The Critical Role of Power Conversion in the Hydrogen Economy
Hydrogen is one of the few storage media that can absorb intermittent renewable output and store that energy for weeks or months, making it useful for grid balancing in ways that batteries are not. Power electronics are one of the key enablers for hydrogen to become a controllable energy carrier in the modern electricity grid.
For example, electrolysis, used for the industrial production of hydrogen, requires high direct current (DC) with low current ripple to avoid large electrical and operational inefficiencies. To satisfy these present needs, power supply technologies are generally used, such as traditional thyristor rectifiers or more advanced converters.
Similarly, fuel cells typically require a power-conditioning system to step up the low-voltage DC to match the high-voltage AC grid and to properly re-inject into the grid. Without advanced power topologies, wide-bandgap semiconductors, and sophisticated control loops, hydrogen cannot be successfully used for long-duration energy storage.
This article clarifies the importance of power conversion in the hydrogen value chain, describes the most used topologies, and discusses the innovations introduced by wide-bandgap semiconductors.
High-current, low-voltage rectification
Water electrolyzers, which extract hydrogen from water, are massive energy consumers. The two most commercially mature technologies for green hydrogen production are alkaline and proton-exchange membrane (PEM) electrolyzers.
Because the individual electrochemical cells operate at average operating voltages between 1 and 4 VDC, even full multi-cell stacks operate at relatively low total DC voltages (typically a few hundred volts). To deliver multi-megawatts of power at these low voltages, the system needs to draw very high-intensity direct current (DC), often thousands to tens of thousands of amperes. This power must be efficiently converted and controlled from a high-voltage three-phase AC grid connection, as shown in Figure 1.
The usual solution for large installations is the 12-pulse diode rectifier with interleaved choppers. It consists of a 12-pulse AC/DC diode front end and multi-phase DC/DC interleaved switching converters. A phase-shifting transformer supplies two six-pulse diode bridges that cancel low-order harmonics, and an interleaved chopper stage based on IGBTs controls the current delivered to the stack and smooths ripple.
The alternative solution is thyristor-based rectifiers that produce considerably lower-order harmonics, unlike 12-pulse diode rectifiers, which require large passive filters. However, they are very robust and can operate reliably in tough industrial environments, even with severe overload conditions.
Conventional line-commutated diode and thyristor front ends are inherently nonlinear loads from a power electronics perspective. They inject non-sinusoidal currents, which introduce high total harmonic distortion (THD) and a distorted power factor to the AC grid.
Especially in large industrial green hydrogen projects (e.g., electrolyzer plants >1 MW), these distortion levels can exceed the harmonic limits defined in the power quality standards IEEE 519 or IEC 61000-3-12.
Active front-end (AFE) converters and modular multi-level converters have been proposed to alleviate the risk of harmonics. Unlike passive rectifying, AFE employs controlled switching devices (e.g., IGBTs) to shape a nearly sinusoidal input current. This allows them to have nearly unity power factor and low THD.
Jeju Island, South Korea (Figure 2), is becoming a testbed for an industrial-scale hydrogen production project. Last year, for the first time, 100% of Jeju’s total electricity was generated from renewable energy produced by wind and solar power plants (for a four-hour period, 11 a.m. to 3 p.m., under favorable wind and solar conditions). The large electrolyzers and the power converters needed to run them struggle to integrate renewables.
KraftPowercon, a provider of industrial power supply solutions, supplied its 5-MW rectifier solution for Jeju’s project. The system, based on the containerized H2Kraft concept, supplies a third-party electrolyzer, providing controlled DC power from the medium-voltage grid interface. Its rectifier system is based on AFE architecture, and the company claims a THDI below 3% (meeting strict IEEE 519 grid codes).
Often, the rectifier stage is followed by a DC/DC conversion stage. This is especially true for electrolyzers fed by variable renewables, where accurate current control is required to maintain stack stability and reduce current ripple to an absolute minimum. Ohmic I2R heating is not the only undesirable effect of high current ripple. It also accelerates catalyst degradation and increases gas crossover (hydrogen leaking into the oxygen channel), which is a major safety hazard and degrades gas purity during low-load operation.
At this stage, the main megawatt-scale industrial installations are dominated by non-isolated multi-phase interleaved buck (chopper) converters in which the galvanic isolation is provided by the upstream line-frequency AC transformer. However, point-to-point DC/DC topologies such as dual active bridge and LLC resonant converters are gaining popularity for next-generation modular and solid-state transformer architectures. These designs use medium- or high-frequency transformers in small DC/DC modules to achieve local galvanic isolation without bulky, heavy AC magnetics.
These isolated and high-frequency topologies are quickly moving away from conventional silicon IGBTs to silicon carbide (SiC) MOSFETs. At higher voltages, SiC devices can switch much faster with low switching losses, allowing engineers to reduce the size of passives and transformers and significantly increase power density and efficiency.
Using SiC devices, engineers can design converters that switch much faster without serious thermal issues. The higher switching frequencies directly translate into smaller, lighter passive components (L and C filters), allowing a multi-megawatt power converter to be packaged in standard, weatherized outdoor enclosures located immediately adjacent to electrolyzer stacks or fuel-cell blocks.
Prodrive Technologies’ Hydra platform is a next-generation, SiC MOSFET–based high-power rectifier system, targeting green hydrogen electrolyzers. It can deliver 6-MW power in a scalable configuration that can be expanded to a 24-MW system, with high peak efficiency (98.7%) and a small footprint suitable for large-scale industrial hydrogen plants (Figure 3). Hydra provides sub-1% THDI, fault ride-through, and reactive power control in about 1.7 MW/m2.
Fuel cells: the reverse problem
In fuel cells, the converter problem is the reverse of electrolyzers. A PEM fuel-cell stack produces a DC voltage that drops significantly as the load current increases along its polarization curve. The converter must accept this sagging, wide-ranging input voltage and raise it up to a stable, higher-regulated output for grid or bus connection (Figure 4).
Importantly, conventional fuel-cell stacks are strictly unidirectional energy sources, and subjecting a fuel-cell stack to reverse current results in severe, irreversible electrode damage and membrane degradation. Therefore, the converter port directly connected to the fuel-cell stack must be strictly unidirectional.
In hybrid DC microgrids, bidirectional power conversion is used only at the system level, where a single multi-port or multi-stage converter connects the unidirectional fuel cell and a bidirectional energy storage port (e.g., battery array or supercapacitors) for dynamic load changes and energy buffering. A notable exception to this is reversible solid oxide cell systems, where the same electrochemical stack is explicitly used in both the fuel-cell-generation and water-electrolysis modes.
Two basic technologies can be used to address these needs. The first is the multi-phase interleaved boost converter, which shares current among parallel phases to reduce ripple and distribute thermal stress across devices. The second is high-gain topologies such as quasi-Z-source and dual-Z-source networks, switched-inductor/switched-capacitor boost cells, and dual-switch or dual-input topologies.
For instance, a dual-Z-source, grid-connected topology for megawatt-scale hydrogen fuel cells has recently been proposed. This topology provides a higher output voltage of approximately 21.3% and a lower switch-voltage stress of approximately 8.3% at the rated output power in comparison with the traditional Z-source boost converter. It is aimed at the high-power, grid-connected fuel-cell installations now under pilot for stationary power generation.
Siemens is partnering with FuelCell Energy to accelerate the deployment of fuel-cell power generation. Siemens will provide the electrical balance of plant (EBOP) system design and supply for FuelCell Energy’s installations under a memorandum of understanding, with the aim to develop projects at a scale of 100+ MW (Figure 5).
The partnership tackles a growing bottleneck: As grid interconnection queues and permitting delays stall traditional buildouts, energy-intensive industries are increasingly looking for on-site power. Siemens’s electrical infrastructure expertise combined with FuelCell Energy’s fuel-cell technology can smooth the path from fuel-cell stack to grid-ready power, according to the companies.
Familiar renewable territory
As electrolyzer plants become large enough to be significant grid loads, and fuel cells become large enough to operate as grid-connected generation or ancillary-service assets, both must meet the same fault-ride-through, reactive power, and harmonic distortion requirements that historically have been associated with wind and solar inverters. That, in fact, takes hydrogen power conversion into territory that power electronics engineers already know from renewables, even if load and source characteristics on the DC side are distinctly their own.
As hydrogen production and consumption scales from demonstration projects to gigawatt-class infrastructure, the power converter is no longer a commodity accessory bolted onto the electrolyzer or fuel-cell stack but a first-order design variable. A few-tenths-of-a-percent efficiency gain will make a real difference to project economics, and the industry’s ability to hit its cost and scale targets will be as much a function of power electronics engineering as it is of catalyst chemistry.
The post The Critical Role of Power Conversion in the Hydrogen Economy appeared first on Power Electronics News.