Segrate (Milan), February 20, 2025
Green hydrogen is one of the most promising solutions for the energy transition and the decarbonization of important industrial sectors. Unlike other types of hydrogen, this energy carrier is produced using renewable sources, through a process of water electrolysis that does not involve CO₂ emissions.
Even if the production costs are still higher than those of fossil alternatives, technological progress and the increasing adoption of renewable energy sources are making this solution more and more competitive. Power converters play a crucial role in this context. These electronic devices convert and regulate the electrical energy from the electrolyzers, thus having a significant impact on efficiency and reliability and making the production of green hydrogen increasingly attractive for many sectors, from industry to transportation.
Green hydrogen and the electrolysis process
The electrolysis process allows the production of green hydrogen by separating water (H₂O) into its fundamental components, oxygen (O₂) and hydrogen (H₂), using electricity. The device that carries out this separation is the electrolyser, which consists of:
- Two electrodes (anode and cathode), immersed in an electrolyte solution or separated by an ion-conducting membrane.
- A power supply that provides the necessary direct current.
When electrical energy is applied to the electrodes:
- At the anode (positive electrode): the water molecules give up electrons, producing oxygen gas (O₂) and hydrogen ions (H⁺).
- At the cathode (negative electrode): the hydrogen ions (H⁺) acquire electrons and are reduced, forming hydrogen gas (H₂).
The overall chemical equation of the process is: 2H2O(l)→2H2(g)+O2(g)2H_2O(l)
where (l) indicates the liquid state and (g) the gaseous state of the products.
Types of electrolyzers
There are several electrolytic cell technologies, each with specific characteristics:
- Alkaline electrolyser (AEL): uses potassium hydroxide (KOH) or sodium hydroxide (NaOH) solutions. This is a well-established, economical technology with efficiencies of 60-70%.
- PEM (Proton Exchange Membrane) electrolysers: these use a proton exchange membrane, with greater current density and rapid response to energy variations. They have efficiencies of 70-80%, but higher costs.
- Solid oxide electrolysers (SOEC): these operate at high temperatures (600-800°C), using solid oxides as electrolytes. They can reach efficiencies of over 80%, using industrial waste heat, but are still in the development phase.
- AEM (Anion Exchange Membrane) electrolysers: these combine some of the advantages of alkaline and PEM electrolysers, using an anion exchange membrane. They offer potential cost reductions compared to PEMs thanks to the use of non-precious materials, while maintaining good performance and efficiency.
The role of converters in green hydrogen production
The production of green hydrogen by electrolysis requires a large amount of energy and, above all, a stable and optimised power supply. Power converters are essential to ensure the correct functioning of the electrolysers, converting, regulating and optimising the electrical energy to improve the overall efficiency of the process. The main task of the conversion units (or PSU, Power Supply Unit) is to adapt the input power to the electrolysers, ensuring a stable supply that is compatible with the specific needs of the electrolysis process.
The main functions of converters include:
- Converting alternating current (AC) to direct current (DC): Electrolysers operate with direct current, while the electrical grid supplies alternating current. Converters transform this energy to make it usable.
- Regulating voltage and current: Electrolysis requires precise parameters to ensure the efficiency and durability of the electrodes. Converters regulate voltage and current.
- Reduction of energy losses: High-efficiency converters minimise energy dispersion during transformation.
- Adaptation to variable loads: Renewable energy availability and hydrogen demand can fluctuate. Converters guarantee flexible operation, responding to these variations.
- Protection and safety: The converters protect the electrolysers from overvoltage, overcurrent and other electrical problems.
Types of converters used in green hydrogen production
Depending on the characteristics of the plant and the specific needs of the electrolyzers, different types of converters are used:
- AC/DC converters (power rectifiers):
- They transform alternating current into direct current, which is essential for the operation of electrolyzers.
- They can be diodes, thyristors or advanced semiconductors (IGBT, SiC).
- They offer high efficiency and low harmonic distortion (THD).
- DC/DC converters:
- They adjust voltage and direct current to match the requirements of electrolyzers.
- They optimize operation in the presence of energy variations, such as from solar panels or wind turbines.
- By providing a low ripple, they ensure a stable current, which is essential for preserving electrodes.
- Multilevel converters:
- Employed in medium-power systems, they improve performance with respect to power quality to the distribution grid.
- By providing a low ripple, they ensure a stable current, which is essential for preserving electrodes.
FRIEM designs and produces various solutions for the production of green hydrogen. Our conversion units are modular and designed to meet the needs of the main electrolytic cell technologies. They mainly consist of a transformer, a rectifier, DC isolators, a cooling unit, a polariser and a control system and a possible harmonic filtering system if required by network constraints (power quality).
The benefits of green hydrogen converters
Increased energy efficiency
As explained, converters help to improve the overall efficiency of the electrolysis system by reducing energy losses and optimizing power transfer. The main ways in which this is accomplished are:
- Minimisation of conversion losses: specific design solutions and the use of advanced semiconductors such as silicon carbide (SiC) reduce energy dissipation.
- Optimisation of voltage and current: Converters regulate the energy supplied to the electrolyzers, improving the efficiency of the electrical conversion into hydrogen and reducing losses due to the Joule effect.
- Natural switching technologies and multilevel converters: These improve efficiency and reduce harmonic distortion, guaranteeing a high quality electrical energy supply.
These improvements reduce energy consumption per kg of hydrogen produced, helping to make the process more economical and sustainable.
Operational stability and longer life of electrolyzers
Proper operation of electrolyzers depends on a stable power supply. Fluctuations in voltage or current can damage electrodes and reduce efficiency. Power converters ensure:
- Stability of direct current (DC): The converters reduce ripple and protect the electrodes from electrical stress.
- Precise power control: Optimising the operation of the electrolyzers extends their lifespan.
- Protection from overvoltage and overcurrent: The converters reduce the risk of breakdowns and lower maintenance costs.
Flexibility and adaptability to renewable sources
Renewable energy, such as solar and wind, is intermittent and difficult to predict. Converters make it possible to:
- Adapting to fluctuations in renewable energy: they dynamically regulate voltage and current based on energy availability.
- Integrating energy storage systems: using batteries or supercapacitors, excess energy can be stored and used when production is low.
- Manage the modulation of hydrogen production: optimise the operation of the electrolysers based on the availability of energy and electricity prices.
This flexibility maximizes the use of renewable sources and reduces the cost of green hydrogen, making it more competitive with fossil alternatives.
Reduced operating and maintenance costs
The use of advanced converters helps reduce operating costs through:
- Lower energy losses: Reduction in the cost of electricity needed for hydrogen production.
- Greater reliability: The systems become more resistant to breakdowns and require less technical intervention, guaranteeing continuity in hydrogen production.
- Reduction in maintenance costs: thanks to the protection of the electrolysers and optimisation of their operation, the costs associated with routine maintenance are reduced.
- Long life: thanks to their specific design, they guarantee an extended operating life of over 30 years, maximising the initial investment.
In addition, increased electrolyzer lifetime and improved efficiency allow the initial investment in converters to pay off quickly, improving the overall profitability of green hydrogen plants.
Support for power grid stability
Large-scale production of green hydrogen, powered by renewable sources, can affect the stability of the electric grid. Converters contribute to grid management through:
- Frequency and load regulation: Converters support the stability of the electrical system.
- Demand response functionality: They allow energy consumption to be modulated according to the availability of renewable energy.
- Management of microgrids: They optimise the integration between production, storage and use of energy for hydrogen production.
In this way, the converters not only improve the production of green hydrogen, but also promote greater integration of renewable energy into the overall energy mix.
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As technologies evolve and innovative materials are introduced, converters are achieving ever higher levels of performance, helping to reduce operating costs and improve the sustainability of green hydrogen production.
Investing in highly efficient and reliable converters is a strategic choice that makes green hydrogen increasingly competitive and economically viable, solidifying it as a key solution in the global energy landscape.