The production of green hydrogen requires renewable energy to power the electrolysers that split water molecules into H2 and oxygen.
And under the “additionality” rules to access the much-needed subsidies in the US and EU (under current guidelines), it will be necessary for developers to use power from new renewables projects — which, on the basis of cost and ease of construction, are most likely to be intermittent wind and solar power.
Given the importance of wind and solar to the green hydrogen sector, it may be surprising to learn that the impact of intermittent (or variable) power supply on electrolysers is not yet fully understood, according to a new “comprehensive review” of 130 previously published studies into alkaline and PEM electrolysis published in the International Journal of Hydrogen Energy.
The knowns
The study does point out that there is a known negative impact on the performance of electrolysis systems when powered by variable renewable supply.
“Both the performance and durability of electrolysers are impacted by the frequency of intermittent cycles during intermittent operation,” says the report, entitled Impacts of intermittency on low-temperature electrolysis technologies: A comprehensive review.
“Quantitative studies incorporating intermittent profiles from renewable energies have shown that the average efficiency of electrolyzers is lower than under steady nominal conditions.
“There is unanimous agreement among studies that variations in temperature and electrical load strongly impact efficiency, gas purity, and durability.”
In other words, the continuous ramping up and down of electrolysers from wind- and solar-based input reduces the amount of green hydrogen that electrolyser systems can produce in both the short and long term.
It adds that intermittency does not just impact the electrolyser stack — the part where water molecules are split into hydrogen and oxygen — but the balance of plant (BoP) too, including purification units, circulation pumps, fans and control systems, “leading to an overall decrease of the system efficiency during intermittent operation”.
“The highest efficiency of 59.88% HHV [higher heating value] was achieved at full capacity, compared to 43.96% HHV under wind-type conditions,” the report explains.
It points to one study from 2023 that found that a 60kW PEM electrolyser required 67kWh to produce 1kg of hydrogen at maximum load, but this fell to 80kWh/kg when operating at 60% of load, and 140kWh/kg at 30%.
“Performance losses were mainly attributed to the energy consumption of the BoP components, where 80% were associated to the recirculation pump.”
The report explains that higher currents lead to a higher amount of heat generation in the stack, and that operating the stack at lower than optimal temperatures results in significant losses in system.
“When supplied with intermittent power conditions, the electrolyzer struggles to maintain its nominal temperature due to frequent start-up, shut-down periods and fluctuations coming from the electrical load profile,” it says.
“Beyond the thermal aspect, efficiency decrease is also attributed to hydrogen losses, arising from unwanted mechanisms occurring at the stack level, eg, crossover phenomena [in which oxygen and water residues appear in the hydrogen], shunt current [which can reduce energy efficiency and cause corrosion] in the case of alkaline technology etc, but also from regeneration processes during the operation of the purification units and hydrogen venting and leaks at the system level.”
The paper explains that one study found that the gas dryer operation [to remove water vapour] accounted for 90% of the total hydrogen losses at the system level.
“In the case of PEM electrolysers, an additional issue related to filling and draining phenomena arises when the system operates intermittently.
“Transport mechanisms of water from the oxygen to the hydrogen side through the polymer membrane rises the water level in the hydrogen separator. To counterbalance these phenomena, drainages are scheduled during the system operation when the water level in the hydrogen separator exceeds the tolerated threshold. However, these draining phenomena lead to pressure drops and consequently, variation of the hydrogen flow at the system outlet.”
Regular start-ups and shutdowns of electrolysers also cause long-term performance issues, the study adds (see panel below for detailed explanation).
“Intermittent operation involves fluctuations in the electrical load profile, with variations in current density, voltage, and operating conditions (temperature, pressure, flow rate …)... These dynamic operational features generate thermal and mechanical stresses, including shocks and vibrations, affecting the electrochemical performance and long-term mechanical stability of the system,” the report explains.
One study on PEM electrolysis found “a significant performance decay with increasing number of short-time scale fluctuations, which suggests that the impact on durability is closely linked to the frequency of intermittent events”.
Another PEM-focused study found that the “irreversible degradation rate” under a PV-only scenario simulation was four times higher than when PV was integrated with a back-up battery to reduce starts and stops in operation.
The “same outcomes” were found in alkaline electrolysis studies, in which on-off cycles resulted in a drop in hydrogen production at the stack, which was attributed to “the progressive formation of a barrier to active sites caused by the accumulation and coalescence of gas bubbles on the upper part of the electrodes, resulting in increased kinetic resistance”.
When gas bubbles are not evenly distributed, local hotspots can form on the membrane and catalysts that can “induce thermal stress to the mechanical structure”.
Uncertainties
But while most studies reviewed by researchers from Engie’s Crigen Lab and the University of Franche-Comté in eastern France suggested that intermittency negatively impacted durability, “some authors have investigated the effect of specific dynamic conditions and have shown that intermittency may have in some cases positive effects on performance”.
Some studies found that giving rest periods to electrolysis cells that make up stacks allowed them to partially recover some of their performance degradiation, suggesting that integrating rest periods into operation would help reduce irreversible decay — something that would naturally be factored into solar-only operations, as the sun doesn’t shine at night.
It concludes that there is “a lack of consensus in the literature regarding the impacts of intermittency on the decay rate of electrolysers”.
Unknowns
The researchers argue that their review “has uncovered several gaps in the existing literature”.
“Most publications examining the effects of intermittency on the performance indicators of electrolyzers are grounded in dynamic tests under part-load conditions or synthetic profiles mimicking the behavior of renewable energy sources.
“However, very few quantitative studies of intermittency based on actual operational data profiles, and especially at the system level, are available to date.”
They add: “It is also worth noting that no long-term study based on renewable profiles has been identified in the literature.
“However, a comprehensive understanding of the impacts of intermittency relies on analyzing intermittent events at various time scales. Applying annual renewable profiles would, in this sense, help to capture the effects of seasonal intermittencies on the degradation of electrolyzers.”
The study also point out that there have been no studies into the impact of intermittency on the degradation of BoP components, and how this might affect gas purity.
In addition to this, there is an “absence of standardized methods and test protocols for assessing the impacts of intermittency”.
“Up to now, most studies rely on comparing the average performances of electrolyzers under intermittent and steady nominal conditions. However, this approach raises two fundamental issues that are not considered: i) the average capacity factor of the tested intermittent profile is not equal to the nominal operating point of the electrolyzer. Thus, it amounts to comparing the system performances at two distinct average operating points, which fails to provide additional insights into the dynamic effects of intermittency on system performance.
It continues: “ii) for studies that have incorporated dynamic effects of intermittency into their analysis, the parameters often studied include variations in the frequency and amplitude of intermittent cycles endured by the system. However, the majority of identified publications that have adopted this approach are related to durability and degradation studies. Integrating additional parameters may offer further insights into the performance and behavior of electrolyzers in response to specific dynamic effects. They may also aid in distinguishing the impact factor associated with various inherent features of intermittent operation.”
The paper argues that understanding the impact of intermittent operation is “crucial to enhance the level of knowledge on electrolysis technologies and improve system design and operation”.
“Ultimately, these efforts may promote the extensive development of renewable electrolyzers and improve the cost-effectiveness of large-scale projects in the coming years.”
And it adds that there should be further investigation into the impact of intermittency on “additional indicators, such as system diagnosis and control design… to elaborate effective mititgation strategies”.
