Hydrogen production

Considering that it is the world’s cleanest energy source, hydrogen is set to play a key role in the decarbonization of several industrial sectors. Crucial to this rapidly developing process is the development of electrolyzer power supplies, which must be capable of managing unregulated low DC voltage under megawatt-scale power levels. This article presents an overview of state-of-the-art power supplies for electrolyzers, while examining the pros and cons behind each configuration.

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Ahmed Abdelhakim ABB Corporate Research Västerås, Sweden, ahmed.abdelhakim@se.abb.com; Tero Viitanen ABB Motion System Drives Helsinki, Finland, tero.viitanen@fi.abb.com; Francisco Canales ABB Corporate Research Baden, Switzerland, francisco.canales@ch.abb.com

Decarbonization of major energy-consuming sectors is a top priority of the 2015 Paris Agreement and the Intergovernmental Panel on Climate Change (IPCC) climate change 2022 report. Perhaps the most promising strategy for addressing this challenge is the implementation of hydrogen production technologies. These are seen to be a solution for many sectors, including so-called “hard-to-abate” areas in which it can be used as a feedstock or directly as a fuel [1].

Hydrogen is the world’s cleanest energy source, with around 80 million tons currently being produced annually [2]. Furthermore, production is expected to exceed 200 million tons by 2030, and 500 million tons by 2050 [3].

There are different ways of producing hydrogen, but water electrolysis, which today accounts for only around 0.3 GW, is expected to account for more than 60 percent of global production by 2050 [3]. In view of this, global electrolyzer capacity is expected to reach 850 GW by 2030 and 3,600 GW by 2050 →01.

01 Hydrogen production by water electrolysis is expected to grow considerably.
01 Hydrogen production by water electrolysis is expected to grow considerably.

Among the many important factors that must be considered in terms of enhancing electrolyzer performance while reducing cost is power supply. Different power supply configurations can be utilized, with each configuration having pros and cons as seen from both the grid and the electrolyzer perspectives.

With a view to making sense of these complex circumstances, this article presents a general classification and review of the state-of-the-art power supplies that can be utilized with grid-connected water electrolysis systems, while highlighting the pros and cons behind each configuration.

Water electrolysis technology and types of electrolyzers
Electrolyzers and fuel cells have a lot in common, but they also have a fundamental difference. The former use electrical energy to split the bonds of water and oxygen to release hydrogen →02, whereas fuel cells use hydrogen to produce electricity.

02 Illustration of water electrolysis theory.
02 Illustration of water electrolysis theory.

There are different types of electrolyzers, with alkaline, proton exchange membrane (PEM), and solid-oxide systems being the most common technologies. Alkaline electrolyzers represent a mature technology compared to PEM systems and have lower investment costs. Currently, PEM electrolyzers are behind their alkaline counterparts in terms of efficiency and cost due to their dependence on noble material. However, this is expected to change over the next few years. On the other hand. PEM electrolyzers have better dynamics and offer higher power density, meaning that they require 20 – 25 percent less space than alkaline systems [1].

Compared to alkaline and PEM electrolyzers, solid-oxide systems can offer higher efficiencies and can also be reversed to be fuel cells, thus generating electricity from hydrogen by using about 25 percent of the electrolyzer’s capacity [1]. On the other hand, solid-oxide electrolyzers operate at much higher temperatures compared to their alkaline and PEM counterparts, which still operate in the kW scale.

Classification of power supplies for electrolyzers
Electrolyzers are characterized as low-voltage (LV) equipment, where a typical 5 MW unit can have a maximum DC voltage of 1 kV by end-of-life. Hence, connecting it to a medium-voltage (MV) network is typically accomplished through a step-down transformer along with an AC/DC converter. Such a converter can be a single-stage solution, where the AC power is converted to DC in a single step as shown in →03a, where the DC power should meet the electrolyzer’s requirements.

On the other hand, the converter can be a two-stage solution, where the AC power is converted to DC and then this DC power is converted to another DC level that meets an electrolyzer’s requirements as shown in →03b.

  • 03a Single-stage solution.
  • 03b Two-stage solution.

03 General classification of electrolyzer power supplies.

Each of these configurations can utilize different converters, as illustrated by the state-of-the-art options highlighted in this article. In addition to the above-mentioned AC supplied systems, electrolyzers can be coupled to a DC distribution system. This requires only DC/DC converters for the voltage level matching.

Single-stage power supplies
As described in the prior section, single-stage solutions convert AC power from a transformer to DC power that meets the electrolyzer’s requirements under different operating conditions.

Different converters can be utilized for this purpose, as illustrated in →04 where four configurations are utilized as state-of-the-art options.

  • 04a Six-pulse thyristor rectifier.
  • 04b Twelve-pulse thyristor rectifier.
  • 04c Two-level voltage source rectifier.
  • 04d Three-level active neutral point-clamped voltage source rectifier.

04 State-of-the-art single-stage power supplies for electrolyzers.

The first option is the six-pulse thyristor rectifier, whose topology is shown in →04a. Despite its simplicity, this option introduces higher harmonic current content on the grid-side and high current ripples on the electrolyzer side along with high reactive power requirements. Such harmonic content can be reduced using the twelve-pulse thyristor rectifier option, which is presented in →04b. Moreover, the twelve-pulse thyristor rectifier can enable higher power through the parallel operation of two six-pulse rectifiers, although this requires a more complex transformer.

On the other hand, fully controlled options can be utilized, where, eg the two-level or three-level active voltage source rectifiers can be utilized as shown in →04c – d. Both options provide smoother electrolyzer current, whereas the prior options present a low frequency component in the electrolyzer current. Furthermore, both options significantly improve grid-side performance in terms of reactive power and current harmonics. However, lower efficiency is expected under these options due to increased semiconductor losses. In addition, both active options suffer from higher current stress due to the boosting nature of these topologies.

Two-stage power supplies

Compared to the prior single-stage options, the two-stage options convert the AC power from the transformer to DC power using an AC/DC converter, and then convert this DC power to another DC voltage level that matches the electrolyzer requirements as shown in →05.

  • 05a Twelve-pulse diode rectifier-fed interleaved buck converter.
  • 05b Two-level voltage source rectifier-fed interleaved buck converter.

05 State-of-the-art two-stage power supplies for electrolyzers.

The first option is presented in →05a. It is based on a twelve-pulse diode rectifier as an uncontrolled AC/DC converter, followed by an interleaved DC/DC buck converter. This solution enhances the reactive power content on the grid side compared to a 12-pulse thyristor rectifier, resulting in a compatible, yet slightly less efficient solution to power the electrolyzer.

In spite of these positive results, the current harmonic remains a challenge and additional harmonic filters must be utilized especially for weak grids. These reactive power and current harmonic challenges can be further addressed by replacing the twelve-pulse diode rectifier with a two-level active voltage source rectifier as shown in →05b. This solution, compared to the prior boosting single-stage two-level voltage source rectifier, avoids the increased current stresses in the AC/DC conversion stage. In other words, the single-stage two-level active rectifier requires a lower voltage on the AC side compared to the two-stage option that is depicted in →05b. Furthermore, the two-stage solution introduces an additional current stress to the DC capacitors as the AC/DC and the DC/DC stages both share the common DC link.

Conclusion
Power electronics will play an important role in terms of coping with and accelerating current and future plans for hydrogen production. Although the state-of-the-art power supply architectures that have been reviewed here can cope with the dynamic behavior of an electrolyzer, solutions with higher power density and efficiency that also offer lower weight at lower cost will be fundamental.

Special attention should be paid to applications in which hydrogen is produced with renewable energy. The inherent fluctuations of such energy sources will also demand new electrical layouts. Control methods, together with new semiconductor materials such as silicon carbide and novel power converters may result in solutions that are compatible with both the grid and the electrolyzer, while simultaneously reducing complexity. 

References
[1] “Green hydrogen supply, a guide to policy making.” International Renewable Energy Agency (IRENA) May 2021, p. 39.
[2] IEA. The future of hydrogen, seizing today’s opportunities. June, 2019. Available: https://iea.blob.core.windows.net/assets/9e3a3493-b9a6-4b7d-b499-7ca48e357561/The_Future_of_Hydrogen.pdf [Accessed August 11, 2022].
[3] IEA. Net zero by 2050, a roadmap for the global energy sector. October, 2021. Available: https://iea.blob.core.windows.net/assets/deebef5d-0c34-4539-9d0c-10b13d840027/NetZeroby2050-ARoadmapfortheGlobalEnergySector_CORR.pdf [Accessed August 11, 2022].

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