to the provided data. Operating based on the DC link voltage setpoint, the 3L-DNPC design ensures a minimal DC link voltage ripple of 1%. It boasts a grid-side current total.
Three phase natural active Point clamped converter.
The Three-Phase Three-Level T-Type Converter, as depicted in Fig. 16, represents a noteworthy advance in power electronic systems, comprising four switches per leg akin to traditional two-level rectifiers but uniquely includes an inductor connecting the grid to the midpoint, along with two capacitors and a resistor in each leg. Through diverse switching schemes, this converter generates three distinct pole voltages: positive, neutral, and negative, offering several advantages. It enables the formation of a five-level line-to-line voltage, enhancing power quality and minimizing harmonic distortion, while achieving unity power factor for efficient energy conversion as noted in references76,83. Its design simplicity, using fewer components than other multilevel architectures, results in cost and complexity reduction. T-type rectifiers contribute to smaller EMI filter requirements and provide fault tolerance. Performance analyses, both through modeling and experiments, have shown promising results76,83. The implemented sliding mode control (SMC) technique effectively manages DC voltage, line current, and DC capacitor voltage balance, showcasing rapid dynamic response, ease of implementation, and robustness to disturbances and parameter variations. Despite a low theoretical total harmonic distortion (THD) in line currents, estimated at 1.75%, experimental conditions yield a slightly higher THD of 2.4%, still within internationalstandards. However, challenges include the SMC’s sensitivity to disturbances and steady-state inaccuracies in DC voltage regulation, suggesting the SMC design differs significantly from previous methods and may require further investigation and validation76,83.
Three phase T-type converter configuration for electrolyzer applications.
In Table 9, front-end rectifiers emerge as an optimal choice for high and medium power applications. Initially, passive three- phase two-level rectifiers demonstrate satisfactory system efficiency but exhibit significantly high Total Harmonic Distortion (THD). Transitioning to active rectifiers can markedly enhance system efficiency, leading to superior power factor and a substantial reduction in THD. Additionally, alternative topologies such as the 3L-DNPC and T-type converters offer notable improvements in efficiency and power factor levels while maintaining THD within acceptable limits. These insights underscore the critical role of rectifier selection in optimizing performance metrics for power conversion systems.
In the realm of DC-DC converters for linking photovoltaic systems (PV) to hydrogen electrolyzers (H2), achieving high energy efficiency, power density, and reliability while minimizing cost and electromagnetic interference is paramount. Non-isolated converters, such as buck converters, are commonly utilized for their simplicity and affordability. However, they suffer from significant drawbacks like high output current ripple and a medium voltage conversion ratio. Overcoming these requires oversizing inductor components, which increases size, cost, and reduces efficiency, limiting their application to scenarios with low conversion ratios.
To enhance efficiency, literature sources84,85 suggest synchronous DC-DC converters that replace the freewheeling diode with an active switch, mitigating reverse recovery issues and reducing conduction losses due to lower voltage drop and thermal resistance. Additionally, quadratic converters, as outlined in Refs.86,87 offer improved voltage step-up/down ratios and reduced output current ripple using a single power switch across two series-connected converters. Further advancements include double and triple quadratic buck converters88,89,90, which lower voltage stress on the switch and decrease output current ripple, enhancing performance.
Isolated DC-DC converters, essential for electrolyzer applications, feature electrical insulation between input and output, utilizing high-frequency transformers for galvanic insulation. Operating frequencies range from tens to hundreds of kilohertz, significantly reducing converter size and volume despite potential eddy current and skin effect penalties91. Optimally sized transformers in these converters minimize current and voltage stress on switches, boosting energy efficiency without the voltage ratio limitations seen in non-isolated converters. The half-bridge topology, suggested in Ref.92 for electrolyzers, employs capacitive snubbers for soft switching, reducing switching losses but is challenged by transformer turn ratio issues affecting magnetic flux and increasing leakage inductance and power losses. Full-bridge topologies, explored in Refs.93,94,95,96,97, are favored for high power applications due to reasonable voltage stress on switches, yet they are hindered by their complexity, component count, conduction losses, and the need for a current control loop to prevent transformer saturation, leading to lower energy efficiency and voltage ratios compared to half-bridge converters and reduced fault tolerance after power switch failures.
In Ref.98, an interleaved bidirectional buck-boost DC-DC converter is introduced, effectively reducing input current ripple without requiring additional components, thus lowering the converter element count and minimizing losses. Figure 17 illustrates the converter’s design, which includes four switches equipped with reverse-parallel diodes, two inductors, and a capacitor. It operates based on a duty cycle (D) and receives 180-degree phase-shifted signals. This setup offers several benefits: reduced ripple, lower losses, the capability for bidirectional power transfer, and interleaved functionality. Empirical data indicate peak efficiencies of 95.6% in boost mode and 95.4% in buck mode.The converter demonstrates a marked improvement in performance by achieving a substantially lower total harmonic distortion (THD) of the input current amplitude, measuring at an impressive 0.12. This achievement surpasses traditional converters, which typically exhibit a THD of around 0.277, highlighting a significant operational efficiency and effectiveness enhancement.
Continuous input current non-isolated bidirectional interleaved buck-boost DC-DC converter for electrolyzer applications.
The marginal variance of approximately 0.5% between the actual and theoretical efficiency can be primarily attributed to factors such as conduction losses, fluctuations in component properties, and potential inaccuracies in measurement methodologies. Despite this slight deviation, the converter’s overall performance remains commendable, showcasing notable advancements in mitigating harmonic distortions and optimizing energy conversion processes. Ultimately, this proposed converter stands out for its high efficiency and effective THD minimization.
In Ref.99, an Interleaved Buck Converter (IBC) topology is introduced, designed for high-input-voltage applications with operating duties below 50%. The architecture of the proposed IBC, depicted in Fig. 18, shares similarities with conventional IBCs but incorporates two series-connected active switches and a coupling capacitor within the power pathway. These switches are controlled with a 180° phase shift, and the output voltage is regulated by adjusting the duty cycle at a set switching frequency. Operating in Continuous Conduction Mode (CCM), this IBC configuration ensures reduced current stress. It significantly diminishes voltage stress across both the active switches and the freewheeling diodes, thereby lowering capacitive discharge and switching losses. Employing Schottky diodes with low breakdown voltages further mitigates reverse-recovery and conduction losses. Compared to standard IBCs, this design offers a better conversion ratio and less output current ripple. The layout mirrors that of traditional IBCs but with the addition of two series-connected active switches and a coupling capacitor, as shown in Fig. 18. Steady-state operational waveforms indicate that these switches are synchronized with a 180° phase difference. The operation of this IBC divides into four distinct modes within each switching cycle, detailed in the operational diagrams. Assumptions include large output and coupling capacitors treated as constant voltage sources, equal inductance for both inductors, and ideal semiconductor behavior. Advantages of this IBC include efficient power conversion with reduced capacitive discharge and switching losses, and lower voltage stress on components, allowing for the use of Schottky diodes with lower breakdown voltages. This results in improved conversion ratios and reduced output current ripple. Nevertheless, challenges such as the requirement for two series-connected active switches and an additional coupling capacitor may complicate the circuit design and layout. Efficiency metrics for the proposed IBC vary from 89.18% to 96.25%, influenced by the switching frequency and load conditions.
Interleaved buck converter with extended duty cycle configuration for electrolyzer applications.
In the context of electrolyzer applications, the proposed symmetric 3-level buck-boost converter with a coupled inductor in Ref.100 presents a compelling solution for enhancing performance and reducing common-mode (CM) noise. The converter’s design, as depicted in Fig. 19, integrates two separate inductors into a 1:1 coupled inductor configuration to boost power density and minimize winding loss. By leveraging quadrangle modulation for soft-switching and zero-voltage switching (ZVS) operation, the converter achieves peak power efficiencies ranging from 97.5% to 98.9% across varying voltage levels, demonstrating its effectiveness in optimizing energy conversion. This design not only improves efficiency but also offers up to a 25 dB reduction in CM noise compared to traditional setups, thereby enhancing overall electromagnetic compatibility. With simplified control mechanisms and real-time operation capabilities, the converter provides a practical and resource-efficient solution without the need for complex additional circuits, making it well-suited for demanding electrolyzer applications.
3-level buck-boost converter with a coupled inductor for electrolyzer applications.
Moreover, the experiments conducted on the 50 kW 3-level buck-boost converter underscore its promising performance metrics. The converter exhibits impressive power delivery capabilities, with output powers ranging from 12.5 kW to 50 kW under different voltage settings, accompanied by corresponding currents that align well with operational demands. The utilization of soft switching techniques, precise deadtime control, and optimized output capacitance results in efficient ZVS operation, further enhancing energy conversion efficiency. The observed linear relationship between ZVS currents and output currents validates the design methodology and emphasizes the topology’s effectiveness in both power delivery and common- mode noise reduction. While challenges exist, such as the need for active balance control to ensure consistent performance under diverse conditions, the converter’s strengths in efficient power delivery, robust noise suppression, and soft switching capabilities position it as a promising candidate for electrolyzer applications, where high efficiency and reliable operation are paramount.
In the realm of reconfigurable quadratic converters for electrolyzers in DC microgrids, the study in Ref.101 delves into two key topologies: the quadratic buck and semi-quadratic buck-boost converters through proposing three topologies of quadratic buck DC-DC converters were proposed that are capable of reconfiguring to semi-quadratic buck-boost converters. These versatile structures, as depicted in Fig. 20, are meticulously designed to ensure fault tolerance, seamlessly transitioning between configurations to uphold continuous operation even in the event of switch failures. By merging back-to-back cells, these converters boost their voltage conversion capabilities, offering a level of reliability and fault tolerance that surpasses conventional converter designs. This resilience is particularly vital for applications like electrolyzers in microgrids, where uninterrupted power supply is fundamental for equipment longevity. Leveraging graph theory concepts, these topologies amalgamate traditional converter architectures, creating adaptable systems adept at handling diverse operating conditions. This approach not only fortifies system resilience but also addresses the demands posed by high current and low voltage loads, notably prevalent in hydrogen production setups.
Quadratic converters for electrolyzers utilized in DC microgrids.
The experimental and numerical insights gleaned from these reconfigurable quadratic converters for electrolyzers in DC microgrids shed light on their operational prowess. Initial tests on a 0.5 Ω load with a 24 V input voltage showcased the converters’ adeptness at stepping down to 6 V. Notably, during these tests, the transition between the two switches revealed the converters’ flexibility, with the current through inductor L2 oscillating between 12 and 20 A, underscoring their adaptability across various operational scenarios. Employing components like Rohm RGTH00TK65GC11 switches, IXYS DPF30I300PA diodes, and Fair-Rite inductors, these converters operated at a frequency of 64 kHz facilitated by TI’s F28069M. Efficiency evaluations indicated levels ranging from 92 to 88% under specific conditions, emphasizing the influence of power ratings and component currents on overall efficiency. Furthermore, the closed-loop testing with a PI controller demonstrated stable responses to load and input voltage fluctuations, ensuring minimal disruption to the system’s output voltage stability. The converters’ seamless transition between different topologies, exemplified by the dynamic adjustment of duty cycles during shifts, underscores their versatility in meeting the evolving demands of DC microgrid applications. However, challenges such as efficiency drops under specific load conditions and switch failures advocate for further optimization to bolster overall performance and reliability in demanding operational contexts.
The three-level interleaved buck converter (TLIBC) showcased in Ref.102, exemplifies a groundbreaking advancement in power converter technology, specifically tailored for hydrogen production through PEM electrolyzers. The architecture of the TLIBC, as depicted in Fig. 21, is optimized for the unique low-voltage demands of electrolyzers, offering exceptional operational efficiency and fault tolerance. By leveraging a fault-tolerant design with dual pairs of complementary switches, this converter ensures uninterrupted performance, even in the face of component failures, underpinning its suitability for critical applications in hydrogen production. Through interleaved switch operation, the TLIBC not only mitigates thermal stress and minimizes current ripple but also significantly enhances energy efficiency, addressing the pressing need for reliable and fault-tolerant power converters in hydrogen generation setups. Its ability to reduce conduction losses, distribute current evenly among phases, and maintain low ripple current renders it ideal for medium-power applications linking renewable energy sources and electrolyzers, promising a sustainable energy future. The fault-tolerant control strategies discussed, as detailed in the document, further underscore the TLIBC’s robustness by effectively minimizing output current ripple and augmenting electrolyzer performance during prolonged operation. While the simulations and experiments affirm the converter’s strengths in fault identification, rapid recovery, and overall performance enhancement, they also reveal areas for improvement, notably in managing increased ripple during compensation. Nonetheless, the TLIBC’s demonstrated capabilities in enhancing system efficiency, ensuring continuous operation, and reducing degradation effects on electrolyzers position it as a promising solution for advancing hydrogen production technologies within renewable energy integration frameworks.
Three-level interleaved buck converter (TLIBC) for electrolyzer applications.
The DC-DC converter for electrolyzers proposed in Ref.102 introduces a zero-ripple circuit mechanism, as depicted in Fig. 22, ensuring minimal output current fluctuations and reduced voltage stress on switches. This design achieves a high conversion ratio of D/3 while maintaining near-zero output current ripple levels. By incorporating a passive ripple elimination circuit, the converter operates efficiently without the need for additional control or feedback systems, streamlining its functionality. Its operational principle revolves around effectively stepping down the output voltage from high to low levels by controlling power semiconductors and diodes. With a focus on zero-voltage switching to enhance efficiency, the converter offers benefits such as consistent and relatively low voltage stress on components, ultimately improving reliability. Rigorous validation under demanding electrolyzer conditions confirms its practical feasibility, positioning it as a promising solution for applications necessitating high step-down ratios with minimal ripple, especially in electrolyzers powered by renewable energy sources.
DC-DC Converter for electrolyzer with low ripple and high step down.
The numerical results of the converter’s performance underscore its strengths and weaknesses. Achieving a high step- down ratio, generating 56 V output voltage and 29 A electrolyzer current from a 400 V input, demonstrates its efficacy in hydrogen production. However, challenges such as increased switch voltage stress, surpassing alternative topologies with more components but lacking near-zero output current ripple, highlight areas for improvement. Hardware-in-Loop (HIL) experiments validate the converter’s high step-down ratio and minimal current ripple, essential for electrolyzer efficiency. Physical tests confirm the converter’s effectiveness, maintaining a stable output of 14 V and 5 A from a 90 V input, consistent with theoretical predictions. An efficiency curve analysis, showcasing a peak efficiency of 92.87% using a WT1600 power analyzer, along with power loss decomposition charts, emphasizes the converter’s operational efficiency under full-load conditions. Nonetheless, challenges like high voltage stress on switches and capacitors point towards optimization needs to enhance reliability and longevity in practical applications.
In Ref.92, a galvanically isolated step-down DC/DC converter is introduced, featuring a half-bridge inverter on the primary side, a high-frequency transformer for stepping down voltage, and a full-bridge phase-shifted active rectifier with reverse blocking (RB) switches on the secondary side, as depicted in Fig. 23. This design leverages a phase-shifted synchronous rectification technique to suppress ringing, enhance efficiency, and enable zero voltage switching (ZVS) across the converter’s switches, utilizing non-dissipative capacitive snubbers within the inverter for stable, constant frequency operations and straightforward control mechanisms. The rectification segment is implemented as a phase-shifted full-bridge configuration, offering adjustable output voltage by modulating the delay in switch activation between the rectifier and the inverter IGBTs, ensuring transformer current directionality and enabling ZVS for the inverter’s transistors. This setup incorporates non-dissipative capacitive snubbers in the inverter and proposes using the transformer’s leakage inductance as an inductive snubber in the rectifier to mitigate potential energy backflow during switch delay periods, which could otherwise increase inverter input voltage, alter the midpoint potential of the capacitor input voltage divider, and reduce both power factor and efficiency due to heightened conduction losses. Addressing the drawbacks associated with traditional phase-shifted synchronous rectifiers, the rectifier’s control algorithm can be fine-tuned to retain phase-shifted control benefits while minimizing energy return effects through the introduction of additional switching states for the rectifier switches, effectively reducing power source backflow. This proposed topology delivers several advantages, including diminished ringing, improved efficiency, and ZVS for converter switches, by curbing energy return intervals. It simultaneously minimizes potential rises in input voltage, deviations in midpoint potential, and deteriorations in power factor and efficiency, with the inclusion of non-dissipative capacitive and inductive snubbers simplifying the design. However, specific performance metrics like efficiency of this topology remain unspecified in the documentation, necessitating further empirical data or testing results for a comprehensive performance evaluation.
Half-bridge converter for electrolyzer applications with controlled RB switches at the secondary side.
The proposed bidirectional power converter topology, outlined in Ref.103 and depicted in Fig. 24, offers a solution for hydrogen generation systems, particularly in electrolyzer applications. By integrating a full-bridge inverter with a push–pull setup, the system ensures low ripple and high step-down capabilities essential for stable power conversion. Through cyclic alteration of the transformer’s polarity using the full-bridge inverter, bidirectional power flow, galvanic isolation, and soft switching operation are achieved across a wide range of input power and voltage variations. This approach prioritizes current stability for the electrolyzer, optimizing hydrogen production efficiency in correlation with the applied current density. The utilization of a Proportional Integral (PI) controller in the control system enhances regulation by managing the converter’s phase-shift to maintain the desired current output accurately.
Push–Pull isolated DC-DC converter for electrolyzer applications.
The converter’s design emphasis on current stability for the electrolyzer, rather than solely focusing on power extraction from the PV array, showcases a distinctive strategy in hydrogen generation technology. The system’s robust performance under varying radiation inputs, as demonstrated by simulation results, underscores its reliability and precise current control capabilities. While the topology excels in current stability and adaptability to changing radiation levels, the trade-off emerges in its limited potential for power optimization due to this prioritization. However, the converter’s advantages in efficiency, bidirectional power flow, and soft switching operation position it as a viable choice for applications requiring consistent current input, such as hydrogen generation systems.
Regarding the numerical results of the push–pull buck converter system for hydrogen generation, the PV array’s dynamic response to radiation changes, generating peaks of 200W and 420W against a maximum power of 213W, reflects its adaptability and efficiency. The hydrogen electrolyser’s characteristics, including a rated input voltage of 380 V, power consumption of 19,000W, and hydrogen production rate of 10Nm3/h, highlight the system’s capability to ensure stable current provision to the electrolyzer cells, thereby guaranteeing a consistent output despite radiation fluctuations. The converter’s adaptability to varying radiation inputs showcases its versatility in meeting load demands efficiently, while hardware-in-the-loop testing further confirms its dynamic behavior and robust performance in practical scenarios.
The proposed bidirectional power converter topology presents a promising solution for advancing hydrogen generation technologies, with significant potential contributions to the sustainability and innovation of the energy sector. Its applicability extends beyond electrolyzer applications to encompass renewable energy integration, grid-tied systems, and other settings requiring reliable and efficient power conversion. Despite the trade-off between efficiency and flexibility due to its focus on current stability, this converter remains a reliable choice for stable hydrogen production, highlighting its potential for driving advancements in sustainable energy solutions.
The proposed bidirectional power converter topology, as detailed in Ref.104, integrates a full-bridge inverter with a push–pull configuration, showcasing a versatile design tailored for medium power ranges up to 5 kW. This approach offers galvanic isolation and seamless integration with grid-connected inverters, eliminating the need for complex PV array reconfigurations. Utilizing a push–pull topology shown in Fig. 25 provides distinct advantages such as simplified driving requirements, elimination of bulky input capacitors, efficient thermal management with air-forced heatsinks, straightforward PWM control, and well-understood stability characteristics. Despite potential drawbacks like reduced efficiency due to hard-switching and higher blocking voltages, the push–pull configuration is strategically chosen for its overall benefits. By alternating the states of its switches to manage core magnetization effectively, this design ensures efficient energy transfer, making it particularly suitable for applications demanding efficient power conversion, including solar energy systems and electrolysis processes. The converter’s stability, simplicity, and performance at medium power levels make it a promising solution for such applications. The meticulous selection of key components like the IGBT IRGPS60B120KDP and the rectifier diode UFB200FA40P, along with the carefully designed magnetic devices and filters, contributes to the converter’s impressive performance. Operating at a switching frequency of 50 kHz and achieving an effective frequency of 100 kHz at the output, the converter demonstrates a peak efficiency exceeding 90%, indicating robustness in power conversion. Through extensive simulations and experimental validations, the converter’s reliability and efficiency in maintaining stable operation are evident, underscoring its potential as a well-rounded solution for grid-connected photovoltaic applications and electrolyzer systems. This topology’s balance between performance and practicality in power electronics design positions it as a compelling choice for various renewable energy applications.
Bidirectional full bridge converter based on push–pull configuration for electrolyzer applications.
The Isolated Full Bridge Boost DC-DC Converter proposed in Ref.105, presents a robust solution for bidirectional energy conversion in fuel cell and electrolyzer applications within grid-tie scenarios. This converter’s ability to efficiently handle the fluctuating power output from renewable sources makes it a pivotal component in energy storage systems. The design, illustrated in Fig. 26, operates by converting low voltage and high current from fuel-electrolyzer cells into high voltage suitable for grid integration. With reported efficiencies ranging from 96.5 to 97.8%, the converter boasts remarkable performance, ensuring minimal energy wastage during operation. Moreover, its galvanic isolation feature enhances safety, while scalability enables flexibility in accommodating diverse power levels by parallel connection of cells and converters. These attributes position the converter as a versatile tool for applications in smart grids and energy storage systems, where efficiency and adaptability are paramount.
Isolated full-bridge boost converter (IFBBC) for electrolyzer applications.
The experimental evaluation of a 6 kW prototype of the Isolated Full-Bridge Boost Converter in Solid Oxide Fuel Cell (SOFC) and Solid Oxide Electrolyzer Cell (SOEC) modes offers valuable insights into its operational efficiency and losses. Notably, at 30 V input voltage, efficiencies are constrained to 95.9% in SOFC and 93.3% in SOEC, primarily due to substantial conduction and switching losses induced by high current. However, at the peak input voltage of 80 V, efficiencies soar to 97.8% in SOFC and 96.5% in SOEC, underlining the converter’s capability to deliver exceptional performance across varying voltage levels. The disparity in efficiency between SOFC and SOEC modes can be attributed to losses stemming from IGBTs and active rectification, with SOEC mode consistently exhibiting lower efficiency metrics. Loss analysis underscores the critical role of the boost inductor in SOFC mode and the significant contribution of high-voltage power semiconductors, particularly IGBTs, to losses in SOEC mode, with switching losses accounting for up to 45% of total losses. The study emphasizes the influence of component temperatures on efficiency and advocates for the potential integration of SiC devices to alleviate losses, especially in SOEC mode marked by substantial losses from high-voltage IGBTs. Overall, the Isolated Full Bridge Boost Converter stands as a promising solution for integrating fuel and electrolyzer cells into grid-tie applications, showcasing its potential to advance renewable energy utilization and storage technologies effectively.
The study presents a meticulous investigation into soft-switched DC-to-DC converters tailored for electrolyzer applications in renewable energy systems, highlighting three distinct high-frequency transformer isolated converter topologies. Central to the analysis is a two-stage methodology aiming for optimal performance, with particular focus on the LCL-type series resonant converter (SRC) integrated with a capacitive output filter. This topology, as illustrated in Fig. 21, showcases remarkable versatility in maintaining zero-voltage switching (ZVS) capabilities across a wide spectrum of input voltage and load current variations. By leveraging advanced soft-switching techniques, these converters not only manifest heightened efficiency but also exhibit reduced dimensions, weight, cost implications, and mitigated electromagnetic interference concerns, making them exceptionally well-suited for electrolyzer applications within renewable energy frameworks. The amalgamation of the ZVT boost converter with the LCL SRC, as demonstrated in Fig. 27, ensures consistent ZVS functionality throughout the entire range of line and load variations, underscoring the robustness and adaptability of this design. The converters, meticulously engineered to meet critical specifications like a maximum 7.2 kW output power, input/output voltages spanning 40 to 60 V, and requisite electrical isolation, hold significant promise for fostering efficient power flow control between renewable energy sources and electrolyzers. This strategic integration not only streamlines the power management process but also contributes substantially to the seamless assimilation of renewable energy resources into existing grids.
Two-stage ZVT boost converter with LCL-Type SRC for electrolyzer applications.
The experimental results and numerical analyses detailed in the study shed light on the efficacy of the proposed topology, emphasizing its superiority over alternative configurations. Specific component values derived through rigorous calculations underscore the critical role of component selection in optimizing performance metrics. Noteworthy equations for determining vital parameters like Lr, Cr, and other normalized quantities further accentuate the meticulous design considerations that underpin the operational efficiency of these converters. The study’s conclusion, bolstered by practical validations, unequivocally positions the LCL SRC with a capacitive output filter as the premier choice for electrolyzer applications, extolling its virtues such as simplified design, inherent soft-switching capabilities, and consistent efficiency across diverse operational scenarios. In essence, this topology stands poised as a transformative solution for advancing electrolyzer applications within the realm of renewable energy systems, promising substantial benefits and operational enhancements in real-world deployments.
The study in Ref.106, explores the implementation of a multiphase interleaved DC-DC converter for power-to-hydrogen systems, particularly focusing on its applicability to large-scale hydrogen production at the MW power level. The system’s core, the P2H power stage, adeptly transforms medium voltage electrical power into a robust DC current stream tailored for the hydrogen electrolyzer, dynamically adjusting hydrogen output in response to renewable energy sources (RES) generation. Notably, this system, introduces an isolated DC-DC stage with a multiphase interleaved configuration, as depicted in Fig. 28, to curtail specific energy consumption and fortify system reliability. By integrating parallel stages within the converter, this approach achieves a commendable minimum load current ripple factor, fostering stable operation and prolonging equipment longevity. Impressively, power losses are impressively curtailed to 0.4% for load current ripple factor and a mere 1.8% for overall power losses, spotlighting its prowess in augmenting efficiency and system performance. The adept deployment of this cutting-edge topology not only streamlines energy transfer and operational robustness but also heralds promising prospects for upscale electrolysis applications, showcasing a more effective and dependable solution for the burgeoning realm of large-scale hydrogen production.
Isolated DC-DC stage with a multiphase interleaved configuration for electrolyzer applications.
The three-port DC/DC converter design proposed in Ref.107 and illustrated in Fig. 29, introduces a versatile and efficient solution for applications in electrolyzers and other energy systems. By incorporating a three-winding transformer for electrical isolation and voltage transformation, this converter facilitates bidirectional power flow among its input, fuel cell/batteries, and Electrolyzer ports. The utilization of a dual active bridge (DAB) topology for Port 2 offers significant advantages, including high efficiency, soft switching capabilities, and the implementation of advanced control strategies such as single-phase-shift (SPS) and extended phase-shift (EPS) control. These features enable precise management of power exchange between sources like batteries, fuel cells, and the grid, enhancing operational flexibility for renewable energy systems, electrolyzers, and batteries.
Three-port isolated DC/DC converter for electrolyzer applications.
In experimental validation, the converter exhibited peak efficiencies exceeding 95%, demonstrating its robust performance characteristics. Notable strengths of the topology include its rapid response to Electrolyzers (EL), a streamlined single-stage conversion structure, comprehensive system integration capabilities, and the ability to facilitate bidirectional power transfer across ports. Despite these strengths, observations of duty cycle losses in the Pulse-Width Modulation (PWM) stage highlight a potential area for improvement. The converter effectively regulated power flow between renewable energy sources, electrolyzers, and fuel cells/batteries, maintaining RMS current values within optimal ranges. While minor inefficiencies and current ripples were noted, the converter excelled in tightly controlling output voltages and power flows across different operational modes, showcasing its adaptability to varying load conditions.
The experimental efficiency curves further underscore the converter’s reliability and efficiency, with exceptional performance demonstrated at 1 kW output power. These results position the proposed converter as a promising solution for Hybrid Energy Storage System (HESS) applications, given its exceptional efficiency, bidirectional power transmission capabilities, and adaptability to diverse operational scenarios. The converter’s unique design, control mechanisms, and soft switching operations not only enhance system reliability but also hold the potential to elevate the integration and performance of various energy sources within a unified system, making it a compelling option for modern energy management applications, particularly in electrolyzer systems.
The quest for efficient energy conversion in power-to-hydrogen systems has spurred the development of various DC-DC converter topologies, each exhibiting unique characteristics regarding efficiency, control complexity, scalability, and the number of semiconductor devices employed. Table 10 highlights both isolated and non-isolated converter types, illustrating their performance metrics and operational implications. Non-isolated converters, such as the quadratic buck converter, achieve an efficiency of 93% with a simple configuration of 1 MOSFET and 3 diodes, making it ideal for small-scale applications where cost and simplicity are essential. The multi-device boost converter achieves 96.45% efficiency with 2 MOSFETs and 2 diodes, effectively converting energy in small systems while maintaining moderate complexity.
In terms of performance, the multi-phase interleaved boost converter stands out with an efficiency of 97.68% while utilizing 2 MOSFETs and 2 diodes. Its interleaved configuration reduces ripple and improves load distribution, making it particularly suitable for applications requiring stable output. Similarly, the multi-device multi-phase interleaved boost converter reaches 97.76% efficiency with 4 MOSFETs and 4 diodes, balancing improved performance with increased control complexity appropriate for medium-scale applications. The continuous input current non-isolated bidirectional converter displays versatility, achieving 95.6% efficiency in boost mode and 95.4% in buck mode with 4 MOSFETs. Additional notable designs include the interleaved buck-boost DC-DC converter, which offers variable efficiency ranging from 89.18 to 96.25%, and the three-level buck-boost converter with a coupled inductor, showcasing efficiencies between 97.5 and 98.9% with 8 MOSFETs. The quadratic converters for electrolyzers achieve efficiencies between 88 and 92% using 2 MOSFETs and 2 diodes, while the non-isolated interleaved DC-DC converter for zero-current-ripple circuits achieves 92.87%, designed specifically for applications requiring minimal ripple.
Isolated DC-DC converters generally offer higher efficiencies, making them more suitable for larger systems.The half-bridge converter circuit, featuring 6 MOSFETs and 4 diodes, presents increased complexity due to a higher component count, yet it is suitable for intermediate applications with enhanced power handling capabilities. The isolated full-bridge boost converter (IFBBC) achieves efficiencies ranging from 96.5 to 97.8% with 8 MOSFETs, justifying its complexity due to superior performance. The push-pull isolated bidirectional converter enhances safety in high-power applications with its use of 8 MOSFETs. The isolated DC-DC stage with a multiphase interleaved configuration boasts an impressive 98.2% efficiency but requires a large number of components, necessitating advanced control strategies. The three-port isolated DC-DC converter operates at less than 95% efficiency with 8 MOSFETs and 4 diodes, providing flexibility for multiple outputs, though its control complexity can be challenging. The integrated multiport DC-DC converter, while having variable efficiency, employs 6 MOSFETs and 4 diodes, allowing for versatile applications but also introducing control complexities.
In summary, the choice of DC-DC converter topology for power-to-hydrogen systems must balance efficiency, control complexity, and scalability. Isolated converters usually provide higher efficiencies, making them ideal for demanding applica- tions, whereas non-isolated converters offer simplicity and cost-effectiveness for smaller-scale systems. Control complexity varies significantly among designs; simpler configurations facilitate easier control, while advanced multi-phase and interleaved converters require sophisticated techniques to manage their interactions effectively. Additionally, scalability is crucial, as non-isolated converters are generally more suitable for smaller applications, while isolated converters excel in larger systems. Understanding these trade-offs is essential for optimizing energy conversion processes in hydrogen production, ultimately contributing to the advancement of sustainable energy technologies. This comprehensive analysis underscores the necessity for tailored solutions that align with specific application requirements, ensuring the successful integration of DC-DC converters in future hydrogen energy systems.
The elimination of the line-frequency transformer (LFT) is a transformative step in the evolution of power-to-hydrogen (P2H) systems, aiming to enhance efficiency and reduce the physical footprint of these technologies. Traditionally, the LFT has been essential for voltage transformation and isolation in P2H applications, but its bulkiness poses challenges for transportation, installation, and overall system integration. By removing the LFT, P2H systems can achieve a more streamlined design, allowing for greater scalability and flexibility to meet the demands of modern energy infrastructures. This shift paves the way for innovative converter topologies and advanced control strategies that can maintain performance while minimizing size and complexity.
Recent advancements emphasize the transition from traditional converter topologies to modular multicell rectifier systems. These modular systems shown in Fig. 30, comprised of discrete converter cells implementing an AC/DC stage followed by a DC/DC stage, facilitate efficient interfacing with medium voltage (MV) grids through configurations such as input-series–output-parallel (ISOP) and input-parallel–output-parallel (IPOP). The ISOP configuration utilizes 1200/1700 V Si IGBT and SiC MOSFET devices to optimize performance for higher voltage levels, while the IPOP configuration, adaptable for both MV and low-voltage power grids, employs 10-kV SiC MOSFETs, effectively addressing challenges like flashover faults and high switching losses112.
Modular multi-cell rectifier’s circuit diagram112. (a) ISOP configuration (b) IPOP configuration.
Significant technological advancements also manifest in the realization of AC/DC and DC/DC stages within these modular systems. The AC/DC stage can be implemented using a single-phase full-bridge converter, potentially incorporating soft-switching techniques to boost efficiency112. Additionally, integrating high-frequency transformers (HFTs) and medium-frequency transformers (MFTs) in the DC/DC stage contributes to reducing the overall size and weight of the converter cells. Compared to traditional Si IGBTs, SiC MOSFETs are preferred for their ability to operate at higher switching frequencies, enhancing overall system efficiency. However, this modular design introduces complexities in control, necessitating that each converter cell self-regulate and share power effectively. This shift toward modular, scalable designs not only promises improved efficiency and reduced footprint but also lays a foundation for further research and development in P2H technologies, positioning them as critical components in the transition to sustainable hydrogen production112.
Various topologies, such as the multi-stage AC configurations referenced113,114,115,116,117,118,119,120, present compelling solutions to circumvent the need for cumbersome line transformers, especially when integrating electrolyzers with wind power systems. These converters offer a host of advantages for hydrogen production, boasting heightened efficiency through reduced switching losses and improved thermal performance. By ensuring the delivery of sinusoidal currents with a unity power factor, these converters significantly enhance power quality and curtail harmonic distortion. Their compact design caters well to space-constrained settings, while operational flexibility, diminished maintenance requirements, refined control mechanisms, scalability, and cost-effectiveness collectively position them as robust choices for diverse energy requirements.
An illustrative instance of such a topology is the multi-stage serial configuration, as proposed in Ref.85, incorporating a Vienna rectifier and a synchronous buck converter to interface electrolyzers with the AC grid, thereby amplifying hydrogen production via water electrolysis. The architecture, depicted in Fig. 31, seamlessly combines the Vienna rectifier with the synchronous buck converter to yield a resilient system adept at converting AC voltage into regulated DC voltage suitable for electrolyzer operation. The Vienna rectifier excels in furnishing sinusoidal currents with a unity power factor, effectively mitigating harmonic distortion and elevating overall power quality. Its streamlined design, featuring fewer active switches compared to conventional rectifiers, leads to diminished switching losses and enhanced thermal efficiency.
Operating in grid-connected and off-grid modes, this system accommodates varying power demands with agility. While the Vienna rectifier stabilizes the DC voltage level in grid-connected mode, the synchronous buck converter modulates the voltage supplied to the electrolyzer in off-grid scenarios, ensuring optimal performance across diverse operating conditions. This adaptability proves pivotal for applications reliant on intermittent renewable energy sources, facilitating efficient energy management and storage.
A standout feature of this multi-stage converter topology, over conventional line transformer-based setups, lies in its capacity to furnish high-quality power with minimal losses. The compact nature of the Vienna rectifier contrasts sharply with the bulkiness of traditional transformers, curbing energy wastage attributed to size and weight. Advanced control functionalities integrated into the system enable precise voltage and current regulation, bolstering both reliability and efficiency.
The incorporation of a Vienna rectifier not only enhances the power factor but also streamlines the system architecture by obviating the necessity for bulky transformers. Consequently, a lighter, more cost-effective solution emerges, readily scalable to diverse applications. Furthermore, the direct AC-DC conversion process minimizes reliance on extensive filtering apparatus, thereby reducing maintenance outlays and operational expenses.
Power electronic converters play a pivotal role in meeting these demands, offering voltage regulation, surge protection, and adaptive power adjustments. Industrial electrolyzers, in particular, necessitate a stable DC voltage range and substantial current, highlighting the importance of these converters. For those powered by grid or wind farm sources, AC-DC converters are essential, whereas solar panel-backed ones rely on DC-DC converters for Maximum Power Point Tracking (MPPT) and voltage regulation.
A widely employed solution in power-to-hydrogen systems, the 6-pulse diode bridge rectifier, unfortunately, introduces challenges such as a reduced power factor and harmonic disturbances. These harmonics can adversely affect the power system, control de-vices, and other electrical equipment. Recognizing this, international standards like IEEE-519 and DO-160G have been formulated to mitigate these current harmonics. Solutions like filters and multi-pulse rectifiers have been devised to counteract these harmonic problems. Although filters, both active and passive, can enhance power quality, they bring with them challenges concerning design complexity, cost, and efficiency variations. Meanwhile, increasing the converter’s number of pulses, such as with the 12-pulse rectifiers, emerges as a more efficient method to improve power quality. These multi-pulse rectifiers are becoming the preferred choice in industrial settings due to their simplicity and cost-effectiveness. Techniques combining both active and passive auxiliary harmonic-reducing circuits have been proposed to meet the rigorous standards set by the likes of DO-160G and IEEE-519 without significantly increasing weight and dimensions. Among these approaches, multi-pulse rectifiers, especially when combined with pulse multiplication circuits, stand out for balancing simplicity, efficiency, and affordability. Future efforts should concentrate on refining the dimensions, weight, and cost of these rectifiers, perhaps incorporating the merits of autotransformers in non-isolated setups. Diving into the realm of AC-DC converter topologies, there’s a spectrum of designs, each having its unique benefits, drawbacks, and efficiencies. Key among them is the 12-pulse thyristor-based rectifier, which incorporates a three-phase, three-winding trans-former and an active power filter (APF) for advanced harmonic filtering. Noteworthy is the 20-pulse asymmetric non-isolated Multi-Step Auto-connected Transformer, which amplifies power quality by producing an almost sinusoidal waveform. A notable inclusion is the Active Front End (AFE) converters, with the B6 rectifier particularly standing out for superior current shaping and voltage control.
Beyond these, the 3L-DNPC rectifier, applied by companies like ABB in their electrolyzer systems, boasts of features like a high-power factor and unmatched efficiency, although its complex design with 18 active switches might be a limiting factor for some. The Three-Phase Three-Level T-Type Converter offers improved power quality and a simpler construction. On the DC-DC front, while traditional converters sometimes grapple with efficiency challenges, the Isolated DC-DC Converters and the Continuous Input Current Non-Isolated Bidirectional Interleaved Buck-Boost DC-DC Converter present promising solutions, though they are not without their disadvantages.
Recent advancements such as the 3-level buck-boost converter with a coupled inductor, Reconfigurable quadratic converters, three-level interleaved buck converter, DC-DC Converter with zero-ripple circuit mechanism, Push-Pull isolated DC-DC converter, and Isolated Full-Bridge Boost Converter (IFBBC) represent significant strides in improving fault tolerance, operational efficiency, and energy conversion in electrolyzer applications.
The Two-stage ZVT boost converter with LCL-Type SRC emerges as a standout choice for electrolyzer applications, celebrated for its simplified design, inherent soft-switching capabilities, and consistent efficiency under varying operational conditions. This converter not only streamlines the design process but also ensures optimal performance, making it a promising solution for the evolving needs of electrolyzer systems.
Furthermore, the multiphase interleaved DC-DC converter represents a significant development in power-to-hydrogen systems, with a specific focus on enabling large-scale hydrogen production at the MW power level. Its innovative design and scalability position it as a key player in advancing the efficiency and capacity of hydrogen generation technologies.
The Three-port DC/DC converter introduces a versatile solution facilitating bidirectional power flow among multiple sources, including inputs, fuel cells/batteries, and electrolyzers. This flexibility in power management is crucial for optimizing energy utilization and ensuring seamless operation within integrated energy systems.
A notable trend in the field is the progressive elimination of bulky Frequency line transformers in favor of more efficient, reliable, and scalable solutions such as modular multi-cell rectifiers and multistage converter rectifiers. By embracing these advancements, the industry is poised to achieve higher levels of efficiency, reliability, and adaptability in electrolyzer systems, paving the way for sustainable energy production and utilization. Future research should focus on investigating advanced converter topologies that go beyond 12-pulse rectifiers and active front end converters. Additionally, techniques for mitigating harmonics should be explored to effectively reduce distortion. Optimization of DC-DC converters is necessary to improve efficiency and reliability. Integration of renewable energy sources into electrolyzer systems should be studied, along with the research on smart grid integration for grid-friendly operation. Power electronics for energy storage applications should also be explored. Furthermore, fault-tolerant converter designs and reliability enhancement techniques should be investigated. Lastly, efforts should be made to contribute to the development of industry standards and regulations governing power electronic converters in electrolyzer systems.
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.
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Department of Electrical Power Engineering, Egypt-Japan University of Science and Technology, Alexandria, 5221241, Egypt
AlAmir Hassan & Omar Abdel-Rahim
Department of Electrical Engineering, Aswan University, Aswan, 81542, Egypt
Hourani Center for Applied Scientific Research, Al-Ahliyya Amman University, Amman, Jordan
Department of Electrical Engineering, Graphic Era (Deemed to be University), Dehradun, 248002, India
College of Engineering, University of Business and Technology, 21448, Jeddah, Saudi Arabia
Department of Theoretical Electrical Engineering and Diagnostics of Electrical Equipment, Institute of Electrodynamics, National Academy of Sciences of Ukraine, Beresteyskiy, 56, Kyiv-57, 03680, Ukraine
Center for Information-Analytical and Technical Support of Nuclear Power Facilities Monitoring, National Academy of Sciences of Ukraine, Akademika Palladina Avenue, 34-A, Kyiv, India
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AlAmir Hassan: Conceptualization, Methodology, Software, Visualization, Investigation, Writing- Original draft preparation, Omar Abdel-Rahim: Data curation, Validation, Supervision, Resources, Writing—Review & Editing, Mohit Bajaj & Ievgen Zaitsev: Project administration, Supervision, Resources, Writing—Review & Editing. All authors reviewed the manuscript.
Correspondence to omar abdel-Rahim or Ievgen zaitsev.
The authors declare no competing interests.
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Hassan, A., Abdel-Rahim, O., Bajaj, M. et al. Power electronics for green hydrogen generation with focus on methods, topologies, and comparative analysis. Sci Rep 14, 24767 (2024). https://doi.org/10.1038/s41598-024-76191-6
DOI: https://doi.org/10.1038/s41598-024-76191-6
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