Green hydrogen | 'Solid-oxide electrolysers can handle variable wind power without shortening lifespan': study




Danish scientists test Topsoe stack for 12 weeks, but warn that further data needed to assess system-level durability


Danish scientists claim to have proven that a solid-oxide electrolyser (SOE) stack can operate using an intermittent and variable wind power supply, without any extra degradation that would shorten the machine’s lifetime and push up costs.


In a peer-reviewed study, published last week in the journal Applied Energy, academics from the Technical University of Denmark and engineers from Danish electrolyser manufacturer Topsoe tested one of the company’s 7kW solid-oxide stacks with a power supply that mimicked the profile of one of Denmark’s onshore wind farms.


The stack was also operated with a steady flow of power, in order to compare the effects.


In total, the machine was operated for 2,104 hours (the equivalent of around 88 days), of which around 70% (1,493 hours) was with a fluctuating supply.


The tests revealed that operating with a fluctuating power supply made no difference to the stack degradation (which affects how long the stack lasts before it needs to be replaced).


“Compared to steady operation, the stack shows no signs of additional degradation in dynamic operation,” the study said. “Thus the [stack] has been proven robust and flexible enough to handle fluctuating wind power supplies under both operating strategies.”


SOEs operate at high temperatures of up to 700°C, meaning they can take hours to heat up from cold. As a result they are often dismissed as unsuitable for operating with variable renewables such as wind and solar, which require fast ramp-up and ramp-down in response to fluctuating supply.


However, once hot, SOEs can be kept warm (on “hot standby”) in order to ramp up in a matter of seconds in response to variable renewable supply — in fact this is how green hydrogen and ammonia producer First Ammonia plans to operate its SOE plant in Texas, the company told Hydrogen Insight in a recent interview.


Even so, the authors warned that further testing would be required to assess the system-level degradation effects of operating with fluctuating supply.


A stack is an electrochemical cell in which the chemical reaction that splits water into hydrogen and oxygen takes place. An electrolyser is often made up of multiple stacks, configured into a larger system for hydrogen production.


The report comes as interest in SOE machines is on the rise, on account of their higher efficiency compared to alkaline or proton exchange membrane (PEM) equivalents — several manufacturers claim around 25% — when co-located with industrial processes such as ammonia production.


But SOEs are significantly more expensive to install than other types of electrolyser, and the ceramic stacks that make up the machines are notoriously short-lived.


In fact, fertiliser producer Yara recently invested in a Danish tech start up that claims to be developing an SOE that can last for ten years, compared to what the company says is currently a typical two-year lifetime.


However, US manufacturer Bloom has disputed this baseline, arguing that its stacks last longer than two years (although it has not yet revealed its estimated lifetime), while Topsoe has in the past told Hydrogen Insight that its stacks last for four years.



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